Movement I


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Movement I

  1. 1. Movement Part I Spinal Control of Movement
  2. 2. Overview <ul><li>Alpha motor neurons, which innervate the skeletal muscle fibers, are the final common pathway for behavior. </li></ul><ul><li>They are wired into a complex set of reflex loops in the spinal cord. </li></ul><ul><li>These reflex loops are supplemented by locomotor programs in the spinal cord which provide the basic rhythmic aspects activities such as walking. </li></ul>
  3. 3. Types of Muscle <ul><li>Smooth muscle </li></ul><ul><ul><li>digestive system & arterioles </li></ul></ul><ul><ul><li>innervated by adrenergic autonomic nervous system </li></ul></ul><ul><li>Cardiac muscle (striated) </li></ul><ul><ul><li>heart muscle </li></ul></ul><ul><ul><li>modulated by autonomic nervous system </li></ul></ul><ul><li>Skeletal muscle (striated) </li></ul><ul><ul><li>body and eye movement </li></ul></ul><ul><ul><li>breathing </li></ul></ul><ul><ul><li>controlled by lower motor neurons in spinal cord </li></ul></ul>
  4. 4. Skeletal muscles are the effectors of movement .
  5. 5. Categories of Muscles <ul><li>Categories based on direction of motion </li></ul><ul><li>Categories based on body location </li></ul>
  6. 6. Types by Body Location <ul><li>Axial muscles </li></ul><ul><ul><li>move trunk </li></ul></ul><ul><li>Proximal muscles </li></ul><ul><ul><li>move shoulder, elbow, pelvis, knee </li></ul></ul><ul><li>Distal muscles </li></ul><ul><ul><li>move hands, feet, digits </li></ul></ul>
  7. 7. Muscles Are the Effectors of Movement <ul><li>All animal movement is based on contraction of muscles working against some type of skeleton </li></ul><ul><li>The action of a muscle is always to contract </li></ul><ul><ul><li>Muscles extend only passively </li></ul></ul><ul><li>To move body parts in opposite directions, muscles are attached in antagonistic pairs </li></ul><ul><li>Example: </li></ul><ul><ul><li>Bicep contracts  arm flexes </li></ul></ul><ul><ul><li>Bicep relaxes; triceps contracts  arm extends </li></ul></ul>
  8. 9. Types by Direction of Motion <ul><li>Flexors </li></ul><ul><ul><li>reduce angle of joints </li></ul></ul><ul><li>Extensors </li></ul><ul><ul><li>increase angle of joints </li></ul></ul><ul><li>Synergists </li></ul><ul><ul><li>all flexor muscles working together on one joint </li></ul></ul><ul><ul><li>all extensors working together on one joint </li></ul></ul><ul><ul><li>muscles that work in parallel </li></ul></ul><ul><li>Antagonists </li></ul><ul><ul><li>flexors and extensors for one joint </li></ul></ul><ul><ul><li>muscles that work in opposition </li></ul></ul>
  9. 10. Structure of Skeletal Muscle <ul><li>Formed from a hierarchy of smaller & smaller parallel units </li></ul><ul><li>Each muscle consists of a bundle of long fibers, the length of the muscle </li></ul><ul><ul><li>Each fiber is a single cell with many nuclei </li></ul></ul><ul><li>Each fiber is a bundle of smaller myofibrils </li></ul><ul><li>Myofibrils are formed from 2 types of myofilaments: </li></ul><ul><ul><li>Thick & thin </li></ul></ul><ul><li>Myofilaments are formed from 2 key proteins: </li></ul><ul><ul><li>Actin & myosin </li></ul></ul>
  10. 12. Myofilaments <ul><li>Thin filaments </li></ul><ul><ul><li>Two strands of actin and one of regulatory protein </li></ul></ul><ul><li>Thick filaments </li></ul><ul><ul><li>Staggered arrays of myosin molecules </li></ul></ul>
  11. 13. Sarcomeres <ul><li>Skeletal muscle is striated: </li></ul><ul><li>The regular arrangement of myofilaments creates a repeating pattern of light & dark bands </li></ul><ul><li>Each repeating unit = sarcomere </li></ul><ul><li>The basic contractile unit of muscle </li></ul>
  12. 14. Z-Lines <ul><li>The borders of the sarcomere = Z-lines </li></ul><ul><li>These are lined up in adjacent myofibrils </li></ul><ul><li>Thin filaments are attached to the Z-lines and project toward the center of the sarcomere </li></ul><ul><li>Thick filaments are centered in the sarcomere </li></ul>
  13. 16. Banding <ul><li>At rest, thick & thin filaments don’t overlap completely </li></ul><ul><li>The area at the edge of the sarcomere where there are only thin filaments = I band </li></ul><ul><li>The broad region of thick filaments = A-band </li></ul><ul><li>H - zone is in the center of the A-band and contains only thick filaments </li></ul><ul><li>The arrangement of thick & thin filaments is the key to muscle contraction </li></ul>
  14. 18. Filaments & Contraction <ul><li>When a muscle contracts, the length of each sarcomere is reduced </li></ul><ul><li>The distance from one Z-line to the next gets shorter </li></ul><ul><li>The A-bands don’t change, but the I-bands shorten </li></ul><ul><li>The H-zone disappears </li></ul>
  15. 19. The Sliding Filament Model <ul><li>Neither group of filaments changes length when a muscle contracts </li></ul><ul><li>Rather, the filaments slide past each other, so the overlap increases </li></ul><ul><li>If the overlap increases, the area of only thin filaments (I-band) and the area of only thick filaments (H-zone) decreases </li></ul>
  16. 20. The sliding filament model of muscle contraction.
  17. 21. Actin & Myosin <ul><li>Thick & thin filaments are formed from actin & myosin </li></ul><ul><li>The myosin “head” is the site of bioenergetic reactions that power muscle contraction </li></ul>
  18. 22. Interaction of Actin & Myosin <ul><li>Myosin head binds ATP and hydrolyzes it to ADP </li></ul><ul><li>The energy released is transferred to myosin </li></ul><ul><li>The myosin changes shape </li></ul><ul><li>The energized myosin binds a specific site on the actin molecule, forming a cross-bridge </li></ul><ul><li>This releases energy, relaxing the myosin head </li></ul>
  19. 23. Actin & Myosin (continued) <ul><li>The myosin changes shape and bends inward on itself </li></ul><ul><li>This exerts tension on the thin filaments to which it is bound </li></ul><ul><li>Which pulls the thin filaments toward the center of the sarcomere </li></ul><ul><li>When a new ATP molecule binds the myosin head, the bond between myosin & actin is broken </li></ul><ul><li>The cycle repeats </li></ul>
  20. 24. The Repeating Cycle <ul><li>Each of the ~ 350 myosin heads of a thick filament forms and reforms 5 cross-bridges/sec </li></ul><ul><li>Producing muscle contractions </li></ul>
  21. 25. Actin & Myosin Interaction
  22. 26. Energy <ul><li>Muscle cells store only enough ATP for a few muscle contractions </li></ul><ul><li>They store glycogen between myofibrils </li></ul><ul><li>Most energy for muscles is stored in phosphagens </li></ul><ul><ul><li>In vertebrates = creatine phosphate </li></ul></ul>
  23. 27. Motor Neurons & Movement <ul><li>A muscle contracts only when stimulated by a motor neuron </li></ul><ul><li>An action potential in a motor neuron connected to muscle causes it to contract </li></ul><ul><li>Ca ++ ions and regulatory proteins control muscle contractions </li></ul>
  24. 28. Regulatory Proteins <ul><li>When a muscle is at rest, myosin binding sites on actin are blocked by regulatory proteins, tropomyosins </li></ul><ul><li>The position of tropomyosin on the thin filaments is controled by troponin complex . Another set of regulatory proteins </li></ul><ul><li>For a muscle to contract, the myosin binding site on actin must be exposed </li></ul>
  25. 29. The Role of Ca ++ <ul><li>When Ca ++ binds to troponin alters the tropomyosin-troponin complex, exposing the mysosin binding sites on actin. </li></ul><ul><li>When Ca ++ is present, filaments can slide and muscles contract </li></ul><ul><li>When Ca ++ levels decrease, contraction stops </li></ul>
  26. 31. The Sarcoplasmic Reticulum <ul><li>Ca ++ in the cytosol of a muscle cell is regulated by the sarcoplasmic reticulum (specialized type of ER) </li></ul><ul><li>Surrounds myofibrils; sequesters and releases calcium </li></ul><ul><li>The membrane of the sarcoplasmic reticulum (SR) actively transposrt Ca++ from the cytosol to the interior of the SR </li></ul><ul><ul><li>An interior storehouse for Ca++ </li></ul></ul>
  27. 32. Motor Neurons <ul><li>Spinal organization </li></ul><ul><ul><li>Lower motor neurons </li></ul></ul><ul><li>Alpha motor neurons </li></ul><ul><li>Motor units </li></ul><ul><li>Motor neuron pools </li></ul>
  28. 33. Spinal Organization Lower Motor Neurons <ul><li>Motor neuron fibers exit the spinal cord in the ventral root of each spinal segment </li></ul><ul><ul><li>cell bodies in ventral horn </li></ul></ul><ul><li>Cell bodies have a somatotopic arrangement </li></ul><ul><li>There are bulges in the ventral horn because of the large number of motor neurons for the arms and for the legs </li></ul>
  29. 34. Alpha Motor Neurons <ul><li>Neuron directly responsible for synapsing on muscle fibers and causing movement </li></ul><ul><ul><li>final common pathway for behavior </li></ul></ul><ul><li>Sources of direct input </li></ul><ul><ul><li>Sensory input from muscle spindles </li></ul></ul><ul><ul><li>Input from spinal interneurons </li></ul></ul><ul><ul><li>Descending input from upper motor neurons (e.g. motor cortex) </li></ul></ul><ul><li>Controlling the force of muscle contraction </li></ul>
  30. 35. The Neuromuscular Junction <ul><li>Action potential in a motor neuron connected to a muscle causes contraction </li></ul><ul><li>The synaptic terminal of the motor neuron releases acetylcholine at the neuromuscular junction, depolarizing the muscle cell </li></ul><ul><li>Sarcolemma </li></ul><ul><ul><li>external, electrically excitable membrane of a muscle fiber </li></ul></ul>
  31. 36. Excitation  Contraction <ul><li>How the action potential in a motor neuron causes muscle contraction </li></ul><ul><li>Nicotinic ACh receptors (transmitter-gated ion channel) open Na + channels </li></ul><ul><li> EPSP </li></ul><ul><li>Muscle fiber generates action potential which sweeps down the sarcolemma </li></ul>
  32. 37. Transverse Tubules <ul><li>Transverse (T) tubules = infoldings of sarcolemma (membrane) </li></ul><ul><li>Conduct the action potential inward </li></ul><ul><li>Depolarization of T-tubules activates a voltage sensitive protein that plugs Ca ++ channels in SR </li></ul><ul><li>Where the T-tubules touch SR, the action potential changes the permeability of the SR, causing release of Ca ++ </li></ul><ul><ul><li>Calcium is released and floods myofibrils </li></ul></ul><ul><li>Ca ++ binds to troponin, allowing the muscle to contract </li></ul>
  33. 39. Relaxation <ul><li>Contraction stops when the SR pumps Ca++ out of the cytosol and troponin-tropomyosin complex blocks myosin binding sites as Ca++ concentration decreases </li></ul><ul><li>Calcium ions are sequestered by SR via an ATP-driven Ca ++ pump </li></ul><ul><li>Myosin binding sites on actin are covered by troponin </li></ul>
  34. 41. Graded Contractions <ul><li>Muscle contractions are graded </li></ul><ul><ul><li>some are strong, some are weak </li></ul></ul><ul><li>We can voluntarily alter the strength of a contraction </li></ul><ul><li>At a cellular level, the response is all or none </li></ul><ul><li>Any stimulation that depolarizes the plasma membrane of a single muscle fiber triggers a contraction </li></ul><ul><ul><li>Like in a neuron </li></ul></ul><ul><li>So how are contractions graded? </li></ul>
  35. 42. Creating Graded Responses <ul><li>Nervous system can vary the frequency of action potentials in motor neurons </li></ul><ul><li>Action potential summation  gradation </li></ul><ul><li>Rate coding </li></ul><ul><ul><li>each action potential produces a muscle twitch </li></ul></ul><ul><ul><li>fire faster and produce stronger contraction </li></ul></ul><ul><li>If the rate of stimulation is fast enough, individual twitches become one smooth contraction = tetanus </li></ul><ul><ul><li>Not the same as the disease! </li></ul></ul>
  36. 43. Temporal summation of muscle contraction: muscle tension resulting from 1, 2, or a series of action potentials.
  37. 44. The Motor Unit <ul><li>One alpha motor neuron and all the muscle fibers it innervates </li></ul><ul><li>Each muscle fiber is innervated by only one motor neuron </li></ul><ul><li>Each motor neuron may synapse with many muscles cells </li></ul><ul><ul><li>Motor units range in size from 1:3 (fine control) to 1:1000 (leg muscles) </li></ul></ul>
  38. 45. Structure of a vertebrate motor unit.
  39. 46. The Role of Motor Units <ul><li>When a motor neuron fires, all of the muscle fibers it controls contract as a group </li></ul><ul><li>Graded contraction then depends on how many motor units are activated and whether they are small or large motor units </li></ul><ul><li>Motor units are recruited in the order of increasing size </li></ul><ul><ul><li>i.e. small units are always recruited first </li></ul></ul>
  40. 47. Motor Neuron Pool <ul><li>All of the motor neurons that innervate a single muscle </li></ul><ul><li>All the muscle fibers enclosed in a single sheath with a single tendon </li></ul><ul><ul><li>e.g. biceps brachii, gastrocnemius </li></ul></ul>
  41. 48. Recruitment <ul><li>Muscle tension can be increased by activating more of the motor neurons controlling a muscle = recruitment </li></ul><ul><li>The brain recruits motor neurons based on the task </li></ul><ul><li>Recruiting synergists </li></ul><ul><ul><li>activate more motor units that work to move in same direction, produce more force </li></ul></ul>
  42. 49. Duration <ul><li>An action potential triggers a muscle to contract </li></ul><ul><li>The duration is controlled by how long the Ca ++ concentration in the cytosol is elevated </li></ul><ul><li>Muscle fibers are specialized for fast or slow contraction </li></ul><ul><li>The type of motor neuron determines the type of muscle fiber </li></ul>
  43. 50. Types of Motor Units <ul><li>Fast motor units </li></ul><ul><ul><li>Muscle fibers used for short, rapid, powerful contractions </li></ul></ul><ul><ul><li>rapidly fatiguing, white muscle fibers </li></ul></ul><ul><ul><li>burst firing patterns in motor neuron </li></ul></ul><ul><li>Slow motor units </li></ul><ul><ul><li>slowly fatiguing, red muscle fibers </li></ul></ul><ul><ul><li>slow, steady firing patterns in motor neuron </li></ul></ul><ul><ul><li>Can sustain long contractions </li></ul></ul><ul><ul><li>Often found in muscles that maintain posture </li></ul></ul>
  44. 51. Specialization of Slow Muscle Fibers <ul><li>Slow muscle fibers must sustain long contractions </li></ul><ul><li>Have less SR </li></ul><ul><li>Slower Ca ++ pumps </li></ul><ul><li>Many mitochondria for a steady energy supply </li></ul><ul><li>Contain myoglobin – </li></ul><ul><ul><li>Specialized oxygen storing protein </li></ul></ul><ul><ul><li>Greater affinity for oxygen than hemoglobin, so it can extract oxygen from the blood </li></ul></ul>
  45. 52. Motor Units & Activity <ul><li>Activity (exercise, athletic training) may change the type of motor neuron </li></ul><ul><li>Patterns of activity may change motor unit type </li></ul><ul><li>Levels of activity increase muscle bulk (especially isometric exercise) </li></ul>
  46. 53. Spinal Control of Motor Units <ul><li>How a motor neuron is controlled </li></ul><ul><li>Sensory feedback from the muscles </li></ul><ul><li>Muscle spindles </li></ul><ul><ul><li>Specialized structures within skeletal muscles </li></ul></ul><ul><ul><li>Specialized muscle fibers contained in a fibrous capsule </li></ul></ul><ul><ul><li>Muscle fibers are wrapped in the middle with with Ia sensory axons </li></ul></ul><ul><li>Spindles & their Ia axons are specialized to detect changes in muscle length (stretch) </li></ul>
  47. 55. Proprioception <ul><li>Proprioception = “body sense” </li></ul><ul><ul><li>Understanding how our body is positioned and moving in space </li></ul></ul><ul><li>Muscle spindles and Ia axons are proprioceptors </li></ul><ul><li>Part of the somatic sensory system </li></ul><ul><li>Myotactic reflex provides one path of sensory input to the spinal cord </li></ul>
  48. 56. Myotatic or Stretch Reflex <ul><li>When a muscle is stretched by an external force, the opposite muscle is also stretched </li></ul><ul><li>Stretching a muscle spindle increases firing rate of the associated nerve </li></ul><ul><li>Nerve makes excitatory synapse with a motor neuron </li></ul><ul><li>Alpha motor neuron increases firing rate </li></ul><ul><li>Muscle fibers contract, muscle spindle is no longer stretched, firing rate decreases, alpha motor neuron excitation is reduced, muscle contraction is reduced </li></ul><ul><li>Serves to maintain muscle tone and compensate for the effects of gravity during movement </li></ul>
  49. 58. Intra & Extrafusal Muscle Fibers <ul><li>Extrafusal skeletal muscle fibers </li></ul><ul><ul><li>The bulk of muscle fibers </li></ul></ul><ul><ul><li>Outside the muscle spindle </li></ul></ul><ul><ul><li>Innervated by alpha motor neurons </li></ul></ul><ul><li>Intrafusal skeletal muscle fibers </li></ul><ul><ul><li>Modified skeletal muscle fibers found only in the muscle spindle </li></ul></ul><ul><ul><li>Innervated by gamma motor neurons at ends to control length of spindle </li></ul></ul>
  50. 59. Gamma Motor Neurons <ul><li>Motor neuron for the muscle spindle </li></ul><ul><li>If not for gamma motor neurons, contraction of muscle would turn off muscle spindles </li></ul><ul><li>During voluntary movements, alpha and gamma motor neurons are co-activated </li></ul><ul><li>The gamma loop: gamma motor neuron  muscle fiber  afferent neuron  alpha motor neuron  opposite muscle fiber </li></ul><ul><li>The gamma loop controls the set point of the myotatic reflex feedback control loop </li></ul>
  51. 60. Golgi Tendon Organs <ul><li>Another sensor of proprioception </li></ul><ul><li>Monitors muscle tension </li></ul><ul><li>Wired in series with whole muscles in tendons </li></ul><ul><li>Excite inhibitory interneurons which inhibit alpha motor neurons in the motor neuron pool for that muscle </li></ul><ul><li>Mediates reverse myotatic reflex </li></ul><ul><ul><li>When force being generated is too great, the alpha motor neurons are turned off </li></ul></ul><ul><ul><li>Reduces force toward the limits of extension of a joint </li></ul></ul><ul><ul><li>Reduces force when limb hits an immovable object </li></ul></ul><ul><ul><li>Regulate fine motor movements of fragile objects such as picking up an empty egg shell </li></ul></ul>
  52. 62. Proprioception from Joints <ul><li>Receptors in joint capsules </li></ul><ul><li>Most are rapidly adapting (movement) </li></ul><ul><ul><li>a few are slowly adapting (stationary position) </li></ul></ul><ul><li>Input is combined with information from muscle spindles and Golgi tendon organs </li></ul><ul><li>Replacement-joint patients still have ability to determine position of limbs </li></ul>
  53. 63. Spinal Interneurons <ul><li>Inhibitory interneurons </li></ul><ul><ul><li>Mediate inverse myotatic reflex </li></ul></ul><ul><ul><li>Mediate coordination of synergists and antagonists by reciprocal inhibition </li></ul></ul><ul><li>Excitatory interneurons </li></ul><ul><ul><li>Mediate polysynaptic flexor reflex - withdrawal of foot when one steps on a tack </li></ul></ul><ul><li>Sometimes excitatory and inhibitory interneurons work together </li></ul><ul><ul><li>Crossed-extensor reflex which tends to keep you from falling when you step on a tack </li></ul></ul>
  54. 64. Spinal Locomotor Programs <ul><li>Circuits of neurons which produce rhythmic motor activity </li></ul><ul><ul><li>central pattern generators </li></ul></ul><ul><li>Different circuits use different mechanisms </li></ul><ul><li>Simplest pattern generators are neurons that serve as pacemakers </li></ul><ul><li>One proven example: </li></ul><ul><ul><li>swimming in a lamprey </li></ul></ul><ul><li>Results from activation of NMDA receptors on spinal interneurons </li></ul>
  55. 65. NMDA Receptors <ul><li>NMDA (N-methyl-D-asparate) receptors </li></ul><ul><li>Glutamate-gated ion channels </li></ul><ul><li>Allow more current to flow into the cell when postsynaptic membrane is depolarized </li></ul><ul><li>Admit Ca ++ as well as Na + into the cell </li></ul>
  56. 66. NMDA Receptors & Locomotion <ul><li>Glutamate activates NMDA receptors </li></ul><ul><li>Na + and Ca ++ flow into cell as membrane depolarizes </li></ul><ul><li>Ca ++ activates Ca ++ activated K + channels </li></ul><ul><li>K + flows out of cell - cell hyperpolarizes </li></ul><ul><li>Ca ++ stops flowing into cell </li></ul><ul><li>K + channels close - ready for another cycle </li></ul><ul><li>Central pattern generators for walking are in spinal cord </li></ul><ul><ul><li>modulated by higher motor neurons </li></ul></ul>