When a skeletal muscle with an intact nerve supply is stretched, it responds by contracting and increasing muscle tone. This reflexive action helps maintain posture and protects muscles from overstretching or injury.

Mechanism Behind Increased Muscle Tone on Stretch

The muscle spindle, a specialized sensory receptor located within the muscle, detects changes in muscle length when stretched. Stretching the muscle lengthens the muscle spindle fibers, stimulating afferent nerve fibers (type Ia sensory neurons).

This sensory information is transmitted to the spinal cord, where it creates a reflex arc that leads to:

• Activation of α-motor neurons that stimulate the extrafusal muscle fibers to contract, countering the stretch.

• Modulation by γ-motor neurons, which adjust the sensitivity of the muscle spindles by contracting the intrafusal fibers within the spindle, ensuring the spindle remains responsive during muscle contraction.

Evaluating the Statements

• A. Golgi tendon organ was stimulated by the stretching of muscle: Incorrect. The Golgi tendon organ detects tension, not muscle stretch, and generally inhibits contraction to prevent damage.

• B. γ-motor neurons were excited by the stimulated afferent nerve fibres from the stretched muscle: Not the primary pathway. Gamma neurons adjust spindle sensitivity and often co-activate with alpha motor neurons but are not directly excited by stretch afferents.

• C. Muscle spindle was stimulated by stretching of muscle: Correct. Muscle spindles are the receptors that detect stretch.

• D. α-motor neurons were excited by the stimulated afferent nerve fibres from the stretched muscle: Correct. Alpha motor neurons cause muscle contraction in response to spindle afferent signals.

Correct Combination

The correct combination is:

(3) C and D