DEPOLARIZATION OF PRESYNAPTIC FIBERS IN THE CUNEATE NUCLEUS

Abstract

Two standard experimental procedures have been employed in further investigating this postulated depolarization of the presynaptic terminals of cuneate tract fibers: excitability testing of the presynaptic fibers at their terminals and along their length, and intracellular recording from the fibers. The increments in excitability of the terminals of cutaneous fibers in the cuneate nucleus had time courses corresponding closely to the time courses of the P-waves produced by the various inputs. The increments in excita-bilities were greatest where there are most synaptic terminals of cuneate fibers and there was a rapid decrement in the hyper-excitability more caudally along the lengths of fibers. By intracellular recording from the presynaptic fibers of the cuneate nucleus the depolarization was directly demonstrated and shown to conform with other more indirect observations. There was evidence that with some intracellular recordings the microelectrode was within or close to the large synaptic knobs of the cuneate tract fibers. In some cases at least it was shown that impulses were generated near to the synaptic terminals of the afferent fibers in the cuneate nucleus and not in the spinal cord, hence the designation, dorsal column reflexes.