Through the use ofthe "reazione nera" (black reaction), the method discovered by Camillo Golgi (1843-1926) in 1873, he became the first histologist to propose the existence of two morphologically and physiologically different types of neurons in the nervous system: motor (type I) and sensory (type II) neurons. Type I neurons had long axons that left the gray matter (projection neurons), whereas type II neurons had short axons whose arbors remained near the parent cell and did not leave the gray matter (intrinsic neurons). Santiago Ramón y Cajal (1852-1934) argued that a number of situations arose in the nervous system that made this physiological distinction inaccurate. Furthermore, the axon of the type I motor neurons gave rise to axonal collaterals (local plexus) in their trajectory out of the site where the parent neurons were localized. That is, the axon of the motor neurons differed morphologically from those of the sensory type in its length. Thus, Cajal named Golgi's two types as cells with a long axon and cells with a short axon. Since then, the term short-axon cell has commonly been considered as synonymous with the term interneuron (DeFelipe 2002). However, because the axon of all neurons gives rise to local axonal arborization, referring to interneurons as "local circuit neurons" is incorrect.
The collective work of numerous laboratories has shown that in the cerebral cortex, two main classes of interneurons can be distinguished. These are commonly referred to as: spiny non-pyramidal or stellate cells and aspiny or sparsely spiny non-pyramidal cells. Spiny stellate cells are morphologically heterogeneous and are typically found in the middle cortical layers (especially layer IV). The axons of these cells branch within layer IV where their parent cell bodies are located, or in the layers above or below. These neurons form asymmetrical synapses with dendritic spines and shafts, and are considered to be excitatory. Aspiny or sparsely spiny non-pyramidal cells are highly varied in terms of their morphology, and they are found in all cortical layers. The axons of these neurons maintain their arbors near the parent cell, but they may additionally give rise to collaterals that course along a horizontal and/or vertical trajectory (descending and/or ascending). These aspiny or sparsely spiny non-pyramidal cells form symmetrical synapses with a variety of postsynaptic elements, but some specific morphological types can be grouped together due to their unique patterns of connectivity. These non-pyramidal cells constitute the majority of short-axon cells and approximately 15-30% ofthe total neuron population. Most of them appear to be GABAergic and thus inhibitory. In this chapter, we address certain anatomical, biochemical, and physiological characteristics of aspiny inter-neurons in the neocortex, which for simplicity we will refer to as interneurons.
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