Afferent and Efferent Impulses
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In acute experiments on anesthetized and on spinal cats, three major mechanisms of divergent-convergent interrelationships in the spinal cord responsible for the formation of electrical responses in the efferent and afferent links of the spinal reflex apparatus, were electrophysiologically established: 1) afferent-efferent, 2) afferent-interneuron-efferent, and 3) afferent--interneuron-afferent mechanisms each having special features of its structural and functional organization. The first two mechanisms are actualized through mono- and poly-synaptic discharges in the ventral roots, resp., and the third one--through electrotonic potentials and reflexes of the dorsal roots. Intracellular recording of potentials revealed that single interneurons participated in the actualization of the second and third mechanisms of the divergent-convergent interrelationships.
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It has been hypothesized that normal pruning of exuberant branching of afferent neurons in the developing cochlea is caused by the arrival of the olivocochlear efferent neurons and the resulting competition for synaptic sites on hair cells. This hypothesis was supported by a report that afferent innervation density on mature outer hair cells (OHCs) is elevated in animals deefferented at birth, before the olivocochlear system reaches the outer hair cell area (Pujol and Carlier [1982] Dev. Brain Res. 3:151-154). In the current study, this claim was evaluated quantitatively at the electron microscopic level in four cats that were de-efferented at birth and allowed to survive for 6-11 months. A semiserial section analysis of 156 OHCs from de-efferented and normal ears showed that, although de-efferentation essentially was complete in all four cases, the number and distribution of afferent terminals on OHCs was indistinguishable from normal, and the morphology of afferent synapses was normal in both the inner hair cell area and the OHC area. Thus, the postnatal presence of an efferent system is not required for the normal development of cochlear afferent innervation, and the synaptic competition hypothesis is not supported.
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It has been hypothesized that normal pruning of exuberant branching of afferent neurons in the developing cochlea is caused by the arrival of the olivocochlear efferent neurons and the resulting competition for synaptic sites on hair cells. This hypothesis was supported by a report that afferent innervation density on mature outer hair cells (OHCs) is elevated in animals deefferented at birth, before the olivocochlear system reaches the outer hair cell area (Pujol and Carlier [1982] Dev. Brain Res. 3:151–154). In the current study, this claim was evaluated quantitatively at the electron microscopic level in four cats that were de-efferented at birth and allowed to survive for 6–11 months. A semiserial section analysis of 156 OHCs from de-efferented and normal ears showed that, although de-efferentation essentially was complete in all four cases, the number and distribution of afferent terminals on OHCs was indistinguishable from normal, and the morphology of afferent synapses was normal in both the inner hair cell area and the OHC area. Thus, the postnatal presence of an efferent system is not required for the normal development of cochlear afferent innervation, and the synaptic competition hypothesis is not supported. J. Comp. Neurol. 423:132–139, 2000. © 2000 Wiley-Liss, Inc.
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