Physiology and morphology of visual movement detector neurons in a crab (Decapoda: Brachyura).

Although visually elicited behaviors have been extensively studied in crabs, their investigation at the neurophysiological level is scant. The present study is a physiological and morphological description of intracellularly recorded and dye injected visual movement detector neurons that respond to the same stimulus that elicits the escape response in the crab Chasmagnathus granulatus. The neurons were investigated in intact animals. The response of movement detector neurons to the danger stimulus (an object moving above the animal) consists of a strong discharge of action potentials frequently superimposed on noisy graded potentials, whereas the response to stationary changes in illumination is weak or undetectable. The response to the moving stimulus is relatively independent of the background intensity and of the contrast between target and background. Repeated presentations of the moving stimulus produce rapid habituation of the neural response. Some of the neurons also respond to mechanical stimulation. These physiological results coincide with those from early studies on visual movement detector fibers of crustaceans achieved by extracellular recordings. However, there are no previous morphological studies of these neurons. Intracellular injection with Lucifer Yellow revealed that these neurons in Chasmagnathus arborize extensively in the internal medulla and in the lateral protocerebrum. They have their somata located in the cell body cluster laying beneath the internal medulla. Their axons project centripetally across the protocerebral tract.