What Is an Interneuron? Structure, Function, and Role in the Nervous System

An interneuron is a neuron that talks mainly to other neurons inside the central nervous system, the brain and spinal cord. Sensory neurons carry information in, and motor neurons carry commands out to muscles, but interneurons sit in between. They receive, process, and pass on information among sensory neurons, motor neurons, and other interneurons. They get less attention than their better-known cousins, yet they make up the vast majority of neurons in the central nervous system, and reflexes, movement, sensation, learning, memory, and thinking all depend on them.

The neurons in the middle

Neurons are often sorted into three functional classes: sensory, or afferent, neurons; motor, or efferent, neurons; and interneurons. Interneurons generally don’t connect directly with sensory receptors or muscles. Instead, they form the networks that process information inside the brain and spinal cord. They have also been called association neurons or relay neurons, although interneuron is the preferred term today.

If sensory neurons are reporters bringing in news and motor neurons are messengers carrying out orders, interneurons are the newsroom, where information is compared, edited, and turned into a decision. The analogy is imperfect, because there is no single editor in charge, but it captures why interneurons are far more than relays.

Interneurons are found almost entirely within the central nervous system, including the cerebral cortex, basal ganglia, thalamus, cerebellum, brainstem, and spinal cord. They are not spread throughout the peripheral nervous system. As the most abundant neuron type in the central nervous system, they form the complex circuits responsible for information processing.

How they shape a signal

An interneuron receives signals from one or more neurons, combines them, and sends signals on to others. In a simple pathway, a sensory receptor activates a sensory neuron, which passes the signal to an interneuron, which activates a motor neuron that moves a muscle. Most real circuits are more elaborate, with many interconnected interneurons that analyze, modify, and coordinate information before any output is produced. Interneurons don’t just pass messages along; they integrate, amplify, suppress, and coordinate neural activity throughout the central nervous system.

Their roles show up at every level. By combining signals from many neurons, interneurons let the central nervous system compare different sources of information before responding. Many spinal reflexes use them. In the withdrawal reflex, for example, interneurons quickly activate flexor muscles to pull a limb away while at the same time inhibiting the opposing extensor muscles. Not every reflex has one, though. The monosynaptic stretch reflex, such as the knee-jerk reflex, is a well-established exception, with the sensory neuron connecting directly to the alpha motor neuron.

Interneurons also coordinate movement by regulating the timing and strength of motor neuron firing, which is essential for smooth action. In the brain, they take part in circuits for learning, memory, attention, and decision-making, helping organize processing across large networks.

Turning activity up or down

Interneurons can either increase or decrease the activity of other neurons. Excitatory interneurons make another neuron more likely to fire an action potential, and the main excitatory neurotransmitter is glutamate. Inhibitory interneurons reduce neuronal activity, and the main inhibitory neurotransmitter in the central nervous system is GABA, or gamma-aminobutyric acid. Inhibitory interneurons are essential for preventing excessive activity and keeping circuits balanced, much like brakes that make a fast car safe to drive.

Without interneurons, the nervous system could not process information efficiently. They allow the brain and spinal cord to interpret sensory information, coordinate movement, produce reflexes, regulate muscle activity, support learning and memory, and balance excitation and inhibition. Nearly every higher brain function depends on interneuron circuits.

When interneurons don’t work properly

Abnormal interneuron function has been implicated in several neurological and neuropsychiatric conditions, including epilepsy, schizophrenia, autism spectrum disorder, Parkinson’s disease, where interneuron circuits in the basal ganglia are altered, and Huntington’s disease. These conditions involve different mechanisms, and interneuron dysfunction is only one part of their complex biology.

This article is educational background and does not replace medical evaluation.

Related: What Is a Sensory Neuron? · What Is a Motor Neuron? · What Is a Reflex Arc? · What Is a Synapse? · What Are Neurotransmitters?

References

1. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2020.
2. Caire MJ, Reddy V, Varacallo MA. Physiology, Synapse. StatPearls. Updated March 27, 2023.
3. Neuroanatomy, Spinal Cord Myotatic Reflex. StatPearls. NCBI Bookshelf.