2/19/2023 0 Comments Muscle spindle![]() Both of the stretches start and end at the same muscle length. Figure 4 shows the dynamic response to two different stretches performed at two different rates (A is slow while B is fast). If the change in length is slow the firing rate of the Ia fibers tend to more closer resemble the firing rate of the static type II fibers. However if the difference is large there was a rapid change in length. If the difference is small this means there was a slow change in muscle length. The difference in the firing of rates of the muscle spindle before and after a stretch or a change in length is called the dynamic response. These fibers are slowing adapting therefore they fire while the muscle is stretching and continue to fire after the muscle has stopped moving. The type II fibers which are attached to the chain intrafusal fibers monitor the static stretch and length of the muscle. These fibers are rapidly adapting so there is a quick change in their firing rate during muscle stretch but once the stretch is completed the Ia adapts and stops firing (seen in Figure 3 above). ![]() The rate at which the length changes is monitored by the Ia fibers around the dynamic bag fibers because these fibers are more compliant and less sensitive to stretch. Therefore as the muscle contracts gamma motor neurons are also firing in order to keep the spindle taut, which allows the spindle to function at all muscle lengths during movement and postural adjustments. When the nervous system sends information to the alpha motor neurons it also sends information to the gamma motor neurons causing them to fire through alpha-gamma coactivation. There is also a synapse in the spinal cord with an inhibitory alpha motor neuron that goes to the antagonist muscle causing reciprocal inhibition. ![]() This causes autogenic excitation and results in muscle contraction. Once the signal reaches the spinal cord it has a monosynaptic synapse with an alpha motor neuron that leads to the muscle where the spindle is located. This allows for an influx and a signal carrying information about the muscle length is propagated on a Ia or II afferent to the dorsal root of the spinal cord. When a change in muscle length occurs the intrafusal fibers in the spindle are lengthened which pulls on the sensory nerve endings and causes mechanically gated ion channels in the spindle to open. It has been established that this is not the case by direct visual observation of intrafusal contraction in isolated living spindles, by intracellular recording from intrafusal fibres, and by light and electron microscopy of cat muscle spindles.The muscle spindle works primarily through the myotatic stretch reflex. Complete resolution of the problem was delayed until 1980 because the glycogen depletion technique suggested that static γ axons frequently innervated the bag₁ fibre (Fig. The other one, the 'bag₁ fibre', is separately controlled by dynamic axons (Fig. One of them, the 'bag₂ fibre', is controlled by static γ axons which sometimes share this innervation with the nuclear chain fibres. The controversy was to a large extent resolved by the discovery in the early 1970s that there were two types of nuclear bag fibres which are structurally and mechanically quite different. Barker and colleagues maintained that the same motor axon frequently terminated on both types of intrafusal fibre. Boyd maintained that two types of intrafusal muscle fibre, nuclear bag fibres and nuclear chain fibres, were separately innervated by different types of small γ motor axons. The controversy in the 1960s about the motor innervation of mammalian muscle spindles is described.
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