Neuroanatomy - Composition

Composition

At the tissue level, the nervous system is composed of neurons, glial cells, and extracellular matrix. Both neurons and glial cells come in many types (see, for example, the nervous system section of the list of distinct cell types in the adult human body). Neurons are the information-processing cells of the nervous system: they sense our environment, communicate with each other via electrical signals and synapses, and produce our thoughts and movements. Glial cells maintain homeostasis, produce myelin, and provide support and protection for the brain's neurons. Some glial cells (astrocytes) can even propagate intercellular calcium waves over long distances in response to stimulation, and release gliotransmitters in response to changes in calcium concentration. The extracellular matrix also provides support on the molecular level for the brain's cells.

At the organ level, the nervous system is composed of brain regions, such as the hippocampus in mammals or the mushroom bodies of the fruit fly. These regions are often modular and serve a particular role within the general pathways of the nervous system. For example, the hippocampus is critical for forming memories. The nervous system also contains nerves, which are bundles of fibers that originate from the brain and spinal cord, and branch repeatedly to innervate every part of the body. Nerves are made primarily of the axons of neurons, along with a variety of membranes that wrap around and segregate them into nerve fascicles.

The vertebrate nervous system is divided into the central and peripheral nervous systems. The central nervous system (CNS) consists of the brain, retina, and spinal cord, while the peripheral nervous system (PNS) is made up of all the nerves outside of the CNS that connect it to the rest of the body. The PNS is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of "afferent" neurons, which bring sensory information from the sense organs to the CNS, and "efferent" neurons, which carry motor instructions out to the muscles. The autonomic nervous system also has two subdivisions, the sympathetic and the parasympathetic, which are important for regulating the body's basic internal organ functions such as heartbeat, breathing, digestion, etc.