Lateral Inhibition In The Eye Of Limulus

Much of the early work on CE vision was done by Hartline (1949), Hartline and Ratliff (1957; 1958), Ratliff et al., 1963; 1966, and Tomita (1958) on the lateral eyes of the horseshoe crab, L. polyphemus. Limulus, a chelicercate arthropod, has two relatively simple, CEs and is an ideal subject to study the basic anatomy, organization, and neural signal processing of a primitive CE. Limulus eyes are elipsoidal bulges on the sides of the animal's dorsal carapace, about 12 mm long by 6 mm wide. Each ommatidium has an optical aperture about 0.1 mm in diameter; the facets are spaced about 0.3 mm center-to-center on the surface of the eye. The optical axes of the ommatidia diverge from one another, so that the combined visual fields of the ommatidia view about a hemisphere (2n sterradians).

A vertical section through a Limulus ommatidium is shown in Figure 5.3-1. Note the eccentric cell (E-cell); more will be said about this cell below. Note that in cross-sections of ommatidia, 11 retinula cells (from 7 to 15 have been reported; Wulff and Mueller, 1975) are arranged around the center like the sections of an orange. The striated parts of the retinula cells at the center of the ommatidium are the rhabdoms, where photon trapping and light-to-depolarization transduction take place. Rhabdoms are composed of closely packed microvilli, at right angles to the cell surface and continuous with the cytoplasmic reticulum. The E-cell dendrite at the center of the retinula cell cluster also has a layer of microvilli arranged radially around its circumference. The microvilli (tubules) are about 140 |im in diameter (Bullock and Horridge, 1965).

FIGURE 5.3-1 Schematic drawing of a section through an ommatidium in a Limulus CE. Note the eccentric cell, which sends a dendrite into the center of the rhabdom. Pigment cells are omitted from surrounding spaces. Cell nucleus; CL, crystaline lens; XC, glassy cells, RC, retinula cells; EC, eccentric cell; Dt, dendrite; Rh, rhabdoms; LP lateral plexus.

The number of ommatidia in a CE depends on the animal and its lifestyle. Limulus has from 700 to 1000 ommatidia per eye, while a dragonfly has about 9000 to 10,000 ommatidia per eye. There is neurophysiological and anatomical evidence that Limulus CEs are very primitive; they may have evolved earlier and separately from those of insects and crustaceans (Bullock and Horridge, 1965).

Directly beneath the ommatidia of each CE, Limulus has a lateral plexus of nerve fibers emanating from the approximately 11 retinula cells and the E-cell of each ommatidium. These fibers spread laterally in all directions at the base of the eye to form the lateral plexus; the lateral fibers evidently synapse in an inhibitory manner with the spiking eccentric cell of each neighboring ommatidium. Intracellular recording

Limulus Eye Retinula Eccentric Cell

FIGURE 5.3-1 Schematic drawing of a section through an ommatidium in a Limulus CE. Note the eccentric cell, which sends a dendrite into the center of the rhabdom. Pigment cells are omitted from surrounding spaces. Cell nucleus; CL, crystaline lens; XC, glassy cells, RC, retinula cells; EC, eccentric cell; Dt, dendrite; Rh, rhabdoms; LP lateral plexus.

Dt with glass micropipette electrodes revealed that the retinula cells depolarize in a logarithmic manner when subjected to a flash of light, but do not spike Figure 5.3-2 illustrates a depolarizing receptor potential from a Limulus retinula cell. Figure 5.3-3 illustrates the logarithmic response of the instantaneous spike frequency of one E-cell at ON of light to one ommatidium. The frequency log-linearity at ON is exists over about three decades of source intensity before it saturates. The E-cell spike frequency shown in curve B, taken 3.5 s after ON, shows little log-linearity. The E-cell is evidently a true neuron rather than a photoreceptor (Ratliff, 1964).

Transmembrane Potential Gragh
FIGURE 5.3-2 Transmembrane potential change of a Limulus retinula cell given a 0.5-second flash of light. Note sharp overshoot at ON. (From Wulff & Meuller, 1975, in The Compound Eye and Vision in Insects. G.A. Horridge, Ed., Oxford University Press, with permission.
Stopping Potential Frequency Graph

logio (Intensity/Imax)

FIGURE 5.3-3 Graph of typical instantaneous frequency (IF), r, vs. log light intensity ratio for a Limulus eccentric cell. Open circles: the IF at ON; dark circles: the IF 4 s after ON. A single ommatidium was illuminated.

logio (Intensity/Imax)

FIGURE 5.3-3 Graph of typical instantaneous frequency (IF), r, vs. log light intensity ratio for a Limulus eccentric cell. Open circles: the IF at ON; dark circles: the IF 4 s after ON. A single ommatidium was illuminated.

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