Classically, neurons between a sensory neuron and a motoneuron; more commonly, neurons with short axons and part of local neural circuits. Found exclusively in the brain and spinal cord and sometimes referred to as association (or connection) neurons, it has been estimated that the human brain contains 100 billion (1011) interneurons averaging 1000 synapses on each (i.e., some 1014 connections). They often contain neuromodulators that alter the direct excitatory or inhibitory connections of other cells. Most CNS neurons are interneurons; the only ones that are not are the sensory neurons and the motoneurons that drive the muscles. An interneuron receives input from about 2,000-10,000 ‘upstream’ neurons and transmits its output to a similar number of ‘downstream’ neurons, occasionally even one of its own input neurons, creating a loop. The term ‘interneuron’ was originally introduced to denote cells that mediated between input and output neurons in invertebrates. With the introduction of the concept of ‘synaptic inhibition’ by John C. Eccles (1903-1997) in 1964, it came to stand for a unifying principle that inhibitory cells with short axons have an important role in regulating the excitability of local neuronal circuits in contrast to excitatory cells with long axons that project to distant brain regions. In contemporary neurosciences, it is recognized that the term ‘interneuron’ contains a great diversity of structural and functional types, and so there are ongoing attempts to devise schemes of interneuron classification (e.g., with regard to the hippocampus) that capture the cellular diversity of interneurons and their functional roles in neuronal circuits.
See Basket cells, Central pattern generator, Cerebellar cortex, Corticospinal tract (CST), Direct corticomotoneuronal Dorsal commissural interne Golgi tendon organ, Golgi type II cells, Granule cells, Grasp response, Hippocampus, Molecular layer, Monosynaptic and polysynaptic reflexes, Motoneuron, Motor cortex, Mossy fibers, Neuron, Purkinje cells, Synapse