The nervous system contains two types of cells: neurons and neuroglia. Neurons are the basic nerve cell, specialized to carry electrical signals to and from the brain. Neuroglia are the support cells of the nervous system, supporting neuron function. Some subtype of each cell type is found in both the central (spinal cord and brain) and peripheral nervous systems.
Neurons – nerve cells
The nerve cells consist of a cell body that contains the nucleus, an axon that reaches out from the cell body, and dendrites that send signals to the cell body. Where the axon of one neuron meets a dendrite of another is called a synapse. Chemical signals travel from the axon (presynapse) of one neuron to receptors on the dendrite of another (postsynapse). Neural axons in the peripheral nervous system are protected by a myelin sheath made of Schwann cells a type of support cells.
The various types of neurons can be classified based on the number of processes extending from the cell body, also known as the soma (see Neuroscience for Kids for illustrations):
Bipolar neurons – traditional two processes; one axon, one dendrite (with branches)
Pseudounipolar cells – two axons, no dendrites; postsynaptic interaction occurs with the cell body
Multipolar neurons – one axon, many dendrites
Neurons can also be classified based on the role they play, i.e. the type of information they carry. Sensory neurons, also known as afferents, are in the peripheral nervous system and relay information from sensory receptors to the central nervous system (spinal cord or brain). Motor neurons, also known as efferents, relay motor responses to the peripheral tissues from the central nervous system. Interneurons are the central nervous system cells that allow communication between the afferents and efferents.
Neuroglia – support cells
Glial cells comprise the majority of cells in the nervous system. Oligodendrocytes have few processes, wrapping around neuron processes in the central nervous system, whereas Schwann cells form the myelin sheath in the peripheral nervous system. Junctions between the support cells forming the myelin sheath are called nodes of Ranvier, allowing amplification of the signals relayed down the axon. Peripheral neurons are further supported physically by satellite cells.
Astrocytes (also known as astroglia) are star-shaped cells that provide support, both physical and nutritional, to neurons via their various processes, or feet, which contact all tissue surfaces that meet neurons from the central nervous system. Astrocytes act like macrophages, engulfing debris, but also transport nutrients and regulate the extracellular space between neurons. Microglia have similar debris “cleaning” functions, acting as brain-specific immune cells.
Besides the cells that form the myelin sheath, astroglia are one of the most important support cells in the body. The blood-brain barrier is maintained by astrocytes, as is the potassium and water gradient between neurons. Nervous system repair and axon regrowth is directed by astrocytes, and they direct neuron outgrowth in development. Essentially, the nervous system would not be able to function without the neuroglia.