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Зміст10.2 Organization of the autonomic nervous system j 175
176 10 The autonomic nervous system
Organ Effect of sympathetic activation Effect of parasympathetic activation
10.3 Chemical transmission in the autonomic nervous system
178 1 10 The autonomic nervous system
Principles of neural science
180 10 The autonomic nervous system
The autonomic nervous system
The autonomic nervous system regulates the operation of the internal organs to support the activity of the body as a whole. It is not a separate nervous system but is the efferent (motor) pathway that links those areas within the brain concerned with the regulation of the internal environment to specific effectors such as blood vessels, the heart, the gut, and so on.
Unlike the efferent fibers of the skeletal muscles, those of the autonomic nervous system do not pass directly to the effector organs, rather they pass to autonomic ganglia which are located outside the CNS. Furthermore, the autonomic nervous system is not under voluntary control. The fibers that project from the CNS to the autonomic ganglia are called preganglionic fibers and those that connect the ganglia to their target organs are called postganglionic fibers.
10.2 Organization of the autonomic nervous system
The autonomic nervous system is divided into two parts:
The sympathetic nervous system
The sympathetic division originates in the cells of the inter-mediolateral column of the thoracic and lumbar regions of the spinal cord, between segments Tl and L2 or L3. These neurons are called sympathetic preganglionic neurons. The axons of these neurons (sympathetic preganglionic fibers) pass from the spinal cord via the ventral root together with the somatic motor fibers. Shortly after the dorsal and ventral roots fuse, the sympathetic preganglionic fibers leave the spinal nerve trunk to travel to sympathetic ganglia via the white rami communicantes, as shown in Fig. 10.1. The preganglionic fibers synapse on sympathetic neurons within the ganglia and the postganglionic sympathetic fibers project to their target organs mainly via the gray rami communicantes and the segmental spinal nerves (Fig. 10.1).
Fig. 10.1 A comparison of the arrangement of the sympathetic preganglionic and postganglionic neurons with the organization of the somatic motor nerves. Note that sympathetic preganglionic fibers may terminate in the sympathetic ganglion of the same segment or pass to another ganglion in the sympathetic chain. Some also terminate in prevertebral ganglia, such as the celiac ganglion.
The majority of sympathetic ganglia are found on each side of the vertebral column and are linked together by longitudinal bundles of nerve fibers to form the two sympathetic trunks, as shown in Figure 10.2. With the exception of the cervical region, the sympathetic ganglia are distributed segmentally down as far as the coccyx. The sympathetic preganglionic fibers to the abdominal organs join Together to form the splanchnic nerves which pass to the celiac, superior and inferior mesenteric ganglia, where they synapse. Postganglionic sympathetic fibers then pass to the various abdominal organs as shown in Fig. 10.2.
The sympathetic innervation to the adrenal medulla is an exception to the general rule. The preganglionic fibers from the thoracic spinal cord pass to the adrenal glands via rhe splanchnic nerves where they synapse directly with the chromaffin cells of the adrenal medulla. The chromaffin cells of the adrenal medulla are homologous with sympathetic postganglionic neurons and share many of their physiological properties, including the generation of action potentials and the secretion of catecholamines (Section 10.3).
Although the sympathetic preganglionic fibers are myelinated, the sympathetic postganglionic fibers are unmyelinated.
10 The autonomic nervous system
Fig. 10.2 The organization of the autonomic nervous system. In addition to the innervation of the principal organ systems, segmental sympathetic fibers also innervate blood vessels, piloerector muscles, and sweat glands. Parasympathetic preganglionic fibers are found in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus), scg, meg, and icg refer to superior, middle and inferior cervical ganglion; eg, celiac ganglion; smg and img refer to the superior and inferior mesenteric ganglia. Pterygopal. gangl refers to the Pterygopalatine ganglion.
This explains the difference in appearance of gray and white rami. As shown in Fig. 10.2, sympathetic postganglionic fibers innervate many organs, including the eye, the salivary glands, the gut, the heart, and the lungs. They also innervate the smooth muscle of the blood vessels, the sweat glands, and the piloerector muscles of the skin hairs. As the sympathetic ganglia are located close to the spinal cord, most sympathetic preganglionic fibers are relatively short while the postganglionic fibers are much longer, as indicated in Fig. 10.3.
The parasympathetic nervous system
The preganglionic neurons of the parasympathetic division of the autonomic nervous system have their cell bodies in two regions: the brainstem and in sacral segments S3-S4 of the spinal cord. Thus the parasympathetic preganglionic fibers emerge as part of the cranial outflow in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus), and from the sacral outflow.
Fig. 10.3 Comparison of the organization of the sympathetic and parasympathetic divisions of the autonomic nervous system with that of somatic motor innervation. The lower part of the figure shows a simplified plan of the innervation of the gut by sympathetic and parasympathetic nerve fibers. (Note - indicates an inhibitory action, an +excitatory action.)
The parasympathetic ganglia are usually located close to the target organ or even embedded within it. Thus the parasympathetic innervation is characterized by long preganglionic fibers and short postganglionic fibers, in contrast to the organization of the sympathetic nervous system (see Fig. 10.3). Parasympathetic postganglionic fibers innervate the eye, the salivary glands, the genitalia, the gut, the heart, the lungs, and other visceral organs, as shown in Fig. 10.2. Parasympathetic innervation of blood vessels is confined to vasodilator fibers supplying the salivary glands, the exocrine pancreas, the gastrointestinal mucosa, the genital erectile tissue, and the cerebral and coronary arteries. Other blood vessels are innervated exclusively by sympathetic fibers.
Many organs receive a dual innervation
from sympathetic and parasympathetic
Most visceral organs (but not all) are innervated by both divisions of the autonomic nervous system. The specific actions of the autonomic nerves on the various organ systems of the body are discussed at length in the relevant chapters of this book. In many cases, the actions of the sympathetic and parasympathetic divisions are antagonistic, so that the actions of the two divisions provide a delicate control over the functions of the viscera. Thus, activation of the sympathetic nerves to the heart increases heart rate and the force of contraction of the heart muscle, while activation of the vagus nerve (parasympathetic) slows the heart. Activation of the parasympathetic supply to the gut enhances its motility and secretory functions, while activation of the sympathetic supply inhibits the digestive functions of the gut and constricts its sphincters.
Some organs only have a sympathetic supply. Examples are the adrenal medulla, the pilomotor muscles of the skin hairs, the sweat glands, and the spleen. In humans, it is probable that most
blood vessels are also exclusively innervated by the sympathetic nerves. The parasympathetic supply has exclusive control over the focusing of the eyes by the ciliary muscles and of pupillary constriction by the constrictor pupillae muscle of the iris. Sympathetic stimulation dilates the pupil of the eyes by its action on the dilator pupillae muscle of the iris. Thus, the iris provides an example of functional antagonism rather than dual antagonistic innervation of a specific smooth muscle. The effects of activation of the sympathetic and parasympathetic divisions of the autonomic nervous system on various organs are summarized in Table 10.1.
Autonomic nerves maintain a basal level of tonic activity
The autonomic innervation generally provides a basal level of activity in the tissues it innervates, called tone (see also Chapter 7). The autonomic tone can be either increased or decreased to modulate the activity of specific tissues. Thus the blood vessels are generally in a partially constricted state as a result of sympathetic tone. This partial constriction restricts the flow of blood. If sympathetic tone is increased, the affected vessels become more constricted and this results in a decrease in blood flow. Conversely, if sympathetic activity is inhibited, tone decreases and the affected vessels dilate, so increasing their blood flow (Chapter 15).
The heart in a resting person is normally under the predominant influence of vagal tone. If the vagus nerves are cut, the heart rate rises. During the onset of exercise, the tonic parasympathetic inhibition of the heart declines and sympathetic activation increases, with a resulting elevation in heart rate.
The enteric nervous system
Sympathetic and parasympathetic nerve fibers act on neurons that are present in the walls of the gastrointestinal tract. These
10 The autonomic nervous system
Table 10.1 The main actions of sympathetic and patasympathetic stimulation on various otgan systems
This rable summarizes rhe main effects of activation of the sympathetic of parasympathetic nerves to various otgans. More detailed accounts of the regulation of the function of each organ by the autonomic nervous system will be found in the relevant chapters of this book.
neurons have been considered to form a separate division of the nervous system—the enteric nervous system. The enteric neurons are organized as two interconnected plexuses—the submucosal plexus (also known as Meissner's plexus), which lies in the submucosal layer beneath the muscularis mucosae, and the myenteric plexus (or Auerbach's plexus), which lies between the outer longitudinal and the inner circular smooth muscle layers of the muscularis. The enteric nervous system can function independently of its autonomic supply and its neurons play an important part in the regulation of the motility and secretory activity of the digestive system (Chapter 18).
Within the autonomic ganglia, the synaptic contacts are highly organized and similar in structure to the other neuronal synapses described in Chapter 6. In the target tissues, however, the synaptic contacts are more diffuse than those of the CNS or those of the neuromuscular junction of skeletal muscle. The postganglionic fibers have varicosities along their length which secrete neurotransmitters into the space adjacent to the target cells rather than onto a clearly defined synaptic region. In multi-unit smooth muscle, however, each varicosity is closely associated with an individual smooth muscle cell.
The main neurotransmitters secreted by the neurons of the autonomic nervous system are acetylcholine and norepinephrine (also
known as noradrenaline). Within the ganglia of both the sympathetic and parasympathetic divisions, the principal transmitter secreted by the preganglionic fibers is acetylcholine. The postganglionic fibers of the parasympathetic nervous system also secrete acetylcholine onto their target tissues. The postganglionic sympathetic fibers secrete norepinephrine, except for those fibers that innervate the sweat glands and pilomotor muscles. These fibers secrete acetylcholine. The transmitters utilized by the different neurons of the autonomic nervous system are summarized in Fig. 10.4.
Acetylcholine activates nicotinic
receptors in the autonomic ganglia but
muscarinic receptors in the target tissues
The acetylcholine receptors of the postganglionic neurons in autonomic ganglia are called nicotinic receptors because they can also be activated by the alkaloid nicotine. They are similar in structure to the nicotinic receptors of the neuromuscular junction but have a different response to various drugs and toxins. For example, they can be blocked by mecamylamine, which has no action at the neuromuscular junction, but not by α-bungarotoxin, which is a potent blocker of the nicotinic receptors of the neuromuscular junction. Activation of nicotinic receptors leads to the opening of an ion channel and rapid excitation, as described in Chapter 6.
The acetylcholine receptors of the target tissues of both parasympathetic and sympathetic postganglionic fibers are mus-
10.3 Chemical transmission in the autonomic nervous system
Fig. 10.4 The role of cholinergic and adrenergic innervation in the autonomic nervous system.
carinic receptors as they can be activated by muscarine. Muscarinic receptors are also present at sympathetic nerve endings in sweat glands, pilomotor muscles, and, in some animal species, at the nerve endings of vasodilator fibers in skeletal muscle. They can be inhibited by low concentrations of atropine. Muscarinic receptors are linked to G-proteins so that activation of these receptors leads to modulation of the intracellular levels of IP3 or cyclic AMP (Chapter 5).
Although five different types have been identified by the techniques of molecular biology, muscarinic receptors can be grouped into three main classes on the basis of their physiological actions:
Adrenergic receptors belong to two main classes
The receptors for norepinephrine are called adrenoceptors. Two main classes are known, α-adrenoceptors and β-adrenoceptors. As for muscarinic receptors, the adrenoceptors are coupled to second-messenger systems via a G-protein. Each group is further subdivided so that five subtypes are presently recognized: a}, a2, ft, ft, and βy
5. ^-Adrenoceptors are present in adipose tissue, where they initiate lipolysis to release free fatty acids and glycerol into the circulation.
Although the diversity in both adrenoceptors and cholinergic receptors is somewhat bewildering, the development of agonists and antagonists that act on specific subtypes of these receptors has been of considerable clinical benefit in the treatment of diseases such as asthma and hypertension.
The adrenal medulla secretes epinephrine
and norepinephrine, which have a
sympathomimetic action, into the
Although the activation of the autonomic nerves provides a mechanism for the discrete regulation of specific organs, activation of the splanchnic nerve results in the secretion of epinephrine and norepinephrine (also known as adrenaline and noradrenaline) from the adrenal medulla into the circulation. About 80 per cent of the secretion is epinephrine and 20 per cent is norepinephrine. These catecholamines exert a hormonal action on a variety of tissues (Chapter 12), which forms part of the overall sympathetic tesponse. Their release is always associated with an increase in the secretion of norepinephrine from sympathetic nerve terminals.
The response of a tissue to circulating epinephrine or norepinephrine will depend on the relative proportions of the different types of adrenoceptor it possesses. Epinephrine activates /3-adrenoceptors more strongly than α-adrenoceptors, while norepinephrine is more effective at activating α-adrenoceptors than /3-adrenoceptors. During exercise, increased activity in the sympathetic nerves will result in an increased heart rate and vasoconstriction in the splanchnic circulation. The circulating norepinephrine will have a similar effect but the actions of circulating epinephrine will lead to relaxation of the smooth muscle that possesses a high proportion of/3-adrenoceptors, such as that of the blood vessels of skeletal muscle. The increased levels of circulating epinephrine cause bronchodilatation and a vasodilatation in the skeletal muscle, thus favoring increased gas flow to the alveoli and blood flow to the exercising muscles.
Circulating catecholamines affect virtually every tissue, with the result that the metabolic rate of the body is increased. Indeed, maximal sympathetic stimulation may double the metabolic rate. The major metabolic effect of epinephrine and norepinephrine is to increase the rate of glycogenolysis within cells by activating adenylyl cyclase via /3-adrenoceptors, as described in Chapter 5. The result is a rapid mobilization of glucose from glycogen and an increased availability of fatty acids for oxidation as a result of lipolysis occurring in adipose tissue. The increased availability of substrates for oxidative metabolism is important both in exercise and during cold stress, where an increase in metabolic rate is important for generating the heat required to maintain body temperature (Chapter 26).
] 0.4 Central nervous control of autonomic activity
Like the somatic motor system discussed in the previous chapter, the activity of the autonomic nervous system varies according to the information it receives from both visceral and somatic afferent fibers. It is also subject to regulation by the higher centers of the brain, notably the hypothalamus.
The internal organs are innervated by afferent fibers that respond to mechanical and chemical stimuli. Some visceral afferents reach the spinal cord by way of the dorsal roots and enter the dorsal horn together with the somatic afferents. These fibers synapse at the segmental level and the second-order fibers ascend the spinal cord in the spinothalamic tract. They project to the nucleus of the tractus solitarius (NTS), various motor nuclei in the brainstem, and to the thalamus and hypothalamus. Other visceral afferents, such as those from the arterial baroreceptors, reach the brainstem by way of the vagus nerves.
Information from the visceral afferents elicits specific visceral reflexes which, like the reflexes of the somatic motor system, may be either segmental ot may involve the participation of neurons in the brain. Examples of autonomic reflexes are the baroreceptor reflex, the lung inflation reflex, and the micturition
reflex. These are discussed in detail in the relevant chapters of this book.
In response to a perceived danger there is a behavioral alerting that may result in aggressive or defensive behavior. This is known as the defense reaction, which has its origin in the hypothalamus. During the defense reaction there are marked changes in the activity of the autonomic nerves, in which normal reflex control is overridden. Further details may be found in Chapter 15.
The hypothalamus regulates the
homeostatic activity of the autonomic
Both the activity of the autonomic nervous system and the function of the endocrine system are under the control of the hypothalamus, which is the part of the brain mainly concerned with maintaining the homeostasis of the body. If the hypothalamus is destroyed, the homeostatic mechanisms fail. The hypothalamus receives afferents from the retina, the chemical sense organs, somatic senses, and from visceral afferents. It also receives many inputs from other parts of the brain, including the limbic system and cerebral cortex. Hypothalamic neurons play important roles in thermoregulation, in the regulation of tissue osmolality, in the control of feeding and drinking, and in reproductive activity.
Barr, M. L. (1979). The human nervous system: an anatomical viewpoint, Chapter 24. Harper International, Hagerstown, Maryland.
Brodal, P. (1992). The central nervous system. Structure and function, Chapters 14 and 15. Oxford University Press, Oxford.
Rang, H. P., Dale, M. M., and Ritter, J. M. (1995). Pharmacology, (3rd edn), Chapters 5—7. Churchill-Livingstone, Edinburgh.
Dodd, J. and Role, L. W. (1991). The autonomic nervous system. In ^ (3rd edn), (eds E. R. Kandel, J. H. Schwartz, and T. M. Jessell), Chapter 49- Prentice-Hall International, London.
Jordan, D. and Marshall, J. M. (ed.) (1995). Cardiovascular regulation. Portland Press, London.
Koizumi, K. and Brooks, C. M. (1980). The autonomic nervous system and its role in controlling body functions. In Medical physiology, (14th edn), (ed. V. B. Mountcastle), Chapter 33. Mosby, St Louis.
Shepherd, G. M. (1994). Neurobiology, (3rd edn), Chapter 18. Oxford University Press. Oxford.
Each sratement is either true or false. The answers are given below.
1. a. Sympathetic preganglionic neurons are found in spinal
segments from Tl to L2.
b. The sympathetic chain extends from the cervical to the
sacral regions of the spinal cord.
c. Sympathetic preganglionic fibers secrete norepineph
d. Acetylcholine is secreted by some sympathetic post
e. The diameter of the blood vessels is regulated entirely
by the sympathetic nervous system.
2. a. Parasympathetic preganglionic fibers are found in
cranial nerve III (oculomotor).
b. The vagus nerves provide the parasympathetic
innervation to the heart.
c. Parasympathetic vasoconstrictor fibers are present in the
d. Parasympathetic postganglionic fibers secrete nor
epinephrine onto their target organs.
e. Parasympathetic preganglionic fibers secrete acetyl
3. a. Stimulation of the sympathetic nerves to the eyes
causes pupillary constriction.
b. Epinephrine secreted by the adrenal medulla causes
glycogen breakdown in the liver.
c. Srimulation of the vagus nerves increases the motility of
the gastrointestinal tract.
d. Activation of the sympathetic system causes vaso
constriction in the viscera and skin but vasodilatation
in skeletal muscle.
e. Srimulation of the vagus nerves slows the heart.
4. a. The acetylcholine receptors in both parasymparhetic
and sympathetic ganglia are nicorinic.
b. Acetylcholine secreted by parasympathetic post
ganglionic fibers acts on muscarinic receptors.
c. Norepinephrine secreted by symparheric postganglionic
fibers acts preferentially on /3-adrenoceptors.
d. The adrenal medulla secretes epinephrine in response to
stimulation of sympathetic postganglionic fibers in the
1. Although the sympathetic preganglionic neurons are found in the thoracic and upper lumbar spinal segments, paired sympathetic ganglia are found on both sides of the
10 The autonomic nervous system
spinal cord from the cervical region to the sacral region. In the cervical region only three pairs of ganglia are found (superior, middle, and inferior cervical ganglia) while from Tl to L3 there is a pair of ganglia for each spinal segment. Sympathetic preganglionic fibers secrete acetylcholine. Although the majority of blood vessels are innervated by sympathetic vasoconstrictor fibers, some, such as those in the salivary glands and in the exocrine pancreas, also have a vasodilator parasympathetic innervation.
2. Parasympathetic preganglionic fibers are found in cranial nerves III, VII, IX, and X. The salivary glands are innervated by parasympathetic vasodilator fibers (parasympathetic stimulation promotes salivary secretion). Both parasympathetic preganglionic and parasympathetic postganglionic fibers secrete acetylcholine.
3. Stimulation of the sympathetic nerves to the eyes results in
dilation of the pupils. Although activation of the sym
pathetic nerves results in generalized vasoconstriction,
epinephrine secreted by the adrenal medulla acts on the
/3-adrenoceptors of the blood vessels of skeletal muscle to
4. Norepinephrine acts preferentially on α-adrenoceptors,
epinephrine acts preferentially on /3-adrenoceptors. It
is, however, important to realize that norepinephrine and
epinephrine will activate both types of adrenoceptor.
Epinephrine (and norepinephrine) are secreted by the
adrenal medulla in response to stimulation of sympathetic
preganglionic fibers traveling in the splanchnic nerves.
The chromaffin cells of the adrenal medulla are homo
logous with sympathetic postganglionic neurons.
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