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REFLEXES: Tendon & Other 1

Tendon reflex
  General
  Physiology
H-reflex
Axon reflex

Gowers


Tendon Reflexes: General Clinical Patterns

  • Stimulus
    • Elicited by a short, sharp blow to tendon with a tendon hammer
    • Potentiation: When muscle is mildly stretched
    • Inhibition: Active muscle contraction; Flaccid muscle
  • Typical tendon reflexes tested & major Afferent root level
    • Jaw: Trigeminal
    • Biceps: C5
    • Brachioradialis: C5
    • Pronator: C6
    • Triceps: C7
    • Finger flexor (Hoffman): C6/C7
    • Quadriceps: L4
    • Ankle (Gastrocnemius): S1
  • Reflexes are lost before weakness with
  • Reflexes are lost with weakness
    • Reflexes are generally lost in proportion to weakness
    • Reflexes may be lost proximally but normal distally in muscular dystrophies
  • Potentiation of Tendon reflexes
    • Clinical features
      • Tendon reflex may be evoked by low amplitude stimulus
      • Motor response with potentiation
        • Occurs at somewhat shorter latency
        • May have increased amplitude
      • Spread: Motor activity occurs in groups of nerighboring neurons
    • Jendrassik maneuver (Reinforcement)
      • Produces excitatory effect on α-motor neurones
    • Upper motor neuron lesion
      • Due to reorganization of segmental spinal connections: Especially interneurons  
      • May be associated with spasticity
      • Biceps reflex increased with lesions at or above C3/C4
    • Other causes: Anxiety; Hyperthyroidism; Tetany
  • Other
    • Aging: Tendon reflexes at ankles absent in 6% of normals over age 65
    • Absent ankle reflexes with upgoing toes (Babinski)
      • Suggests combination of spinal & peripheral nerve lesion
  • Comparison to muscle contraction after direct muscle percussion
    • Percussion response > Tendon reflex: Often muscle irritability due to denervation
    • Sustained contraction after muscle percussion: See


Reflex hammer
Pine handled

Tendon Tap: Physiology

  • Nosology: Phasic stretch reflex; Short latency reflex to stretch
  • Initial phase: Phasic stretch stimulates axon terminals in muscle spindles
  • Afferent conduction of impulses
    • Axon type
      • Fastest conducting (Ia) afferents: Terminals respond to phasic changes in muscle length
      • NOTE: Group II afferents in spindles respond to static muscle length
    • Impulses: Synchronized central conduction
  • Motor neuron excitation
    • Monosynaptic
    • Location: Proximal dendrites & cell body
  • Other central pathways involved in tendon reflex
    • Stimulation of spinal interneurons by afferents: At same & neighboring segmental levels as motor neuron
    • Excitation & Inhibition of segmental neurons: Via reticulospinal, vestibulospinal & corticospinal pathways

H-reflex2

  • Definition: Electrical equivalent of the tendon jerk
    • 2-neuron, monosynaptic pathway
    • Elicited by: Electrical stimulation of afferent Ia axons
      • Stimulus: Low amplitude (submaximal) & Long duration (1 ms)
      • Stimulate the IA afferent axons but not efferent motor axons
      • Ia axons have lower electrical threshold than motor axons
      • No role of muscle spindle or fusimotor drive in stimulation of H reflex
    • Efferent limb of H-reflex
      • Anatomical: α-motor neurons, Smallest motor neurons 1st
      • Electrophysiology
        • Consistent latency & configuration
        • Normal latency ~30 ms
        • Initial deflection: Downward (Positive)
        • Measure: H-reflex with shortest latency
        • Normal latency increases with greater height
      • Pathology
        • Unilateral lesion: Diference of ≥ 1.5 ms between sides
        • Upper motor neuron lesion: High H-reflex amplitude to M-wave amplitude (H/M) ratio
    • Modulated by
      • Central excitation & inhibition
      • Jendrasik maneuver: May potentiate H-reflex
      • Operant conditioning: Via corticospinal tract
      • Down-regulation: May be related to increased number of GABAergic terminals on motoneurons
      • Immobilization: Increases H-reflex amplitude via reduced presynaptic inhibition
      • Antagonistic muscles: Contraction reduces H-reflex
      • Golgi tendon organ afferents (Ib): Inhibit H-reflex
      • Whole body vibration: Reduces H-reflex
      • High amplitude stimulus
        • Stimulates motor neurons as well as Ia afferents
        • Inhibits H reflex: Collision of antidromic motor volley with orthodromic afferent volley
      • Spinal cord injury
        • Increased H-reflex: Onset 7 to 21 days after injury
        • Assessmenrt: Decreased low frequency-dependent depression of H-reflex
    • Discovered by: Hoffman in 1918
  • Upper extremity: Flexor carpi radialis H reflex
    • Most easily obtainable H-reflex in upper extremity
    • Elicited by: Median nerve stimulation in antecubital fossa
    • Record over: Flexor carpi radialis muscle
    • Abnormal in
      • Radiculopathies: C6 & C7
      • Proximal median nerve lesions
      • Brachial plexus lesions
  • Lower extremity: Posterior tibial H-reflex
    • Elicited by: Tibial nerve stimulation in popliteal fossa
    • Record over: Soleus muscle

Axon reflex (A-wave)

  • Features
    • Late potential
      • Timing: Occurs between M-response & F-response
      • Same latency & configuration with each stimulation
    • Mechanisms
      • Antidromic potential travels back orthodromically down collateral branch
      • Antidromic potential ephaptically stimulates neighboring axon
  • Clinical associations


TENDON REFLEX ARCS


Click on labels for links
From: Washington University Neuroscience

   
Efferent
  limb

Ramon y Cajal
Afferent
  limb

Return to Myopathy & NMJ Index

1. J Neurol Neurosurg Psychiatry 2003;74:150153
2. J Neurosci Methods. 2008;171:1-12

4/29/2013