Which white matter structure carries information from the cerebral cortex to the cerebellum?

Connections of the cerebellum

The cerebellar peduncles contain the afferent and efferent tracts of the cerebellum. The inferior cerebellar peduncle contains four afferent tracts (posterior spinocerebellar, vestibulocerebellar, olivocerebellar and reticulocerebellar) and one efferent tract (the cerebellovestibular tract). The middle cerebellar peduncle is the largest and contains only afferent fibres from the pontine nucleus. This pontocerebellar tract provides an important connection between the cerebral cortex and cerebellum and modulates skilled activities of hands and fingers. The superior cerebellar peduncle contains one afferent, anterior cerebellar tract and one efferent tract from the cerebellar nuclei to the red nucleus, thalamus and medulla.

Invasion of the Brain Stem and Cerebellar Peduncle

Invasion of the brain stem and cerebellar peduncle is found in up to 40% of newly diagnosed medulloblastoma.4,42 In another study, brain stem or peduncle involvement is identified in 34% of cerebellar pilocytic and diffuse astrocytomas.43 Cerebellar peduncle involvement is suggested by an ill-defined border on the affected side on postcontrast MRI (Fig. 55-5). On MR midsagittal images without/with contrast infusion, a cerebrospinal fluid space between the tumor and the floor of the fourth ventricle may be found, which can indicate that there is no invasion of the floor of the fourth ventricle (Fig. 55-1C and D). However, absence of a cerebrospinal fluid space does not always indicate the invasion of tumor. As such, it is difficult to affirm unequivocally that the tumor is compressing or invading the floor of the fourth ventricle by imaging studies alone.

Vision

The optic nerve, cervical cord, brainstem and cerebellar peduncles are most commonly affected. Frequently at onset the first feature is of optic neuritis of varying severity. Pain is felt behind the affected eye with some element of visual disturbance occurring. In 58% of cases acuity is affected without pain. However, overall more than 90% of cases recover the majority of their visual acuity. Later on, other symptoms affecting eye movement are dysmetria, nystagmus, internuclear ophthalmoplegia (slowing of adduction) and occular flutter. Diplopia might be experienced.

Cerebellar Peduncles

The cerebellum is connected to the brainstem by three pairs of cerebellar peduncles (Fig. 27.1A-C). The inferior cerebellar peduncle is composed of a larger part, the restiform body, and a smaller portion, the juxtarestiform body (Fig. 27.1B, C). The restiform body is the large ridge on the dorsolateral aspect of the medulla rostral to the level of the obex. This bundle contains mainly fibers that arise in the spinal cord or medulla. The juxtarestiform body is located in the wall of the fourth ventricle. This bundle is composed primarily of fibers that form reciprocal connections between the cerebellum and vestibular structures (Table 27.1).

The basilar pons, which is located inferior to the exiting roots of the trigeminal nerve, is continuous into the middle cerebellar peduncle (brachium pontis), which is located superior (dorsal) to the exiting roots of the fifth cranial nerve (Figs. 27.1A-C and 27.2). These exiting roots represent the boundary between the basilar pons and the middle cerebellar peduncle. This large peduncle mainly conveys pontocerebellar fibers arise from the pontine nuclei of the basilar pons and enter the cerebellum.

The superior cerebellar peduncle (brachium conjunctivum) sweeps rostrally out of the cerebellum and penetrates into the midbrain just caudal to the exit of the trochlear nerve (Fig. 27.1A-C). Within the midbrain, these fibers cross the midline as the decussation of the superior cerebellar peduncle at the level of the inferior colliculus. This bundle contains predominantly cerebellar efferent fibers that originate from neurons of the cerebellar nuclei and distribute to the diencephalon and brainstem.

ANATOMY

The cerebellum consists of a central median region, the vermis, named for the wormlike contortions it presents caudally and a hemisphere laterally on each side of the vermis (Figs. 13-8, 13-9). The cerebellum is divided into two disproportionate regions: (1) the large body of the cerebellum and (2) the small flocculonodular lobe. These two regions are separated by the uvulonodular fissure (see Fig. 13-9). The flocculonodular lobe, also known as the archicerebellum or vestibular cerebellum, is confined to the ventral aspect of the cerebellum near its center. The nodulus is the most rostral part of the caudal vermis that is adjacent to the fourth ventricle. It connects laterally by a peduncle on each side to the flocculus, which is a small lobule on the ventral aspect of the cerebellar hemisphere. The much larger body of the cerebellum, consisting of the vermis and two hemispheres, is divided into rostral and caudal lobes by the primary fissure. Within each lobe, the folia are grouped into named lobules that reside in different portions of the vermis and hemispheres.

The cerebellum is attached to the brainstem by three groups of neuronal processes on each side of the fourth ventricle (Fig. 13-10). These processes are the cerebellar peduncles (see Figs. 2-9 through 2-14Fig 9Fig 14). Although arranged in a medial to lateral plane, they are named from rostral to caudal based on their connections with the brainstem. The caudal cerebellar peduncle connects the spinal cord and medulla with the cerebellum. It contains primarily afferent processes projecting to the cerebellum. The middle cerebellar peduncle connects the transverse fibers of the pons with the cerebellum, which is entirely afferent to the cerebellum. The rostral cerebellar peduncle connects the cerebellum with the mesencephalon and contains mainly efferent processes passing out of the cerebellum.

When the cerebellum is sectioned transversely or longitudinally, an extensive area of white matter is visible in the center. This area is the cerebellar medulla, which should not be confused with the medulla (oblongata) of the brainstem. The cerebellar medulla has extensions of white matter into the overlying folia. As a group, these extensions appear similar to tree branches and are called the arbor vitae. Individually, each is the white lamina of a folium. The arbor vitae are covered by the three layers of the cerebellar cortex. In the cerebellar medulla are situated collections of neuronal cell bodies that comprise the cerebellar nuclei (see Fig. 2-13). These cell bodies are organized into three nuclei on each side of the median plane. From medial to lateral, these nuclei are the fastigial, interposital, and lateral cerebellar nuclei. For some reason, students have difficulty with this concept of cerebellar nuclei embedded in the white matter of the cerebellar medulla. This arrangement of cerebellar nuclei is directly comparable to the cerebrum, where the neuronal cell bodies are located either on the surface in the cerebral cortex or deep to the surface in basal nuclei. Their arrangement in both the cerebellum and cerebrum is determined by the degree and pattern of migration of the cells from the germinal layer.

The cerebellar cortex, which is composed of three layers, forms the outer portion of each folium and is similar throughout the cerebellum (Fig. 13-11). The folial and sulcal surfaces in the adult are covered by the leptomeninges. Adjacent to these leptomeninges is the most external of the three cortical layers, the relatively cell-free molecular layer. It is composed mostly of the axons and telodendria of granule neurons and the dendritic zones of the Purkinje neurons and a small population of interneurons and astrocytes. The molecular layer covers the middle layer that is a narrow single layer of large flask-shaped neurons, which comprise the Purkinje neuron layer. The deepest of the three layers is the granule neuron (granular) layer. This layer is thick and is composed of a remarkably large number of small neuronal cell bodies and their dendritic zones. This cell layer varies from 5 to 6 neurons thick where the cortex is continuous from one folium to another at the depth of a sulcus to 15 to 20 neurons thick at the top of a folium. Figs. 13-12 through 13-19 illustrate gross and microscopic features of the cerebellum.

The cerebellar cortex is uniquely organized for the distribution of afferent information (see Fig. 13-11). Two major types of afferents to the cerebellum exist based on the morphology of their telodendrons: mossy fibers and climbing fibers. The more abundant mossy fibers have a widespread origin in the brainstem and spinal cord. As they pass into the cerebellum, collaterals of these axonal processes synapse on the cell bodies and dendritic zones of neurons in the cerebellar nuclei. The main axon continues through the cerebellar medulla into the white lamina of a folium and enters the granular layer, where it terminates on the dendritic zones of the granule neurons. These synapses are sometimes called glomeruli. They are adjacent to the small neuronal cell body of the granule neuron. The axon of this granule neuron projects externally through the Purkinje neuronal layer into the molecular layer, where this axon forms two branches that course in opposite directions parallel to the longitudinal axis of the folium. The dendritic zone of the Purkinje neuron is arranged in the molecular layer as a maze of branched axons that are oriented in a flat plane transverse to the axis of the folium. By this arrangement in the molecular layer, the axon of the granule neuron traverses the dendritic zone of numerous Purkinje neurons. Synapse occurs between these processes. This network can be likened to telephone wires (granule neuronal axons) coursing from one telephone pole (dendritic zone of Purkinje neurons) to another. Climbing fibers are the axons of olivary neurons that enter the cerebellum through the caudal cerebellar peduncle.34 Collaterals of these axons synapse on neurons in the cerebellar nuclei. The main axon continues through the cerebellar medulla into a white lamina of a folium and passes through the granule neuronal layer and the Purkinje neuronal layer into the molecular layer, where it entwines around the dendritic zone of the Purkinje neuron and terminates there in synapses.

The mossy and climbing fibers are facilitory at their synapse with neurons of the cerebellar nuclei and the granule and Purkinje neurons, respectively.58 Acetylcholine is the neurotransmitter released at the synapses of the mossy fibers and aspartate at the synapses of the climbing fibers. The granule neurons are facilitory to the Purkinje neurons with glutamate released at the synapses. Within the molecular layer are stellate neurons (outer and basket) that are inhibitory to the Purkinje neurons. Large stellate neurons (Golgi) are scattered through the granular layer. These neurons are inhibitory to granule neurons.56 The only axon that projects from the cerebellar cortex (an efferent axon) is that of the Purkinje neuron. These axons pass through the granular layer into the folial white matter lamina and continue into the cerebellar medulla. The majority of these terminate on the dendritic zones of the neurons in the cerebellar nuclei. A small population of Purkinje neuronal axons, the cell bodies of which are primarily located in the flocculonodular lobe, leave the cerebellum through the caudal cerebellar peduncle and terminate on the dendritic zones of neurons in the vestibular nuclei. At all of the telodendria of these Purkinje neurons, the inhibitory neurotransmitter gamma-aminobutyric acid is released.29 With the exception of these direct cerebellovestibular Purkinje neuronal projections, the efferent axons that project from the cerebellum to the brainstem are all from the cerebellar nuclei.54 This anatomy supports a major role of the cerebellar cortex in modulating the continual facilitation of the neurons in the cerebellar nuclei via Purkinje neuronal inhibition.

The cerebellum plays a major role in the control of motor activity. Therefore, logically, it must receive afferent information to provide it with knowledge of where the head, neck, trunk, and limbs are in space via connections with the general and special proprioceptive systems. It also needs information on what voluntary motor activity is to occur, and therefore it must receive afferents from the upper motor neuron (UMN) systems. The cerebellum is often called the great regulator of movement.

Key Terms: Brain and Cranial Nerves

abducens nerve

accessory nerve

anterior commissure

arachnoid mater

arbor vitae

central canal

cerebellar hemispheres

cerebellar peduncle, anterior, middle, and posterior

cerebellum

cerebral aqueduct (aqueduct of Sylvius)

cerebral hemispheres

cerebral peduncle

cerebrum

corpora quadrigemina

corpus callosum

diencephalon

dura mater

epithalamus

facial nerve

flocculonodular lobe

folia

fornix

genu

glossopharyngeal nerve

gyrus (plur., gyri)

habenula

habenular commissure

hypoglossal nerve

hypophysis

hypothalamus

inferior colliculi

infundibulum

intermediate mass (massa intermedia)

interventricular foramen (foramen of Munro)

lamina quadrigemina

lamina terminalis

lateral olfactory stria

lateral ventricle

longitudinal cerebral fissure

mammillary body

medial olfactory stria

medulla oblongata

medullary velum

meninx (plur., meninges)

metencephalon

myelencephalon

oculomotor nerve

olfactory bulbs

olfactory tract

optic chiasm

optic nerves

pia mater

pineal body

pineal gland

piriform lobe

pons

posterior commissure

pyramids

rhinal sulcus

septum pellucidum

splenium

sulcus (plur., sulci)

superior colliculi

tectum

tela choroidea of diencephalon

tela choroidea of myelencephalon

telencephalon

thalamus

third ventricle

trapezoid body

trigeminal nerve

trochlear nerve

trunk of corpus callosum

tuber cinereum

vagus nerve

ventral fissure

vermis

vestibulocochlear nerve (auditory nerve, octaval nerve)

14.1 ANATOMY

Anterior Inferior Cerebellar Artery Syndrome

Brainstem infarction may occur after damage to the AICA, the vascular supply to the pons and cerebellar peduncle. Mechanisms of injury include disruption, cauterization, and arteriospasm with thrombosis. A full-fledged AICA syndrome is extremely serious and is often fatal because it results in the loss of respiratory center control.48 Partial interruption of flow in the AICA system, avulsion of one or more of its branches, or obstruction of a nondominant AICA may result in an incomplete AICA syndrome.17 More recently, we have recognized several patients operated on for acoustic neuromas greater than 3 cm in diameter in whom gadolinium-enhanced MRI detected an infarction in the region of the middle cerebellar peduncle. These patients had unilaterally impaired cerebellar function and required prolonged physical therapy rehabilitation.49

4 What are the principal afferent and efferent pathways of the cerebellum?

The afferent and efferent pathways to and from the cerebellum course through three pairs of tracts (cerebellar peduncles) that connect the cerebellum to the brain stem:

The inferior cerebellar peduncle (restiform body) consists of mainly afferent fibers. A single efferent tract, the fastigiobulbar tract, goes to the vestibular nucleus from the flocculonodular lobe. Afferent fibers enter the inferior cerebellar peduncle from at least five sources, including: (1) the vestibulocerebellar tract, (2) the olivocerebellar tract, (3) the dorsal spinocerebellar tract, (4) the cuneocerebellar tract, and (5) the reticulocerebellar tract.

The middle cerebellar peduncle (brachium pontis) consists almost entirely of crossed afferent fibers from the pontine nuclei that transmit impulses from the cerebral cortex to the intermediate and lateral zones of the cerebellum (corticopontocerebellar tract).

The superior cerebellar peduncle (brachium conjunctivum) consists principally of efferent projections from the cerebellum. Rubral, thalamic, and reticular projections arise from the dentate and interposed nuclei. The fastigiobulbar tracts run with this peduncle for a short distance before it enters the inferior cerebellar peduncle. Afferent fibers include the ventral spinocerebellar tract and the trigeminocerebellar and tectocerebellar projections.

Cerebellar Peduncles and Pathways

The cerebellum is connected to the rest of the nervous system by three pairs of peduncles, or feet. The cerebellar peduncles anchor the cerebellum to the brainstem. All afferent and efferent fibers of the cerebellum pass through the three peduncles and the pons to the other levels of the nervous system. The pons, which means bridge, is aptly named; it is literally a bridge from the cerebellum to the rest of the nervous system (Fig. 6-8).

The inferior cerebellar peduncle, or restiform body, carries primary afferent fibers from the structures close to it: the medulla, spinal cord, and cranial nerve VIII. Thus spinocerebellar, medullocerebellar, and vestibular fibers pass through the inferior peduncle.

The middle cerebellar peduncle, or brachium pontis, connects the cerebellum with the cerebral cortex by the pathways that traverse it. The middle peduncle is easily recognized; it is the largest of the three peduncles and also conveys the largest number of fibers from the cerebral cortex and pons. It carries pontocerebellar fibers as well as the majority of the corticopontocerebellar fibers. These fibers convey afferent information from the temporal and frontal lobes of the cerebrum to the posterior lobe of the contralateral cerebellum.

The superior cerebellar peduncle, or brachium conjunctivum, conveys the bulk of efferent fibers that leave the cerebellum. The primary efferent fibers arise from an important nucleus deep in the cerebellum called the dentate nucleus. The rubrospinal and dentatothalamic pathways, along with several other tracts, leave by way of the superior peduncle and terminate in the contralateral red nucleus and ventrolateral nucleus of the thalamus. From here impulses are relayed to the cerebral cortex.