muscle spindles


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Related to muscle spindles: Golgi tendon organs, joint receptors
Simplified diagram of a muscle spindle.

muscle spindles

specialized sensory structures within skeletal muscles, consisting of small intrafusal muscle fibres which do not contribute to load-bearing or power generation, but participate in control of the working (extrafusal) fibres. The intrafusal fibres are innervated towards their ends by gamma motor neurons, but are also invested more centrally by both primary and secondary sensory endings; these signal changes in length, and also rates of change in length of the muscle, to the central nervous system. The gamma motor neurons control the sensitivity of the sensory endings to stretching. They also compensate for the shortening of muscle spindles during extrafusal contraction by causing additional intrafusal contraction; this ensures that sensory feedback continues during muscle contractions. The overall system mediates both the stretch reflex and, in voluntary movement, appropriate adjustment of force-generation to load, provided the load is not varying too rapidly. See also gamma motor system, tendon jerk reflex.
References in periodicals archive ?
Such a system would control the prosthetic hand via muscle contraction and would sense the tension and length changes transmitted from the prosthetic hand via muscle spindles and skin sensors, allowing more natural sensory feedback and control of the prosthetic device.
The patella tendon reflex is a monosynaptic reflex that included 1a-afferent and an alpha-motorneuron, and should be the best mechanism to study the sensitivity of the muscle spindles.
Spinal manipulation has been shown to stimulate the muscle spindle and Golgi tendon organ afferents evoking paraspinal reflexes that can inhibit motorneuron excitability and, thus, relax skeletal muscle (8).
Liu JX, Thornell LE and Pedrosa-Domell F (2003): Muscle spindles in the deep muscles of the human neck: A morphological and immunocytochemical study.
The second type of muscle fiber, intrafusal fibers, functions within muscle spindles and is innervated by gamma motor neurons and sensory Ia neurons (see Figure 1 ).
Since the muscles of velopharyngeal closure are not well endowed with muscle spindles, vibration is contraindicated as a treatment and has not been used to date (Clark, 2003).
In limb musculature, the presence and function of muscle spindles and the associated stretch reflexes are consistent relative to muscle size [19-21].
1986) found a decrease in the ability to reproduce joint angles after a series of interval running sprints to fatigue, suggesting that this decreased ability was due to a loss of efficiency of muscle spindles.
Strong evidence exists that in addition to ligamentous and capsular receptors, muscle spindles and possibly golgi tendon organs located in muscles, tendons and at musculotendinous junctions mediate joint positional and joint movement sense (Grigg 1994, Jami 1992, Jozsa et al, 1993, Wilkerson and Nitz 1994).
However, vibration may cause an indirect involvement of muscle spindles, where other sensory inputs may influence gamma (y) motoneuron activity, causing changes to spindle input (Gandevia 2001).
One of these differences is that the discharge rate of Ia afferents from muscle spindles during eccentric contraction is higher than that during other types of muscle contraction, since an eccentric contraction involves the development of tension in the muscle spindle while the whole muscle is being lengthened (Burke, 1978; Ribot-Ciscar and Roll, 1998).