Physio Monosynaptic Reflex (10)

the single restricted region of the VENTRAL horn of the spinal cord where all the motoneurons innervating a single muscle are clustered together

True - each single motor neuron has to innervate multiple muscle fibers

• consist of one motor neuron, its axon, & the collection of muscle fibers said neuron innervates
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• 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

• 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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• 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

when both ends of a muscle are fixed & no movement occurs in the joint(s) involved

• 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)

• 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

• 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

• 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

large motor neurons of the brainstem & spinal cord that innervate extrafusal skeletal muscle fibers & are responsible for their contraction

• 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

• 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

• 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

• 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)

• 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

• 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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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

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

• 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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