Distributed feedback systems for muscle control

Suppression of motor seizures after specific thalamotomy in chronic epileptic monkeys

In the monkey with a chronic motor epileptogenic focus, thalamic lesions restricted to the anterior part of the ventro-postero-lateral nucleus produced a long-lasting improvement, leading in most cases to almost complete seizure suppression. This improvement was attributed to the interruption of a long-loop recurrent excitatory mechanism, involving muscular afferents which are known to relay in the thalamic region where lesions were produced. The specific nature of this effect was suggested by the preliminary results of lesions performed in thalamic regions with non-specific cortical projections. Compared with the pre-lesional situation in which focal activity resulted in permanent motor troubles, the post-lesional state did not show any additional motor deficit. It is concluded that such lesions could be a new approach to controlling intractable motor epilepsies in man.

Operant conditioning in motor and neural integration

Empirical and theoretical reasons were given to investigate operant conditioning in a new, integrative approach within motor control physiology. Elements of inborn and learned behavior were presented in a framework specifying their stimuli and responses. The operant was redefined as a controlling discriminative stimulus, Sd, together with the response, R, it produces, on the basis of a previous literature of operant and instrumental research. Complex motor and neural activity were reviewed in accordance with partitioning of: responses, controlling stimulation, reinforcement, and functions of movement-produced stimulation. Schematics portrayed reinforcement principles through analysis of a fast pathway from Ia muscle spindle afferents to motor outflow. Methods were suggested to minimize operant units through selective reinforcement and establish them to defined end points of learning within composite, ongoing behavior. It was argued that operant neural mechanisms can be investigated efficiently only by starting with individual operants that are thoroughly characterized.

Is the hindlimb representation of the rat's cortex a 'sensorimotor amalgam'?

In the rat, the hindlimb representation of the sensorimotor cortex is characterized by the presence of large pyramids in the fifth layer and a dense granular layer ('sensorimotor amalgam'). The objective was to investigate in the rat, whether or not the efferent zones to the gastrocnemius muscle and the proprioceptive feedback projection from that muscle to the cortex are co-extensive. To this end, the proprioceptive zone was mapped by means of field potentials and single unit discharges evoked by controlled longitudinal displacements of the gastrocnemius tendon. The efferent zones to the gastrocnemius muscle were mapped by means of intracortical microstimulation (ICMS; less than 30 microA). The proprioceptive zone occupied a territory extending from 1.0 to 2.5 mm caudal to the bregma and from 2.0 to 3.0 mm lateral from the midline. The response properties were similar to those observed previously in area 3a of monkeys. For sinusoidal displacements threshold amplitude decreased with increasing stretch frequency. The modal value of response latency was 7 ms, the shortest latency 4 ms. The ICMS zone lay 0.5 to 1.5 mm caudal to bregma having an overlap of 0.5 mm with the proprioceptive region. The proprioceptive as well as the motor areas lay within the granular cortex, but overlapped only to a small extent.

The frontal agranular cortex and the organization of purposeful movements

A critical review of the traditional concepts of cortical association and motor areas is followed by a description of the functional organization and intrinsic and extrinsic cortical connectivity of the arcuate premotor area (APA). It is concluded that the frontal cortical organization of externally triggered purposeful movements is made possible by the associative character of Brodmann's area 6 and by its peculiar pattern of intra-areal connectivity.

Modulation of proprioceptive transcortical reflexes in the cat with a penicillin epileptic motor focus

Mechanisms responsible for the triggering of paroxysmal events by proprioceptive afferents, previously described in the monkey with a chronic epileptic focus, were studied in more detail in the cat with a penicillin focus. To analyse the topical organization of this reflex triggering, the focus was restricted to very small areas of the motor cortex; in this study only pericruciate areas were considered in which stimulation elicited a motor response in one of the several forelimb muscles tested, and which received afferents from that muscle. When the focus was located in the post-sigmoid gyrus, stimulation (usually by stretch) of the given (target) muscle first elicited a cortical spike following the evoked response, and secondly a late phasic EMG response (about 40 msec latency) quite distinct from purely spinal reflexes. Cortical spikes and late EMG responses were closely correlated, especially considering their probability of occurrence or their parallel latency fluctuations. In most cases, this effect was limited to the muscle whose motor area had been treated with penicillin: stretching muscles in the vicinity was ineffective, nor were these muscles activated when the target muscle was stimulated. Evidence is given for the participation of a transcortical reflex in the generation of the late phasic response and for the involvement of the pyramidal tract in this reflex.

Operant conditioning of tonic firing patterns from precentral neurons in monkey neocortex

This report presents a single neurons operant conditioning paradigm which allows the quantification of operant control between neurons and monkeys. The operant task is for the monkey to change firing patterns of the neuron from phasic to tonic. In 60 neurons conditioned after the protocol had been standardized the following results were obtained. (1) For a fixed interspike interval target range, the time off target may be considered error, and this may be used as a parameter with which to judge operant neuronal control. (2) The degree to which the monkey could control a neuron was not correlative with the neuron's initial firing rate, firing rate variance, or pattern. (3) Neurons coactivated by distal arm muscle groups were, as a group, more highly controlled in comparison to neurons coactivated by proximal muscle groups. (4) Pyramidal tract neurons, as a group, appear more accurately controlled than non-pyramidal tract neurons. (5) The role of proprioception in response acquisition is discussed.

Effects of dentate cooling on precentral unit activity following torque pulse injections into elbow movements

(1) The effects were studied of brief, reversible cooling of the dentate nucleus of two Cebus monkeys on the activity of 69 precentral neurons that were related to self-paced elbow flexions or extensions. Sixty-two neurons were studied when movements were randomly perturbed by transient load changes. (2) During unperturbed movements dentate cooling altered precentral activity and movement parameters in the same direction. Extension movements and extension-related precentral neurons developed oscillations and rhythmical activity at about 4 Hz with increased peak values of acceleration and discharge intensity. Flexion peak accelerations and dynamic properties of flexion-related precentral neurons decreased. (3) Dentate cooling had no major effects on the "early" precentral responses to torque pulses and on the initial rapid movement adjustment. (4) Dentate cooling decreased intensity of "late" precentral responses and slowed their frequency together with associated movement oscillations from 5-6 Hz to 3-4 Hz. EMG bursts of biceps and triceps were prolonged accordingly. (5) It is concluded that the tightly coupled interaction between precentral motor cortex and the evolving movement was independent of dentate function in the early stage of the cortical load compensation. The dentate nucleus was involved, however, in adjusting intensity and frequency of subsequent late precentral responses and of movement oscillations.

Motor responses to sudden limb displacements in primates with specific CNS lesions and in human patients with motor system disorders

Central feedback pathways for motor control were studied by recording EMG responses to sudden upper limb displacements in humans and monkeys using a precision torque motor to generate step load changes. Normal human subjects showed three short-latency EMG responses (M1, M2 and M3) which appear to correspond to those recorded from trained monkeys. The M2 and M3 components, thought to represent feedback in supraspinal pathways, were significantly increased when the subjects were instructed to actively compensate for the load changes. Parkinsonian patients with rigidity showed evidence of markedly increased feedback over the interval for the M2 and M3 responses and appeared to have lost the ability to modulate feedback according to the motor task being performed. The results are discussed with reference to recent research on motor control mechanisms in primates and a tentative model for the basis of Parkinsonian rigidity is proposed.

Roles of cerebellum and basal ganglia in initiation and control of movements

Theories of function of the cerebellum and basal ganglia are examined in the light of recent experimental findings obtained with the local cooling method, and both are matched against clinical observations. Evidence is summarized for a programming and initiating role in monkeys' elbow movements of the lateral, and to a lesser degree, intermediate, cerebellum. Cooling either nuclei affected movements, but neither seemed to be important for precentral cortical unit discharge accompanying compensation for suddenly applied load pulses. The globus pallidus seemed to be importantly involved in movement guidance in the absence of vision.

Synchronization of human motor units: possible roles of exercise and supraspinal reflexes

Some normal human subjects show definite synchronization of the motor units in hand muscles (i.e., the impulses from two or more motor units coincide in time more frequently than expected for independent random processes). Subjects who show synchronization tend to use their hands to exert large, brief forces, either in their work (e.g., manual laborers) or recreational activities (e.g., weightlifters). In this study all seven weightlifters examined showed a significant degree of synchronization. Furthermore, after 6 weeks of using the first dorsal interosseus muscle of the hand to exert maximal, voluntary contractions, the level of synchronization increased substantially in four control subjects, and the average level became significantly different from zero. Weightlifters also showed greater late reflex responses than control subjects, but no significant difference in earlier spinal reflexes. Two late reflex waves are described which probably involve fast pathways to and from motor cortes. We suggest that supraspinal connections from motor cortex directly to spinal motoneurons may be enhanced as a result of training to the point where they produce a significant synchronization of motor units during steady, voluntary contractions.