Neurons

Definition

A neuron is a specialized cell of the nervous system designed to rapidly communicate with other neurons and organs by sending chemical and electrical signals.

Description

The nervous system contains two major types of cells, neurons and glia. Neurons are specialized cells of the central and peripheral nervous systems that play key roles in transmitting and propagating information from one neuron to another. The role of glial cells is less clear, but they are involved in supporting the functions of the neuron. There are many different types of neurons, such as motor neurons, sensory neurons, and interneurons. Each class of neuron is specially designed to perform certain functions, and therefore neuronal populations differ in structure and chemical composition. Most neurons are polarized, which means that fibers extend from the cell in a certain direction or orientation. Polarization is determined by the direction and length of structures unique to neurons, which are axons, dendrites, and the cell body.

Neurons are similar to other types of cells in that they contain all the basic cell organelles such as a nucleus, mitochondria, ribosomes, lysosomes, endoplasmic reticulum, and Golgi apparatus. However, neurons differentiate into polarized cells that contain three basic structural components: cell body (soma), axon, and dendrites. The cell body contains the nucleus and other cellular organelles and is the major place where protein synthesis occurs.

Dendrites are branched fibers extending from the cell body. The number and organization of dendrites is unique to each neural population and most neurons extend multiple dendrites that are relatively short processes. Dendrites contain small protrusions called spines. These spines express protein receptors on the surface that are capable of responding to chemical neurotransmitters such as acetylcholine. The dendritic spines contact axon terminals of other neurons at a connection point called a synapse. The dendrites send this chemical information to the cell body. The cell body integrates the chemical signal from all the dendrites and generates an electrical signal called an axon potential that is sent down the length of the axon to signal the next neuron.

The axon is a fiber neurite process that extends from the cell body and can be up to a meter in length. The axon protrudes from a bulge at the base of the cell body at a region called the axon hillock. Most neurons have only one axon but may have hundreds of dendrites. The axon is specially designed to send electrical signals known as action potentials down the length of the axon to the axon terminals. The axon terminal releases chemical neurotransmitters in response to the action potential onto the dendrite of another neuron. The place where the axon terminal of one neuron meets the dendrite of another neuron is called a synapse. The axon contains a cytoskeletal structure designed to transport proteins and other molecules down the length of the axon to the axon terminal and from the axon terminal back up to the cell body. This cytoskeletal structure is composed of actin filaments, neurofilaments, and microtubules.

Function

Neurons are specially designed to communicate with other neurons by converting chemical signals into electrical ones. This is accomplished by the axon. A covering called myelin insulates the outside of the axon. Myelin is a sheath of stacked membranes and is very high in lipid. Axon myleination is conduction by the glia, oligodendrocytes, and Schwann cells. There are periodic interruptions in the myelin at the nodes of Ranvier. Electrical signals referred to as action potentials are rapidly transmitted down the axon by jumping from one node of Ranvier to the next. The action potential induces the release of chemical neurotransmitters from the axon terminal. The axon terminal contains vesicles containing packaged neurotransmitters. The action potential triggers the release of neurotransmitters onto the next neuron that then generates an axon potential to propagate the signal for cell-cell communication. This process allows signaling to occur over very long distances within milliseconds.

Common diseases and disorders

Neurons are implicated in numerous nervous system diseases from Alzheimer's disease to Huntington's disease to certain types of brain cancer. In many neural diseases, neurons degenerate due to abnormalities in basic cellular function. Populations of neurons can also become cancerous, such as in neuroblastomas.

KEY TERMS

Axon— A fiber process extended from the neuronal cell body that carries action potentials.

Dendrite— A branch-like projection from the cell body of a neuron.

Synapse— Meeting place between the axon terminal of one neuron and the dendrite of another neuron.

Resources

BOOKS

Behbehani, Michael M. "Biology of Neurons." In Cell Physiology. Edited by Nicholas Sperelakis. San Diego, CA: Academic Press, 1998, pp. 429-434.

Zigmond, M.J., F.E. Bloom, S.C. Landis, J.L. Roberts, and L.R. Squire. Fundamental Neuroscience. San Diego, CA: Academic Press, 1999.