The nervous system is the body’s complex communication network, and the neuron is the fundamental cell responsible for transmitting information. The interneuron acts as a specialized intermediary in processing information. These cells reside exclusively within the central nervous system (CNS), meaning they are found only in the brain and the spinal cord. Their primary role involves neither sensing the external environment nor directly controlling muscles, but rather connecting and integrating signals between other neurons.
Defining the Local Circuit Neuron
Interneurons differ structurally and functionally from the two other main types of neurons: sensory and motor neurons. Sensory (afferent) neurons carry information from the body’s periphery toward the central nervous system, while motor (efferent) neurons transmit signals away from the CNS to muscles and glands. Interneurons act as a link between these two pathways.
The term “local circuit neuron” describes a key characteristic: their axons are typically short. These short axons connect nearby neurons to form intricate local circuits within a specific brain region, allowing them to analyze and process small pieces of information. Interneurons are the most numerous class of neurons, accounting for approximately 99% of all neurons in the human nervous system.
The Role of Integration and Modulation
The central function of the interneuron is to integrate and modulate neural signals, acting as a computational hub within the nervous system. Interneurons receive input from multiple sources, including sensory neurons and other interneurons. They then determine the ultimate output sent to motor neurons or other processing centers. This process is crucial for complex functions such as learning, decision-making, and simple reflex arcs.
A large portion of this modulatory role involves regulating the balance between excitation and inhibition in neural circuits. Inhibitory interneurons release neurotransmitters that dampen the activity of their target cells, preventing them from firing an electrical signal. This inhibitory control prevents runaway excitation, which could otherwise lead to uncontrolled neural activity. Inhibitory interneurons ensure that neural networks are stable and only fire signals when appropriate.
Interneurons organize neural activity through complex circuit patterns, such as feedforward and feedback inhibition. In feedforward inhibition, an excitatory input simultaneously activates a principal neuron and an inhibitory interneuron, where the interneuron then suppresses the activity of the principal neuron. Feedback inhibition occurs when a principal neuron, after being excited, activates an inhibitory interneuron that then loops back to suppress the activity of the original principal neuron. These intricate arrangements allow the nervous system to precisely sculpt the timing and magnitude of electrical signals.
Diverse Classes of Interneurons
Interneurons are highly diverse and are primarily classified based on the chemical signals they use. The major functional classification is based on their neurotransmitter identity, which dictates whether they have an excitatory or inhibitory effect on their target cells. The most prominent class is the inhibitory interneuron, which typically uses the neurotransmitter gamma-aminobutyric acid (GABA). GABAergic interneurons are responsible for the vast majority of inhibition in the brain, regulating network excitability. Conversely, a smaller population of interneurons utilizes the excitatory neurotransmitter glutamate, which can increase the probability of a target neuron firing an action potential.
Beyond chemical identity, interneurons are also categorized by their morphology, which refers to their shape and where they connect to other cells. Examples include basket cells, which form a dense network of connections around the cell body of their targets, and chandelier cells, which uniquely target the axon initial segment—the point where the action potential is generated. Other subtypes are identified by the proteins they express, such as parvalbumin (PV) or somatostatin (SST), each specializing in targeting different parts of the principal neuron.
Interneuron Dysfunction and Neurological Health
The proper function of interneurons is important to neurological health, and their dysfunction is implicated in numerous disorders. When the balance between excitation and inhibition is disrupted, particularly due to the failure of inhibitory interneurons, the result is often neural hyperexcitability. This can lead to uncontrolled and excessive electrical activity within the brain.
Defects in inhibitory interneuron development or function are linked to conditions characterized by aberrant neural activity, such as epilepsy. In this disorder, the failure of GABAergic interneurons to sufficiently dampen signals contributes to synchronized, excessive firing that results in seizures. Research suggests that subtle perturbations in interneuron circuitry are a factor in neuropsychiatric conditions like schizophrenia and autism spectrum disorder. The parvalbumin-expressing interneurons, in particular, have been identified as vulnerable cells whose impairment may contribute to the cognitive and sensory processing deficits seen in these conditions.