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FIGURE 5.1-7 Graph of normalized, peak retinula cell depolarization vs. log flash intensity. (Light from a xenon flash lamp was attenuated with neutral density filters.) Note that the assymptote follows the model, Vlpk = k log(1 + I). The depolarization curve exhibits hard saturation at high light levels. (Figure drawn from data in Naka and Kishida, 1966.)

The structure and function of the lamina has been most widely investigated, and is best known among the three ganglion masses in an OL. The lamina is a thin, curved plate lying at the outer boundary of an OL. It is the region where the nonspiking retinula cells synapse with various types of interneurons. In each lamina cartridge of the fly Calliphora are found six types of neurons:

1. All but one or two retinula cells from the kth ommatidium send their axons to the kth lamina cartridge (Osorio et al., 1997).

2. There are two nonspiking large monopolar cells (LMCs) that have their cell bodies (somata) distal to the lamina, lying between the outer margin of the lamina cartridges and the outer OL membrane. They send signals to the medulla. Their axons are large, from 3 to 5 |im in diameter.

3. There are also two small monopolar cells (SMCs); their somata are also found with the LMC somata. SMC axons also go to the medulla.

4. A single T cell has its soma between the lamina and medulla, and sends an axon to the medulla and to the lamina.

5. In analogy with the vertebrate retina, amacrine cells, whose somata lie on the inner margin of the lamina, send fibers horizontally between neighboring cartridges. They also synapse with a small bipolar cell (SBC) and a T cell (Osorio et al., 1997).

6. One to three (centrifugal) C cells, whose somata lie between the medulla and the lobula, send efferent feedback along their axons to a lamina cartridge.

Drawing Synapse

FIGURE 5.1-8 Phantom drawing of the head of the lubber grasshopper, Romalea microptera, showing the outlines of the surfaces of the CEs and the underlying OLs and the protocerebrum. The "fingerlike" projections from the OLs toward the eyes are bundled retinula cell axons. They are enclosed by a regular system of hemolymph channels lying between the pigmented layer and the lamina. (From Northrop, R.B. and E.F. Guignon, J. Insect Physiol., 691, 1970. With permission.)

FIGURE 5.1-8 Phantom drawing of the head of the lubber grasshopper, Romalea microptera, showing the outlines of the surfaces of the CEs and the underlying OLs and the protocerebrum. The "fingerlike" projections from the OLs toward the eyes are bundled retinula cell axons. They are enclosed by a regular system of hemolymph channels lying between the pigmented layer and the lamina. (From Northrop, R.B. and E.F. Guignon, J. Insect Physiol., 691, 1970. With permission.)

Intracellular recordings from the large axons of the LMC neurons reveals that these cells hyperpolarize in response to the retinula cell depolarization from flashes of light to the eye. Figure 5.1-13 shows a series of these responses in a dragonfly lamina in response to flashes of increasing intensity. Figure 5.1-14 shows a vm vs. log(I) plot for the LMC responses to flashes. The log-linear range covers about 2.5 log units (Laughlin, 1975.)

There is a 1:1 mapping of the retinula cell axons of an ommatidium's to a lamina cartridge, which then projects LMC and SMC axons centrally to the medulla. The medulla has nine or ten anatomically distinct layers of densely packed neuropil. Interneuron fibers interconnect the medullary layers both horizontally and perpendicularly. There is also anatomic and electrophysiological evidence that complex visual feature extraction takes place in the medullar neu-ropile (Northrop, 1975).

FIGURE 5.1-9 Schematic vertical cross section through an OL of Romalea. (drawn from silver-stained sections). Figure is intended to give an overview of the neuroanatomy. Key: O, ommatidial layer of the CE; B, basement membrane; P, pigment layer; H, hemolymph channel; LG, lamina ganglionaris; OC, outer chiasma; M, medulla externa; IC, inner chiasma; L, lobula. (From Northrop, R.B. and E.F. Guignon, J. Insect Physiol., 16: 691, 1970. With permission.)

FIGURE 5.1-9 Schematic vertical cross section through an OL of Romalea. (drawn from silver-stained sections). Figure is intended to give an overview of the neuroanatomy. Key: O, ommatidial layer of the CE; B, basement membrane; P, pigment layer; H, hemolymph channel; LG, lamina ganglionaris; OC, outer chiasma; M, medulla externa; IC, inner chiasma; L, lobula. (From Northrop, R.B. and E.F. Guignon, J. Insect Physiol., 16: 691, 1970. With permission.)

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