Granular layer

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

Granular 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.

Deep cerebellar nuclei

The deep nuclei of the cerebellum act as the main centers of communication, and the four different nuclei of the cerebellum (dentate, interpositus, fastigial, and vestibular) receive and send information to specific parts of the brain. In addition, these nuclei receive both inhibitory and excitatory signals from other parts of the brain which in turn affect the nucleus's outgoing signals.

Together, molecular, Purkinje cells and granular layers constitute the cortex of the cerebellum. Deep in the granular layer is the white matter. As in the cerebrum, it contains nerve fibers, supporting neuroglial cells, and small blood vessels, but no neural cell bodies. The fibrous cover on the cerebellar surface is the pia mater. Cerebellar blood vessels travel in this layer.

Blood brain barrier

Figure 3. Schematic drawing of blood brain barrier

More than 100 years ago it was discovered that if blue dye was injected into the bloodstream of an animal, that tissues of the whole body EXCEPT the brain and spinal cord would turn blue. To explain this, scientists thought that a "Blood-Brain-Barrier" (BBB) which prevents materials from the blood from entering the brain existed. More recently, scientists have discovered much more about the structure and function of the BBB

Дата добавления: 2015-11-14