Receptive field organization and response properties of spinal neurones with axons ascending the dorsal columns in the cat.

Micro-electrode recordings were made from single post-synaptic axons in the dorsal columns of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The recordings were made from the L5 segment and the axons were shown to project to the upper cervical level. Forty-eight units were recorded and the axons had conduction velocities of 22-61 ms-1, averaging 38.3 ms-1. Excitatory receptive fields were complex in many units, being made up of clearly defined, separate, low and high threshold areas. The receptive fields were often discontinuous. Only a few units behaved as if they received excitatory input from a single class of mechanoreceptors. A minority (13%) of units had labile, excitatory receptive fields that expanded in size during the recording period. About 40% of the units had inhibitory receptive fields. These were of two main types: either small and within or adjacent to the excitatory field, or large and separated from or adjacent to the excitatory field. The great majority of units had resting discharges upon isolation and these consisted of single impulses or bursts of impulses at short intervals separated by longer, irregular periods. The time course of inhibition produced by electrical stimulation of cutaneous nerves suggested presynaptic inhibitory components to the inhibition. Some inhibitory curves were very prolonged with maxima at about 100 ms and total durations of up to 400 ms. The complexity of the receptive field organization in these dorsal horn neurones is discussed, as is their possible significance as input neurones to the dorsal column nuclei.