Book - Vertebrate Zoology (1928) 31
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXXI The Autonomic Nervous System
It has been mentioned that the smooth muscles and glands of the body are innervated by fibres of the general visceral efferent component system. It is characteristic of such fibres that they do not reach all the way from the central nervous system to the effector in question, but they make synaptic connexions with other neurons which carry the impulses on to the muscle or gland as the case may be. There are, there- fore, two members in each efferent circuit of this kind : a connector neuron and an exciter neuron. The cell-body of the exciter neuron may be in a sympathetic ganglion, or it may be by itself near the muscle which it innervates. In the former case, the connector neuron is often called the pre- ganglionic fibre, and the exciter the postganglionic fibre. Impulses conveyed in this way through the visceral efferent system to smooth muscles and glands are involuntary, and the neurons and ganglia concerned in the conduction of these impulses form the autonomic or involuntary nervous system. It may be noticed that the autonomic system is essentially efferent. Although the afferent visceral neurons run up from the viscera through the ramus communicans, and accompany the efferent neurons, they conform to the type of the somatic afferent fibres in that their cell-bodies are in the ganglia on the dorsal roots, and that they stretch all the way from the sense-organ to the central nervous system. After separating off the autonomic nervous system, what is left is called the cerebro-spinal nervous system, including the brain, spinal cord, and the somatic fibre-systems.
The autonomic nervous system can be separated into two divisions, each of which works against the other. The visceral efferent fibres which come out from the spinal cord in the neck, thorax, and lumbar regions together constitute the sympathetic system ; those which leave the central nervous system in the head from the brain, and from the spinal cord in the sacral region, constitute the parasympathetic system. The word " sympathetic " is sometimes loosely used as synonymous with " autonomic," which introduces confusion. The sympathetic system may be called the " thoracico-lumbar " outflow, and the parasympathetic system the " cranio-sacral outflow."
The autonomic system may now be described in greater detail, in a typical mammal, and commencing with its sympa- thetic constituent.
The visceral efferent fibres in the cervical, thoracic and lumbar regions of the spinal cord run out through the ventral roots and down the rami communicantes to the sympathetic ganglia situated on each side of the aorta. These fibres are preganglionic or connectors, and their cell-bodies are in the grey matter of the spinal cord ; they are surrounded by medullary sheaths and these rami communicantes are conse- quently white.
Some of the preganglionic fibres stop in the sympathetic ganglion corresponding to the segment in which they emerge from the spinal cord, others continue to the next sympathetic ganglia in front or behind and end there. In this way, the sympathetic ganglia of each side become connected together forming the lateral sympathetic chains, and the ganglia on them are called the lateral ganglia. In the region of the neck, several of these lateral ganglia join up close together, forming the large anterior and posterior cervical ganglia and the stellate ganglion.
Yet other preganglionic fibres run out through the lateral ganglia, but do not stop there. Instead, they run on and end in groups of ganglia situated near the base of the coeliac, anterior and posterior mesenteric arteries. The most im- portant of these ganglia, which are called collateral, are the anterior mesenteric and the posterior mesenteric ganglia. The long rami communicantes which connect these ganglia with the spinal nerves are the splanchnic nerves.
In the lateral and collateral sympathetic ganglia are the cell-bodies of the postganglionic or exciter neurons. These run out of the ganglia as non-medullated and therefore grey fibres, to the muscles of the blood-vessels, heart, stomach, intestine, oviduct, bladder, and skin ; and some of them run to the ciliary and iris muscles inside the eye.
The effect of stimulation through the sympathetic system is to slacken the ordinary muscles surrounding the gut, but to tighten the sphincters, to tighten the heart and artery muscles, to tighten the muscles under the skin (which make hair stand " on end "), to tighten and slacken the muscles of the oviduct, to slacken the sphincter and tighten the radial muscles of the iris so that the pupil enlarges.
The structures enumerated above are also innervated by the parasympathetic system (except the muscles of the oviduct). The visceral branch of the vagus contains connector fibres which run to exciter neurons situated on the lungs, heart, and the muscles of the gut as far as the end of the small intestine. In the region of the intestine, the exciter neurons lie between the muscle coats of the gut, forming the plexus of Auerbach. The remainder of the gut is innervated by connector fibres which leave the spinal cord in the sacral region through the ventral nerve-roots, and form the pelvic nerve. These con- nector fibres run to excitor neurons on the muscles of the large intestine, on the bladder, on the skin round the anus, and on the blood-vessels near the urethra.
The ciliary and iris eye-muscles receive innervation by means of connector fibres which run in the oculomotor nerve to the ciliary ganglion. This ganglion contains the cell- bodies of the exciter neurons which run to the muscles in question in the eye. Two sets of autonomic connector fibres run through the facial nerve. One goes down the palatine branch (" greater superficial petrosal ") to the spheno-palatine ganglion from which exciter neurons run to the lachrymal glands and the glands of the nose. The other set runs in the chorda tympani (ramus mandibularis internus facialis of the dogfish) to the submaxillary ganglion, whence exciter neurons run to the submaxillary salivary glands. Another set of connector fibres runs out in the glossopharyngeal nerve through the lesser superficial petrosal nerve to the otic ganglion, from which exciter neurons innervate the parotid salivary glands.
A very interesting feature of the connector fibres of the parasympathetic autonomic nervous system is, that while those of the oculomotor (midbrain outflow) and of the sacral outflow connect with the central nervous system through ventral nerve- roots, those of the facial, glossopharyngeal, and vagus (hindbrain outflow) run in dorsal nerve-roots.
The anatomy of the autonomic system in the head is slightly complicated. The anterior prolongation of the lateral sympathetic chain of the trunk continues forwards, accompany- ing the internal carotid artery as the internal carotid nerve. A branch of it (the deep petrosal) joins the palatine nerve (forming the Vidian nerve) and runs to the spheno-palatine ganglion. This ganglion is also connected to the maxillary branch of the trigeminal. Another sympathetic branch runs to the ciliary ganglion, which is also connected to the ophthalmic branch of the trigeminal. The sympathetic exciter neurons from the anterior cervical ganglion are thus able to make their way into the eye to the iris-muscles. The mandibular branch of the trigeminal connects with the chorda tympani and the sub- maxillary ganglion.
Further mention must be made of Auerbach's plexus, which lies between the circular and longitudinal coats of muscles on the intestine. The neurons which compose it are the exciters of the parasympathetic outflow through the vagus, and these neurons branch, the two axon fibres having different destinations. A mass of food in the intestine stimu- lates the muscles above it to contract, and those below it to slacken, thus causing peristaltic action. This is particularly interesting because peristalsis can occur when all the nerves to the intestine are cut, which means that local reflex arcs are formed in Auerbach's plexus. Another plexus (Meissner's), which lies within the muscle-coats of the intestine, has an unknown function.
The effect of impulses travelling out through the parasympathetic outflows is to contract the ordinary muscles round the gut, but to slacken the sphincters, to slacken the muscles of the heart and of the blood-vessels near the urethra (causing erection of the penis), to tighten the ciliary muscle and the sphincter of the iris, to slacken the radial muscles of the iris (which allows the pupil to be contracted, and to secrete saliva and tears).
The antagonism between the effects of the sympathetic and parasympathetic systems is remarkable. It may be expressed in the form of a table.
Ordinary muscles Sphincters Radial muscles Sphinctex
of the gut. of the gut. Heart. of the iris. of the iris.
Sympathetic. Slackens. Tightens. Tightens. Tightens. Slackens.
Parasympathetic. Tightens. Slackens. Slackens. Slackens. Tightens.
It is also interesting to note that the action of the sym- pathetic system can partly be simulated by the injection of adrenalin, and that of the parasympathetic by injection of acetyl-cholin. The similar effects of adrenalin and the sym- pathetic are less surprising when it is remembered that the supra-renals and the medulla of the adrenal bodies are derived from cells similar to sympathetic neurons, and which like them have migrated out from the spinal cord.
The case of the gut is particularly interesting, because the ordinary muscles of its coat are antagonistic in their effects to those of the sphincters. It stands to reason, that if the ordinary gut-musculature contracts and propels the contents of the gut along, contraction of the sphincters would prevent this movement of the contents. Now the parasympathetic system tightens the ordinary musculature and slackens the sphincters, and the sympathetic system contracts the sphincters and slackens the ordinary musculature. Further, the cell- bodies of the neurons which tighten the sphincters and slacken the ordinary muscles are in the same ganglion (anterior or posterior mesenteric ganglion, according to the region of the gut). It is possible that it is one and the same neuron which produces two axon fibres, one tightening the sphincters and the other slackening the ordinary muscles. This provides an explanation of how the co-ordination between antagonistic sets of muscles may be brought about.
No autonomic system is known in Amphioxus. In Petro- myzon, neurons are found along the gut, connected with the vagus and probably with " pelvic " nerves. The parasympa- thetic system is therefore present. The sympathetic system is, on the other hand, not well developed, and imperfectly differentiated from the supra-renal elements. Groups of these cells are found near the spinal nerves and the blood-vessels, but they are not joined together by sympathetic chains. Parallel with this poor development of the sympathetic component of the autonomic system in Petromyzon, it may be mentioned that that animal has no oviduct or bladder, and no smooth muscle under the skin. In the head the eyes are degenerate, and there are no salivary glands, and this is parallel with the absence of differentiated cranial autonomic ganglia. In Selachians, the sympathetic ganglia are joined together by the longitudinal lateral chains, and the ciliary ganglion is present in the head. With the land-vertebrates the full development of the autonomic system appears.
It is not easy to see why the exciter neurons for smooth muscles and glands should migrate out of the central nervous system as they do, and take up positions outside it. It is also very remarkable that some of them should connect with the central nervous system through dorsal nerve-roots (hindbrain outflow of parasympathetic), while others should connect through ventral nerve-roots (midbrain and sacral outflow of parasympathetic and the entire sympathetic). In this connexion it may be noted that in Amphioxus the smooth muscles of the body are innervated through the dorsal nerve- roots, while the ventral roots contain only fibres belonging to the somatic system. The primitive course for fibres innervat- ing smooth muscle, therefore, appears to be through the dorsal nerve-roots, and this primitive feature is retained in the case of the hindbrain (facial, glossopharyngeal and vagus) outflow of the parasympathetic system, but lost in all the rest.
Literature
luntary Nervoi Lanoley, J. N. Autonomic Nervous System. Heffer, London, 1921.
Gaskell, W. H. The Involuntary Nervous System. Longmans, Green, London, 1920.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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