Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

17. Cerebellum

17.1 Cerebellum, External Structure

A Isolated cerebellum

a Inferior view, b superior view, c anterior view. The cerebellum has been removed from the posterior cranial fossa and detached from the brainstem below the tentorium at the cerebellar peduncles (see also B).

The cerebellum is part of the motor system. It cannot initiate conscious movements by itself but is responsible for unconscious coordination and fine control of muscle actions (see B, p. 244). Crossly, the cerebellar surface presents a much finer arrangement of gyri and sulci than the cerebrum, providing an even greater expansion of its surface area. Externally the cerebellum consists of two large lateral masses, the cerebellar hemispheres, and a central part called the vermis (see a). Cerebellar fissures further subdivide the cerebellum into lobes. In particular:

 The primary fissure separates the anterior lobe of the cerebellum from the posterior lobe (see b).

 The posterolateral fissure separates the posterior lobe of the cerebellum from the flocculonodular lobe (see B).

Other, less important fissures have no clinical or functional significance and are not described here. Besides these anatomical divisions, the parts of the cerebellum can also be distinguished according to phylogenetic and functional criteria (see C; also B, p. 244). The cerebellum is connected to the brainstem by the three cerebellar peduncles (superior, middle, and inferior, see c), through which its afferentand efferent tracts enter and leave the cerebellum. The superior medullary velum stretches between the superior cerebellar peduncles and forms part of the roof of the fourth ventricle (see c). The cerebellar tonsils protrude downward near the midline on each side, almost to the foramen magnum at the base of the skull (not shown). Increased intracranial pressure may cause the cerebellar tonsils to herniate into the foramen magnum, impinging upon vital centers in the brainstem and posing a threat to life (see D, p. 189). Functionally, the medial part of the cerebellum (red) is distinguished from the intermediate part (pale red) and lateral part (gray). This functional classification does not conform to the anatomically defined lobar boundaries. Each of these parts projects to a specific cerebellar nucleus (see p. 240).

В Relationship of the cerebellum to the brainstem

Left lateral view. The cerebellum overlies the dorsal aspect of the pons. Only the middle cerebellar peduncle can be identified in this external view. The cerebellopontine angle is clearly displayed. It has great clinical importance because it is the site where cerebellopontine angle tumors develop—most commonly acoustic neuromas (see D, p.149).

C Synopsis of cerebellar classifications

Phylogenetic classification

Anatomical classification

Functional classification based on the origin of afferents

• Archicerebellum

• Flocculonodular lobe

• Vestibulocerebellum: maintenance of equilibrium

• Paleocerebellum

• Anterior lobe of cerebellum

• Portions of the vermis

• Medial portions of the posterior lobe

• Spinocerebellum: regulation of muscle tone

• Neocerebellum

• Lateral portions of the posterior lobe

• Pontocerebellum (= cerebrocerebellum): skilled movements

17.2 Cerebellum, Internal Structure

A The cerebellum, brainstem, and diencephalon

Midsagittal section viewed from the left side, displaying the internal structure of the cerebellum. The interior of the cerebellum is composed of white matter and its exterior of gray matter (cerebellar cortex, whose layers are shown in D). This section again shows how the cerebellum abuts the fourth ventricle, in which the choroid plexus can be seen. The superior medullary velum forms the upper portion of the roof of the fourth ventricle; the lingula is closely apposed to its dorsal surface. The lower portion of the roof of the fourth ventricle is in contact with the cerebellar nodule. This section demonstrates how the cerebellar cortex is deeply folded into folia (gyri, not individually labeled), producing a tree-like outline of the white matter called the arbor vitae (“tree of life").

В Nuclei of the cerebellum

Section through the superiorcerebellar peduncles (plane of section shown in A), viewed from behind. Deep within the cerebellar white matter are four pairs of nuclei that contain most of the efferent neurons of the cerebellum:

 Fastigial nucleus (green)

 Emboliform nucleus (blue)

 Globose nuclei (blue)

 Dentate nucleus (pink)

The cortical regions have been color-coded to match their target nuclei. The dentate nucleus is the largest of the cerebellar nuclei and extends into the cerebellar hemispheres. The cerebellar nuclei receive projections from Purkinje cells in the cerebellar cortex (see D). While the efferent fibers of the cerebellum can be assigned rather easily to anatomical structures, this is not true of the afferent fibers. Their sources are examined on p. 244.

C Cerebellar nuclei and the regions of the cortex from which they receive projections (see also p. 238)

Cerebellar nucleus

Synonyms

Region of the cerebellar cortex that send axons to the nucleus

Dentate nucleus

Lateral cerebellar nucleus

Lateral part

(lateral portions of the cerebellar hemispheres)

Emboliform nucleus

Anterior interpositus nucleus

Intermediate part

(medial portions of the cerebellar hemispheres)

Globose nuclei

Posterior interpositus nucleus

Intermediate part

(medial portions of the cerebellar hemispheres)

Fastigial nucleus

Medial cerebellar nucleus

Median part (cerebellar vermis)

D Cerebellar cortex

The cerebellar cortex consists of three layers:

 Molecular layer: outer layer; contains parallel fibers, which are the axons of granule cells (blue) from the granular layer. They run parallel to the cerebellar folia and terminate in the molecular layer, where they synapse into the dendrites of the Purkinje cells. This layer also contains axons from the inferior olive and its accessory nuclei (climbing fibers) and a small number of inhibitory interneurons (basket and stellate neurons).

 Purkinje layer: contains the cell bodies of Purkinje cells (purple).

 Granular layer: contains mostly granule cells (blue), as well as mossy and climbing fibers (green and pink, respectively), and Golgi cells (not shown; the cell types are viewed in F).

The white matter of the cerebellum is located under the granular layer.

Note: The Purkinje cells are the only efferent cells of the cerebellar cortex. They project to the cerebellar nuclei.

E Synaptic circuitry of the cerebellum

(after Bahr and Frotscher)

The cerebellum comprises 10% of the mass of the brain, but contains upto 50% of its neurons. This enormous population (cerebellar granule cells alone may number in excess of 100 billion) is composed of a few cell types arranged in a repetitive, highly ordered array. This repetition of simple elements has led to the description of the cerebellum as an intricate synaptic computer for motor coordination.

The basic cerebellar circuitry involves afferents including climbing and mossy fibers. Climbing fibers originate from the inferior olivary complex and form multiple excitatory synapses on the cell bodies and proximal dendritic tree of Purkinje cells (see D); collateral branches synapse in the (deep) cerebellar nuclei. Mossy fibers originate in the vestibular and pontine nuclei and the spinal cord to form excitatory contacts with granule cells in synaptic complexes called cerebellar glomeruli (see D); some branches excite local inhibitory neurons, and collaterals also enter the cerebellar nuclei. The axons of granule cells form parallel fibers that form excitatory synapses on the dendritic trees of Purkinje cells. The Purkinje cells in turn send their axons mostly to the cerebellar nuclei (see B, above; also to vestibular nuclei), where they make inhibitory synapses. The identities of some neurotransmitters in this pathway have been established: local inhibitory neurons, and Purkinje cells themselves, use gamma-aminobutyric acid (GABA), while granule cells employ glutamate. Glutamate is probably also involved at mossy and climbing fiber synapses. The principal cerebellar efferent axons arise from the cerebellar nuclei. This circuitry combines direct activation (afferents to granule cells to Purkinje cells) and indirect inhibition (afferents to inhibitory interneurons to Purkinje cells), which may be integrated in a complex spatial pattern and temporal sequence in the cerebellar cortex and deep nuclei to provide indirect feedback control for motor coordination.

F Principal neurons and fiber types in the cerebellar cortex

Name

Definition

Climbing fibers

Axons of neurons of the inferior olive and its associated nuclei

Mossy fibers

Axons of neurons of the pontine nuclei, the spinal cord, and vestibular nuclei (pontocerebellar, spinocerebellar, and vestibular tracts)

Parallel fibers (see D)

Axons of granule cells

Granule cells

Interneurons of the cerebellar cortex

Purkinje cells

The only efferent cells of the cerebella r cortex; exert an inhibitory effect

17.3 Cerebellar Peduncles and Tracts

A Cerebellar peduncles

a Left lateral view with the upper portion of the cerebellum and lateral portions of the pons removed. This dissection, which has been prepared to show fiber structure, clearly shows the course of the cerebellar tracts. The size of the cerebellar peduncles, and thus the mass of entering and emerging axons, is substantial and reflects the extensive neural connections in the cerebellum (see p.241). The cerebellum requires these numerous connections because it is an integrating center for the coordination of fine movements. In particular, it contains and processes vestibular and proprioceptive afferents and it modulates motor nuclei in other brain regions and in the spinal cord. The principal afferent and efferent connections of the cerebellum are reviewed in B.

b Left lateral view. Here the cerebellum has been sharply detached from its peduncles to demonstrate the complementary cut surface of the peduncles on the brainstem (compare with Ac, p. 238).

В Synopsis of the cerebellar peduncles and their tracts

Tracts made up of afferent and efferent axons enter or leave the cerebellum through the cerebellar peduncles. The afferent axons originate in the spinal cord, vestibular organs, inferior olive and pons, while the efferent axons originate in the cerebellar nuclei (see p.240). The representation of the body in the cerebellum, unlike in the cerebrum, is ipsilateral. Ascending cerebellar tracts thus cross (decussate) to the opposite side.

Cerebellar peduncle and constituent parts*

Origin **

Site of Termination

Superior cerebellar peduncle: contains mostly efferent tracts from the cerebellar nuclei. Some tracts cross in the decussation of the superior peduncle, then divide into a descending limb (to the pons) and an ascending limb (to the midbrain and thalamus).

Descending parts (e)

Fastigial and globose nuclei

Reticular formation and vestibular nuclei (projection is mostly contralateral)

Ascending parts (e)

Dentate nucleus

Red nucleus and thalamus (both contralateral)

Anterior spinocerebellar tract (a)

Secondary neurons in intermediate gray matter, lumbosacral spinal cord. Relays proprioception (muscle spindles, tendon receptors, etc.) from dorsal root (spinal) ganglion cells, lower limb and trunk. Fibers cross locally and then ге-cross in the pons to return to the ipsilateral side.

Vermis and intermediate partofantenor lobe of cerebellum (ipsilateral-, terminates as mossy fibers)

Middle cerebellar peduncle: contains only afferent tracts.

Pontocerebellar fibers (a)

Basal pontine nuclei. Relay cerebropontine to pontocerebellar projection (source of 90% of axons in middle peduncle)

Lateral regions of posterior and anterior lobes of cerebellum (contralateral; terminate as mossy fibers; branches to contralateral dentate nucleus)

Inferior cerebellar peduncle: contains both afferent and efferent tracts.

Posterior spinocerebellar tract (a)

Posterior thoracic nucleus and thoracic spinal cord. Relays proprioception and cutaneous sensation from the lower limb. Contains large axons with high conduction velocity.

Vermis and nearby anterior lobe of cerebellum, pyramid and nearby posterior lobe of cerebellum.

(ipsilateral; terminates as mossy fibers)

Cuneocerebellar tract (a)

Nucleus cuneatus and external cuneate nucleus. Relays proprioception (external cuneate nucleus) and cutaneous sensation (nucleus cuneatus) from the upper limb, with fast transmission, functionally corresponding to the posterior spinocerebellar tract.

Posterior part of anterior lobe of cerebellum (ipsilateral; terminates as mossy fibers).

Olivocerebellar tract (a)

Inferior olivary nuclear complex. Inferior olive receives numerous inputs from sensory and motor systems, including a large contralateral projection from the cerebellum itself (dentate nucleus, see below).

Molecular layer of cerebellar cortex (contralateral, terminates as climbing fibers)

Vestibulocerebellar tract (a)

Semicircular canal (vestibular ganglion) and vestibular nuclei. Transmits balance and body position/motion information either directly (vestibular axons via vestibulocochlear nerve [CN VIII], ipsilateral) or via synaptic relay in vestibular nuclei (bilateral).

Nodule, flocculus, anterior lobe, and vermis of cerebellum (bilateral, see left; terminates as mossy fibers)

Trigeminocerebellar fibers (a)

Trigeminal sensory nuclei in the brainstem. Relay proprioception and cutaneous sensation from the head.

Rostral part of posterior lobe of cerebellum (ipsilateral; terminate as mossy fibers)

Cerebello-olivary fibers (e)

Dentate nucleus

Inferior olive (contralateral)

*Subentries for constituent parts are classified as efferent (e) or afferent (a).

**ln the case of afferents, the type of afferent is listed along with the site of origin. 243

17.4 Cerebellum, Simplified Functional Anatomy and Lesions

A Simplified functional anatomy of the cerebellum (after Klinke and Silbernagl)

Two-dimensional representation of the cerebellum. Left: afferent inputs to the cerebellar cortex; Right: paths of cerebellar (efferent) output.

The coordination of motor activity by the cerebellum can be divided into three broad categories corresponding to the areas responsible for the coordination:

 Maintenance of posture and balance (“vestibulocerebellum”)

 Dynamic control of muscle tone under various loads (“spinocere- bellum")

 Integration of activity of various muscle groups during complex tasks (“pontocerebellum”)

These categories of cerebellar function require different types of afferent information, and have different output (efferent) paths. Although afferent inputs and their corresponding tracts are not segregated by obvious anatomical boundaries in the cerebellum, there is a functional division that correlates with the evolution of the cerebellar structures (see B). The phylogenetically ancient part of the cerebellum (archicerebellum) receives vestibular input, projects to the fastigial nucleus and lateral vestibular nucleus, and controls trunk musculature through a “medial motor system” (see p.282). Dynamic control of muscle tone requires feedback from muscle and tendon proprioceptors entering the cerebellum through spinocerebellar tracts. This “spinocerebellar” function utilizes more recently evolved paleocerebellar structures, the emboliform and globose cerebellar nuclei, and modulates muscle activity through a “lateral motor system” that involves muscles in the extremities. The most recent evolutionary developments in the cerebellum include the significant expansion of cerebral cortical projections via a relay inthe pons, and a reciprocal massive cerebellar projection, through the dentate nucleus, back to the cerebral cortex via the thalamus. The neocerebellum thus sends information back to the cerebral cortex, which controls some musculature directly through corticonuclear projections to lower motor neurons controlling the tongue and face (see p. 232), and corticospinal projections to spinal motor neurons controlling the hands. This “pontocerebellar” pathway and function involves complex anticipatory activation of muscle groups to accept a load or limit a motion, and so can be characterized in part as a “planning” or “programming” function.

Note: this simplified outline of cerebellar function does not take into account the complexity of cerebellar contributions to a variety of other tasks. Visual inputs and oculomotor functions, specifically, have not been considered here.

В Synopsis of cerebellar classifications and their relationships to motor deficits

Some cerebellar lesions cause subtle cognitive deficits that cannot be explained simply as a loss of muscle coordination.

Functional classification

Phylogenetic classification

Anatomical classification

Deficit symptoms

Vestibulocerebellum

Archicerebellum

Flocculonodular lobe

• Truncal, stance and gait ataxia

• Vertigo

• Nystagmus

• Vomiting

Spinocerebellum

Paleocerebellum

Anterior lobe, parts of vermis; Posterior lobe, medial parts

• Ataxia, chiefly affecting the lower limb

• Oculomotor dysfunction

• Speech disorder (asynergy of speech muscles)

Pontocerebellum (= cerebrocerebellum)

Neocerebellum

Posterior lobe, hemispheres

• Dysmetria and hypermetria (positive rebound)

• Intention tremor

• Nystagmus

• Decreased muscle tone

C Cerebellar lesions

Cerebellar lesions may remain clinically silent for some time because other brain regions can functionally compensate for them with reasonable effectiveness. Exceptions are direct lesions of the efferent cerebellar nuclei, which cannot be clinically compensated.

Cerebellar symptoms:

Asynergy

Lack of coordination among different muscle groups, especially in the performance of fine movements.

Ataxia

Uncoordinated sequence of movements. Truncal ataxia (patient cannot sit quietly upright) is distinguished from stance and gait ataxia (impaired limb movements, such as an unsteady gait in inebriation). The patient stands with the legs spread apart and places his hand on the wall for stability (a).

Decreased muscle tone

Ipsilateral muscle weakness and rapid fatigability (asthenia).

Intention tremor

Involuntary, rhythmical wavering movement of the hand when a purposeful movement is attempted, as in the finger-nose test: normal test (b), test indicating a cerebellar lesion (c).

Rebound phenomenon

The patient, with eyes closed, is told to move the arm against a resistance from the examiner (d). When the examiner suddenly releases the arm, it forcefully “rebounds” toward the patient (hypermetria).