Motor systems
Introduit| English | Sens |
|---|---|
| corticospinal tract | corticospinal tract |
| lower motor neuron | The only path to the muscle, so its loss gives floppy weakness, wasting, fasciculations (visible twitches) and reduced or absent reflexes. |
| upper motor neuron | upper motor neuron |
| basal ganglia | basal ganglia |
| cerebellum/ˌserɪˈbeləm/ | A region of the brain involved in coordinating movement, balance and motor learning. |
Two kinds of weak legs
- Two patients cannot walk well. One has stiff legs with jumpy reflexes; the other has floppy, thin legs with muscles that twitch under the skin.
- Movement runs through two neurons in series, with two control systems attached, and each part fails with its own signs.
The corticospinal tract
- The corticospinal tract 皮质脊髓束 runs from the motor cortex through the internal capsule and brainstem to the pyramids in the medulla, where about 85–90% of fibres cross.
- It ends on the lower motor neuron 下运动神经元 in the anterior horn, whose axon runs to the muscle.
Put the corticospinal route in order.
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Upper motor neuron down to the anterior horn, lower motor neuron out to the muscle.
Each corticospinal tract has about 1 000 000 fibres, and 85% cross at the pyramids. How many do not cross?
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15% of 1 000 000 = 150 000.
Upper motor neuron signs
- Upper motor neuron 上运动神经元 damage removes control from above, so the lower neurons are released: stiffness that builds with speed of stretch (spasticity), brisk reflexes and an upgoing big toe (the Babinski sign).
- Muscles waste little, because their lower motor neurons still work.
Stroking the sole makes the big toe go up. What does this suggest?
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The Babinski sign appears when corticospinal control is lost.
Lower motor neuron signs
- The lower motor neuron is the only path to the muscle, so its loss gives floppy weakness, wasting, fasciculations (visible twitches) and reduced or absent reflexes.
- UMN or LMN is the first question in every motor examination.
Basal ganglia: the size of movement
- The basal ganglia 基底神经节 scale movements up or down. Too little dopamine (Parkinson disease) gives slowness, rigidity and a tremor at rest.
- Loss of striatal neurons (Huntington disease) gives unwanted dance-like movements, chorea.
Cerebellum: the timing
- The cerebellum 小脑 times and coordinates movement. Damage causes overshooting (dysmetria), tremor as the hand nears a target, and clumsy rapid alternating movements.
- The signs are on the SAME side as the lesion, because the cerebellum's pathways cross twice.
Match each sign to the system it points to.
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Each part of the motor system has its own signature.
A right cerebellar lesion causes clumsiness of the left arm.
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Cerebellar pathways cross twice, so the signs are on the same side as the lesion.
Worked example: a left cord lesion at C5
- (Fictional) A disc presses on the left side of the cord at C5. At that level: LMN signs in C5 muscles, with a wasted biceps and an absent biceps reflex.
- Below it: UMN signs. On which side? The corticospinal fibres crossed at the medulla, so the UMN signs are on the LEFT, the same side as the lesion.
The left half of the cord is compressed at C5. Where are the UMN signs?
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The corticospinal tract crossed at the medulla, so in the cord it serves the same side.
Worked example: the tremor fork
- (Fictional) Patient A's hand shakes at rest and steadies when reaching, and all movements are slow: basal ganglia, the Parkinson pattern.
- Patient B's hand is steady at rest but shakes more and more as the finger nears the target: cerebellum. Same word, two circuits.
A tremor at rest that improves when the hand moves, with slow movements, points to —
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This is the Parkinson pattern of too little dopamine.
Three common slips
- Expecting weakness on the opposite side below a cord lesion. The tract has already crossed.
- "Spastic means strong." Spasticity is tone, not power.
- Putting cerebellar signs on the opposite side. They are on the same side.
Recap
- Cortex → internal capsule → pyramids (85–90% cross) → anterior horn → muscle.
- UMN: spastic, brisk reflexes, Babinski, little wasting. LMN: floppy, wasting, fasciculations, reflexes lost.
- Basal ganglia scale movement (Parkinson, Huntington); the cerebellum times it, on the same side.