Types of Neurons
The nervous system consists of different types of neurons. These neurons are classified according to where they transmit information. An afferent neuron carries sensory information from sensory organs (e.g., eyes, ears, and skin) to the central nervous system (CNS). An efferent neuron carries motor information, such as when the body should move, from the CNS to effector organs including muscles and glands. The effector organ responds to a stimulus from a nerve. An interneuron relays information between an afferent (sensory) neuron and an efferent (motor) neuron. Interneurons are located in the brain and spinal cord to help process sensory information and coordinate motor activities. For example, if a person’s hand touches a hot stove, a signal is first sent through an afferent neuron to an interneuron, which interprets the information. The interneuron transmits this signal to an efferent neuron, which stimulates the muscles in the hand to move.
Individual neurons also vary in structure and are named based on how dendrites and axons are arranged around the cell body. A unipolar neuron is a sensory neuron that has a single, long axon extending from the cell body. It is found in the spine and cranial nerve ganglia. A bipolar neuron consists of a cell body with one dendrite and one axon extending off of it in opposite directions from each end. Bipolar neurons are found in the retina, the inner ear, and the nasal cavity. Neurons are most commonly multipolar. A multipolar neuron has a single axon and many dendrites that extend from the cell body. These neurons are typically motor neurons found in the CNS. An anaxonic neuron has multiple dendrites and often no axons. These cells are found in the brain and retina.
Neuroglia, also known as glial cells, are cells that support and protect neurons. Unlike neurons, they do not carry neural impulses. Different types of neuroglia are found in the central nervous system (CNS) and peripheral nervous system (PNS). The following glial cells are found in the CNS:
- An oligodendrocyte is a glial cell that generates myelin, which wraps around axons in the CNS.
- An ependymal cell lines the spinal cord and ventricles of the brain. It produces and secretes cerebrospinal fluid (CSF).
- A microglia cell mediates immune responses in the CNS. It can transform into a special type of macrophage that can clear up neuronal debris via phagocytosis.
- An astrocyte is a star-shaped glial cell that supports neurons by connecting them to nutrient supplies and repairing nervous tissue after injury.