STRETCH REFLEX

STRETCH REFLEX/ MYOTACTIC REFLEX

MECHANISM OF REFLEX ACTION:

• Stimulus: Passive stretch.
• On stretching, skeletal muscle responds by contracting (becomes shorter & stiffer).
• Sensitizes “Muscle Spindles”-
  • – Stretch receptor within a muscle.
• Only known example of “Monosynaptic reflex”.
  • – Single-synapse.
  • – Located in anterior horn of spinal cord.
  • – Lowest most level of integration.

1. To maintain muscle tone.

2. To make muscles respond to stretch & release.

• Ie., by contracting & relaxing.

REFLEX ARC OF STRETCH REFLEX:

On stretching muscle length increases.

• Detected by muscle spindle.

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Spindles get stretched.

• especially stretch-sensitive central part.

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Increases afferent nerve activity from muscles spindles.

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Conveyed monosynaptically to alpha motor neurons in spinal cord.

• Stimulating the same.
• α-motor neurons supply extrafusal muscle fibers.

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Results in muscle contraction (extrafusal fibers)

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Subsequently, releases/reduces stretch on intrafusal fibers (muscle spindle)

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Eventually reflex stops.

 

TYPE OF STRETCH RESPONSE:

• Depending upon stimulated afferent type:

1. Dynamic response:

• Afferents respond with frequency proportional to rate at which fiber stretches.
• Eg: Seen with afferents originating from nuclear bag fibers.

2. Static response :

• Afferents respond to absolute length of spindle.
• Eg: Seen with afferents originating from nuclear chain fibers.

NEURONS INVOLVED:

Two types of α (Aα) neurons involved:

1. Sensory – 

• Afferent (Aα/Iα).
• Originate from intrafusal fibers (bag & chain fibers).
• At annulospiral (primary) endings.

2. Motor –

• Efferent (Aα).
• Supply extrafusal fibers.
• Causes muscle contraction.

ROLE OF GAMMA(y) -MOTOR NEURON:

• Stretch reflex arc involves only alpha motor neurons.
  • – Supplying extrafusal fibers.
  • – Thus, α-motor neuron stimulation causes muscle contraction directly.

• y-motor neuron (Ay-fibers) alter spindle sensitivity to stretch.
  • – Causes muscle contraction indirectly.
  • – By stimulating stretch reflex via intrafusal fibers contraction.

• If both muscle stretching & y-motor neuron stimulated simultaneously,
  • – Causes intrafusal fiber contraction & Iα activation.
  • – Also, increases Iα discharge rate.

• Stimulation of motor cortex & other higher centers send descending excitatory input to spinal circuit.
• – Results in simultaneous activation of alpha & gamma motor neurons.
• – In turn, produces sustained muscle contraction maintaining posture.

Factors increasing y-motor neuron discharge:

• Stimulation of skin 
• – Especially noxious agents
  • Anxiety
  • Unexpected movement

Jendrassik’s maneuver:

• Trying to pull hands apart when flexed fingers are hooked together facilitating knee-jerk reflex.

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