CNS

Meaning: The central nervous system (CNS) is the brain and spinal cord. It integrates sensory information and sends out motor commands. The peripheral nervous system (PNS) is the nerves and ganglia outside them, carrying afferent (sensory) signals toward the CNS and efferent (motor) signals away from it. Within the CNS, neuron cell bodies cluster as gray matter and nuclei, while myelinated axons form white matter tracts. The CNS develops from the neural tube, which arises from ectoderm.

On the MCAT: Questions ask you to sort structures by location or trace a pathway. In a reflex arc, a sensory neuron enters the spinal cord, synapses with an interneuron, and then with a motor neuron, so a response can occur before the brain registers the stimulus. Glia are also tested: oligodendrocytes myelinate CNS axons, while Schwann cells myelinate PNS axons. Passages on injury may contrast the CNS, whose axons regenerate poorly, with the PNS, whose axons regenerate better.

Common confusion: Students place the autonomic nervous system in the CNS. The autonomic system (sympathetic and parasympathetic) and the somatic system are divisions of the PNS, even though CNS centers such as the hypothalamus and medulla regulate them. Also keep the terms straight: clusters of cell bodies are nuclei in the CNS but ganglia in the PNS.