Motor Control: From Intention to Muscle
Movement uses interacting cortical, brainstem and spinal circuits. Motor cortex contributes to muscle commands, brainstem circuits support posture, and motor neurons activate muscle fibres. Cerebellar circuits refine coordination; basal ganglia circuits influence action selection and movement size. Isolated dysfunction can impair movement without primary paralysis.
What a learner can do afterwards
- Distinguish the roles of spinal, brainstem and cortical levels in a single reaching movement
- Explain what a motor unit is and how force is graded by recruitment
- Contrast the movement disorder caused by cerebellar damage with the one caused by basal ganglia damage
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Movement uses interacting circuits. Cortical areas help choose goals and plan actions; motor cortex also contributes commands that influence muscles. Brainstem circuits support posture and balance. Spinal circuits contain reflexes and pattern generators for repeated movements. Motor neurons in the spinal cord or brainstem send the final neural signals to skeletal muscle fibres. Commands need not pass through every control region in a single chain.
Reach for a cup: cortical areas help select the target and direct the arm. Brainstem pathways contribute to postural adjustments. Sensory feedback and other circuits also help maintain posture. Spinal circuits combine descending commands with local sensory input, and their motor neurons activate arm muscles. These systems cooperate; the brainstem does not control posture alone.
A motor unit is one motor neuron and all the muscle fibres it supplies. Recruitment activates additional units. As force demand rises gradually, smaller, lower-threshold units generally join before larger, higher-threshold units. Force can also rise when active neurons fire more often. If a new muscle twitch starts before full relaxation, twitch forces add. Recruitment changes the active set; firing rate changes how often active neurons signal.
This shows recruitment order during a gradual rise in demand. Firing rate is a separate control.
Cerebellar circuits help coordinate timing and accuracy using feedback and predictions. Isolated damage can cause overshoot and unsteady balance, often with largely preserved strength. Basal ganglia circuits help select actions and regulate movement initiation and size. Parkinsonian dysfunction can cause slowness, rigidity and rest tremor. Other basal ganglia disorders can cause unwanted movements. These are movement impairments, not signs of unwillingness.
An isolated cerebellar or basal ganglia disorder can impair coordination or movement regulation while motor output pathways still activate muscles. Movement can remain possible without primary paralysis. This depends on the structures affected. Damage that also involves descending motor pathways or motor neurons can cause weakness or paralysis. A symptom pattern suggests a circuit; it does not establish a diagnosis.
Interacting circuits plan and regulate movement. Recruitment and firing rate control muscle output. Cerebellar and basal ganglia dysfunction affect movement in different ways.
Lesson, questions, worked solutions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked video by Khan Academy.
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Recruitment and firing rate
There are two different ways to change neural drive to a muscle. Recruitment adds motor units to the active set. Each unit consists of one motor neuron and all the muscle fibres it supplies. During a gradual increase in demand, smaller, lower-threshold units generally become active before larger, higher-threshold units.
Firing rate changes how often an active neuron sends impulses. The active set can remain unchanged while its neurons fire more frequently. If a new twitch begins before the previous twitch has fully relaxed, their forces can add.
To distinguish the controls, ask two questions: did additional units become active, and did active neurons send impulses more often?
Cooperating circuits in a reach
Consider the tasks involved in reaching for a cup. The nervous system must select a target, direct the arm and adjust posture. Cortical areas contribute to planning the action. Motor cortex also contributes descending commands that influence muscle activity.
Brainstem pathways participate in postural control. Spinal circuits combine descending commands with local sensory information and include reflex circuits. Motor neurons connect neural control to muscle activation at neuromuscular junctions.
These functions interact. Commands do not have to pass through every control region in one serial route. Posture also depends on sensory feedback and participation from several circuits, rather than the brainstem acting alone.
Coordination and movement regulation
Cerebellar circuits contribute to movement timing and accuracy using feedback and predictions. Isolated dysfunction can make a reach overshoot its target or make balance unsteady. Strength may remain largely preserved despite poor coordination.
Basal ganglia circuits influence action selection, initiation and movement size. A Parkinsonian pattern can include slowness, rigidity and tremor at rest. Other disorders of these circuits can produce unwanted movements. These are changes in movement control, not evidence that a person is unwilling to move.
The extent of a lesion matters. When motor output pathways are spared, muscles may still be activated despite impaired coordination or regulation. Damage involving descending pathways or motor neurons can also cause weakness or paralysis. A symptom pattern provides clues; it does not establish a diagnosis on its own.
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