Book - Vertebrate Zoology (1928) 29

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXIX The Functional Divisions Of The Nervous System

It is usual to describe and to refer to a nerve with regard to the segment of the body in which it finds itself. So one may speak of the facial (7th) nerve, or of the 2nd spinal nerve, and designate by these terms well-marked structures, visible by dissection. Nerves are composed of fibres formed of long filaments (or axons) which are produced by cells (neurons), the " bodies " and nuclei of which are situated in the brain and spinal cord, or in the swellings on certain nerves, called ganglia. But all the fibres of any given segmental nerve do not serve the same function. The function of a nerve is to conduct impulses. If the conduction is towards the brain and spinal cord (which together are called the central nervous system) from sense-organs, the fibres are called afferent or sensory. If the conduction is from the central nervous system outwards towards muscles or glands, the fibres are called efferent or motor. Sense-organs may be of many different kinds and appreciate various sorts of stimuli, such as light, sound, pressure, vibration, pain, etc., but from the fact that they do receive these stimuli they are called receptors. On the other hand, muscles and glands are structures which " do something," and are consequently called effectors.


A large part of the life of an animal is taken up with adjust- ing itself to different conditions, and these conditions may be of two kinds. There is the outside world with which the animal keeps in touch by means of its receptors at or near the skin : eyes, ears, lateral-line organs, and the skin itself. These are the exteroceptors. The movements which the animal makes in response to the outside world are largely locomotory, and brought about by the muscles of the body- wall and limbs.


These muscles are striated and voluntary. In order that such movements may be properly coordinated, the animal must have some information (unconscious, of course) of the existing state of its muscles, tendons, and joints. This is supplied by sense-organs which are situated in these structures, and are called proprioceptors.


At the same time, there is a " world " within the animal, and sensations arise from stimuli which start from organs such as the stomach, intestine, or bladder, and the functions connected with them. The sense-organs of taste are largely of use in connexion with what is about to enter the alimentary canal, and they also belong here. Such sense organs are called interoceptors. The reactions to these stimuli take the form of secretions on the part of glands, and contractions of the muscles of the alimentary canal, bladder, arteries, or oviduct. Such muscles are always smooth and involuntary.



Fig. 169. — Diagrammatic transverse section through the trunk of a verte- brate showing the relations of the nerve-roots, sympathetic ganglia, and the functional components.


ama, anterior mesenteric artery ; amg, anterior mesenteric ganglion ; da, dorsal aorta ; dr, dorsal nerve-root ; g, gut ; n, notochord ; re, ramus communicans ; sg, spinal ganglion ; sm, somatic motor region of grey matter ; ss, somatic sensory region ; sy, sympathetic ganglion ; vm, visceral motor region ; vr, ventral nerve-root ; vs, visceral sensory region.




It is possible, therefore, to make out four main divisions of the nerves according to their function : those which convey sensory impulses from the outside world, somatic sensory, or afferent ; those which convey sensory impulses from the inner world, visceral sensory, or afferent ; those which convey motor impulses to the smooth muscles of the viscera, visceral motor, or efferent ; those which convey motor impulses to the striped muscles of the body-wall and limbs, somatic motor, or efferent. Each of these functional systems are called components, and as the same components can be found in several different nerves, it is interesting to study the nerves according to the components which they contain. In this way a classification of nerves is obtained which, as it were, runs at right angles to the classification according to the segment of the body in which they lie. Further, the different components occupy special parts of the central nervous system, and the evolution of the latter, and especially of the brain, has been largely controlled by the positions and relations of these " centres." In an ordinary spinal nerve of any vertebrate above the Cyclostomes, there are two roots : one dorsal and one ventral, and they join to form a mixed nerve. The mixed nerve also sends a branch (ramus communicans) to a sympathetic ganglion. Now, the dorsal root is made of fibres of afferent (sensory) neurons, and the ventral root is composed of efferent (motor) ones. Accompanying the anatomical division into dorsal and ventral roots, there is therefore an important physiological distinction.


The cell-bodies of the afferent neurons are situated in the ganglion which is always present on the dorsal root in all chordates above Amphioxus. This means that the receptor cell itself does not convey the impulse to the central nervous system, this function being served by the afferent neuron of the ganglion of the dorsal root. (In Amphioxus, and in the nose of all vertebrates, on the other hand, the primitive con- dition characteristic of many invertebrates persists : that is, the receptor sensory cell itself produces an axon which runs into the central nervous system and conveys the impulse thither. There is, therefore, no ganglion on the dorsal root of the nerves of Amphioxus, nor on the olfactory nerve in any vertebrate.)


After running into the central nervous system through the dorsal root, the afferent fibres terminate and make synaptic connexions with other neurons. Now the neurons in the spinal cord have their cell-bodies in the grey matter which is central, while the surrounding white matter is made up of the axons (fibres) which pass up and down the cord to higher or lower levels. The grey matter of the cord can be separated into four longitudinal regions on each side. The most dorsal strip is where the fibres of somatic afferent neurons terminate. Beneath this is the place where the visceral afferent neurons end. Under this again is the region which contains the cell- bodies of the efferent visceral neurons ; and lastly the most ventral part of the grey matter contains the cell-bodies of the efferent somatic neurons. Thus the dorsal half of the spinal cord is related to afferent and the ventral half to efferent fibres. As will be seen later, this arrangement is also the fundamental plan on which the brain is built.


The axons of the efferent neurons run out of the spinal cord through the ventral root. The somatic efferent neurons go straight to the striped voluntary muscles of the body- wall, and to the muscles of the limbs (or fins) and end in them. All muscles which are innervated direct in this way by ventral roots are somatic, striped, voluntary muscles derived from the segmented myotomes. On the other hand, the visceral efferent fibres leave the mixed nerve by the ramus communi- cans, and end in the sympathetic ganglia. There they make synaptic connexions with other neurons which run to the smooth muscles of the viscera and form the sympathetic (autonomic) nervous system. The sympathetic system will be dealt with in greater detail below, but it may be noticed now that the visceral efferent fibres belonging to this system never run all the way to the smooth muscle or gland. There is always another neuron intercalated in the circuit, and carrying the impulses on from the sympathetic ganglion. The muscles so innervated are never striped, voluntary nor derived from the segmented myotomes.


The ramus communicans serves not only for the passage of the visceral efferent fibres, but also for the visceral afferent fibres, which then continue to the spinal cord through the dorsal root.


In the region of the head, a slight complication is introduced owing to the development of special sense-organs, and to the fact that the anterior region of the alimentary canal is modified in connexion with the jaws and gill-arches. There is further the fact that the dorsal and ventral nerve-roots of the cranial segments remain separated and do not join to form a mixed nerve.


The various nerve-components in the head can conveniently be studied in the dogfish. Leaving aside for the moment the very specialised visual and olfactory organs, the somatic afferent system is divided into two owing to the development of the lateral-line system.


There is, therefore, a general somatic afferent system which receives impulses from simple sense-organs in the skin corre- sponding to those in the region of the trunk and spinal nerves. This component is present in the trigeminal, glossopharyngeal, and vagus, and their fibres end in the dorsal portion of the medulla oblongata in a region which may be called the " skin- brain." The special somatic afferent system is concerned with the lateral-line organs and the special member of these which is the ear. This component is present in the facial (superficial ophthalmic, buccal and hyomandibular branches), auditory, glossopharyngeal and vagus, and its centre is also in the dorsal part of the medulla oblongata. ^3o great is the number of fibres which end in this way, that the neurons in the medulla with which the afferent fibres make connexion are also multi- plied. The result is that this region, which may be called the " ear-brain," bulges out, forming the tuberculum acusticum. The special somatic afferent system is also called the lateralis system, and arises in relation to the dorso-lateral placodes of the 7th, 9th, and 10th cranial nerves (see p. 194).


The proprioceptive organs are innervated by nerves which (in the head) run in to the brain through most of the cranial nerves, including the oculomotor, trochlear, and abducens. The ear, as an organ of balance, can also be considered as belonging to the proprioceptive organs.


The visceral afferent fibres collect impulses from the mucous surface of the pharynx, mouth, and other viscera, and from the taste sense-organs. In fish, the taste sense-organs are not confined to the mouth, but may be found all over the surface of the body. The afferent visceral fibres run in the branches of the facial, glossopharyngeal, and vagus from the pharynx and from the anterior and posterior faces of the gill-slits. In the brain they converge in the medulla oblongata in the visceral lobe or " taste-brain," beneath the centres for the somatic afferent system. The visceral afferent system is also called the communis system. The fibres innervating the sense-organs of taste are sometimes regarded as forming the special visceral afferent system, and they arise in relation to the epibranchial placodes (see p. 194).


The visceral efferent system is complicated by the fact that the anterior end of the alimentary canal enters into relations with the outside world. Its opening, the mouth,. is bounded by the jaws which are under voluntary control, and so enable the animal to aim at its prey and bite it. In connexion with this, it is found that the muscles which actuate the jaws are striated and voluntary, although they are visceral in origin. The muscles attached to the gill-arches and which perform respiratory movements are likewise striated. But although voluntary and striated, these jaw and gill-arch muscles are not innervated by ventral roots, for they are not derived from segmented myotomes. Instead, they are innervated direct by fibres of the special efferent visceral system which run in the branches of the trigeminal, facial, glossopharyngeal, and vagus, that pass down behind the mouth, spiracle, and the several gill-slits respectively. In higher vertebrates, a portion of the fibres of the vagus become grouped together more posteriorly, and form the spinal accessory or nth nerve.


The general efferent visceral system innervates smooth muscles and glands, and forms part of the autonomic (para- sympathetic) system. The fibres run through the oculomotor, facial, glossopharyngeal, and vagus nerves. The centre of origin of the visceral efferent neurons is for the most part in the medulla oblongata, beneath the visceral lobe.


The somatic efferent system is concerned with the innerva- tion of striated voluntary muscles derived from the segmented myotomes. In the head these are represented by the muscles which move the eyeballs, and the hypoglossal muscles. This component is, therefore, to be found in the oculomotor, trochlear, abducens, and hypoglossal nerves. The centres of the oculomotor and trochlear are in the mid-brain, those of the abducens and hypoglossal are in the medulla oblongata.



Fig. 170, G. — The proprioceptive fibres of the general somatic sensory component.


It may be noticed that the arrangement in the medulla oblongata of the centres concerned with the various components, is similar in a general way to that which holds in the spinal cord. The medulla is the least specialised portion of the brain.


The eyes themselves are part of the brain, and therefore the optic nerve is not an ordinary nerve. Its fibres are strictly intra-cerebral throughout their course. They run through the optic chiasma and end in the roof of the midbrain, which is enlarged to form the optic lobes, or " eye-brain." The nasal sacs are lined by sensory epithelium, the cells of which produce axons growing back into the end-brain. The latter becomes enlarged to form the olfactory lobes or " nose-brain." Expressed in tabular form, the component nerve-systems are as follows : —


Somatic. Afferent. General.



Visceral.



Eye.


Nose.



Efferent. Afferent. Efferent.



Special.


Interoceptors. General.



Special.



Exteroceptors Proprioceptors Lateral-line organs To myotomic striped muscles.


Taste organs and mucous surfaces.


Autonomic (sympathetic and para- sympathetic) to smooth muscles and glands.


To striped visceral muscles of jaw and gill-arches.



Literature

Herrick, C. Judson. An Introduction to Neurology. Saunders Co., Philadelphia and London, 1922.

Johnston, J. B. The Nervous System of Vertebrates. John Murray, London, 1907.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
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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