Neural correlates of isometric force in the ?motor? thalamus

Abstract

The relationship between single cell activity in the "motor" thalamus and the generation of isometric force between the fingers has been investigated in 2 monkeys. Neurons related to the task were found in the thalamic motor regions VLo, VPLo, and VA where microstimulation occasionally elicited motor reactions in hand and fingers. 58% of these 55 neurons, designated "typical", showed modulation of their discharge patterns with force similar to neurons in precentral cortex and could be assigned to one of 5 discharge patterns described for the motor cortex. Only a small percentage of the thalamic neurons were found to have phasic activity. The other "atypical" neurons (42%) had discharge patterns with complex sequences of phasic and tonic activation with respect to force. For 18 typical and atypical neurons with tonic and phasic-tonic modulation of their firing rate with force significant regression coefficients between firing rate and static force were observed. The mean index of force sensitivity (rate-force slope) was 54.5 Hz/N for the neurons increasing their discharge rate with force, i.e. approximately that of precentral cells. Neurons tested for their sensory properties had receptive fields located on hand and/or fingers and were activated mainly by stimulation of muscle and joint receptors. The characteristics of these thalamic neurons are compared to those of precentral cells recorded under identical experimental conditions and are discussed in relation to the known input-output relationships of the motor thalamic nuclei. The data strongly support the hypothesis that parameters of movement, in particular force are represented by the activity of neurons in the "motor" thalamus.

Arborisation and termination of single motor thalamocortical axons in the rat

“…If VL neurones do project to multiple cortical representations of the same muscle, then the cortical extent of influence (in terms of number of joints) of individual cerebellar nuclear neurones would be determined solely by the degree of divergence in the cerebellothalamic pathway. This notion is consistent with previous findings that single-joint movements are typically evoked by microstimulation of the motor thalamus (Asanuma and Hunsperger, 1975;Strick, 1976b;Anner-Baratti et al, 1986;Buford et al, 1996;Vitek et al, 1996), whereas multijoint movements are a feature of cerebellar nuclear microstimulation (Schultz et al, 1976(Schultz et al, , 1979Rispal-Padel et al, 1981;Cicirata et al, 1989;Ekerot et al, 1995). It must be stressed, however, that the cerebellothalamocortical pathway was ruled out as the mediator of these cerebellar stimulation effects in most of these studies (Schultz et al, 1976(Schultz et al, , 1979Cicirata et al, 1989;Ekerot et al, 1995).…”

“…It is equally possible, however, that the multiple and widespread motor thalamocortical projection reflects the multiple and widespread representation of individual muscles in the motor cortex (Kwan et al, 1978;Pappas and Strick, 1981a,b;Tanji and Wise, 1981;Donoghue and Wise, 1982;Drew, 1984, 1985;Huang et al, 1988). Indeed, electrical microstimulation of the motor thalamus has been shown to evoke movements predominantly restricted to a single joint in awake monkeys (Strick, 1976b;Anner-Baratti et al, 1986;Buford et al, 1996;Vitek et al, 1996) and cats (Asanuma and Hunsperger, 1975). The functional extent of influence of single motor thalamocortical neurones will be an important area of study for the future, particularly with respect to current ideas concerning the development of topography in the motor system raised in the introduction.…”

Age-related changes in the neural correlates of motor performance

“…Using single cell recording in macaque monkeys (Ashe 1997), the correlation between firing rates of cortical neurones and force of both isometric and dynamic grip tasks has been demonstrated to be mainly monotonic or linear. Linear relationships have been observed primarily in primary motor cortex (Evarts, 1968; Smith et al , 1975; Hepp-Reymond et al , 1978; Evarts et al , 1983; Wannier et al , 1991; Georgopoulos et al , 1992), and also for neurones in somatosensory cortex (Wannier et al , 1991), premotor cortex (Hepp-Reymond et al , 1994), supplementary motor area (Smith et al , 1975) and thalamus (Anner-Baratti et al , 1986). Functional imaging techniques allow the activity in larger populations of neurones to be studied simultaneously across all brain regions, which allows inference about activity relationships between one population of neurones and another.…”

Neuronal activity in the monkey motor thalamus during bicuculline‐induced dystonia

“…We found that microstimulation induced more frequent motor responses in the rostral than in the caudal motor thalamus. Similar results were previously reported (Strick, 1976; Anner‐Baratti et al ., 1986; Vitek et al ., 1994; Buford et al ., 1996; Miall et al ., 1998) although there is no consensus about the percentage and threshold of response to microstimulation in the motor thalamus. One striking result of the present study is that bicuculline injections induced a dramatic increase in the excitability of thalamic neurons.…”