Final answer:
When a muscle lengthens, the intrafusal fibers within a muscle spindle will also elongate. This response is part of maintaining muscle tone and proprioception, aided by the elasticity of the endomysium covering each muscle fiber.
Step-by-step explanation:
The intrafusal fibers within a muscle spindle are proprioceptive stretch receptors that lie within a muscle. When the length of the muscle increases, the intrafusal fibers will also elongate. This elongation is part of a normal reflex arc that helps maintain muscle tone and proprioception. The muscle spindle, which contains the intrafusal fibers, has an important role in detecting the degree of muscle stretch.
The endomysium is a loose and well-hydrated connective tissue covering each muscle fiber in a skeletal muscle, providing an environment that allows for the elasticity and ability to stretch and rebound. This elasticity helps the intrafusal fibers lengthen and return to their original length without damage.
Muscle contraction can occur when the muscle fibers receive a nervous stimulus, signaling through the neuromuscular junction to drive the cross-bridge cycling. Meanwhile, during relaxation or stretching of a muscle, the intrafusal fibers within muscle spindles lengthen and demand less tension, contributing to the overall variation in muscle length.