Physiology of Nervous System PDF

University of Baghdad College of Medicine Department of Physiology Physiology of Nervous System Abdulnasir Hussin Ameer M.B.Ch.B M.Sc. Neuroscience Ph.D. Therapeutic Clinical Neurophysiology  The extra pyramidal system Which includes all those portions of...

University of Baghdad College of Medicine Department of Physiology Physiology of Nervous System Abdulnasir Hussin Ameer M.B.Ch.B M.Sc. Neuroscience Ph.D. Therapeutic Clinical Neurophysiology  The extra pyramidal system Which includes all those portions of the brain and brain stem and their fibers that contribute to motor control but that are not part of the direct pyramidal system This includes the basal ganglia, the reticular formation of the brain stem, the vestibular nuclei, the red nuclei, substantia nigra, tectum, and the subthalamic nucleus.  The red nucleus It receives projection fibers from the motor cortex (the corticorubral tract) and collaterals from the corticospinal tract and from the globus pallidus of the basal ganglia. All these fibers synapse in the lower part of the red nucleus which contains giant pyramidal neurons similar to Betz cells from which Rubrospinal tract originate, cross to the opposite side and descend very closely anterior to the lateral corticospinal tract. The red nucleus, via the rubrospinal tract, exerts a substantial influence over the physiological flexor muscles of the fore- and hindlimb. Lesion or inactivation of the red nucleus, or of the rubrospinal tract leads to deficits in the use of the contralateral distal limb that closely resemble those observed after lesions of the motor cortex or of the corticospinal tract. The tectum of the midbrain It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts: The lateral tectospinal tract: Originates from the superior colliculus (the center of visual reflexes), crosses to the opposite side and terminates in the cervical segments of the spinal cord. It is concerned with directing the eye and turning the head towards a light source (visuospinal reflexes). The ventral tectospinal tract: Originates from the inferior colliculus (the center of auditory reflexes), crosses to the opposite side and terminates in the cervical segments of the spinal cord. It is concerned with turning the head to direct the ears towards a sound source (audiospinal reflexes).  Reticular formation From the upper end of the spinal cord and throughout the entire extent of the brain stem (medulla, pons, mesencephalon) there is an area of diffuse neurons collectively known as the reticular formation. It is made up of small neurons arranged in complex intertwining nets. It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts: The lateral reticulospinal tract: Originates from the inhibitory reticular formation of the medulla. Some fibers cross to the opposite side, but most fibers descend in the same side of the spinal cord. It inhibits the gamma motor neurons, thus inhibiting the stretch reflex and skeletal muscle tone. The ventral reticulospinal tract: Originates from the facilitatory reticular formation of the pons. Fibers descend without crossing to terminate on the gamma motor neurons of the ipsilateral side of the spinal cord. It facilitates the gamma motor neurons, thus facilitating the stretch reflex and the skeletal muscle tone.  The basal ganglia It receives projection fibers from the motor cortex to corpus striatum and then to globus pallidus then to several nuclei in the brainstem. These nuclei include the subthalamus, substantia nigra, red nucleus, tectum of the midbrain, reticular formation, vestibular nucleus and inferior olive.  The vestibular nucleus of the medulla It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts; The lateral vestibulospinal tract: Originates from the vestibular nucleus, descends uncrossed to terminate on the alpha and gamma motor neurons (postural adjustments). The ventral vestibulospinal tract: Originates from the vestibular nucleus, descends on both sides of the spinal cord to terminate on the alpha and gamma motor neurons (adjustments of head position). The vestibulospinal tracts facilitate the stretch reflex and skeletal muscle tone and they mediate some postural reflexes  The inferior olive of the medulla The inferior olive receives input fibers from the motor cortex, the globus pallidus of the basal ganglia and the spinal cord. It sends output fibers to the cerebellum and it projects the fibers of the olivospinal tract which descend in the spinal cord to terminate on the ventral horn cells of the same side. The olivospinal tract is facilitatory to the stretch reflex and the skeletal muscle tone. The inferior olive works in close association with the cerebellum to correct any deviation of the muscle contraction from the preset plan of movement (the servocomparator function of the cerebellum). General functions of the extrapyramidal system 1. Mediation of gross movements which involve a group of large muscles. 2. Provides a weaker alternative to the pyramidal system for mediation of some discrete movements. 3. Mediation of fixation and positioning movements which accompany other fine movements. 4. Adjustment of the skeletal muscle tone through facilitation or inhibition. 5. Adjustment of muscle movements to match preset plans to reach a certain target. Stroke Hemiplegia  Signs of lesion of the upper motor neuron (pyramidal and extrapyramidal systems): Damage to the motor cortex or the pyramidal tracts due to interruption of blood supply to it is called stroke. The term “upper motor neuron” is preferable to “pyramidal tract” because the signs traditionally described under the “pyramidal tract” are not those of a pure pyramidal tract lesion but rather one involving both the pyramidal tract and subcorticospinal pathways (extrapyramidal tracts). A single upper motor neuron with its cell body in the cerebral cortex and its axon making direct synaptic contact with a lower motor neuron is uncommon except for the muscles involved in moving the fingers. Upper motor neuron lesion Lower motor neuron 1. Weakness or paralysis of the involved 1. Weakness in corticospinal muscles. distribution, i.e. shoulder abduction 2. Loss of tone on passive movement and finger movements, hip flexion and (flaccidity). toe dorsiflexion. 3. Absence of reflexes in the involved 2. Spastic increase in muscle tone. muscles. 3. Increased stretch reflexes. 4. A normal flexor planter response unless 4. Extensor planter response (positive the neurons of this reflex are damaged. Babinski sign). 5. Muscle atrophy. 5. Little or no atrophy. 6. Abnormal electrical excitability of the peripheral nerves and muscle in association with fibrillation and fasciculation of the involved muscles.  Lesion of the extrapyramidal system only 1. No paralysis of the muscle but rather a slowness of movements in association with changes in facial expression and loss on the opposite side of some stereotyped movements associated with postural adjustment such as swinging the arm when walking. 2. The muscle tone may be increased or decreased. Hypertonia of extrapyramidal type affects both the gravity and antigravity muscles by the same degree (rigidity). 3. The presence of involuntary movements such tremor, choreiform movements, and athetosis.