Final answer:
Gamma static efferent neurons cause contraction of the polar regions of intrafusal muscle fibers, particularly the nuclear chain regions, to adjust the sensitivity of muscle spindle sensory organs to muscle length changes.
Step-by-step explanation:
The action of intrafusal muscle fibers produced by gamma static efferent neurons is the contraction of nuclear chain regions. More specifically, these neurons adjust the sensitivity of the muscle spindle sensory organs to the length of the muscle. When gamma neurons are activated, they cause the contraction of the polar regions of the intrafusal muscle fibers, making the central region of the spindle taut. This increases the sensitivity of the muscle spindle to changes in muscle length, helping to maintain muscle tone and posture. Although contraction happens at the ends of intrafusal fibers rather than the central regions, the polar regions associated with contraction specifically relate to the nuclear chain fibers.