A type Ia sensory fiber is one of two types of afferent (sensory) nerve fibers innervating muscle spindles - a type of stretch receptor encountered in nearly all striated muscles. Type Ia sensory fibers are fast-conducting, large-diameter, thickly myelinated muscle fibers conveying rapidly adaptating (dynamic) proprioceptive information regarding velocity of muscle stretch from the nuclear bag fibers. The other type of afferent fibres innervating muscle spindles are the slower-conducting type II sensory fibers which convey slowly adaptating (static) information regarding sustained state of muscle stretch.