The responses of cat motor cortical units to electrical cutaneous stimulation during locomotion and during lifting, falling and landing

Abstract

Experiments were performed to examine the influence of cutaneous information on motor cortical cells during movement in intact, awake cats. The movements investigated were locomotion and a sequence in which the animal was repeatedly lifted and dropped. Electrical stimuli to distal skin areas were delivered periodically during the movements and responses of motor cortical cells were examined. The animals used in these experiments were chronically implanted with cortical microelectrodes, a pyramidal tract stimulating electrode, cutaneous stimulating electrodes in the forepaw, and a recording cuff electrode around the median nerve. EMG electrodes were implanted in several forelimb muscles and a length gauge was implanted across the elbow joint. Results included in this report were obtained from three cats. The twenty-two cortical units analysed in this study (seven were PT units) were selected from a larger sample by the following criteria: cutaneous receptive fields which included the distal part of the limb, consistent short latency responses to electrical cutaneous stimulation and spontaneous activity modulated in consistent patterns during the movement investigated. Sixteen units were recorded during locomotion, 12 during the lifting and dropping cycle and 6 of these during both conditions. Most of the cells were influenced by the cutaneous input during locomotion. Three units had no response to peripheral stimulation during locomotion though they were responsive to this stimulus when the animal was sitting quietly. All the cells were responsive to the cutaneous input during the lifting and dropping cycle. The responses to cutaneous stimuli were found to be modulated in relation to phases of the step cycle and the lifting and dropping cycle. In 13 units this modulation did not parallel the modulation of the unit's spontaneous firing during these activities. For these units a common finding during locomotion was that the response to cutaneous stimuli increased throughout stance, reached maximum during the flexion part of the swing, and then declined to a minimum during the beginning of the next stance. During the lifting and dropping cycle, the responses were greatest when the animal was held in the air and when starting to fall, and minimal just prior to and after landing. In both movements, cutaneous responses were reduced when the limb was used to support the animal's weight. There is apparently a movement phase-related modulation of cutaneous input to some motor cortical cells. This modulation of cutaneous input resembled the modulation of cutaneous reflexes during locomotion.(ABSTRACT TRUNCATED AT 400 WORDS)

Central regulation of motor cortex neuronal responses to forelimb nerve inputs during precision walking in the cat

“…When variation was present it did not in any instance parallel in its temporal pattern the variation present in the responses of the cortical neurone simultaneously under study. The excitatory responses generated in motor cortex neurones by nerve stimulation were similar in latency and size to those encountered in awake cats by Palmer et al (1985) who employed stimuli delivered to palmar skin. However, Palmer et al (1985) did not report responses of the kind we have termed inhibitory and we have located no other systematic description of such responses in anaesthetized or awake cats, perhaps because previous studies have not sought them.…”

“…at the elbow andÏor above). Such an interpretation in fact offers at least a partial explanation for the lack of inhibitory responses in the study by Palmer et al (1985): these authors confined their study to cells with cutaneous receptive fields involving the distal part of the limb and the chances would in consequence be high that the electrical stimuli were delivered within the excitatory receptive fields. retrospect such effects merited systematic investigation and in future studies it will be worthwhile systematically to define inhibitory as well as excitatory components of receptive fields (see also later).…”

“…As in the present experiments, peripheral nerve compound action potentials were monitored to establish that the changes were of central origin. However, the response criteria differ between the two studies because Palmer et al (1985) estimated their responses by visual inspection of PSTHs whilst we used measures (see Methods) that were statistical (though necessarily arbitrary). Furthermore, it is clear that we could not claim a response had been abolished even if no feature in a pooled (regardless of step phase) response histogram met our statistical criteria.…”

“…'Suppression' was widespread but these authors used the term as a synonym for response size reduction: complete abolition appears not to have been observed. Note also that like Palmer et al (1985) these authors studied only 'excitatory' responses though in one instance of a response to nerve stimulation that involved an increase in activity followed by a decrease the two components were analysed separately.…”

“…In some neurones these changes did not parallel the step cycle-related variations in cell excitability as judged from the firing rate in steps without stimuli. Palmer et al (1985) studied animals walking on a moving belt and locomotion under other circumstances appears to require different control signals from the motor cortex. During walking on the rungs of a horizontal ladder or over a succession of low barriers individual cortical neurones (including PTNs) discharge at substantially higher rates than during walking on a flat uninterrupted surface (Amos et al 1990;Beloozerova & Sirota, 1993a,b).…”