Muscle stretch and the presynaptic inhibition of the group Ia pathway to motoneurones.

It has been shown that the monosynaptic EPSP which was evoked in a motoneurone by a volley in its muscle nerve could be depressed by conditioning volleys in other muscle nerves without any evidence of postsynaptic inhibitory changes in the motoneurone (Frank & Fuortes, 1957; Eccles, Eccles & Magni, 1960, 1961). Eccles et al. (1961) described the time course of this EPSP depression and related the depression to volleys in nerves from muscles with different functions. In general, activity in nerves from flexors evoked greater depression of the monosynaptic EPSP than volleys in nerves from extensors. It seemed that a correlation existed between the EPSP depression and the dorsal root reflex (DRR) and that both these phenomena are due to the same process, viz. depolarization of the primary afferent fibres within the spinal cord. Eccles, Magni & Willis (1962) provided conclusive evidence of the depolarization of central terminals of group I primary afferents from muscle. This depolarization followed the same time course as the EPSP depression and is evoked under the same experimental conditions. This phenomenon was designated 'presynaptic inhibition'. Devanandan, Eccles & Yokota (1965) observed that depolarization of the central terminals of primary afferent fibres can be produced by muscle stretch. It will be shown in this paper that muscle stretch may effectively depress the monosynaptic EPSP and the monosynaptic reflex. This depression of the monosynaptic reflex was strychnine resistant, thereby establishing that it was not due to post-synaptic inhibition (Bradley, Easton & Eccles, 1953).