Atype Ia sensory fiber (alsogroup Ia afferent fiber, orprimary afferent fiber) is one of two types ofafferent (sensory) nerve fibers innervatingmuscle spindles - a type ofstretch receptor encountered in nearly allstriated muscles. Type Ia sensory fibers are fast-conducting, large-diameter, thicklymyelinated muscle fibers conveying rapidlyadaptating (dynamic)proprioceptive information regarding velocity (rate of change) of muscle stretch from thenuclear bag fibers. The other type of afferent fibres innervating muscle spindles are the slower-conductingtype II sensory fibers which convey slowlyadaptating (static) information regarding sustained state of muscle stretch.[1]
Type Ia fibers innervate both thenuclear bag fibers and thenuclear chain fibers.[2] Type Ia fibers coil around the non-contractive central region of eachintrafusal muscle fiber of a muscle spindle to form so-called annulospinal primary endings[1] or primary sensory endings (type II sensory fibers meanwhile innervate the periphery of the sensory middle portion ofnuclear chainintrafusal fibers on one or both sides of the central area innervated by type Ia fibers).[2] Type Ia fibers convey phasic responses to small changes in stretch. Dynamicgamma motor neurons adjust the tautness of the central intrafusal region to maintain the dynamic responsiveness of type Ia fibers.[1]
Type Ia fibers participate in mediating thestretch reflex by exciting motor neurons innervatingsynergist muscles and inhibiting motor neurons innervatingantagonist muscles viainterneurons. The stretch reflex thus operates as anegative feedback loop maintaining a given muscle length (and thus body position).[1]