Abstract
Purpose :
In primates, the midget system is responsible for our conscious vision, where our brain constructs an internal representation of the world. Wavelength and intensity of light in the retinal image are transformed into lightness and color, representing objects' surface reflectance, allowing us to recognize them and know their internal composition. Studies of lower vertebrates have shown that different components of the visual scene are transformed, discarded, or selected in the inner retina before visual information is communicated centrally. We explored the possibility of amacrine cells (ACs) specific to the midget system that may perform computations often ascribed to the cortex.
Methods :
Serial block-face scanning electron microscopy was used to reconstruct local circuits comprised of A5 and stellate varicose ACs (SVAC) connected to ON midget bipolar cell (BC) terminals. Outgoing ribbon synapses and incoming conventional synapses were identified using classical electron microscopic criteria.
Results :
Earlier, we described a wide-field SVAC specific to midget BC terminals. Here we report a narrow field AC with morphological characteristics and stratification matching the A5 ACs described in humans. These cells are also specific to midget BCs and share many of the same BC terminals with the SVACs. However, the details of the circuity of the two AC types are quite different. The wide-field SVACs are very narrowly stratified and form many reciprocal synapses with the ON midget BC terminals; the narrow-field A5 AC is broadly stratified. In addition to being postsynaptic to midget BC ribbons in sublamina (S) 5, it receives substantial conventional synapse input from ACs in S3 and S4.
Conclusions :
The SVAC is GABAergic, and we assume the A5 AC is glycinergic, with these two fulfilling separate roles in processing visual signals in the midget pathway. GABAergic ACs can shape signals by acting as spatial and temporal filters. Glycinergic ACs may function to elevate the sensitivity of midget ganglion cells to specific stimuli by the process of sensitization, which arises from the adaptation of tonic inhibition. These processes may contribute to the transformation of image intensity to percepts of light and color.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.