Complex sentence: Type Ia Sensory Fiber also called Primary Afferent Fiber is a component of a muscle fiber's muscle spindle which keeps track of the how fast a muscle stretch changes (the velocity of the stretch).
Split into simpler sentences: 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 the how fast a muscle stretch changes (the velocity of the stretch).