Type Ia sensory fiber
![A muscle spindle, with γ motor and Ia sensory fibers](/Images/godic/202502/04/MuscleSpindle.svg2856.png")
A type Ia sensory fiber, or a primary afferent fiber is a type of sensory nerve. It is a component of a muscle fiber's muscle spindle, which constantly monitors how fast a muscle stretch changes. (In other words, it monitors the velocity of the stretch).