Physio Monosynaptic Reflex (10)

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  1. Motor Pool
    the single restricted region of the VENTRAL horn of the spinal cord where all the motoneurons innervating a single muscle are clustered together
  2. True or False: There are more muscle fibers than there are motor neurons?
    True - each single motor neuron has to innervate multiple muscle fibers
  3. Motor Unit
    • consist of one motor neuron, its axon, & the collection of muscle fibers said neuron innervates
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  4. Molecular Basis of Muscle Length-Tension Relationship
    • at very short muscle lengths, it can't develop much tension: thick myosin filaments are already pressed up against actin Z lines, there's no room to contract
    • at very stretched muscle lengths, myosin filaments DON'T overlap w/ actin filaments - contraction is again impossible
    • a muscle's normal working range is controlled by angle of a joint
    • throughout the normal working length range the passive tension is very small
    • the normal working range of muscle is relatively short compared to the total muscle length possible
  5. Muscle Length-Tension Relationship
    • passive tension is highest when the muscle is completely stretch to its abilities (think like a spring - tension is highest when it's completely stretched
    • active tension is lowest when completely stretched
    • like in the bicep: active tension is probably the ability to FLEX (which you can't do well with a straight arm) but can do well in a neutral resting arm position of ~90 degrees or so (this probably corresponds to L = 1)
    • however passive muscle tension is highest when the arm's completely stretched, b/c passively the muscle is as stretched/tense as it can be
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  6. The tension produced by a muscle is under direct control of what?
    • the motoneurons that innervate it
    • a single action potential in a motoneuron gives rise to a single twitch in the muscle fibers it innervates
    • the amplitude of APs is fixed; only changes in frequency are used to encode information
  7. Isometric Contraction
    when both ends of a muscle are fixed & no movement occurs in the joint(s) involved
  8. Tetanus
    • the product of when muscle fibers are activated repetitively, causing the tension produced during each twitch to sum with that produced by earlier twitches
    • a summation of twitch tension
    • unfused tetanus: has visible "jiggles"
    • fused tetanus: when the "jiggles" aren't visible at all (results from a high frequency of APs)
  9. Why does muscle tension increase with increasing frequency of motor nerve stimulation?
    • skeletal muscle responds to a single AP w/ a twitch
    • the free Ca2+ released by that AP is not long lived in the cytoplasm - it's quickly taken up by binding proteins & pumped back into the sarcoplasmic reticulum by ATPases
    • slow muscle stimulation manifests as tetanus
    • Ca2+ present from a previous AP hasn't been fully removed from the intracellular milieu so the next AP that releases another puff of Ca2+ adds to the tail of the Ca2+ concentration from before
    • more Ca2+ is present, continuing the Crossbridge cycle
    • as Ca2+ frequencies overlap e/a other twitch strength is potentiated
  10. How tension be increased in a muscle (in other words, how can the strength of a muscle contraction be increased)?
    • 1. by increasing the rate at which motoneurons fire action potentials to a muscle
    • 2. increasing the number of neurons firing action potentials on a muscle fiber
    • *can NEVER change the intensity of a twitch by grading action potential heights, ONLY by changing their frequency or the number of neurons sending action potentials to a muscle
  11. Muscle Spindle
    • the sensory receptor in skeletal muscle that detects changes in muscle LENGTH (NOT tension)
    • composed of intrafusal fibers; is encapsulated in collagen; runs parallel to normal muscle fibers
    • conveys length information to the CNS via sensory (afferent) neurons to determine the position of body parts
    • also play an important role in regulating the contraction of muscles by activating motoneurons via the stretch reflex to resist muscle stretching
    • is innervated by 2 types of sensory nerves & a special class of motor axons allowing it to respond both to rapid & maintained changes in muscle length throughout the full range of movement
  12. α-motoneuron
    large motor neurons of the brainstem & spinal cord that innervate extrafusal skeletal muscle fibers & are responsible for their contraction
  13. γ-motoneuron
    • motor neurons that innervate spindles responsible for increasing tension only in the intrafusal muscle fibers
    • have smaller axons and conduct action potentials from the CNS more slowly than α-motoneurons
  14. group Ia sensory (afferent) fibers
    • increase their firing frequency when the muscle & subsequently the spindle is stretched
    • innervate every intrafusal fiber by coiling around its equatorial region (annulo-spiral endings)
    • adapt quickly once the length stops changing
    • main type of stretch receptor
  15. group II sensory (afferent) fibers
    • the muscle's instantaneous length is directly proportional to their firing rate
    • they do not respond to rate of length changes as do the Ia fibers
    • are non-adapting: even when there is no change in muscle length they keep responding to stimuli
    • (are the second most highly myelinated fibers)
    • second type of stretch receptor
  16. What types of skeletal muscles have the highest density of muscle spindles?
    • muscle whose length (& position) we care about have more spindles
    • muscles we have fine control of have more stretch feedback
    • eg. finger muscles have more spindles than back muscles (proportionately)
  17. What does the orientation of muscle spindles within a muscle have to do with their function?
    • the orientation of spindle intrafusal fibers in parallel with extrafusal fibers MEANS that spindles report changes in muscle LENGTH, not tension
    • eg. bicep FLEXION has no effect on the stretch reflex b/c the muscle length stays constant
  18. Efferent (α & γ) Control of Spindle Response
    • as extrafusal muscle fibers shorten, so do intrafusal; whenever α-motoneurons are active, so are γ-motoneurons
    • γ-motoneuron allow spindles to maintain a high sensitivity to length changes by controlling the length of the intrafusal muscle fibers in the spindle
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  19. Homonymous Neuron
    synapses with some afferent neuron (eg. Ia sensory) in the CNS then itself projects back to the same muscle where the neuron it synapsed with originated & projects from
  20. Synergistic Neuron
    synapses with some afferent neuron (eg. Ia sensory) in the CNS then itself projects back to a different muscle that produces a similar motor action
  21. Antagonistic Neuron
    • connected by an interneuron to some some afferent neuron in the CNS then itself projects back to a different muscle that produces the OPPOSITE motor action
    • these connections serve to stabilize the position of a joint
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Card Set:
Physio Monosynaptic Reflex (10)
2014-01-25 19:27:37
MBS Physiology
Exam 1
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