The morphology and physiology of a “mini-ommatidium” in the median optic nerve of Limulus polyphemus

Abstract

Examination of the Limulus median optic nerve with low-magnification light microscopy allows clear visualization of an ultraviolet-sensitive mini-ommatidium enshrouded by pigment cells, glial cells, and guanophores. Serial l-/*m sections of median optic nerves containing mini-ommatidia revealed the presence of a single, heavily pigmented photoreceptor (retinular) cell and a single, unpigmented arhabdomeric cell. Computer-assisted serial reconstructions from l-/tm sections confirmed the presence of two cells, each bearing a nucleus, and two axons leaving the mini-ommatidium. The retinular cell is morphologically similar to retinular cells from the median and lateral eyes. Its rhabdomere appears to be a continuous sheet of microvilli with much infolding. The structure of the arhabdomeric cell is nearly identical to those found in the median ocellus. As in other photoreceptors in Limulus, the retinular cell of the mini-ommatidium is innervated by efferent fibers from the brain. Each mini-ommatidium generates a single train of nerve impulses in response to light, presumably from the arhabdomeric cell. Measurement of the spectral sensitivity of the mini-ommatidium based upon a constant-response criterion indicated that the retinular cell is maximally sensitive to near ultraviolet light with X max = 380 nm. Comparison of intensity-response functions revealed that those of the mini-ommatidium are significantly steeper than those of the ocellus almost certainly as the result of neural processing in the ocellus which is absent in the mini-ommatidium.

Photosensitivity of the Central Nervous System of Limulus polyphemus

“…Limulus has four kinds of photoreceptors: lateral eyes, median ocelli, ventral photoreceptors and mini-ommatidia. The latter two kinds of photoreceptors are located internally: the ventral photoreceptors are distributed along the ventral optic nerves and are most sensitive to light of 535 nm (Millecchia et al ., 1966); and the mini-ommatidia are distributed in the median optic nerves and are sensitive to light of 380 nm (Samie et al ., 1995). In addition, it has been suggested that there are photosensitive cells in the telson (Hanna et al ., 1985;1988).…”

Physiological and Morphological Identification of Photosensitive Neurons in the Opisthosomal Ganglia ofLimulus polyphemus

“…All cells responded to light with an increase in the frequency of action potentials and a corresponding increase in the slope of the pacemaker potential, as seen in responses just before and at the beginning of illumination (dotted line under each left trace) and on the right with an expanded time scale. 1966), photoreceptors adjacent to the optic ganglia (Snodderly, 1971), photoreceptors in the median optic nerves (Samie et al, 1995), and photoreceptors in the telson (Hanna et al, 1988). Here, we found new photosensitive neurons in the opisthosomal ventral nerve cord.…”