The stretch reflex, which is also often called the myotatic reflex, is a pre-programmed response by the body to a stretch stimulus in the muscle. It is the monosynaptic reflex and the quickest of all the reflexes. Stretch reflex play an important role in maintaining posture.
When a muscle spindle is stretched an impulse is immediately sent to the spinal cord and a response to contract the muscle is received. This is designed as a protective measured for the muscles, to prevent tearing the muscle spindle is stretched and the impulse is also immediately received to contract the muscle, protecting it from being pulled forcefully or beyond a normal range.
The proper muscle function is decided by the afferent and efferent impulses from the nervous system.
Afferent system
- Muscle spindle
- Golgi tendon
Efferent system
- Alpha motor neurons
- Gamma motor neuron
MUSCLE SPINDLE
Muscle spindles are stretch receptors within the body of muscle that primarily detect changes in the length of the muscle.
STRUCTURE: It has two types of muscle fibers;
- Intrafusal muscle fiber
- Extrafusal muscle fiber
Extrafusal muscle fibers are the skeletal standard muscle fiber that are innervated by alpha motor neurons and generate tension by contracting thereby allowing for skeletal movement.
Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organ (proprioceptors) that detect the amount and rate of change in length of a muscle. These fibers are innervated by gaama efferent motor neuron.
INTRAFUSAL MUSCLE FIBERS DIVIDED INTO TWO VARIETIES :
1)Nuclear bag fibers:
- Nuclear bag fibers contain the nucleus in the center portion of the receptor and it gives the bag like structure.
- Afferent fibers: group 1a fiber( primary afferent fiber) innervatetd in the middle portion.
- Efferent fibers: end portion of the fiber is innervated by the gamma (γ) efferent neurons (γ-d fiber).
- onset stretch(dynamic response).
- The dynamic component lasts for only a movement in response to the initial sudden increase in muscle length.
2) Nuclear chain fiber:
- Nuclear chain fibers look like the chain and the nucleus concentrate more in the center and scatterly present in the receptor part of the fiber.
- Afferent fibers: innervated by the secondary efferent.
- Efferent fibers: end portion of the fiber is innervated by the gamma(γ) efferent neurons(γ-d fibers)
- sustained stretch(static response)
- The static component lasts as long as the muscle is being stretched.
STRETCH REFLEX MECHANISM
Stretch reflex is the reflex contraction of muscle when it is stretched.
Stretching of the muscle causes stretching of the muscle spindle, this stimulates the muscle spindle and it discharge the sensory impulse.
These impulses are transmitted via the primary and secondary sensory nerve fibers to the alpha (α) motor neuron in spinal cord.
Alpha motor neurons in turn send motor impulse to muscles through their fibers and cause contraction of extrafusal fibers.
ROLE OF MUSCLE SPINDLE IN THE MAINTENANCE OF MUSCLE TONE
Muscle tone is defined as the state of continuous and passive partial contraction of muscle with certain vigor and tension. It is due to the continuous discharge of impulses from gamma motor neuron.
Gamma motor neurons innervate the intrafusal fibers. Motor impulses from gamma motor neurons stimulate the intrafusal fibers of muscle spindle, which in turn sends sensory impulses back to spinal cord. Now the alpha motor neurons in spinal cord are activated resulting in contraction of extrafusal muscle fiber of muscle.