Physiology and biochemistry in modern medicine . eling to a sensory surface, the other to some effector organ, asoccurs in the hypogastric nerves to the bladder (see page 883). THE SYNAPSIS At the point of contact between a branch of one neuron and a nervecell of the next, we have seen that there exists a structure known asthe synapsis. Although this is described by histologists as a tuft-like 798 THE CENTRAL NERVOUS SYSTEM branching of the end of the axon (Fig. 207), it may really consist of asort of membrane—the synaptic membrane. It permits the nerve im-pulse to pass in one direction only,
Physiology and biochemistry in modern medicine . eling to a sensory surface, the other to some effector organ, asoccurs in the hypogastric nerves to the bladder (see page 883). THE SYNAPSIS At the point of contact between a branch of one neuron and a nervecell of the next, we have seen that there exists a structure known asthe synapsis. Although this is described by histologists as a tuft-like 798 THE CENTRAL NERVOUS SYSTEM branching of the end of the axon (Fig. 207), it may really consist of asort of membrane—the synaptic membrane. It permits the nerve im-pulse to pass in one direction only, from synapsis to cell. Of what thismembrane may be composed, we do not know, but there can be nodoubt as to its great functional importance in connection with the in-tegration of the central nervous system; for example, failure of an im-pulse to pass between two neurons may be due to retraction of thesynaptic membrane from the cell, or to alteration in its permeability to-wards the nerve impulse, perhaps as a consequence of changes in surface. Fig. 207.—Arborization of collaterals from the posterior root fibers around the cells of theposterior horn. A, ascending fiber in posterior columns; B, collaterals; C, cells of posterior horn;E, synapsis. (From Ramon y Cajal.) tension. Similar changes may also be brought about by the action ofelectrolytes or by chloroform, strychnine, and other drugs. As we shallsee when Ave come to study the reflexes of the higher animals, there canbe little doubt that it is in the synaptic membrane that many of thepeculiarities reside which characterize conduction in a reflex arc ascompared with that in a nerve trunk. The phenomena of summation,of reciprocal inhibition, of facilitation, etc., are undoubtedly depend-ent upon such alterations. The synapsis is also almost certainly theseat of fatigue in the central nervous system, and it is possibly thestructure whose physiologic activity becomes upset in surgical shock. THE PROPERTIES OF EACH PART OF
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