Chapter 2: Spinal Level Reflexes

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Chapter 2: Spinal Level Reflexes
2015-03-08 18:15:24
Spinal Level Reflexes
NPTE: Chapter 2
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  1. Spinal Level Reflexes
    • Involuntary responses to stimuli
    • Basic, specific and predictable
    • Dependent on intact neural pathway (reflex arc)
    • May be monosynaptic or polysynaptic (involving interneurons)
    • Provides basis for unconscious motor function and basic defense mechanisms
  2. Stretch (Myotactic) Reflexes: Stimulus
    Muscle Stretch
  3. Stretch (Myotactic) Reflexes: Reflex Arc
    • 1a fiber from muscle spindle to alpha motor neurons
    • Projecting back to muscle of origin (monosynaptic)
  4. Stretch (Myotactic) Reflexes: Functions
    • Maintenance of muscle tone
    • Support agonist muscle contraction
    • Provide feedback about muscle length
    • Clinically, sensitivity of stretch reflex and intactness of SC segment tested by applying stretch to deep tendons
  5. Stretch (Myotactic) Reflexes: Reciprocal Inhibition
    • Via an inhibitory interneuron
    • Same stretch stimulus inhibits the antagonist muscle
  6. Stretch (Myotactic) Reflexes: Reciprocal Innervation
    Describes the effects of a stretch stimulus on agonist (autogenic facilitation), antagonist (reciprocal inhibition), and synergistic muscles (facilitation)
  7. Inverse Stretch (Myotatic) Reflex: Stimulus
    Muscle Contraction
  8. Inverse Stretch (Myotactic) Reflexes: Reflex Arc
    Afferent 1b fiber from GTO via inhibitory interneuron to muscle of origin (polysynaptic)
  9. Inverse Stretch (Myotactic) Reflexes: Functions
    • Provide agonist inhibition
    • Diminution of force of agonist contraction
    • Stretch-protection reflex
  10. Gamma Reflex Loop: Definition
    • Stretch reflex forms part of this loop
    • Allows muscle tension to come under control of descending pathways (reticulospinal, vestibulospinal, and others)
  11. Gamma Reflex Loop: Mechanism
    Descending pathways excite gamma motor neurons → contraction of muscle spindle → increased stretch sensitivity and increased rate of firing from spindle afferents

    Impulses are often conveyed to alpha motor neurons
  12. Flexor (Withdrawal) Reflex: Stimulus
    Cutaneous sensory stimuli
  13. Flexor (Withdrawal) Reflex: Reflex Arc
    • Cutaneous receptors via interneurons to largely flexor muscles
    • Multisegmental response involving groups of muscles (polysynaptic)
  14. Flexor (Withdrawal) Reflex: Functions
    Protective withdrawal mechanism to remove body part from harmful stimuli
  15. Crossed Extension Reflex: Stimulus
    • Noxious stimuli and reciprocal action of antagonists 
    • Flexors of one side are excited → extensors on same side to be inhibited (opposite responses occur in opposite limb)
  16. Crossed Extension Reflex: Reflex Arc
    Cutaneous and muscle receptors diverging to many spinal cord motor neurons on same and opposite side (polysynaptic)
  17. Crossed Extension Reflex: Function
    Coordinates reciprocal limb activities such as gait