--- module: 028-01 language: en chapter: 28 title: "Motor Control, Reflexes, Cerebellum, and Basal Nuclei" module_title: "Foundations" source_sha256: 81e6bfa0967282eb931e703dab670b3a3a80fb02b5facc6fb097cc5b73890c5e --- # Motor control foundations ## Distributed motor control ### Spinal circuits: reflexes and patterned output ### Brainstem: posture, balance, gaze, orientation ### Motor cortex specifies skilled action ### Basal nuclei select and scale actions ### Cerebellum predicts, compares, adapts ### Localise by weakness, tone, reflexes, gait ## Motor units and force ### Alpha motor neuron plus its muscle fibres #### Small units for fine control, large for power ### Force: faster firing, larger units recruited ### Asynchronous activation smooths contraction ### Neuromuscular transmission via acetylcholine #### Nicotinic end-plate potential, then sodium channels #### Safety factor can fail with receptor or release loss ### Lower motor-neuron loss: atrophy, fasciculations ### Junction disease: fatigable weakness, no sensory loss ### Muscle disease: symmetric proximal weakness ## Muscle spindles and tendon organs ### Spindles in parallel sense length and stretch rate #### Ia excites same muscle, inhibits antagonist #### Gamma drive preserves sensitivity in contraction ### Tendon organs in series sense tension via Ib #### Distribute load, flexible role in locomotion ### No receptor acts in isolation ## Spinal reflexes ### Stretch reflex resists sudden lengthening ### Tendon tap probes the whole reflex arc #### Amplitude varies with attention, age, technique #### Symmetry and other signs outweigh one grade ### Flexor withdrawal with crossed extension ### Renshaw recurrent and reciprocal inhibition ### Spinal shock, then exaggerated reflexes ### Pattern generators need descending initiation ## Descending pathways ### Corticospinal fibres cross in lower medulla #### Lateral tract: contralateral distal skill ### Corticobulbar input largely bilateral #### Lower face and tongue contralateral #### Central lesion spares forehead ### Reticulospinal: tone, posture, locomotion ### Vestibulospinal stabilises against gravity ### Tectospinal orients head to stimuli ### Upper motor-neuron lesion after shock resolves #### Hyperreflexia, clonus, extensor plantar #### Spasticity velocity-dependent, rigidity not ## Motor cortex and planning ### Primary motor: force direction, execution ### Premotor uses external cues ### Supplementary areas: sequences, bimanual ### Parietal cortex turns space into plans ### Maps overlap and change with learning ### Efference copy predicts sensory consequences ### Practice reduces attentional demand ## Cerebellar architecture and function ### Compares intended plans with performance ### Repeated cortical circuit #### Climbing fibres give teaching signals #### Purkinje cells inhibit deep nuclei ### Midline axial, lateral planning and timing ### Internal models support error-based learning ### Hemispheric lesions give ipsilateral signs ### Dysmetria, intention tremor, broad gait ### Sensory ataxia worsens with eyes closed ## Basal nuclei circuits ### Parallel motor, cognitive, limbic loops ### Inhibitory output selects cortical actions ### Direct pathway facilitates selected action ### Indirect pathway suppresses competitors ### Dopamine: D1 direct, D2 reduces indirect ### Parkinsonism from reduced dopamine #### Replacement helps but causes dyskinesia ### Chorea, dystonia, tics, ballism, myoclonus ### Dopamine blockers: dystonia, akathisia ## Posture, gait, and balance ### Quiet stance is active #### Ankle, hip, stepping strategies ### Anticipatory adjustments precede movement ### Ageing, neuropathy, sedatives reduce reserve ### Gait patterns localise the failure #### Parkinsonian short steps and freezing #### Frontal gait impairs initiation ## Motor learning and rehabilitation ### Restitution, compensation, practice, adaptation ### Task-specific, challenging, safe repetition ### Excess external correction impairs independence ### Treat spasticity for pain, contracture, function #### Stiffness may support transfers