Correct option is B
Statement A:
"The specialized intrafusal fibers in muscle spindles have non-contractile polar ends and a contractile centre."
Explanation: Intrafusal fibers indeed have contractile regions at their polar ends (near the nerve endings) and a non-contractile central region where sensory endings wrap around. However, these contractile parts are activated by γ-motor neurons, not α-motor neurons. This design allows the spindle to maintain sensitivity during muscle contraction by adjusting tension at the ends.
It is mostly correct but somewhat incomplete because the central part is non-contractile, not the polar ends. The polar ends are contractile, and the central region is non-contractile. So the wording is slightly off.
Statement B:
"The intrafusal fibers do not contribute to the overall contractile force of the muscle."
Explanation: This is true. Intrafusal fibers are specialized sensory fibers inside muscle spindles that detect stretch but do not generate force to produce movement. Instead, the extrafusal fibers surrounding them are responsible for the contractile force of skeletal muscle.
Statement C:
"The primary sensory ending in a muscle spindle is formed by group Ia afferent fibers."
Explanation: Correct. Group Ia afferent fibers are large-diameter, fast-conducting sensory neurons that form the primary (annulospiral) endings around the central region of intrafusal fibers. They detect both the rate and degree of muscle stretch, sending this info to the spinal cord to initiate reflexes.
Statement D:
"The axons of α-motor neurons having a diameter of 12-20 µm innervate the muscle spindles as the motor nerve."
Explanation: This is incorrect. Muscle spindles are innervated by γ-motor neurons, not α-motor neurons. α-motor neurons innervate extrafusal muscle fibers to generate force. γ-motor neurons have smaller axons and adjust spindle sensitivity by controlling intrafusal fiber contraction.
