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Download Unit One: Introduction to Physiology: The Cell and General Physiology
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Chapter 55: Cortical and Brain Stem Control Of Motor Function Guyton and Hall, Textbook of Medical Physiology, 12 edition Motor Cortex and Corticospinal Tract Fig. 55.1 Motor and somatosensory functional areas of the cerebral cortex Motor Cortex (cont.) • Primary Motor Cortex Fig. 55.2 Degree of representation of the different muscles of the body in the motor cortex Motor Cortex (cont.) • Primary Motor Cortex Fig. 55.3 Representation of the different muscles of the body in the motor cortex and location of other cortical areas responsible for specific types of motor movements Motor Cortex (cont.) • Transmission of Signals from the Motor Cortex to the Muscles a. Corticospinal (Pyramidal) Tract Fig. 55.4 Motor Cortex (cont.) • Red Nucleus As An Alternate Pathway Fig. 55.5 Motor Cortex (cont.) • Function of the Corticorubrospinal System a. Accessory route for transmission of discrete signals from the motor cortex to the spinal cord • Extrapyrimidal System- all portions of the brain and brain stem that contribute to motor control but are not part of the direct scorticospinal-pyramidal system a. Include the basal ganglia, reticular formation, and the vestibular nuclei Motor Cortex (cont.) • Excitation of the Spinal Cord Motor Control Areas by the Primary Motor Cortex and the Red Nucleus a. Vertical columnar arrangement of the neurons in the motor cortex b. Each column functions as a unit, usually stimulating a group of synergistic muscles (sometimes a single muscle) c. Each column operates as an integrative operating system d. Each column can also function as an amplifying system e. Dynamic neurons-excited at a high rate for a short period of time at the beginning of a contraction f. Static neurons-fire at a slower rate but maintain the force of contraction Motor Cortex (cont.) • Somatosensory Feedback a. Vertical columnar arrangement of the neurons in the motor cortex b. Each column functions as a unit, usually stimulating a group of synergistic muscles (sometimes a single muscle) c. Each column operates as an integrative operating system d. Each column can also function as an amplifying system e. Dynamic neurons-excited at a high rate for a short period of time at the beginning of a contraction f. Static neurons-fire at a slower rate but maintain the force of contraction Motor Cortex (cont.) • Stimulation of Spinal Motor Neurons Fig. 55.6 Convergence of different motor control pathways on the anterior motor neurons Role of the Brain Stem in Controlling Motor Function • Brain Stem Provides Special Control Functions a. b. c. d. Control of respiration Control of the cardiovascular system Partial control of GI function Control of many stereotyped movements of the body e. Control of equilibrium f. Control of eye movements Role of the Brain Stem in Controlling Motor Function • Support of the Body Against Gravity a. Reticular Nuclei 1) Pontine reticular nuclei- transmit excitatory signals; receive strong signals from vestibular nuclei and from nuclei of the cerebellum 2) Medullary reticular nuclei- transmit inhibitory signals to the same neurons as the pontine nuclei Role of the Brain Stem in Controlling Motor Function • Support of the Body Against Gravity b. Vestibular Nuclei- transmit strong excitatory signals to control antigravity muscles Role of the Brain Stem in Controlling Motor Function Fig. 55.7 Locations of the reticular and vestibular nuclei in the brain stem Fig. 55.8 Vestibulospinal and reticulospinal descending tracts to excite or inhibit anterior motor neurons Vestibular Sensations and Maintenance of Equilibrium Fig. 55.9 Membranous labyrinth and organization of the crista ampullaris and the macula Vestibular Sensations and Maintenance of Equilibrium • Vestibular Apparatus- sensory organ for detecting sensations of equilibrium a. Encased in the bony labyrinth b. Within are the membranous labyrinth which is the functional part of the vestibular apparatus • Membranous Labyrinth- composed of a. b. c. d. Cochlea Three semicircular canals Utricle Saccule Vestibular Sensations and Maintenance of Equilibrium • Maculae a. Sensory organs of the utricle and saccule for detecting orientation of the head with respect to gravity b. Covered with a gelatinous layer that contains calcium carbonate crystals (statoconia) c. Hair cells project into the gel layer; synapse with sensory endings of the vestibular nerve Vestibular Sensations and Maintenance of Equilibrium • Maculae d. Calcified statoconia have a specific gravity 2-3X the specific gravity of the surrounding fluid and tissues; e. Weight of the statoconia bends the hair cells (cilia) in the direction of gravitational pull Vestibular Sensations and Maintenance of Equilibrium • Directional Sensitivity of the Hair Cells—Kinocilium Fig. 55.10 Hair cell of the equilibrium apparatus and its synapses with the vestibular nerve Vestibular Sensations and Maintenance of Equilibrium • Directional Sensitivity of the Hair Cells—Kinocilium a. Each hair cell has 50-70 small cilia (stereocilia) and one large kinocilium b. The kinocilium is located to one side and the cilia get progressively shorter toward the other side of the cell c. When the stereocilia and the kinocilium bend in the direction of the kinocilium, this opens fluid channels capable of conducting large numbers of positive ions Vestibular Sensations and Maintenance of Equilibrium • Directional Sensitivity of the Hair Cells—Kinocilium d. Positive ions pour into the cell causing receptor membrane depolarization (hyperpolarization occurs the bending is in the opposite direction) e. At rest: impulses are conducted continuously at a rate of 100 per second f. When cilia are bending, impulses increase; as the orientation of the head in space changes and the weight of the statoconia bends the cilia, appropriate signals are transmitted to the brain to control equilibrium Vestibular Sensations and Maintenance of Equilibrium • Semicircular Ducts- arranged at right angles so that they represent all three planes in space a. Anterior b. Posterior c. Lateral (horizontal) Vestibular Sensations and Maintenance of Equilibrium Fig. 55.11 Movement of the cupula and its embedded hairs at the onset of rotation Vestibular Sensations and Maintenance of Equilibrium • Semicircular Ducts (cont.) a. Each has an enlargement at one end called the ampulla b. Ducts and ampulla are filled with endolymph c. Each ampulla has a crest called the crista ampullaris d. On top of the crista is the cupula Vestibular Sensations and Maintenance of Equilibrium • Semicircular Ducts- Rotation a. When a person’s head starts to rotate in any direction, the inertia of the fluid causes the fluid to remain stationary while the duct rotates with the head b. This causes the fluid to flow from the duct through the the ampulla, bending the cupula to one side c. Rotation of the head in the opposite direction causes the cupula to bend to the opposite side Vestibular Sensations and Maintenance of Equilibrium • Semicircular Ducts- Rotation d. Hundreds of cilia project into the cupula from hair cells on the crest e. Kinocilia are oriented in the same direction, bending the cupula in that direction causing depolarization of the hair cells f. Stimulates the vestibular nerve Vestibular Sensations and Maintenance of Equilibrium • Function of the Utricle and Saccule in Static Equilibrium a. Function to maintain equilibrium when the head is in a near vertical position b. Do not operate for the detection of linear velocity • Dectection of Head Rotation by the Semicircular Ducts Vestibular Sensations and Maintenance of Equilibrium Fig. 55.12 Response of a hair cell when a semicircular canal is stimulated first by the onset of head rotation and then by stopping rotation Other Factors Concerned With Equilibrium • Neck Proprioceptors • Visual Information • Proprioception and Exteroceptive Information from Other Parts of the Body