Extraocular muscles (EOM) move the eye and, as such, subserve a diverse range of fine movements from the slowest (smooth pursuit) to the most rapid (saccades) in the body, a breadth of functions virtually unique for a single muscle. Two populations of EOM fibers exhibit a systematic variation in myosin heavy-chain (MHC) expression along their length, with fast MHC centrally and slower MHC peripherally. One fiber population is dually innervated by small multiple endplates peripherally and a twitch-like endplate centrally, suggesting that innervation controls myosin expression along the fiber. However, MHC variation is seen as well in fibers with only a single-twitch endplate. These observations are important because such fibers will have a fast contracting central portion surrounded by slower contracting periphery resulting in fast, rapidly damped contractions of tiny amplitude, possibly underlying the eye's ability to make quick, small and brief movements. We want to describe this heterogeneity at the protein and mRNA levels and determine how it is maintained along the fiber.

Another project is to investigate the nature and origin of the sustained tension present in EOM during passive stretch. This "tone's" source and its function in EOM are unknown; however, like the sustained tension produced by EOM tonic fibers under maintained depolarization (see Figure), this tone requires extracellular calcium. If the tone is triggered by the smooth muscle stretching, it might be expected to be activated in vivo by contracting the opposing EOM and thus acting as a damp to that contraction without the time delay of proprioceptive feedback.