The Nervous System and the Senses

Structural Organization and Cellular Components of the Nervous System

The nervous system is structurally divided into two major components: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord, serving as the primary control and processing center for the body. In contrast, the peripheral nervous system consists of all neural tissue outside the central nervous system, which includes the cranial nerves and spinal nerves that connect the center to the rest of the body. Neural tissue throughout these systems is organized into white matter and gray matter. White matter consists predominantly of myelinated axons, whereas gray matter consists of neuronal cell bodies, dendrites, and unmyelinated fibers. In the brain, gray matter is located primarily on the outer cortex, whereas in the spinal cord, gray matter is located internally.

The primary structural and functional cell of the nervous system responsible for transmitting electrical signals is the neuron. A typical neuron consists of three main parts: dendrites, a cell body or soma, and an axon. Dendrites receive incoming signals from other neurons or receptors and conduct them toward the cell body. The axon conducts electrical impulses away from the cell body toward target cells or subsequent neurons. The axon emerges from the cell body at a specialized region called the axon hillock, and action potentials typically begin in the adjacent axon initial segment.

To ensure electrical signals travel efficiently, many axons are surrounded by a myelin sheath. The myelin sheath is a lipid-rich insulating layer that surrounds axons to increase the speed of electrical conduction. In the central nervous system, oligodendrocytes produce the myelin sheath around axons. In the peripheral nervous system, Schwann cells produce the myelin sheath. Along a myelinated axon, there are unmyelinated gaps where action potentials regenerate, which are known as the nodes of Ranvier.

Beyond neurons, the nervous system relies on specialized glial cells to maintain a healthy environment. Astrocytes are central nervous system glial cells that support neurons and help form the protective blood-brain barrier. Microglia are specialized central nervous system glial cells that function as resident phagocytes to clear cellular debris and pathogens. Ependymal cells line the brain's ventricles and the spinal cord's central canal; specialized cells of the choroid plexus produce cerebrospinal fluid, and ependymal cilia help circulate it.