Popular Science Monthly/Volume 75/August 1909/The Origin of the Nervous System and its Appropriation of Effectors II
|THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS|
II. Receptor-effector Systems
PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY
THE second step in the development of the neuromuscular mechanism is represented by the receptor-effector system, a condition fairly realized in such cœlenterates as the sea-anemones and the jellyfishes and probably recurring in the digestive tubes of the higher metazoans. As an introductory example we may turn to the sea-anemones. Fig. 1. Longitudinal, Section of a Sea-anemone (Metridium); g, gullet; gvs, gastro-vascular space; m, mouth; t, tentacles. Most sea-anemones (Fig. 1.) are cylindrical animals attached to some firm object by their aboral disks and carrying on their oral disks a ring of tentacles surrounding the mouth. This aperture leads inward through a short gullet to a large, somewhat divided, digestive cavity, the gastrovascular space, which extends throughout the whole interior of the animal even to the tips of its tentacles and is the only cavity within the sea-anemone. The body of the animal is made up of walls of extreme thinness; these walls consist of two layers of cells, an outer one next the sea water, the ectoderm, and an inner one next the gastrovascular space, the entoderm. These two layers are separated by a tough, non-cellular sheet, the supporting lamella.
Unlike sponges, sea-anemones are very responsive to changes in their environment. If a fully expanded Metridium is disturbed by mechanical agitation, it will quickly retract its oral disk, discharge through its mouth the water contained in its gastrovascular cavity, and finally cover its tentacles and month by puckering in the oral edge of its column. In this contracted state it may remain hours at a time, and when it eventually expands it does so by relaxing its muscles and refilling its body with sea water. A beam of strong sunlight, if thrown upon an expanded Metridium several feet under water, will usually call forth the same contraction as mechanical stimulation does.
When the exterior of a Metridium is tested locally, its receptiveness for certain stimuli is found to be quite diverse. The animal makes no movements when dissolved food-substances are cautiously discharged upon the external surface of its Fig. 2. Ectoderm from the Tentacle of a Sea-anemone (Metridium); e, epithelial layer; m, muscular layer; n, nervous layer; s, supporting lamella. column, though this very area is sensitive to mechanical stimulation. Precisely the reverse is true of the lips; these organs are easily stimulated by dissolved food-products, but no reaction occurs even when they are punctured by a needle. Both mechanical and chemical stimulation, however, are effective on the tentacles and vigorous responses can be called forth from even distant parts of the body by the application of either of these forms of stimuli to the tentacles. Since these reactions, as just intimated, often involve responses in very different parts of the animal from those to which the stimulus is applied, it follows that we are dealing with a process justly regarded as nervous for transmission in this case is not accompanied with any observable motion. The surface of a sea-anemone may then be pictured as a true receptor surface partly differentiated in different regions for particular classes of stimuli, but not so far specialized that it can be described as made up of sense organs.
An examination of the structure of the ectoderm (Fig. 2) will do much to make clear the mechanism by which the reactions of sea-anemones are carried out. The ectoderm of these animals is a modified epithelium in which three definite layers can be distinguished. The outermost of these forms more than half the thickness of the total layer and is a true columnar epithelium. It contains, in addition to ordinary epithelial cells, gland-cells and nettle-cells, and, what is of more importance to us, sense-cells. These sense-cells are long, narrow bodies whose distal ends are armed with a sensory bristle which, under ordinary conditions, projects into the surrounding sea water and whose proximal ends run out into finely branched, nervous processes which intermingle with similar processes from other cells. The complex made by the interweaving of immense numbers of these processes constitutes the second layer of the ectoderm, the nervous layer, and this layer often contains in addition to the large amount of fibrillar material derived from the sense-cells, numerous multipolar ganglion-cells whose processes add to the fibrillar material already mentioned. A careful study of this fibrillar material has recently been made with the result that a true nervous network has been demonstrated in hydroids (Wolff, 1904; Hadzi, 1909), siphonophores (Schaeppi, 1904) and sea-anemones (Wolff, 1904; Groselj, 1909). In the sea-anemones in particular this network appears to be a perfectly continuous and diffuse one, notwithstanding Havetfs previous declaration (1901) to the contrary. The third layer is composed of parallel muscle-fibers that rest against the supporting lamella on one side and are in contact with the nervous network on the other side. The muscle-cells of this layer are much elongated, spindle-shaped cells. These three layers, the epithelial layer, the nervous layer and the muscular layer, constitute the structural elements in the ectodermic neuromuscular mechanism of a sea-anemone.
The nervous type of ectoderm just described covers practically the whole surface of a sea-anemone and has been designated as a diffuse nervous system in contrast to a centralized one. The fact that the nervous layer is more fully developed on the oral disk than elsewhere has given anatomical grounds for the assumption that this portion is a central nervous organ, but, as will be shown later, the physiological evidence in favor of this opinion is so slight that the designation of the nervous system as a diffuse one is more consistent with facts.
From the standpoint of our original analysis, it is quite plain that in the sea-anemones we are dealing with at least two elements of the typical neuromuscular mechanism, namely, receptors as represented by the sense-cells, and effectors as seen in the muscle-fibers. Whether the fibrillar material that intervenes between these two structures represents an adjustor or central apparatus will be discussed after the action of this nervous mechanism has been more fully described.
The feeding habits of the sea-anemones throw considerable light on the physiology of their nervous structures. If particles of meat are dropped on the tentacles of an expanded Metridium, they become entangled in the mucus on these organs and are quickly delivered to the mouth, where they are swallowed. If fragments of clean filter-paper soaked in sea water are similarly dropped on the tentacles, they are usually discharged from the edge of the oral disk without having been brought to the mouth. Thus the animal appears to discriminate between what is good for food and what is not. If, however, pieces of filter-paper soaked with meat juice are put on the tentacles, they are usually swallowed as though the sea-anemone had been deceived. On the basis of these simple experiments a still more striking combination can be devised. If a sea-anemone is provided alternately with pieces of meat and pieces of filter-paper soaked in meat juice it will in the beginning swallow in sequence both materials, but after ten or a dozen trials it will regularly swallow the meat but usually discard the filter-paper. Thus it would appear that the sea-anemone had detected the deception practised on it in the beginning and had learned to circumvent the experimenter. But further observations show how erroneous this interpretation is. If the experiment just described is performed on a limited group of tentacles on one side of the oral disk and, after the animal has arrived at the stage of discriminating between meat and paper, the experiment is repeated on another and distant group of tentacles, it is found that these tentacles and the part of the mouth next them will accept both meat and paper as the first group did and the same process as was used on this group must be repeated on the second group in order to bring it to the stage of discrimination. Thus it is clear that, however we may regard these acts, Metridium shows no marked power of making the experience of one part of its body serve another; in other words, it shows no decided evidence of a central nervous organ.
This conclusion is in substantial accord with the recent results obtained by Fleure and Walton (1907) from experiments on Actinia except that they believe that the repeated trials on the tentacles of one side of the circle had in this form a slight influence on those of the other. This influence, however, was so slight that they declared that experience of this kind certainly did not become the possession of the animal as a whole.
Not only is there in these reactions absence of any strong evidence in favor of well-marked central nervous functions in anemones, but it is very doubtful if we are justified in regarding the local reaction just described as a true discrimination. Jennings (1905) has suggested that sea-anemones possess sensations of hunger and that as the experiment proceeds the animal's hunger diminishes and it finally discards when less hungry what it at first accepted. But Allabach (1905) has shown that the same so-called discrimination is arrived at if the sea-anemone is not allowed to swallow anything, but is robbed of meat and paper alike by having these materials picked out of its gullet just as they are about to be swallowed. In fact it seems quite clear that this process of apparent discrimination is in no sense due to centralized nervous functions, but is merely the result of exhaustion. At the beginning of each experiment the receptors are stimulated by the strong juices of the meat and the weaker juice of the paper. As they run down in efficiency, they come to a stage where they no longer react to the weaker stimulus of the paper and respond only to the meat. At this stage apparent discrimination takes place.
Not only do these experiments show no evidence of central nervous functions, but they indicate a decided looseness of nervous articulation. The activity of one side of the body of the sea-anemone has very little, if any, influence on the other side. Moreover, the fact of intimate local relations between nerve and muscle, as seen in the anatomy of these animals, supports the idea of neuromuscular independence instead of centralized relations. This is well exemplified in the reactions of the tentacles. If a tentacle of Metridium is stimulated by food, it turns and twists irregularly and then points toward the mouth. If the same tentacle is cut off and held filled with water so that its original relations in the animal as a whole can be kept in mind, it will be found to react to food as it formerly did, in that it will finally turn toward that side which was originally next the mouth. Fig. 3. Longitudinal Section of the Intestinal Wall of a Vertebrate, showing the nervous and muscular constituents; ap, Auerbach's plexus; cm, circular muscles; lm, longitudinal muscles; m, mucous layer; mp, Meissner's plexus; s, serous layer. Hence we may conclude that the tentacle has within itself all that is necessary by way of neuromuscular mechanism for its characteristic reactions and is not dependent for these on such other parts of the sea-anemone as have been regarded as central nervous organs. Physiologically as well as anatomically the sea-anemone seems to possess a diffuse rather than a centralized nervous system, and its neuromuscular mechanism consists of receptors and effectors connected by a nervous net which is composed partly of the nervous processes of the receptor cells and partly of similar processes from ganglion cells.
The type of neuromuscular mechanism found in the sea-anemones probably also recurs in the digestive tube of vertebrates. This view is supported not only by the action of the intestine, but also by its structure (Fig. 3). Omitting for the moment the outer serous layer and the inner mucous layer of the intestine, both of which have little or nothing directly to do with its neuromuscular mechanism, there are left the outer or longitudinal muscular layer, followed internally by a nervous layer, Auerbach's plexus, which in turn is followed by the cular muscles on which rests a second nervous layer, Meissner's plexus. Each plexus, so far as is known, is a true nervous net as intimately related to the adjacent muscle fibers as is the case of the sea-anemones. In fact one of the muscle layers and the adjacent plexus in the intestine reproduce very accurately all the essentials of the neuromuscular mechanism of a sea-anemone except the epithelial sense-cells.
Not only is there this anatomical similarity between the neuromuscular mechanisms of the sea-anemone and of the vertebrate intestine, but there is also a physiological similarity as seen in the movements of the digestive tube. The essentials of these movements are well exemplified in the small intestine. In this part of the digestive tube the characteristic movements are segmentation and peristalsis. Segmentation consists in a series of temporary, ring-like constrictions in the intestinal wall that come and go in such a way that the enlarged region of the tube between any two constrictions is the site of the constriction next to appear, and so on. As a result of segmentation, the food is most thoroughly churned and mixed. Peristalsis is a wave-like movement whereby the food is carried posteriorly through the intestine. Usually these two movements go on together in such a way that the peristalsis is combined with segmentation in that the latter becomes somewhat unsymmetrical and cuts each food mass into two unequal parts the larger of which is on the posterior side of the constriction. Hence the food is not only churned but is at the same time moved posteriorly through the intestine.
The small intestine receives nerve-fibers from two extraneous sources, the vagus and the splanchnic nerves, and it might be supposed that these were essential for the movements of the intestine. But as Cannon (1906) has demonstrated, both sets of nerves may be cut, and yet after recovery from the immediate effects of the operation segmentation and peristalsis will be found to go on in the digestive tube in an essentially natural manner. It is thus clear that the vertebrate intestine, like the tentacle of a sea-anemone, contains a complete neuromuscular mechanism within its own wall, and though there is no histological evidence of the presence of receptors reaching from the mucous surfaces of the intestine to the nervous nets within, yet there are sound physiological grounds for assuming the presence of such organs. In that case the type of neuromuscular mechanism in the intestine would be practically identical with that in the sea-anemone.
A second example of a receptor-effector system in ccelenterates is seen in the jellyfishes. In these animals as contrasted with the seaanemones, locomotion is a well-developed activity, and it is the neuromuscular mechanism concerned with this function that must be considered. The structures involved in locomotion are well exemplified in Aurelia (Fig. 4). This common jellyfish possesses on the edge of its bell eight clusters of sense-organs. Each cluster contains an ocellus, two sensory pits that are probably concerned with the chemical sense, and a sense-club which may be a pressure organ. The sensory portions of all these organs are modified ectoderm and from these portions nervefibers pass out as radiating bundles to the ectoderm of the subumbrellar surface. Here they merge into a nervous net which overlies the ectodermic musculature as in the sea-anemones. Fig. 4. Aurelia, subumbrellar surface; s, cluster of sense-organs. This musculature forms a circular sheet concentrically disposed with reference to the symmetry of the jellyfish. When the bell of an Aurelia is pulsing, the movement is carried out by the more or less general contraction of this circular band of muscle, which is brought back to its original position on relaxation by the elasticity of the gelatinous mass of the bell. The locomotor muscle, then, is a gigantic sphincter that works against an elastic resistance.
The significance of the various parts of the neuromuscular mechanism in such an animal as Aurelia can be determined by experiment. If the eight sense-bodies are removed, the animal will no longer pulse spontaneously, though its muscles may be made to contract by direct stimulation. If all but one sense-body are removed, the bell will pulse with regularity and by artificially stimulating the single remaining body a wave of muscular contraction can be sent over it. It is therefore evident that the sense-bodies act like extremely delicate triggers and thus touch off the contractile mechanism. In this respect, then, the jellyfish is more highly developed than the sea-anemone, for the latter possesses no such specialized and delicate receptors.
The wave of contraction that passes over a bell when one of its sense-bodies is stimulated, may be either a purely muscular phenomenon or may be the result of nervous transmission through the nervous net whereby one region after another of the musculature is brought into action. The fact that this wave is not checked when the bell is cut even in a most irregular way provided the subumbrellar epithelium is still continuous, favors the nervous rather than the muscular interpretation. But stronger evidence on the nervous side than this has come from an entirely different direction. Mayer (1906) has shown that the subumbrellar epithelium of Cassiopea after removal will readily regenerate, and that in regeneration the nervous net forms earlier than the muscles. By taking jellyfishes at the appropriate stage in regeneration, it was found that a stimulus applied to one side of a regenerated area was followed by a muscular response on the other side of this area without any observable movement in the area itself. Hence transmission through the regenerated region must have been by nervous means, doubtless by the nervous net.
In jellyfishes the nervous net will transmit apparently in any direction and in this respect it is in strong contrast with the central nervous organs of the higher metazoans, where, especially in the vertebrates, Fig. 5. Neuromuscular Cell (black) In place In a columnar epithelium. a polarized condition generally prevails. Thus in the spinal nerves of vertebrates, it is easy to send impulses through from a dorsal root to a ventral one, but impossible to send them in the reverse direction. Apparently the cord contains some structure on its path of conduction that is valve-like and allows impulses to pass in one direction only. Such a condition does not exist in the nervous net of the jellyfishes.
The neuromuscular organs of the coelenterates have been considered by so many investigators as the most primitive in the animal, Fig. 6. Differentiation of Neuromuscular Constituents from an Indifferent Epithelium. The upper figure represents an indifferent condition containing three cells which subsequently (lower figure) differentiate into a sense-cell (1), a ganglion-cell (2), and an epithelial muscle cell (3). kingdom that it is not inappropriate to consider at this place the relations of some of the older views on this subject to those expressed in these articles.
The discovery by Kleinenberg (1872) of the so-called neuromuscular cells (Fig. 5) in Hydra led this investigator to the belief that these cells represented a complete neuromuscular apparatus in that each cell-body could be regarded as a receptor and its fibrous portion as an effector. By growth and cell division, according to Kleinenberg, separate receptors and effectors would be differentiated simultaneously from such single cells.
The simultaneous differentiation of nervous and muscular elements (Fig. 6) was also accepted by the brothers Hertwig (1878), but in their opinion the two types of tissue did not arise from a common cell as claimed by Kleinenberg, but from separate cells which became simultaneously differentiated, some to form nerve-cells (sense-and ganglion cells) and others to form muscle-cells. This view has come to be commonly accepted by the majority of investigators.
The independent origin of the nervous system and its secondary connection with the musculature has been advocated by Claus (1878) and by Chun (1880), but a nervous system without effectors is, as Samassa (1892) and Schaeppi (1904) declare, scarcely conceivable.
The opinion about the origin of nervous and muscular tissues as expressed in these articles is opposed to the various theories stated in the preceding paragraphs in that muscular tissue is regarded as the ancestral tissue and nervous tissue is supposed to have formed secondarily and as a means of bringing muscular tissue into action with greater certainty than direct stimulation would do. According to this view the primitive state of the neuromuscular mechanism is to be seen in such animals as sponges, which possess muscles but no true nervous organs; and the neuromuscular or, better, epithelial-muscular cells of the coelenterates represent these primitive effectors to which have been added a diffuse system of receptors as seen in the sea-anemones or a specialized system as in the jellyfishes. In both instances the receptors and effectors are related through a nervous net.
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