The Central and Peripheral Nervous Systems:

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else. The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.

Nervous tissue consists of neurons and glial cells, found in both the CNS and PNS. Neurons are the essential units responsible for communication in the nervous system. They have a cell body (soma) and processes, including dendrites for receiving signals and axons for transmitting signals to target cells. Gray matter, made up of cell bodies and dendrites, can appear pinkish or tan, while white matter, composed of axons surrounded by myelin, appears white due to its lipid-rich composition. These distinctions are observed in the brain and spinal cord, with gray matter often found on the outer layers and white matter deeper within or on the periphery.

Gray Matter and White Matter: A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain.