The spinal cord carries signals between the brain and the body and handles reflexes on its own. In adults it ends near the L1 to L2 vertebrae, and below that the nerve roots form the cauda equina.
Gray and white matter
In cross section the butterfly-shaped gray matter holds nerve cell bodies: the dorsal horns handle sensation and the ventral horns hold motor neurons. The white matter around it contains the tracts running up and down.
Motor pathway
The corticospinal tract carries voluntary movement from the motor cortex, and most of its fibers cross in the lower medulla. Upper motor neuron damage causes weakness with brisk reflexes, while lower motor neuron damage causes weakness, wasting and reduced reflexes.
Sensory pathways
The dorsal columns carry fine touch, vibration and position sense and cross in the medulla. The spinothalamic tract carries pain and temperature and crosses within a segment or two of entering the cord.
Reflex arc
A stretch reflex needs only a sensory neuron, a motor neuron and one cord segment, so it works without the brain. That is why tendon reflexes can test specific cord levels.
| Tract | Carries | Where it crosses |
|---|---|---|
| Corticospinal | Voluntary movement | Lower medulla |
| Dorsal columns | Fine touch, vibration, position sense | Medulla |
| Spinothalamic | Pain and temperature | Spinal cord, near the level of entry |
| Spinocerebellar | Position information to the cerebellum | Mostly stays on the same side |
At the bedside
Because the tracts cross at different levels, an injury to one side of the cord causes weakness and loss of position sense on that side but loss of pain and temperature on the other side. Protect the spine in trauma until injury is ruled out.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).