Physio Monosynaptic Reflex (10)
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the single restricted region of the VENTRAL horn of the spinal cord where all the motoneurons innervating a single muscle are clustered together
True or False: There are more muscle fibers than there are motor neurons?
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
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
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
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
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)
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
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
- 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
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
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
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)
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
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
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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