Cerebellum or Little Brain

Cerebellum=Little brain: • 10% of total volume of the brain. • Contains >half of all neurons. • Its complete removal does not impair sensory perception or muscle strength. • Regulates movements and postures indirectly by adjusting output of the major descending motor systems. • It has an important role in motor learning. Lesions of cerebellum • Disrupt coordination of limb and eye movements. • Impair balance. • ↓ tone of muscles. • Impair motor learning and some type of associative learning and certain cognitive functions. • Scanning speech (one word at a time) due to vermis lesion How does cerebellum adjust the output of motor system? • Receives information about the programming and execution of motor functions. • Receives information during movement through sensory feedback from the periphery. • Compares central information with motor response and makes necessary corrections. Functions of cerebellum • 1-Planning of the rapid and sequential movements • 2- Coordinate between agonists and antagonists muscles • 3- Equilibrium Cerebellum The Cerebellum Has Three Functionally Distinct Regions B. The main body of cerebellum has three functional regions: the central vermis and the lateral and intermediate zones in each hemisphere. The Cerebellum Has Three Functionally Distinct Regions The Cerebellum Has Three Functionally Distinct Regions The three functional regions of the cerebellum have different inputs and outputs. Afferent Pathways 1 • An extensive and important afferent pathway is the cortico pontocerebellar pathway • originates in the cerebral motor and premotor cortices and cerebral somatosensory cortex. • It passes pontile nuclei and ponto cerebellar tracts to the lateral divisions of the cerebellar hemispheres on the opposite side of the brain from the cerebral areas. 2- Input from brain stem (1)olivocerebellar tract, from inferior olive to all parts of cerebellum and is excited in olive by fibers from cerebral motor cortex, basal ganglia, widespread areas of reticular formation, and spinal cord (2) vestibulocerebellar fibers, originate in vestibular apparatus and others from brain stem vestibular nuclei terminate in the flocculonodular lobe and fastigial nucleus of the cerebellum (3) Reticulocerebellar fibers, originate in different portions of the brain stem reticular formation and terminate in the midline cerebellar areas (mainly in the vermis). 3-Input from spinal cord • dorsal spinocerebellar The signals transmitted from muscle spindles and somatic receptors, 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 • (4) forces acting on surfaces of body. • The ventral spinocerebellar • they are excited mainly by motor signals arriving in anterior horns of the spinal cord from • (1) the brain through the corticospinal and rubrospinal tracts • (2) the internal motor pattern generators in the cord itself. • Thus, this ventral fiber pathway tells the cerebellum which motor signals have arrived at anterior horns; this feedback is called the efference copy of the anterior horn motor drive. input& Deep nuclei • 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 • (2) to a corresponding area of the cerebellar cortex overlying the deep nucleus . cerebellar cortex relays an inhibitory output signal to the deep nucleus. • all input signals that enter the cerebellum end in the deep nuclei in the form of initial excitatory signals followed a fraction of a second later by inhibitory signals. Output (1) • From the deep nuclei, output signals leave the cerebellum and are distributed to other parts of the brain. • A pathway that originates the vermis (fastigial nuclei ) into medullary and pontile regions of brain stem. • 1- With the equilibrium apparatus and brain stem vestibular nuclei to control equilibrium • 2- With the reticular formation of brain stem to control postural attitudes of the body and equilibrium. Output(2) • A pathway that originates in intermediate zone interposed nucleus • (a) the ventrolateral and ventroanterior nuclei of the thalamus • (b) the cerebral cortex • (c) several midline structures of the thalamus • (d) the ganglia basal • (e) the red nucleus and reticular formation of upper portion of brain stem. • This complex circuit helps to coordinate mainly the reciprocal contractions of agonist and antagonist muscles in the peripheral portions of limbs, especially in hands, fingers, and thumbs. Output(3) • A pathway that begins in the cerebellar cortex of the lateral zone(dentate nucleus) • 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 Cerebellar Cortex • The cerebellum has about 30 million nearly identical • functional units. • This functional unit centers on a single, very large Purkinje cell (30 million of which are in the cerebellar cortex) and a corresponding deep nuclear cell. • 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 cerebellar mass, are the deep cerebellar nuclei that send output signals to other parts of the nervous system. Neuronal Circuit of Functional Unit. • 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 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. Neurons in the Cerebellar Cortex Are Organized into Three Layers The cerebellar cortex is organized into three layers and contains five types of neurons. A vertical section of a single cerebellar folium, in both longitudinal and transverse planes, illustrates the general organization of the cerebellar cortex. The detail of a cerebellar glomerulus in the granular layer is also shown. A glomerulus is a clear space where the bulbous terminal of a mossy fiber makes synaptic contact with Golgi and granule cells. Neurons in the Cerebellar Cortex Are Organized into Three Layers The Purkinje Cells Receive Excitatory Input From Two Afferent Fiber Systems and Are Inhibited by Three Local Interneurons Synaptic organization of the basic cerebellar circuit module. Mossy and climbing fibers convey output from the cerebellum via a main excitatory loop through the deep nuclei. This loop is modulated by an inhibitory side-loop passing through the cerebellar cortex. This figure shows the excitatory (+) and inhibitory (-) connections among the cell types. Geometrical Plan of Parallel and Climbing fibers The geometry of the mossy and parallel fiber system contrasts with that of the climbing fiber system. Mossy fibers excite granule cells whose parallel fibers branch transversely to excite hundreds of Purkinje cells several millimeters from the branch point, both medially and laterally. By contrast, climbing fibers branch in the sagittal dimension to excite 10 or so Purkinje cells anterior and posterior to the branch point. The transverse connections of the parallel fibers and the sagittal connections of the climbing fibers thus form an orthogonal matrix. Mossy and Climbing Fibers Encode Peripheral and Descending Information Differently Simple and complex spikes recorded intracellularly from cerebellar Purkinje cells. Complex spikes (right bracket) are evoked by climbing fiber synapses, while simple spikes (left bracket) are produced by mossy fiber input. (From Martinez et al. 1971.) Clinical application • Lesion of floccolonodular lobe leads to disequilibrium. • Lesion of cerebropontine connections leads to hypotonia, diminished muscle jerk, intention tremor, clumsy ‫اﻟﺧرﻗﺎء‬ movements unskilful • Isolated lesions of the vermis are produced in children by medulloblastomas in the roof of the fourth ventricle Clinical application • Anterior lobe lesions leads to ataxia • Dysdiadochokinesia: inability to perform alternating movements regularly and rapidly • Dysarthria occurs in cerebellar disease because of ataxia of the muscles of the larynx Cerebellar Cortex Inputs Molecular Granular Climbing fibers •from Inferior olive Mossy fibers Output Purkinje neurons Interneurons Granule neurons Stellate neurons Basket neurons Golgi neurons Functional divisions of cerebellar cortex --> Deep nuclei Spinocerebellum Vermis Intermediate hemisphere Cerebrocerebellum Lateral hemisphere Spinocerebellum Vermis Intermediate hem Cerebrocerebellum Lateral hemisphere Inter Dentate posed Fastigial Vestibulocerebellum To frontal motor areas Motor Planning +++ To lateral sysetms To medial sysetms Motor execution Vestibulocerebellum Floculo-nodular lobe To vestibular nuclei Eye mvt & balance PNS Fig. 42-3 NTA Fig. 13-1 Medial & lateral systems Interposed nuclei Fastigial Vestibulocerebellum via vestibular nuclei Turn-On/Turn-Off and Turn-Off/TurnOn Output Signals from Cerebellum • • • • • the onset of movement begins with signals from the cerebral cortex. These signals pass through noncerebellar brain stem and cord pathways directly to the agonist muscle to begin the initial contraction. At the same time, parallel signals are sent of mossy fibers into the cerebellum. One branch of each mossy fiber goes directly to deep nuclear cells this instantly sends an excitatory signal back into the cerebral corticospinal motor system ,return signals through the thalamus to cerebral cortex or by way of neuronal circuitry in the brain stem, to support muscle contraction signal that had already been begun by the cerebral cortex. As a consequence, the turn-on signal, after a few milliseconds, becomes even more powerful than it was at the start because it becomes the sum of both the cortical and the cerebellar signals. turn-off signal • all mossy fibers have a second branch that transmits signals of the granule cells to the cerebellar cortex and by way of "parallel“ fibers, to the Purkinje cells. • The Purkinje cells in turn inhibit the deep nuclear cells. This pathway passes through some of the smallest, slowestconducting nerve fibers in the nervous system: that is, the parallel fibers of the cerebellar cortical molecular layer. • the Purkinje cell is excited, it in turn sends a strong inhibitory signal to the same deep nuclear cell. this helps to turn off the movement after a short time Function of Cerebellum in Motor Control • Vestibulocerebellum equilibrium movements. • Spinocerebellum coordinating mainly movements of the distal portions of the limbs, especially the hands and fingers. • Cerebrocerebellum It receives its input from cerebral motor cortex and premotor and somatosensory cortices of cerebrum. • Its output information back to the brain • functioning in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements • This is called development of "motor imagery" of movements to be performed. Clinical Abnormalities of the Cerebellum • Hypotonia • Ataxia • Action or Intention tremors • Dysmetria • Dysarthria • Dysgraphia • Dysdiadochokinesia Conclusions • Cerebellar lesions produce – Incoordination & errors not weakness – Lose ability to anticipate errors – Lose ability to correct • Motor learning – Requires sensory awareness – Implemented via the descending cortical and brain stem pathways • Cognitive and emotional disturbances – Anatomical connections to prefrontal and cingulate cortex (via thalamus) • No single function – Clearly mostly motor; learning, optimizes – Functions may apply to cognitive and emotional behaviors