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  1. A stereotyped involuntary motor response to a specific stimulus involving a receptor, afferent neuron, CNS involvement influenced by the brain, efferent neuron (alpha motor neuron) and effector muscle
  2. Reflexes of muscular origin – responses to stretch involve two receptor organs – the neuromuscular spindle and the Golgi tendon organ and 3 reflex circuits – Ia, Ib, and II
    1. neuromusclular spindles – See Figure 7
      1. composed of a group of small intrafusal muscle fibers enclosed within a connective tissue capsule and innervated by sensory and motor nerve fibers; muscles with the most fine movements have the greatest number of spindles; spindles are sensitive to muscle tension, length and velocity of change
        1. there are 2-12 intrafusal muscle fibers within the capsule; these are either nuclear bag fibers or nuclear chain fibers (nuclear chain fibers are long and thin and more numerous)
      2. spindles are located within the muscle belly; the connective tissue capsule of the intrafusal muscle fibers (nuclear bag fibers) attach to the connective tissue of extrafusal muscle fibers
        1. intrafusal fibers are organized in parallel with extrafusal fibers which in turn are in series with the collagen fibers in the tendon
      3. each spindle is innervated by one group Ia afferent fiber which spiral around the equatorial regions of the nuclear bag and chain fibers
      4. each spindle is also innervated by 1-5 group II afferent fibers that are adjacent to the equatorial region of the nuclear bag and chain fibers
      5. each spindle has 7-25 efferent fibers which terminate on the contractile polar regions of the nuclear bag and chain fibers
    2. Golgi tendon organs
      1. Located at the muscle-tendon junction in series with extrafusal muscle fibers and composed of collagen bundles of the tendon and innervated by a sensory nerve fiber
      2. Innervated by one group Ib afferent fiber that branches around the surface of the collagen bundle
      3. Golgi tendon organs detect changes in tension and active contraction
    3. Ib circuit
      1. Have high threshold to externally applied stretches and are usually stimulated by contraction (i.e. tension changes not length changes) of the extrafusal muscle fibers that are in series with the Golgi tendon organ
      2. The Ib fibers cause inhibition of alpha motor neurons in the muscle from which the stimulus arose and cause excitation of alpha motor neurons to antagonist muscles
    4. Ia circuit
      1. Have very low threshold to stretch thus when a stretch is applied anywhere along the muscle tendon complex (e.g. tapping for tendon jerk reflex or by slow sustained stretches) it is transmitted to the extrafusal fibers and their associated intrafusal fibers causing an increase in Ia discharges
      2. Central processes of Ia fibers enter the spinal cord and give off branches that ascend and descend in the gray matter (Clarke’s column) for projection to the cerebellum and dorsal white columns; other branches of Ia fibers go to alpha motor neurons that innervate the muscle from which they arose producing excitatory effects
        1. Short, sharp stretches elicit a short burst of impulses in the Ia fibers which elicit a short burst of impulses in the target alpha motor neurons – the phasic stretch reflex
        2. Long, sustained stretches such as squeezing the muscle produce a sustained contraction – the tonic stretch reflex
      3. Other branches of Ia fibers cause inhibition of alpha motor neurons to the muscles which are antagonists of the stretched muscle
      4. Reflex induced contraction of a stretched muscle leads to shortening of the extrafusal muscles and release of stretch on their associated intrafusal muscles; the slackening of the intrafusal fibers decreases impulse activity in the Ia and II fibers
    5. II circuit
      1. Have low threshold to stretch and responds mostly to slow sustained stretches (tonic response)
      2. Behave almost identically to Ia fibers and cause similar effects
      3. As a greater length in the muscle is achieved, the II fibers fire at greater frequency (Ia fibers do the same thing but also increase their firing rate as the muscle is actively stretched providing information to the brain about the velocity of the muscle stretch – the dynamic response)
    6. gamma motorneuron control of the spindle receptors
      1. when activated, gamma motor neurons cause shortening of the contractile portions of the intrafusal muscle fibers just like externally applied stretches cause activity in both Ia and II circuits; because intrafusal fibers slacken when a muscle shortens, the spindle discharge should cease when the muscle shortens but if this were to occur the spindle would fail to transmit information; gamma motor neurons prevent this from happening
      2. gamma motor neurons function to adjust the length of the intrafusal muscle fibers in order to keep the spindle receptors on a sensitive part of their response scale
      3. two types of gamma motor neurons
        1. dynamic which enhances dynamic response; increased in activity when muscle length may change rapidly and unpredictably such as walking on a balance beam; dynamic gamma motor neurons innervate dynamic nuclear bag fibers only
        2. static which enhances static response (keeps muscle taut even when contracted); may be part of the spasticity pathology; static gamma motor neurons innervate nuclear chain fibers and static nuclear bag fibers; Remember: chains generally don’t stretch and are static
        3. activity of gamma motor neurons is decreased by lesions of the cerebellum causing hypotonia
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