Tendon Reflexes: General Clinical Patterns

• Nosology: Phasic stretch reflex; Short latency reflex to stretch; Myotatic reflex
• Physiology
  • Tendon tap
  • H-reflex
• 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
  
  • Demyelinating neuropathies
  • Loss of large myelinated sensory axons: Ankle reflex commonly lost early
• 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
    • Myotonia
    • Myoedema

Tendon Tap: Physiology

• 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-reflex ²

• 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
      • ↑ numbers 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: via Collision
        • Antidromic motor volley vs Orthodromic afferent volley
    • Spinal cord injury
      • Increased H-reflex
        • Onset 7 to 21 days after injury
      • Assessment
        • 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
• Also see: F-wave

Axon reflex (A-wave)

• Features
  • Late potential
    • Timing: Occurs between M-response & F-response
    • Same latency & configuration with each stimulation
  • Mechanisms after Antidromic potential (AP)
    • AP Travels back orthodromically down collateral branch
    • AP Ephaptically stimulates neighboring axon
• Clinical associations
  • Reinnervation
  • Demyelination: 1st days in Guillain-Barré syndrome