PNS

Meaning: The PNS is all nervous tissue outside the brain and spinal cord (the CNS): nerves and ganglia. The sensory (afferent) division carries information toward the CNS; the motor (efferent) division carries commands away. The motor division splits into the somatic system (voluntary control of skeletal muscle) and the autonomic system (involuntary control of smooth muscle, cardiac muscle and glands). Autonomic pathways use a two-neuron chain, preganglionic then postganglionic, and have sympathetic and parasympathetic branches. Schwann cells myelinate PNS axons; oligodendrocytes do so in the CNS.

On the MCAT: Questions ask you to classify a pathway, often a reflex arc: the sensory neuron enters the spinal cord by the dorsal root, and the motor neuron leaves by the ventral root. Sensory cell bodies lie in dorsal root ganglia, outside the CNS. You may also be asked to predict the effect of damaging a nerve or its Schwann cells.

Common confusion: Students equate the PNS with the autonomic system, but the autonomic system is only one part of the motor PNS. Afferent and efferent are defined relative to the CNS, not by the target tissue. Parasympathetic is usually abbreviated PSNS, so PNS normally means peripheral.