An interneuron is a type of nerve cell that sits between other neurons, relaying and modifying signals rather than directly sensing the environment or commanding muscles to move. While sensory neurons carry information inward and motor neurons carry commands outward, interneurons handle everything in between. They make up the vast majority of neurons in your brain and spinal cord, forming the complex circuits responsible for processing, decision-making, reflexes, and coordinated movement.
How Interneurons Fit Into the Nervous System
The simplest way to understand interneurons is to picture the three main types of neurons in the spinal cord: sensory, motor, and interneurons. Sensory neurons detect things like pain, temperature, and pressure, then send that information toward the central nervous system. Motor neurons carry signals from the central nervous system out to muscles. Interneurons connect these two, but they do far more than just pass messages along. They communicate with each other in layered circuits of varying complexity, filtering, amplifying, or suppressing signals before they reach their destination.
Unlike sensory neurons, which have long fibers stretching from your skin to your spinal cord, or motor neurons, which extend from the spinal cord to distant muscles, most interneurons keep their connections local. Their axons (the cable-like projections that transmit signals) tend to stay within a small region rather than projecting to faraway targets. This is why they’re sometimes called “local circuit neurons,” in contrast to projection neurons, which send their axons across long distances to specific brain regions.
What Interneurons Actually Do
Interneurons shape nearly every signal your nervous system processes. Some are inhibitory, meaning they quiet down or block activity in neighboring neurons. Others are excitatory, meaning they boost signals and help them spread. This balance between excitation and inhibition is fundamental to how your brain and spinal cord work. Roughly one-third of all synapses in the brain use the inhibitory chemical messenger GABA, and the majority of those synapses belong to interneurons. In the spinal cord, about half of inhibitory connections rely on a different chemical messenger called glycine, with GABA handling most of the rest.
Excitatory interneurons play an equally important role. In the spinal cord’s pain-processing region, for example, excitatory interneurons dominate sensory integration. They act as signal amplifiers and coincidence detectors, increasing their firing rate as stimulation intensifies and linking information across different layers of the spinal cord. This means interneurons don’t just relay pain signals; they actively shape how intense or widespread a pain sensation becomes before that information ever reaches your brain.
Interneurons in Reflexes and Movement
One of the clearest examples of interneurons at work is the withdrawal reflex. When you touch something hot, sensory neurons fire and connect to interneurons in the spinal cord. Those interneurons activate motor neurons to pull your hand away, but they simultaneously inhibit the motor neurons controlling the opposing muscle group. This process, called reciprocal inhibition, was first described by Sir Charles Sherrington in the early 1900s. It ensures that when one muscle contracts, its opposing muscle relaxes, so your movements are smooth rather than fighting against themselves.
A specialized type of spinal interneuron called the Renshaw cell provides a feedback loop for motor control. Renshaw cells receive signals from motor neurons and then send inhibitory signals back to those same motor neurons and their partners. Think of it as a volume knob: when a motor neuron fires, the Renshaw cell dials the signal back down, preventing excessive or runaway muscle contraction. This recurrent inhibition was first reported by Birdsey Renshaw in 1941 and remains one of the best-understood interneuron circuits in the body.
Types of Interneurons in the Brain
While spinal cord interneurons are relatively straightforward to classify, brain interneurons are far more diverse. In the cerebral cortex alone, researchers have identified dozens of distinct types based on their shape, firing patterns, and chemical markers. Two well-studied examples are basket cells and chandelier cells, both of which fire extremely rapidly (40 to 200 times per second) and are classified as “fast-spiking” interneurons.
Basket cells get their name from the basket-like shape their axon terminals form around the cell bodies of neighboring excitatory neurons. They target the main body and nearby branches of those neurons, giving them powerful control over whether the target cell fires or stays silent. Chandelier cells are more unusual. Their axon terminals hang in vertical clusters that resemble the arms of a chandelier, and they connect exclusively to a very specific spot on their target neurons: the point where outgoing signals originate. This gives chandelier cells a uniquely strategic position to control the output of entire columns of brain cells.
Where Interneurons Come From
During embryonic development, most cortical interneurons don’t actually originate in the cortex. They’re born in a deeper brain structure called the medial ganglionic eminence, then migrate long distances to reach their final positions in the cortex, hippocampus, striatum, and thalamus. This tangential migration pattern is highly conserved across species, from rodents to pigs to humans.
Progenitor cells in the medial ganglionic eminence are guided by a specific set of molecular signals, including transcription factors that act as identity tags. Depending on which signals they receive, these progenitors develop into different interneuron subtypes. The two major subtypes produced are those expressing parvalbumin (the fast-spiking type) and those expressing somatostatin. Both play distinct roles in cortical circuits once they arrive at their destination and wire themselves into existing networks.
What Happens When Interneurons Malfunction
Because interneurons are responsible for keeping the balance between excitation and inhibition, their dysfunction has far-reaching consequences. When inhibitory interneurons fail to do their job, the result is too much uncontrolled excitatory activity, which can manifest as seizures. This excitation-inhibition imbalance is a core feature of epilepsy.
In schizophrenia, the connection to interneurons is particularly well documented. Postmortem studies of people with schizophrenia have found reduced concentrations of GABA in the cortex, along with lower levels of the enzyme needed to produce it. Fast-spiking, parvalbumin-positive interneurons appear to be especially affected. These are the same cells responsible for synchronizing activity across widespread cortical circuits, a function critical for working memory, attention, and coherent thought. Researchers have found that selectively disrupting a specific receptor on cortical interneurons in mice produces a constellation of molecular, physiological, and behavioral changes that closely resemble human schizophrenia symptoms.
The vulnerability of these interneurons may partly explain why schizophrenia has so many different risk factors. Genetic predispositions, prenatal malnutrition, infections during pregnancy, and birth complications can all converge on the same outcome: damage to fast-spiking interneurons during critical developmental windows. Similar interneuron disruptions have been implicated in mood disorders and autism spectrum disorders, reinforcing how central these cells are to healthy brain function.