The central nervous system (CNS) regulates the loading through alpha motor neurons, which modulate spindle activity and thereby make the transition from stretch to contraction smoother. 25Īlthough the muscle spindle reacts to stretch, it does not simply 'turn off' when the muscle is no longer stretched the fibers continue to send messages when the muscle has begun to concentrically contract and shorten. Thus, the muscle spindle is sensitive to changes in muscle length, as well as to the speed and magnitude of the stretch. Both primary and secondary afferent fibers are present, and these fibers contribute to the ability of the spindle to detect small changes in length. When the stretch is released or lessened, the firing rate diminishes. This stretch increases the firing rate of the afferent fibers innervating the intrafusal fibers, thus 'loading' the muscle spindle. Based on the architecture of the muscle spindle, stretching of the skeletal (extrafusal) fibers also stretches the intrafusal fibers. Muscle spindles are innervated by myelinated afferent nerve fibers, which enter the capsule of and spiral around the intrafusal fibers. Muscle spindles are located within extrafusal (skeletal) muscle fibers and consist of connective tissue surrounding intrafusal fibers in a capsular structure.

The brain is not involved in this spinal reflex loop, contributing to the speed at which the stretch-contraction cycle occurs. These afferent nerve fibers conduct the impulse directly to the spinal cord, where they are immediately conducted via interneurons to alpha motor neurons, which stimulate muscle contraction. Muscle spindles are sensitive to changes in velocity and are innervated by type 1a nerve fibers. This minimal latency time between the quick stretch and subsequent contraction is mediated at the level of the spinal cord as a monosynaptic reflex. When the patellar tendon is tapped with a reflex hammer, the muscle spindles are stimulated, which causes an immediate concentric contraction of the quadriceps muscle group. Muscle spindles function primarily as stretch receptors, as observed clinically in the performance of standard reflex testing (e.g., the knee jerk). Tyler PT, MS, ATC, in Physical Rehabilitation of the Injured Athlete (Fourth Edition), 2012 Neuromuscular Physiology: Muscle Spindles and Golgi Tendon Organs In monkey (rhesus monkey and cynomolgus monkey), very few muscle spindles (2–6) have been observed in some EOMs, and none in the others.Īnthony Cuoco DPT, MS, CSCS, Timothy F. The density of human EOM spindles is comparable to that of muscle spindles in finely controlled skeletal muscle such as the hand lumbrical and deep dorsal neck muscles. Only in the inferior oblique muscle has a lower number of muscle spindles been counted (3–7). The number of human EOM spindles varies between 13 and 42. Specifically, muscle spindles are located predominantly in the proximal and distal parts of the EOM, and each muscle has a spindle-free zone approximately in the middle. In man the distribution of muscle spindles exhibits differences when compared with that in even-toed ungulates. The number of muscle spindles is remarkably high, and counts per muscle yield between 146 and 333 muscle spindles in pig between 100 and 181 muscle spindles in camel and more than 200 muscle spindles in cow. In the EOMs of even-toed ungulates, muscle spindles are uniformly distributed throughout the entire muscle length. b So far not analyzed for palisade endings. A So far, palisade endings have only been demonstrated in sheep.