Organization of the central nervous system

In the brain, the gray matter forms an outer covering or cortex; the white matter forms an inner core or medulla.

The cortex of gray matter in the brain contains nerve cell, bodies, axons, dendrites, and glial cells and in the site of synapses.

In addition to the gray matter of the cortex, islands of gray matter, nuclei, are found in the deep portions of the cerebrum and cerebellum.

The white matter contains only axons of nerve cell plus the associated glial cells and blood vessels.

Whereas many of the axons going to or coming from a specific location are grouped into bundles called tracts.

Cells of the gray matter

Each functional region of the gray matter has a characteristic variety of cell bodies associated with a meshwork of axonal, dendritic, and glial processes.

Cerebral cortex layers

The cerebral cortex is described as containing six distinguishable layers. No sharp boundaries separate these layers, rather, they are distinguished on the basis of predominant cell type and fiber (axons and dendrites) arrangement. From outside (pial surface) to inside (white matter) of the cerebral cortex these layers are:

The plexiform or molecular layer I consists largely of fibers, most of which travel parallel to the surface (extensions of apical dendrites and horizontally-oriented axons), and relatively few cells, such as some Cajal-Retzius and spiny stellate neurons

The external granular layer II, which contains small pyramidal neurons and numerous stellate neurons

The external pyramidal layer III, is not sharply demarcated from layer II and contains predominantly small and medium-size pyramidal neurons, as well as non-pyramidal neurons with vertically-oriented intracortical axons;

The internal granular layer IV, which contains different types of stellate (granule) and pyramidal neurons, and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents

The internal pyramidal layer V, which contains large pyramidal neurons (such as the Betz cells in the primary motor cortex); it is the principal source of subcortical efferents

The layer of polymorphic cells (the multiform layer) VI contains cells with diverse shapes, many of which have a spindle or fusiform shape. These cells are called fusiform cells. layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus

Figure 1. Simple scheme of intracortical circuits: a, cortical afferent fibers; and b, cortical efferent fibers.

Three cell types have been referred to above: pyramidal cells, stellate cells (or granule cells), and fusiform cells.

Two other cell types are the horizontal cells of Cajal, which are present only in layer I and send their processes laterally, and cells of Martinotti, which send their axons toward the surface (opposite to that of pyramidal cells).

It is important to note that the cortical layers are not simply stacked one over the other; there exist characteristic connections between different layers and neuronal types, which span all the thickness of the cortex. The basic functional and structural unit of cortex is called cortical column or cortical module. A cortical column is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields.

Cortical areas that lack a layer IV are called agranular. Cortical areas that have only a rudimentary layer IV are called dysgranular.

Cerebellum

There are three layers to the cerebellar cortex; from outer to inner layer, these are the molecular, Purkinje, and granular layers.

Molecular layer

This outermost layer of the cerebellar cortex contains two types of inhibitory interneurons: the stellate and basket cells. It also contains the dendritic arbors of Purkinje neurons and parallel fiber tracts from the granule cells. Both stellate and basket cells form synapses onto Purkinje cell dendrites.

Purkinje layer

The middle layer contains only one type of cell body—that of the large Purkinje cell. It is extremely large flask-shaped cell bodies. These cells are characteristic of the cerebellum. Purkinje cells are the primary integrative neurons of the cerebellar cortex. Purkinje cell dendrites are large arbors with hundreds of branches reaching up into the molecular layer. These dendritic arbors are flat—nearly all of them lie in planes—with neighboring Purkinje arbors in parallel planes. Each parallel fiber from the granule cells runs orthogonally through these arbors, like a wire passing through many layers. Purkinje neurons have inhibitory synapses—with the neurons of the deep cerebellar and vestibular nuclei in the brainstem. Each Purkinje cell receives excitatory input from 100,000 to 200,000 parallel fibers. Parallel fibers are said to be responsible for the simple spiking of the Purkinje cell.

Purkinje cells also receive input from the inferior olivary nucleus via climbing fibers. Each Purkinje cell receives input from exactly one climbing fiber; but this single fiber "climbs" the dendrites of the Purkinje cell, winding around them and making a large number of synapses as it goes. The net input is so strong that a single action potential from a climbing fiber is capable of producing a "complex spike" in the Purkinje cell: a burst of several spikes in a row, with diminishing amplitude, followed by a pause during which simple spikes are suppressed.

Figure 2. Cytoarchitecture of the cerebellar cortex a, granule cell; b, Purkinje cell; c, basket cell; d, stellate cell; e, Golgi cell; f, mossy fiber; and g climbing fiber

G ranular layer

The innermost layer contains the cell bodies of two types of cells: the numerous and tiny granule cells, and the larger Golgi cells. Incoming (mossy) fibers enter the granular layer. They contact granule cells in the lightly stained areas called gromeruli.These fibers form excitatory synapses with the granule cells and the cells of the deep cerebellar nuclei. The granule cells send their T-shaped axons—known as parallel fibers—up into the superficial molecular layer, where they form hundreds of thousands of synapses with Purkinje cell dendrites. The human cerebellum contains on the order of 60 to 80 billion granule cells, making this single cell type by far the most numerous neuron in the brain (roughly 70% of all neurons in the brain and spinal cord, combined). Golgi cells provide inhibitory feedback to granule cells, forming a synapse with them and projecting an axon into the molecular layer.

The function of the cerebellar cortex is essentially to modulate information flowing through the deep nuclei. Mossy and climbing fibers carry sensorimotor information into the deep nuclei, which in turn pass it on to various premotor areas, thus regulating the gain and timing of motor actions. Mossy and climbing fibers also feed this information into the cerebellar cortex, which performs various computations, resulting in the regulation of Purkinje cell firing. Purkinje neurons feed back into the deep nuclei via a potent inhibitory synapse. This synapse regulates the extent to which mossy and climbing fibers activate the deep nuclei, and thus control the ultimate effect of the cerebellum on motor function. The synaptic strength of almost every synapse in the cerebellar cortex has been shown to undergo synaptic plasticity. This allows the circuitry of the cerebellar cortex to continuously adjust and fine-tune the output of the cerebellum, forming the basis of some types of motor learning and coordination. Each layer in the cerebellar cortex contains the various cell types that comprise this circuitry.