Cerebellum: Gross anatomy and main connections


This note describes the gross features of cerebellum and its main connections.

Developmental source: It develops from the metencephalon division of rhombelcephalon (hind brain).


Location – posterior cranial fossa
Separated from the cerebrum by a fold of dura mater called tentorium cerebelli
Separated from pons and medulla by the cavity of fourth ventricle



Presenting parts
Vermis – near the midline
Two lateral hemisphere
Two surfaces: superior and inferior surfaces separated by horizontal fissure

On the inferior aspect, the two hemispheres are separated by a depression called the valeculla.

On each side the vermis is separated from the corresponding cerebellar hemisphere by a paramedian sulcus

Anteriorly and posteriorly, the hemisphere extends beyond the vermis and are separated by anterior and posterior cerebellar notch.

The surface of the cerebellum is marked by a series of fissures that run more or less parallel to one another. The fissures subdivide the surface of the cerebellum into narrow leaf like bands or folia.

Further, these fissures divide the cerebellum into lobes and lobules.

The deepest fissures in the cerebellum are
The primary fissure – running transversely across the superior surface
Posterolateral fissure – present in the inferior aspect

Lobes:
Anterior lobe – the part anterior to the primary fissure
Posterior lobe – the part between the two fissures (also called the middle lobe)
The remaining part is the flocculonodular lobe.

The vermis also consists of (from before backward)
  • Lingual, central lobule and culmen (in the anterior lobe)
  • Declive, folium, tuber, pyramid and uvula (in the middle lobe)
  • Nodule (in flocculonodular lobe)


Except lingula, each subdivision of vermis is laterally related to a part of the hemisphere.

In the anterior lobe,
  • Central lobule is related to Ala
  • Culmen is related to anterior quadrangular lobule


In the middle lobe
  • Delive is related to posterior quadrangular lobule
  • Folium is related to superior semilunar lobule
  • Tuber is related to inferior semilunar lobule and gracile
  • Pyramid is related to biventral lobule and
  • Uvula is related to tonsil


Nodule is laterally continuous with the flocculus.



Grey matter and deep nuclei of Cerebellum

Most of the grey matter of the cerebellum is arranged as a thin layer covering the central core of white matter. Embedded within the white matter are masses of grey matter called deep cerebellar nuclei and they are:
  1. Dentate nucleus – lies in the center of each cerebellar hemisphere
  2. Emboliform nucleus – lies on the medial side of the dentate nucleus
  3. Globose nucleus – lies medial to the emboliform nucleus
  4. Fastigial nucleus – lies close to the midline 
White matter of the cerebellum
The central core of each cerebellar hemisphere is made up of white matter which is continuous with the cerebellar peduncles. The two sides of the white matter is connected by a thin lamina of fibers that forms the roof of the fourth ventricle. The upper part of this lamina forms the superior medullary velum and lower part forms the inferior medullary velum.

The white matter consists of:
1.     Afferent fibers entering the cerebellum from outside
2.     Projection fibers from the cerebellar cortex to the cerebellar nuclei
3.     Association fibers interconnecting different parts of the cerebellar cortex
4.     Commissural fibers connecting the two cerebellar hemisphere
5.     Fibers from the cerebellar nuclei to centers outside the cerebellum

Cerebellar peduncles: the fibers entering or leaving the cerebellum pass through three thick bundles known as cerebellar peduncles : superior middle and inferior

Superior cerebellar peduncle
Consists of the fibers mainly arising in the cerebellar nuclei (mainly dentate nucleus) that enter the midbrain and cross to the opposite side before ending in the red nucleus (mainly). Many of the fibers also ascend to the thalamus.

The right and left superior peduncles are connected to each other by a thin lamina of white matter called the superior or anterior medullary velum.

Middle cerebellar peduncle
It begins as a lateral continuation of the ventral part of the pons. The fibers in this peduncle arise from the pontine nuclei which cross to the opposite side

Inferior Cerebellar peduncle
Also called the restiform body, it is a thick bundle of fibers that connect the posterolateral part of the medulla with the cerebellum.

Main connection of cerebellum
The fundamental points to be considered regarding the connections of the cerebellum are
1.     Afferent fibers terminate in the cortex
2.     Efferent fibers arising in the cortex end in the cerebellar nuclei and
3.     Fibers arising in the nuclei project to centers outside the cerebellum
Exception:  some fibers especially vestibular project directly to the deep nuclei.

Cerebellar Afferent Fibers
Cerebellar Afferent Fibers From the Cerebral Cortex
The cerebral cortex sends information to the cerebellum by three pathways:
(1) the corticopontocerebellar pathway,
(2) the cerebro-olivocerebellar pathway, and
(3) the cerebroreticulocerebellar pathway.

Corticopontocerebellar Pathway
The corticopontine fibers arise from nerve cells in the frontal, parietal, temporal, and occipital lobes of the cerebral cortex and project to the pontine nuclei after descending through the corona radiata and internal capsule.The pontine nuclei give rise to the transverse fibers of the pons, which cross the midline and enter the opposite cerebellar hemisphere as the middle cerebellar peduncle.

Cerebro-olivocerebellar Pathway
The cortico-olivary fibers arise from nerve cells in the frontal, parietal, temporal, and occipital lobes of the cerebral cortex to terminate bilaterally on the inferior olivary nuclei after passing through the corona radiata and internal capsule. The inferior olivary nuclei give rise to fibers that cross the midline and enter the opposite cerebellar hemisphere through the inferior cerebellar peduncle.

Cerebroreticulocerebellar Pathway
The corticoreticular fibers arise from nerve cells from many areas of the cerebral cortex, particularly the sensorimotor areas. They descend to terminate in the reticular formation on the same side and on the opposite side in the pons and medulla. The cells in the reticular formation give rise to the reticulocerebellar fibers that enter the cerebellar hemisphere on the same side through the inferior and middle cerebellar peduncles.

Cerebellar Afferent Fibers From the Spinal Cord
The spinal cord sends information to the cerebellum from somatosensory receptors by three pathways:

(1) the anterior spinocerebellar tract,
(2) the posterior spinocerebellar tract, and
(3) the cuneocerebellar tract


Anterior Spinocerebellar Tract
The axons entering the spinal cord from the posterior root ganglion terminate by synapsing with the neurons in the nucleus dorsalis (Clarke's column) at the base of the posterior gray column. Most of the axons of these neurons cross to the opposite side and ascend as the anterior spinocerebellar tract in the contralateral white column. The fibers enter the cerebellum through the superior cerebellar peduncle and terminate as mossy fibers in the cerebellar cortex. It is believed that those fibers that cross over to the opposite side in the spinal cord cross back within the cerebellum.

The anterior spinocerebellar tract fibers convey muscle joint information from the muscle spindles, tendon organs, and joint receptors of the upper and lower limbs. It is also believed that the cerebellum receives information from the skin and superficial fascia by this tract.

Posterior Spinocerebellar Tract
The axons entering the spinal cord from the posterior root ganglion enter the posterior gray column and terminate by synapsing on the neurons at the base of the posterior gray column. The axons of these neurons enter the posterolateral part of the lateral white column on the same side and ascend as the posterior spinocerebellar tract to the medulla oblongata. Here, the tract enters the cerebellum through the inferior cerebellar peduncle and terminates in the cerebellar cortex. The posterior spinocerebellar tract receives muscle joint information from the muscle spindles, tendon organs, and joint receptors of the trunk and lower limbs.

Cuneocerebellar Tract
These fibers originate in the nucleus cuneatus of the medulla oblongata and enter the cerebellar hemisphere on the same side through the inferior cerebellar peduncle. The cuneocerebellar tract receives muscle joint information from the muscle spindles, tendon organs, and joint receptors of the upper limb and upper part of the thorax.

Cerebellar Afferent Fibers From the Vestibular Nerve
The vestibular nerve receives information from the inner ear concerning motion from the semicircular canals and position relative to gravity from the utricle and saccule. The vestibular nerve sends many afferent fibers directly to the cerebellum through the inferior cerebellar peduncle on the same side. Other vestibular afferent fibers pass first to the vestibular nuclei in the brainstem, where they synapse and are relayed to the cerebellum. They enter the cerebellum through the inferior cerebellar peduncle on the same side.

Other Afferent Fibers
In addition, the cerebellum receives small bundles of afferent fibers from the red nucleus (rubrocerebella tract) and the tectum (tectocerebellar tract).

Cerebellar Efferent Fibers
The entire efferent outflow of the cerebellar cortex is through the axons of the Purkinje cells via deep cerebellar nuclei. A few Purkinje cell axons pass directly out of the cerebellum to the lateral vestibular nucleus.

The efferent fibers from the cerebellum connect with the red nucleus, thalamus, vestibular complex, and reticular formation.

Globose-Emboliform-Rubral Pathway
Axons of neurons in the globose and emboliform nuclei travel through the superior cerebellar peduncle and cross the midline to the opposite side in the decussation of the superior cerebellar peduncles. The fibers end by synapsing with cells of the contralateral red nucleus, which give rise to axons of the rubrospinal tract. Thus, this pathway crosses twice, once in the decussation of the superior cerebellar peduncle and again in the rubrospinal tract close to its origin. By this means, the globose and emboliform nuclei influence motor activity on the same side of the body.

Dentothalamic Pathway
Axons of neurons in the dentate nucleus travel through the superior cerebellar peduncle and cross the midline to the opposite side in the decussation of the superior cerebellar peduncle. The fibers end by synapsing with cells in the contralateral ventrolateral nucleus of the thalamus. The axons of the thalamic neurons ascend through the internal capsule and corona radiata and terminate in the primary motor area of the cerebral cortex. 

 By this pathway, the dentate nucleus can influence motor activity by acting on the motor neurons of the opposite cerebral cortex; impulses from the motor cortex are transmitted to spinal segmental levels through the corticospinal tract. Since most of the fibers of the corticospinal tract cross to the opposite side in the decussation of the pyramids or later at the spinal segmental levels, the dentate nucleus is able to coordinate muscle activity on the same side of the body.

Fastigial Vestibular Pathway
The axons of neurons in the fastigial nucleus travel through the inferior cerebellar peduncle and end by projecting on the neurons of the lateral vestibular nucleus on both sides. Some Purkinje cell axons project directly to the lateral vestibular nucleus. The neurons of the lateral vestibular nucleus form the vestibulospinal tract. The fastigial nucleus exerts a facilitatory influence mainly on the ipsilateral extensor muscle tone.

Fastigial Reticular Pathway
The axons of neurons in the fastigial nucleus travel through the inferior cerebellar peduncle and end by synapsing with neurons of the reticular formation. Axons of these neurons influence spinal segmental motor activity through the reticulospinal tract.


 Test Yourself


1)    What is the developmental source of cerebellum?

     a)     Telencephalon
     b)    Diencephalon
     c)     Metencephalon
     d)    Myelencephalon

2)    Which of the following structures connect the cerebellum with the mid brain?

     a)     Superior cerebellar peduncle
     b)    Middle cerebellar peduncle
     c)     Inferior cerebellar peduncle
     d)    Crus cerebri

3)    Which of the following structures connect the cerebellum with the pons?

     a)     Superior cerebellar peduncle
     b)    Middle cerebellar peduncle
     c)     Inferior cerebellar peduncle
     d)    Cerebral aqueduct

4)    Which of the following structures connect the cerebellum with the medulla oblongata?

     a)     Superior cerebellar peduncle
     b)    Middle cerebellar peduncle 
     c)     Inferior cerebellar peduncle
     d)    Stria medullaris

5)    Which of the following cavity is found in relation to cerebellum?

     a)     Lateral ventricles
     b)    Third ventricle
     c)     Cerebral aqueduct
     d)    Fourth ventricle

6)    Which of the following structures separate the cerebellum from cerebrum?

     a)     Falx cerebri
     b)    Falx cerebelli
     c)     Tentorium cerebelli
     d)    Valeculla

7)    What is the midline structure of cerebellum called

     a)    Ala
     b)    Flocculonodular lobe
     c)     Hemisphere
d) Vermis

REFERENCES

Gray’s Anatomy, 39th Edition
Snell’s Clinical Neuroanatomy 7th Edition