Cerebellar Function and Cerebellar Syndromes · 251 5

the cerebellorubral tract crosses the midline as soon as it enters the brainstem from behind, and the rubrospinal tract crosses the midline again just after its origin from the red nucleus (in the decussation of Forel). Similarly, the cerebellothalamic fibers travel from one side of the cerebellum to the opposite side of the thalamus and then proceed to the ipsilateral cerebral cortex, whose efferent fibers enter the pyramidal tract and decussate once more before they reach the spinal cord on the original side.

Cerebellar Function and Cerebellar Syndromes

Three important points must be grasped for a proper understanding of cerebellar function:

The cerebellum receives a very large amount of general and special sensory input, but does not participate to any significant extent in conscious perception or discrimination.

Although the cerebellum influences motor function, cerebellar lesions do not produce paralysis.

The cerebellum is unimportant for most cognitive processes but nonethe-

less plays a major role in motor learning and memory.

Essentially, the cerebellum is a coordination center that maintains balance and controls muscle tone through regulatory circuits and complex feedback mechanisms, and assures the precise, temporally well-coordinated execution of all directed motor processes. Cerebellar coordination of movement occurs unconsciously.

The individual components of the cerebellum (vestibulocerebellum, spinocerebellum, and cerebrocerebellum) have different functions in the coordination of movement. These particular functions can be determined from experimental studies in animals on the one hand, and from clinical studies of patients with cerebellar lesions on the other. The constellations of signs and symptoms accompanying cerebellar disease that will be described here are seldom observed in pure form, both because it is rare for only one of the functional components of the cerebellum to be affected in isolation, and because slowly expanding processes (such as benign tumors) may induce functional compensation. Other portions of the brain can apparently assume some of the functions of the cerebellum, if necessary. Yet, if the disturbance affects not just the cerebellar cortex but also the deep cerebellar nuclei, only minimal recovery is likely to occur.

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5252 · 5 Cerebellum

This being said, it is still best, from the didactic point of view, to consider the functions and typical clinical syndromes of each of the three parts of the cerebellum separately.

Vestibulocerebellum

Function. The vestibulocerebellum receives impulses from the vestibular apparatus carrying information about the position and movements of the head. Its efferentoutputinfluencesthemotorfunctionoftheeyesandbodyinsuchaway that equilibrium can be maintained in all positions and with any movement.

Synaptic connections. The following reflex arcs participate in the maintenance of equilibrium (balance). From the vestibular organ, impulses travel both directly and indirectly (by way of the vestibular nuclei) to the vestibulocerebellar cortex, and onward to the fastigial nucleus. The vestibulocerebellar cortex transmits impulses back to the vestibular nuclei, as well as to the reticular formation; from these sites, the vestibulospinal and reticulospinal tracts and the medial longitudinal fasciculus enter the brainstem and spinal cord to control spinal motor and oculomotor function (Fig. 5.5). These reflex arcs assure stability of stance, gait, and eye position and enable the fixation of gaze.

Lesions of the Vestibulocerebellum

Functional impairment of the flocculonodular lobe or fastigial nucleus renders the patient less capable of orienting himself or herself in the Earth’s gravitational field, or of keeping his or her gaze fixed on a stationary object when the head is moving.

Dysequilibrium. The patient has difficulty standing upright (astasia) and walking (abasia), and the gait is broad-based and unsteady, resembling the gait of a drunken individual (truncal ataxia). Heel-to-toe walking can no longer be performed. The unsteadiness is not due to a deficiency of proprioceptive impulses reaching consciousness, but rather to faulty coordination of the musculature in response to gravity.

Oculomotor disturbances, nystagmus. Cerebellar disturbances of oculomotor function are manifest as an impaired ability to hold one’s gaze on a stationary or moving target (lesions of the flocculus and paraflocculus). The result is saccadic pursuit movements and gaze-evoked nystagmus: if the patient tries to follow a moving object with his or her eyes, square-wave jerks can be observed, i.e., the amplitude of the microsaccades that normally occur in ocular pursuit is abnormally increased, so that they become visible to the examiner. Gaze-

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Cerebellar Function and Cerebellar Syndromes · 253 5

evoked nystagmus is more prominent when the eyes move toward the side of the cerebellar lesion and diminishes somewhat if the gaze is held to that side; if the eyes are then brought back to the midline, nystagmus in the opposite direction may be seen (rebound nystagmus).

Lesions of the vestibulocerebellum may impair the patient’s ability to suppress the vestibulo-ocular reflex (VOR, p. 191), in which turning the head produces saccadic jerks of the eyes. A healthy individual can suppress this reflex by fixing the gaze upon an object, but a patient with a vestibulocerebellar lesion cannot (impaired suppression of the VOR by fixation). Furthermore, lesions of the nodulus and uvula impair the ability of the VOR (rotatory nystagmus) to habituate and may lead to the appearance of periodic alternating nystagmus that changes directions every 2­4 minutes.

Cerebellar lesions can also produce various types of complex nystagmus, such as opsoclonus (rapid conjugate movements of the eyes in multiple planes) or ocular flutter (opsoclonus in the horizontal plane only), whose precise localization has not yet been determined.

Spinocerebellum

Function. The spinocerebellum controls muscle tone and coordinates the actions of antagonistic muscle groups that participate in stance and gait. Its efferent output affects the activity of the anti-gravity muscles and controls the strength of forces induced by movement (e. g., inertia and centrifugal force).

Connections. The cortex of the spinocerebellum receives its afferent input from the spinal cord by way of the posterior spinocerebellar tract, the anterior spinocerebellar tract, and the cuneocerebellar tract (from the accessory cuneate nucleus). The cortex of the paravermian zone mainly projects to the emboliform and globose nuclei, while the vermian cortex mainly projects to the fastigial nucleus. The efferent output of these nuclei then proceeds through the superior cerebellar peduncle to the red nucleus and the reticular formation, from which modulatingimpulsesareconveyedovertherubrospinal,rubroreticular,andreticulospinal tracts to the spinal motor neurons (Fig. 5.5). Each half of the body is servedbytheipsilateralcerebellarcortex,butthereisnoprecisesomatotopicarrangement.Recentstudiessuggestthattheneuralorganizationofthecerebellar cortex resembles a patchwork rather than an exact somatotopic map.

Some of the efferent output of the emboliform nucleus travels by way of the thalamus to the motor cortex—mainly the portion of it that controls the proximal musculature of the limbs (pelvic and shoulder girdles) and the trunk. By this means, the spinocerebellum also exerts an influence on voluntary, directed movements of these muscle groups.

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5254 · 5 Cerebellum

Lesions of the Spinocerebellum

The major manifestations of lesions of the cerebellar vermis and paravermian zone are as follows.

Lesions of the anterior lobe and of the superior portion of the vermis in and near the midline produce ataxia of stance and gait. The gait ataxia (abasia) produced by such lesions is worse than the ataxia of stance (astasia). Affected patients suffer from a broad-based, unsteady gait that deviates to the side of the lesion, and there is a tendency to fall to that side. The ataxia of stance is revealed by the Romberg test: when the patient stands with eyes closed, a gentle push on the sternum causes the patient to sway backwards and forwards at a frequency of 2­3 Hz. If the lesion is strictly confined to the superior portion of the vermis, the finger­nose test and the heel­knee­shin test may still be performed accurately.

Lesions of the inferior portion of the vermis produce an ataxia of stance

(astasia) that is more severe than the ataxia of gait. The patient has difficulty sitting or standing steadily, and, in the Romberg test, sways slowly back and forth, without directional preference.

Cerebrocerebellum

Connections. The cerebrocerebellum receives most of its neural input indirectly from extensive portions of the cerebral cortex, mainly from Brodmann areas 4 and 6 (the motor and premotor cortex) via the corticopontine tract (Fig. 5.6), but also, to a lesser extent, from the olive via the olivocerebellar tract (Fig. 5.7). The cerebellum receives advance notice of any planned voluntary movement initiated in the cerebral cortex, so that it can immediately send modulating and corrective impulses back to the motor cortex through the dentatothalamocortical pathway (Fig. 5.5, p. 247, and Fig. 5.6). The dentate nucleus also projects to the parvocellular portion of the red nucleus. Unlike the rest of the red nucleus, this part does not send fibers to the spinal cord by way of the rubrospinal tract. Rather, it projects through the central tegmental tract to the inferior olive, which then projects back to the cerebrocerebellum. This den- tato-rubro-olivo-cerebellar neural feedback loop plays an important role in neocerebellar impulse processing.

Function. The complex connections of the cerebrocerebellum enable it to regulate all directed movements smoothly and precisely. By way of the very rapidly conducting afferent spinocerebellar pathways, it continuously receives real-time information about motor activity in the periphery. It can thus

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take action to correct any errors in the course of voluntary movement to ensure that they are executed smoothly and accurately. The executive patterns of a large number of different types of movement are probably stored in the cerebellum, as in a computer, over the life of the individual, so that they can be recalled from it at any time. Thus, once we have reached a certain stage of development, we can perform difficult learned movements rapidly, relatively effortlessly, and at will by calling upon the precise regulatory function of the cerebellum.

The functions of the cerebellum extend beyond the coordination of movement to the processing of sensory stimuli and of information that is relevant to memory. A further discussion of these aspects is beyond the scope of this book.

Lesions of the Cerebrocerebellum

It follows from the discussion of cerebellar function in the preceding sections that lesions of the cerebrocerebellum do not produce paralysis but nonetheless severely impair the execution of voluntary movements. The clinical manifestations are always ipsilateral to the causative lesion.

Decomposition of voluntary movements. The movements of the limbs are atactic and uncoordinated, with dysmetria, dyssynergia, dysdiadochokinesia, and intention tremor. These abnormalities are more pronounced in the upper than in the lower limbs, and complex movements are more severely affected than simple ones. Dysmetria, i.e., the inability to stop a directed movement on time, is manifested (for example) by a moving finger going past the location of its target (past-pointing, overshoot; hypermetria). Dyssynergia is the loss of the precise cooperation of multiple muscle groups in the execution of a particular movement; each muscle group contracts, but the individual groups fail to work together correctly. Dysdiadochokinesia is an impairment of rapid alternating movements caused by a breakdown of the precisely timed coordination of antagonistic muscle groups: movements such as rapid pronation and supination of the hand are slow, halting, and arrhythmic. Intention tremor,or—more prop- erly—action tremor, is seen mainly in directed movements and becomes more intense the nearer the finger comes to its target. There may also be a postural tremor at a frequency of 2­3 Hz, particularly when the patient tries to hold the pronated hands directly in front, with arms extended.

Rebound phenomenon. When the patient presses against the examiner’s hand with maximum strength and the examiner suddenly pulls his or her own hand away, the patient’s movement fails to be braked as normal, and the arm lurches toward the examiner.

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