Muscle Spindle



Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurons via the stretch reflex to resist muscle stretch.

Extrafusal muscle fibers are innervated by alpha motor neurons(highly myelinated) and generate tension by contracting, thereby allowing for skeletal movement.

Intrafusal muscle fibers are innervated by gamma motor neurons(less myelinated) and thus serve as a sensory proprioceptor.

The static axons innervate the chain or bag2(largest) fibers. They increase the firing rate of Ia(primary) and II(secondary) afferents at a given muscle length (see schematic of fusimotor action below).

The dynamic axons innervate the bag1(smaller) intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia(primary) afferents by stiffening the bag1 intrafusal fibers.

The alpha motor neuron and the extrafusal muscle fibers it innervates make up the motor unit. The connection between the alpha motor neuron and the extrafusal muscle fiber is a neuromuscular junction, where the neuron's signal, the action potential, is transduced to the muscle fiber by the neurotransmitter acetylcholine.

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