Electrophysiological and sensory properties of the thalamic reticular neurones related to somatic sensation in rats.

1. In urethane-anaesthetized rats, studies were made on unit discharges of neurones of the thalamic reticular nucleus (tr) receiving the somatosensory input. Such units were found distributed in a part of the tr (s-tr) surrounding the anterior and anterolateral borders of the ventrobasal nuclear complex (vb). 2. A series of five to ten bursts of grouped discharges were elicited in the s-tr neurones as orthodromic responses to single shock stimulation of the medial lemniscus (ml) or the somatosensory cortex (smc). In contrast to this, the vb relay cells were fired by ml impulses only once at short latencies, though later there occurred a series of two to three bursts of grouped discharges. The same response pattern was seen by stimulating the smc, except that the initial response to smc shocks was of antidromic origin. 3. The mean response latency of the s-tr neurones to ml shocks was about 0.6 msec longer than that of the vb relay cells. Ml fibres connected to the s-tr neurones had the same distribution of conduction velocity as those connected to the vb relay cells. It is suggested that most, if not all, of the s-tr neurones are innervated by axon collaterals of the vb relay cells. 4. About half the s-tr units were activated from vibrissae. For most of the remaining half either movements of hairs or touch to the skin were effective. In the vb, the units discharged by movements of hairs were most numerous and those activated from vibrissae were less frequent. While responses of the s-tr neurones to adequate stimuli were almost exclusively of phasic type, 30% of the responses of the vb relay cells were tonic and the remainder were phasic. 5. By exploring the tr extensively, the s-tr was found to be contiguous caudally to the perigeniculate reticular nucleus which is immediately adjacent to the dorsal lateral geniculate nucleus and contains reticular neurones activated by visual stimuli. Though few in number, units were found in the tr which received a convergence of somatosensory and visual inputs or of somatosensory and auditory inputs.