11. The Cerebellum
Revised January 25, 2021
The objectives of this chapter are to:
- Describe cerebellar anatomy, afferents, internal circuitry, efferents and blood supply
- Compare the effects of lesions of the hemisphere, vermis and vestibulocerebellum (flocculonodular lobe).
I. Cerebellar Anatomy
A. Anatomic Divisions of the Cerebellum: Like the Balkans, the cerebellum has been divided, redivided, and parcelled into various divisions and regions. In this section we will talk about the two kinds of anatomic divisions: 1) vermis and hemispheres, and 2) three lobes.
Three lobes: The cerebellum is divided into three lobes by two fissures (fig 11a). The posterolateral fissure separates the flocculonodular lobe from the rest of the cerebellum. The flocculonodular lobe is composed of the small nodulus (#15111), which is part of the midline vermis, and the left and right flocculi (#5260, #5283), which are small lobules of the hemispheres. This lobe is functionally related to the vestibular system, especially via its efferents.
The rest of the cerebellum is further divided by the primary fissure into an anterior lobe and a posterior lobe. The anterior lobe is rostral to the primary fissure (#11721). The primary fissure is hard to find by examining the surface of the cerebellum (#5585), but it is the deepest fissure that is seen in midsagittal sections of the cerebellum (#11721). The posterior lobe, the largest cerebellar lobe, is caudal to the primary fissure (#5317). What are the cerebellar tonsils?
The anterior lobe just might be of some interest because it degenerates in alcoholics (#5335). It atrophies, leaving the individual with ataxia. What are some other indications of cerebellar disturbance?
Many textbooks mistakenly try to functionally equate anatomic divisions with afferent regions and with evolutionary divisions. E.g., posterior lobe = pontocerebellum = neocerebellum. However, 1) Afferent regions overlap. Afferent regions are not discrete divisions and do not correspond to anatomic divisions. 2) Evolutionary terms do not refect current knowledge of the comparative anatomy and evolution of the cerebellum. Evolutionary terms (neocerebellum, paleocerebellum and pontocerebellum) are outdated despite their continued frequent use in textbooks and even journal articles.
B. Cerebellar peduncles: The cerebellar peduncles have been referred to incidentally in previous chapters. Now they are to be
studied. There are three on each side: the inferior cerebellar peduncle (#4025, #6172), the middle cerebellar peduncle (brachium pontis) (#8361, #6553), and the superior cerebellar peduncle (brachium conjunctivum) (#6554).
The middle and inferior cerebellar peduncles contain most of the cerebellar
afferents. The superior cerebellar peduncle contains the majority of the cerebellar efferent
fibers. What attaches the cerebellum to the brain stem (#4629)?
II. Cerebellar Afferents
A. Afferents via the inferior cerebellar peduncle: Proprioceptive information from muscle spindles (#7606) in the trunk and lower limb travels to the nucleus dorsalis (Clarke's column, Clarke's nucleus) (fig 11b). This nucleus extends from T1 to L2. Axons from this nucleus form the ipsilateral dorsal spinocerebellar tract (fig 11b, #6525), which ascends in the lateral funiculus of the spinal cord to the medulla (#6530 #4369). The dorsal spinocerebellar fibers enter the cerebellum via the inferior cerebellar peduncle (#6172) and end as mossy fibers in the cerebellar cortex.
[The lateral cuneate nucleus, also called the accessory or external cuneate nucleus (fig 11c), is rarely mentioned clinically or pathologically. It is similar to the nucleus dorsalis, and serves as a relay nucleus for proprioceptive information entering the spinal cord from the neck and upper limb. Axons of this nucleus, the cuneocerebellar fibers, join the dorsal spinocerebellar tract and enter the inferior cerebellar peduncle.]
Click for the Movie of proprioceptive input to the cerebellum from "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charleston, South Carolina.
It is now time to introduce the inferior olivary nucleus (#6541). The cells of the inferior olivary nucleus send their axons across the midline (#6542). These olivocerebellar axons (#6540, #7977) travel in the inferior cerebellar peduncle (#6317) to terminate as climbing fibers in the cerebellar cortex.
[Other axons in the inferior cerebellar peduncle come from the vestibular nerve and nuclei, reticular formation and trigeminal nuclei.]
B. Afferents via the middle cerebellar peduncle: The middle cerebellar peduncle is by far the largest peduncle (#5599) and contains only afferents. The axons in this peduncle come from the pontine nuclei of the opposite side (#6545). What is the afferent supply of the pontine nuclei? What course do the afferent axons follow in reaching the pons? What is the vascular supply of the pons? Pontocerebellar (transverse pontine) axons terminate as mossy fiber endings.
The cerebellum receives sensory information of all modalities, not just proprioception. Indirect pathways bring auditory, visual, somatosensory, and cortical information to the cerebellum.
III. Cerebellar Internal Circuitry
A. Cerebellar cortex: The surface of the cerebellum is a sheet of gray matter that is folded quite regularly into folia (#4959). The structure of the cerebellar cortex is uniform throughout and it is difficult to distinguish - even histologically - one region from another. This is not true for the cerebral cortex.
Study fig 11e. Identify the molecular layer (#8321) and granular layer (#8323). Cell bodies of the Purkinje cells occur along the boundary between these layers (#8322). The complex dendrites (#4486) of the Purkinje cells radiate into the molecular layer. What is the orientation of these dendrites with respect to the granule cell axons (parallel fibers)?
B. Cerebellar Nuclei: The cerebellar nuclei are located deep in the white matter adjacent to the roof of the fourth ventricle (fig 11f) and receive axons from Purkinje cells as well as collaterals from all climbing fibers and some mossy fibers. The nuclei are the main source of cerebellar efferent fibers. The largest is the dentate nucleus (#6567). The Purkinje cells of the lateral part of the hemisphere project to the dentate nucleus. The medial part of the hemisphere projects to the emboliform (#6566) and globose (#7926) nuclei (together called the interposed nuclei).The fastigial nucleus (#6565) receives axons from Purkinje cells of the vermis.
IV. Cerebellar Efferents
A. Hemisphere efferents: The superior cerebellar peduncle (#15178) originates from the nuclei of the cerebellar hemisphere (#8438), and carries most of the cerebellar efferent fibers. It begins on the medial side of the dentate nucleus (#6575) and forms the lateral wall of the rostral part of the fourth ventricle (fig 11d, #6578). The superior cerebellar peduncles converge (#6544) to cross the midline in the decussation of the superior cerebellar peduncles (#6336) in the tegmentum of the midbrain (fig 11g) at the level of the inferior colliculus. (In Fig. 11g, the superior colliculus is seen rather than the inferior colliculus because the slice is oriented coronally.)
Axons of the superior cerebellar peduncle project to the red nucleus and the thalamus, but there is little clinical correlation for the fibers to the red nucleus. Axons of the cerebellothalamic tract (or dentatothalamic tract) pass through and around the red nucleus to the thalamus. They terminate in the posterior part of the ventral lateral nucleus (VLp) of the thalamus (#6592), also called the ventral intermediate nucleus (Vim) in humans. This thalamic nucleus (fig 11h) projects mainly to the primary motor cortex (#4321) and the premotor cortex (#4322) of the frontal lobe to affect limb coordination. What is the route of these thalamocortical fibers? A constellation of findings, sometimes called the cerebellar hemisphere syndrome, occurs with disease of the cerebellar hemisphere.
B. Vermis efferents: Other cerebellar efferents arise in the vermis and its fastigial nuclei and leave the cerebellum in the inferior cerebellar peduncle. They end in the vestibular nuclei and reticular formation, which give rise to vestibulospinal and reticulospinal tracts. These projections all affect axial muscles. Disease of the vermis is associated with 1) truncal ataxia: impaired equilibrium (balance) with unsteady stance and gait (though these also occur with hemisphere disease) and unsteadiness while sitting, 2) titubation, and 3) inaccurate (dysmetric) saccades.
C. Vestibulocerebellum (flocculonodular lobe) efferents: Finally, other cerebellar efferents arise from the Purkinje cells in the vestibulocerebellum, which is almost equivalent to the flocculonodular lobe. These projections again leave the cerebellum in the inferior cerebellar peduncle. They end in ipsilateral vestibular nuclei that project via the medial longitudinal fasciculus (fig 11d) to the nuclei of cranial nerves III, IV and VI to affect eye movements. Vestibulocerebellar (flocculonodular lobe) disease causes many eye movement disorders, especially gaze-evoked nystagmus, impaired smooth pursuit (eyes following a moving object), and downbeat nystagmus.
Think about the functional differentiation of each region as deduced from its connections, particularly its efferents. From what has been studied, you should now know if the influence of the cerebellum on skeletal muscle is ipsilateral or contralateral. Is the right cerebral cortex "working" with the right or left cerebellar cortex? Do the clinical signs of cerebellar disease generally occur on the same side or opposite side of the lesion? Does isolated cerebellar disease cause loss of strength? Loss of sensation? A positive Romberg sign?
V. Blood Supply to the Cerebellum
The blood supply to the cerebellum comes from branches of the vertebral and basilar arteries. The three cerebellar arteries are the posterior inferior cerebellar artery (#8469), anterior inferior cerebellar artery (#8453), and the superior cerebellar artery (#8455). What is the relationship of the superior cerebellar artery to cranial nerve III?
As these arteries go around the circumference of the brain stem, they give off branches that vascularize discrete portions of the tegmentum. For example, branches of the posterior inferior cerebellar artery nourish the dorsolateral quadrant of the medulla. The constellation of findings resulting from a lesion here is called the lateral medullary syndrome. What sensory pathways will be interrupted if this artery is occluded as in (#2826)? Here the right side of the medulla is affected. On which side(s) will the sensory deficits occur? What other neurologic impairments will be found on examination of the patient? What area of the brain stem is vascularized by the anterior inferior cerebellar artery? By the superior cerebellar artery?
Click for the Syllabus Quiz.
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