Motor Output from the Brain and Spinal Cord

Abstract

All skeletal muscles are controlled by activation and inhibition of motor neurons in the spinal cord and brainstem. Motor neurons integrate information from a wide variety of sources in the brain and spinal cord. Simple limb reflexes and basic patterns of limb movements, including locomotion, are coordinated by spinal cord interneurons and sensory neurons. Descending inputs from the reticular formation, the vestibular nuclei and the superior colliculus in the brainstem are important in the control of automatic, axial, postural and locomotor movements. Descending inputs from several areas of the cerebral cortex and from the red nucleus are important in the control of voluntary, distal limb and fine movements. The basal ganglia in the basal forebrain and the cerebellum in the hindbrain are involved in the modification and learning of movements. These structures act indirectly on motor output, via outputs through the thalamus to widespread areas of the cerebral cortex. Key Concepts All voluntary movements involve motor neurons in the spinal cord and brainstem that cause contraction of skeletal muscles. Neurons in many areas of the brain and spinal cord are active at overlapping times to control voluntary movements. The spinal cord can produce coordinated movements of the limbs, such as locomotion, as well as simple reflexes. The superior colliculus can trigger orienting movements, especially of the eyes and head. Voluntary limb movements involve neurons in several parts of the cerebral cortex, including especially the frontal lobe, which project axons to the spinal cord. Cortical movement control signals are modified by loops to and from the basal ganglia and the cerebellum, going through the thalamus. The basal ganglia modify the speed and amplitude of movements and contribute to learning new movement sequences. The cerebellum adapts movements to altered sensory feedback and may also contribute to a wide variety of other functions, including memory, language and emotion. Patients with movement disorders may in the future be able to control a limb or prosthetic device with ‘their thoughts’, via a brain–machine interface that monitors the activity of some of their cortical neurons.