Popular Science Monthly/Volume 65/October 1904/Correlation of Reflexes and the Principle of the Common Path
|CORRELATION OF REFLEXES AND THE PRINCIPLE OF THE COMMON PATH.|
PRESIDENT OF THE PHYSIOLOGICAL SECTION OF THE BRITISH ASSOCIATION.
PHYSIOLOGY studies the nervous system from three main points of view. One of these regards its processes of nutrition. Nerve cells, as all cells, lead individual lives, breathe, dispense their own stores of energy, repair their own substantial waste, are, in short, living units, each with a nutrition more or less centered in itself. The problems of nutrition of the nerve-cell and of the nervous system, though partly special to this specially differentiated form of cell life, are, on the whole, accessible to the same methods as is nutrition in other cells and in the body as a whole.
But besides the essential functions common to all living cells, the cells of the nervous system present certain which are specialized. Among properties of living matter, one by its high development in the nerve-cell may be said to characterize it. I mean the cell's transmission of excitement spatially along itself and thence to other cells. This 'conductivity' is the specific physiological property of nerve-cells wherever they exist. Its intimate nature is, therefore, a problem coextensive with the existence of nerve-cells, and enters as a factor into every question concerning the specific reactions of the nervous system.
Thirdly, physiology seeks in the nervous system how by its 'conductivity' the separate units of an animal body are welded into a single whole, and from a mere collection of organs there is constructed an individual animal.
This third line of inquiry, though greatly needing more data from the second and the first, must in the meantime go forward of itself. It is at present busied with many questions that seem special—hence its work is generally catalogued as special physiology. But it includes general problems. In the time before us I would venture to put before you one of these.
When we regard the nervous system as to this, which I would term its integrative function, we can distinguish two main types of system according to the mode of union of the conductors—(i.) the nerve-net system, such as met in medusa and in the walls of viscera, and (ii.) the synaptic system, such as the cerebro-spinal system of arthropods and vertebrates. In the integrative function of the nervous system the unit mechanism is the reflex. The chain of conduction in the reflex is a nervous arc, running from a receptor organ to an effector organ, e. g., from a sense-organ to a limb-muscle. We may still, I think, conveniently accept the morphological units termed neurones as units of construction of the reflex arc. It may be that these neurones are in some cases not unicellular, but pericellular. That question need not detain us now. Accepting the neurone as the unit of structure of the reflex chain, the characteristic of the synaptic system is that the chain consists of neurones jointed together in such a way that conduction along the chain seems possible in one direction only. These junctions of the neurones are conveniently termed synapses. The irreversible direction of the conductivity along the neurone chain is probably referable to its synapses. This irreciprocity of conduction especially distinguishes the synaptic nervous system from the nerve-net system. That neurone forms the sole avenue which impulses generated at its receptive point can use whithersoever may be their distant destination. That neurone is therefore a path exclusive to the impulses generated at its own receptive points, and other receptive points than its own can not employ it.
But at the termination of every reflex arc we find a final neurone, the ultimate conductive link to an effector organ, gland or muscle. This last link in the chain, e. g., the motor neurone, differs obviously in one important respect from the first link of the chain. It does not subserve exclusively impulses generated at one single receptive source alone, but receives impulses from many receptive sources situate in many and various regions of the body. It is the sole path which all impulses, no matter whence they come, must travel if they would reach the muscle fibers which it joins. Therefore, while the receptive neurone forms a private path exclusive for impulses of one source only, the final or efferent neurone is, so to say, a public path, common to impulses arising at any of many sources in a variety of receptive regions of the body. The same effector organ stands in reflex connection not only with many individual receptive points, but even with many various receptive fields. Reflex arcs arising in manifold sense organs can pour their influence into one and the same muscle. A limb muscle is the terminus ad quem of nervous arcs arising not only in the right eye, but in the left; not only in the eyes, but in the organs of smell and hearing; not only in these, but in the geotropic labyrinth, in the skin and in the muscles and joints of the limb itself and of the other limbs as well. Its motor nerve is a path common to all these.
Reflex arcs show, therefore, the general feature that the initial neurone is a private path exclusive for a single receptive point; and that finally the arcs embouch into a path leading to an effector organ, and that this final path is common to all receptive points wheresoever they may lie in the body, so long as they have any connection at all with the effector organ in question. Before finally converging upon the motor neurone arcs usually converge to some degree by their private paths embouching upon internuncial paths common in various degree to groups of private paths. The terminal path may, to distinguish it from internuncial common paths, be called the final common path. The motor nerve to a muscle is a collection of such final common paths.
Certain results flow from this arrangement. One seems the preclusion of qualitative differences between nerve-impulses arising in different afferent nerves. If two conductors have a tract in common, there can hardly be qualitative difference between their modes of conduction.
A second result is that each receptor being independent for communication with its effector organ upon a path not exclusively its own, but common to it with certain other receptors, that nexus necessitates successive and not simultaneous use of the common path by various receptors using it to different effect.
The first link of each reflex chain is a neurone which starts in a receptor organ, e. g., a sense-organ. A receptive field, e. g., an area of skin, is always analyzable into receptive points, and the initial nerve-path in every reflex arc starts from a receptive point or points. A single receptive point may play reflexly upon quite a number of different effector organs. It may be connected through its reflex path with many muscles and glands in various parts. Yet all its reflex arcs spring from the one single shank, so to say; that is, from the one afferent neurone that conducts from the receptive point at the periphery into the central nervous organ. This neurone dips at its deep end into the great central nervous organ, the cord or brain. There it enters a vast network of conductive paths. In this network it forms manifold connections. So numerous are its potential connections there, that, as shown by the general convulsions induced under strychnia-poisoning, its impulses can discharge practically every muscle and effector organ in the body. Yet under normal circumstances the impulses conducted by it to this central network do not irradiate there in all directions. Though their spread over the conducting network does, as judged by the effects, increase with increase of stimulation of the entrant path, the irradiation remains limited to certain lines. Under weak stimulation of the entrant path these lines are sparse. The conductive network affords, therefore, to any given path entering it some communications that are easier than others. This canalization of the network in certain directions from each entrant point is sometimes expressed, borrowing electrical terminology, by saying that the conductive network from any given point offers less resistance along certain circuits than along others. This recognizes the fact that the conducting paths in the great central organ are arranged in a particular pattern. The pattern of arrangement of the conductive network of the central organ reveals somewhat of the integrative function of the nervous system. It tells us what organs work together in time. The impulses are led to this and that effector organ, gland or muscle, in accordance with the pattern. The success achieved in the unraveling of the conductive patterns of the brain and cord is shown by the diagrams furnished by the works of such investigators as Edinger, Exner, Flechsig, van Gehuchten, v. Lenhossek, v. Monakow, Ramon and Schäfer. Knowledge of this kind stands high among the neurological advances of our time.
But we must not be blind to its limitations. The achievement may, though more difficult, be likened to tracing the distribution of blood vessels after Harvey's discovery gave them meaning, but before the vasomotor mechanism was discovered. The blood vessels of an organ may be turgid at one time, constricted almost to obliteration at another. With the conductive network of the nervous system the temporal changes are even greater, for they extend to absolute withdrawal of nervous influence. Our schemata of the pattern of the great central organ take no account of temporal data. But the pattern of the web of conductors is not really immutable. Functionally its details change from moment to moment. In any active part it is a web that shifts from one pattern to another, from a first to a second, from a second to a third, then back perhaps to the first and then to a fourth and so on backwards and forwards. As a tap to a kaleidoscope, so a new stimulus that strikes the central organ causes it to assume a partially new pattern. The pattern in general remains, but locally the patterns are in constant flux of back and forward change. These time-changes offer, I venture to think, a study important for understanding the integrative function of the nervous system.
If we regard the nervous system of any higher organism from the broad point of view, a salient feature in its architecture is the following: At the commencement of every reflex arc is a receptive neurone, extending from the receptive surface to the central nervous organ.