There are many structural and functional types of both neurons and glia, meaning they have different shapes and they perform different jobs. Some of these are only located in the central nervous system, which is mainly the brain and spinal cord, or the peripheral nervous system, which is mainly the nerves. Neurons are found in both the central and peripheral nervous systems. Most types of glial cells are found only in the central nervous system, and a few are found only in the peripheral nervous system.
The most common structural type of neuron is called multipolar, which has one axon and multiple dendrites attached to the soma. Dendrites are processes that are usually short and branched close to the soma, while axons are processes that are usually long without branching until their end, when they branch into endings called axon terminals. Less common structural types of neurons include: bipolar neurons, which have one axon and one dendrite; unipolar neurons, which have one axon as their only process; and pseudounipolar neurons, which have a single short process attached to the soma that splits into two long axons.
In the peripheral nervous system, neurons are divided into the functional types of afferent neurons, which transmit information from the periphery into the central nervous system, and efferent neurons, which transmit information away from the central nervous system into the periphery. Afferent neurons are also called sensory neurons because they usually carry information that is sensed about different kinds of stimuli from either inside or outside the body. Motor neurons are a type of efferent neuron that control skeletal muscle, the main type of muscle tissue in our body that is mostly attached to our skeleton. Autonomic neurons are a type of efferent neuron that control smooth muscle, cardiac muscle, or gland cells, which are cells involved in the functions of things like organs, blood vessels, and glands. Most neurons in the central nervous system are of the functional type called interneurons, which are neurons that connect other neurons together. The term innervation refers to connections of neurons and their target cells; a target cell is innervated by the neuron.
The most important glial cells of the peripheral nervous system are Schwann cells, which help many neurons transmit information by wrapping an insulating material called myelin around their axons. Oligodendrocytes also perform this function, but they do so in the central nervous system. Astrocytes perform many functions in the central nervous system, including: providing structural support; forming scar tissue after injury; regulating the concentration of many substances, which are important for neuron function, in the interstitial fluid between cells; as well as contributing to a barrier between the interstitial fluid and the blood in blood vessels. Microglia kill and remove foreign cells, abnormal neural cells, or debris in the central nervous system, and they work with the immune system to fight off infections. Ependymal cells line fluid-filled cavities in the central nervous system, and also play a role in its secretion. Glial cells also perform many other functions as they interact with neurons, other glial cells, and cells of other systems of the body.