The Role of Primary afferent Depolarisation in Presynaptic Inhibition of Group I Fibres
In a previous study, declining inhibitory potentials, ascribed to the action of Ib afferents, were recorded in homonymous and synergic motoneurones during sustained subtotal isometric contractions of gastrocnemius medialis (GM) muscle (Zytnicki, Lafleur, Horcholle-Bossavit, Lamy & Jami, 1990). In a subsequent study, contraction-induced primary afferent depolarisations (PADs) were recorded intra-axonally from the intraspinal portion of homonymous Ib fibres during similar GM contractions (Lafleur, Zytnicki, Horcholle-Bossavit & Jami, 1992). Since PAD is known to be the electrophysiological correlate of presynaptic inhibition (Eccles, Schmidt & Willis, 1962), this observation supported the assumption that presynaptic inhibition of Ib fibres accounted for the decline of contraction-induced inhibition in motoneurones (see Zytnicki & L’Hôte, 1993). The aim of the present computer study was to evaluate the efficacy of the mechanism of presynaptic inhibition received by myelinated afferent fibres. Activation of an axo-axonic synapse may partly shunt the action potentials travelling along the fibre (Segev, 1990), but as recently suggested, the PAD might also contribute by itself to presynaptic inhibition by reducing the spike height (Graham & Redman, 1994). In the present paper, potentiation of the effects of an axo-axonic synapse by the PAD is demonstrated and explained on a simple model of the myelinated axon in terms of the dynamics of ionic currents.
KeywordsGastrocnemius Medialis Presynaptic Inhibition Action Potential Amplitude Peak Conductance Afferent Action Potential
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