Lissauer's tract: why pain and temperature loss starts below the level of injury
Lissauer's tract is a small bundle at the tip of the dorsal horn (where pain fibres relay) that lets pain and temperature fibres move a short distance before they synapse.
Remember
Pain and temperature fibres cross the midline within the cord, so after a hemisection (one half of the cord is damaged) the loss is on the opposite side. The fibres may also move a short distance in Lissauer's tract before they synapse, which helps explain why the loss of pain and temperature starts a few segments (levels of the cord) below the injury.
Memory aid
a short climb, then a cross; the numbness starts a little lower
The fibres take a short climb before they cross, so the loss is on the opposite side and starts a little lower than the injury.
Pain and temperature signals travel in the spinothalamic pathway, the route that carries them to the thalamus. Its first order neurones are the ones that enter the spinal cord, and they relay (synapse) with second order neurones in the dorsal horn.
First order neurones may not relay where they enter the cord. They can climb or descend a short distance in Lissauer's tract, a small bundle at the tip of the dorsal horn, and only then relay.
After the relay, the second order neurones decussate (cross the midline) in the ventral white commissure, which lies in the centre of the cord. They then ascend on the opposite side.
A hemisection at C5 cuts fibres that carry pain and temperature from the opposite side of the body, so the loss is contralateral. Because some fibres climb a little before they cross, the cut also catches fibres that entered the cord a few segments lower. The loss begins a few segments below the injury, not exactly at it.
The exact pattern depends on how much of the cord is damaged. A hemisection can also cause a band of lost sensation in the single dermatome (area of skin served by one spinal nerve) at the level of the cut, on the same side as the injury, but that is a separate finding.
Key facts
- Pathway
- pain and temperature travel in the spinothalamic pathway, not in the dorsal columns, which are a separate pathway for senses such as fine touch and vibration.
- Lissauer's tract
- a small bundle at the tip of the dorsal horn, where first order pain and temperature fibres may climb or descend a short distance before they relay.
- Crossing
- second order neurones cross the midline in the ventral white commissure, then ascend on the opposite side.
- Side
- after a hemisection, pain and temperature are lost on the opposite side.
- Level
- the loss begins a few segments below the injury, not at it.
- Caution
- the exact pattern depends on how much of the cord is damaged.
Exam traps
- An ipsilateral loss of pain and temperature beginning below the level of a hemisection is wrong, because the second order neurones cross the midline within the cord, so the loss is on the opposite side.
- Contralateral loss that begins exactly at the level of the injury is wrong, because pain and temperature fibres may climb a short distance in Lissauer's tract before they relay and cross, so the loss begins a few segments below.
- A loss limited strictly to the single dermatome (area of skin served by one spinal nerve) at the level of the injury is not the expected pattern, because fibres from the segments just below the injury climb in Lissauer's tract before crossing, so the loss extends below that level.
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