Visual processing in the crayfish medulla
Pfeiffer-Linn, Cindy L.
Glantz, Raymon M.
Doctor of Philosophy
The crayfish visual system consists of a neural network which acts to convey and process sensory information to the brain. One previously established pathway in this network involves a class of visual interneuron known as sustaining fibers. These third order interneurons make a functional connection with oculomotor neurons in the brain which mediate a light elicited behavior known as the dorsal light reflex. This behavior represents one of the few sensory examples where the physiological input and behavioral output are known. However, little is known about what type of information is being conveyed and processed to the sustaining fibers from the photoreceptors. As a result, I have used a neuropharmacological approach in this thesis to verify synaptic connections to the sustaining fibers and to analyze the mechanisms of action for conveying visual information. Each cell type analyzed in this study was found to receive two antagonistic neurotransmitter inputs. In two instances, I discovered rather unconventional mechanisms used by the crayfish to generate the cell's light response. One of these occurs in a class of neurons known as tangential cells. Here, acetylcholine initiates the initial hyperpolarizing phase of the light response, whereas gamma-aminobutryric acid is actively responsible for the repolarizing phase of the response. These two antagonistic neurotransmitters were found to act on the exact same set of chloride channels. A second unconventional mechanism is used by the crayfish to generate the sustaining fiber's light response. In this case, two different glutamate receptors act jointly to shape the light response. One type of glutamate receptor (non-NMDA) mediates the initial transient depolarization of the sustaining fiber light response and the subsequent plateau response may be due to activation of a second class of glutamate receptor (NMDA). The results of this pharmacological study elucidated many of the synaptic connections in the crayfish visual system and illustrated the mechanisms of information transfer. These studies have direct implications for a number of general aspects of visual processing such as contrast and motion detection.