5 - Muscle tone and movement

Abstract

The frontal, postcentral, and temporal cortex play a part in the planning of voluntary movements. Subcortical structures also participate in the programming of movement, the basal ganglia for slow smooth movements and the cerebellum for fast movements. The outflow from the sensorimotor cortex comprises pyramidal fibers and parapyramidal fibers, the latter being distributed to basal ganglia, thalamus, cerebellum, and brainstem. The vestibulospinal and facilitatory reticulospinal tracts facilitate the motor neurons of antigravity muscles and the stretch reflex. The inhibitory reticulospinal tract, which opposes these actions, is driven from the motor cortex, thus forming an inhibitory cortico­reticulospinal pathway, the interruption of which increases muscle tone. The pyramidal tract, which arises from the sensorimotor cortex and continues into the spinal cord as crossed and uncrossed corticospinal tracts, provides the monosynaptic as well as polysynaptic control of motor neurons responsible for skilled movements. It facilitates extension and abduction movements in the upper limbs and the flexor synergy in the lower limbs of man. The pyramidal tract also regulates afferent inflow to the cortex from the posterior columns by the presynaptic inhibition of the cuneate and gracile nuclei. Pyramidal and extrapyramidal pathways are complementary in the control of movement.