Mechanosensory afferent input and neuronal firing properties in rodent spinal laminae III-V: re-examination of relationships with analysis of responses to static and time-varying stimuli.

Relationships between neuronal firing pattern and mechanosensory input in the deep dorsal horn were investigated using whole-cell recordings from isolated hamster spinal cord with innervation from an attached skin patch. Neurons that fired repetitively to depolarizing current (tonic cells) responded to both moving and static stimulation of their cutaneous receptive fields, and discharged continuously for the duration of stimulus application. Neurons responding to depolarizing current with transient, rapidly adapting firing (phasic cells) were significantly more responsive to stimulus movement than to static skin contact. Phasic cells typically issued a brief discharge at the onset or termination of a stimulus; their responses during static skin contact were weaker than tonic cells. Tonic cells were activated during both ramp and steady-state skin indentations, whereas phasic cells responded with their strongest excitation to displacement velocities exceeding 8 microm/ms. Mechanosensory input to phasic cells originated primarily from low threshold receptors, whereas tonic cells demonstrated a mixture of inputs from both low and high threshold sources. A third class of neurons responded to depolarizing current with a pronounced firing delay and displayed a sensitivity to cutaneous stimuli that was similar to tonic cells except they showed a modest decrease in firing as skin indentation velocity increased. The results suggest a correlation between functional properties of mechanoreceptive afferent fibers and intrinsic discharge properties of laminae III-V neurons that may significantly influence integration of cutaneous mechanosensory information at the first spinal relay.