The somatosensory thalamus of the raccoon: properties of single neurons responsive to light mechanical stimulation of the forepaw

Abstract

These studies were undertaken to characterize the discharge properties of single neurons of the raccoon thalamic ventrobasal complex (VB) that respond to light mechanical stimulation of the glabrous surfaces of the forepaw. Microelectrodes were used to record the extracellular activity of 146 cells in anesthetized raccoons, and all neurons were histologically verified as falling within or along the boundaries of VB. Sixty-one neurons were tested for activation by electrical stimulaton of primarily somatosensory cortex. Of these, 88% were antidromically activated, 5% were synaptically activated, and the remaining 7% were unresponsive.Out of the total sample of 146 neurons, 136 had peripheral receptive fields (RFs) that were restricted to glabrous skin and revealed properties of modality and place-specificity predictable through knowledge of properties of primary mechanoreceptive afferents. Rapidly adapting (RA) neurons accounted for 77% of this modality-place-specific sample, while 19% were slowly adapting (SA), and 4% revealed properties indicative of input from Pacinian afferents (PC). Absolute displacement thresholds were comparable for RA and SA neurons (range = 6-415 pm). Palmar RF areas (range = 3.3-328 mmZ) were significantly larger than digital RF areas (range = 0.5-98.2 mm*).As defined by exponents ( A small number of neurons (7% of total sample) revealed "complex" properties, not predictable from knowledge of properties of primary afferents. These included five neurons whose RFs encompassed both glabrous and hairy skin, and several linear orientation, or "tactile edge," detectors.The present results, in conjunction with those of earlier stud-

The Human Thalamic Somatic Sensory Nucleus [Ventral Caudal (Vc)] Shows Neuronal Mechanoreceptor-Like Responses to Optimal Stimuli for Peripheral Mechanoreceptors

“…RA-like neurons were defined by their frequency response, which was similar to the response of RA-selective neurons in the marmoset and monkey forebrain (Mountcastle et al 1990;Warren et al 1986). The present von Frey thresholds were Ͼ1.4 g in the case of all RA-like neurons, much greater than human RA mechanoreceptors (Ͻ0.1 g; Table 1) (Gybels and Van Hees 1972) or marmoset RA-specific neurons (Ͻ0.5 g; Table 1).…”

“…In the ventral posterior thalamus of anesthetized marmosets and raccoons, neuronal response patterns have been compared with those of the mechanoreceptor classes (Hirai and Jones 1989;Warren et al 1986;Zhang et al 2001b), and each was found to be similar to that of the peripheral mechanoreceptors (Pubols and Pubols 1973). Studies of neuronal responses to cutaneous stimuli have also been reported in old world monkeys (Mountcastle et al 1969;Sinclair et al 1991).…”

Neuronal responses to tactile stimuli and tactile sensations evoked by microstimulation in the human thalamic principal somatic sensory nucleus (ventral caudal)

“…Anesthesia was carried out with 1) halothane induction and chloralose or barbiturate maintenance (Byl et al 1996;Pons et al 1991;Rasmusson. 1996b), 2) ketamine plus xylazine induction and ketamine maintenance, and 3) induction and maintenance on flurane or pentothal (Warren et al 1986;Zhang et al 2001b). These methods producing small changes in VP neuronal response patterns at low doses of barbiturates but greater changes with other agents (Dougherty et al 1997).…”

“…These methods producing small changes in VP neuronal response patterns at low doses of barbiturates but greater changes with other agents (Dougherty et al 1997). Recordings are carried out with standard techniques including handheld devices for search stimuli followed by precisely controlled mechanical stimuli in a small number of studies as reviewed below (Hirai and Jones 1989;Warren et al 1986;Zhang et al 2001b). All animals are euthanized with recovery of the brain for histological processing, which produces greater accuracy of the recording site location.…”

“…In VP of anesthetized marmosets and raccoons, neuronal response patterns have been compared with those of the mechanoreceptor classes using precisely controlled mechanical cutaneous stimuli (Hirai and Jones 1989;Warren et al 1986;Zhang et al 2001b). Each stimulus produces the optimal response for one class of cutaneous mechanoreceptor, and all were applied during the study of individual thalamic neurons (see Techniques of Thalamic Electrophysiological Mapping in Humans, Monkeys, and Raccoons).…”