THE important part played by the neuromuscular apparatus in defence, and the prominence allotted by modern theories to a nervous element in the causation of many clinical conditions in the alimentary tube, render it essential that the nervous arrangements of the tube should be studied carefully.
In an elementary manual it is.impossible to discuss these in much detail, but a brief outline of the factors involved may indicate the lines of further study.
The various neuromotor reactions taking place in the alimentary tube are difficult to investigate by experimental methods. Much, therefore, of the theory of the nervous mechanisms of the alimentary tube is based on the (reason- able) assumption that the reflex actions occurring there are similar to those of the somatic system. For this reason a very short summary of some salient features of the somatic nervous system may help in the understanding of the auto- nomic nervous system.
The somatic nervous system is a great concourse of reflex arcs that are combined and integrated to control the working of the general body musculature. In its ultimate analysis each reflex arc consists of three neurons, namely, (i) an afferent neuron, (2) a connector neuron, and (3) an efferent neuron. These neurons are not in structural continuity; at the region where two neurons meet—termed a synapse— branching fibrils of one neuron make contact with similar branching fibrils of the other neuron. Nerve impulses are transmitted only in one direction through the arc, namely, from the periphery along the afferent neuron, through the connector neuron to the efferent neuron, and thence to a responsive mechanism. These impulses do not evoke a continuous nerve current; an impulse from the afferent neuron initiates a new impulse in the connector neuron, and this in turn initiates an impulse in the efferent neuron.
In the vertebrate somatic system the afferent impulses travel to the central nervous system mainly by the sensory nerves; the connector neurons are located in the central nervous system] the efferent impulses are carried from the central nervous system to the peripheral responsive mechanisms by the motor nerves.
An important anatomical difference between these reflex arcs and those of the autonomic system is that in this latter system the connector neurons are not located wholly in the central nervous system. In the case of the alimentary tube the con- nector neurons extend from the central nervous system to distal ganglia situated in or near the wall of the tube. It is these connector neurons that form the vagus and splanchnic nerves of the autonomic system.
It may be noted in passing that there is no apparent anatomical difference between the afferent neurons of the somatic system and those of the autonomic system. Each afferent neuron has a cell-body in a posterior root ganglion, or corresponding ganglion of a cerebral nerve; from this a long process extends to a receptor structure in the periphery of the body or in the wall of the alimentary tube, while shorter processes from the cell-body form synapses with connector neurons in the cerebrospinal axis.
The impulses that travel inwards along the sensory nerves are rearranged and co-ordinated through connector neurons until the destination of a final efferent impulse is decided. This final impulse is sent down a specific motor nerve which is described as the final common path. It will be obvious that the final common paths of the autonomic system are the short (usually very short) fibres in or close to the wall of the alimentary tube. Experimental stimulation of the vagus or splanchnic nerves, therefore, is not comparable with stimulation of somatic motor nerves.
The study of the phenomena exhibited in the reflex arc such as summation of impulses, irradiation, refractory period, etc., that are all essential to orderly co-ordination of reflexes, does not fall within the scope of the present inquiry. A close study of them, however, is recommended, as it will prove helpful in understanding many of the problems of gastro-enterology.
Special attention, however, may be drawn to the phenom- enon of reciprocal innervation. Many of the efferent impulses that traverse the somatic final common paths are not translated into active motor responses. Instead, their effect may be to inhibit the actions of the muscles to which they are distributed. This is, of course, necessary for the production of specific movements as opposed to specific muscle contractions; without the inhibition of antagonistic muscles it would be impossible for groups of muscles to effect co-ordinated movements. In this relation attention may be directed to the fact that in the somatic system there are no instances where direct stimulation of a motor nerve evokes inhibition of the muscles to which it is distributed.
In other words, there are no inhibitory nerves; inhibition is a function of co-ordinating reflex mechanisms situated in the cerebro- spinal centres. Another interesting fact is that when a motor nerve is stimu- lated through a reflex arc only a proportion of its fibres respond. An important physiological difference between the reflexes of the somatic and autonomic systems is dependent on the difference between the adequate stimuli required for the respective responsive muscles. In the somatic system the adequate stimulus for the contraction of a skeletal muscle is the impulse transmitted to it through the efferent neuron. In the autonomic system, on the other hand, the adequate stimulus that evokes the contraction of the involuntary muscles is a mechanical stretching of the muscle-fibres. Impulses, then, travelling down to the short efferent neurons of the autonomic reflex arcs do not normally initiate contrac- tion of the muscular tunic of the bowel wall—they control it.
Receptor End-organs.—Some features of the collecting mechanisms associated with afferent neurons may now be reviewed briefly.
Many forms of specialized end-organs collect the stimuli for the impulses conveyed by the sensory nerves, and they may be classified in various ways. A useful grouping of them is under the headings of exteroceptive, proprioceptive, and interoceptive. The exteroceptive end-organs are situated in the surface layers of the body, the proprioceptive in the walls of the body, and the interoceptive in the lining mem- branes of the viscera.
Exteroceptive.—As might be expected, the exteroceptive group embraces the greatest number and variety of sensory end-organs, since these are brought most directly into contact with the many external agencies impingeing on the surface of the body. This group may be classified according to the forms of stimuli for which the receptors are designed. Thus there are chemo-receptors for smell, taste, and the chemical senses; mechano-receptors for touch, pressure, etc.; radio- receptors for heat and cold; noci-receptors for harmful stimuli, etc.
For practical purposes a subdivision of the exteroceptors into distance and local receptors is helpful. The distance receptor end-organs include the eyes, ears, and nose, and they receive their stimuli from vibrations or materials emanating from distant objects. These end-organs are located in the head end of the animal and have very elaborate nerve stations and communications in the brain. The cell-bodies of their associated afferent neurons are connected intimately with groupings of nerve-cells in the cerebral cortex. Thus, impulses from the distance receptors may be transmitted readily to the higher cerebral centres mediating the emotions, memories, and sensations of pleasure, disgust, etc. Further, since the distance receptors are utilized to recognize food or danger at a distance, the reflex motor responses that they call forth are large movements involving the whole body or considerable parts of it. Examples are the turning move- ments of the head or body, movements of locomotion towards or away from the exciting stimuli, grasping movements, etc.
On the other hand, the majority of the local exteroceptive end-organs have their first connector stations in the spinal cord. Their arrangement is more o f a segmental type, a n d correspondingly the muscles that they activate are local segmental groups.
Proprioceptive.—Several types o f proprioceptive end-organs are found in the somatic muscles, tendons, and fasciae. They resemble those o f the local exteroceptive group in that their disposition is primarily segmental a n d th e motor reflexes associated with them thus exhibit a local segmental type.
No specialized proprioceptive end-organs are demonstrable in the muscular tunic of the alimentary tube.
Interoceptive.—It is only at the ports of entry and exit of the alimentary tube that sensory receptor end-organs similar to those of the local exteroceptive group are found.
A small exception is that a few receptor end-organs have been demonstrated in the mesenteries. Otherwise, there are no specialized sensory end-organs in the wall of the alimentary tube.
For the present purpose attention is concentrated on such of these general considerations as have an apparent bearing on the defence mechanisms of the alimentary tube. A group of nerve-cells in the hypothalamic region of the brain constitutes a motor area that is in close communication with sensory cells in the thalamus. Together, these cell-groupings form what may be termed “the autonomic brain” (Fig. 17). This represents the highest co-ordinating centre for the autonomic nervous system. It must not be regarded as a completely independent part of the central nervous system, but merely as a subsidiary part working in harmony with the somatic system. The principles involved in both systems are alike. Starting with a series of short reflex arcs that are of a segmental nature, more complex arcs are gradually introduced; these involve longer reflex pathways that extend ultimately to the brain and convey impulses to larger and more widely distributed groups of responsive mechanisms.
In the anatomical arrangements of the reflex pathways there are important differences between the somatic and autonomic systems.
Somatic Reflex Pathways.—Impulses entering by the somatic sensory nerves (whatever be the types of receptor end-organs involved) find their way to connector tracts in the cerebrospinal axis. Some of these tracts are short and involve only a few segments, but others are long enough to reach the cerebrum. The tracts connected with the local exteroceptive and proprioceptive end-organs have, in the main, shorter and less complex communications than those associated with the distance receptors. A more rapid but more local response to stimuli, therefore, may be obtained.
Since the receptor end-organs are all very sensitive, many of the ingoing impulses reach the cerebral cortex, where they are analysed and appreciated as specific sensations.
Outgoing impulses associated with the somatic system travel down through the connector tracts in the cerebro- spinal axis, and ultimately via the final common paths (somatic motor nerves) activate the somatic muscles.
Autonomic Reflex Pathways.—In the autonomic system, as already described, the reflex pathways are arranged in two groups, namely, sympathetic and parasympathetic. Each of these has its own ingoing and outgoing paths, but in neither are the connector neurons for the outgoing impulses confined to the cerebrospinal axis.
Sympathetic.—If ingoing impulses from the alimentary tube be directed along the sympathetic nerves, they are transferred to groups of connector neurons in the spinal cord. Such groups have, in the first instance, local segmental communica- tions—but some impulses may be carried as far as the autonomic brain through longer spinal tracts.
The efferent connector neurons of the sympathetic system form the splanchnic nerves from the 5th to the 12th thoracic and the ist to the 3rd lumbar segments of the spinal cord. Through these nerves outgoing impulses of the sympathetic reflex pathways reach the ganglia in or near the wall of the alimentary tube and are thence distributed to the musculature of the bowel wall.
The points to which special attention is drawn are: (1) That the sympathetic reflex pathways are essentially of a segmental nature. (2) That they are closely associated with the local exteroceptors and with the (segmental) somatic motor nerves. (3) That the responsive mechanisms of this system are largely muscular and are not associated with the more highly specialized secretory apparatus; it is probable, however, that they may influence the secretion of mucus, and, by their connexions with the blood-vessels, the outpouring of diluting fluids for defence purposes. (4) That the ingoing impulses, under normal conditions, are not translated into sensations.
Parasympathetic.—If ingoing impulses from the alimentary tube be directed along the parasympathetic nerves, they travel mainly by the vagus nerves to the root ganglia of these nerves and thence to the medullary nuclei of the vagi. Here they are transferred by the associated connector neurons, and the impulses conveyed by these have a choice of several pathways. For instance, they may be transferred to nerve- cells higher in the cerebral hemispheres; they may be trans- ferred to the autonomic brain; they may be carried down the long connector neurons that constitute the vagus nerves to activate the motor and secretory neurons in the wall of the alimentary tube.
The points to which special attention is drawn in the parasympathetic system are : (ι) That the outgoing connector neurons are not segmental—they control long stretches of the alimentary tube, i.e., viscera or compartments. (2) That they are intimately associated with the distance receptors. (3) That they control not only the muscles of the alimentary tube, but all the highly specialized secretory apparatus. (4) That, under normal conditions, the ingoing impulses are not translated into sensations.
It is interesting to note that in the part of the bowel where there is no highly specialized secretory activity a portion of the parasympathetic efferent system is divorced from the main system. Instead of the more complex vagus (medullary) system to Compartments III and IV, the parasympathetic supply to Compartment V by the pelvic splanchnic nerves is simpler and its primary cell-stations are in the spinal cord. Presumably its muscular activities are similar to those of the vagus system, i.e., it controls large stretches of the musculature rather than short segments.
If this view of the sympathetic and parasympathetic systems be taken, then the two systems should not be regarded as antagonistic but rather as complementary. Each system has its own field of control—one (sympathetic) controlling local segmental regions of the alimentary tube, the other
(parasympathetic) controlling larger stretches, namely, viscera or compartments of the tube.
It is interesting to note, in this connexion, that experi- menters have found that it is not possible to explain the results of their experiments on the cardiac (oesophageal) sphincter if an antagonistic action of the vagi and splanchnics be postulated. It is suggested that careful study might reveal other instances.
Salivary Secretion.—A brief study of the nervous arrangements of the salivary glands may serve to illustrate the manner in which the sympathetic and parasympathetic systems are co-ordinated.
In Part II (pp. 40, 42, 56) it was stated that in the glands of the alimentary tube secretion may be stimulated by both nervous and chemical influences. The usual procedure is for the secretion to be started by nerve impulses and then con- tinued by chemical substances—a process that conserves a considerable amount of nervous energy. The chemical substances (hormones, etc.) require time for their elaboration, and so this method is unsuitable where foodstuffs are passing rapidly through a part of the tube. It will be obvious that in the mouth the secretion of saliva for digestive purposes must be rapid (and therefore largely nervous), and thus there is presented an opportunity of studying nervous secretion with a minimum of chemical interference. Further, there is easy anatomical access to the nerves of the salivary glands and direct (experimental) stimulation of them can be accom- plished readily.
As a result of experiments it is found that stimulation of the parasympathetic nerves (derived from salivary nuclei in the brain-stem) evokes a watery secretion, whilst stimulation of the sympathetic nerves (derived from nuclei in the spinal cord) is followed by a secretion of a thick mucous material. A normal saliva contains both elements, and observations have demonstrated that the relative proportion of the two elements is regulated by both external and local factors co-ordinated in a purposeful manner.
The portion of the parasympathetic system involved in salivary secretion has been shown to be fundamentally, related to the distance receptors; it is normally stimulated by memories, emotions, and sensations of sight, smell, and hearing. The output of a watery secretion in these circum- stances gives rise to the phenomenon of 4’watering of the mouth Conversely inhibitory impulses travelling down the parasympathetics lead to a marked ” dryness of the mouth “. This generalized response is supplemented by the responses from the local interoceptors and the proprioceptors.
The different foodstuffs actually in the mouth evoke responses that are aroused by mechanical and chemical stimuli. Solid masses that are swallowed quickly call forth a (sympathetic) secretion of lubricating mucus, whilst powdery foodstuffs demand a more watery (parasympathetic) flow that contributes to the building up of a bolus; these secre- tions are of course modified by the chemical constituents of the food. At the same time purely mechanical stimuli of differing physical nature, such as small stones or powdered inert substances, evoke similar differences in the character of the secretion.
The important part played by these factors in defence when abnormal conditions are present now becomes apparent. An increase in the amount of watery saliva, for example, is required to remove small particles of harmful materials or micro-organisms that might accumulate in the recesses of the teeth, etc. Larger inedible foreign bodies may require a coating of mucus to minimize local trauma. Acids or caustic substances in solution are dealt with by a combined secretion ; increased protein in the saliva exerts a buffer action on acids, the watery secretion is diluent, whilst the mucous content protects the surfaces from contact with the noxious materials.
Prominent points relevant to the present discussion seem to be: (1) the co-ordination of the sympathetic and para- sympathetic nerves (probably achieved in the autonomic brain) ; (2) the preponderating influence of the distance receptors on the parasympathetics and of the local receptors on the sympathetics ; (3) the existence of receptive mechanisms (as yet purely hypothetical) by which stimuli are directed either into the sympathetic reflex pathways or into those of the parasympathetic—there is no anatomical evidence, however, of any such selective apparatus. An alternative suggestion is that the differences in the character of the autonomic responses may be due to differences in the intensity of the exciting stimuli.
The student should give careful consideration to the results obtained by experimental stimulation of autonomic nerves. When preganglionic endings (connector neurons) are stimu- lated, acetylcholine is liberated. When, however, post- ganglionic endings are stimulated, those of the sympathetic system liberate adrenaline instead. The theory advanced is that these chemical substances excite a receptive substance in the tissue to which the nerves are distributed.
It seems a reasonable assumption that in the autonomic brain there is a process of selection that determines the route to be followed by emissive impulses. The corollary would be that the process of selection is guided by information which is derived from the hypothetical receptors postulated above.