2 Neuroscience2.7 Neural circuits
2.7.1.6 Motor control and extrapyramidal side effects
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Written by Resident Doctor Committee · Published
Upper and lower motor neurone signs
The cerebellum times and grades voluntary movement, and the lower motor neurone (anterior horn cell and motor axon) is the final output.
Level | Signs | Dysarthria |
|---|---|---|
Upper motor neurone | Weakness, pronator drift, spasticity, brisk reflexes, extensor plantar | Spastic, from bilateral corticobulbar damage (pseudobulbar) |
Lower motor neurone | Weakness, wasting, fasciculation, flaccidity, reduced reflexes | Flaccid, with weak low-tone muscles |
Spasticity is a velocity-dependent, clasp-knife increase in tone.
Fasciculations localise a lesion to the lower motor neurone, whereas weakness and mild wasting occur at either level.
Cerebellum
Cerebellar damage causes incoordination with preserved power: gait ataxia, broad-based gait, nystagmus, intention tremor, dysmetria and dysdiadochokinesia.
Zone | Function | Lesion signs |
|---|---|---|
Vestibulocerebellum (flocculonodular lobe) | Posture, balance and gaze stability through vestibular input | Truncal ataxia, nystagmus |
Spinocerebellum (vermis and paravermal zones) | Ongoing trunk and limb movement | Gait ataxia, limb dysmetria, intention tremor |
Ataxic (scanning) dysarthria breaks speech into syllables of equal stress, with irregular rhythm and variable loudness.
Lead-pipe rigidity and athetosis are basal ganglia signs, not cerebellar ones.
Dopamine D2 blockade
Antipsychotic extrapyramidal side effects come from D2 blockade in the nigrostriatal pathway (dorsal striatum), not at mesolimbic (nucleus accumbens) or mesocortical (prefrontal) targets.
D2 blockade strengthens the indirect pathway, so the thalamus is more inhibited.
Higher striatal D2 occupancy raises the risk, and anticholinergic activity can offset it.
Metoclopramide and prochlorperazine also block dopamine.
Extrapyramidal syndromes
Syndrome | Onset | Features |
|---|---|---|
Acute dystonia | Hours to days | Painful sustained spasm of neck, jaw, tongue or eyes; laryngeal spasm is dangerous |
Parkinsonism | Days to weeks | Bradykinesia, rigidity, hypomimia, shuffling, often symmetrical |
Akathisia | Days to weeks or later | Inner restlessness, pacing, rocking |
Tardive dyskinesia | Months to years, sometimes after dose reduction | Oro-bucco-lingual choreiform movements, which may spread to limbs and trunk |
Akathisia mistaken for relapse can lead to a harmful dose increase.
Parkinsonism can resemble depression or negative symptoms.
D2 receptor supersensitivity is a proposed but incomplete explanation for tardive dyskinesia.