Type Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes (the velocity of the stretch).
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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, whose cell bodies are located in dorsal root ganglia located next to the spinal cord.
The two kinds of sensory fibers are different in respect to the kind of potentials they generate:
| Type | Primary/secondary | Response |
| Type Ia | primary | Respond to the rate of change in muscle length, as well to change in length |
| Type Ib | N/A | in Golgi tendon organ |
| Type II | secondary | Respond only to changes in length |
The first of the two main groups of stretch receptors wrapping the intrafusal fibers are the Ia fiber, which are the largest and fastest fibers, and they fire when the muscle is stretching. They are characterized by their rapid adaptation, because as soon as the muscle stops changing length, the Ia stop firing and adapt to the new length. Ia fibers essentially supply proprioceptive information about the rate of change of its respective muscle: the derivative of the muscle's length (or position).
Type Ia fibers connect to both nuclear bag fibers and nuclear chain fibers. These connections are also called "annulospiral endings ".
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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