The Cerebellum and Approach to Ataxia

INTRODUCTION

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The output of the motor cortex (and the adjacent premotor and supplementary cortices) is shaped by input from the basal ganglia and cerebellum. The basal ganglia and cerebellum participate in corticocortical loops that begin and end in motor regions. The basal ganglia are involved in the initiation and patterning of movements (see Ch. 7), and the cerebellum is involved in the coordination of movements. Both structures are also involved in cognition and eye movements. Lesions of the cerebellum can lead to incoordination of movements (ataxia), imprecision of movements (dysmetria), difficulty with rapid alternating movements (dysdiadochokinesia), truncal and gait instability, and difficulty with articulation of speech (dysarthria). Due to cerebellar involvement in oculomotor and vestibular function, cerebellar lesions can also cause nystagmus, vertigo, nausea, and vomiting.

ANATOMY AND FUNCTION OF THE CEREBELLUM

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Overall Structure of the Cerebellum

Like the brain, the cerebellum has two hemispheres consisting of a cortex, deep white matter, and deep gray matter (the bilateral dentate, emboliform, globose, and fastigial nuclei) (Fig. 8–1). At the midline, a cerebellar structure called the vermis lies between the hemispheres. The midline vermis controls coordination of the middle of the body, so pathology of the vermis leads to truncal and gait instability. The laterally placed cerebellar hemispheres control the lateral parts of the body: the limbs. Therefore, lesions of the cerebellar hemisphere can cause limb ataxia. Lesions in the cerebellar hemispheres cause deficits in the arm and/or leg ipsilateral to the affected hemisphere (in contrast to lesions of the cerebral hemispheres which cause deficits in the arm and/or leg contralateral to the affected hemisphere).

FIGURE 8–1

Anatomy of the cerebellum. A: Schematic posterior view of the cerebellum. B: Schematic posterior view of the brainstem with the cerebellum removed, demonstrating the three cerebellar peduncles. C: Schematic anterior view of the cerebellum with the brainstem removed. D: Schematic midsagittal view of the cerebellum. E: T1-weighted MRI, sagittal view. F: Schematic axial section of the cerebellum at the level of the pons and middle cerebellar peduncle demonstrating the deep cerebellar nuclei. Reproduced with permission from Martin J: Neuroanatomy Text and Atlas, 4th ed. New York: McGraw-Hill Education; 2012.

The left and right flocculi and the midline nodulus (together referred to as the flocculonodular lobe) are anterior cerebellar structures involved in vestibular function and eye movements.

The Cerebellar Peduncles

In order to coordinate movements, the cerebellum must have access to two types of information (Figs. 8–2, 8–3, and 8–4):

What the brain wants the body to do
Where the body is in space

FIGURE 8–2

Schematic of the flow of information through the cerebellar peduncles.

FIGURE 8–3

Schematic of the pathways that traverse the cerebellar peduncles demonstrating projections to and from the cerebellum. Reproduced with permission from Aminoff M, Greenberg D, Simon R: Clinical Neurology, 9th ed. New York: McGraw-Hill Education; 2015.

FIGURE 8–4

Axial T1-weighted postcontrast MRI demonstrating the cerebellar peduncles. A: At the level of the upper pons showing the superior cerebellar peduncles. B: At the level of the midpons showing the middle cerebellar peduncles. C: At the level of the medulla showing the inferior cerebellar peduncles.

After determining what needs to be done to get the body from where it is to where the brain wants it to go, the cerebellum computes a plan and sends it back to the motor regions to carry out the appropriate adjustments to coordinate ongoing movements.

To accomplish these goals, the cerebellum needs two inputs (one with information about what the brain wants the body to do, one with information about where the body is in space) and one output (to tell the brain how to guide further movements). This information travels in the three paired cerebellar peduncles (superior, middle, and inferior cerebellar peduncles on each side), which are the conduits of information into and out of the cerebellum. The cerebellar peduncles should not be confused with the cerebral peduncles (the name given to the corticospinal tracts at the level of the midbrain).

Inferior Cerebellar Peduncles: Input From Below

The inferior cerebellar peduncles mostly carry input in from inferiorly: The vestibulocerebellar tracts (vestibular information about where the head is in space), spinocerebellar tracts (proprioceptive information about where the body is in space), and olivocerebellar tracts (involved in motor learning) all travel through the inferior cerebellar peduncles to the cerebellum. The inferior cerebellar peduncles enter the cerebellum at the most inferior level of the brainstem: the medulla. All pathways traveling through the inferior cerebellar peduncles project to the ipsilateral cerebellar hemisphere except the olivocerebellar tracts, which cross. There is one minor exception to the inferior cerebellar peduncles being input pathways: The cerebellum sends output back to the vestibular system and this passes through the inferior cerebellar peduncles.

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