Sensory innervation of myosin heavy chain-based types of chicken intrafusal fiber

Chicken intrafusal fibers that had been categorized as slow or fast contracting by their reactions with monoclonal antibodies against myosin heavy chains (MHC) at polar regions were examined at the equator and neighboring juxtaequator to determine if fiber type-specific morphologies were also present at the sensory region. The parameters chosen for examination were contractile filaments, distribution of acetylcholinesterase and sensory innervation. Evaluation of acetylcholinesterase was included to define the proximal limit of motor terminal distribution. At the equator actin was primarily restricted to a thin layer beneath myosensory junctions. Myosin heavy chains at the equator and the juxtaequator were more evenly distributed. Although absent from the synaptic side of the sensory region, acetylcholinesterase activity was present on the extrasynaptic side. Plots of diameters of sensory axon collaterals and of sensory endings yielded continuous spectra instead of distinct peaks. Taken together, these features indicated a uniform equatorial morphology. However, sensory terminals to fast intrafusal fibers were longer and narrower than those to slow intrafusal fibers, and typically more terminals impinged on fast than on slow intrafusal fibers. Moreover, in the larger spindles short sensory terminals were most prevalent near the equatorial-juxtaequatorial junction, the region where in mammalian intrafusal fibers secondary sensory axons make contact. The differences seen in sizes and distributions of terminals suggest that, despite the lack of type-specific morphologies at the sensory region of chicken intrafusal fiber types, some level of functional separation is maintained there.