The thalamocortical motor system and stability: observation of a possible regulatory mechanism.

Abstract

1. A simplified model of the thalamocortical motor system (VL-MC) is proposed, containing as principal components, an integrator and a regulator. The integrator generates ramp functions, with the slope of the ramp determined by sensory (peripheral) input. The duration of the ramp function is controlled by the regulator. The regulator is assumed to be a higher order control system whose overall goal is to prevent the VL-MC system from shifting to an unstable (e.g. epileptiform) mode of operation. 2. The VL-MC system has been investigated in cats utilizing 3 anesthetic agents; pentobarbital, chloralose and ketamine. The experimental results, when explained within the constraints of the model, were as follows: a) In the pentobarbital cases, ramp slopes were truncated by a negative slope of approximately equivalent magnitude. The ramp truncation mechanism (regulator) operates to limit the maximum amplitude of the ramp, resulting in a compensatory reduction in ramp duration for steeper ramp slopes. No cortical epileptiform activity was observed. b) In the chloralose cases, at a level of anesthesia where active nociceptive reflex action was present, steeper ramp slopes were observed which were not truncated by a negative slope of equivalent magnitude. Epileptiform spike activity was observed on motor cortex. The ramp truncation or regulatory mechanism appears to be significantly reduced in capacity, when chloralose is used as an anesthetic agent, as compared to pentobarbital. c) In the ketamine cases, the ramp generation mechanism appears to be completely disabled. No epileptiform activity was observed. 3. It is suggested that a regulatory mechanism exists as a functional part of the thalamocortical motor system of the cat and that this regulatory mechanism plays a key role in preventing the system from shifting to an unstable mode, characterized by epileptiform cortical activity. 4. A simplified model of the system has been implemented on an analog computer and suggests one possible design for the regulatory mechanism in terms of the variables measured and the parameters controlled.