The sensura neglecta in the pigeon: a scanning electron and light microscope study.

Abstract

AbstractBoth scanning electron and light microscopy have been used to observe the tiny sensura neglecta which is located in the inferior utricular sinus of the vestibular labyrinth in the pigeon. The following new observations were made. The end organ, which occupies one‐third of the floor of the inferior utricular sinus, is ovoid, measuring 200 μm in length × 80 μm in width and 30 μm in height. The neuroepithelium of the neglecta is uniformly innervated by the neglectar nerve which contains about 60 to 90 myelinated nerve fibers that fasciculate from the posterior ampullary nerve. The fiber spectrum is unimodal with a mode diameter of 1–2 μm. From 150 to 200 Type I and Type II hair cells are found on the surface of the neglecta. The surface features of the hair cells, i.e., the pipe organ patterns of the stereocilia are identical to that of the other vestibular sensory regions. No hair cells are found on the sides of the neglecta and all hair cells project their tufts upward from the top surface. The sensory region of the neglecta is covered by a gelatinous membrane. In no instance are statoconia observed on this covering. The non‐sensory epithelium contiguous to the sensura neglecta is comprised only of “light cells” (Dohlman, '67).The above observations indicate that the sensura neglecta in the pigeon has both crista‐ and macula‐like features. It is hypothesized that this is reasonable from an evolutionary and teleological point of view. It is further suggested that regardless of its similarities to the statolithic and canalicular organs, the mode of stimulation of the neglecta can best be described by analogy to the culmen type of system associated with the auditory papilla of the lizard (Wever, '71).