Neurochemical stratification in the inner plexiform layer of the vertebrate retina

Abstract

Most regions of metazoan nervous systems can be idealized as laminated structures in which layers of neuropil are sandwiched between layers of neuronal somas and the structure pierced by afferent or efferent fibers whose trajectories are roughly orthogonal to the layers. Within very complex systems, such as retina, mammalian cerebral cortex or invertebrate visual medulla and lobula plate, a finer lamination exists within individual layers of neuropil. The inner plexiform layer (IPL) of the vertebrate retina has long been known as a structure composed of many sublayers. As one passes along an axis normal to the plane of the IPL*, the local composition varies in terms of the kinds and numbers of synaptic complexes, axonal or dendritic processes, and glial profiles. We will consider the composition within a plane at a given level to be homogeneous, though this presumption is dependent on the scale over which one samples the composition. This review is intended to summarize current knowledge, and an extensive historical account is inappropriate. Nevertheless, the observations of Ramon y Cajal and his contemporaries are central to interpreting more recent data. Ramon y Cajal ( 1933) gives Dogiel (I 891) credit for the attribution of IPL banding to the processes of amacrine and ganglion cells but notes that many