Neurophysiology Lecture 6 PDF

Neurophysiology Lecture 6 Prof.dr.heba shawky  Functions of The Brain Stem The brain stem The brain stem is a complex extension of the spinal cord, which performs sensory, motor and reflex functions. It also contains centers that: 1- Regulate cardiovascular, respiratory and...

Neurophysiology Lecture 6 Prof.dr.heba shawky  Functions of The Brain Stem The brain stem The brain stem is a complex extension of the spinal cord, which performs sensory, motor and reflex functions. It also contains centers that: 1- Regulate cardiovascular, respiratory and gastrointestinal functions. 2- Play a major role in the control of eye movements. 3-Support the body against gravity. The Arousal Mechanism “Wakefulness” (The reticular formation of the brain stem): The reticular formation is a large structure occupying the core of the brain stem. Functional divisions of the reticular formation: 1- The pontine reticular system, often called the "facilitatory RF". This area gives rise to: a. Ascending branches: Pass to non-specific thalamic nuclei and to the cerebral cortex via the thalamus. This is known as "Reticular Activating System = RAS", which plays a major role in the control of brain activity, consciousness and alertness. b. Descending branches: The ventral reticulo-spinal tract which transmits facilitatory impulses to the gamma motor neurons. 2- The medullary reticular system: Often called the "inhibitory RF". The lateral reticulo-spinal tract from it transmits inhibitory impulses to gamma motor neurons. Factors affecting the activity of the RAS A. Factors that increase RAS activity: 1. Impulses from all the sensory pathways. Pain and proprioceptive stimuli are particularly effective and can arouse a person from sleep. 2. Descending impulses from the cerebral cortex have a strong excitatory effect on RAS. Emotions and voluntary movements help in keeping a person awake. 3. Epinephrine and norepinephrine secreted from the adrenal medulla produce alerting response.  B. Factors that decrease the RAS activity: 1. Impulses from the sleep centers. 2. Lesions that damage the brain stem cells e.g. vascular lesions, poisons, tumors and hypoxia. Drugs e.g. barbiturates as they cause hyperpolarization of the neurons  The Motor Cortex The motor cortex sends their impulses through descending tracts ,the pyramidal and extrapyramidal tracts. I- The Pyramidal tract ( Corticospinal tract) Functions of the Pyramidal System 1. It is responsible for complex fine skilled voluntary movements of the fingers, toes and face 2. It is facilitatory to muscle tone and deep reflexes. II-The extrapyramidal system Functions of extrapyramidal tract: 1. Fixation of the body in a position suitable for the performance of fine movements. 2. Some fibers are facilitatory, others are inhibitory to muscle tone. It is apparent that, for the performance of normal voluntary movement, two sets of neurons are important. Upper motor neurons (UMN). - They include the motor neurons originating from the cerebral cortex and the brain stem and synapse with the AHCs - UMN are grouped into pyramidal and extra-pyramidal tracts. Lower motor neurons (LMN). They include; the AHCs and the axons of motor nerves i.e. the motor nerves to the skeletal muscles