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Movement I

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  • 1. Movement Part I Spinal Control of Movement
  • 2. Overview
    • Alpha motor neurons, which innervate the skeletal muscle fibers, are the final common pathway for behavior.
    • They are wired into a complex set of reflex loops in the spinal cord.
    • These reflex loops are supplemented by locomotor programs in the spinal cord which provide the basic rhythmic aspects activities such as walking.
  • 3. Types of Muscle
    • Smooth muscle
      • digestive system & arterioles
      • innervated by adrenergic autonomic nervous system
    • Cardiac muscle (striated)
      • heart muscle
      • modulated by autonomic nervous system
    • Skeletal muscle (striated)
      • body and eye movement
      • breathing
      • controlled by lower motor neurons in spinal cord
  • 4. Skeletal muscles are the effectors of movement .
  • 5. Categories of Muscles
    • Categories based on direction of motion
    • Categories based on body location
  • 6. Types by Body Location
    • Axial muscles
      • move trunk
    • Proximal muscles
      • move shoulder, elbow, pelvis, knee
    • Distal muscles
      • move hands, feet, digits
  • 7. Muscles Are the Effectors of Movement
    • All animal movement is based on contraction of muscles working against some type of skeleton
    • The action of a muscle is always to contract
      • Muscles extend only passively
    • To move body parts in opposite directions, muscles are attached in antagonistic pairs
    • Example:
      • Bicep contracts  arm flexes
      • Bicep relaxes; triceps contracts  arm extends
  • 8.  
  • 9. Types by Direction of Motion
    • Flexors
      • reduce angle of joints
    • Extensors
      • increase angle of joints
    • Synergists
      • all flexor muscles working together on one joint
      • all extensors working together on one joint
      • muscles that work in parallel
    • Antagonists
      • flexors and extensors for one joint
      • muscles that work in opposition
  • 10. Structure of Skeletal Muscle
    • Formed from a hierarchy of smaller & smaller parallel units
    • Each muscle consists of a bundle of long fibers, the length of the muscle
      • Each fiber is a single cell with many nuclei
    • Each fiber is a bundle of smaller myofibrils
    • Myofibrils are formed from 2 types of myofilaments:
      • Thick & thin
    • Myofilaments are formed from 2 key proteins:
      • Actin & myosin
  • 11.  
  • 12. Myofilaments
    • Thin filaments
      • Two strands of actin and one of regulatory protein
    • Thick filaments
      • Staggered arrays of myosin molecules
  • 13. Sarcomeres
    • Skeletal muscle is striated:
    • The regular arrangement of myofilaments creates a repeating pattern of light & dark bands
    • Each repeating unit = sarcomere
    • The basic contractile unit of muscle
  • 14. Z-Lines
    • The borders of the sarcomere = Z-lines
    • These are lined up in adjacent myofibrils
    • Thin filaments are attached to the Z-lines and project toward the center of the sarcomere
    • Thick filaments are centered in the sarcomere
  • 15.  
  • 16. Banding
    • At rest, thick & thin filaments don’t overlap completely
    • The area at the edge of the sarcomere where there are only thin filaments = I band
    • The broad region of thick filaments = A-band
    • H - zone is in the center of the A-band and contains only thick filaments
    • The arrangement of thick & thin filaments is the key to muscle contraction
  • 17.  
  • 18. Filaments & Contraction
    • When a muscle contracts, the length of each sarcomere is reduced
    • The distance from one Z-line to the next gets shorter
    • The A-bands don’t change, but the I-bands shorten
    • The H-zone disappears
  • 19. The Sliding Filament Model
    • Neither group of filaments changes length when a muscle contracts
    • Rather, the filaments slide past each other, so the overlap increases
    • If the overlap increases, the area of only thin filaments (I-band) and the area of only thick filaments (H-zone) decreases
  • 20. The sliding filament model of muscle contraction.
  • 21. Actin & Myosin
    • Thick & thin filaments are formed from actin & myosin
    • The myosin “head” is the site of bioenergetic reactions that power muscle contraction
  • 22. Interaction of Actin & Myosin
    • Myosin head binds ATP and hydrolyzes it to ADP
    • The energy released is transferred to myosin
    • The myosin changes shape
    • The energized myosin binds a specific site on the actin molecule, forming a cross-bridge
    • This releases energy, relaxing the myosin head
  • 23. Actin & Myosin (continued)
    • The myosin changes shape and bends inward on itself
    • This exerts tension on the thin filaments to which it is bound
    • Which pulls the thin filaments toward the center of the sarcomere
    • When a new ATP molecule binds the myosin head, the bond between myosin & actin is broken
    • The cycle repeats
  • 24. The Repeating Cycle
    • Each of the ~ 350 myosin heads of a thick filament forms and reforms 5 cross-bridges/sec
    • Producing muscle contractions
  • 25. Actin & Myosin Interaction
  • 26. Energy
    • Muscle cells store only enough ATP for a few muscle contractions
    • They store glycogen between myofibrils
    • Most energy for muscles is stored in phosphagens
      • In vertebrates = creatine phosphate
  • 27. Motor Neurons & Movement
    • A muscle contracts only when stimulated by a motor neuron
    • An action potential in a motor neuron connected to muscle causes it to contract
    • Ca ++ ions and regulatory proteins control muscle contractions
  • 28. Regulatory Proteins
    • When a muscle is at rest, myosin binding sites on actin are blocked by regulatory proteins, tropomyosins
    • The position of tropomyosin on the thin filaments is controled by troponin complex . Another set of regulatory proteins
    • For a muscle to contract, the myosin binding site on actin must be exposed
  • 29. The Role of Ca ++
    • When Ca ++ binds to troponin alters the tropomyosin-troponin complex, exposing the mysosin binding sites on actin.
    • When Ca ++ is present, filaments can slide and muscles contract
    • When Ca ++ levels decrease, contraction stops
  • 30.  
  • 31. The Sarcoplasmic Reticulum
    • Ca ++ in the cytosol of a muscle cell is regulated by the sarcoplasmic reticulum (specialized type of ER)
    • Surrounds myofibrils; sequesters and releases calcium
    • The membrane of the sarcoplasmic reticulum (SR) actively transposrt Ca++ from the cytosol to the interior of the SR
      • An interior storehouse for Ca++
  • 32. Motor Neurons
    • Spinal organization
      • Lower motor neurons
    • Alpha motor neurons
    • Motor units
    • Motor neuron pools
  • 33. Spinal Organization Lower Motor Neurons
    • Motor neuron fibers exit the spinal cord in the ventral root of each spinal segment
      • cell bodies in ventral horn
    • Cell bodies have a somatotopic arrangement
    • There are bulges in the ventral horn because of the large number of motor neurons for the arms and for the legs
  • 34. Alpha Motor Neurons
    • Neuron directly responsible for synapsing on muscle fibers and causing movement
      • final common pathway for behavior
    • Sources of direct input
      • Sensory input from muscle spindles
      • Input from spinal interneurons
      • Descending input from upper motor neurons (e.g. motor cortex)
    • Controlling the force of muscle contraction
  • 35. The Neuromuscular Junction
    • Action potential in a motor neuron connected to a muscle causes contraction
    • The synaptic terminal of the motor neuron releases acetylcholine at the neuromuscular junction, depolarizing the muscle cell
    • Sarcolemma
      • external, electrically excitable membrane of a muscle fiber
  • 36. Excitation  Contraction
    • How the action potential in a motor neuron causes muscle contraction
    • Nicotinic ACh receptors (transmitter-gated ion channel) open Na + channels
    •  EPSP
    • Muscle fiber generates action potential which sweeps down the sarcolemma
  • 37. Transverse Tubules
    • Transverse (T) tubules = infoldings of sarcolemma (membrane)
    • Conduct the action potential inward
    • Depolarization of T-tubules activates a voltage sensitive protein that plugs Ca ++ channels in SR
    • Where the T-tubules touch SR, the action potential changes the permeability of the SR, causing release of Ca ++
      • Calcium is released and floods myofibrils
    • Ca ++ binds to troponin, allowing the muscle to contract
  • 38.  
  • 39. Relaxation
    • Contraction stops when the SR pumps Ca++ out of the cytosol and troponin-tropomyosin complex blocks myosin binding sites as Ca++ concentration decreases
    • Calcium ions are sequestered by SR via an ATP-driven Ca ++ pump
    • Myosin binding sites on actin are covered by troponin
  • 40.  
  • 41. Graded Contractions
    • Muscle contractions are graded
      • some are strong, some are weak
    • We can voluntarily alter the strength of a contraction
    • At a cellular level, the response is all or none
    • Any stimulation that depolarizes the plasma membrane of a single muscle fiber triggers a contraction
      • Like in a neuron
    • So how are contractions graded?
  • 42. Creating Graded Responses
    • Nervous system can vary the frequency of action potentials in motor neurons
    • Action potential summation  gradation
    • Rate coding
      • each action potential produces a muscle twitch
      • fire faster and produce stronger contraction
    • If the rate of stimulation is fast enough, individual twitches become one smooth contraction = tetanus
      • Not the same as the disease!
  • 43. Temporal summation of muscle contraction: muscle tension resulting from 1, 2, or a series of action potentials.
  • 44. The Motor Unit
    • One alpha motor neuron and all the muscle fibers it innervates
    • Each muscle fiber is innervated by only one motor neuron
    • Each motor neuron may synapse with many muscles cells
      • Motor units range in size from 1:3 (fine control) to 1:1000 (leg muscles)
  • 45. Structure of a vertebrate motor unit.
  • 46. The Role of Motor Units
    • When a motor neuron fires, all of the muscle fibers it controls contract as a group
    • Graded contraction then depends on how many motor units are activated and whether they are small or large motor units
    • Motor units are recruited in the order of increasing size
      • i.e. small units are always recruited first
  • 47. Motor Neuron Pool
    • All of the motor neurons that innervate a single muscle
    • All the muscle fibers enclosed in a single sheath with a single tendon
      • e.g. biceps brachii, gastrocnemius
  • 48. Recruitment
    • Muscle tension can be increased by activating more of the motor neurons controlling a muscle = recruitment
    • The brain recruits motor neurons based on the task
    • Recruiting synergists
      • activate more motor units that work to move in same direction, produce more force
  • 49. Duration
    • An action potential triggers a muscle to contract
    • The duration is controlled by how long the Ca ++ concentration in the cytosol is elevated
    • Muscle fibers are specialized for fast or slow contraction
    • The type of motor neuron determines the type of muscle fiber
  • 50. Types of Motor Units
    • Fast motor units
      • Muscle fibers used for short, rapid, powerful contractions
      • rapidly fatiguing, white muscle fibers
      • burst firing patterns in motor neuron
    • Slow motor units
      • slowly fatiguing, red muscle fibers
      • slow, steady firing patterns in motor neuron
      • Can sustain long contractions
      • Often found in muscles that maintain posture
  • 51. Specialization of Slow Muscle Fibers
    • Slow muscle fibers must sustain long contractions
    • Have less SR
    • Slower Ca ++ pumps
    • Many mitochondria for a steady energy supply
    • Contain myoglobin –
      • Specialized oxygen storing protein
      • Greater affinity for oxygen than hemoglobin, so it can extract oxygen from the blood
  • 52. Motor Units & Activity
    • Activity (exercise, athletic training) may change the type of motor neuron
    • Patterns of activity may change motor unit type
    • Levels of activity increase muscle bulk (especially isometric exercise)
  • 53. Spinal Control of Motor Units
    • How a motor neuron is controlled
    • Sensory feedback from the muscles
    • Muscle spindles
      • Specialized structures within skeletal muscles
      • Specialized muscle fibers contained in a fibrous capsule
      • Muscle fibers are wrapped in the middle with with Ia sensory axons
    • Spindles & their Ia axons are specialized to detect changes in muscle length (stretch)
  • 54.  
  • 55. Proprioception
    • Proprioception = “body sense”
      • Understanding how our body is positioned and moving in space
    • Muscle spindles and Ia axons are proprioceptors
    • Part of the somatic sensory system
    • Myotactic reflex provides one path of sensory input to the spinal cord
  • 56. Myotatic or Stretch Reflex
    • When a muscle is stretched by an external force, the opposite muscle is also stretched
    • Stretching a muscle spindle increases firing rate of the associated nerve
    • Nerve makes excitatory synapse with a motor neuron
    • Alpha motor neuron increases firing rate
    • Muscle fibers contract, muscle spindle is no longer stretched, firing rate decreases, alpha motor neuron excitation is reduced, muscle contraction is reduced
    • Serves to maintain muscle tone and compensate for the effects of gravity during movement
  • 57.  
  • 58. Intra & Extrafusal Muscle Fibers
    • Extrafusal skeletal muscle fibers
      • The bulk of muscle fibers
      • Outside the muscle spindle
      • Innervated by alpha motor neurons
    • Intrafusal skeletal muscle fibers
      • Modified skeletal muscle fibers found only in the muscle spindle
      • Innervated by gamma motor neurons at ends to control length of spindle
  • 59. Gamma Motor Neurons
    • Motor neuron for the muscle spindle
    • If not for gamma motor neurons, contraction of muscle would turn off muscle spindles
    • During voluntary movements, alpha and gamma motor neurons are co-activated
    • The gamma loop: gamma motor neuron  muscle fiber  afferent neuron  alpha motor neuron  opposite muscle fiber
    • The gamma loop controls the set point of the myotatic reflex feedback control loop
  • 60. Golgi Tendon Organs
    • Another sensor of proprioception
    • Monitors muscle tension
    • Wired in series with whole muscles in tendons
    • Excite inhibitory interneurons which inhibit alpha motor neurons in the motor neuron pool for that muscle
    • Mediates reverse myotatic reflex
      • When force being generated is too great, the alpha motor neurons are turned off
      • Reduces force toward the limits of extension of a joint
      • Reduces force when limb hits an immovable object
      • Regulate fine motor movements of fragile objects such as picking up an empty egg shell
  • 61.  
  • 62. Proprioception from Joints
    • Receptors in joint capsules
    • Most are rapidly adapting (movement)
      • a few are slowly adapting (stationary position)
    • Input is combined with information from muscle spindles and Golgi tendon organs
    • Replacement-joint patients still have ability to determine position of limbs
  • 63. Spinal Interneurons
    • Inhibitory interneurons
      • Mediate inverse myotatic reflex
      • Mediate coordination of synergists and antagonists by reciprocal inhibition
    • Excitatory interneurons
      • Mediate polysynaptic flexor reflex - withdrawal of foot when one steps on a tack
    • Sometimes excitatory and inhibitory interneurons work together
      • Crossed-extensor reflex which tends to keep you from falling when you step on a tack
  • 64. Spinal Locomotor Programs
    • Circuits of neurons which produce rhythmic motor activity
      • central pattern generators
    • Different circuits use different mechanisms
    • Simplest pattern generators are neurons that serve as pacemakers
    • One proven example:
      • swimming in a lamprey
    • Results from activation of NMDA receptors on spinal interneurons
  • 65. NMDA Receptors
    • NMDA (N-methyl-D-asparate) receptors
    • Glutamate-gated ion channels
    • Allow more current to flow into the cell when postsynaptic membrane is depolarized
    • Admit Ca ++ as well as Na + into the cell
  • 66. NMDA Receptors & Locomotion
    • Glutamate activates NMDA receptors
    • Na + and Ca ++ flow into cell as membrane depolarizes
    • Ca ++ activates Ca ++ activated K + channels
    • K + flows out of cell - cell hyperpolarizes
    • Ca ++ stops flowing into cell
    • K + channels close - ready for another cycle
    • Central pattern generators for walking are in spinal cord
      • modulated by higher motor neurons