The avian oculomotor system provides an important animal model to better understand the development and control of eye movements. My work describes and quantifies critical morphological and functional parameters of the developing and maturing extraocular muscles (EOMs) of chicken. It also determines the long-term effects of botulinum neurotoxin on developing extraocular muscle; a question that has clinical significance for the treatment of strabismus.

The first project describes the structural changes associated with the developing neuromuscular junctions of extraocular muscles from embryonic day 14 (E14) through posthatch day 2 (P2). Changes in motor endplate density/size and the progression of synapse elimination are quantified. Motor endplates increased in size and synapse elimination was completed by P2.

The second project compares two common approaches to measure the force of EOM: in situ and in vitro methods. Specifically, the effect of confounding parameters is determined: blood supply, nerve stimulation, direct muscle stimulation, muscle size, oxygenated or non-oxygenated buffer solutions, and the time after interruption of vascular circulation. Blood supply is found to be critical, and absolute forces of extraocular muscle are substantially lower when examined in vitro. EOM should be examined by in situ methods to gain valid absolute forces.

The third project determines how extraocular muscle fibers change in their structural and functional properties during development from post-hatch day 2 (P2) to adulthood. We describe and quantify critical parameters of the developing superior oblique extraocular muscle, including contractile force, muscle mass, myofiber diameters, classification of fiber types, and distribution and quantification of mitochondria. Results show similarities with mammalian EOMs and reveal surprisingly protracted changes in myofiber type composition into adulthood.

The fourth project determines whether botulinum neurotoxin's therapeutic effects in the treatment of infantile strabismus are more likely due to changes in the periphery or due to central mechanisms. Quantification of physiological parameters (contractile force measurements) and morphological parameters (myofiber morphometry, innervation, quantitative transmission electron microscopy of mitochondria/fiber types) demonstrates that the effects of botulinum neurotoxin on developing extraocular muscle are transient. Thus, permanent therapeutic effects of botulinum neurotoxin appear to be caused by central (CNS) adaptive responses rather than permanent changes at the level of the peripheral effector organ.