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Neurons are the excitable cells of the brain that function by communicating with other neurons and interneurons (via synapses), in neural circuits and larger brain networks. The two main neuronal classes in the cerebral cortex are excitatory projection neurons (around 70-80%) and inhibitory interneurons (around 20–30%). [2]
Projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord. In human neuroanatomy, bundles of axons (nerve fibers) called nerve tracts , within the brain, can be categorized by their function into association tracts , projection tracts , and commissural tracts .
Local interneurons have short axons and form circuits with nearby neurons to analyze small pieces of information. [5] Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions. [5] However, interneurons are generally considered to operate mainly within local brain areas. [6] The ...
Each of these interneuron class can be further divided into diverse subpopulations of neurons with distinct neurotransmitter phenotype, axonal projection and function during locomotion. [38] For example, V2 interneurons are ipsilaterally projecting that can be further classified as excitatory V2a and inhibitory V2b.
Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells, which, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the ...
Afferent neurons are pseudounipolar neurons that have a single process leaving the cell body dividing into two branches: the long one towards the sensory organ, and the short one toward the central nervous system (e.g. spinal cord). These cells do have sensory afferent dendrites, similar to those typically inherent in neurons. [1]
Betz cells are not the sole source of direct connections to those neurons because most of the direct corticomotorneuronal cells are medium or small neurons. [3] While Betz cells have one apical dendrite typical of pyramidal neurons, they have more primary dendritic shafts, which can branch out at almost any point from the soma (cell body). [4]
Association fibers are axons (nerve fibers) that connect cortical areas within the same cerebral hemisphere. [1]In human neuroanatomy, axons within the brain, can be categorized on the basis of their course and connections as association fibers, projection fibers, and commissural fibers. [1]