Photoreceptor axon terminals

Structure and Function of Gap Junctions in the Photoreceptor Axon Terminals of the Fly... 

Figure 3. Interference contrast micrograph of a 1-fim-thick section of the lamina that shows photoreceptor axon terminals surrounding the paired mono-polar neurons in each cartridge.

Figure 5. Light micrograph of a few facets of a fly s compound eye. Dark spots that represent the rhabdomeres of the photoreceptors have been superimposed onto each corneal lens to demonstrate the principle of neuro-superposition. The central corneal facet has been removed from the photograph to depict the underlying lamina cartridge. Anatomically, six peripheral photoreceptor axon terminals (R1-R6) synapse with two second-order cells (LI and L2) in the underlying neuropil that is called the lamina. Each of these six photoreceptors is illuminated by a different lens, but optically they share the same visual axis that is, they look at the same point in space. This lamina subunit is known as neuroommatidium. Axons of the central receptor cells (R7 and R8) from the overlying ommatidium pass close to this cartridge, but simply bypass the lamina and do not contribute synapses at this neural level.

Figure 7. Fluorescence micrograph of a 1-pm-thick section of the lamina of a male fly that shows stained photoreceptor axon terminals cut parallel to their long axis. Gap junctions between the six peripheral cells occur in the outer 10 pirn of the axon terminal, just below the somas of the monopolar neurons. Because recorded cells are located in reference to the corneal facet matrix, the location and orientation of that cell in the underlying neuropil is predetermined, so it can be sectioned in any given plane and recovered for histological and histochemical examination.

Figure 9. Traces of intracellular recordings from photoreceptor axon terminals (a and b) while the soma of the cell and the soma of a coupled cell were stimulated by a laser and from a monopolar cell (c) while two coupled photoreceptors were stimulated. Half-second stimuli were used in all three cases. In a and b the photoreceptor that contributes its axon terminal to the same cartridge was stimulated by a laser with increasing intensity, and the voltage was recorded from the impaled terminal. The recordings show that coupling is strong between adjacent axon terminals and also that disparate information is retained and encoded by the monopolar cell.

Below the inner segment is the soma and nucleus, which connects at its base to the axon and synaptic terminal. Photoreceptors release glutamate at ribbon synapses (Heidelberger et al., 2005). Synaptic ribbons are specialized for sustained release of neurotransmitter and are also found in the terminals of retinal bipolar cells, as well as vestibular and cochlear hair cells. Synaptic ribbons receive their name because of their planar strnctnre in photoreceptor terminals, although bipolar and hair cell ribbons are more spherical in shape. 

Invertebrate photoreceptor neurons The membrane properties of nerve terminals, axons, soma, and microvflH of Type-B photoreceptors of the invertebrate Hermissenda were modeled using a fast sodium current, an A-type potassium current, a calcium-dependent potassium current, a delayed rectifier potassium current, a noninactivating calcium current, hght-induced sodium and calcium currents, and a leakage current by Post and Clark [1996]. 

The retina contains photoreceptors which are connected to the ganglion cells via neuronal circuits. The ganglion cells axons form the optical nerve. Besides the optical nerve, the visual pathway encloses the optic chiasm and the LGN (Lateral Geniculate Nucleus). The main processing unit for visual perception is the terminal of the optical path The visual cortex, which contains visual stimuli responsive cells performing the final formation of mental representations of the perceived scene (see Fig. 1). It shall be noted that the fibres of the inner (nasal) and outer (temporal) part of the retina of each eye are separated in the optic chiasm and conducted to the same terminal in the visual cortex. Thus, the left hemisphere processes the right visual field of both eyes, whereas the right hemisphere processes the left visual... 

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