What Is an Interneuron? Structure, Function, and Role in the Nervous System
What Is an Interneuron?
Introduction
When you touch a hot object, your hand pulls away almost instantly. When you solve a math problem, recognize a familiar face, or learn a new skill, billions of neurons work together to process information.
The neurons responsible for most of this information processing are interneurons.
Unlike sensory neurons, which carry information into the central nervous system, or motor neurons, which send commands to muscles, interneurons communicate with other neurons. They integrate incoming information, make neural circuits possible, and coordinate appropriate responses.
Although they receive less attention than sensory or motor neurons, interneurons make up the vast majority of neurons within the central nervous system and are essential for nearly every brain function.
Quick Answer
An interneuron is a neuron that communicates with other neurons within the central nervous system (CNS). Interneurons receive, process, and transmit information between sensory neurons, motor neurons, and other interneurons, making them essential for reflexes, movement, sensation, learning, memory, and cognition.
What Is an Interneuron?
Interneurons are one of the three major functional classes of neurons.
Unlike sensory neurons (afferent neurons) and motor neurons (efferent neurons), interneurons generally do not connect directly with sensory receptors or muscles.
Instead, they form networks that process information inside the brain and spinal cord.
Because of this role, interneurons are often called association neurons or relay neurons, although the term interneuron is preferred in modern neuroscience.
Where Are Interneurons Found?
Interneurons are located almost entirely within the central nervous system, including:
- Cerebral cortex
- Basal ganglia
- Thalamus
- Cerebellum
- Brainstem
- Spinal cord
They are the most abundant neuronal type in the CNS and form the complex neural circuits responsible for information processing.
How Interneurons Work
Interneurons receive signals from one or more neurons, integrate that information, and send signals to other neurons.
A simplified pathway is:
Sensory Receptor
↓
Sensory Neuron
↓
Interneuron
↓
Motor Neuron
↓
Muscle
However, many neural circuits involve multiple interconnected interneurons, allowing the nervous system to analyze, modify, and coordinate information before generating an output.
Functions of Interneurons
Interneurons perform several essential functions.
Information Integration
Interneurons combine signals arriving from multiple neurons.
This allows the central nervous system to compare different sources of information before producing an appropriate response.
Reflex Processing
Many spinal reflexes involve interneurons that connect sensory neurons to motor neurons.
For example, the withdrawal reflex uses interneurons to rapidly activate flexor muscles while simultaneously inhibiting extensor muscles.
Some reflexes, such as the monosynaptic stretch reflex (patellar reflex), are an important exception because they do not include an interneuron between the sensory neuron and the alpha motor neuron.
Motor Coordination
Interneurons help coordinate muscle activity by regulating the timing and strength of motor neuron firing.
They are essential for smooth, coordinated movement.
Learning and Memory
Within the brain, interneurons participate in neural circuits involved in learning, memory, attention, and decision-making.
They help organize information processing across large neuronal networks.
Excitatory and Inhibitory Interneurons
Interneurons can either increase or decrease the activity of other neurons.
Excitatory Interneurons
Excitatory interneurons increase the likelihood that another neuron will generate an action potential.
The principal excitatory neurotransmitter is glutamate.
Inhibitory Interneurons
Inhibitory interneurons reduce neuronal activity.
The principal inhibitory neurotransmitter in the central nervous system is GABA (gamma-aminobutyric acid).
Inhibitory interneurons are essential for preventing excessive neuronal activity and maintaining balanced neural circuits.
Why Interneurons Are Important
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
- Balance excitation and inhibition within neural networks
Nearly every higher brain function depends on interneuronal circuits.
Clinical Relevance
Abnormal interneuron function has been implicated in several neurological and neuropsychiatric disorders.
Examples include:
- Epilepsy
- Schizophrenia
- Autism spectrum disorder
- Parkinson disease (altered basal ganglia interneuronal circuits)
- Huntington disease
These disorders involve different mechanisms, and interneuron dysfunction represents only one component of their complex pathophysiology.
Common Misunderstandings
“Every reflex contains an interneuron.”
Incorrect.
Many reflexes involve interneurons, but the monosynaptic stretch reflex is a well-established exception.
“Interneurons only relay information.”
Not entirely.
Interneurons do much more than relay signals. They integrate, modify, amplify, suppress, and coordinate neural activity throughout the central nervous system.
“Interneurons are found throughout the peripheral nervous system.”
Incorrect.
Interneurons are located almost exclusively within the central nervous system.
Key Takeaways
- Interneurons communicate with other neurons within the central nervous system.
- They connect sensory neurons, motor neurons, and other interneurons.
- Most information processing in the brain and spinal cord depends on interneurons.
- Interneurons may be excitatory or inhibitory.
- They play essential roles in reflexes, movement, learning, memory, and cognition.
Related Articles
- What Is a Sensory Neuron?
- What Is a Motor Neuron?
- What Is a Reflex Arc?
- What Is a Synapse?
- What Are Neurotransmitters?