Accommodation Motor Neurons in the Foveate Teleost <i>Paralabrax clathratus: </i>Horseradish Peroxidase Labeling and Axonal Morphometry, with Comparisons to Other Ciliary Nerve Components

Abstract

Although much is known about the optics and mechanism of ocular accommodation in teleost fishes, there is to date no description of the neurons innervating the muscle of accommodation, the lens retractor. I have identified accommodation motor neurons in the kelp bass, <i>Paralabrax clathratus, </i>by backfilling the lens retractor nerve (a branch of the short ciliary nerve) with the retrograde tracer horseradish peroxidase. These neurons comprise a subpopulation of relatively large unipolar neurons in the ciliary ganglion. Backfilling either of the remaining ciliary nerve branches (which innervate mainly cornea and iris) labels smaller cells in the ciliary ganglion as well as sensory neurons in the profundus ganglion and sympathetic neurons in the trigeminal sympathetic ganglion. No labeled cells were found in the brain in any of these experiments. I have also examined the lens retractor nerve and the corneal-iridal branch of the short ciliary nerve by electron microscopy. Counts of axons in these nerves from animals of different sizes suggest postembryonic growth of axon number in the corneal-iridal branch but not in the lens retractor nerve. The latter comprises approximately 100 myelinated and a few unmyelinated axons. Its diameter spectrum shows a preponderance of large-diameter axons, but the myelin sheaths are unusually thin (mean axon diameter: 7.5 &#181;m; mean ratio axon diameter/fiber diameter: g <i>= </i>0.81 for 830-gram animal). The results indicate that kelp bass accommodation motor neurons lie primarily if not entirely within the ciliary ganglion. Some of their axons are the largest in the short ciliary nerve, but their sheath thicknesses are apparently not optimal with respect to conduction velocity.