CHAPTER 7. THE CEREBELLUM

OBJECTIVE:
  1. BE ABLE TO IDENTIFY THE VERMIS AND HEMISPHERES AND THE LOBES AND THEIR BOUNDARIES.
  2. BE ABLE TO IDENTIFY THE LAYERS OF THE CEREBELLAR CORTEX AND THE MORPHOLOGICAL CHARACTERISTICS OF EACH.
  3. BE ABLE TO IDENTIFY THE CEREBELLAR NUCLEI.
  4. BE ABLE TO IDENTIFY THE TRACTS, NUCLEI, AND CEREBRAL CORTICAL AREAS ASSOCIATED WITH THE INPUT AND THE OUTPUT OF THE CEREBRAL CEREBELLUM, SPINAL CEREBELLUM, AND VESTIBULAR CEREBELLUM.
Gross Anatomy

The cerebellum consists of two hemispheres connected in the median plane by the vermis. It is the largest part of the hindbrain and it occupies most of the posterior cranial fossa, being separated from the occipital lobes by the tentorium cerebelli (MRIs 6, 7, 14, 16). Its median part forms the roof of the fourth ventricle (Pl. 13).  It is connected to the brainstem by the superior, middle, and inferior cerebellar peduncles (Pl. 9; Sl. 19).The surface of the cerebellum is marked by the transversely-directed folia (Pls. 16, 17). 

The superior surface, which is in contact with the tentorium, is somewhat flattened and its anterior edge is notched by the shallow anterior incisure. The inferior surface is related to the posterior surface of the petrous part of the temporal bone, the sigmoid sinus, the mastoid part of the temporal bone, and the squamous part of the occipital bone. This surface is convex and presents deep grooves where the hemispheres meet the vermis. Here the inferior parts of the hemispheres are actually separated by a deep fossa, the vallecula cerebelli, whose floor is formed by the vermis. The vallecula is continuous with the posterior incisure, the deep notch which normally contains the falx cerebelli.

The primary fissure is the most conspicuous groove on the superior surface (Pl. 16).  It is located at about the junction of the anterior third with the posterior two-thirds. The anterior lobe lies in front of the primary fissure, the posterior lobe behind it.

The most inferior part of the vermis is the nodulus (Pl. 13).  It lies next to the inferior medullary velum and continues laterally into the flocculus (Pl. 17).  The two flocculi and the nodulus form the flocculonodular lobe which is separated from the posterior lobe by the posterolateral fissure.  A conspicuous eminence seen medially in the most inferior part of the posterior lobe is the tonsil.

The tonsil is of medical importance because in a condition called "tonsillar herniation" the brainstem shifts downward lodging the tonsil and medulla oblongata in the foramen magnum.  Because of the pressure exerted on the medulla, this condition is rapidly fatal.

Cerebellar Cortex

Each cerebellar folium is comprised of an outer cortical part and an inner white matter part (Sl. 76).  The cortex (Sl. 77) consists of three layers which are, from external to internal:

  1. Molecular layer - relatively devoid of neurons;
  2. Purkinje cell layer - a single row of cell bodies of Purkinje neurons; and
  3. Granular layer - densely packed granule cells.

The Purkinje neuron (Sls. 78, 79) is unique to the cerebellar cortex.  Its globular cell body gives off a massive dendritic tree that virtually fills the molecular layer.  The surfaces of the smaller dendrites contain numerous gemmules (Sl. 79).  The axon emerges from the base of the cell body and proceeds internally, through the granular layer to the white matter.

The axons entering the cerebellum become either mossy or climbing fibers. The mossy fibers branch repeatedly in the white matter, enter the granular layer, and continue to branch. Eventually the mossy fiber loses its myelin and forms a large lobulated terminal, called a mossy fiber rosette. A single rosette terminates in relation to the dendrites of about 20 granule cells. In addition to these dendritic endings, the mossy rosette is also in contact with the axonal terminals and the proximal parts of the dendrites of Golgi neurons. This entire conglomeration is surrounded by a layer of glial processes and forms a cerebellar glomerulus. These glomeruli are located in "islands" between the granule cells (Sl. 77).  The granule cells send their axons vertically into the molecular layer where they divide into two branches each of which runs parallel with the long axis of the folium, that is, from ear-to-ear. The Purkinje dendritic tree spreads at a right angle to the long axis of the folium and, hence, the granule cell axons in the parallel plexus are strung along the Purkinje dendrites like wires on telephone poles. Each parallel fiber extends about 1.5 mm and contacts the dendritic trees of about 500 Purkinje neurons. In turn, due to its enormous dendritic tree, each Purkinje neuron is contacted by about 80,000 granule cells. The granule cell axons in the parallel plexus terminate not only on Purkinje neurons, but also on the basket cells located in the deeper part of the molecular layer, the stellate cells located more superficially, and the dendrites of the Golgi cells. The cell body of the Golgi neuron is usually located in the superficial part of the granular layer and most of its dendrites extend into the molecular layer.

Axons of the basket neurons terminate in relation to the cell bodies of Purkinje neurons while stellate neuron axons terminate in relation to the Purkinje dendrites. Axons from Golgi neurons terminate at the mossy fiber-granule cell junction in the cerebellar glomeruli. Since all three of these neurons are inhibitory, as are the Purkinje neurons, the only excitatory neuron in the cerebellar cortex is the granule cell.

The other type of fiber in the cerebellar cortex is the climbing fiber. These arise from the inferior olivary nuclear complex. They pass through the medullary white matter and the granular layer of the cortex and, upon reaching the Purkinje cell layer, they climb upon the Purkinje cell body and pass onto the dendrites. They make multiple synapses on the Purkinje cell and are strongly excitatory.

Cerebellar Nuclei

Embedded in the cerebellar white matter near the roof of the fourth ventricle are the cerebellar nuclei (Sls. 16, 17).  From medial to lateral, these are the fastigial, globose, emboliform, and dentate.  The globose and emboliform nuclei are referred to functionally as the interposite (interposed) nucleus.

Cerebral  Cerebellum

The cerebellar hemispheres receive most of their input from the cerebral cortex and this occurs through the massive cortico-ponto-cerebellar pathway. Impulses from the frontal lobe descend as frontopontine fibers in the anterior and posterior limbs of the internal capsule and the medial part of the cerebral crus (Sl. 31).  Impulses from the other lobes of the cerebrum descend as parieto-temporo-occipitopontine fibers through the posterior limb of the internal capsule and the lateral part of the cerebral crus (Sl. 31).  Both groups of fibers terminate ipsilaterally in the pontine nuclei (Sls. 21, 22, 23, 24, 25, 26, 27, 28, 29).  The pontine nuclei give rise to the deep and superficial transverse pontine fibers (Sls. 21, 22, 23, 24, 25, 26, 27, 28, 29), which, after decussating, traverse the contralateral basilar pons and form the contralateral middle cerebellar peduncle (Pl. 30; Sls. 19, 20, 21, 22, 23, 24, 25).  From here they are distributed mainly to the lateral parts of the hemisphere.

The Purkinje neurons of the lateral part of the hemisphere project ipsilaterally to the dentate nucleus (Sls. 16, 17).  This huge, convoluted nucleus is the chief source of the fibers in the superior cerebellar peduncle.  These dentatofugal fibers pass rostrally in the superior cerebellar peduncle as it courses initially in the roof of the fourth ventricle (Sls. 19, 21), then in the roof and wall (Sls. 23, 24), and finally enters the tegmentum of the rostral pons (Sls. 25, 26, 27, 28).  The superior cerebellar peduncle decussates at the level of the inferior colliculus (Pl. 29; Sls. 29, 30), and the dentatofugal components of the cerebellothalamic projection continue rostrally through and adjacent to the red nucleus (Sls. 33, 34) to the prerubral field part of the subthalamus (Sls. 36, 37, 38).  From here they enter the thalamic fasciculus (Sls. 37, 38, 39, 40, 41) through which they reach the ventral lateral nucleus (Sls. 37, 38, 39, 40, 41, 42).  The ventral lateral nucleus projects to the motor cortex.  Thus, the influence of the cerebral cerebellum on skilled movements is directed rostrally through the contralateral thalamus to the motor area of the cerebral cortex.

Spinal Cerebellum

Information from individual mechanoreceptors in muscles and tendons of the lower limb reaches the cerebellum via a 2-neuron path.  The first neurons are the large unipolar cells in the lumbosacral ganglia.  The central branches of their axons sweep into the spinal cord through the medial divisions of the dorsal roots, ascend in the gracile tract (Sls. 1, 2, 3, 4), and synapse in the dorsal nucleus of Clarke.  Identify this nucleus in the dorsomedial part of lamina VII (Sl. 5). The large myelinated axons from the second order neurons in the dorsal nucleus pass laterally and form the dorsal spinocerebellar tract, which is located at the surface of the posterior part of the lateral funiculus (Sls. 5, 6, 8).

Two interesting features related to the termination of the primary axons and the origin of the secondary axons of this spinal-cerebellar path are: 1) the smaller size of the gracile tract in the cervical region as compared to the lumbar because of the large numbers of axons that terminate between L.2 and C.8 (compare slides 3 and 6), and 2) the absence of the dorsal spinocerebellar tract below L.2 places the lateral corticospinal tract at the dorsolateral surface of the lumbosacral spinal cord (Sls. 1, 2, 3).

Upon reaching the brainstem the heavily myelinated dorsal spinocerebellar tract moves from the ventral to the lateral surface of the spinal trigeminal tract (Sls. 9, 10, 11, 12) and reaches the ipsilateral inferior cerebellar peduncle at the level of the inferior olivary nucleus (Sls. 12, 13, 14, 15).

A similar fast conducting monosynaptic path for discrete information from the upper limb includes:  1) cervical spinal ganglion cells and their axons that ascend in the cuneate tract (Sls. 6, 7, 8, 9, 10), and 2) neurons in the accessory cuneate nucleus (Sls. 10, 11, 12, 13) whose axons enter the closely related inferior cerebellar peduncle as its cuneatocerebellar components. Thus, discrete proprioceptive information from both ipsilateral limbs reaches the spinocerebellum through the inferior cerebellar peduncle. Therefore, a lesion of the inferior cerebellar peduncle results in ataxia of the ipsilateral upper and lower limbs.

The spinal cerebellum Purkinje neurons then influence movements chiefly through the fastigial and interposite nuclei.  Those Purkinje cells located more medially project chiefly to the fastigial nucleus and the vestibular nuclei.  The spinal cerebellum also has direct (Purkinje-vestibular) connections.  Much of the influence of the spinal cerebellum, however, is exerted through the lateral vestibular nucleus (Sls. 20, 21, 22) and its lateral vestibulospinal tract, as well as through the medullary reticular formation and its lateral reticulospinal tract. Through these routes the spinal cerebellum and the fastigial nuclei coordinate ongoing movements in the more proximal parts of the limbs.

The paravermal parts of the spinal cerebellum project chiefly to the interposite (globose and emboliform) nucleus (Sls. 16, 17).  In turn, each interposite nucleus projects to the contralateral red nucleus (Sls. 31, 32) and adjacent reticular formation via the superior cerebellar peduncle and its decussation (Sl. 29).  Hence, through the red nucleus and reticular formation;, the spinal cerebellum and its interposite nuclei coordinate ongoing movements, especially in the more distal parts of the limbs.

Vestibular Cerebellum

The flocculonodular lobe and adjacent parts of the vermis, as well as parts of the fastigial nuclei, are concerned chiefly with equilibrium.  These cerebellar structures strongly influence the axial or trunk muscles and, in turn, they are strongly influenced by the vestibular apparatus.  Information concerning changes in the position of the head reaches the cerebellum directly from the vestibular nerve and indirectly from the vestibular nuclei.  Both the direct and indirect fibers reach the cerebellum in the medial part of the inferior cerebellar peduncle, the juxtarestiform body (Sls. 20, 21, 22).

The Purkinje cells of the vestibulocerebellum influence the vestibular nuclei (and adjacent reticular formation) mainly through the fastigial nuclei (Sl. 16).  Some Purkinje axons, however, have direct connections with the vestibular nuclei.  Both reach the vestibular nuclei (Sls. 17, 18, 19, 20, 21, 22) and the adjacent reticular formation through the juxtarestiform body (Sls. 19, 20, 21, 22). 

Inferior Olivary and Nucleocortical Influences

Two major sources of input to the cerebellar cortex are the inferior olivary complex and the cerebellar nuclei themselves.  The inferior olivary complex consists of the large convoluted principal or main nucleus (Sls. 12, 14, 15, 17, 18, 19) and two accessory nuclei, the dorsal and medial (Sls. 15, 17).  The accessory nuclei receive their input chiefly from the spinal cord, while the main nucleus receives impulses primarily from the sensori-motor cortex. The massive olivocerebellar projections pass medially, decussate, and sweep through the opposite olive and medullary tegmentum (Sls. 14, 15, 18) to form the major components of the inferior cerebellar peduncle. The olivocerebellar axons become the climbing fibers which are distributed to all parts of the cerebellar cortex.

All parts of the cerebellar cortex also receive impulses from the cerebellar nuclei. These nucleocortical projections are reciprocally organized with the corticonuclear projections. They terminate in the granule cell layer and represent an important feedback mechanism modulating efferent cerebellar activity.