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Interneurons are nerve cells located in the central nervous system that act as “go-betweens” for sensory and motor neurons. They receive and process information, and can either send a signal to a motor neuron for action or transmit the impulse to the brain for further processing. Interneurons come in a variety of forms and produce inhibitory signals that modulate electrical stimuli between afferent and efferent nerve cells. They play important roles in higher brain functions such as memory, perception, and emotion.
An interneuron, also known as a paired neuron, is a neuron, or nerve cell, located entirely within the central nervous system that conducts signals between other nerve cells. The central nervous system (CNS) is made up of nerve cells within the brain and spinal cord, as opposed to the peripheral nervous system, which is all system outside these areas. An interneuron acts as a “go-between” between afferent, or sensory, neurons that receive signals from the peripheral nervous system, and efferent, or motor, neurons that transmit signals from the brain. It also connects to other interneurons, allowing them to communicate with each other.
Neuron structure
A neuron is a type of cell specialized for receiving and transmitting nerve impulses. It has two types of extension extending from the main body, or soma. Dendrites are branching projections that usually receive information via electrochemical signals from the axon of another neuron; however, they can also send certain types of signals. The axon is another long extension of the soma, more like a cable, that transfers information from the cell body. All nerve cells have an axon, a cell body, and one or more dendrites.
Interneurons are multipolar nerve cells, which means they have more than one dendrite. Although they are found throughout the brain, each is confined to a particular region—they don’t connect different parts of the brain to each other. They come in a much greater variety of forms than afferent or efferent nerve cells, but, as of 2013, there is no standard method for classifying them into types.
How neurons work
Signals carried to the central nervous system via afferent neurons convey information about sensations felt on or within the body, such as visual and auditory stimuli, pressure, and pain. Efferent neurons, in contrast, send signals from the central nervous system to the body. For example, if a person touches a hot stove with their hand, afferent nerve cells will carry sensory impulses to the central nervous system, registering pain. After processing the impulse, the central nervous system sends a message to the body via efferent nerve cells to move the hand.
A nerve impulse occurs when a sensory receptor causes the nerve’s normal negative electrical charge, or resting potential, to become positive. This charge change is called depolarization. If the depolarization reaches a certain level, an action potential is created. This travels along the nerve cell to the synapse, or space, between the end of the axon and another cell’s dendrite. The positive charge at the end of the axon causes a series of reactions that allow “messenger” chemicals called neurotransmitters to enter the synapse and bind to receptors on the dendrite of the neighboring neuron. If this nerve cell is an interneuron, then it will have to decide what to do with the information it receives.
This type of signal is called excitatory as it causes the receiving nerve cell to generate an impulse. It usually involves chemicals called glutamates. The opposite type of signal is called inhibitory as it acts to suppress an impulse by generating a negative electrical charge in the receiving nerve. These signals generally involve the neurotransmitter gamma-amino butyric acid (GABA). The behavior of interneurons is most commonly inhibitory.
The role of interneurons
This type of nerve cell can be stimulated by an efferent or afferent neuron, or by another interneuron. It can receive information from the external or internal environment of the body and transmit it to the brain for further processing, or it can process the information itself and send a signal to a motor neuron to take action. In the latter case, it 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 about how to react.
In the above example of someone touching a hot stove, the interneuron processes information from the sensory nerve cell itself and allows a signal to pass to a motor neuron to take action. This is called a spinal reflex. Other signals, however, may require a higher brain scan and are sent from afferent neurons to one or more interneurons, which transmit the impulse to the brain. In this case, the brain is considered the center of integration.
As of 2013, the various functions of interneurons are an active area of research and much remains to be learned. The inhibitory signals they produce may serve to modulate electrical stimuli between afferent and efferent nerve cells, but they also appear to play many other essential roles. Large groups of different types of these nerve cells appear to interact in complex ways that are important for higher brain functions such as memory, perception and emotion.
