Chapter 10: Control of Body Movement

The motor control hierarchy comprises three integrated levels: higher brain centers that formulate movement intentions based on memory and motivation, middle-level structures including the sensorimotor cortex, basal nuclei, cerebellum, and brainstem that translate intentions into detailed motor programs, and local spinal and brainstem circuits that activate motor neurons and coordinate reflexes. Proprioceptive feedback systems—including muscle spindles that sense stretch, Golgi tendon organs that monitor force, and joint receptors—continuously inform the nervous system about body position and muscle tension, enabling real-time adjustments. The stretch reflex exemplifies local control, wherein spindle activation directly triggers motor neuron firing to resist lengthening, while alpha-gamma coactivation preserves spindle sensitivity during voluntary contraction. Descending pathways from the brain convey motor commands through distinct channels: corticospinal tracts mediate fine, voluntary control of distal limbs through direct pyramidal projections, whereas brainstem pathways including vestibulospinal and reticulospinal tracts coordinate posture, balance, and locomotion. The basal nuclei suppress unwanted movements and facilitate desired ones through dopaminergic signaling; degeneration of dopamine neurons in Parkinson's disease causes bradykinesia, rigidity, and tremor, addressed through dopamine replacement therapy and deep brain stimulation. The cerebellum integrates sensory feedback with descending commands to refine movement timing, smoothness, and accuracy, with cerebellar damage producing intention tremor and ataxia. Muscle tone reflects the balance between excitatory and inhibitory influences; spasticity and rigidity characterize upper motor neuron lesions, while flaccidity indicates lower motor neuron damage. Central pattern generators in the spinal cord drive rhythmic locomotion, modulated by descending control and sensory input. Clinical pathology like tetanus, caused by bacterial toxin blocking inhibitory neurotransmitter release, illustrates how disrupted motor control balance produces life-threatening rigidity and spasms.