ROLE OF AUTONOMIC NERVOUS SYSTEM
IN REGULATION OF VISCERAL FUNCTIONS.
Morpho-functional organization of autonomic nervous system
a)
Sympathetic nervous system (Sympathetic part of autonomic nervous system
includes paravertebral ganglions, prevertebral ganglions,
sympathetic nerves. In the lateral parts of the spinal cord on the thoracic-lumbal level are present sympathetic centre of Yakobson, whose activity regulated by brain stem. Axons of
neurons of sympathetic centre go out from the spinal cord in ventral roots and
form white branches with the ganglions of sympathetic stems. From these stems
go out postganglionic axons and go to the organs of brain, thorax, abdominal
cavity and pelvis. Preganglionic axons, which goes out on the level of segments
of spinal cord, innerevate a few paravertebral and prevertebral ganglions; that is why provide multiplicative
central regulation of different visceral functions
This review examines how the sympathetic nervous
system plays a major role in the regulation of cardiovascular function over
multiple time scales. This is achieved through differential regulation of
sympathetic outflow to a variety of organs. This differential control is a
product of the topographical organization of the central nervous system and a
myriad of afferent inputs. Together this organization produces sympathetic
responses tailored to match stimuli. The long-term control of sympathetic nerve
activity (SNA) is an area of considerable interest and
involves a variety of mediators acting in a quite distinct fashion. These
mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity,
and a host of humoral factors. A key feature of many
cardiovascular diseases is increased SNA. However, rather than there being a
generalized increase in SNA, it is organ specific, in particular to the heart
and kidneys. These increases in regional SNA are associated with increased
mortality. Understanding the regulation of organ-specific SNA is likely to
offer new targets for drug therapy. There is a need for the research community
to develop better animal models and technologies that reflect the disease
progression seen in humans. A particular focus is required on models in which
SNA is chronically elevated.
Historically, the sympathetic nervous system (SNS) has
been taught to legions of medical and science students as one side of the
autonomic nervous system, presented as opposing the parasympathetic nervous
system. This review examines the evidence that over the past decade a new and
more complex picture has emerged of the SNS as a key controller of the
cardiovascular system under a variety of situations. Studies have revealed some
of the central nervous system pathways underlying sympathetic control and where
or how a variety of afferent inputs regulate sympathetic outflow. Our
understanding of how sympathetic nerve activity regulates end organ function
and blood pressure has increased along with the development of new technologies
to directly record SNA in conscious animals and humans. Most importantly,
increasing clinical evidence indicates a role for sympathoactivation
in the development of cardiovascular diseases. Such information highlights the
need to better understand how the SNS interfaces with the cardiovascular system
and how this interaction may result in increased morbidity or mortality.
Aspects of the SNS have been the subject of reviews in the past, and with
between 1,300 and 2,000 publications published per year for the past 5 years
involving various aspects of the SNS, it is not possible to cover in detail the
wealth of recent information on this area. The accent of this review is on the
nature of the activity present in sympathetic nerves, how it affects
cardiovascular function, and how it is implicated in disease processes. It aims
not to simply catalog the studies surrounding these areas, but rather attempts
to distill down observations to provide future directions and pitfalls to be
addressed.
SNS activity provides a critical aspect in the control
of arterial pressure. By rapidly regulating the level of activity, the degree
of vasoconstriction in the blood vessels of many key organs around the body is
altered. This in turn increases or decreases blood flow through organs,
affecting the function of the organ, peripheral resistance, and arterial
pressure. In contrast to the activity present in motor nerves, sympathetic
nerves are continuously active so all innervated blood vessels remain under
some degree of continuous constriction. Since its first description in the
1930s sympathetic nerve activity (SNA) has engendered itself to researchers in
two camps; neurophysiologists have seen its inherent properties as an
opportunity to understand how areas of the central nervous system may be
“wired” to generate and control such activity, while cardiovascular
physiologists saw its regulation of blood flow as a means to measure the
response to different stimuli, drugs, and pathological conditions. However, the
innervation to almost all arterioles and actions on specific organs such as the
heart and kidney is not sufficient to justify its importance. What
distinguishes the SNS is the emerging evidence that overactivity
is strongly associated with a variety of cardiovascular diseases. A key
question is, Does this increased SNA act as a driver of the disease progression
or is it merely a follower? Furthermore, how does increased SNA accelerate the
disease progression? Is it simply that it results in increased vascular
resistance or are there subtle structural changes induced by elevated SNA or
specific actions on organs such as the kidney through its regulation of the
renin-angiotensin system and/or pressure natriuresis?
It was Walter Cannon who portrayed the SNS as central
to the regulation of homeostasis. Cannon showed that when an animal is strongly
aroused, the sympathetic division of its autonomic nervous system “mobilizes
the animal for an emergency response of flight or fight. The sympathico-adrenal system orchestrates changes in blood
supply, sugar availability, and the blood's clotting capacity in a marshalling
of resources keyed to the violent display of energy.” In this setting, the SNS
and parasympathetic nervous system were presented as two opposing forces with
the parasympathetic endorsing “rest and digest” while the SNS “flight and
fight.” An unintended side effect advanced in some textbooks has been to
portray the actions of sympathetic nerves as confined to extreme stimuli. As
will be advanced in this review, the SNS plays a key role in the
moment-to-moment regulation of cardiovascular function at all levels from quiet
resting to extreme stimuli. While SNA can be quite low under quiet resting
conditions, removal of all sympathetic tone via ganglionic blockade
significantly lowers blood pressure. Furthermore, removal of SNA to only one
organ such as the kidney can chronically lower blood pressure in some animals,
indicating its importance in maintaining normal cardiovascular function.
Evidence that
sympathetic nerves are tonically active was
established from the 1850s with the observation that section or electrical
stimulation of the cervical sympathetic nerve led to changes in blood flow in
the rabbit ear. However, it was not until the 1930s that Adrian, Bronk, and Phillips published the first description of
actual sympathetic discharges. They observed two obvious features: 1)
that discharges occur in a synchronized fashion, with many of the nerves in the
bundle being active at approximately the same time, and 2) that
discharges generally occur with each cardiac cycle in a highly rhythmical fashion.
They also noted that by no means was the overall activity level constant as it
was increased by asphyxia or a small fall in blood pressure. This was the first
direct evidence supporting Hunt's assertion in 1899 that “the heart is under
the continual influence of sympathetic impulses.” These early studies answered
a number of questions on the nature of multifiber
discharges, such as whether the activity present in the nerve bundle reflected
that of single fibers firing very rapidly, or groups of fibers firing more or
less synchronously. They also showed that the synchronized activation of
postganglionic nerves was not a function of the ganglia as it could be observed
in preganglionic nerves and that activity was bilaterally synchronous, that is,
that activity in right and left cardiac nerves was the same.
The origin of
the rhythmical discharges was considered in the 1930s to be a simple
consequence of phasic input from arterial baroreceptors, which had been shown
to display pulsatile activity. This proposal had the effect of diminishing the
role of the central nervous system to that of a simple relay station and may go
some way to explaining the lack of further interest in recording SNA until the
late 1960s. Green and Heffron then reexamined the
question of the origin of SNA after noting a rapid sympathetic rhythm (at ∼10 Hz) under certain conditions (mainly reduced
baroreceptor afferent traffic) that was far faster than the cardiac rhythm.
This indicated that the origin of bursts of SNA could not simply be a product
of regular input from baroreceptors. Their suggestion that the fast rhythm did
not have a cardiac or ganglionic origin, but was of brain stem origin,
stimulated interest from neurophysiologists, who could use this phenomena for
the study of the central nervous system.
Postganglionic
sympathetic nerves are composed of hundreds to thousands of unmyelinated
fibers, whose individual contributions to the recorded signal are exceedingly
small. But fortunately, their ongoing activity can be measured from whole nerve
recordings because large numbers of fibers fire action potentials at almost the
same time (synchronization) to give discharges of summed spikes. Although it is
possible to perform single unit recordings from postganglionic nerve fibers, the
favored approach is a multiunit recording. This is obviously a much easier
experimental preparation, which allows recordings in conscious animals.
However, several important points can only be shown from single-unit
recordings. First, while multifiber discharges can
occur at quite fast rates (up to 10 Hz), the frequency of firing in the single
unit is much lower. Average rates in anesthetized rabbits have been recorded
between 2 and 2.5 spikes/s for renal nerves, ∼1.2
spikes/s for splenic nerves in the cat, and between 0.21 and 0.5
spikes/s in the human. This slow firing rate means that the rhythmical
properties of the single-unit discharges are not seen unless their activity is
averaged over time against a reference such as the cardiac cycle or respiration.
Single unit recordings also show the minimal firing interval for postganglionic
neurons is between 90–100 ms. This indicates it is
unlikely that multifiber discharges represent high
frequency impulses from a single neuron, but rather the summation of impulses
from multiple fibers that fire synchronously. These properties have
subsequently been confirmed with single unit recordings in the human. The low
firing rate of individual nerves seems to preclude the same neuron being
activated more than once in each multifiber
discharge. Rather, it would seem that the activated neurons are drawn from a
neuronal pool. It is unlikely that the low firing rate is due to a long
refractory period for the nerves, since the individual nerves can be induced to
fire at quite fast rates by stimuli such as from chemoreceptors or nociceptors.
b)
Parasympathetic nervous system (Parasympathetic patr
of autonomic nervous system includes ganglions (present near organs-effectors
or inside them), parasympathetic nerves. Bodies of the preganglionic
parasympathetic neurons are in the brain stem and in the sacral level of spinal
cord. Axons of preganglion neurons go to the postganglion neurons, which are present in ganglions. The
parasympathetic fibers are in n.oculomotorius, n.facialis, n.glossopharyngeus, n.vagus, sacral nerves. Parasympathetic nervous system also
innervates muscles of vessels, exept sex organs and
may be brain.)
c) Metasympathetic nervous system (Metasympathetic
patr of autonomic nervous system is intramural
ganglions, which are in the organs walls. Reflector arc are present in the wall
of organs too. It regulated by sympathetic and parasympathetic system. It has
sensory, interneuronal, moving chain and own
mediators.)
The autonomic nervous system, like the somatic nervous
system, is organized on the basis of the reflex arch. Impulses initiated in
visceral receptors are relayed via afferent autonomic
pathways to the central nervous system, integrated within it at various levels, and transmitted via efferent pathways
to visceral effectors.
Regulation of the internal environment of the body with regard to
temperature and body fluids is a normal function of the autonomic system. Emotional states are supported by very extensive bodily
changes. A state of fear can induce the desire to run, but one cannot run far
unless physiological adjustments
support the effort. Anger can mean that one is prepared to fight, but it will not be an efficient battle unless one's
circulatory system makes necessary adjustments to provide strength and
endurance for the contest. The situation need not be a highly emotional one;
work and exercise are supported by the same physical adjustments.
Physical adjustments to an emergency are largely controlled by the autonomic
nervous system. Preparations to strengthen the body for a critical
situation include acceleration and strengthening of the heartbeat, a rise in blood pressure, release of glucose
from the liver, and the secretion of a small amount of epinephrine by the adrenal
glands. Breathing is made easier by the relaxation of muscles in the bronchial tubes. During an emergency digestion can
wait, and so the activity of the
digestive system is altered and depressed; the blood supply is largely diverted
from the digestive system to the
skeletal muscles. These effects are obtained mainly by the stimulation of one
division of the autonomic nervous system,
the sympathetic, or thoracolumbar, portion.
The autonomic system is divided
somewhat artificially into a thoracolumbar, or sympathetic, portion and a craniosacral, or
parasympathetic, part.
The thoracolumbar division is composed of a chain of ganglia and nerves
on either side of the spinal cord, extending
from the cervical region through the thoracic and lumbar regions. Throughout the thoracic and lumbar regions each ganglion
Is connected to a
spinal nerve by a communicating branch. Fibers extend upward to the head from (he superior cervical ganglion; they also extend downward from sacral ganglia, thus increasing the distribution of
sympathetic fibers.
The craniosacral, or parasympathetic, division is associated with
certain cranial and sacral nerves and will
be discussed later. The terms thoracolumbar or
craniosacral appear well adapted for anatomical
considerations; the terms sympathetic and parasympathetic
seem better adapted when referring to the physiology of the autonomic system.
Anatomic organization of autonomic outflow. The peripheral motor portions of
the autonomic nervous system are
made up of preganglionic
and postganglionic
neurons. The cell bodies
of the preganglionic neurons are located
in the visceral efferent (intermediolateral) column
of the spinal cord or the homologous motor nuclei of the cranial nerves. Their axons are mostly myelinated, relatively slow-conducting B fibers. The axons synapse on the cell bodies of postganglionic neurons that are located in all
cases outside the central nervous system. Each preganglionic axon diverges
to an average of 8-9 postganglionic neurons.
In this way, autonomic output is
diffused. The axons of the postganglionic neurons, mostly unmyelinated C fibers, end on the visceral effectors.
Characteristic of reflector arc of
autonomic nervous system
a) Sensory
part (Receptors are present in inner organs, walls of blood vessels and
lymphatic vessels of skin, muscles. They named interoreceptors.
Their stimulus: mechanical, chemical, and temperature irritans.
Afferent part of autonomic reflex consists of interoreceptors,
dendrites of sensory neurons, which are in autonomic and spinal ganglions.
From interoreceptors afferent informations
enter to sensitive neurons, whose body are in autonomic and spinal ganglions.
So, in afferent part of autonomic reflex, sensitive information transmit in two
ways: 1) from interoreceptors to sensitive neurons of
spinal ganglions of dorsal roots of spinal cord; 2) from interoreceptors
to sensitive neurons of autonomic ganglions of dorsal, and then to sensitive
neurons of spinal ganglion of posterior roots. From spinal ganglions,
transmitters of autonomic sensitivity enter in spinal cord. Signal from interoreceptors may enter in brain pass spinal cord.
Crossing of afferent signals on interneurons is on spinal' and bulbar' level.)
b) Central
part (Spinal level. When the neurons enter in spinal cord one part of the
afferent fibers interact with segmental interneurons, which interact with
preganglionic neurons. This is polysynaptic arc. Part of the afferent fibers
end in grey substance of upper segments and medulla oblongata. Part of the
afferent fibers lower and connect by synapses with interneurons of lower
segments.
Supraspinal level. Then the
afferent signals go to reticular formation of brein
stem. Interaction of afferent visceral and somatic signals activates reticular
formation. From reticular formation descending signals transmit to preganglion neurons of arc of autonomic reflex. Ascending
signals transmited to mid-brain, dyencephalon
and cortex.
c) Efferent
part. Peculiarities of mediator transmition in
efferent part of autonomic nervous system (scheme):
Transmission
at the synaptic junctions between pre- and postganglionic neurons and between
the postganglionic neurons and the autonomic effectors is chemically mediated.
The principal transmitter agents involved are acetylcholine and norepinephrine,
although dopamine is also secreted by intemeurons in
the sympathetic ganglia.
Anatomically, the autonomic outflow
is divided into 2 components: the sympathetic and parasympathetic divisions of the autonomic
nervous system. Sympathetic, or thoracolumbar, division. Motor impulses from the spinal cord
to smooth muscles are conveyed over two sets of visceral efferent fibers
instead of one, as in somatic motor nerves. A
synaptic connection ordinarily is
made in a ganglion of the
thoracolumbar chain, although this is not
necessarily so. There are preganglionic neurons,
with cells bodies located in the intermediolateral
column of gray matter of the
spinal cord and with fibers extending, ordinarily, from the cell body to the autonomic ganglion
outside the cord, and a postganglionic neuron.
With its cell body located in a ganglion and with its fiber extending to visceral muscle. The preganglionic fiber can extend through the autonomic ganglion to a collateral ganglion, in which
case there is a short postganglionic fiber
to the organ supplied.
PREGANGLIONIC NEURON
The cell body of the preganglionic neuron is smaller than that of a motor neuron of the central nervous system. The
particles of its Nissle substance are finer and more
rounded. The axon emerges from the spinal
cord as a part of the motor root of a spinal nerve but soon leaves it to enter
the autonomic ganglion. The
majority of these axons are myelinated. A group of myelinated fibers presents a white appearance, and so this
connection of preganglionic
fibers between the spinal nerve and
the sympathetic ganglion is called the white branch, or white ramus communicants. When the preganglionic neuron enters the sympathetic ganglion, it makes a synaptic connection with many postganglionic neurons. This arrangement is significant, since it provides for the
rapid, widespread response characteristic of the sympathetic system. The
thoracic and the first three lumbar nerves are connected with the autonomic chain of ganglia by a white ramus; hence
the name thoracolumbar for this division.
Cervical ganglia are supplied by preganglionic fibers extending upward from the
thoracic nerves through the lateral chains of ganglia. The lower lumbar and
sacral ganglia are supplied by fibers extending downward.
POSTGANGLIONIC NEURON The postganglionic neuron of the thoracolumbar division
has its cell body in a lateral chain ganglion or in a collateral ganglion. The
fiber extends to involuntary muscle tissue or to glandular cells; thus the
cell bodies of the ganglia of the lateral chain are entirely motor. Postganglionic fibers may take two courses
extending beyond the lateral ganglia. They may proceed inward by way of a
visceral branch to terminate in the muscles of the viscera, or they may rejoin
the spinal nerve by way of the gray root, usually called the
gray ramus communicants, and terminate in
the involuntary muscles of the peripheral
region, such as the muscles in the walls of blood vessels, or in sweat glands of the skin. Since these postganglionic fibers are not myelinated, the nerve appears gray in contrast with the white branch of myelinated preganglionic fibers.
While the white rami are limited
to the thoracolumbar region, each spinal
nerve is connected with the
sympathetic trunk by a gray root. Each spinal nerve, therefore, receives postganglionic fibers.
SYMPATHETIC PLEXUSES. The great plexuses of the autonomic system
are the cardiac; celiac,
or solar; and hypogastric plexus.
While these plexuses are regarded as
essentially sympathetic, they also receive fibers from the parasympathetic system. The cardiac
plexus lies under the arch of the aorta just above the heart. It receives
branches from the cervical sympathetic ganglia and from the right and left vagal nerves (parasympathetic)
and has a regulatory effect on the heart. The celiac, or solar, plexus is the largest network of cells and fibers
of the autonomic system. It lies behind
the stomach and is associated with the aorta and the celiac arteries. The ganglia
receive the splanchnic nerves from the sympathetic system and branches of the vagus from the parasympathetic
system. A blow to this region may slow
the heart, reduce the flow of blood to the head, and depress the breathing mechanism.
The hypogastric plexus forms a
connection between the celiac plexus
above and the two pelvic plexuses below. It is located in front of the fifth
lumbar vertebra and continues downward in front
of the sacrum, forming the right and left pelvic plexuses. These plexuses supply
the organs and blood vessels of the pelvis.
PARASYMPATHETIC,
OR CRANIOSACRAL, DIVISION. The craniosacral, or parasympathetic, division of the autonomic nervous system is associated with
certain cranial and sacral nerves in which autonomic
fibers are incorporated; hence the name craniosacral
division (Figure 3). The oculomotor (Hid cranial) nerve, arising in the midbrain, innervates certain voluntary muscles
that move the eyeball; in
addition, it carries parasympathetic fibers
to involuntary muscles within the eyeball. Preganglionic
fibers are distributed to the ciliary’s
ganglion located behind the eyeball. Postganglionic
fibers arising in the ganglion extend to the ciliary’s
muscles of the eye and to the sphincter of the pupil. The facial (Vll th cranial), glossopharyngeal
(IX th cranial), vagus
(X th cranial), and accessory (Xl th
cranial) nerves constitute a group of cranial nerves arising from the medulla. Since they also contain parasympathetic fibers, they are a part of the craniosacral division. The vagus
supplies the viscera of the thorax and abdomen; this may be the reason why
there are no parasympathetic fibers
arising from the thoracic or lumbar regions of the cord.
The sacral portion of this system is identified with certain sacral
nerves that carry parasympathetic fibers to the pelvic viscera.
PREGANGLIONIC
AND POSTGANGLIONIC FIBERS. Typically the parasympathetic preganglionic fiber
extends from its nucleus in the brain or sacral region of the spinal cord to
the organ supplied. The postganglionic fiber
is often a very short fiber located within
the organ itself. The preganglionic fiber
can end in a collateral ganglion, as in
the case of preganglionic fibers
extending out to the ciliary’s ganglion of the
eye. The postganglionic fibers in this
case are longer than those
incorporated within certain organs.
The parasympathetic system
functions as an antagonist of the sympathetic system if an organ is supplied by
both systems. If the sympathetic system is the accelerator system, as in the heart, for example, then the parasympathetic system is the inhibitor. Its function in this case is to slow
the accelerated heart and thus restore the normal heart rate. Even though it
acts as an inhibitor, it does not ordinarily depress the heart rate below
normal unless unduly stimulated, as from the action of drugs or pressure on a nerve.
d)
Difference between autonimuc and somatic nervous
system (1. Nervous centres in autonimuc
nervous system are present in mesencephalon, bulbar part of brain, thoraco-lumbal and sacral part of spinal cord, in somatic –
diffuse in all sentral nervous system;
2. Efference ways of reflector arc in autonimuc nervous system consist of two neurons, in somatic
– of one;
CRANIAL NERVES THAT CARRY PARASYMPATHETIC FIBERS. If the oculomotor nerve is cut experimentally,
the pupil dilates. The parasympathetic fibers
within the oculomotor nerve carry nervous impulses
that cause the pupil to constrict. Cutting the nerve destroys the balance
between parasympathetic and sympathetic
innervation. The sympathetic
nervous impulses then cause the pupil to
dilate. The "drops" placed in the eye for optical examination apparently act in much the same way by blocking the parasympathetic nerve endings.
It has been
mentioned that there are four cranial nerves arising from the medulla that
carry autonomic fibers and therefore are
a part of the craniosacral system. These
nerves are the facial glossopharyngeal,
vagus, and accessory nerves. The facial nerve includes parasympathetic fibers that are secretary to the lacrimal gland and to the sublingual and submaxillary salivary glands. The lacrimal gland is supplied with postganglionic
fibers from the sphenopalatine ganglion.
The sublingual and submaxillary salivary glands receive postganglionic fibers arising in the submaxillary
ganglion.
Preganglionic fibers in the glossopharyngeal nerve extend outward to the optic ganglion. Postganglionic fibers arise in the otic ganglion and supply the parotid salivary gland. These
glands, including the lacrimal, have a
double innervation. They derive their
sympathetic innervation by way of the superior
cervical sympathetic ganglion and carotid plexuses. The action of the two sets
of nerves is not clear. Apparently they both contain secretory fibers, but the
secretory action of the parasympathetic system
seems to be dominant. The vagus nerve contains both motor and visceral afferent
fibers. The motor fibers are long preganglionic fibers that extend out to the
organ supplied. Very short postganglionic fibers
are contained within the organ. Motor fibers are supplied to the larynx,
trachea, bronchioles, heart, esophagus, stomach, small intestine, and some parts of the large intestine. Stimulation of the vagus acts as an inhibitor to the heart,
causing its rate of beating to slow or to stop. To the muscles of the wall of the digestive tract, branches of the vagus act as accelerator nerves. Peristalsis is increased
by parasympathetic stimulation. Parasympathetic fibers to the glands of the
digestive tract have regulatory function on secretion, but food content of the
stomach or intestine and hormones circulating in the blood can also stimulate secretion.
Parasympathetic fibers from
both the right and left vagus nerves enter the great plexuses of the sympathetic system. There is, however,
a definite parasympathetic nerve supply to such organs as the pancreas, liver,
and kidneys. Nervous stimulation of these organs is, for the most part, merely
regulatory. Hormones in the blood normally cause the pancreas and liver to
secrete, but stimulation of the vagus increases the flow of pancreatic juice and bile.
While sympathetic stimulation of the kidneys by way of the splanchnic nerves
results in vasoconstriction and therefore reduced flow of
urine, there are many other
physiological factors that affect the function of the kidneys. A part of the accessory nerve contains visceral motor
and cardiac inhibitory fibers. Certain types
of allergy offer examples of overstimulation of
the parasympathetic system. Epinephrine can be used to counteract these effects,
since it is associated with the action
of the sympathetic system.
THE SACRAL AUTONOMICS. The sacral portion of the craniosacral system is composed
of preganglionic fibers incorporated in
the second, third, and fourth sacral nerves. The fibers extend out to the
pelvic plexuses, where they enter into close relationship with fibers of the
sympathetic system. Parasympathetic fibers
innervate the urogenital organs and the
distal part of the colon. Postganglionic fibers are considered to be in the organs supplied or
in small ganglia located close by. These parasympathetic fibers are motor to the muscles
of the distal two-thirds of the colon, to the rectum, and to the urinary
bladder. They carry vasodilator impulses to the
penis and clitoris. Inhibitory impulses pass to the internal sphincter muscle of the bladder and to the internal sphincter of
the anus.
PARASYMPATHETIC PLEXUSES. Enteric
Plexuses the digestive tube has its own
intrinsic nerve supply, consisting of the myenteric
plexus, located between the
longitudinal and circular muscles and a
submucous plexus, located under the mucous layer in the sub mucosa. This part of the nervous system extends
the entire length of the digestive tube. It can be assumed that parasympathetic fibers entering the wall of the digestive tract are preganglionic fibers that make synaptic connections with neurons of the
enteric system. Sympathetic fibers entering the muscular wall, however, are postganglionic fibers and terminate in the
tissues that they supply without making synaptic
connections.
The enteric plexuses function in maintaining rhythmic peristaltic
movement along the digestive tract.
Peristalsis is maintained if both sympathetic and parasympathetic nerve supply is cut. The nerves of the autonomic system, however, exert a regulatory
effect.
SYMPATHETIC AND PARASYMPATHETIC RELATIONSHIPS. Autonomic effects are
usually conditioned by other factors such as the presence of hormones in the bloodstream or by circulatory effects.
The secretion of a gland can be depressed by
the stimulation of an inhibitor nerve; secretion can also be depressed by vasoconstriction of blood vessels supplying the
gland, thus limiting its blood supply. While the sympathetic system
can be considered as an accelerator to the
heart, the situation is reversed in the case of the action of the autonomic system upon the digestive tract. Here
the action of sympathetic nerves depresses peristalsis and the secretion of digestive glands during emotional
excitement, while the parasympathetic system, as an accelerator, effects
a return to normal. When we speak of the sympathetic and parasympathetic nerves as being antagonistic,
we mean this in the sense of antagonistic muscles. The nerves from the
sympathetic and parasympathetic systems
can produce opposite effects, but they
provide a correlated adjustment to meet many physiological conditions. Autonomic effects are not always clearly
antagonistic. The accommodation reflex of
the eye whereby the lens and iris are adjusted to facilitate clear vision
appears to be primarily a parasympathetic function so far as the ciliary’s muscle and the muscles of the iris are concerned. The two sets of muscles of the iris
seem to have a
synergistic relationship, which
causes them to contract or dilate the pupil smoothly in a mild state of
opposition to each other. The pupil can also dilate in response to an emotional state such as fear or
pain. This is due to stimulation of the sympathetic system.
CHEMICAL TRANSMISSION AT AUTONOMIC FUNCTIONS
Transmission at
the synaptic junctions between pre- and postganglionic neurons and between the postganglionic neurons
and the autonomic effectors is chemically
mediated. The principal transmitter agents
involved are acetylcholine and norepinephrine, although dopamine is also secreted by interneurons in
the sympathetic ganglia.
Chemical Divisions of the Autonomic Nervous
System
On the basis of the chemical mediator released, the autonomic nervous system can be divided into cholinergic and noradrenergic
divisions. The neurons that are cholinergic are (1)
all preganglionic neurons; (2) the
anatomically parasympathetic postganglionic neurons; (3) the anatomically sympathetic postganglionic neurons which innervate sweat
glands; and (4) the anatomically sympathetic neurons which end on blood vessels
in skeletal muscles and produce vasodilatation when
stimulated. The remaining postganglionic sympathetic
neurons are noradrenergic. The adrenal
medulla is essentially a sympathetic ganglion in which the postganglionic cells have lost their axons and become specialized for secretion directly into the bloodstream. The cholinergic preganglionic neurons to these
cells have consequently become the
secret motor nerve supply of this gland.
RESPONSES OF EFFECTOR ORGANS TO
AUTONOMIC NERVE IMPULSES
General Principles
On the basis of the chemical mediator released, the autonomic nervous system can be divided into cholinergic and noradrenergic
divisions. The neurons that are cholinergic are (1)
all preganglionic neurons; (2) the
anatomically parasympathetic postganglionic neurons; (3) the anatomically sympathetic postganglionic neurons which innervate sweat
glands; and (4) the anatomically sympathetic neurons which end on blood vessels
in skeletal muscles and produce vasodilatation when
stimulated. The remaining postganglionic sympathetic
neurons are noradrenergic. The adrenal medulla
is essentially a sympathetic ganglion in which the postganglionic cells have lost their axons and become specialized for secretion directly into the bloodstream. The cholinergic
preganglionic neurons to these cells have consequently become the secret motor nerve supply of this gland.
The smooth
muscle in the walls of the hollow viscera is generally innervated by both
noradrenergic and cholinergic fibers, and activity in one of these systems
increases the intrinsic activity of the smooth muscle whereas activity in the
other decreases it. However, there is no uniform rule about which system
stimulates and which inhibits. In the case of sphincter muscles, both
noradrenergic and cholinergic innervations are excitatory, but one supplies the
constrictor component of the sphincter and the other the dilator.
There is usually no acetylcholine in
the circulating blood, and the effects of localized cholinergic discharge are
generally discrete and of short duration because of the high concentration of acetylcholinesterase at cholinergic nerve endings.
Norepinephrine spreads farther and has a more prolonged action than
acetylcholine. The epinephrine and some of the dopamine come from the adrenal
medulla, but much of the norepinephrine diffuses into the bloodstream from norad-renergic nerve endings.
Cholinergic Discharge
In a general
way, the functions promoted by activity in the cholinergic division of the
autonomic nervous system are those concerned with the vegetative aspects of
day-to-day living. For example, cholinergic action favors digestion and
absorption of food by increasing the activity of the intestinal musculature,
increasing gastric secretion, and relaxing the pyloric sphincter. For this
reason, and to contrast it with the ''catabolic'' noradrenergic division, the
cholinergic division is sometimes called the anabolic nervous system.
Noradrenergic Discharge
The noradrenergic division discharges
as a unit in emergency situations. The effects of this discharge are of
considerable value in preparing the individual to cope with the emergency,
although it is important to avoid the teleologic
fallacy involved in the statement that the system discharges in order to do
this. For example, noradrenergic discharge relaxes accommodation and dilates
the pupils (letting more light into the eyes), accelerates the heartbeat and
raises the blood pressure (providing better perfusion of the vital organs and
muscles), and constricts the blood vessels of the skin (which limits bleeding
from wounds). Noradrenergic discharge also leads to lower thresholds in the
reticular formation (reinforcing the alert, aroused state) and elevated blood
glucose and free fatty acid levels (supplying more energy). On the basis of
effects like these, Cannon called the emergency-induced discharge of the
noradrenergic nervous system the ''preparation for flight or fight.''
The emphasis on mass discharge in
stressful situations should not obscure the fact that the noradrenergic
autonomic fibers also subserve other functions. For
example, tonic noradrenergic discharge to the arterioles maintains arterial
pressure, and variations in this tonic discharge are the mechanism by which the
carotid sinus feedback regulation of blood pressure is effected. In addition,
sympathetic discharge is decreased in fasting animals and increased when fasted
animals are refed. These changes may explain the
decrease in blood pressure and metabolic rate produced by fasting and the
opposite changes produced by feeding.
Adrenergic Fibers The
terminal filaments of most sympathetic postganglionic
neurons produce an adrenalin-like substance and are classified as adrenergic. Sympathetic fibers to sweat glands, blood vessels of the skin, and to the arrestors pylorus muscles are exceptions. These postganglionic fibers enter spinal nerves through the gray rami and reach
the skin incorporated in peripheral nerves.
The effects of norepinephrine, in
conjunction with epinephrine, can be general and widespread. There is experimental evidence that
the chemical substance resulting from excitation of sympathetic postganglionic fibers is carried by the
bloodstream and can affect organs remote from the point of origin. It is
interesting to note that the sympathetic
ganglia and the modularly portion of the adrenal gland have the same embryonic origin. They both arise from neural crest cells.
Cholinergic Fibers Parasympathetic
fibers also produce a chemical mediating substance. In this case the substance is acetylcholine,
which is promptly converted to choline and
acetic acid by the action of an enzyme called cholinesterase. Since acetylcholine does
not remain in its most active state for any great length of time, it is
probable that its effects are entirely local. Unlike norepinephrine, it is probably not carried by the bloodstream.
All preganglionic fibers, whether sympathetic or parasympathetic, have been shown to liberate
a cholinergic substance, probably
identical with acetylcholine. This
means that the transmission of the nervous impulse across the point of synapse between the preganglionic and postganglionic fiber
is accomplished by the production of acetylcholine.
As we have indicated, postganglionic sympathetic
fibers to the sweat glands and to smooth muscles of the skin are cholinergic. These fibers are carried by peripheral nerves. Voluntary motor nerves to skeletal
muscles are also cholinergic. On the basis of chemical transmitter substances it
appears that the division of the autonomic system
into sympathetic and parasympathetic is
somewhat artificial.
MEDULLA
OBLONGATA
Control of
Respiration, Heart Rate, & Blood Pressure
The
medullary centers for the autonomic reflex control of the circulation, heart,
and lungs are called the vital centers because damage to them is usually fatal.
The afferent fibers to these centers originate in a number of instances in
highly specialized visceral receptors. The specialized receptors include not
only those of the carotid and aortic sinuses and bodies but also receptor cells
that are apparently located in the medulla itself. The motor responses are
graded and delicately adjusted and include somatic as well as visceral
components.
Other
Medullary Autonomic Reflexes
Swallowing,
coughing, sneezing, gagging, and vomiting are also reflex responses integrated
in the medulla oblongata. Coughing is initiated by irritation of the lining of
the respiratory passages. The glottis closes and strong contraction of the
respiratory muscles builds up intrapulmonary pressure, whereupon the glottis
suddenly opens, causing an explosive discharge of air. Sneezing is a somewhat
similar response to irritation of the nasal epithelium. It is initiated by
stimulation of pain fibers in the trigeminal nerves.
References:
1. Review of
Medical Physiology // W.F. Ganong. – Twentieth
edition, 2001. – P. 217-223, 226-229, 232, 233, 242.
2. Textbook of
Medical Physiology // A.C. Guyton, J.E. Hall. – Tenth edition, 2002. – P. 364,
632, 681-684, 697-707, 736.