The cerebellum has traditionally been considered as a sensory motor integration centre involved in monitoring and modulating motor function in the spine, head, and limbs. The cerebellum receives afferent input from the spinal cord and sensory receptors via the spinocerebellar tracts as well as from the brainstem and cerebral cortex.

The input and output connections flow through the superior, inferior, and middle cerebellar peduncles which connect the cerebellum to the brainstem. The flocculonodular lobe is involved in the control of posture, eye movement, and certain autonomic responses via its connections with vestibular nuclei. The anterior lobe and posterior parts of the vermis receive input from the axial regions of the body and project to medial descending pathways. The lateral parts of the cerebe-llum and the central vermis are considered the ’neocerebellum’ and are thought to play a role in the planning of movement rather than the execution of movement.

It is now widely accepted that the cerebellum also plays a part in controlling affect, emotion, and cognition, especially the lateral component of the cerebellum, which is referred to as the neocerebellum or cerebrocerebellum.

The prefix ’neo’ indicates that this component of the cerebellum is the newest region to develop in human evolution. It is therefore the most advanced region of the cerebellum and its development parallels the growth of the lateral aspects of the cerebral hemispheres, the association cortices, and those areas associated with advanced communication, higher consciousness, and skilled use of the digits.

It is now clear that the cerebellum and vestibular systems also play a role in the integration of sensory information that is essential for generating appropriate responses to environmental stimuli and for a variety of other functions including constructing a perception of ourselves in the universe; controlling muscle movement; maintaining balance; maintaining internal organ and blood flow functionality; maintaining cortical arousal; and developing active plasticity in neural networks which allows environmental conditioning to occur. The importance of the cerebellum in the overall function of the neuraxis is demonstrated by the fact that there are approximately 20 million corticopontocerebellar fibres projecting between the cerebellum and the cortex, compared to only about 1 million corticospinal fibres supplying the cortical output to the voluntary muscles of the body. The integration function of the cerebellum is evident, as the input-to-output or afferent-to-efferent ratio in the cerebellum is approximately 40:1.

The cerebellum is composed of an outer covering of grey matter, the cerebellar cortex, the internal white matter, and three pairs of deep cerebellar nuclei arranged on either side of the midline. The deep nuclei are the fastigial, interposed, and the dentate nuclei. The bulk of the output of the cerebellum emerges from these three nuclei.

The cerebellum lies behind the pons and medulla in the posterior cranial fossa (Fig. 14.1). It is separated from the cerebrum by an extension of dura mater, the tentorium cerebelli, and from the pons and medulla by the fourth ventricle (Fig. 14.2). It is somewhat smaller than the cerebrum but this difference varies with age, being 1/8 the size of the adult cortex but only 1/20 the size of the infant cortex.

Figure 14.1 The location of the cerebellum. The cerebellum lies behind the pons and medulla in the posterior cranial fossa.

(from Drake et al. 2010. Gray’s Anatomy for Students 2nd edn. Churchill Livingstone, with permission)

Figure 14.2 The relationship of the cerebellum to the cortex.

(from Drake et al. 2010. Gray’s Anatomy for Students 2nd edn. Churchill Livingstone, with permission)

The cerebellum is derived from the rhombencephalon, along with its homologues the pons and medulla, and is connected to the brainstem via three peduncles. These peduncles, together with the anterior and posterior medullary velum, are the main routes of entry or exit from the cerebellum. The inferior cerebellar peduncle, also known as the restiform body, conveys a number of axon projections into the cerebellum including (Figs 14.3 and 14.4):

Figure 14.3 The anatomical relationships of the inferior, middle, and superior cerebellar peduncles.

Figure 14.4 The pathways of the cerebellar peduncles from a superior (A) and lateral (B) view.

The middle cerebellar peduncle or brachium pontis is the largest of the cerebellar peduncles, and contains the massive afferent corticopontocerebellar pathways. The middle peduncle is composed of axon projection fibres from the pontine nuclei to the contralateral cerebellum (Figs 14.3 and 14.4). The primary projections from the cerebral cortex to the cerebellum include premotor and supplementary motor areas (area 6), primary motor areas (area 4), primary sensory areas (areas 3, 1, and 2), and association and limbic cortices.

The superior peduncle or brachium conjunctivum supports both afferent and efferent fibres. The efferent fibres compose the following tracts:

Noradrenergic neurons in the locus ceruleus project to Purkinje dendrites in the molecular layer and with granule cells in the granular layer (Bloom et al. 1971; Kimoto et al. 1981). Dopaminergic fibres arising from the neurons in the ventral mesencephalic tegmentum project to the Purkinje and granule neurons of interposed and lateral cerebellar nuclei (Simon et al. 1979). Serotonergic afferent fibres arise from the raphe nuclei of the brainstem and terminate in both the molecular and granule layers (Takeuchi et al. 1982).

The anterior and posterior medullary velum also supports some decussating fibres of the superior cerebellar peduncles and trochlear nerve. The velum also supports fibres originating in the peduncles of the flocculus.

The cerebellar surface is a striking example of natural economics in that it contains parallel convolutions or folia running in a transverse direction on the surface of the cerebellum that increase the surface area of the cerebellar cortex and give the cerebellum a tree-like appearance (Fig. 14.6). There are three primary lobes, anterior, posterior, and flocculonodular, on the cerebellar surface which are further dived into ten lobules. The cerebellum can be divided into nine regions along the vermis, which is a small unpaired structure in the median portion of the cerebellum separating the two large lateral masses. These nine regions are listed in Table 14.1 from anterior to posterior (see also Fig. 14.7). There are two major fissures that divide the cerebellum into three main lobes, and a number of other fissures that divide each lobe into its respective lobules.

Figure 14.6 (A) A midline section through the vermis leaving the right cerebellar hemisphere intact. Note the components of the lobes including the central lobule, culmen, declive, tuber, pyramid, uvula, and nodule. (B) Note the superior, middle and inferior peduncles.

Table 14.1 Lobes and lobules of the cerebellum

Figure 14.7 The anatomical relationships of the different phylogenic areas of the cerebellum.

Early in the third month of development, the cerebellum appears as a dumbbell-shaped mass on the roof of the hindbrain vesicle. A number of transverse grooves representing the fissures begin to appear on the dorsal surface of the cerebellum. Later in the third month the posterolateral fissure becomes the first landmark to demarcate its adjacent lobes from one another and results in the separation of the flocculonodular lobe from the remainder of the cerebellum.

At the same time, the primary fissure begins to cut into the surface of the cerebellum, separating the anterior from the posterior lobe and other smaller fissures develop on the inferior surface.

Quick facts 14.2

Figure 14.5 Superior peduncle.

The cerebellum expands dorsally and the inferior aspects of the hemispheres undergo the greatest increase in size, causing the inferior vermis to be buried between them, thus forming the vallecula, which is a deep groove on the inferior surface.

From a functional point of view, we need to consider three main regions within the cerebellum. These three regions are derived from the archicerebellum, palaeocerebellum, and neocerebellum, based on their time of appearance through evolutionary history (phylogeny).

The archicerebellum is the first region to appear in phylogeny and comprises the flocculi, their peduncles, the nodulus, and the lingula. The archicerebellum is the oldest and most medial portion of the cerebellum. In humans, archicerebellum contributes to the vestibulocerebellum, which comprises the flocculonodular lobe and the lingula (see Fig. 14.7) (Brodal 1981). As its new name would indicate, the vestibulocerebellum is the region of the cerebellum that communicates most intimately with the vestibular system. In fact, the vestibular nuclei of the brainstem share similar relationships with the cortex of the archicerebellum as the deep cerebellar nuclei share with the cortex of the palaeo- and neocerebellum. They therefore serve functionally as a cerebellar nuclear complex. The vestibulocerebellum also contains the only cerebellar cortical cells that leave the body of the cerebellum before synapsing. In the other regions of the cerebellum, the output cells of the cerebellar cortex synapse on neurons of the deep cerebellar nuclei. The stimuli from these neurons evoke inhibitory postsynaptic potentials (IPSPs) in the deep cerebellar nuclei.

Phylogenically, the palaeocerebellum is next to develop. Apart from the lingula, it comprises the anterior lobe, the pyramid, and uvula of the posterior vermis. This separated the archicerebellum into two parts, the lingula anteriorly and the flocculonodulus posteriorly (Brodal 1981). The palaeocerebellum contributes to the spinocerebellum, which is involved in a variety of parameters associated with movement.

The neocerebellum was the most recent component to arise in phylogeny and comprises the posterior lobes apart from the pyramid and uvula. This developed in parallel with the expansion of the neopallium and neocortex of the brain and the posterior aspects of the thalamus, which reflects the extent of the association cortices of the brain. Both anterior and posterior lobes of the cerebellum have sensory motor maps of the complete body surface, which overlap each other exactly. The neocerebellum contributes to the cerebrocerebellum, which is thought to be involved in a wide range of activities including memory and learning.

The cerebellum was traditionally seen as a sensory motor integration centre involved in monitoring and modulating motor function in the spine, head, and limbs. It is now widely accepted that the cerebellum also plays a part in controlling affect, emotion, and cognition – especially the lateral component of the cerebellum, which is referred to as the neocerebellum or cerebrocerebellum.

The prefix ’neo’ indicates that this component of the cerebellum is the newest region to develop in human evolution. It is therefore the most advanced region of the cerebellum and its development parallels the growth of the lateral aspect of the cerebral hemispheres (the association cortices) and those areas associated with advanced communication, higher consciousness, and skilled use of the digits.

The cerebellar cortex is divided into three distinct layers: the molecular, Purkinje, and granule layers. The molecular layer is composed of axons of granule cells, known as parallel fibres, running parallel to long axis of the folia, Purkinje dendrites, basket cell interneurons, and stellate cell interneurons, both of which are inhibitory interneurons. The basket cells have long axons that run perpendicular to parallel fibres and synapse with Purkinje dendrites. They also synapse directly to some Purkinje cell bodies. The basket cells form synapses anterior and posterior to the parallel fibre beams, therefore resulting in inhibition of neighbouring parallel fibres. This is thought to produce a type of centre-surround antagonism. Stellate cells have smaller axons that inhibit local Purkinje cells and synapse on the distal aspect of their dendrites. Both basket cells and stellate cells are excited by parallel fibres.

The next layer, the Purkinje layer, consists of the only output fibres of the cerebellar cortex, the Purkinje cells. The dendrites of Purkinje cells project outwards to the molecular layer. Purkinje cells have recurrent collaterals that inhibit adjacent Purkinje cells and Golgi type II neurons.

Quick facts 14.4

Figure 14.8 Spinocerebellum.

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Figure 14.9 Spinocerebellum.

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Figure 14.10 Cerebrocerebellum.

The granule layer consists of Golgi neurons and granule cells. Granule cell bodies form the core of the cerebellar glomeruli (they are activated by mossy and climbing fibres) and receive axodendritic synapses from Golgi cells. Golgi cells promote inhibition of up to 10 000 granule cells and are also activated by mossy and climbing fibres in the granule cell layer. This promotes the sharpening of inputs in the cerebellar cortex by suppressing weak excitatory postsynaptic potentials (EPSPs).

The cellular interactions of the cortical cells result in one inhibitory output tract, from Purkinje cells which synapses on the deep cerebellar and vestibular nuclei. There are two main input tracts to the cerebellum, each of which is excitatory. The two input tracts consist of climbing fibres, which are actually the axons of neurons which reside in the contralateral inferior olive, and the mossy fibres, which are axons of neurons of a variety of pontomedullary reticular nuclei and axons of neurons in laminae VI and VII of the spinal cord (Ito 1984). As discussed earlier, aminergic neurons in the brainstem also project to the cerebellum.

Quick facts 14.8

Figure 14.11 Five cell types.

The climbing fibres first give off collateral projections to the deep cerebellar nuclei before synapsing on granule, Golgi, basket, and Purkinje cells in the cerebellar cortex (Gilman et al. 1981; Van der Want et al. 1989). Only one synapse per Purkinje cell occurs; however, many Purkinje cells are innervated by a single climbing fibre so that a single climbing fibre spike produces a burst of Purkinje cell activity. The mossy fibres influence the Purkinje activity indirectly via synapses on granule cells. The parallel fibres of the granule cells in turn synapse on the Purkinje cells directly or via basket or stellate interneurons. Each parallel fibre excites a long array of about 500 Purkinje neurons, whereas each Purkinje neuron receives input from approximately 200 000 parallel fibres (Gilman 1992) (Fig. 14.12).

Figure 14.12 The cellular connections of the cerebellar cortex.

All of the cerebellar neurons of the cortex are inhibitory except the granule cells. The Purkinje inhibitory output is exerted on the spontaneously active nuclear cells. Thus, the nuclear cells must have a strong ’pacemaker’ potential or a powerful excitatory input to match the inhibition resulting from the Purkinje cells. The latter excitatory input may be manifested in excitatory impulses from axon collaterals of mossy and climbing fibres that each give off before synapsing in the cerebellar cortex.

The cerebellum receives information about all commands originating in the motor and association areas of the brain via the climbing fibres of the inferior olive. These olivary neurons also receive input from descending midbrain and telencephalic structures. Climbing fibres detect differences between actual and expected sensory inputs rather than simply monitoring afferent information. Neurons in the inferior olive are electronically coupled through dendrodendritic synapses and therefore can fire in synchrony. The synchronous inputs produce complex spikes in multiple Purkinje cells. In turn, the electrotonic coupling is under efferent control by GABA-ergic fibres from the deep cerebellar nuclei so that they can be functionally disconnected. This results in the selection of specific combinations of Purkinje cells. Climbing fibres modulate synaptic efficacy of parallel fibres by reducing strength of EPSPs of parallel fibres and by inducing selective long-term depression in synaptic strength of parallel fibres active concurrently (within 100–200 ms). Long-term depression depends on prolonged voltage-gated calcium influx.

Damage to the cerebellar cortex or inferior olive leads to inability to adapt. The largest input to the cerebellum is from the contralateral cerebral cortex. This is known as the corticopontocerebellar pathway. There are approximately 20 million neurons in this pathway compared to only 1 million neurons in the corticospinal pathway of the spinal cord.

The axons of the Purkinje cells project to the deep cerebellar nuclei as well as to the vestibular nuclei. The cerebellum, via these output nuclei, is able to exert descending influences on the spinal cord as well as ascending influences on the cerebral cortex. These outputs can be separated into three components:

The first component originates in the cortex of the vermis and flocculonodular lobes and acts on the fastigial and vestibular nuclei. The fastigial nucleus and its efferent targets, the vestibular nuclei, are often referred to as the vestibulocerebe-llum and are involved in limb extension and muscle tone in the neck and trunk to maintain posture, as well as eye movements. Being the earliest to arise in evolutionary history and embryological development, the vestibulocerebellum serves the most primitive function of the cerebellum. It receives extensive inputs from sensory receptors throughout the head and body that provide us with spatial coordinates for the purpose of spatial orientation and self-awareness.

This includes information from the retina and advanced visual processing systems of the brain, auditory and vestibular neurons including mono- and polysynaptic connections from the inner ear, and muscle and joint receptors particularly from the spine via the vestibular nuclei.

Fastigial efferent fibres can be crossed and uncrossed. The crossed fibres project via the uncinate fasciculus of the superior cerebellar peduncle to the super colliculus bilaterally, and to the interstitial nucleus of Cajal (Ito 1984). Some fibres also project to the ventral lateral and ventral posterior lateral nuclei of the thalamus via the superior cerebellar peduncle. The uncrossed fibres form the fastigiobulbar projections whose bulk forms the smaller juxtarestiform body. These fibres project to all four of the ipsilateral vestibular nuclei and the ipsilateral reticular formation (Batton et al. 1977). The fastigial nuclei fire after the commencement of movement. This nucleus receives large inputs from the periphery and sends few projections to the motor cortex. The fastigial nucleus is involved in the feedback mechanisms of the cerebellum.

The second component originates in the intermediate areas of the cerebellar hemispheres and projects to the interposed nuclei. Some neurons from the interpositus nuclei project through the superior peduncles to synapse on magnocellular neurons of the contralateral red nucleus (Asanuma et al. 1983). The majority, however, synapse on neurons of the contralateral thalamic nuclei of the ventral lateral pars caudalis and ventral posterolateral pars oralis. These thalamic neurons fire almost simultaneously to the primary motor cortex and are involved in efferent copy mechanisms. Efferent copy mechanism compares the intended programme from the cortex to the cerebellum’s knowledge of the state of the organism. Corrections are sent to the brain prior to the movement being carried out. This minimises the time delay in regulating evolving movements in a changing environment.

The inability to carry out a motor programme because of apparent weakness in muscles may in fact be due to a disturbance in midline cerebellar function. An example can illustrate the concept. As an arm or leg is raised or moved away from the body a greater demand is placed on postural mechanisms, which then may fail because of poor reinforcement by medial descending motor pathways from the brainstem and cerebellum.

The third component originates in the lateral cortical areas and projects to the dentate nuclei. The axons of neurons of the dentate nucleus project through the brachium conjunctivum to the contralateral red nucleus where they terminate on the parvocellular neurons in the rostral third of the nucleus (Gilman 1992) as well as via the ventral lateral and ventral anterior nuclei of the thalamus. The dentate nucleus is more heavily activated in tasks requiring the conscious evaluation of sensory information, for example, tasks requiring processing of sensory input to solve or programme complex spatial and temporal motor programmes. The dentate nucleus receives little input from the periphery and is involved in feedforward responses. The feedforward mechanisms are important for fast movements.

The lateral cerebellum is involved in preprogramming of learned volitional movements. It regulates tone and movement of the ipsilateral limbs and is also important in cognition. Stimulation results in facilitation of ipsilateral flexor tone. When an action is carried out without the need for prior sensory guidance, the lateral component of the cerebellum becomes more active in readiness for delivering a self-motivated plan of action to the cerebral cortex.

Quick facts 14.9

Figure 14.13 The canalith repositioning manoeuvre (Epley’s manoeuvre).

The fastigial and vestibular projections control the proximal limb muscles mostly through an excitatory action on proximal extensor muscles. The interposed nuclei control limb movements of the upper and lower extremities via the rubrospinal tract and through ascending fibres that project to motor cortex via the ventral posterior lateral nucleus of the thalamus. The dentate nuclei mainly project to the motor cortex via the ventral lateral and ventral anterior nuclei of the thalamus (Figs 14.14 and 14.15).

Figure 14.14 The efferent (input) projections to the cerebellum.

Figure 14.15 The afferent (output) projections of the cerebellum.

Various functions of the cerebellum have been described simply as a ’damping, clamping’ system that smooths out irregularities from start and braking movements. Other possible functions proposed include the initiation of movements, both simple and compound, the correction of movement trajectory after perturbation, the control of the vestibulo-ocular reflex, and the stopping or braking of movements.

The function of the cerebellum may best be described if the various interacting components are considered as a circuit. The cerebellar circuit consists of a system of interconnecting brain parts that transform and combine messages on intent and the results of those actions into an optional set of instructions for motor execution appropriate at that time. Thus the cerebellum may be described as an implementer of higher brain functions which detects the variation between the programme demands and the actual muscular actions. In order to accomplish this, the cerebellum requires a feedforward projection map of the intended actions which emanates from association cortex or other higher centres. This feedback most probably arises via coactivation of descending pathways to alpha motor neurons, and through conventional feedback mechanisms.

Feedforward mechanisms are carried out through the dentato-rubro-thalamocortical pathway, which conveys a motor or cognitive plan from the cerebellum to the cerebral cortex, allowing the cerebral cortex to carry out a precise action. This is referred to as the ’feedforward’ pathway of the cerebe-llum. Feedforward processes are anticipatory movement plans such as contraction of triceps after biceps reflex contraction. The feedforward pathway is not feedback and is necessary during anticipatory and ballistic movements where feedback mechanisms either are not available or are too slow to evoke an appropriate response. Damage of these processes therefore leads to defective anticipatory control of limb motion.

The actions of the fastigial nucleus, which increase sympathetic activity as a result of input from the labyrinthine systems via the vestibular apparatus in posture-righting movements, and the actions of the cerebellum on the vasomotor centres, which alter blood flows to limb muscle before initiation of movement of those muscles, are also important aspects of cerebellar function.

Efference copy mechanisms integrate the sensory information concerning real-time status of the individual and the anticipated or programmed information from lateral brain and cerebellar regions in order to minimise error as the movement is evolving – or as the environment in which the individual is performing the movement is changing.

The interposed nuclei and the intermediate zone of the cerebellar cortex serve as a key link between areas of the cerebellum involved in motor planning and those areas that respond reflexively to sensory inputs from the spine and midline structures. Consider a basketball player shooting for goal from outside the 3-point line. Just before the player extends his elbows and flexes his wrists to shoot the ball, an opponent nudges him from the side. If the player did not react and change the motor programme initially set before shooting for goal, he would more than likely miss the goal because his body was pushed off line. However, because of the feedback of sensory information from the spine, limbs, and the vestibular system, the player is able to alter the original motor programme sent from the lateral hemisphere of the cerebellum and cortex of the brain.

The fastigial nucleus and the vermis of the cerebellum are chiefly involved in feedback mechanisms of sensorimotor programming. This means that these areas receive large inputs from muscles, joints, and connective tissue, particularly from midline structures. For example, alterations in an individual’s centre of mass leads to reflex changes in muscle tone that compensate for the anticipated perturbation in stability. Therefore, sensory inputs largely determine the output from the midline cerebellar nuclei and fastigial nuclei.

During learning of new tasks, feedback input is utilised first until the dentate and lateral cerebellum can begin firing to promote feedforward processes. In other words, we learn by trial and error. This is why during the learning phase our execution of tasks is slower and it takes more conscious effort to perform the task. The cerebellum provides a signal to the brain that promotes the closest learned response and this is constantly updated, based on judgement of degree of error and evolving changes in the environment, posture, etc. (Fig. 14.16).

Figure 14.16 Cerebellar functional systems. The functional projections between the cerebellum, vestibular nuclei, pontomedullary reticular formation (PMRF), the mesencephalic reticular formation (MES), and the cortex. The cerebellum receives input through the PMRF via the pontine projections from the contralateral cortex. The cerebellum projects back to the contralateral cortex via the red nuclear projections in the MES. Reciprocal projections exist between the ipsilateral vestibular nuclei and the cerebellum.

Lesions of the cerebellum can result in a multitude of symptoms based on the area of involvement within the cerebellum. There are, however, some general signs and symptoms of cerebellar damage that can be detected through a thorough history and examination. The following are clinical pearls that signal cerebellar involvement:

Dysfunction in the lateral cerebellum also leads to delayed initiation and timing of movement (decomposition) and poor coordination between distal and proximal joints and independent finger manipulation. The lateral cerebellum is also heavily involved in verb association tasks, especially the right side of the cerebellum, which shares reciprocal communication with Broca’s speech area.

Medial cerebellar lesions interfere only with accurate execution of a response, whereas lateral cerebellar lesions interfere with the timing of serial events. This applies not only to motor tasks but also judgement of elapsed time in mental or cognitive tasks. For example, a patient may have decreased ability to judge the difference between the length of two tones. This could result in poor judgement of prosodic speech or keeping time to music. A patient may have difficulty in detecting or responding to differences in speed of moving objects, such as optokinetic stimuli or judging the speed of oncoming traffic while crossing the road.

The cerebellum is active during all forms of learning including those associated with motor and cognitive function. It is particularly active during the early stages of learning when the individual is exposed to a novel stimulus. The cerebellum assumes increasing responsibility during learning until it gains essentially complete control of the motor tasks. The cerebellum becomes less active once the acquisition of the skill or task has been completed.

The cerebellum is responsible for recognising the context for action and automatically triggering an appropriate response to the stimulus. This also occurs in word association tasks. From a cognitive point of view, the cerebellum is involved in cognitive planning, associative learning, classical conditioning, instrumental learning, and voluntary shifts of selective attention between sensory modalities.

The cerebellum is involved in two basic operations involving eye control. The first involves its role in both real-time positional eye control with respect to visual acquisition and the second involves long-term adaptive control mechanisms regulating the oculomotor system (Leigh & Zee 1991). The cerebellum functions to ensure that the movements of the eye are appropriate for the stimulation that they are receiving. The flocculus of the vestibulocerebellum contains Purkinje cells that discharge in relation to the velocity of eye movements during smooth pursuit tracking, with the head either stationary or moving. For example, you can keep your head still and fixate your gaze on a moving object, in which case your eyes should smoothly follow the object across your visual field, or you could keep your eyes fixed on a stationary object and rotate your head, in which case your eyes should still smoothly track in the opposite direction and at the same speed as the rotation of your head to maintain the target in focus (Zee et al. 1981). Other neurons discharge during saccadic eye movement in relation to the position of the eye in the orbit. Individual control of eye movement is accomplished for the most part by the contralateral cerebellum although intimate bilateral integration is also important. For example, the smoothness of pursuit activity and the return to centre function of saccadic movement of the right eye are under left cerebellar modulatory control.

Lesions of the dorsal vermis and fastigial nucleus of the cerebellum result in saccadic dysmetria, especially hypermetria of centripetal saccades (Optican & Robinson 1980). Lesions of the flocculus result in a variety of eye movement dysfunctions including (Berthoz & Melvil-Jones 1985; Optican et al. 1986):

From a developmental perspective the vestibular system is one of the oldest parts of the nervous system. It consists of a peripheral and central portions. The peripheral portion includes the vestibular apparatus (semicircular canals and otolithic organs) and the vestibular nerve. The central portion includes the vestibular nuclei and flocculonodular lobe of the cerebellum.

Peripheral vestibular system

The vestibular components of the inner ear and the vestibular nerve make up the peripheral vestibular system. In the inner ear there are three semicircular canals: the horizontal, anterior and posterior, and two otolithic organs, the utricle and saccule.

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Midline cerebellar lesions

• Disordered stance and gait

• Truncal titubation

• Rotated postures of the head

• Disturbed extraocular movements

• Normal limb movements with isolated testing

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Paravermal lesions

Paravermal lesions usually presents with a combination of medial and lateral cerebellar signs.

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Lateral cerebellar lesions

• Abnormalities of stance and gait (except if it is an isolated lesion)

• Disturbed extraocular movements

• Decomposition of limb movements

• Dysmetria

• Dysdiadochokinesia/dysrhythmokinesis

• Appendicular ataxia

• Impaired check and excessive rebound

• Kinetic and static tremor

• Dysarthria and hypotonia

• Cognitive disturbances

The function of the semicircular canals is to detect rotational movements of the head. The separate canals are orientated in different planes so as to detect the different directions of movement (Fig. 14.17). They are paired with a canal from the opposite side, that is the horizontal canals, the right anterior/left posterior, and the left anterior/right posterior are paired so as to detect movement in their respective planes. Rotating the head horizontally to the right increases the rate of firing of receptors in the right horizontal canal, and decreases the rate of firing in the left. Similar paired responses are produced in the other opposing canals when they are moved in their planes. The canals are surrounded by a thin layer of fluid called perilymph, which is essentially the same composition as cerebrospinal fluid that cushions them from the surrounding bony labyrinth. They are filled with another fluid referred to as endolymph, which is very high in potassium. The horizontal canal is orientated 30° from the horizontal in the neutral position and is thus in the vertical plane when the head is tilted backwards by about 60°.

Figure 14.17 Summary of vestibular and cochlear structures.

Each canal has an expansion called the ampulla, which contains a specialised structure called the cupola and hair cells (Fig. 14.17). The receptors on the hair cells are polarised to respond to movement in one direction by firing, and to hyperpolarise (decrease firing) with movement in the other, with respect to the single kinocilium present on the hair cell (Fig. 14.18). As the head rotates, the fluid within the canal stays still due to inertia. This exerts pressure on the cupola in the ampulla, distorting it and its associated hair cells, either increasing or decreasing the rate of firing, depending on the direction of movement.

Figure 14.18 The receptors on the hair cells are polarised to respond to movement in one direction only, with respect to a single kinocilium also present on the hair cell.

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Blepharospasm

• Is spasms of the muscles around the eye so that the eye remains in a constant state of partial closure for prolonged periods

• Presents with other cranial dystonias

• Spasmodic dysphonia

• Meige’s syndrome

• Frequently observed in patients with chronic vestibulocerebellar disorders

The otolithic organs are designed to detect linear or translational movements. The utricle is orientated approximately horizontally and detects lateral and anterior/posterior linear movements. The saccule is vertically orientated and detects inferior/superior (vertical) and anterior/posterior linear movements. They consist of hair cells with their stereo and kinocilia embedded in a gelatinous membrane. Embedded in the top of this membrane are otoliths. During linear movements of the head, the inertia of these otoliths causes the cilia on the hair cells to be distorted, thus increasing or decreasing the firing rate of the hair cells, depending on the direction of movement. Sometimes the otoliths can become dislodged, usually due to trauma or degeneration of the gelatinous membrane. If they become trapped in the semicircular canals they will produce the condition known as benign paroxysmal positional vertigo.

The axons from the hair cells of the utricle synapse in the superior vestibular ganglion. The axons of the neurons in the superior vestibular ganglion then form the superior vestibular nerve. These axons contribute along with the axons of the inferior vestibular nerve from the saccule and the cochlear nerve to form the ipsilateral vestibulocochlear nerve (CN VIII). Some axons pass directly from the vestibular apparatus to synapse directly into the cerebellum. It is worth noting that there is a tonic discharge from the hair cells of the vestibular apparatus, with a phasic increase or decrease in discharge overlaid depending on the direction of movement to which they are subjected. Lesions which artificially disrupt the rate of tonic discharge will create a change on vestibular output including the sensation of movement, even when movement is not taking place.

The central vestibular system

The vestibular nuclei receive afferent information from a variety of sources including:

1. The vestibular apparatus, which includes the semicircular canals and otolithic organs and vestibular ganglia;

2. Proprioceptive information, which is relayed to the cerebellum via the climbing fibres which arise in Clark’s column of the spinal cord;

3. Visual information from the striate cortex, the superior colliculus, and lateral geniculate nucleus of the thalamus;

4. Tactile information, which projects from the thalamus and somatosensory cortex; and

5. Auditory information from the inferior colliculus and medial geniculate body of the thalamus.

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Diaschisis

• This occurs when there is absent or decreased input from a neurological group or pool that is ‘upstream’ (presynaptic) to the affected area.

• It is potentially reversible functional hypometabolism.

• Broca’s aphasia nearly always creates right cerebellar diaschisis.

The vestibular projections from cranial nerve VIII travel primarily to the vestibular nuclei and the flocculonodular lobe of the cerebellum. There are four vestibular nuclei on each side located at the pontomedullary junction in the floor of the fourth ventricle, just inferior and medial to the inferior cere-bellar peduncles (Fig. 14.19). They are the:

1. Medial vestibular nucleus;

2. Lateral vestibular nucleus;

3. Inferior vestibular nucleus (Deiter’s nucleus);

4. Superior vestibular nucleus.

Figure 14.19 The anatomical relationship of the vestibular nuclei (purple) in the brainstem.

All of the vestibular nuclei receive projection axons from the vestibular nerve. All of the vestibular nuclei form reciprocal projections with the flocculus and nodule of the posterior lobe of the cerebellum. These reciprocal projections form the cerebellovestibular fibres.

Projections from the medial vestibular nucleus ascend in the medial longitudinal fasciculus to synapse in the ipsilateral abducent, and contralateral trochlear, and oculomotor nuclei. Descending projections from the medial vestibular nucleus form the medial vestibulospinal tract, which descends in the anterior spinal cord and synapses on ventral horn neurons in the cervical and thoracic spinal cord areas. These projections probably do not reach the lumbar areas of the cord and thus are involved with the postural corrections of neck and upper limb muscles exclusively (Fig. 14.20).

Figure 14.20 The functional projections between the vestibular nuclei and other structures of the brainstem and cerebellum.

Projections from the lateral vestibular nuclei descend in the lateral vestibulospinal tracts to all levels of the cord (Fig. 14.20). It is thought this tract is involved in postural responses.

The integration system is composed of a complex array of projection systems involving (see Fig. 14.19):

1. The vestibular nuclei;

2. Areas of the brainstem reticular formation;

3. Areas of the mesencephalic reticular system;

4. All functional areas of the cerebellum (Shimazu & Smith 1971; Horak & Diener 1994);

5. Various nuclei of the thalamus;

6. Multiple areas of the cerebral cortex (Tomasch 1969; Uemura et al. 1977).

Assessment of the vestibular system

Because of the large number of connections the vestibular system has in the nervous system, its evaluation is important for the functional neurologist. Bedside evaluation of the vestibular system can be achieved with the following:

1. Assessment of posture—Head tilts and certain postural reactions can be due to altered vestibulo-ocular and vestibulo-spinal reflexes. In lesions of the graviceptive pathways from the otoliths and semicircular canals, patients may present with ocular tilt reactions which involves head tilt, skew deviation, and ocular torsion (Brodsky et al. 2006).

2. Stabilometry—The practitioner assesses the patient’s ability to stand unaided with eyes open and closed. By closing the eyes, the practitioner forces that patient to rely on vestibular and proprioceptive information to determine balance. If either of those systems are defective, the patient should experience noticeably greater difficulty staying upright and may fall (mildly increased sway is normal). If their balance improves with the eyes closed, they may have a visually mediated balance problem. By having the patient stand on a compliant surface, such as a block of high density foam, proprioceptive information is reduced, and the patient further must rely on vestibular information. Stabilometry may be done with force plates to further measure and quantify postural and balance changes.

3. Stepping test of Fukuda (Fukuda 1953) —The practitioner has the patient march on the spot for 50 steps with the eyes closed. The patient will usually deviate to the side of vestibular hypofunction. More than 30^° rotation is considered a positive test.

4. Headthrust test—The patient is asked to look at a point in the distance in front of them. The practitioner then quickly moves the head to the side. The vestibulo-ocular (VOR) reflex should keep the eyes on the target straight ahead. A deficiency of the VOR will mean the eyes travel with the head and the patient must make a quick saccade eye movement back to the target.

5. Nystagmus—Nystagmus can be due to many causes, but in vestibular dysfunction a discrepancy in the tonic vestibular discharge will result in the eyes being driven relatively slowly to the side of lower vestibular output. This will then result in a quick saccade eye movement back to the target. Nystagmus is named according to the direction of the fast movement. Normally, vestibular lesions result in decreased vestibular output; hence the nystagmus is usually away from the lesion side. In cases of subtle nystagmus, it can be best seen by viewing the movement of the blood vessels in the retina with an ophthalmoscope. Nystagmus due to peripheral lesions can usually be reduced with having the patient fixate their vision on something. Classically, this does not occur with central vestibular lesions. Hence the ability to fixate should be removed through Freznal or video-electronystagmography goggles, or by having the patient cover their other eye when using the ophthalmoscope.

6. Dix Hallpike test—This is a test for Benign Positional Paroxysmal Vertigo (BPPV). The test is designed to move any loose otoliths in the posterior semicircular canals, and thereby induce vertigo and nystagmus. The nystagmus should be toward the affected ear, normally down toward the floor. Steps A to C of Fig. 14.21 demonstrate the Dix Hallpike test for the right posterior canal.

Figure 14.21 The canalith repositioning manoeuvre (Epley’s manoeuvre).

Quick facts 14.18

The canalith repositioning manoeuvre (Epley’s manoeuvre)

Epley’s manoeuvre is a simple and long-lasting treatment for benign paroxysmal positional vertigo (BPPV) (see Fig. 14.21).

Patient is supine with their head hanging over the edge of the table with the effected side down. In this position the patient will usually experience vertigo and nystagmus for the few seconds. Once the vertigo and nystagmus stops, the head is kept extended and rolled away from the effected side until the patient is facing the opposite direction. The head and body are then rolled as a unit to the unaffected side. A neck brace or collar is then applied and the patient told not to move their head or neck vigorously for 1 or 2 days. During this period they should also sleep sitting up in a chair. This manoeuvre is effective in as many as 90–95% of cases of BPPV.