The nature of the monosynaptic excitatory and inhibitory processes in the spinal cord

Abstract

Recently it has been possible to record electrically from motoneurones in the spinal cord of the anaesthetized cat by means of an intracellular electrode (Brock, Coombs & Eccles 1951, 1952). As with investigations on isolated nerve and muscle fibres (Ling & Gerard 1949; Nastuk & Hodgkin 1950; Weidmann 1951; Fatt & Katz 1951) the micro-electrode is a fine glass tube filled with 3 m-KCI and with a tip diameter of about 0-5 µ .Necessarily it has to be inserted ‘blindly’ into a motoneurone lying some 2 mm deep in the spinal cord. However, the position of a pool of motoneurones belonging to any one muscle is now fairly well known (Romanes 1951), and the motoneurones are made to signal their position electrically during the process of insertion by firing impulses into them antidromically and also by monosynaptically activating them. The entry of a micro-electrode into a motoneurone is immediately and unambiguously signalled by two events: the recording of the resting membrane potential (about 70 mV); the inversion and large increase in the antidromic spike potential, which gives a reversal of membrane potential of as much as 35 mV. In general, both during rest and in the propagation of impulses, the electrical properties of the motoneurone closely resemble those of the isolated giant axon.