Chapter 8 Notes

Chapter 8 – Control of Movement Skeletal Muscle  The muscles that move our skeletons  Most are attached to bones at each end  Fastened to bones via tendons  Movement types: o Flexion: contraction of a flexor muscle  The drawing in of a limb o Extension: contraction of extensor muscles  Antigravity muscles o Muscles contract, limbs flex  Anatomy o Skeletal muscles are composed of 2 types of muscle fibers  Extrafusal muscle fibers: served by axons of the alpha motor neurons  Provide the muscle’s motive force  Found outside muscle spindles  Intrafusal muscle fibers: sensory organs that are served by 2 axons (1 sensory, 1 motor)  Called muscle spindles (within the spindles)  Central region (capsule) contains sensory endings sensitive to stretch  Gamma motor neuron causes intrafusal muscle fiber to contract (very little force) to modify the sensitivity to stretch o 1 myelinated axon of alpha motor neuron serves many Extrafusal muscle fibres (from 1 per 10 to 1 per 100s) o motor unit: 1 alpha motor neuron + its axon + associated extrafusal muscle fibres  1 muscle fiber has:  a bundle of myofibrils (with overlapping strands of actin and myosin) (overlap zone is called a striation)  Muscular Contraction o Neuromuscular junction: the synapse between the terminal button of an efferent neuron and the membrane of a muscle fiber  Terminal buttons synapse on motor endplates  Endplate potential: Acetylcholine is released from terminal buttons and depolarizes the postsynaptic membrane  Very large potential, always causes muscle fiber to fire, induces a twitch/contraction  Opens the calcium channels so calcium ions enter the cytoplasm, triggering the contraction  Ca2+ are a cofactor that permits myofibrils to take energy from the ATP in the cytoplasm  Cross bridges row along actin filaments * Muscles not covered in course Control of Movement by the Brain  Primary Motor Cortex o On the precentral gyrus, rostral to the central sulcus o Shows somatotopic organization  Disproportionate amount of cortical area devoted to fingers and speech muscles  Organized in terms of movements o Frontal association cortex: principal cortical input to primary motor cortex (rostral to PMC) o Supplementary motor area (SMA) and premotor cortex important for control of movement  Both regions receive sensory info from the parietal and temporal lobes, both send efferent axons to PMC  SMA is on medial surface of the brain, rostral to PMC  Premotor cortex is on the lateral surface, rostral to the PMC  The Descending Pathways o Lateral group: independent limb movements, incl. hands and fingers  1. Corticospinal tract  Has axons of cortical neurons from PMC that terminate in the gray matter of the spinal cord  Also has neurons from parietal and temporal lobes  Path: cortex –> subcortical white matter –> ventral midbrain (cerebral peduncles in medulla) –> form the pyramidal tracts –> most fibers decussate (cross over) to contralateral spinal cord (form the lateral corticospinal tract)  The rest of the fibers descend thru the ipsilateral spinal cord (form the ventral corticospinal tract) * actually part of the ventromedial group  Most axons in lateral Cor-Spi tract control distal parts of limbs (arms, hands, fingers, lower legs, feet, toes)  Form synapses with motor neurons in the gray matter of the spinal cord (in lateral part of ventral horn)  Axons in ventral Cor-Spi tract control muscles of upper legs and trunk  send terminal buttons to both sides of gray matter in spinal cord  2. Corticobulbar tract  Projects to the medulla th th  Terminates in the motor nuclei of 5 , 7 , 9 , 10 , 11 thand 12 thcranial nerves  Control movements of the face, neck, tongue and some eye muscles  3. Rubrospinal tract  Originates in the red nucleus of the midbrain  Red nucleus receives most important inputs from motor cortex via corticorubral tract from cerebellum  Controls independent movements of the forearms and hands (not fingers) o Independent limb movements: right and left limbs make different movements, or 1 limb moves while the other stays still o Ventromedial group:  1. Vestibulospinal tract  Cell bodies are in vestibular nuclei  Controls posture  2. Tectospinal tract  Cell bodes in superior colliculus  Coordinate head and trunk w/ eye movements  3. Reticulospinal tract  Cell bodies in nuclei in brain stem & midbrain reticular formation  Control automatic functions (muscle tonus, respiration, coughing, sneezing) and walking  4. Ventral corticospinal tract o Control automatic movements: gross movem