Anatomically, the cerebellum is divided into three lobes by two deep fissures, as shown in Figures 56-1 and 56-2: (1) the anterior lobe, (2) the posterior lobe, and (3) the flocculonodular lobe. The flocculonodular lobe is the oldest of all portions of the cerebellum; it developed along with (and functions with) the vestibular system in
Somatosensory projection areas in the cerebellar cortex.
controlling body equilibrium, as discussed in Chapter 55.
Longitudinal Functional Divisions of the Anterior and Posterior Lobes. From a functional point of view, the anterior and posterior lobes are organized not by lobes but along the longitudinal axis, as demonstrated in Figure 56-2, which shows a posterior view of the human cerebellum after the lower end of the posterior cerebellum has been rolled downward from its normally hidden position. Note down the center of the cerebellum a narrow band called the vermis, separated from the remainder of the cerebellum by shallow grooves. In this area, most cere-bellar control functions for muscle movements of the axial body, neck, shoulders, and hips are located.
To each side of the vermis is a large, laterally protruding cerebellar hemisphere, and each of these hemispheres is divided into an intermediate zone and a lateral zone.
The intermediate zone of the hemisphere is concerned with controlling muscle contractions in the distal portions of the upper and lower limbs, especially the hands and fingers and feet and toes.
The lateral zone of the hemisphere operates at a much more remote level because this area joins with the cerebral cortex in the overall planning of sequential motor movements. Without this lateral zone, most discrete motor activities of the body lose their appropriate timing and sequencing and therefore become incoordinate, as we discuss more fully later.
Topographical Representation of the Body in the Vermis and Intermediate Zones. In the same manner that the cerebral sensory cortex, motor cortex, basal ganglia, red nuclei, and reticular formation all have topographical representations of the different parts of the body, so also is this true for the vermis and intermediate zones of the cerebellum. Figure 56-3 shows two such representations. Note that the axial portions of the body lie in the vermis part of the cerebellum, whereas the limbs and facial regions lie in the intermediate zones. These topographical representations receive afferent nerve signals from all the respective parts of the body as well as from corresponding topographical motor areas in the cerebral cortex and brain stem. In turn, they send motor signals back to the same respective topographical areas of the cerebral motor cortex, as well as to topographical areas of the red nucleus and reticular formation in the brain stem.
Note that the large lateral portions of the cerebellar hemispheres do not have topographical representations of the body. These areas of the cerebellum receive their input signals almost exclusively from the cerebral cortex, especially from the premotor areas of the frontal cortex and from the somatosensory and other sensory association areas of the parietal cortex. It is believed that this connectivity with the cerebral cortex allows the lateral portions of the cerebellar hemispheres to play important roles in planning and coordinating the body's rapid sequential muscular activities that occur one after another within fractions of a second.
The human cerebellar cortex is actually a large folded sheet, about 17 centimeters wide by 120 centimeters long, with the folds lying crosswise, as shown in Figures 56-2 and 56-3. Each fold is called a folium. Lying deep beneath the folded mass of cerebellar cortex are deep cerebellar nuclei.
Afferent Pathways from Other Parts of the Brain. The basic input pathways to the cerebellum are shown in Figure 56-4. An extensive and important afferent pathway is the corticopontocerebellar pathway, which originates in the cerebral motor and premotor cortices and also in the cerebral somatosensory cortex. It passes by way of the pontile nuclei and pontocerebellar tracts mainly to the lateral divisions of the cerebellar hemispheres on the opposite side of the brain from the cerebral areas.
In addition, important afferent tracts originate in each side of the brain stem; they include (1) an extensive olivocerebellar tract, which passes from the inferior olive to all parts of the cerebellum and is excited in the olive by fibers from the cerebral motor cortex, basal ganglia, widespread areas of the reticular formation, and spinal cord; (2) vestibulocerebellar fibers, some of which
Anterior Superior cerebellar lobe peduncle
Ventral spinocerebellar tract
Anterior Superior cerebellar lobe peduncle
Middle cerebellar peduncle Vestibulocerebellar tract Olivocerebellar and reticulocerebellar tract Inferior cerebellar peduncle Ventral spinocerebellar tract Dorsal spinocerebellar tract originate in the vestibular apparatus itself and others from the brain stem vestibular nuclei—almost all of these terminate in the flocculonodular lobe and fastigial nucleus of the cerebellum; and (3) reticulocerebellar fibers, which originate in different portions of the brain stem reticular formation and terminate in the midline cerebellar areas (mainly in the vermis).
Afferent Pathways from the Periphery. The cerebellum also receives important sensory signals directly from the peripheral parts of the body mainly through four tracts on each side, two of which are located dorsally in the cord and two ventrally. The two most important of these tracts are shown in Figure 56-5: the dorsal spinocere-bellar tract and the ventral spinocerebellar tract. The dorsal tract enters the cerebellum through the inferior cerebellar peduncle and terminates in the vermis and intermediate zones of the cerebellum on the same side as its origin. The ventral tract enters the cerebellum through the superior cerebellar peduncle, but it terminates in both sides of the cerebellum.
The signals transmitted in the dorsal spinocerebellar tracts come mainly from the muscle spindles and to a lesser extent from other somatic receptors throughout the body, such as Golgi tendon organs, large tactile receptors of the skin, and joint receptors. All these signals apprise the cerebellum of the momentary status of (1) muscle contraction, (2) degree of tension on the muscle tendons, (3) positions and rates of movement of the parts of the body, and (4) forces acting on the surfaces of the body.
Conversely, the ventral spinocerebellar tracts receive less information from the peripheral receptors. Instead, they are excited mainly by motor signals arriving in the anterior horns of the spinal cord from (1) the brain through the corticospinal and rubrospinal tracts and (2) the internal motor pattern generators in the cord itself. Thus, this ventral fiber pathway tells the cerebellum
Principal afferent tracts to the cerebellum.
which motor signals have arrived at the anterior horns; this feedback is called the efference copy of the anterior horn motor drive.
The spinocerebellar pathways can transmit impulses at velocities up to 120 m/sec, which is the most rapid conduction in any pathway in the central nervous system. This extremely rapid conduction is important for instantaneous apprisal of the cerebellum of changes in peripheral muscle actions.
In addition to signals from the spinocerebellar tracts, signals are transmitted into the cerebellum from the body periphery through the spinal dorsal columns to the dorsal column nuclei of the medulla and then relayed to the cerebellum. Likewise, signals are transmitted up the spinal cord through the spinoreticular pathway to the reticular formation of the brain stem and also through the spino-olivary pathway to the inferior olivary nucleus. Then signals are relayed from both of these areas to the cerebellum. Thus, the cerebellum continually collects information about the movements and positions of all parts of the body even though it is operating at a subconscious level.
Output Signals from the Cerebellum Deep Cerebellar Nuclei and the Efferent Pathways. Located deep in the cerebellar mass on each side are three deep cerebellar nuclei—the dentate, interposed, and fastigial. (The vestibular nuclei in the medulla also function in some respects as if they were deep cerebellar nuclei because of their direct connections with the cortex of the flocculonodular lobe.) All the deep cerebellar nuclei receive signals from two sources: (1) the cerebellar cortex and (2) the deep sensory afferent tracts to the cerebellum.
Each time an input signal arrives in the cerebellum, it divides and goes in two directions: (1) directly to one of the cerebellar deep nuclei and (2) to a corresponding area of the cerebellar cortex overlying the deep nucleus. Then, a fraction of a second later, the cerebellar cortex relays an inhibitory output signal to the deep nucleus. Thus, all input signals that enter the cerebellum eventually end in the deep nuclei in the form of initial excitatory signals followed a fraction of a second later by inhibitory signals. From the deep nuclei, output signals leave the cerebellum and are distributed to other parts of the brain.
The general plan of the major efferent pathways leading out of the cerebellum is shown in Figure 56-6 and consists of the following:
1. A pathway that originates in the midline structures of the cerebellum (the vermis) and then passes through the fastigial nuclei into the medullary and pontile regions of the brain stem. This circuit functions in close association with the equilibrium apparatus and brain stem vestibular nuclei to control equilibrium, and also in association with the reticular formation of the brain stem to control the postural attitudes of the body. It was discussed in detail in Chapter 55 in relation to equilibrium.
2. A pathway that originates in (1) the intermediate zone of the cerebellar hemisphere and then passes through (2) the interposed nucleus to (3) the ventrolateral and ventroanterior nuclei of the thalamus and then to (4) the cerebral cortex, to (5) several midline structures of the thalamus and then to (6) the basal ganglia and (7) the red nucleus and reticular formation of the upper portion of the brain stem. This complex circuit helps to coordinate
To thalamus - Red nucleus
Reticulum of mesencephalon
Superior cerebellar peduncle o ^ Fastigioreticular tract
0 \ ^ Fastigial nucleus
Fastigioreticular tract Paleocerebellum
mainly the reciprocal contractions of agonist and antagonist muscles in the peripheral portions of the limbs, especially in the hands, fingers, and thumbs.
3. A pathway that begins in the cerebellar cortex of the lateral zone of the cerebellar hemisphere and then passes to the dentate nucleus, next to the ventrolateral and ventroanterior nuclei of the thalamus, and, finally, to the cerebral cortex. This pathway plays an important role in helping coordinate sequential motor activities initiated by the cerebral cortex.
Functional Unit of the Cerebellar Cortex— The Purkinje Cell and the Deep Nuclear Cell
The cerebellum has about 30 million nearly identical functional units, one of which is shown to the left in Figure 56-7. This functional unit centers on a single, very large Purkinje cell (30 million of which are in the cerebellar cortex) and on a corresponding deep nuclear cell.
To the top and right in Figure 56-7, the three major layers of the cerebellar cortex are shown: the molecular layer, Purkinje cell layer, and granule cell layer. Beneath these cortical layers, in the center of the cere-bellar mass, are the deep cerebellar nuclei that send output signals to other parts of the nervous system.
Neuronal Circuit of the Functional Unit. Also shown in the left half of Figure 56-7 is the neuronal circuit of the functional unit, which is repeated with little variation 30 million times in the cerebellum. The output from the functional unit is from a deep nuclear cell. This cell is continually under both excitatory and inhibitory influences. The excitatory influences arise from direct connections with afferent fibers that enter the cerebellum from the brain or the periphery. The inhibitory influence arises entirely from the Purkinje cell in the cortex of the cerebellum.
The afferent inputs to the cerebellum are mainly of two types, one called the climbing fiber type and the other called the mossy fiber type.
The climbing fibers all originate from the inferior olives of the medulla. There is one climbing fiber for
Input (inferior olive)
cells ^HKni I Deep
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