Popular Science Monthly/Volume 75/July 1909/The Origin of the Nervous System and its Appropriation of Effectors I
|THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS|
I. Independent Effectors
PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY
THE physiological unit in the operations of the nervous system is the reflex. Broadly understood, this consists of the chain of consequences that begins with the reception of a stimulus on the surface of the animal and, leading through the central nervous organs, ends in the excitation of a reaction by some such organ as a muscle. The term reflex is made to apply nowadays to nervous operations involving conscious states as well as to those that are carried out unconsciously. In its greatest simplicity the conventional reflex involves at least two nervous cells or neurones and some form of reacting organ
Fig. 1. Transverse Section of the Ventral Nervous Chain and surrounding Structures of an Earthworm (modified from Retzlus). cm, circular muscle; ep, epidermis; lm, longitudinal muscle; mc, motor cell-body; mf, motor nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion.
such as a muscle-fiber. The first neurone, as exemplified in the nervous structure of such an animal as the earthworm, is often the body of a sense-cell on the surface of the animal and the sensory nerve-fiber to which this cell body gives rise and which leads to the central nervous organ. The second neurone is a nerve-cell whose body lies within the central nervous organ and whose process, a motor nerve-fiber, extends from the central organ to the muscle-cells which it controls. The first neurone as it enters the central organ breaks tip into a large number of delicate branches which are in physiological continuity with similar branches from the second neurone. It is over these delicate branches that the nerve-impulse passes from one neurone to the other and it is the structure of this system of branches that has been a matter of so much discussion within recent years. Anatomically, then, this simplest form of central nervous organ consists of motor cell-bodies and fibrillations from these bodies and from sensory neurones. Of course most central organs include additional neurones, such for instance as association neurones, which connect one part of the central organ with another and do not participate directly as sensory or motor constituents. The simplest conceivable reflex mechanism, however, does not include these, but only the sensory and the motor neurone as described. Such a chain reaching from the periphery of the animal through its central nervous organ to and including its muscles is usually regarded as the primary type of neuromuscular mechanism.
From a physiological standpoint this simplest type of reflex mechanism falls into three parts. The first of these is the sense organ or receptor, which, as its name implies, receives the external stimulus; the receptor is also the seat of the production of the nerve-impulse. The second is the central nervous organ or, as it may be called, the adjuster, which is concerned with directing the impulse toward the appropriate end-organ and with modifying it in accordance with the particular reaction to be obtained. The third and last is the effector or organ brought into action by the impulse, such as a muscle or gland. Thus a simple reflex may be said to involve at least three special classes of mechanisms: receptors, adjusters and effectors. These mechanisms, however, do not correspond exactly to the three histological elements already named, for, though the receptive function is an activity limited entirely to the first neurone in such an animal as the earthworm, and the effector is the muscle-fiber, the adjuster is a part of the first as well as of the second neurone and is made up of at least the fine fibrillar material contributed by these two neurones to the central nervous organ. The neuromuscular mechanism even in this its simplest type has probably not sprung into being fully formed, but it has had without doubt a slow and gradual growth. It is one of the objects of these articles to trace as far as possible the steps in this growth.
It is to be noted that every reflex mechanism is in the nature of a physiologically continuous span of living substance which reaches from the receptive surface on the one hand to the effector organ on the other. At no point in this span can there be a real interruption, for a physiologically continuous thread of protoplasm must connect the two extremes. It is, therefore, conceivable that a reflex mechanism might exist in the body of a protozoan and in fact there is experimental evidence to show that in certain infusorians the superficial protoplasm is somewhat differentiated as a receptive surface and that this protoplasm also serves as a conducting organ whereby, for instance, the activity of certain groups of specialized cilia in these animals is coordinated. These conditions, however, are found within the substance of a single cell and are so remote from those of a true nervous mechanism that, interesting and significant as they are, they had better be termed neuroid than nervous. They show at best that the protoplasm of the protozoan harbors operations that may develop in the multicellular animals into reflex processes Fig. 2. Diagram of the Canal System of a Calcareous Sponge (modified from Haeckel). The lateral Inlet pores receive water from the exterior, as shown by the arrows on the sides; the osculum at the apex discharges water to the exterior. rather than that the protozoans possess these processes, and that we must look among the simplest metazoans for the beginnings of a true neuromuscular mechanism.
In making a quest for the first stages in the development of the nervous system, it is important to keep in mind the relative significance of the three physiological elements already pointed out: the receptors, the adjusters and the effectors. A little reflection will show that these three are not likely to prove all of primary significance.
A receptor or sense organ alone would be of no service whatever to an animal; it would resemble a telephone receiver disconnected from the rest of the system. In a similar way the adjustor or central organ is useless without at least some other element in the reflex apparatus. The only mechanism sufficient in itself is the effector, which, if it can be brought into action by direct stimulation, may accomplish something serviceable to the animal. It is therefore improbable that we shall find multicellular animals that possess either receptors or adjusters without effectors, but it is conceivable that primitive metazoans may have effectors without other parts of the typical neuromuscular mechanism.
In a search for the earliest traces of the neuromuscular mechanism, we may turn first to those very primitive metazoans, the sponges. The body of one of the simpler sponges is a more or less goblet-shaped, multicellular mass, whose surface is covered with an enormous number of minute pores; these lead into tubes which in turn communicate with a relatively large central cavity that opens to the exterior by an aperture of considerable size, the osculum. In a living undisturbed sponge, water is continually passing into the lateral pores, through the tubes and central cavity, and out at the osculum. This current is produced by means of numerous cells, the choanocytes, which are provided with vibratile lashes and are variously distributed through the internal chambers and tubes of the sponge. Apparently these choanocytes work incessantly, and the current generated by them carries food, etc., to the sponge and removes waste products. Although frequent efforts have been made to show that nervous structures occur in sponges, nothing of this nature has been conclusively demonstrated and it is now generally believed that these animals are without differentiated nervous organs, either sensory or central. Nevertheless, sponges are capable of a certain amount of response. Merejkowsky (1878) observed that when he pricked with a needle the inner face of the. osculum of Rinalda, this aperture quickly closed, not to open again for several minutes. The same reaction occurs with the lateral pores of many sponges (Vosmaer and Pekelharing, 1898). This power of closing the pores seems to be the only means by which a sponge may check the current which ordinarily flows through its canals, for, as already mentioned, the choanocytes apparently lash the water incessantly.
When a search is made for the organs concerned with the closing of the pores and oscula, they are found to consist of rings of elongated contractile cells or myocytes, which surround these apertures. These rings of cells form veritable sphincters and their action is often efficient enough to bring about a complete temporary closure of the aperture. Whether the pores and oscula open by the counteraction of radial, contractile myocytes or by the simple elasticity of the surrounding tissue does not seem to have been determined.
Since these sphincters lie very close to the epithelium that bounds the surfaces of the pores or oscula and in fact probably often form a part of this very epithelium, and since no nervous mechanism is known to be connected with them, it seems very probable that they are brought into action by direct stimulation and that the sponge is a metazoan in which there are functional effectors unassociated with receptors or adjusters. Thus the sponge would represent the first stage in the differentiation of a neuromuscular mechanism, i. e., one in which the effector in the form of a primitive muscle-cell is the only element present. In my opinion it is around these contractile cells that the nervous organs of the higher metazoans have developed and I therefore believe that these effector elements are the most primitive members in the typical neuromuscular mechanism.
That there is absolutely no trace of nervous activity in sponges is probably not true, but their extreme inertness shows that this function is certainly in a most primitive state and corresponds at best probably only to that sluggish form of reception and transmission that Kraft (1890) demonstrated for ciliated epithelium and that is probably characteristic of other epithelia. Taking all in all, the only element of the neuromuscular mechanism that is really present in sponges is the effector as represented by the sphincters of the pores and oscula.
If independent effectors occur in sponges, it is not unlikely that they may be present in the higher animals, and as possible examples of these the sphincter pupillæ of the eye in vertebrates and the heart-muscle may be considered. The sphincter pupillæ is a ring of muscle imbedded in the iris and surrounding the pupil in the eyes of most vertebrates. Its contraction would naturally reduce the size of the pupil and thereby diminish the amount of light that enters the eye. In the higher vertebrates it is well known that this reaction has the character of a simple reflex in which the retina is the receptor, with the optic nerve as its transmitting organ, and the stem of the brain is the adjustor from which the oculomotor nerve transmits peripherally to the effector, the sphincter pupillæ. In the lower vertebrates, particularly in the fishes and amphibians, it has long been known that the sphincter pupillæ will react in a characteristic way even in extirpated eyes. This fact has been explained by those who cling to the idea of a reflex as due to intraocular nervous connections between the retina and the sphincter. But Steinach (1892) demonstrated the contraction of the pupil in the extirpated eyes of lower Vertebrates from which the retina had been removed and moreover he showed that when a minute beam of light was thrown on a part of the sphincter, that part contracted first and was followed later by the rest of the muscle, an observation recently confirmed by Hertel (1907) in the eyes of higher vertebrates, including man. It therefore seems quite certain that the sphincter pupillæ of the vertebrate eye, though usually controlled by nerves, is a muscle that can be directly stimulated and in this respect is an independent effector like the sphincters of the pores in sponges.
A second case of independent muscle action in the higher metazoans is the heart-muscle. This muscle for a long time past has been the occasion of much discussion. In the vertebrates it is still an open question whether the beat of the heart is primarily nervous or muscular in its origin and the neurogenic and the myogenic theories of heart action have had a lengthy history (Engelmann, 1904; Howell, 1906). To Harvey we owe not only the discovery of the circulation of the blood, but the first true ideas of the action of the heart, for he showed that the active phase of the heart-beat was during contraction, not during expansion, as had been generally supposed, and that the heart was in reality a muscular force pump. Harvey seems likewise to have had the idea, though perhaps not very clearly expressed, that the heart-beat was dependent upon the heart-muscle and not upon some extra-cardiac mechanism. In this sense he may be regarded as the founder of the myogenic theory. Later Willis pointed out that the stomach, intestine, and heart received nerves from the brain and he believed that the movements of these parts were controlled by such nerves; he therefore may be looked upon as the originator of the neurogenic theory. To account for the fact that the heart would continue to beat for some time after its removal from the body, it was assumed by the neurogenists that the branches of the nerves left in the substance of the heart when this organ was cut from the body were sufficient to maintain the heartbeat for some time, but Haller opposed this view and declared that the heart-muscle itself was directly stimulated by the blood that coursed through it. The older form of the neurogenic theory, however, was entirely swept away by the discovery of the brothers Weber that the vagus nerve when stimulated, instead of increasing the heart-beat brought this organ to a standstill. At about this time Remak described nerve ganglia within the substance of the heart and these have been accepted by the modern neurogenists as the nervous mechanism for the heart-beat. The fact that it is practically impossible to get adult, vertebrate heart-muscle free from nerve-cells has left the problem of the heart-beat in these animals in a situation difficult for experimental approach. That the heart-muscle in vertebrates is always a continuous one, the auricles and ventricles being connected by at least a slender bridge of muscle, favors the myogenic theory, as does also the fact that the beat can be reversed in that the ventricle can be made to contract first and the auricle afterwards. In fact the general proposition, clearly expounded by Gaskell (1900), that the vertebrate heart is a muscular tube over which a myogenic wave of contraction proceeds from the posterior to the anterior end, has much in its favor and yet there are facts enough to show that the neurogenic interpretation of the action of the adult vertebrate heart is not an impossibility. The unfavorable conditions that surround the study of the vertebrate heart have forced investigators to seek evidence concerning the nature of the heart-beat in other animals and as a result two remarkably clear sets of cases have been obtained. The first of these is the heart of the king-crab, Limulus. The heart of this animal, as Carlson (1904) has pointed out, possesses the unique feature of a complete anatomical separation of nervous and muscular parts. The heart itself is a long, segmented, muscular tube situated near the dorsal line of the animal. On the dorsal face of the heart is a median nerve-cord containing ganglion-cells and connected with two parallel lateral nerve-strands that lie near the sides of the heart. This whole nervous mechanism may be dissected off from the heart, leaving this organ in other respects intact. Fig. 3. Dorsal View of the Heart of Limulus (after Carlson). The anterior end is uppermost; In, lateral nerve-strand; mn, median nerve-cord.
If a vigorous Limulus is opened from the dorsal side and the heart exposed, it will be seen to contract at the rate of about twenty beats per minute, and this is likely to continue under the conditions of simple exposure for some twelve to fifteen hours. If now the median nerve-cord and the lateral strands are dissected away, the heart comes to a standstill and never again shows a natural beat, though a stimulus applied directly to its substance will cause it to contract. If instead of removing the nerves, the median and lateral strands are cut through at any plane, care being taken not to injure the underlying heart-muscle, the two regions of the heart thus established beat independently and coordination of the heart as a whole is lost. If the nervous connections are left intact but the muscular heart is completely cut across in several places, the whole organ continues to beat in complete coordination. It is quite clear from these observations that the heart-beat of Limulus is absolutely dependent upon an extra-cardiac nervous mechanism and that this beat is carried out in exact accordance with the neurogenic theory. Since the artificial stimulation of a cardiac nerve in Limulus is followed by tetanus in the region of the heart under the control of this nerve, the conclusion is justified that the heart-muscle of Limulus is comparable rather with the skeletal muscles of this animal than with the so-called organic muscles, for skeletal muscles show tetanus when thus stimulated.
As Carlson himself remarks, however, the fact that the heart-beat of Limulus is neurogenic does not prove that the heart in other animals necessarily functions in a like way. In fact it is comparatively easy to point to another example in which the evidence for the myogenic beat is just as strong as that already presented for the neurogenic beat. This example is the tunicate heart. The tunicate heart, as for instance that of Salpa, is a muscular tube over which peristaltic waves run from end to end. As is well known, the direction of these waves reverses from moment to moment, running for a short interval toward the visceral end of the heart, advisceral waves, and then forward the respiratory end, abvisceral waves. In Salpa africana-maxima, to take a single instance, according to Schultze (1901), after 16 abvisceral waves had passed over the heart in some 20 seconds, a resting period of
2 seconds ensued, whereupon 18 advisceral waves occupying 25 seconds preceded another resting period, etc. When the heart is removed from the body of a Salpa, it continues to beat with characteristic reversal. Stimulation of the central nervous ganglion of a normal Salpa has no effect upon the heart-beat, and though a removal of this organ is followed by a reduction in the rate, the same reduction is to be observed when other parts of the body than the central nervous organ are cut
out. Small fragments of the heart of Salpa also beat rhythmically when entirely isolated, a fact recently confirmed by Hunter (1903) on Molgula, and a most careful search of these fragments has failed to reveal nerve-cells or nerve-fibers. It seems therefore clear that the rhythmic heart-beat of the tunicates is myogenic in origin. This seems also to be true of the embryonic, vertebrate heart, for His (1891) has shown that this organ beats at a time when no trace of nervous tissue can be discovered in it.
From this general discussion it is quite evident that the cardiac muscles of different animals act in very different ways and that while some, like the heart of Limulus, have a neurogenic beat, others like that of the tunicates have a myogenic beat.
From this rather lengthy digression we may return to the question raised in the earlier part of this lecture, namely, the possibility of the existence of physiologically independent muscles. This I believe to have been demonstrated in part at least in the sphincter pupillæ of the lower and perhaps all vertebrates, and wholly so in the tunicate heart and the embryonic vertebrate heart. The complete freedom of such muscles from nervous control and their dependence on direct stimulation for normal action is a repetition of a process that, in my opinion, characterized all primitive muscles such as we now meet with in the sphincters of sponges. Such muscles as these sphincters I believe to represent the original and primitive elements around which the other members of the neuromuscular mechanism, the sense organs and the central nervous organs, subsequently developed. In my opinion then, effectors in the form of muscles preceded in an evolutionary sense the receptors and adjustors, and formed the centers around which these organs developed later.
|Carlson, A. J. The Nervous Origin of the Heart-beat in Limulus, and the Nervous Nature of Coordination or Conduction in the Heart. Amer. Journ. Physiol, Vol. 12, pp. 67-74. 1904.|
|Engelmann, T. W. Das Herz. Leipzig, 8vo, 44 pp. 1904.|
|Gaskell, W. H. The Contraction of Cardiac Muscle. In E. A. Schäfer, Textbook of Physiology, Vol. 2, pp. 169-227. 1900.|
|Hertel, E. Experimenteller Beitrag zur Kenntnis der Pupillenverengerung auf Lichtreize. Graefe's Arch. Ophthalmol, Bd. 65, pp. 107-134. 1907.|
|His, W., Jr. Die Entwickelung des Herznervensystems bei Wirbelthieren. Abhandl. Kgl Sächs. Ges. Wiss., mathem.-phys. CL, Bd. 18, pp. 1-64, Taf. 1-4. 1891.|
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|Hunter, G. W. Notes on the Heart Action of Molgula manhattensis (Verrill). Amer. Journ. Physiol, Vol. 10, pp. 1-27. 1903.|
|Kraft, H. Zur Physiologie des Flimmerepithels bei Wirbelthieren. Arch, ges, Physiol, Bd. 47, pp. 196-235. 1890.|
|Merejkowsky, C. Etudes sur les éponges de la Mer Blanche. Mém. Acad. Imp. Sc., St. Pétersbourg, Sér. 7, Tome 26, No. 7, 51 pp., 3 pis. 1878.|
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- The four articles in this series represent four lectures given at the University of Illinois between March 30 and April 3, 1909.