Neural mechanisms underlying the segmental and generalized cord shortening reflexes in the leech
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The neural mechanisms responsible for contraction of the nerve cord sheath muscles were investigated in the leech,Hirudo medicinalis.
Two kinds of cord contractions could be distinguished: one, of small amplitude and slow time course, is not preceded by discharge of the fast conducting system (FCS); the second has higher amplitude and faster time course and is always preceded by the discharge of the FCS (Fig. 2).
Electric stimulation of the nerve cord before and after selective lesions of Faivre's nerve or of the lateral connectives showed that “generalized” shortening of the nerve cord involving all the connectives requires firing of an intact FCS. Only “local” shortening, limited to the two connectives adjacent to the stimulating electrode, can be elicited after lesion of the FCS (Figs. 3, 4, 5).
Impulse discharge of the primary mechanoceptive neurons T, P, and N causes local shortening without FCS firing. Firing of the T cell at a rate sufficient to activate the FCS transsynaptically, is always followed by generalized cord contraction (Fig. 6).
Increases or decreases in the firing rate of the L motoneurone, either spontaneously occurring or experimentally imposed by altering its membrane potential, are consistently followed by contractions and relaxations of the nerve cord, respectively (Fig. 8).
Single FCS action potentials are followed by all-or-none depolarizing PSPs in the L motoneurone, at a latency of less than 1 ms, and are able to fire the cell. After a single potential is fired by the FCS, the PSPs appearing in the L motoneurons located in different ganglia are separated by a latency corresponding to the conduction time of the FCS (Figs. 9, 10, 11). The PSPs are Mg++ resistant and are increased by hyperpolarization and decreased by depolarization of the post-synaptic membrane (Figs. 12, 13).
It is concluded that the L motoneurone, which subserves body shortening, innervates also the muscle cells of the cord sheath and that the rôle of the FCS is to bring about the generalization of both body and cord shortening reflexes.
KeywordsMembrane Potential Electric Stimulation Firing Rate High Amplitude Neural Mechanism
fast conducting system
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