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The nervous system Ahmed L3 Dr.Abdul-Aziz MOTOR NEUROPHYSIOLOGY Overview of the somatic motor system The human skeleton is a system of levers that are moved by contraction of skeletal muscles. The motor system is comprised of skeletal muscles and the neurons that control them. Muscle contraction only occurs in response to action potentials in alpha motor neurons, which originate in the ventral gray matter of the spinal cord (and brainstem nuclei) and constitute the final common path for motor control. The hierarchy of motor control within the CNS is as follows: 1.The highest level of control resides in areas of association cortex and the basal ganglia, which determine the goal of movements. 2.The primary motor cortex and cerebellum determine the correct sequence of commands that will allow the goal to be achieved. 3.Neuronal circuits in the spinal cord are the lowest level of control and function to implement descending commands. Cerebellum The cauliflower-like cerebellum “small brain”, accounts for about 11% of total brain mass. It is located dorsal to the pons and medulla (and to the intervening fourth ventricle). It protrudes under the occipital lobes of the cerebral hemispheres, from which it is separated by the transverse cerebral fissure, it has two lateral hemispheres and central vermis. figure :Anatomy of cerebellum. 1 The nervous system Ahmed L3 Dr.Abdul-Aziz cerebellar peduncles The cerebellum connected to the brain stem by three bundles of axons called the cerebellar Peduncles: 1. Superior cerebellar peduncle, connect the cerebellum with the midbrain. 2. Middle cerebellar peduncle, connects the cerebellum with the pons. 3. Inferior cerebellar peduncle, connect the cerebellum with the medulla oblongata and the spinal cord. Anatomy The cerebellum is bilaterally symmetrical; its two apple-sized cerebellar hemispheres connected medially by the wormlike vermis. Its surface is heavily convoluted, with fine, transversely oriented pleat like gyri known as folia (“leaves”). Deep fissures subdivide each hemisphere into anterior, posterior, and flocculonodular lobes. From midline out: It is divided into the central part the vermis, and a cerebellar hemisphere on each side. Each cerebellar hemisphere divided into intermediate, and lateral zone. figure: lobes of cerebellum The cerebellum has a thin outer layer of gray matter, the cerebellar cortex, and a thick, deeper layer of white matter. Located within the white matter a collection of neurons form the deep cerebellar nuclei. the dentate, interposed, and fastigial. 2 The nervous system Ahmed L3 Dr.Abdul-Aziz Figure:layers of cerebellum. Layers of the cerebellar cortex,The cerebellar cortex has three layers: I.The granular layer is the innermost layer. It contains granule cells, Golgi II cells, and glomeruli. The axons of mossy fibers synapse on dendrites of granule and Golgi type II cells. II.The Purkinje cell layer is the middle layer. It contains Purkinje cells, and its output is always inhibitory. III.The molecular layer is the outermost layer. It contains stellate cells, basket cells, dendrites of Purkinje and Golgi II cells, and axons of granule cells. The axons of granule cells form parallel fibers, which synapse on the dendrites of Purkinje cells, basket cells, stellate cells, and Golgi type II cells. figure: Layers of the Cerebellar Cortex 3 The nervous system Ahmed L3 Dr.Abdul-Aziz Functional division Functionally the cerebellum divided into three subdivisions : 1) the vestibulocerebellum composed of the fluculonodular lobe, the Purkinje cells from this lobe synapse on vestibular nuclei. 2) The spinocerebellum is composed of the vermis and the intermediate zone of cerebellum hemisphere.. Purkinje cells, from the vermis, synapse on the fastigial nuclei. Those from the intermediate cerebellum, synapse on the interpositus nuclei. 3) The cerebrocerebellum, the newest part, projects to the motor and premotor cortex via the dentate nuclei. 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 . figure: Functional division of cerebellum. Input to the Cerebellar Cortex Two systems provide excitatory input to the cerebellar cortex: 1.Climbing fibers originate in the inferior olive of the medulla (Olivocerebellar fibers) and project directly onto Purkinje cells, each Purkinje cell receives input from only one climbing fiber. It is believed that climbing fibers "condition" the Purkinje cells and modulate their 4 The nervous system Ahmed L3 Dr.Abdul-Aziz responses to mossy fiber input. Climbing fibers also may play a role in cerebellar learning. 2.Mossy fibers constitute the majority of the cerebellar input: a. Cerebropontocerebellar fibers--arise from pyramidal cells in the cerebral cortex, synapse on pontine nuclei which send their axons to the contralateral cerebellar cortex via pontocerebellar fibers (form middle peduncle). Alerts cerebellum regarding anticipated movements. b. Vestibulocerebellar fibers--arise mainly from the vestibular nerve and vestibular nuclei; project to flocculonodular lobe and fastigial nucleus (coordinate head and eye movement). c. Spinocerebellar Fibers- arise from spinal cord and go to rostral lobe; makes cerebellum aware of ongoing movements via proprioceptive input from muscle spindles and joint receptors. The vestibulocerebellar, spinocerebellar, and cerebropontocerebellar afferents project to granule cells, which are excitatory interneurons located in collections of synapses called glomeruli. Axons from these granule cells then ascend to the molecular layer, where they bifurcate and give rise to parallel fibers. Parallel fibers from the granule cells contact the dendrites of many Purkinje cells, producing a "beam" of excitation along the row of Purkinje cells. (Note:The climbing fiber system and the mossy fiber also sends collateral branches directly to deep cerebellar nuclei, in addition to their projections to the cerebellar cortex. Major Cerebellar Cerebellar Output (efferents) Outputs (arise from neurons in deep cerebellar nuclei): 1. Fastigial Nucleus Projections: to --> vestibular nuclei and reticular formation--> vestibulospinal & reticulospinal tracts influence spinal motor neurons--> effect extensor muscles related to maintaining posture and balance. 2. Interpositus Nucleus Projections: -->go to red nucleus to influence rubrospinal tract--> correct errors related to the gross movements . 3. Dentate Nucleus Projections: --> projects to thalamus to influence the output from the motor cortex- ->makes delicate adjustments related to fine, skilled movements. Summery of cerebellar function 1.Cerebellum helps to sequence the motor activities and also monitors and makes corrective adjustments in the body’s motor activities while 5 The nervous system Ahmed L3 Dr.Abdul-Aziz they are being executed so that they will conform to the motor signals directed by the cerebral motor cortex and other parts of the brain. 2.The cerebellum receives continuously updated information about the desired sequence of muscle contractions from the brain motor control areas; it also receives continuous sensory information from the peripheral parts of the body, giving sequential changes in the status of each part of the body—its position, rate of movement, forces acting on it, and so forth. The cerebellum then compares the actual movements as depicted by the peripheral sensory feedback information with the movements intended by the motor system. If the two do not compare favorably, then instantaneous subconscious corrective signals are transmitted back into the motor system to increase or decrease the levels of activation of specific muscles. 3.The cerebellum also aids the cerebral cortex in planning the next sequential movement a fraction of a second in advance while the current movement is still being executed, thus helping the person to progress smoothly from one movement to the next. 4. the cerebellum damp movements and prevent overshoot of movements 5. Another important function of the lateral zones of the cerebellar hemispheres is to provide appropriate timing for each succeeding movement. . Cerebellar disorders usually result from: lesion of cerebellum may be due to 1. Tumors (i.e., cerebellar cystic meningioma) 2. Viral Infections 3. Heavy metal poisoning 4. Genetic Disorders: cerebellar degeneration The main abnormalities include lesions of cerebellum usually lead to 1.dysmetria and ataxia, in the absence of the cerebellum, the subconscious motor control system cannot predict how far movements will go. Therefore, the movements ordinarily overshoot their intended mark; then the conscious portion of the brain overcompensates in the opposite direction for the succeeding compensatory movement.This effect is called dysmetria, and it results in uncoordinated movements that are called ataxia. 6 The nervous system Ahmed L3 Dr.Abdul-Aziz 2. Dysdiadochokinesia, when the motor control system fails to predict where the different parts of the body will be at a given time, it “loses” perception of the parts during rapid motor movements. As a result, the succeeding movement may begin much too early or much too late, so that no orderly “progression of movement” can occur. 3. Dysarthria, another example in which failure of progression occurs is in talking because the formation of words depends on rapid and orderly succession of individual muscle movements in the larynx, mouth, and respiratory system 4. Intention tremor occur when a person performs a voluntary act. 5. Hypotonia. Loss of the deep cerebellar nuclei, particularly of the dentate and interposed nuclei, causes decreased tone of the peripheral body musculature on the side of the cerebellar lesion. 7 The nervous system Ahmed L3 8 Dr.Abdul-Aziz