Design and control of the head retractor muscle in a turtle, Pseudemys (Trachemys) scripta: II. Efferent innervation.

The head retractor muscle (RCCQ) of Pseudemys scripta is a useful model in which to study the mechanisms animals use to vary the force and timing of movement. Single fibers in this muscle differ significantly in attachments, length, diameter, taper characteristics, and histochemical properties, suggesting that they may be energetically and architecturally specialized for different roles in head movement. In the present paper, we report the peripheral and central efferent innervation of these diverse muscle cells, and we ask how the design of the neural apparatus is matched to the properties of its target muscle fibers. Three out of four bellies in RCCQ are supplied by multiple segmental nerves. The territories of these nerves are separated rostrocaudally within the muscle belly; thus, long muscle fibers cross the territories of two or more segmental nerves. Motor terminals in RCCQ resemble those on frog twitch muscles. Their sizes (length, bouton number) are correlated with the diameters of their target muscle fibers. Each muscle fiber bears 2-14 terminals evenly spaced (approx. 5 mm apart) along its length. Thus, single muscle fibers in RCCQ are multiterminally, and long fibers are multisegmentally innervated. Control experiments indicate that the axons in each segmental nerve arise from different motor neuron populations. Thus, short, in-series fibers are supplied by different motor neurons, and individual long fibers in RCCQ are polyneuronally innervated. These data help explain how long muscle fibers with relatively slow conduction speeds can generate rapid head movements, and they raise questions about the central mechanisms that coordinate the recruitment of RCCQ motor neurons.