FUNCTIONAL ANATOMY OF CEREBELLUM

 

FUNCTIONAL ANATOMY OF CEREBELLUM

Cerebellum is made up of outer gray matter or cerebellar cortex and an inner white matter. White matter is formed by afferent and efferent nerve fibers of cerebellum. Gray masses called cerebellar nuclei are located within the white matter.

GRAY MATTER

Gray matter or cerebellar cortex is made up of structures arranged in three layers. Each layer of gray matter is uniform in structure and thickness, throughout the cerebellum.

Layers of gray matter:

1. Outer molecular or plexiform layer

2. Intermediate Purkinje layer

3. Inner granular layer.

Molecular or Plexiform Layer

Molecular or plexiform layer is the outermost layer of cortex having the cells arranged in two strata. Superficial stratum contains few star-shaped cells known as stellate cells. Deep stratum contains basket cells. In addition to stellate and basket cells, the molecular layer contains

the following structures:

i. Parallel fibers, which are the axons of granule cells, present in granular layer

ii. Terminal portions of climbing fibers (afferents from medulla)

iii. Dendrites of Purkinje cells and Golgi cells.

Cell junctions in molecular layer

Molecular layer contains the following cellular junctions:

i. Dendrites of stellate cells and basket cells synapse with parallel fibers, which are the axons

of granule cells

ii. Axons of stellate cells end on the dendrites of Purkinje cells. However, the axon of basket cell

descends down into the Purkinje layer and forms the transverse fiber, that ends on the soma of

Purkinje cells.

iii. Dendrites of Purkinje cells synapse with climbing fibers and parallel fibers

iv. Dendrites of Golgi cells situated in inner granular layer enter the molecular layer and end on

parallel fibers.

2. Purkinje Layer

Purkinje layer is situated in between outer molecular layer and inner granular layer. It is the thinnest layer, having a single layer of flask-shaped Purkinje cells. Purkinje cells are the largest neurons in the body. Dendrites of these cells ascend through the entire thickness of molecular layer and arborize there. These dendrites terminate either on climbing fibers or the parallel fibers. Axons of the basket cells form the transverse fibers, which descend down and end on the soma of Purkinje cells. Axons of Purkinje cells descend into the white matter and terminate on the cerebellar nuclei and vestibular

nuclei via cerebellovestibular tract. Purkinje cells are termed as ‘final common path’ of cerebellar cortex. It is because the impulses fromdifferent parts of cerebellar cortex are transmitted to other parts of brain only through Purkinje cells.

3. Granular Layer

Granular layer is the innermost layer of cerebellar gray matter and it is in between Purkinje layer and the

cerebellar white matter. It is formed by interneurons called granule cells and Golgi cells. Total number of

interneurons in this layer is about half the number of all neurons in the whole nervous system.

Axon of granule cell ascends into molecular layer and forms the parallel fiber, which synapses with dendrites of Purkinje cells, stellate cells, basket cells and Golgi cells. Dendrites of granule cells and the axon and few dendrites of a Golgi cell synapse with Mossy fiber. The synaptic area of these cells is called glomerulus and it is encapsulated by the processes of glial cells.

Afferent Fibers to Cerebellar Cortex

Cerebellar cortex receives afferent signals from other parts of brain through two types of nerve fibers:

1. Climbing fibers

2. Mossy fibers.

1. Climbing fibers

Climbing fibers arise from the neurons of inferior olivary nucleus, situated in medulla and reach the

cerebellum via olivocerebellar tract. Inferior olivary nucleus relays the output signals from motor areas

of cerebral cortex and the proprioceptive signals from different parts of the body to the cerebellar cortex via climbing fibers. Proprioceptive impulses from different parts of the body reach the inferior olivary nucleus through spinal cord and vestibular system. After reaching the cerebellum, the climbing fibers

ascend into molecular layer and terminate on the dendrites of Purkinje cells. While passing through

cerebellum, climbing fibers of olivocerebellar tract send collaterals to cerebellar nuclei. So, impulses

from cerebral cortex and proprioceptors of the body are conveyed not only to cerebellar cortex, but also to

the cerebellar nuclei through the climbing fibers. Each climbing fiber innervates one single Purkinje cell.

Post a Comment

0 Comments