Single-unit analysis of the human ventral thalamic nuclear group: somatosensory responses

Abstract

1. We have studied the functional and somatotopic properties of 531 single mechanoreceptive thalamic neurons in humans undergoing stereotactic surgery for the control of movement disorders and pain. The majority of these somatosensory cells had small receptive fields (RFs) and were activated in a reproducible manner by mechanical stimuli applied to the skin or deep tissues. These neurons, which we termed "lemniscal," could be further classified into those responding to stimulation of cutaneous (76% of lemniscal sensory cells) or deep (24%) structures. 2. The incidence of neurons having cutaneous or mucosal RFs in the perioral region, thumb, and fingers (66%) was much higher than that of neurons having RFs elsewhere on the body. Most of the deep cells were activated by movements of and/or mechanical stimuli delivered to muscles or tendons controlling the elbow, wrist, and fingers. 3. Sequences of cells spanning several millimeters in the parasagittal plane often exhibited overlapping RFs. However, RFs changed markedly for cells separated by the same distances in the mediolateral direction. This suggests that the cutaneous somatotopic representation of each region of the body is organized into relatively thin sheets of cells oriented in the parasagittal plane. 4. By comparing neuronal RFs in different parasagittal planes in thalamus of individual patients we have identified a mediolateral representation of body surface following the sequence from: intraoral structures, face, thumb through fifth finger to palm, with forearm and leg laterally. 5. Along many trajectories in the parasagittal plane the sequence of cells with overlapping RFs was interrupted by another sequence of cells with RFs corresponding to a different body region. The RFs of the intervening sequence characteristically represented body regions known to be located more medially in thalamus (see 3 above). These findings could be explained if the lamellae postulated above were laterally convex. 6. Cells responding to deep stimulation (deep cells) could be further classified into those responding to joint movement (63%), deep pressure (15%), or both (22%). Deep cells were found usually at the anterior-dorsal border and sometimes at the posterior border of the region containing cells responding to cutaneous stimuli. Although there was some overlap in the RFs, deep cells representing wrist were found medial to those representing elbow, and both of these were found medial to cells representing leg.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropathic Pain and Deep Brain Stimulation

“…A guiding principle is the established somatotopic organization of the somaesthetic thalamic and PAG regions. Human microelectrode studies reveal a mediolateral somatotopy in the contralateral ventroposterior thalamus, the head of the homunculus being medial and the feet lateral [83]. Subjective observation of a rostrocaudally inverted sensory homunculus in contralateral PAG has been confirmed objectively by our human macroelectrode recordings of somatosensory evoked potentials [84,85].…”

Dynamics of tremor-related oscillations in the human globus pallidus: A single case study

“…One possibility is that tremor-related activity is generated in a globally synchronous manner by the motor thalamus (8,10) and is transmitted to the globus pallidus. The plausibility of a thalamic origin is supported by the presence of tremorrelated activity in the motor thalamus (4,(12)(13)(14). This activity has been proposed to result from a low-threshold calcium current in thalamocortical neurons that can sustain an oscillation in membrane potential and can be triggered by hyperpolarizing input from the globus pallidus (8,10).…”

“…Several lines of evidence support this hypothesis. First, tremor-related neural activity has been observed in the motor thalamus (4,(12)(13)(14) and the posteroventral globus pallidus (11). Second, tremor is effectively ameliorated by making a lesion in the motor thalamus.…”

“…''Tremor-related activity'' is defined as rhythmic neural activity at a frequency in the range of parkinsonian tremor; it does not imply that the neural activity is phase-locked to the tremor. Although tremor-related activity in the basal ganglia and motor thalamus has been reported by many authors, in only a few cases has the temporal relationship between the neural and muscle activity been rigorously examined (4,5).…”

Neuronal activity and outcomes from thalamic surgery for spinocerebellar ataxia

“…It has been reported that a larger lesion in Vim is required to alleviate tremor that is of a lower frequency and involves proximal body parts 27. One reason for this may be found in the somatotopic organization in Vim, which is similar in humans28 and nonhuman primates;29 the leg is represented in the most rostral and lateral regions, the orofacial structures in the most caudal and medial locations, and the fingers, forearm and proximal arm are located in between from medial to lateral and from caudal to rostral directions. Therefore, amelioration of tremor in SCA, which involves both the distal and proximal arm would need to include a larger area of the Vim as compared with that of tremor in ET, which may involve relatively more distal portions of the arm.…”