Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive ‘tactile’ neurones

Abstract

Responses were recorded in decereberate, unanethetized cats from individual cuneate neurons in order to determine firstly, the afferent sources of inhibition on cuneate neurons and secondly, the influence of afferent-induced inhibition on those response features of dynamically sensitive tactile neurons which determine their capacity to code information about parameters of tactile stimuli. For all cuneate neurons which displayed afferent-induced inhibition from areas surrounding or within their excitatory receptive field (71% of the sample) it was consistently found that 300 Hz vibration at low amplitudes (< 25-50 .mu.m) which selectively engages Pacinian corpuscles was an effective source of inhibition. Steady indentation which activates slowly adapting tactile afferents was quite ineffective, as was low frequency vibration (30 Hz) at amplitudes of < 50-100 .mu.m. The latter stimulus can be used to engage rapidly adapting receptors either within glabrous skin (presumed to be Meissners corpuscles) or in association with hair follicles. Afferents from Pacinian corpuscles are apparently the dominant or exclusive source of afferent-induced inhibition of cuneate neurons. For dynamically sensitive neurons responsive to low frequency cutaneous vibration (30 Hz) there was a reduction in the slope of stimulus response relations with afferent-induced inhibition but no expansion of the range of stimulus amplitudes over which the neuron responded. The influence of afferent-induced inhibition on the phase-locking of impulse activity to a cutaneous vibratory wave form was examined by constructing post-stimulus time histograms and cycle histograms. Measures of dispersion of impulse activity around the preferred point of firing in the vibratory waveform indicated that the capacity of individual cuneate neurons to code information about the frequency of the cutaneous vibration was not systematically changed in the presence of afferent induced inhibition.