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How do muscle spindles detect changes in muscle length?

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Meissner's corpuscle; mechanoreception — Mechanoreception In human skin, specialized nerve endings known as Meissner's corpuscles (or tactile corpuscles) are sensitive to touch. They are one of several different types of mechanoreceptors found in human skin.

Muscle spindles are specialized proprioceptors arranged in parallel with ordinary skeletal muscle fibers. They detect changes in muscle length through a noncontractile sensory midsection located within specialized intrafusal muscle fibers.12

When a muscle is stretched, the intrafusal midsection is also stretched, which increases the frequency of nerve impulses sent to the nervous system. Conversely, when the muscle shortens (relaxes), the midsection slackens, causing a decrease in impulse frequency.12

The spindle utilizes two types of sensory endings to monitor these changes:
* Primary (annulo-spiral) endings: Highly sensitive to the rate of change in length and small stimuli.2
* Secondary (flower spray) endings: Respond more slowly and are less sensitive to rapid changes.12

The sensitivity of these receptors is regulated by fusimotor nerve fibers (gamma efferents), which allow the spindle to adjust its response range independently of the contraction of the main muscle fibers. This mechanism is essential for kinesthesia and the regulation of compensatory reflexes, such as the knee-jerk reflex.21