1. ROLE OF MEDULLA IN REGULATION OF FUNCTIONS OF THE ORGANISM.
2. FUNCTION OF LARGE HEMISPHERES OF THE BRAIN AND CEREBELLUM.
3. PHYSIOLOGY OF CEREBRAL CORTEX.
Metencephalon (We mean that metencephalon or hind-brain is combain medulla oblongata and pons.)
a) Neuronal composition, nuclei (Medulla oblongata is a continuation of spinal cord. In medulla oblongata are present efferent neurons, interneurons, neurons of asdendens and descendens tracts, ending of afferents neurons. Functions of metencephalon are reflectory, conductive. The grey matter of hind-brain is present in the look of congestion (nucleus). Classification of nucleus of medulla oblongata according to functional properties: moving (n. nervus hypoglossus, n. nervus accessorius), combain (n. nervus vagus, n. nervus glossopharyngeus), substantia formatio reticularis. Classification of nucleus of pons: moving (n. nervus abducens), sensetive (n. nervus vestibulocochlearis), combain (n. nervus facialis, n. nervus trigeminus), substantia formatio reticularis.)
b) Reflexes (1. Chain reflexes are the compound reflector acts, in which one reflexs is a direct cause of rise of future reflexes. In these reactions, take place moving nucleus of medulla oblongata. These reflexes provide chewing and swallowing of food.
Reflexes, which are direct on supporting of muscle tone are neck and vestibular. Neck reflexes send up in the case of excitement of proprioreceptors of neck muscles. In these cases change the tone of extensor muscles. If the head throw at backward the tone of muscles extensor of upper extremities increase and tone of muscles extensor of lower extremities decrease. If the head put down the tone of muscles extensor of upper extremities decrease and tone of muscles extensor of lower extremities increase. Turning of the head in right side add to the change of tone of muscles extensor in the side of turhing the head. In this case eyeballs are moving in the opposite side. All neck reflexes are polisynaptic.
Vestibular reflexes are static. Static reflexes of position provide supporting of pose in the space.
c) Conductive system (All nervous impulses from tracts of Goll’s and Burdach’s about deep muscle-joint sensitivity transmit to cortex of big hemisphere. Lateral corticospinalis tract begin from the big pyramidal cells of Bets and cross in the hind-brain. To hind-brain go tractus corticobulbaris, which transmit impulses from cortex to mooving nucleus of the cranial nerves. Substantia reticularis of hind-brain give impulses to spinal cord.)
Mesencephalon (We mean that that mesencephalon is combain corpora quadrigemina and pedunculi cerebri.)
a) Funtions, nuclei (Functions: reflectory, conductive. Nucleus of mesencephalon: n. nervus oculumotorius, n. nervus trochlearis, substantia nigra, nucleus ruber, nuclei substantia reticularis.)
b) Reflexes (The anterior quadrigeminal bodies are the primary optic centres and involved in certain reflexes responding to light stimuli, including the visual orientation reflexes. Reflex movements of the eyes are induced by impulses conveyed to the eye musvles from the nuclei of the oculomotor and trochlear nerves. The anterior quadrigeminal bodies take part in the pupillary reflexes. The posterior quadrigeminal bodies are the primary auditory centres. They are involved in the performance of sound orientation reflexes: the pricking up of the ears of animals, turning of the head and body towards a new sound.
Vestibular reflexes are static and statokinetic. Static reflexes of straight provide restore of pose. Statokinetic reflexes direct on supporting of pose in the case of act the change of speed moving. These may be horizontal, vertical (in the lift; increase tone of muscles extensor in the go up mowing and increase tone of muscles flexor in the go down mowing and), angular.)
c) Conductive system (Fibres of the mid-brain connect cortex, mid-brain with hind-brain and spinal cord. If the brain stem of a cat is severed above the medulla oblongata so that the red nuclei are above the incision a special state of the body musculature develops called decerebrate rigidity. This state is characterized by sharply increased tone of the extensor muscles. The extremities are greatly extended; the head is tilted back and the tail raised.)
Diencephalon (We mean that that diencephalon is combain thalamencephalon and hypothalamus.)
a) Specific nuclei of thalamus (Specific nuclei have connection with the projected zones of cortex. They are sensetive (geniculate bodies; transmit impulses of tractus opticus, sound stimulus proprioreceptors of scin to the cortex) and motor (transmit impulses to moving centres of cortex).
b) Associative nuclei of thalamus (Information are goes to them from periferal parts and specific nuclei of thalamus. There are connections betweeuclei and zones. Associative nuclei of thalamus are sensory. For example, nucleus of pillow: it lateral part transmits information about vision to associative zones of occipital part, it medial part transmits information about hearing to associative zones of temporal part of cortex.)
c) Nonspecific nuclei of thalamus (Their neurons are polysensetive. They give the answers of excitement on any stimulus. They have high connection with reticular formation, that’s why the answers are in all part of the cortex.)
d) Morpho-funtional peculiarities of hypothalamus (Hypothalamus has 48 pairs of nucleus. According to the functional meaning, it may be divided on 3 parts: anterior, middle and posterior. Anterior part of hypothalamus produced two kinds of substances: liberins and statins. Middle and posterior parts of hypothalamus are zones without hematoencephalic baarrier. In these parts are present neurons, which are sensetive to the change of temperature, chemical components of blood.)
e) Role of hypothalamus in regulation of behavior (Anterior part of hypothalamus is responsible for increase of muscle tone, aggression. Middle part of hypothalamus is responsible for beggining of complex of somatic reactions, which direct at search of water. Posterior part of hypothalamus is responsible for beggining of complex of somatic reactions, which direct at search of food; in this part present the centers of satisfaction.)
Biceps-reflex
Make a stroke by neurologic hammer on the tendon of biceps above radial bend. The hand of observed person must be semibent and maximally relaxed. With this aim his forearm you must lie on forearm of observer. Call rellexon two hands. Compare reactions.
Triceps-reflex
Make a stroke by neurologic hammer on the tendon of m. triceps brachii above radial process. The hand of observed person must be relaxed, bend in radial articulation and abducted by observer on the back and outside. Call reflex on two hands. Compare reactions.
Patellar reflex
Make a stroke by neurologic hammer on the tendon of quadriceps of hip under the patella. Observed person is sitting with compound legs. Muscle must be relaxed. Call reflex on two legs, with and without test of Yendrasyk. Compare reactions.
Ahill reflex.
Offer observed person to stand on chair with freely hanging feet. Make a stroke by nurologic hammer on Ahill’s tendon to legs, with and without test of Yendrasyk. Compare reactions.
Superciliar reflex
Make a light stroke by neurologic hammer on margin of superciliar arc. Call reflex on both sides. Compare reactions.
Lid reflex
Touch by small piece of cotton wool to the cornea. Compare reactions.
Mandibular reflex
Make the light stroke on chin (the mouth must be open). Pay attention to the reaction.
Physiological role of reticular formation of metecephalon, mesencephalon (Most of the various sensory pathways relay impulses from sense organs via 3- and 4-neuron chains to particular loci in the cerebral cortex. The impulses are responsible for perception and localization of individual sensations. Impulses in these systems also relay via collaterals to the reticular activating system (RAS) in the brain stem reticular formation. Activity in this system produces the conscious, alert state that makes perception possible.
The reticular formation occupies the midventral portion of the medulla and midbrain. It is made up of myriads of small neurons arranged in complex, intertwining nets. Located within it are centers that regulate respiration, blood pressure, heart rate, and other vegetative functions. In addition, it contains ascending and descending components that play important roles in the adjustment of endocrine secretion, the formation of conditioned reflexes, the regulation of sensory input, and consciousness. The reticular activating system is a complex polysynaptic pathway. Collaterals funnel into it not only from the long ascending sensory tracts but also from the trigeminal, auditory, and visual systems and the olfactory system. The complexity of the neuroet and the degree of convergence in it abolish modality specificity, and most reticular neurons are activated with equal facility by different sensory stimuli. The system is therefore nonspecific, whereas the classic sensory pathways are specific in that the fibers in them are activated only by one type of sensory stimulation. Part of the RAS by passes the thalamus to project diffusely to the cortex. Another part of the RAS ends in the intralaminar and related thalamic nuclei, and from them is projected diffusely and nonspecifically to the whole neocortex. The RAS is intimately concerned with the electrical activity of the cortex. It has inhibitory (excitement of the Rentshow cells add to the inhibition of motoneurons; and direct inhibitory influences of motoneurons of spinal cord) and excitive (increase tone of exrensor muscles, contraction of sceletal muscles) influences.)
Structure-functional characteristic of the subcortical nucleus – basal ganglions
a) Components, functions (Physiologically, the basal ganglia are considered to be comprised of the caudate nucleus, putamen, and globus pallidus. However, the substantia nigra, subthalamus, and important portions of both the thalamus and reticular formation operate in close association with these and therefore are actually part of the basal ganglia system for motor control.
b) Afferent and efferent connection (
c) Circulation of excitement in the basal ganglion (cycle of putamen and nucleus caudatum) (Function of the Caudate Nucleus and Putamen (The Neostriatum). The caudate nucleus and putamen seem to function together to initiate and regulate gross intentional movements of the body. To perform this function they transmit impulses through two different pathways: (1) into the globus pallidus, thence by way of the thalamus to the cerebral cortex, and finally downward into the spinal cord through the corticospinal pathway; (2) downward through the globus pallidus and the substantia nigra by way of short axons into the reticular formation and finally into the spinal cord mainly through the reticulospinal tracts.
In summary, the neostriatum helps control gross intentional movements that we normally perform subconsciously. However, this control also involves the motor cortex, with which the neostriatum is closely connected.)
d) Notion about extrapyramidal system
During subsequent development, the three primary brain vesicles develop into five secondary brain vesicles. The names of these vesicles and the major adult structures that develop from the vesicles follow (see Table 1 ):
· The telencephalon generates the cerebrum (which contains the cerebral cortex, white matter, and basal ganglia).
· The diencephalon generates the thalamus, hypothalamus, and pineal gland.
· The mesencephalon generates the midbrain portion of the brain stem.
· The metencephalon generates the pons portion of the brain stem and the cerebellum.
· The myelencephalon generates the medulla oblongata portion of the brain stem
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A second method for classifying brain regions is by their organization in the adult brain. The following four divisions are recognized (see Figure 1 ).
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· The cerebrum consists of two cerebral hemispheres connected by a bundle of nerve fibers, the corpus callosum. The largest and most visible part of the brain, the cerebrum, appears as folded ridges and grooves, called convolutions. The following terms are used to describe the convolutions:
o A gyrus (plural, gyri) is an elevated ridge among the convolutions.
o A sulcus (plural, sulci) is a shallow groove among the convolutions.
o A fissure is a deep groove among the convolutions.
The deeper fissures divide the cerebrum into five lobes (most named after bordering skull bones)—the frontal lobe, the parietal love, the temporal lobe, the occipital lobe, and the insula. All but the insula are visible from the outside surface of the brain.
A cross section of the cerebrum shows three distinct layers of nervous tissue:
o The cerebral cortex is a thin outer layer of gray matter. Such activities as speech, evaluation of stimuli, conscious thinking, and control of skeletal muscles occur here. These activities are grouped into motor areas, sensory areas, and association areas.
o The cerebral white matter underlies the cerebral cortex. It contains mostly myelinated axons that connect cerebral hemispheres (association fibers), connect gyri within hemispheres (commissural fibers), or connect the cerebrum to the spinal cord (projection fibers). The corpus callosum is a major assemblage of association fibers that forms a nerve tract that connects the two cerebral hemispheres.
o Basal ganglia (basal nuclei) are several pockets of gray matter located deep inside the cerebral white matter. The major regions in the basal ganglia—the caudate nuclei, the putamen, and the globus pallidus—are involved in relaying and modifying nerve impulses passing from the cerebral cortex to the spinal cord. Arm swinging while walking, for example, is controlled here.
· The diencephalon connects the cerebrum to the brain stem. It consists of the following major regions:
o The thalamus is a relay station for sensory nerve impulses traveling from the spinal cord to the cerebrum. Some nerve impulses are sorted and grouped here before being transmitted to the cerebrum. Certain sensations, such as pain, pressure, and temperature, are evaluated here also.
o The epithalamus contains the pineal gland. The pineal gland secretes melatonin, a hormone that helps regulate the biological clock (sleep-wake cycles).
o The hypothalamus regulates numerous important body activities. It controls the autonomic nervous system and regulates emotion, behavior, hunger, thirst, body temperature, and the biological clock. It also produces two hormones (ADH and oxytocin) and various releasing hormones that control hormone production in the anterior pituitary gland.
The following structures are either included or associated with the hypothalamus.
o The mammillary bodies relay sensations of smell.
o The infundibulum connects the pituitary gland to the hypothalamus.
o The optic chiasma passes between the hypothalamus and the pituitary gland. Here, portions of the optic nerve from each eye cross over to the cerebral hemisphere on the opposite side of the brain.
· The brain stem connects the diencephalon to the spinal cord. The brain stem resembles the spinal cord in that both consist of white matter fiber tracts surrounding a core of gray matter. The brain stem consists of the following four regions, all of which provide connections between various parts of the brain and between the brain and the spinal cord. (Some prominent structures are illustrated in Figure 2 ).
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o The midbrain is the uppermost part of the brain stem.
o The pons is the bulging region in the middle of the brain stem.
o The medulla oblongata (medulla) is the lower portion of the brain stem that merges with the spinal cord at the foramen magnum.
o The reticular formation consists of small clusters of gray matter interspersed within the white matter of the brain stem and certain regions of the spinal cord, diencephalon, and cerebellum. The reticular activation system (RAS), one component of the reticular formation, is responsible for maintaining wakefulness and alertness and for filtering out unimportant sensory information. Other components of the reticular formation are responsible for maintaining muscle tone and regulating visceral motor muscles.
· The cerebellum consists of a central region, the vermis, and two winglike lobes, the cerebellar hemispheres. Like that of the cerebrum, the surface of the cerebellum is convoluted, but the gyri, called folia, are parallel and give a pleated appearance. The cerebellum evaluates and coordinates motor movements by comparing actual skeletal movements to the movement that was intended.
The limbic system is a network of neurons that extends over a wide range of areas of the brain. The limbic system imposes an emotional aspect to behaviors, experiences, and memories. Emotions such as pleasure, fear, anger, sorrow, and affection are imparted to events and experiences. The limbic system accomplishes this by a system of fiber tracts (white matter) and gray matter that pervades the diencephalon and encircles the inside border of the cerebrum. The following components are included:
· The hippocampus (located in the cerebral hemisphere)
· The denate gyrus (located in cerebral hemisphere)
· The amygdala (amygdaloid body) (an almond-shaped body associated with the caudate nucleus of the basal ganglia)
· The mammillary bodies (in the hypothalamus)
· The anterior thalamic nuclei (in the thalamus)
· The fornix (a bundle of fiber tracts that links components of the limbic system)
Functional assymetry of big hemispheres:
a) Manifestations (somatic, sensory, psychological)
b) Methods of investigation
c) Practical meaning
Structure-functional characteristic of cerebellum (The cerebellum has long been called a silent area of the brain principally because electrical excitation of this structure does not cause any sensation and rarely any motor movement. However, as we shall see, removal of the cerebellum does cause the motor movements to become highly abnormal. The cerebellum is especially vital to the control of very rapid muscular activities such as running, typing, playing the piano, and even talking. Loss of this area of the brain can cause almost total incoordination of these activities even though its loss causes paralysis of no muscles.
But how is it that the cerebellum can be so important when it has no direct capability of causing muscle contraction? The answer to this is that it both helps plan the motor activities and also monitors and makes corrective adjustments in the motor activities elicited by other parts of the brain, it receives continuously updated information on the desired program of muscle contractions from the motor control areas of the other parts of the brain. And it receives continuous information from the peripheral parts of the body to determine the instantaneous status of each part of the body – its position, its rate of movement, forces acting on it, and so forth. It is believed that the cerebellum compares the actual instantaneous status of each part of the body as depicted by the peripheral information with the status that is intended by the motor system. If the two do not compare favorably, then appropriate corrective signals are transmitted instantaneously back into the motor system to increase or decrease the levels of activation of the specific muscles.
Since the cerebellum must make major motor corrections extremely rapidly during the course of motor movements, a very extensive and rapidly acting cerebellar input system is required both from the peripheral parts of the body and from the cerebral motor areas. Also, an extensive output system feeding equally as rapidly into the motor system is necessary to provide the necessary corrections of the motor signals.)
a) Afferent system (cells of cortex, afferent fibres and their cooporation)
b) Efferent system (nucleus, efferent connection)
c) Zones of cerebellum between cortex and nucleus
d) Functions and their neurons securing
FUNCTIONS OF THE BASAL GANGLIA
Before attempting to discuss the functions of the basal ganglia in human beings, we should speak briefly of the better known functions of these ganglia in lower animals. In birds, for instance, the cerebral cortex is poorly developed while the basal ganglia are highly developed. These ganglia perform essentially all the motor functions, even controlling the voluntary movements in much the same manner that the motor cortex of the human being controls voluntary movements. Further more, in the cat, and to a lesser extent in the dog, decortication removes only the discrete types of motor functions and does not interfere with the animal’s ability to walk, eat, fight, develop rage, have periodic sleep and wakefulness, and even participate naturally in sexual activities. However, if a major portion of the basal ganglia is destroyed, only gross stereotyped movements remain, which were discussed earlier in relation to the mesencephalic animal.
Finally, in the human being, cortical lesions in very young individuals destroy the discrete movements of the body, particularly of the hands and distal portions of the lower limbs, but do not destroy the person’s ability to walk crudely, to control equilibrium, or to perform many other subconscious types of movements. However, simultaneous destruction of a major portion of the caudate nucleus almost totally paralyzes the opposite side of the body except for a few stereotyped reflexes integrated in the cord or brain stem.
With this brief background of the overall function of the basal ganglia, we can attempt to dissect the functions of certain portions of the basal ganglia system, realizing that the system actually operates, along with the motor cortex and cerebellum, as a total unit and that individual functions cannot be ascribed to the different individual parts of the basal ganglia.
Inhibition of Motor Tone by the Basal Ganglia. Though it is wrong to ascribe a single function to all the basal ganglia, nevertheless, one of the general effects of diffuse basal ganglia excitation is to inhibit muscle tone throughout the body. This effect results from inhibitory signals transmitted from the basal ganglia to both the motor cortex and the lower brain stem. Therefore, whenever widespread destruction of the basal ganglia occurs, this causes muscle rigidity throughout the body. For instance, when the brain stem is transected at the mesencephalic level, which removes the inhibitory effects of the basal ganglia, the phenomenon of decerebrate rigidity occurs.
Yet, despite this general inhibitory effect of the basal ganglia, stimulation of certain specific areas within the basal ganglia can elicit positive muscle contractions and at times even complex patterns of movements.
Function of the Globus Pallidus. It is alreadyclear that almost all the outflow of signals from the basal ganglia are channelled through the globus pallidus en route back to the cortex or on their way to lower brain centers. However, in addition to this motor relay function of the globus pallidus, the globus pallidus seems to have still another function that operates in close association with the subthalamus and brain stem to help control the axial and girdle movements of the body. These movements provide the background positioning of the body and proximal limbs so that the more discrete motor functions of the hands and feet can then be performed. That is, a person wishing to perform an exact function with a hand first positions the body, next positions the legs and arms, and finally tenses all the axial and girdle muscles to provide background positioning and stability of all the proximal portions of the body. These associated tonic contractions are supposedly initiated by circuits in the globus pallidus but operate also through the axial and girdle motor control areas of the brain stem. Lesions of the globus pallidus seriously interfere with the attitudinal movements that are necessary to position the hand and, therefore, make it difficult or impossible for one to use the hand for discrete activities.
Electrical stimulation of the globus pallidus while an animal is performing a gross body movement often will stop the movement in a static position, the animal holding that position for many seconds while the stimulation continues. This fits with the concept that the globus pallidus is involved in some type of servo feedback motor control system that is capable of locking the different parts of the body into specific positions.
THE MOTOR CORTEX—THE PRIMARY AND PREMOTOR AREAS
The posterior part of this area, the somatic sensory cortex, we have already discussed. Lying directly anterior to the somatic sensory area in front of the central sulcus, as illustrated in the figure, and occupying approximately the posterior one half of the frontal lobes is the motor cortex. Nerve signals originating from this region cause muscle contractions in different parts of the body.
The motor cortex is divided into two separate divisions, the primary motor area and the premotor area. The primary area contains very large pyramidal motor neurons that send their fibers all the way to the spinal cord through the corticospinal tract and therefore have almost direct communication with the anterior motor neurons of the cord for control of either individual muscles or small groups of muscles.
This area is frequently called areas VI and VIII because it occupies both these areas in the Brodmann classification of brain topology. The premotor cortex has very few neurons that project nerve fibers directly to the spinal cord. Instead, most of the nerve signals generated in this area cause more complex muscle movements, usually involving groups of muscles performing some specific task, rather than individual muscles. To achieve these results, the premotor area mainly sends its signals into the primary motor cortex to excite multiple groups of muscles. Some of these signals pass directly to the motor cortex through subcortical nerve fibers, but the premotor cortex also has extensive connections with the basal ganglia and cerebellum, both of which transmit signals back by way of the thalamus to the motor cortex. Thus the premotor cortex, the basal ganglia, the cerebellum, and the primary motor cortex constitute a complex overall system for voluntary control of muscle activity.
The functional part of the cerebral cortex is composed mainly of a thin layer of neurons 2 to
Neurohistologists have divided the cerebral cortex into almost 100 different areas, which have slightly different architectural characteristics. Yet in all these different areas except the hippocampal region there still persist representations of all the six major layers of the cortex.
The posterior part of this area the somatic sensory cortex, we have already discussed. Lying directly anterior to the somatic sensory area in front of the central sulcus, as illustrated in the figure, and occupying approximately the posterior one half of the frontal lobes is the motor cortex. Nerve signals originating from this region cause muscle contractions in different parts of the body.
The motor cortex is divided into two separate divisions, the primary motor area and the premotor area. The primary area contains very large pyram-idal motor neurons that send their fibers all the way to the spinal cord through the corticospinal tract and therefore have almost direct communication with the anterior motor neurons of the cord for control of either individual muscles or small groups of muscles. Even very weak electrical stimuli in this primary motor area usually will elicit a muscle contraction somewhere in the body. The primary motor cortex is also frequently referred to as area IV of the cortex because this area containing the large pyramidal cells is area IV in Brod-mann’s histological classification of the different cortical areas, as will be explained in Chapter 55. This area is frequently called areas VI and VIII because it occupies both these areas in the Brod–mann classification of brain topology. The premotor cortex has very*few neurons that project nerve fibers directly to the spinal cord, instead, most of the nerve signals generated in this area cause more complex muscle movements, usually involving groups of muscles performing some specific task, rather than individual muscles. To achieve these results, the premotor area mainly sends its signals into the primary motor cortex to excite multiple groups of muscles. Some of these signals pass directly to the motor cortex through subcort-ical nerve fibers, but the premotor cortex also has extensive connections with the basal ganglia and cerebellum, both of which transmit signals back by way of the thalamus to the motor cortex. Thus the premotor cortex, the basal ganglia, the cerebellum, and the primary motor cortex constitute a complex overall system for voluntary control of muscle activity.
The Motor Topographical Map in the Human Being. The topographical map of the motor cortex in the human being is quite different from that of lower animals. The reason for this is that the human being has developed two important capabilities involving the motor cortex that are not found in lower animals. These are (1) an exceptional capability to use the hand, the fingers, and the thumb to perform highly dexterous manual tasks, and (2) use of the mouth, lips, tongue, and facial muscles to talk. Therefore, there are very high degrees of representation of the hand, mouth, and facial regions in the human motor cortex. * Figure 53-3 illustrates the degrees of representation of the different muscle areas of the body in the motor cortex as mapped by Penfield and Rasmussen. This was done by stimulating the different areas of the motor cortex in human beings undergoing neurosurgical operations, Note that more than one half of the entire primary motor cortex is concerned with controlling the hands and the muscles of speech. Point stimulations in these areas of the motor cortex will cause contraction of a single muscle or even a portion of a single muscle. On the other hand, in those areas of the primary cortex with less intense degree of representation such as in the trunk area, electrical stimulation will usaalTy contract a group of muscles instead.
The Supplemental Motor Area
Area Ms II in Figure 53-2, located on the medial surface of the frontal lobe slightly anterior to the primary motor cortex, is called the supplemental motor area. This area requires considerably stronger electrical stimuli to cause muscle contraction than does the primary motor area. Also, the movements involve coordinate contractions of many muscles in contradistinction to the much more discrete movements elicited from the primary area. Furthermore, many of the movements are bilateral rather than unilateral, and the contractions occur most often in the trunk or proximal portions of the limbs, causing the animal to position itself in some special attitude. Also, there may be rotation of the head, movement of the eyes, vocalization, or yawning.
THE RED NUCLEUS AND THE RUBROSPINAL TRACT—THEIR RELATIONSHIP TO THE SYSTEM
The red nucleus, illustrated in Figure 53-6, is located in the mesencephalon and functions in close association with the pyramidal tract. This nucleus has two parts, a superior portion, called the parvocellidar portion, that is composed of small neurons, and an inferior portion, called the mag-nocellular portion, that contains many large neurons. The large neurons of the magnocellular portion give rise to the rubrospinal tract that crosses to the opposite side in the lower brain stem and
follows a course parallel to the corticospinal tract into the lateral columns of the spinal cord. This tract partially overlaps the cortieospinal tract but on the average lies slightly anterior to it. The rahrospinal fibers terminate mainly on the inter-neurons of the intermediate areas of the cord gray matter along with the cortieospinal fibers, and a few of the rubrospinal fibers also terminate on the anterior motor neurons, along with some of the cortieospinal fibers.
The red nucleus receives two major input pathways. One of these is from the motor cortex via the corticornbral tract, terminating mainly in the magnocellular portion of the red nucleus and thus stimulating the fibers of the rubrospinal tract. The second source of input fibers is from the cerebellum, which provides fibers to both the parvocellular and magnocellular portions of the red nucleus.
Function of the Corticorubrospinal System. The magnocellular portion of the red nucleus has a somatotopical representation of all the muscles of the body, as is true of the motor cortex. Therefore, stimulation of a single point in this portion of the red nucleus will cause contraction of either a single muscle or small group of muscles. However, the fineness of representation of the different muscles is far less developed than is true in the motor cortex. This is especially true in human beings who have a relatively small red nucleus.
The corticorubrospinal pathway serves as an accessory route for the transmission of relatively discrete signals from the motor cortex to the spinal cord. When the pyramidal fibers are destroyed without destroying this other pathway, discrete movements can still occur, except that the movements of the fingers and hands are considerably impaired. Wrist movements are still well developed, which is not true wrhen the corticorubrospinal pathway is also blocked. Therefore, the pathway through the red nucleus to the spinal cord is associated far more with the pyramidal system than with the vestibuloreticulospinai system that controls mainly the axial and girdle muscles of the body. Furthermore, the rubrospinal tract lies in the lateral columns of the spinal cord, along with the cortieospinal tracts, and terminates more on the interneurons and motor neurons that control the distal muscles of the limbs. Therefore, the cortieospinal and rubrospinal tracts together are frequently calledthe lateral motor system of the cord, in contradistinction to the vestibuloreticulo–spinal system that lies mainly medially in the cord and is called the medial motor system of the cord.
THE CEREBELLUM AND ITS MOTOR FUNCTIONS
The cerebellum has long been called a silent area o/the brain principally because electrical excitation of this structure does not cause any sensation and rarely any motor movement. However, as we shall see, removal of the cerebellum does cause the motor movements to become highly abnormal. The cerebellum is especially vital to the control of very rapid muscular activities such as running, typing, playing the piano, and even talking. Loss of this area of the brain can cause almost total incoordination of these activities even though its loss causes paralysis of no muscles.
But how is it that the cerebellum can be so important when it has no direct capability of causing muscle contraction? The answer to this is that it both helps plan the motor activities and also monitors and makes corrective adjustments in the motor activities elicited by other parts of the brain. It receives continuously updated information on the desired program of muscle contractions from the motor control areas of the other parts of the brain. And it receives continuous information from the peripheral parts of the body to determine the instantaneous status of each part of the body—its position, its rate of movement, forces acting on it, and so forth. It is believed that the cerebellum compares the actual instantaneous status of each part of the body as depicted by the peripheral information with the status that is intended by the motor system. If the two do not compare favor-ably, then appropriate corrective signals are transmitted instantaneously back into the motor system to increase or decrease the levels of activation of the specific muscles.
Since the cerebellum must make major motor corrections extremely rapidly during the course of motor movements, a very extensive and rapidly acting cerebellar input system is required both from the peripheral parts of the body and from the cerebral motor areas. Also, an extensive output system feeding equally as rapidly into the motor system is necessary to provide the necessary corrections of the motor signals.
THE ANATOMICAL FUNCTIONAL AREAS OF THE CEREBELLUM
Anatomically, the cerebellum is divided into three separate lobes by two deep fissures, as shown in Figure 53-8: (a) the anterior lobe, (b) the posterior lobe, and (c) ft\Q flocculonodular lobe. The flocculonodular lobe is the oldest of all portions of the cerebellum; it developed along with (and functions with) the yestibular system in controlling equilibrium, as was discussed in the previous chapter. Because of its ancient heritage, it is frequently called the archicerebellum. The anterior lobe and part of the midportion of the posterior lobe are also old; this is called the paleocerebellum. On the other hand, almost 90 per cent of the posterior lobe is recent in origin and is especially highly developed in primates and human beings; this is called the neocerebellum.
The Longitudinal Functional Divisions of the Anterior and Posterior Lobes.
From a functional point of view, the anterior and posterior lobes are organized not by lobes but instead along the longitudinal axis, as illustrated in Figure 53-9 which shows the human cerebellum after the lower end of the posterior cerebellum has been rolled downward from its normally hidden position. Note down the center of the cerebellum a narrow band separated from the remainder of the cerebellum by shallow grooves. This is called the vermis. In this area most cerebellar control functions for the muscle movements of the axial body, the neck, and the shoulders and hips are located.
To each side of the vermis is a large, laterally protruding cerebellar hemisphere, and each of these hemispheres is divided into an intermediate zone and a lateral zone. The intermediate zone of the hemisphere is concerned with the control of muscular contractions in the distal portions of both the upper and lower limbs, especially of the hands and fingers and feet and toes. On the other hand, the lateral zone of the hemisphere operates at a much more remote level, for this area seems to join into the overall planning of sequential motor movements. Without this lateral zone, most discrete motor activities of the body lose their appropriate timing and therefore become highly incoordinate, as we shall discuss more fully later.
Topographical Representation of the Body in the Cerebellum. In the manner that the sensory cortex, the motor cortex, the basal ganglia, the red nuclei, and the reticular formation all have topograph– ical representations of the different parts of the body, so also is this true for parts of the cerebellum. Figure 53-10 illustrates two separate such representations in a small monkey, showing one to be located in the anterior lobe and the other in the posterior lobe. Note that the axial portions of the body lie in the vermal part of the cerebellum whereas the limbs and facial regions lie in the intermediate zones of the two hemispheres. These topographical representations receive afferent nerve fibers from all the respective parts of the body. In turn, they send motor signals into the same respective topographical areas of .the motor cortex, the basal ganglia, the red nucleus, and the reticular formation.
However, note that the large lateral portions of the cerebellar hemispheres do not have topographical representations of the body. These areas of the cerebellum connect mainly with the association areas of the brain, especially the premotor area 10
of the frontal cortex and the somatic sensory and sensory association areas of the parietal cortex. Presumably this connectivity with the association areas allows the lateral portions of the cerebellar hemispheres to play important roles in planning and coordinating the sequential patterns of muscular activities.
The Input Pathways to the Cerebellum
Afferent Pathways from the Brain. The basic input pathways to the cerebellum are illustrated in Figure 53—11. An extensive and important afferent pathway is the corticopontocerebellar pathway, which originates mainly in the motor cortex but to a lesser extent in the sensory cortex as well and then passes by way of the pontile nuclei and pontocerebellar tracts to the contra-lateral hemisphere of the cerebellum. In addition, important afferent tracts originate in the brain stem; they include (a) an extensive olivocerebellar tract, which passes from the inferior olive to all parts of the cerebellum; this tract is excited by fibers from the motor cortex, the basal ganglia, widespread areas of the reticular formation, and the spinal cord; (b) vestibulocerebellar fibers, some of which originate in the vestibular apparatus itself and others from the vestibular nuclei; most of these terminate in the flocculonodular lobe and fas-tigial nucleus of the cerebellum; and (c) reticulocerebellar fibers, which originate in different portions of the retic-ular formation and terminate mainly in the midline cerebellar areas (the vermis).
Afferent Pathways from the Periphery. The cerebellum also receives important sensory signals directly from the peripheral parts of the body through four separate tracts, two of which are located dorsally in the cord and two ventrally. The two most important of these tracts are illustrated in Figure 53-12: the dorsal spi-nocerebe!iar tract and the ventral spinocerebellar tract These two tracts originate in the sacral, lumbar, and thoracic segments of the cord. Similar tracts, not shown in Figure 53-12, originate in the neck segments of the cord and course roughly along with the dorsal and ventral spinocerebellar tracts in their passage to the cerebellum. These are the cuneocerebellar tract that joins the dorsal spinocerebeliar tract and the rostral spinocer-ebellar tract that joins the ventral spinocerebeilar tract. The dorsal tracts enter the cerebellum through the inferior cerebellar peduncle and terminate in the cerebellum on the same side as their origin. The two ventral tracts enter the cerebellum through the superior cerebellar peduncle, but they terminate in both sides of the cerebellum.
The signals transmitted in the dorsal spinocerebellar tracts come mainly from the muscle spindles and to a lesser extent from other somatic receptors throughout the body, such as from the Golgi tendon organs, the large tactile receptors of the skin, and the joint receptors. All these signals apprise the cerebellum of the momentary status of muscle contraction, degree of tension on the muscle tendons, positions and rates of movement of the parts of the body, and forces acting on the surfaces of the body.
On the other hand, the ventral spinocerebellar tracts receive less information from the peripheral receptors. Instead, they are excited mainly by the motor signals arriving in the spinal cord from the brain through the corticospinal and rubrospinal tracts. Thus, this ventral fiber pathway tells the cerebellum that the motor signals have indeed arrived at the cord, and it also apprises the cerebellum of the intensity of the signals.
The spinocerebellar pathways can transmit impulses at velocities as great as 100 meters per second, which is the most rapid conduction of any pathway in the entire central nervous system. This extremely rapid conduction is important for the instantaneous apprisal of the cerebellum of changes that take place in the status of the body.
In addition to the signals in the spinocerebellar tracts, other signals are transmitted through the dorsal and dorsolateral columns to the medulla and then relayed from there to the cerebellum. Likewise, signals are transmitted through the spinoreticular pathway to the reticular formation of the brain stem and through the spino-olivary pathway to the inferior olivary nucleus and then relayed from both these areas to the cerebellum. Thus, the cerebellum continually collects information about all parts of the body even though it is operating at a subconscious level.
Output Signals from the Cerebellum
The Deep Cerebeflar Nuclei and the Efferent Pathways. Located deep in the cerebellar mass are three deep cerebellar nuclei—the dentate, interpositus, and fasiigial nuclei. The vestibular nuclei in the medulla also function in some respects as if they were deep cerebellar nuclei because of their direct connections with the cortex of the flocculonodular lobe. All the deep cerebellar nuclei receive signals from two different sources: (1) the cerebellar cortex and (2) the sensory afferent tracts to the cerebellum. Each time an input signal arrives in the cerebellum, it divides and goes in two directions: (1) directly to one of the deep nuclei and (2) to a corresponding area of the cerebellar cortex overlying the deep nucleus; then, a short time later, the cerebellar cortex relays its output signals also to the same deep nucleus. Thus, all the input signals that enter the cerebellum eventually end in the deep nuclei. We shall discuss this circuit in greater detail later. Three major efferent pathways lead out of the cerebellum, as illustrated in Figure 53-13:
(1) A pathway that begins in the cortex of the lateral zone of the cerebellarhemisphere, then passes to the dentate nucleus, next to the ventrolateral andventroan-terior nuclei of the thalamus, and finally to the cerebral cortex. Thispathway plays an important role in helping coordinate “voluntary” motor activitiesinitiated by the cerebral cortex.
(2) A pathway that originates in the midline structures of the cerebellum (thevermis) and then passes through the fastigial nuclei into the medullary and pantileregions of the brain stem. This circuit functions in close association with theequilibrium apparatus to help control equilibrium and also, in association with theretic-ular formation of the brain stem, helps control the postural attitudes of thebody. It was discussed in detail in the previous chapter in relation to equilibrium.
(3) A pathway that originates in the intermediate zone of the cerebellahemisphere, between the verrnis and the lateral zone of the cerebellar hemisphere,then passes (a) through the nucleus interpositus to the ventrolateral and ventroanterior nuclei of the thalamus, and thence to the cerebral cortex, (b) to several midline structures of the thalamus and thence to the basal ganglia, and (c) to the red nucleus and reticular formation of the upper portion of the brain stem. This circuit is believed to coordinate mainly the reciprocal contractions of agonist and antagonist muscles in the peripheral portions of the limbs—especially in the hands, fingers, and thumbs.
THE NEURONAL CIRCUIT OF THE CEREBELLUM
The human cerebellar cortex is actually a large folded sheet, approximately 17 cm wide by 120 cm long, with the folds lying crosswise, as illustrated in Figures 53-9 and 53-10. Each fold is called a folium. And lying deep in the folded mass of cortex are the deep nuclei.
The Functional Unit of the Cerebellar Cortex—the Purkinje Cell. The cerebellum has approximately 30 millioearly identical functional units, one of which is illustrated to the left in Figure 53-14, shown mainly in red color. This functional unit centers on the deep nuclear cell and on the Purkinje cell, of which there are also 30 million in the cerebellar cortex.
Note to the right in Figure 53-14 the three major layers of the cerebellar cortex: the molecular layer, the Purkinje cell layer, and the granular cell layer. Then, beneath these layers, the deep nuclei are located far within the center of the cerebellar mass.
The Neuronal Circuit of the Functional Unit As illustrated in Figure 53-14, the output from the functional unit is from a deep nuclear cell. However, this cell is continually under the influence of both excitatory and inhibitory influences. The excitatory influences arise from direct connections with the afferent fibers that enter the cerebellum. The inhibitory influences arise entirely from the Purkinje cells in the cortex of the cerebellum.
The afferent inputs to the cerebellum are mainly of two types, one called the climbing fiber type and the other called the mossy fiber type. There is one climbing fiber for about 10 Purkinje cells. After sending collaterals to several deep nuclear cells, the climbing fiber projects all the way to the molecular layer of the cerebellar cortex where it makes about 300 synapses with the soma and dendrites of each Purkinje cell. This climbing fiber is distinguished by the fact that a single impulse in it will always cause a single, very prolonged, and peculiar oscillatory type of action potential in each Purkinje cell with which it connects. Another distinguishing feature of the climbing fibers is that they all originate in the inferior olive of the medulla, whereas the cerebellar afferent fibers from all other sources are almost entirely of the mossy type.
The mossy fibers also send collaterals to excite deep nuclear cells. Then these fibers proceed to the granular layer of the cortex where they synapse with hundreds of granule cells. These in turn send very small axons, less than 1 micron in diameter, up to the outer surface of the cerebellar cortex to enter the molecular layer. Here the axons divide into two branches that extend 1 to 2 millimeters in each direction parallel to the folia. There are literally millions of these parallel nerve fibers in each small segment of the cerebellar cortex (there are about 1000 granule cells for every Purkinje cell). It is into this molecular layer that the dendrites of the Purkinje cells project, and 80,000 to 200,000 of these parallel fibers synapse with each Purkinje cell; as these fibers pass along their 1 to 2 mm course, each of them contacts about 50 Purkinje cells. Yet, the mossy fiber input to the Purkinje cell is quite different from the climbing fiber input because stimulation of a single mossy fiber will never elicit an action potential in the Purkinje cell; instead, large numbers of mossy fibers must be stimulated simultaneously to activate the Purkinje cell. Furthermore, this activation usually takes the form of prolonged facilitation or excitation that, when it reaches threshold for stimulation, causes repetitive Purkinje cell firing of normal, short-duration action potentials rather than the single prolonged action potential occurring in response to the climbing fiber input.
Thus, the Purkinje cells are stimulated by two types of input circuits—one that causes a highly specific output in response to the incoming signal and the other that causes a less specific but tonic type of response. It should be noted that by far the greater proportion of the afferent input to the cerebellum is of the mossy fiber type, because this represents the afferent input from almost all the cerebellar afferent tracts besides those from the inferior olive. Balance Between Excitation and Inhibition in the Deep Cerebellar Nuclei,
The output signals from the Purkinje cells to the deep nuclei are entirely inhibitory. Therefore, referring again to the circuit of Figure 53-14, one should note that direct stimulation of the deep nuclear cells by both the climbing and the mossy fibers excites them, whereas the signals arriving from the Purkinje cells inhibit them. Normally, there is a continual balance between these two effects so that the degree of output from the deep nuclear cell remains relatively constant at a moderate level of continuous stimulation. On the other hand, in the execution of rapid motor movements, the timing of the two effects on the deep nuclei is such that the excitation appears before the inhibition. Then a few milliseconds later inhibition occurs. In this way, there is first a very rapid excitatory signal fed back into the motor pathway to modify the motor movement, but this is followed within a few milliseconds by an inhibitory signal. This inhibitory signal resembles a “delay-line” negative feedback signal of the type that is very effective in providing damping. That is, when the motor system is excited, a negative feedback signal presumably occurs after a short delay to stop the muscle movement from overshooting its mark, which is the usual cause of oscillation.
Other Inhibitory Cells in the Cerebellar Cortex.
In addition to the granule cells and Purkinje cells, three other types of neurons are also located in the cerebellar cortex; basket cells, stellate cells, and Golgi cells. All these are inhibitory cells with very short axons. Both the basket cells and the stellate cells are located in the molecular layer of the cortex, lying among and stimulated by the parallel fibers. These cells in turn send their axons at right angles across the parallel fibers and cause lateral inhibition of the adjacent Purkinje cells, thus sharpening the signal in the same manner that lateral inhibition sharpens the contrast of signals in many other areas of the nervous system. The Golgi cells lie in the Purkinje cell layer of the cortex, and their dendrites are also stimulated by the parallel fibers of the molecular layer, but the axons from these cells feed back to and inhibit the ‘granule cells instead of the Purkinje cells. The function of this feedback is to limit the duration of the signal transmitted into the cerebellar cortex from the granule cells. That is, within a short fraction of a second after the granule cells are stimulated, they are then inhibited by the feedback. Therefore, a short, transient, pulseiike signal, not a prolonged signal, is transmitted into the parallel fibers and thence to the Purkinje cells.
Special Features of the Cerebellar Neuronal Circuit. A special feature of the cerebellum is that there are no reverberatory pathways in the cerebellar neu-ronal circuits, so that the input-output signals of the cerebellum are very rapid transients that never persist for long periods of time.
Another special feature is that many of the cells of the cerebellum are constantly active. This is especially true of the deep nuclear cells; they continually send output signals to the other areas of the motor system. The importance of this is that decrease of the nuclear cell firing rate can provide an inhibitory output signal from the cerebellum, while an increase in firing rate can provide an excitatory output signal.
FUNCTION Of THE CEREBELLUM IN CONTROLLING MOVEMENTS
The cerebellum functions in motor control only in association with motor activities initiated elsewhere in the nervous system. These activities may originate in the spinal cord, in the reticular formation, in the basal ganglia, or in areas of the cerebral cortex. We will discuss, first, the operation of the cerebellum in association with the spinal cord and lower brain stem for control of postural movements and equilibrium and then discuss its function in association with the motor cortex for control of voluntary movements.
The cerebellum originated phylogeneticaily at about the same time that the vestibular apparatus developed. Furthermore, as was discussed in the previous chapter, loss of the flocculonodular lobes of the cerebellum causes extreme disturbance of equilibrium. Yet, we still must ask the question, what role does the cerebellum play in equilibrium that cannot be provided by the other neuronal machinery of the brain stem? A clue is the fact that in persons with cerebellar dysfunction equilibrium is far more disturbed during performance of rapid motions than during stasis. This suggests that the cerebellum is especially important in controlling the balance between agonist and antagonist muscle contractions during rapid changes in body positions as dictated by the vestibular apparatuses. One of the major problems in controlling this balance is the time required to transmit position signals and kinesthetic signals from the different parts of the body to the brain. Even when utilizing the most rapidly conducting sensory pathways at 100 meters per second, as used by the spinocerebellar system, the delay for transmission from the feet to the brain is still 15 to 20 milliseconds. The feet of a person running rapidly can move as much as 10 inches during this time. Therefore, it is impossible for the brain to know at any given instant during rapid motion the exact position of the different parts of the body.
On The other hand, with appropriate neuronal circuitry, it would be possible for the cerebellum or some other portion of the brain to know how rapidly and in what direction a part of the body was moving 15 to 20 milliseconds earlier and then to predict from this information where the parts of the body should be at the present time. And this seems to be one of the major functions of the cerebellum.
As we have already discussed in relation to the neuronal circuitry of the cerebellum, there are abundant sensory pathways from the somatic areas of the body, especially from the muscles, joints, and skin surface, that feed both into the brain stem and into the older areas of the cerebellum—into the flocculonodular lobes through the vestibular nuclei and into the vermis and intermediate areas of the cerebellum through the dorsal and ventral spinocerebellar tracts and reticulocer-ebellar tracts. Also, the vestibular apparatus is located within a few centimeters of the flocculonodular lobes, allowing no more than a millisecond or so delay in transmission of the vestibular information.
Therefore, during the control of equilibrium, it is presumed that the extremely rapidly conducted vestibular apparatus information is used in a typical feedback control circuit to provide almost instantaneous correction of postural motor signals as necessary for maintaining equilibrium even during extremely rapid motion, including rapidly changing directions of motion. The feedback signals from the peripheral areas of the body help in this process, but their help is presumably contingent upon some function of the cerebellum to compute positions of the respective parts of the body at any given time, despite the long delay time from the periphery to the cerebellum.
Relationship of Cerebellar Function to the Spinal Cord Stretch Reflex
One major component of cerebeilar control of posture and equilibrium is an extreme amount of information transmitted from the muscle spindles to the cerebellum through the dorsal spinocerebel-lar tracts. In turn, signals are transmitted into the brain stem through the cerebellar fastigial nuclei to stimulate the gamma efferent fibers that innervate the muscle spindles themselves. Therefore, a cerebellar stretch reflex occurs that is similar to but more complex than the spinal cord stretch reflex. It utilizes signals that pass all the way to the cerebellum and back again to the muscles. In general, this reflex adds additional support to the cord stretch reflex, but its feedback time is considerably longer, thus prolonging the effect. Through this feedback pathway many of the postural adjustments of the body are believed to occur.
FUNCTION OF THE CEREBELLUM IN VOLUNTARY MUSCLE CONTROL
In addition to the feedback circuitry between the body periphery and the cerebellum, an almost entirely independent feedback circuitry exists between the motor cortex and the cerebellum. This is illustrated in its simplest form in Figure 53-15 and in a much more complex form, involving the basal ganglia also in the control circuit, in Figure 53-16. Most of the signals of this circuit pass from the motor cortex and adjacent cortical areas to the cerebeilar hemispheres and then back to the cortex again, successively, through the dentate and inter-positus cerebeilar nuclei and the ventrolateral and ventroanterior nuclei of the thalamus. These circuits are not involved in the control of the axial and girdle muscles of the body. Instead, they serve two other motor control functions involving respectively (a) the intermediate zone of the cerebeilar hemisphere, and (b) the large lateral zone of this hemisphere. Let us discuss each of these separately.
CEREBELLAR FEEDBACK CONTROL OF THE DISTAL LIMBS THROUGH THE INTERMEDIATE CEREBELLAR ZONE AND INTERPOSITUS NUCLEUS
The intermediate zone of each cerebeilar hemisphere receives information from two sources: (1) direct information from the motor cortex, and (2) feedback information from the peripheral parts of the body, especially from the distal portions of the limbs. After the cerebellum has integrated this information, output signals are then transmitted mainly to the cerebral cortex through relays in the interpositus nucleus and the thalamus. In addition, signals pass directly from the interpositus nucleus to the magnocellular portion (the lower portion) of the red nucleus that gives rise to the rubrospinal tract. The rubrospinal tract in turn innervates especially those portions of the spinal cord gray matter that control the distal parts of the limbs, particularly the hands and fingers.
It is believed that this part of the cerebeilar motor control system provides smooth, coordinate movements of the agonist and antagonist muscles of the distal limbs for the performance of acute purposeful intricate movements. The cerebellum seems to compare the “intentions” of the higher levels of the motor control system, as transmitted to the intermediate cerebeilar hemisphere through the corticopontocerebellar tract, with the “performance” by the respective parts of the body as transmitted back to the cerebellum from the periphery. In fact, the ventral spinothalamic tract even transmits back to the cerebellum a “copy” of the actual motor control signals that reach the anterior motor neurons, and this too is integrated with the signals arriving from the muscle spindles and other pro-prioceptor sensory organs.
Ordinarily, during rapid movements, the motor cortex transmits far more impulses than are needed to perform each intended movement, and the cerebellum therefore must act to inhibit the motor cortex at the appropriate time after the muscle has begun to move. The cerebellum is believed to assess the rate of movement and calculate the length of time that will be required to reach the point of intention. Then appropriate inhibitory impulses are transmitted to the motor cortex to inhibit the agonist muscle and to excite the antagonist muscle. In this way, appropriate “brakes” are applied to stop the movement at the precise point of intention.
Thus, when a rapid movement is made toward a point of intention, the agonist muscle contracts strongly throughout the early course of movement. Then, suddenly, shortly before the point of intention is reached, the agonist muscle becomes completely inhibited while the antagonist muscle becomes strongly excited. Furthermore, the point at which this reversal of excitation occurs depends on the rate of movement and on the previously learned knowledge of the inertia of the system. The faster the movement and the greater the inertia, the earlier the reversal point appears in the course of movement.
Since all these events transpire much too rapidly for the motor cortex to reverse the excitation “voluntarily,” it is evident that the excitation of the antagonist muscle toward the end of a movement is an entirely automatic and subconscious function and is not a “willed” contraction of the same nature as the original contraction of the agonist muscle. We shall see later that in patients with serious cerebellar damage, excitation of the antagonist muscles does not occur at the appropriate time but instead always too late. Therefore, it is almost certain that one of the major functions of the cerebellum is automatic excitation of antagonist muscles at the end of a movement while at the same time inhibiting agonist muscles that have started the movement.
The “Damping” Function of the Cerebellum. One of the byproducts of the cerebellar feedback mechanism is its ability to “damp” muscular movements. To explain the meaning of “damping” we must first point out that essentially all movements of the body are “pendular.” For instance, when an arm is moved, momentum develops, and the momentum must be overcome before the movement can be stopped. And, because of the momentum, all pendular movements have a tendency to overshoot. If overshooting does occur in a person whose cerebellum has been destroyed, the conscious centers of the cerebrum eventually recognize this and initiate a movement in the opposite direction to bring the arm to its intended position. But again the arm, by virtue of its momentum, overshoots, and appropriate corrective signals must again be instituted. Thus, the arm oscillates back and forth past its intended point for several cycles before it finally fixes on its mark, This effect is called an action tremor, or intention tremor.
However, if the cerebellum is intact, appropriate subconscious signals stop the movement precisely at the intended point, thereby preventing the overshoot and also the tremor. This is the basic characteristic of a damping system. All servocon-trol systems regulating pendular elements that have inertia must have damping circuits built into the servomechanisms. In the motor control system of our central nervous system, the cerebellum seems to provide much of this damping function.
Cerebellar Control of Ballistic Movements. Many rapid movements of the body, such as the movements of the fingers in typing, occur so rapidly that it is not possible to receive feedback information either from the periphery to the cerebellum or from the cerebellum back to the motor cortex before the movements are over. These movements are called ballistic movements, meaning that the entire movement is preplanned and is set into motion to go a specific distance and then to stop. Another important example is the saccadic movements of the eyes, in which the eyes jump from one position to the next when reading or when looking at successive points along a road when a person is moving in a car. Much can be understood about the function of the cerebellum by studying the changes that occur in the ballistic movements when the cerebellum is removed. Three major changes occur: (1) the movements are slow to begin, (2) the force development is weak, and (3) the movements are slow to turn off. Therefore, it becomes very difficult to perform the very rapid ballistic movements. Furthermore, it is almost impossible to control how far the movement will go because of the difficulty of turn- ing the movement off once it is begun. Thus, in the absence of the cerebellar circuit the motor cortex has to think very hard to turn ballistic movements on and again has to think hard and take extra time to turn the movement off. Thus, the automatism of ballistic movements is lost.
But how does the cerebellum function in the control of ballistic movements? We do not know the answer to this. The supposition is: When the motor cortex first initiates the movement, it immediately sends signals to the cerebellum at the same time. The first effect of the signals is to excite the deep cerebellar nuclei, and these immediately send an excitatory signal back to the motor cortex, red nucleus, or other motor nuclei to reinforce strongly the onset of the ballistic movement. A few milliseconds later, the signal entering the cerebellum will have had time to go through the delay circuits of the cerebellar cortex and to return by way of the Purkinje cells to the deep cerebellar nuclei, but this time inhibiting these rather than exciting them. Therefore, after this given delay time, this automatic delayed inhibitory signal presumably stops the ballistic movement by turning off the agonist muscle and, because of reciprocal innervation, turning on the antagonist at the same time.
If the student will consider once again the circuitry of the cerebellum as described earlier in the chapter, she or he will see that it is beautifully organized to perform this biphasic, first excitatory and then delayed inhibitory, function that is required for ballistic movements. The student will also see that the time delay circuits of the cerebellar cortex almost undoubtedly are fundamental to this particular ability of the cerebellum.
FUNCTIONS OF CERTAIN SPECIFIC CORTICAL AREAS
Studies in human beings by neurosurgeons have shown that some specific functions are localized to certain general areas of the cerebral cortex. Now present a map of some of these areas as determined by Penfield and Rasmussen from direct electrical stimulation of the cortex or by neurological examination of patients after portions of the cortex had been removed. The lightly shaded areas are primary sensory areas, while the darkly shaded area is the primary motor area (also called voluntary motor area) from which muscular movements can be elicited with relatively weak electrical stimuli. These primary sensory and motor areas have highly specific functions as we have discussed in previous chapters, whereas other areas of the cortex perform more general functions that we call association or cerebration.
THE SENSORY ASSOCIATION AREAS
Around the borders of the primary sensory areas are regions called sensory association areas or secondary sensory areas. In general, these areas extend 1 to
The general function of the sensory association areas is to provide a higher level of interpretation of the sensory experiences.
Destruction of the sensory association area greatly reduces the capability of the brain to analyze different characteristics of sensory experiences. For instance, damage in the temporal lobe below and behind the primary auditory area in the “dominant hemisphere” of the brain often causes a person to lose the ability to understand words or other auditory experiences even though they are heard.
Likewise, destruction of the visual association area in Brodmann’s areas 18 and 19 of the occipital lobe in the dominant hemisphere, or the presence of a brain tumor or other lesion in these areas does not cause blindness or prevent normal activation of the primary visual cortex but does greatly reduce the person’s ability to interpret what is seen. Such a person often loses the ability to recognize the meanings of words, a condition that is called word blindness or dyslexia.
Finally, destruction of the somatic sensory association area in the parietal cortex posterior to primary somatic area I cause the person to lose spatial perception for location of the different parts of the body. In the case of the hand that has been “lost,” the skills of the hand are greatly reduced. Thus, this area of the cortex seems to be necessary for interpretation of somatic sensory experiences.
Possible Mechanisms for Attention and for Searching the Memory Store
We are all aware that we can direct our attention toward certain of our mental activities individually and can also search through our memory store for specific memories. Because of the capability of the generalized thalamocortical system to activate small areas of the cerebral cortex at a time, it is tempting to believe that specific activation of regional portions of the cortex might be the way in which we do indeed direct our attention, and might also be the basis for searching through memory stores.
One other bit of information also suggests that the generalized thalamocortical system might be important in searching for memories: It has been reported that specific lesions in the thalamus are sometimes associated with retrograde amnesia – that is, inability to recall memories that are known to be stored within the brain.
Displays of functional encephalic asymetry:
a) Somatic. Take hand dynamometer, abduct hand from the trunk at right angles. Second hand put down along the trunk. Press with maximum strength fingers and fix the pointer of dynamometer. Do so 5 times with intervals on some minutes. Maximum deflection of the pointer of dynamometer shows maximum strength of the hands. Make determination for both hands.
b) Psychical. Look attentively on proposed table. Determinate where is smile, and where is grief. The key to the test: left – hemispherial person on the first picture shows smile, on the second – grief. Right – hemispherial – the other way round.
Cerebellar tests:
a) Finger – nasal test. Observed person must touch the end of the nose by index finger. The hand must be straight and abduct to the back. Pay attention to the availability of trembling. Fulfil test with opened and closed eyes.
b) Romberg’s test. Offer observed person to stand with put down hands, combined foots and closed eyes. Pay attention is there shaking. If there is no shaking, offer to pick hands up, appraise stability of observed person in present pose.
BRAIN WAVES
Electrical recordings from the surface of the brain or from the outer surface of the head demonstrate continuous electrical activity in the brain. Both the intensity and patterns of this electrical activity are determined to a great extent by the overall level of excitation of the brain resulting from functions in the reticular activating system. The undulations in the recorded electrical potentials, are called brain waves, and the entire record is called an electroencephalogram (EEG).
The intensities of the brain waves on the surface of the scalp range from 0 to 300 microvolts, and their frequencies range from once every few seconds to 50 or more per second. The character of the waves is highly dependent on the degree of activity of the cerebral cortex, and the waves change markedly between the states of wakefulness and sleep and coma.
Much of the time, the brain waves are irregular, and no general pattern can be discerned in the EEG. However, at other times, distinct patterns do appear. Some of these are characteristic of specific abnormalities of the brain, such as epilepsy, which is discussed later. Others occur even iormal persons and can be classified as alpha, beta, theta, and delta waves.
Alpha waves are rhythmic waves occurring at a frequency of between 8 and 13 per second and are found in the EEGs of almost all normal adult persons when they are awake in a quiet, resting state of cerebration. These waves occur most intensely in the occipital region but can also be recorded at times from the parietal and frontal regions of the scalp. Their voltage usually is about 50 microvolts. During sleep the alpha waves disappear entirely, and when the awake person’s attention is directed to some specific type of mental activity, the alpha waves are replaced by asynchronous, higher frequency but lower voltage beta waves. Note that the visual sensations cause immediate cessation of the alpha waves and that these are replaced by low voltage, asynchronous beta waves.
Beta waves occur at frequencies of more than 14 cycles per second and as high as 25 and rarely 50 cycles per second. These are most frequently recorded from the parietal and frontal regions of the scalp. Most beta waves appear during activation of the central nervous system or during tension.
Theta waves have frequencies of between 4 and 7 cycles per second. These occur mainly in the parietal and temporal regions in children, but they also occur during emotional stress in some adults, particularly during disappointment and frustration. They can often be brought out in the EEG of a frustrated person by allowing enjoyment of some pleasant experience and then suddenly removing this element of pleasure; this causes approximately 20 seconds of theta waves. These same waves also occur in many brain disorders.
Delta waves include all the waves of the EEG below 3.5 cycles per second and sometimes as low as 1 cycle every 2 to 3 seconds. These occur in deep sleep, in infancy, and in serious organic brain disease. And they occur in the cortex of animals that have had subcortical transections separating the cerebral cortex from the thalamus. Therefore, delta waves can occur strictly in the cortex independently of activities in lower regions of the brain. Physiologic diagnosis of functional state of mechanisms of regulation in general and particular regulative zones, to realize the functions of cortex of encephalon as the highest regulative CNS level.
Electroencelography
Fix electrodes in frontal, temporal and occipital states. Bioelectric activity of encephalon is registrated when the person is relaxed with eyes closed and later with eyes opened. Define the altitude and frequency of the electroencephalogram. Show it as a scheme in the report.
References:
1. Review of Medical Physiology // W.F. Ganong. – Twentieth edition, 2001. – P. 123-130, 198-216, 224-225.
2. Textbook of Medical Physiology // A.C. Guyton, J.E. Hall. – Tenth edition, 2002. – P. 512-525, 622-634, 659, 663-671.
