It is generally considered that the basic cortical microcircuit is composed of a pyramidal cell and its input-output connections. The inhibitory inputs in these circuits mostly originate from GABAergic interneurons, whose terminations contact with the dendrites, soma, and initial axon segment of pyramidal cells. There is a great diversity of morphological types of interneurons, based on the morphology of their somata, dendritic, and axonal arborization patterns. Interneurons do not necessarily form synapses exclusively with other particular types of neurons, nor are their synapses restricted to pyramidal or non-pyramidal cells, or a single postsynaptic region. However, they do generally show a preference for certain postsynaptic partners. Most interneurons contain GABA, as well as a variety of other neurotransmitters, neuroactive peptides, and calcium-binding proteins. Indeed, one ofthe main conclusions from the collective work of a number of laboratories is that different types of interneurons are characterized by a particular combination of molecular, physiological, and synaptic features. These features enable the different interneuron subtypes to contribute to the generation of synchronized network activity thereby shaping principal cell behavior. Finally, it has become apparent that not all types of interneurons are found in all species and that molecular characteristics of interneurons may differ between species. Therefore, caution should be taken when extrapolating data on interneuronal circuitry from one species to another.

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