Pyramidal versus extrapyramidal motor systems
The pyramidal (corticospinal) system carries the voluntary movement command from the brain to the spinal cord, while the extrapyramidal system, chiefly the basal ganglia (deep nerve cell clusters), refines it.
Remember
The corticospinal (pyramidal) system carries the voluntary command, whereas the extrapyramidal system, chiefly the basal ganglia, modulates (adjusts) and refines movement. Slowness with raised muscle tone but little weakness points to the extrapyramidal system, whereas pyramidal damage raises tone together with weakness.
Memory aid
Pyramidal damage is a cut wire (the message does not get through, so weakness), whereas basal ganglia damage is a badly tuned amplifier
A cut wire gives weakness because the message does not arrive, whereas a badly tuned amplifier gives slow, stiff or distorted movement because the message does arrive.
Movement is controlled mainly by the pyramidal system. Its fibres form the corticospinal tract, the descending pathway that runs from the precentral gyrus (the primary motor cortex) to the spinal cord and carries the command for voluntary movement. On the way it passes through the pyramids, two ridges on the front of the medulla (the lowest part of the brainstem), and that is how the system got its name.
The extrapyramidal system lies outside those pyramids and works alongside the corticospinal tract. It does not send the main command for voluntary movement directly to the spinal cord. Instead it helps the central nervous system modulate (adjust) and refine voluntary movement. Its main structures are the basal ganglia, deep clusters of nerve cell bodies (called nuclei) that influence movement by acting on the motor cortex through the thalamus, a relay station in the centre of the brain. Some textbooks also use the term extrapyramidal for brainstem tracts that control tone, balance and posture, such as the vestibulospinal tract, which starts in the balance centres of the brainstem.
Basal ganglia disease, as in Parkinson's disease, classically gives slow movement (bradykinesia) and rigidity, a raised tone that feels the same throughout a passive movement and at any speed, while strength is largely preserved. Damage to the corticospinal tract also raises tone, but the resistance is spasticity: it depends on how fast the joint is moved, often gives a sudden catch, and comes with weakness, brisk reflexes and a Babinski sign (an upgoing big toe on the sole test).
Key facts
- Pyramidal system
- the corticospinal tract carries the command for voluntary movement from the motor cortex down to the spinal cord.
- Extrapyramidal system
- chiefly the basal ganglia, which modulate and refine voluntary movement instead of sending the main command.
- Medullary pyramids
- two ridges on the front of the medulla made of corticospinal fibres, which give the pyramidal system its name.
- Basal ganglia route
- they influence movement by acting on the motor cortex through the thalamus.
- Rigidity
- uniform resistance to passive movement that does not change with speed, pointing to basal ganglia dysfunction.
- Spasticity
- resistance that depends on speed and often gives a sudden catch, following damage to the corticospinal tract.
Exam traps
- Anterior horn cells, the lower motor neurones in the front of the spinal cord that supply skeletal muscle, are not part of the extrapyramidal system, because they are the final common pathway, the last link, since every descending pathway ends on them.
- The medullary pyramids, two ridges on the front of the medulla, point to the pyramidal system and not the extrapyramidal one, because they are built from corticospinal fibres and extrapyramidal structures lie outside them.
- Raised muscle tone with preserved strength fits pyramidal damage poorly, because damage to the corticospinal tract raises tone but also causes weakness, brisk reflexes and a Babinski sign.
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