spinal nerves
Summary
Spinal nerves are the 31 pairs of peripheral nerves that attach to the spinal cord and connect it to the body. Each is mixed: sensory axons enter through the dorsal root and motor axons leave through the ventral root. Testable point: spinal reflexes can be completed at the cord without the brain.
Detail
Meaning: Spinal nerves are peripheral nervous system (PNS) nerves that connect the spinal cord to the body. Humans have 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each nerve forms where two roots join. The dorsal root carries afferent (sensory) axons, whose cell bodies lie in the dorsal root ganglion. The ventral root carries efferent (motor) axons, whose cell bodies lie in the gray matter of the cord.
On the MCAT: Reflex arcs are the usual setting: sensory neuron, often an interneuron, then motor neuron, as in the knee-jerk reflex. The circuit runs through the cord, so the response is fast and can be completed without the brain. Questions also ask what is lost after dorsal root damage (sensation) versus ventral root damage (movement), and ask you to sort structures into CNS or PNS.
Common confusion: Spinal nerves are not part of the spinal cord; the cord is CNS and the nerves are PNS. They are also not the 12 pairs of cranial nerves, which attach to the brain. The dorsal-sensory, ventral-motor split applies to the roots. Once the roots join, the spinal nerve itself is mixed.
Sources
- Biological Psychology (Kalat)
- Psychology (Myers)
- Biology (Campbell)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.