Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V25, Page 673 of the 1911 Encyclopedia Britannica.
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PHYSIOLOGY OF THE SPINAL CORD The name spinal cord, given by early morphologists to the nervous mass lying in the tubular chamber enclosed by . the vertebral column, was doubtless given under the supposition that the organ so named could be treated as an entity. ' Scientifically, however, it cannot be so treated, either as regards its the brain, the rest of it is called in vertebrates the " spinal cord," in vermes and arthropods the " nerve-cord." The central organ not only receives neurones which converge to it from outside, but many of its own neurones thrust out their conductive arms from it as nerve fibres carrying nervous influence outwards to regulate the activity of glands and muscles. In the vertebrata the ingoing neurones for each segment and similarly the out-going neurone fibres are collected into a segmental nerve. To the spinal cord these are each attached by two roots, one dorsal, consisting of the afferent fibres, the other ventral, consisting of the efferent fibres. The Reflex.—Analysis of function of this nervous system leads to what is termed " the reflex as the unit of its action. The simplest complete reaction of the system is a reflex. There are many reflexes which are extremely complex, being built up of a number of simpler reflexes combined together. A reflex is a reaction started by the environment acting as a stimulus upon some nerve which communicates the excitement thus started in itself to other nerves by means of its corinexions with these in the central nervous organ. The excitement so generated and transmitted finally travels outward from the central organ by one or more of the efferent nerves and through these reaches muscles or glands producing in them its final effect. The muscles and glands are from this point of view termed effector organs. The reaction is therefore " reflected " horn the central organ. The nerve structures along which it runs in its trajectory are spoken of as a nervous arc. The whole purpose of the central nervous organ is therefore to bring afferent neurones into touch with efferent neurones. The whole purpose of reflex arcs is to bind one part of the organism to another part in such a way that what the environment is doing to the organism at one place may appropriately call forth or restrain movement or secretion in the muscles or glands possessed by the organism. Receptor Cells.—There is one condition for the due performance of these reactions which is not provided by the nervous system itself. The afferent neurones are not in most cases so constituted as to be excitable themselves directly by the environment—for instance, they cannot be stimulated by light. Their amenability to the environment, their sensitization to environmental agencies, is effected by special cells adjunct to their peripheral ends. These cells from organs are called receptors. They are delicately adapted to be stimulated by this or that particular agent and are classifiable into various species, so that each species is easily excited by a particular agent which is " adequate " for it, and is quite inexcitable or only excitable with difficulty by agencies of other kinds. Thus in the skin some receptors are adapted for mechanical stimuli (touch) and not for thermal stimuli, while others (cold spots, warm spots) are adapted for thermal stimuli and not for mechanical. As far as it is known each afferent neurone is connected with receptors of one species only. The receptors thus confer upon the reflex arcs selective excitability. Each arc is thus tuned to respond to certain stimuli, while other arcs not having that kind of receptor do not respond. The receptors, therefore, while increasing the responsiveness of the organism to the environment, prevent confusion of reactions (inco-ordination) by limiting to particular stimuli a particular reaction. Propr"ioceptors.—The system of neurones is thus made accessible to the play of the external world acting on the body, And in addition to those receptors which are stimulated directly by the external world, are others lying within the mass of the organism itself, which are excitable by actions occurring in the organism itself. These are called proprioceptors. They are distributed preponderantly in the muscles and structures functionally adjunct to muscle, such as joints, ligaments, fasciae, &c. The reactions induced in such motor structures reflexly in response to environmental stimuli tend therefore secondarily to be followed and accompanied by reflex reactions initiated from proprioceptors. Conduction.—The process by which the excitement generated in the afferent neurone travels along the reflex arc is known as conduction. Conduction along afferent and efferent nerves
PIACENZA (Lat. Placentia)

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