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Muscle Tone

It is due to asynchronous gamma motor neuron discharge which increases sensitivity of muscle spindle and their discharge increases. As a result α Motor neurons are activated and lead to muscle contraction(involuntary).these contractions result in increased tension in muscle even when not contracting called resting muscle tone. Increase gamma discharge leads to increased muscle tone called Hypertonia is of 2 types

Both flexors & Extensors are hypertonic. Seen in Extrepyramidal lesion like Parkinsons

Only 1 group of muscle is hypertonic.eg. Pyramidal lesion

Gamma motor neuron discharge Regulation


1. Descending Tracts: from number of areas in brain – they control the sensitivity of muscle spindle & hence stretch reflexes of various parts of body – to meet the needs of posture control.Mainly inhibitory so when cut leads to increased γ discharge & Rigidity occurs called release phenomenon.eg. Decerebrate rigidity.

2. Anxiety:

γ motor neuron discharge

Hyperactive tendon reflexes

3. Stimulation of Skin by Noxious Agent:
γ motor discharge to ipsilateral flexor muscle spindles, Withdrawl reflex activated.

4. Pain:
γ motor discharge. Eg Gaurding seen in Acute abdomen due to increased γ discharge of abdominal muscles.

Jendrassik’s Maneuver: trying to pull the hands apart when flexed fingers are hooked together – γ efferent discharge facilitates Knee jerk reflex


Summary of Stretch Reflex




Inverse Stretch reflex (
autogenic inhibition)

Golgi Tendon Organ

  1. Receptor for inverse stretch reflex (3-25 muscle fibres / tendon organ )
  2. are in series with muscle fibres high threshold receptors
  3. stimulated by both passive stretch and active contraction of muscle
  4. supplied by Ib afferents
  5. senses muscle tension
  6. It acts as a protective reflex to prevent tearing of muscles. As a strong & potentially damaging muscle force reflexly inhibits the contraction – leading to relaxation instead of trying to maintain the force.
  7. Pneumonic: Golgi tendon organ is for tension (t for tension, t for tendon)
  8. Muscle spindle is for length


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