An interneuron, also known as an associated neuron, is a neuron located entirely within the central nervous system that conducts signals between neurons. The central nervous system consists of nerve cells within the brain and spinal cord, as opposed to the peripheral nervous system, which is completely composed of nerve cells outside the spinal cord and brain. An interneuron acts as a middle-man between neurons, allowing efferent neurons, afferent neurons, and other interneurons to communicate with one another.

A neuron is a type of cell specialized to receive and transmit nerve impulses in the nervous system. The information brought to the central nervous system through afferent, or sensory, neurons relays information about sensations experienced on or within the body, such as pressure, pain, and temperature, amongst other sensations. Efferent neurons, conversely, take signals from the central nervous system out into the body. For example, if a person touches her hand to a hot stove, afferent neurons will carry sensory impulses from the hand to the central nervous system, registering pain. After processing the impulse, the central nervous system sends a message back to the body through efferent motor neurons to move the hand.

An interneuron is a multipolar neuron, or a neuron with more than one dendrite. Dendrites are branched projections from the main body, or soma, of the nerve cell. Dendrites usually receive information via electrochemical signals from the axon of another neuron, but they can also send out certain types of signals. The axon is another, more cable-like, long proj,ection from the soma that takes information away from the cell body. All neurons have one axon, a cell body, and one or more dendrites.

A nerve impulse occurs when a sensory receptor causes the normal negative electrical charge, or resting potential, of the nerve to become positive. This positive change in charge is called a depolarization. If the depolarization reaches a certain level, an action potential is created. The action potential travels along the nerve cell and eventually reaches the synapse, or the gap between the end of one cell's axon and another cell's dendrite. The positive charge at the end of the axon causes a series of reactions that allow little messengers called neurotransmitters to enter the synapse and bind to receptors on the dendrite of the neighboring neuron. If the neighboring neuron is an interneuron, it will then have to decide what to do with the information received.

An interneuron may be stimulated by an efferent neuron, an afferent neuron, or another interneuron. The interneuron may receive information from the body’s outside or inside environment and pass it along to the brain for further processing, or it may process the information itself and send a signal to a motor neuron to act. In the latter instance, the interneuron is considered the integration center, or the place in the central nervous system where information from the environment is processed and a decision is made on how to react.

In the previous example of someone touching her hand to a stove, the interneuron processes the information from the sensory neuron itself and sends a signal to a motor neuron to take action. This is called a spinal reflex. Other signals, however, may require higher brain analysis and are sent from afferent neurons to one or more interneurons, which pass the impulse along to the brain. In this case, the brain is considered the integration center.