The brain contains many billions of neurons that work together to produce sensation, thought, learning, movement, emotion, and many other processes. The coordination of these activities requires rapid and extensive communication among individual neurons and tissues (e.g. muscles). In order to achieve this, neurons use electrical signals to transmit information within a single cell and chemical signals between cells. These unique functions have forced the neuron to adopt a cell structure unlike that of other cells.
Neurons comprise a cell body (or soma), dendrites, and an axon that ends at a terminal. The cell body contains the nucleus and the machinery necessary to synthesize proteins. The cell body is also the region of the neuron in which an electrical impulse is generated. Extending from the cell body are short, branched dendrites which receive chemical signals from other neurons or stimuli that initiate an electrical signal. This electrical impulse (or action potential) propagates from the cell body, along the axon toward its terminal. The axon is an elongated fiber that transmits the impulse by altering the flow of sodium and potassium ions across the neuronal membrane. Many axons are surrounded by a myelin sheath composed of lipids and proteins. Like insulation coating an electrical wire, this fatty layer greatly increases the speed of electrical impulses down the axon.
Though the nerve terminal of one neuron is in close proximity to the dendrites of an adjacent cell, the cells are actually separated by a small space; this connection between the two cells is called a synapse. The synapse represents a true gap between cells; there is no sharing of cytoplasm or cell structures between the pre-synaptic and post-synaptic cells. Communication between neurons is a chemical process that uses neurotransmitters in a process called synaptic transmission.
The neuron consists of a cell body, dendrites, and an axon. Information flows from the dendrites to the cell body, and then on down the axon to its terminal.
When an electrical impulse travels down the axon to the nerve terminals, it triggers the movement of vesicles in the terminal to release their contents, chemicals known as neurotransmitters. After release, the neurotransmitters diffuse across the synaptic space and bind to receptors on the dendrites of post-synaptic cells. The binding of a neurotransmitter to its receptor is specific. Just as a key fits only a certain lock, a neurotransmitter binds only to a certain type of receptor.
There are many types of neurotransmitters in the brain, each having a unique function. The interaction between the receptor and the neurotransmitter produces chemical and/or electrical changes in the post-synaptic cell depending on the exact neurotransmitter bound. Excitatory neurotransmitters promote the propagation of the electrical signal in the receiving cell whereas inhibitory neurotransmitters dampen the transmission of the electrical signal. If the neurotransmitter triggers an action potential in the post-synaptic neuron, the communication process continues. Just a fraction of a second after binding to their receptors, neurotransmitters may be broken down by enzymes or recycled back into the pre-synaptic cell.
An example of neurotransmission is shown for the neurotransmitter acetylcholine binding to acetylcholine receptors. Used with permission from “Animated Neuroscience and the Actions of Nicotine, Cocaine, and Marijuana in the Brain” (www.films.com)