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Type Ia Sensory Fiber also called Primary Afferent Type 1A Fiber is a neuron component of the peripheral sensory system which innervates the muscle spindles, a kind of specialized muscle fiber which is sensitive to muscle length.
In order to control movements, the nervous system must receive continuous sensory information from muscles and joints. For this purpose the body has specialized sensory receptors called proprioceptors. Muscle spindles are a type of proprioceptor, and they are located inside the muscle itself. They are sensitive to muscle length because they are in parallel with the contractile fibers. This change in length of the spindle is transduced (transformed into electric membrane potentials) by two types of sensory afferents, Type Ia (primary) and Type II (secondary). The cell bodies of these sensory neurons are located in the spinal cord.
The two kinds of sensory fibers are different in respect to the kind of potentials they generate. Type Ia fibers respond only to the rate of change in muscle length, as well to change in length. Type II fibers respond only to changes in length. Both types of fiber have a non-linear response to muscle length, i.e., they respond more strongly to small changes in length (less than 1 mm) and reach a plateau at highers lengths.
In addition, the spindle also has a motor efferent innervation carried by gamma motor neurons, which is used by the nervous system to modify the spindle's sensitivity.
Ia afferents from the muscle spindle terminate on the proximal dendrites of motor neurones.
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