Cerebellum

Question 1

Introduction

Answer 1

CEREBELLUM

1. The cerebellum (L. cerebellum little brain) is the largest part of the hindbrain and the second largest part of the brain as a whole.
2. It weighs about 150 g.
3. It is located in the posterior cranial fossa
   - underneath the tentorium cerebelli and
   - behind the pons and medulla oblongata.
   - It is separated from the pons and medulla by the cavity of the 4th ventricle .
4. Its surface is indented by numerous fine slit-like sulci called fissures. Between the fissures lie more or less parallel folds called folia.
   In general, the fissures and folia of the cerebellum lie transversely from side-to-side across the whole extent of the cerebellum.
5. The cerebellum consists of two hemispheres united in the midline by the vermis.
6. Each hemisphere of the cerebellum is connected to three parts of the brainstem by three pairs of the large fibre tracts called cerebellar peduncles.
7. The basic functions of the cerebellum are as follows:
8. **Maintenance of equilibrium.
9. Regulation of muscle tone.
10. Coordination of somatic motor activities.**

The cerebellar disease is manifested by motor disturbances, including
- inability to stand upright,
- staggering gait,
- hypo- tonia, and
- failure of coordination.

Question 2

External features

Answer 2

EXTERNAL FEATURES

The external features of the cerebellum comprise three parts:

• two surfaces,
• two notches, and
• three well-marked fissures

PARTS

The cerebellum consists of three parts:

two large lateral hemispherical lobes, the cerebellar hemispheres, and a narrow median warm-like portion, called vermis

The superior and inferior aspects of vermis are termed superior and inferior vermis, respectively.

The ridge-like superior vermis is continuous on either side with the superior surface of cerebellar hemisphere imperceptively.

The inferior vermis is more clearly demarcated from the hemispheres in the floor of vallecula cerebelli

SURFACES

The superior surface of the cerebellum is convex.

The two cerebellar hemispheres are continuous with each other on this surface.

The inferior surface presents a deep median notch called vallecula which separates the two cerebellar hemispheres

The floor of the vallecula is formed by the inferior vermis and is limited on each side by sulcus valleculae

NOTCHES

The anterior aspect of cerebellum is marked by a wide shallow anterior cerebellar notch which accommodates pons and medulla.

The posterior cerebellar notch is deep and narrow. and lodges the falx cerebelli.

FISSURES

1. The horizontal fissure is most conspicuous and
   - runs along the lateral and posterior margins of the cerebellum.

• It marks the junction between the superior and inferior surfaces of the cerebellum.

2. The posterolateral fissure lies on the
   - inferior surface of the cerebellum and

• separates the flocculonodular lobe from the rest of the cerebellum (corpus cerebelli).

3. The V-shaped fissura prima on the
   - superior surface

• cuts the superior vermis at the junction of its anterior two-third and posterior one-third.
• It divides the corpus cerebelli into anterior and posterior (middle) lobes.

Question 3

Subdivisions

Answer 3

SUBDIVISIONS OF THE CEREBELLUM

ANATOMICAL SUBDIVISIONS

Anatomically the cerebellum is divided into the following three lobes

1. Anterior.
2. Posterior.
3. Flocculonodular.

Anterior lobe: It lies on the superior surface anterior to the fissura prima.

Posterior/middle lobe. It lies between the fissura prima on the superior surface and posterolateral fissure on the inferior surface.

Flocculonodular lobe: It is the smallest of all and lies on the inferior surface in front of the posterolateral fissure.

MORPHOLOGICAL SUBDIVISIONS

Based on phylogenetic and functional criteria, the cerebellum is divided into the following three parts:

1. Archicerebellum.
2. Paleocerebellum.
3. Neocerebellum.

Archicerebellum

• (Vestibular cerebellum):
• Phylogenetically, it is the oldest part of the cerebellum.
• It consists of flocculo- nodular lobe and lingula.
• The archicerebellum is chiefly vestibular in connections
• and concerned with the maintenance of equilibrium, tone, and posture of trunk muscles.

Paleocerebellum
- (Spinal cerebellum):
- Phylogenetically, it is the next part of the cerebellum to appear.
- It consists of anterior lobe (except lingula) and pyramid, and the uvula of inferior vermis.

• The paleocerebellum is chiefly spinocerebellar in connections
• and is concerned with the tone, posture, and crude movements of the limbs.

Neocerebellum

• (Cerebral cerebellum):
• Phylogenetically, it is the most recent part of the cerebellum to develop.
• It is made up of middle lobe, the largest part of the cerebellum (except the pyramid and the uvula of inferior vermis).
• The neocerebellum is chiefly corticopontocerebellar in connections and is
• concerned with the smooth performance of skilled voluntary movements.

Question 4

Internal structure - grey matter

Answer 4

GREY MATTER OF CEREBELLUM

The grey matter of cerebellum is represented by:
• The cerebellar cortex
• The intracerebellar nuclei

STRUCTURE OF CEREBELLAR CORTEX

Most of the grey matter of the cerebellum is arranged as
a thin layer covering the central core of white matter.
This layer is the cerebellar cortex.

It is divided into three layers
as follows
1. Molecular layer (most superficial)
2. Purkinje cell layer
3. Granular layer, which rests on white matter

The neurons of the cerebellar cortex are of five main types:

1. Stellate cells, lying in molecular layer
2. Basket cells, lying in the molecular layer
3. Purkinje cells, forming the layer named after them
4. Granule cells, forming the granular layer
5. Golgi cells, present in the granular layer

(Explanation of layers in copy)

INTRACEREBELLAR NUCLEI

• The intracerebellar nuclei (also called central nuclei)
• are masses of grey matter embedded in the white matter of the cerebellum.
• On each side of the midline they are four in number. From lateral to medial side, these are: dentate nucleus, emboliform nucleus, globose nucleus, and fastigial nucleus

1. The dentate nucleus
   - is the most prominent of the intracerebellar nuclei and
   - largest in primates, especially in humans.

• It is the nucleus of neocerebellum, and therefore receives afferent fibres from it.

2. The emboliform nucleus is
   - oval in shape and
   - situated medial to the dentate nucleus,
   - partially covering its hilum.
   - It is the nucleus of paleocerebellum, hence receives afferent fibres from it.
3. The globose nucleus
   - is rounded in shape and
   - lies between the emboliform and fastigial nuclei.
   - It has similar connections to that of the emboliform nucleus.
   - The globose and emboliform nuclei together are sometimes referred to as nucleus interpositus.
4. The fastigial nucleus
   - lies near the midline in the vermis and close to the roof of the 4th ventricle
   - is the nucleus of archicerebellum, hence receives afferent fibres from the flocculonodular lobe (archicerebellum).

Question 5

White matter

Answer 5

WHITE MATTER OF CEREBELLUM

• The central core of each cerebellar hemisphere is formed by
  the white matter.
• The cerebellar peduncles are continued
  into this white matter.
• The white matter of the two sides is
  connected by a thin lamina of fibres that is closely related
  to the roof of the fourth ventricle. The upper part of this
  lamina forms the superior medullary velum, and its lower
  part forms two crescentic sheets called inferior medullary
  vela.

The white matter consists of two types of fibres—(1)
intrinsic and (2) extrinsic.

Intrinsic fibres:

• Intrinsic fibres remain confined
  within the cerebellum.
• They connect different regions
  of the cerebellum either in the same hemisphere or of
  the two cerebellar hemispheres:

– Projection fibres connect cerebellar cortex to the
cerebellar nuclei.
– Association fibres interconnect different parts of
the cerebellar cortex.
– Commissural fibres connect the two cerebellar
hemispheres.

• Extrinsic fibres:

Extrinsic fibres connect the
cerebellum with other parts of the central nervous
system, i.e. brain and spinal cord through afferent
and efferent fibres.

The fibres entering or leaving the
cerebellum pass through three thick bundles called
the cerebellar peduncles—(1) superior, (2) middle,
and (3) inferior.

AFFERENT FIBRES
Entering the Cerebellar Cortex
The afferent fibres to the cerebellar cortex are of two
different types:
1. Mossy fibres
2. Climbing fibres

Mossy Fibres

Mossy fibres originate from the
- vestibular nuclei
(vestibulocerebellar),
- pontine nuclei (pontocerebellar),
and
- spinal cord (spinocerebellar) and terminate in the
granular layer of the cortex within the glomeruli.

Climbing Fibres

These fibres represent terminations of axons reaching
the cerebellum from the inferior olivary complex
(Olivocerebellar tract and parolivocerebellar tract).
They
pass through the granular layer and the Purkinje cell
layer to reach the molecular layer

EFFERENT FIBRES

The efferent fibres from the cerebellar cortex are axons of
Purkinje cells, which terminate in the cerebellar nuclei.

Some efferents from the flocculonodular lobe bypass the
cerebellar nuclei and terminate in the vestibular nuclei
of brainstem.
Axons of the Purkinje cells are inhibitory to
cerebellar nuclei.

The fibres from dentate, emboliform and globose nuclei
leave the cerebellum through the superior cerebellar
peduncle.

The fibres from the fastigial nucleus leave the
cerebellum through inferior cerebellar peduncle.