Roles of Amacrine Cells by Helga Kolb Webvision

Helga Kolb

1. General characteristics.

Amacrine cells of the vertebrate retina are interneurons that interact at the second synaptic level of the vertically direct pathways consisting of the photoreceptor-bipolar-ganglion cell chain. They are synaptically active in the inner plexiform layer (IPL) and serve to integrate, modulate and interpose a temporal domain to the visual message presented to the ganglion cell. Amacrine cells are so named because they are nerve cells thought to lack an axon (Cajal, 1892). Today we know that certain large field amacrine cells of the vertebrate retina can have long axon-like processes which probably function as true axons in the sense that they are output fibers of the cell (see later section on dopaminergic amacrine cells). However these amacrine axons remain within the retina and do not leave the retina in the optic nerve as do the ganglion cell axons. Figure 1 shows one of the earliest depictions of the retinal cell types including amacrine cells drawn by Ramon y Cajal (circa 1890). These retinal cell types were visualized using the anatomical silver impregnation method devised by the Italian anatomist Camillo Golgi in the nineteenth century (Fig. 2).

Fig. 1. Drawing of the retina made by Cajal

Since the time of Cajal we have known that amacrine cells come in all shapes, sizes and stratification patterns. Since those days many more morphological subtypes have and continue to be described from further Golgi studies, intracellular recordings and immunocytochemical staining. Thus, we presently have a classification of amacrine cells consisting of about 40 different morphological subtypes.

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Roles of Amacrine Cells by Helga Kolb Webvision