Ascending tract neurones processing information from group II muscle afferents in sacral segments of the feline spinal cord.

1. Ascending tract neurones located in the dorsal horn of sacral segments of the spinal cord have been investigated by extracellular and intracellular recording in the anaesthetized cat. The aim was to determine whether information from group II afferents that terminate within the sacral segments is conveyed to supraspinal structures and which types of neurones are involved. 2. A considerable proportion of ascending tract neurones found in the dorsal horn in the same segments as the pudendal (Onuf's) motor nucleus were excited by group II muscle afferents. The great majority (93%) of these neurones had axons ascending in ipsilateral funiculi. Spinocervical tract neurones constituted the largest proportion (82%) of such neurones, while very few spinocerebellar tract and propriospinal neurones and no postsynaptic dorsal column neurones were found among them. 3. In addition to activation by group II muscle afferents all of the neurones were strongly excited by cutaneous afferents. The most potent excitation was evoked by afferents of the posterior biceps-semitendinosus and gastrocnemius muscle nerves and by afferents of the cutaneous femoris, sural and pudendal nerves. The latencies of intracellularly recorded excitatory potentials were indicative of a high incidence of monosynaptic coupling between the afferents and ascending tract neurones. 4. The highly effective monosynaptic excitation of spinocervical tract neurones in the sacral segments by group II afferents is in contrast to the weak disynaptically mediated actions of group II afferents on such neurones in the L6-L7 segments but comparable to the actions of group II afferents on ascending tract neurones in the midlumbar segments. 5. Both the patterns of peripheral input and the latencies of synaptic actions in ascending tract neurones were similar to those in interneurones at the same locations (accompanying report). Similar information is therefore likely to be processed by both categories of neurones. 6. The role of sacral spinocervical tract neurones as a system for transmitting information from group II muscle afferents to supraspinal centres and the potential contribution of this system to the perception of limb position are discussed.