Transfer Function of a Biological Photoreceptor

A full description of the function of a sensory organ, or sensory element, should include definition of the limits of signal transfer. This can be done if the stimulating energy is monitored and, at the same time, the nervous impulses emanating from the sensory receptor are detected and studied. It is believed that the signal that represents the input stimulus, and is transmitted to other centers of the central nervous system, is the average frequency of these nerve impulses. By correlating the instantaneous values of the stimulus and the nerve impulse signal, it is possible to make a rigorous statement regarding the transfer characteristics of the sensory receptor. The task, then, is to find the equation (generally an integro-differential equation) that characterizes the functional relationship between input and output (1-4). To explore the problem, we have chosen to study the photosensitive tail ganglion of Cambarus Astacus, the common crayfish (5-8). Behavioral and reflex studies have demonstrated a physiological role for the receptor (9, 10). Figure XV-1 shows the neuroanatomical structure as drawn by Retzius. The terminal abdominal ganglion, a translucent neuropil of 2-mm diameter receives fibers of several sensory modes from the uropods (7, 8). A small number of neurones, approximately 10, transduces photic energy. Lack of apparent spatial orginization in its structure, absence of lens, and sluggishness of behavioral response suggest that the ganglion serves to detect levels of background illumination only. Through a window in the ventral abdominal exoskeleton, gross electrodes were applied to the ventral nerve cord. The animal was kept in a humidity and temperaturecontrolled environment (100 per cent saturation, 190 ± 0. 5 C). An electronically controlled light source was used to generate transient steps, as well as sinusoidal variations of light (11). Relatively small light fluctuations in a darkened environment permitted small-signal linearization.