Stretch Reflex

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.
PRINCIPLE FUNCTIONS:

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.


Spindles get stretched.

  • especially stretch-sensitive central part.

Increases afferent nerve activity from muscles spindles.

Conveyed monosynaptically to alpha motor neurons in spinal cord.

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

Results in muscle contraction (extrafusal fibers)

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

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.
‘Alpha-gamma coactivation’:
  • 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.
Exam Question
 

STRETCH REFLEX/ MYOTACTIC REFLEX

  • Stretch receptor within a muscle – “Muscle Spindles”.
  • Detects increasing muscle length due to stretching.
  • Stimulus – Passive stretch.
  • “Monosynaptic reflex”.
  • Lowest most level of integration of stretch reflex is spinal cord.
  • Reflex mechanism involves α-motor neurons in spinal cord.
  • In turn, supply extrafusal fibers.
  • Main functions: Mainly maintaining muscle tone & making muscles respond to stretch & release.

ROLE OF GAMMA(y) -MOTOR NEURON:

  • Stretch reflex arc involves only alpha motor neurons.
  • y-motor neuron (Ay-fibers) alter spindle sensitivity to stretch.
  • Activation causes intrafusal fiber contraction

Factors increasing y-motor neuron discharge:

  • Stimulation of skin.
  • Anxiety.
  • Jendrassik’s maneuver.
Don’t Forget to Solve all the previous Year Question asked on Stretch Reflex

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this:
WordPress