PHYSIOLOGY OF LARGE HEMISPHERES
OF THE BRAIN AND CEREBELLUM.
In preceding
lecture we have been concerned with many of the subconscious motor activities integrated in the spinal cord
and brain s especially those responsible for posture and librium.
In the present chapter we will discus control of
motor function by the cerebral o and cerebellum as well as their relationship t
basal ganglia and the other
lower centers. I of this control is "voluntary" in contradistinction to the subconscious control
effected by the lower centers. Yet we will also see that at least motor
functions of the cerebral cortex and cerebellum are not entirely
"voluntary."
THE CORTEX—THE PRIMARY AND PREMOTOR AREAS
Figure
illustrates a broad area of the cerebral cortex that is concerned either with sensation from the somatic areas of the body
or with control of body movement.
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. 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 an exceptional capability to use the hand, the fingers, and the
thumb to perform highly
dexterous manual tasks, and 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, 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 EXTRAPYRAMIDAL
SYSTEM
The term extrapyramidal motor system is widely
used in clinical circles to denote all those portions of the brain and brain stem that contribute to motor
control but that are not part of
the direct pyramidal system. This includes the basal ganglia, the reticular formation of the brain stem, the
vestibular nuclei, and often the red nuclei as well. However, this is such an all-inclusive and
diverse group of motor control areas that it is difficult to ascribe specific
neurophysiological functions to the extrapyramidal system as a whole. For this reason, we have discussed the
functions of the separate
portions of the extrapyramidal system individually.
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: the anterior
lobe, the posterior lobe, and 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, 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. 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.
Afferent
Pathways from the Brain. 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 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; 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 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: the dorsal spi-nocerebeliar
tract and the ventral spinocerebellar tract These two tracts originate in the sacral, lumbar,
and thoracic segments of the cord. Similar tracts 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.
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: the cerebellar cortex and the sensory afferent tracts to the cerebellum. Each time an input
signal arrives in the cerebellum, it divides and goes in two directions: directly to one of the deep nuclei and 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:
(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
The Functional Unit of the Cerebellar Cortex—the Purkinje Cell. The cerebellum has approximately 30 million nearly identical functional
units. 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, 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, 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
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.
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
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.
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.
COMPLEX MOVEMENTS ELICITED BY STIMULATING THE AREA Electrical stimulation of the premotor area will
often elicit complex contractions of groups of muscles. Occasionally,
vocalization occurs, or rhythmic movements such as alternate thrusting of a leg forward and
backward, coordinate moving of the
eyes, chewing, swallowing, or contortions of parts of the body into different postural positions.
Some
neurophysiologists have called this area the motor association area and have ascribed special capabilities to it to control
coordinated movements involving many
muscles simultaneously. In fact, it is peculiarly organized to perform such a function for the following reasons: (1) it has long
subcortical neuronal connections with
the sensory association areas of the parietal lobe; (2) it has direct
subcortical connections with the
primary motor cortex; (3) it connects with areas in the thalamus contiguous with the thalamic areas that
connect with the primary motor cortex;
and (4) most important of all, it has abundant direct connections with the basal ganglia and the cerebellum, both of which
feed back to the primary motor area
through the thalamus.
The Pyramidal Tract (Corticospinaf Tract). The most important output pathway from the motor
cortex is the pyramidal tract, also
called the corticospinal tract, which is illustrated in
Figure 53-5. The pyramidal tract
originates about 60 per cent from the primary motor cortex, 20 per cent from the premotor cortex, and 20 per cent from the
somatic sensory areas posterior to the central sulcus. After leaving the cortex
it passes through the posterior limb of the internal capsule (between the caudate nucleus and
the putamen of the basal ganglia)
and then downward through the brain stem, forming the pyramids of the medulla. By far the majority of the pyramidal fibers then cross to the opposite
side and descend in the lateral corticospinal
tracts of the cord, finally terminating principally on the interneurons in the intermediate
regions of the cord gray matter. However,
some of the fibers in human beings (but not in most lower animals) terminate directly on the anterior motor
neurons.
A few of the
fibers do not cross to the opposite side in the medulla but pass ipsilaterally down the cord in the ventral corticospinal tracts, but
these fibers also cross mainly to
the opposite side of the cord either in the neck or the upper thoracic region.
The most
impressive fibers in the pyramidal tract are a population of large myelinated fibers with mean diameter of about 16 microns.
These originate from the
giant pyramidal cells, also
called Bete cells,
that are found only in the primary motor cortex. These cells are about 60 microns in
diameter, and their fibers transmit
nerve impulses to the spinal cord at a velocity of about
FUNCTION OF THE LARGE LATERAL ZONE OF THE CEREBELLAR HEMISPHERE—THE "PLANNING" AND "TIMING" FUNCTIONS
In human beings,
the lateral zones of the two cerebellar hemispheres have become very highly developed and greatly
enlarged, along with the human capability
to perform intricate movements with the hands and fingers and along with the ability to speak. Yet, strangely
enough, these large lateral portions of the cerebellar hemispheres have no direct input of
information from the peripheral parts
of the body. Also, almost all the communication between these lateral eerebellar areas and the cortex is not with the primary
motor cortex itself but instead
with the premotor area and primary and association somatic sensory areas. Even so, destruction of the lateral portions of
the cerebellar hemispheres along with their deep nuclei, the dentate nuclei,
can lead to extreme in-coordination of the purposeful movements of the hands, fingers, feet,
and speech apparatus. This has
been hard to understand because of lack of direct communication between this part of the cerebellum and the primary motor
cortex. However, recent experimental
studies suggest that these portions of the cerebellum are concerned with two important aspects of motor control: (1) the
"planning" of sequential movements,
and (2) the "timing" of the sequential movements.
1.
Review of Medical Physiology
by W.F.Ganong, 11th ed., 1983. – P. 33-77.
2. Textbook of Medical Physiology by Guyton,
10 ed., 2003. – P. 663-675, 678-687.