Motor Control: Spinal Reflexes to Cortex
Key concepts on motor units, spinal reflexes, descending tracts, and lesion signs in motor control.
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Questions Covered in This Set
12 cards to master
What is the "final common path" and the motor unit?
The alpha motor neuron in the ventral horn is the final common path; one alpha motor neuron plus all the muscle fibres it innervates = a motor unit.
How is muscle force graded?
By recruitment (size principle: small, fatigue-resistant type I units first because small neurons have high input resistance) and rate coding (higher firing frequency → fused, stronger contraction).
What do gamma motor neurons do, and what is alpha–gamma coactivation?
Gamma motor neurons innervate the contractile ends of muscle spindles to keep them taut as muscle shortens; alpha–gamma coactivation means the CNS commands both together so proprioceptive feedback stays informative at all muscle lengths.
Describe the stretch (myotatic) reflex.
Monosynaptic: tendon tap → spindle Ia afferent fires → direct excitation of alpha motor neuron → contraction, with reciprocal inhibition of the antagonist via an inhibitory interneuron. Latency ~20–30 ms; acts as a length servo setting muscle tone.
What does the inverse myotatic (Golgi tendon) reflex sense and do?
Ib afferents from tendon organs sense force (not length) and inhibit the same muscle via interneurons — a tension regulator and protective brake; loss of Ia/Ib balance contributes to the clasp-knife phenomenon.
What is the withdrawal (flexor) reflex with crossed extension?
Nociceptor input drives ipsilateral flexion and contralateral extension so you stay upright while withdrawing; it is polysynaptic, so longer latency and shows habituation.
Lower motor neuron lesion signs?
Hypo/areflexia, flaccidity, atrophy, and fasciculations.
Upper motor neuron lesion signs, and why?
Hyperreflexia, clonus, spasticity, Babinski sign, no significant atrophy — descending inhibition is removed while the spinal reflex arc remains intact (spasticity = disinhibited spinal cord).
Trace the lateral corticospinal tract.
M1 (area 4) and premotor cortex → internal capsule → medullary pyramids → ~85–90% decussate at the pyramidal decussation → lateral cord → mainly interneurons, some direct alpha motor neuron contacts for the hand; carries fine fractionated distal control.
Why does a cortical stroke spare the forehead but drop the lower face?
Corticobulbar fibres to cranial nerve nuclei are largely bilateral for the upper face, so the forehead is spared; lower face receives mainly contralateral input.
Decorticate vs decerebrate posturing?
Decorticate: lesion above midbrain, flexed arms (red nucleus intact). Decerebrate: lesion below red nucleus, extension everywhere, worse prognosis.
Which brainstem tracts favour extensors vs flexors?
Lateral vestibulospinal and pontine reticulospinal facilitate extensors; medullary reticulospinal and rubrospinal favour flexors.
