An in vivo electrophysiological investigation of group Ia afferent fibres and ventral horn terminations in the cat spinal cord.

An extracellular microstimulation technique has been used to investigate and compare the properties of group I primary afferent myelinated fibres in the dorsal column and group Ia unmyelinated terminations in the lumbar spinal cord of cats anaesthetised with pentobarbitone sodium. Fibres were distinguished from terminations on the basis of location, anodic blocking factor and sensitivity to GABAA mimetics. The recovery curves of threshold following an orthodromic impulse provided an estimate of both action potential duration and rate of repolarization. The action potentials of group Ia terminations were of briefer duration (by a factor of approximately 2) with more rapid rates of repolarization (factor of approximately 3) than those of the myelinated fibres. The prolongation of termination but not fibre action potentials by microelectrophoretic tetraethylammonium and 4-aminopyridine indicated the presence of voltage-activated potassium channels in the termination membrane. Differences in the effects on Ia termination action potentials of depolarizations (reductions in threshold) associated with a preceding action potential, synaptically released GABA, microelectrophoretic piperidine-4-sulphonic acid or DL-homocysteic acid suggest that an increase in termination membrane conductance is the major factor in the reduction of transmitter release during the activation of presynaptic GABAA receptors.