WHITE MATTER OF CEREBELLUM

 

WHITE MATTER OF CEREBELLUM

White matter of cerebellum is formed by afferent and efferent nerve fibers. These nerve fibers are classified into three groups.

1. Association fibers

Association fibers connect different regions of same cerebellar hemisphere.

2. Commissural fibers

Commissural fibers connect the areas of both halves of cerebellar cortex.

3. Projection fibers

Projection fibers are the afferent and efferent nerve fibers which connect cerebellum with other parts of central nervous system. Projection fibers of cerebellum are arranged in three bundles :

i. Inferior cerebellar peduncles between cerebellum and medulla oblongata

ii. Middle cerebellar peduncles between cerebellum and pons

iii. Superior cerebellar peduncles between cerebellum and midbrain.

i. Inferior Peduncles

Inferior cerebellar peduncles are otherwise called restiform bodies and contain predominantly afferent

fibers. These nerve fibers transmit the impulses from tactile receptors, proprioceptors and receptors in

vestibular apparatus to cerebellum.

ii. Middle Peduncles

Middle cerebellar peduncles are otherwise called brachia pontis. These penduncles contain predominantly, the afferent fibers. Most of the fibers of the middle cerebellar peduncles are commissural fibers, which connect the areas of both the halves of cerebellar cortex.

 iii. Superior Peduncles

Superior cerebellar peduncles are otherwise called the brachia conjunctivae and contain predominantly,

efferent fibers.

VESTIBULOCEREBELLUM (ARCHICEREBELLUM)

Vestibulocerebellum is connected with the vestibular apparatus and so it is known as vestibulocerebellum. Since vestibulocerebellum is the phylogenetically oldest part of cerebellum, it is also called archicerebellum. It is concerned with the maintenance of posture and

equilibrium.

COMPONENTS

Vestibulocerebellum includes the flocculonodular lobe that is formed by the nodulus of vermis and its lateral extensions called flocculi . Uvula of vermis is also considered as the part of vestibulocerebellum by some physiologists.

CONNECTIONS

Afferent Connections

Vestibulocerebellar tract

Vestibulocerebellar tract is formed by the fibers arising from the vestibular nuclei, situated in pons and medulla. It passes through the inferior cerebellar peduncle of the same side and reaches the cerebellar nuclei, nucleus globosus, nucleus emboliformis and nucleus fastigi . Fibers from these nuclei, reach the

vestibulocerebellum (flocculonodular node). Vestibular nuclei in turn, receive fibers from ves tibular

apparatus situated in the inner ear, through vestibular division of cochlear (VIII cranial) nerve.

Efferent Connections

1. Cerebellovestibular tract

Fibers of cerebellovestibular tract arise from the flocculonodular lobe, pass through the inferior cerebellar

peduncle of the same side and terminate on the vestibular nuclei in brainstem.

Fibers from vestibular nuclei form medial and lateral vestibulospinal tracts, which terminate on the

medial group of alpha motor neurons in the spinal cord. This pathway forms the part of medial system of motor pathway (extrapyramidal system).

2. Fastigiobulbar tract

Fibers of fastigiobulbar tract arise from fastigial nucleus, pass through inferior cerebellar peduncle of the same side and terminate on vestibular nuclei and reticular formation in medulla oblongata. From vestibular nuclei, vestibulospinal tracts arise and terminate on alpha motor neurons. From reticular formation, reticulospinal tract arises and terminates on gamma motor neurons in the spinal cord forming the part of medial motor system (extrapyramidal system).

FUNCTIONS

Vestibulocerebellum regulates tone, posture and equilibrium by receiving impulses from vestibular apparatus. Vestibular apparatus sends information regarding gravity, linear movement and angular acceleration to vestibulocerebellum through vestibulocerebellar tract. Vestibulocerebellum, in turn, sends signals to spinal cord via vestibulospinal and reticulospinal tracts.

Mechanism of Action of Vestibulocerebellum

Normally, vestibular nuclei facilitate the movements of trunk, neck and limbs through vestibulospinal tracts and alpha motor neurons. Medullary reticular formation inhibits the muscle tone through reticulospinal tract and gamma motor neurons. However, vestibulocerebellum inhibits both

vestibular nuclei and medullary reticular formation. As a result, the movements of neck, trunk and limbs are checked and the muscle tone increases. Because of these effects, any disturbance in posture and

equilibrium is corrected.

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