Spinal input to thalamic VL neurons: evidence for direct spinothalamic effects

1. The postsynaptic actions of afferents ascending in the ventrolateral quadrant and dorsal columns of the spinal cord were studied in neurons in the ventrolateral nucleus of the thalamus (VL) (n = 138) by use of intracellular recording procedures. Neurons were identified by their monosynaptic input from the cerebellum and, when possible, their antidromic activation from the motor cortex. The possible occurrence of monosynaptic transmission along spinothalamic fibers was investigated by estimating the intrathalamic delay time of postsynaptic responses and by examining the occurrence of temporal facilitation to double-shock stimulation. The experiments were performed in cats anesthetized with alpha-chloralose. 2. The majority of neurons (86%) responded with excitatory or inhibitory postsynaptic potentials to stimulation of the ascending paths. The response latencies of excitation on stimulation of the ventral quadrants at C3 ranged from 2.9 to 18 ms. Evidence for monosynaptic excitation after stimulation of (spinothalamic) afferents ascending in the ventrolateral quadrants was obtained for a number of neurons (n = 30). For these neurons, estimated intrathalamic delays were less than 1 ms and/or the neurons did not display temporal facilitation to double shocks. All the shortest latency responses (from C3) showed evidence of monosynaptic transmission. It is estimated that approximately 19-39% of neurons sampled may receive monosynaptic input. The spinal conduction velocities of the direct projections ranged from 10 to 35 m/s (median 20 m/s). 3. Much of the ascending input was mediated polysynaptically. For afferents ascending in the ventrolateral quadrant, estimated intrathalamic delays were greater than 1.5 ms and/or the postsynaptic responses displayed temporal facilitation to double shocks. The shortest latency from C3 of a polysynaptic response was 5 ms. Spatial interactions were observed between polysynaptic inputs from the ventrolateral quadrants and the dorsal columns, indicating that at least some of the pathways to VL are shared. 4. The data show that many neurons in the VL receive input ascending from the spinal cord via direct and indirect routes. Somatosensory information reaching VL could serve to adjust, during the course of movement execution, the cerebellar commands relayed by VL to the motor cortex.