Not every neuron projects to distant targets. Interneurons have relatively local axons and primarily influence other neurons within the same region. In the cortex and hippocampus the majority of interneurons release GABA and provide the inhibition that balances excitation.

Interneurons are morphologically and physiologically diverse. Some target the cell bodies of pyramidal cells and control timing with high precision; others target dendrites and regulate input selection; still others form networks among themselves that generate rhythmic activity. This diversity allows fine control of circuit dynamics.

Developmental disorders, epilepsy, and schizophrenia have all been linked to interneuron dysfunction. Because interneurons shape the output of the principal cells that carry information between regions, their proper function is essential for nearly every neural computation. Identifying interneuron subtypes by molecular markers has accelerated research into their specific contributions.

• Local-circuit neurons
• Most are inhibitory in the adult cortex
• Highly diverse in form and function
• Critical for balancing excitation and generating rhythms