The spinal cord

Grey and white matter, horns, roots, ascending and descending tracts: the cord relays and integrates movement.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Grey matter: what is it?
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The butterfly-shaped core, made of neuron cell bodies.
Connections and reflexes happen there.
A lesion cuts the cord at thoracic level. What is lost below it?
- Voluntary movement and sensation
- Voluntary movement, but not sensation
- Sensation, but not voluntary movement
- Only the spinal reflexes
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Voluntary movement and sensation
Both ascending (sensation) and descending (voluntary command) tracts are cut; the reflexes, whose circuits lie below the lesion, return after the spinal shock phase.
White matter: what is it?
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The outer part, made of myelinated axons grouped in tracts.
It carries messages to and from the brain.
Why does a lesion in one hemisphere paralyse the opposite side of the body?
- The ventral roots leave on the opposite side
- The corticospinal tract crosses the midline
- Sensory tracts cross, motor tracts do not
- All cranial nerves cross the midline
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The corticospinal tract crosses the midline
80 to 90% of corticospinal fibres cross in the medulla (pyramidal decussation): the left hemisphere mostly drives the right side.
Ventral horn: what is it?
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The part of the grey matter holding the motor neurons.
The motor neurons' axons leave through the ventral root to the muscles.