Structures in sensory region of snake spindles and their displacment during stretch.

1. Structures within the sensory region of short- and long-capsule snake muscle spindles have been visualized using differential interference contrast microscopy. Profiles seen with Nomarski microscopy have been identified by electron microscopy of the same preparations. 2. Sensory nerve terminals, nuclei and other cytoplasmic inclusions in the intrafusal fiber, collagen bands, and capsular cells may be seen in the living preparation. 3. The length changes of various elements within the sensory region in response to stretch of the spindle have been measured using high-speed ciné photomicrography. This has been corrleated with the impulse response from sensory endings in short-and long-capsule spindles. 4. Short-capsule spindles, which have a high dynamic sensitivity, show length changes in the sensory region in response to ramp-and-hold stretch, which are not parallel to the changes in impulse frequency. The implications for mechanical models of spindle behavior are discussed.