AN IN-VIVO ELECTROPHYSIOLOGICAL INVESTIGATION OF GROUP IA AFFERENT-FIBERS AND VENTRAL HORN TERMINATIONS IN THE CAT SPINAL-CORD

An extracellular microstimulation technique has been used to investiga te and compare the properties of group I primary afferent myelinated f ibres in the dorsal column and group Ia unmyelinated terminations in t he 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 GABA(A) mimetics. The reco very 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 durati on (by a factor of approximately 2) with more rapid rates of repolariz ation (factor of approximately 3) than those of the myelinated fibres. The prolongation of termination but not fibre action potentials by mi croelectrophoretic tetraethylammonium and 4-aminopyridine indicated th e presence of voltage-activated potassium channels in the termination membrane. Differences in the effects on Ia termination action potentia ls of depolarizations (reductions in threshold) associated with a prec eding action potential, synaptically released GABA, microelectrophoret ic piperidine-4-sulphonic acid or DL-homocysteic acid suggest that an increase in termination membrane conductance is the major factor in th e reduction of transmitter release during the activation of presynapti c GABA(A) receptors.