Photoreceptors synaptic terminal

Figure I. Structure of a ciliated photoreceptor cell in vertebrates. (A) Scheme of a rod photoreceptor cell. (B) Transmission electronmicroscopy image of a part of a mouse rod photoreceptor cell. The apical extensions of cells of the retinal pigment epithelium (RPE) evolve the tips of photoreceptors light-sensitive outer segments (OS). The OS is linked via a connecting cilium (CC) to an inner segment (IS) which bears the basal body complex (BB) in its apical region. Synaptic terminals (S) link the photoreceptor cell and the 2nd-order neurons, bipolar and horizontal cells. N = nucleus in B, arrow point to axonemal microtubules projecting into the OS. Bar in B = 0.2 p,m...

Photoreceptor Inner Segment, Soma, and Synaptic Terminal... 

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. 

Glutamate release from synaptic terminals of photoreceptor and bipolar cells is regulated by calcium influx through L-type calcium channels (Heidelberger et al., 2005). The use of F-type channels at ribbon synapses contrasts with the reliance on N, P, and Q type channels for neurotransmission at conventional synapses of spiking neurons. A retina-specific L-type channel, alpha IF (CaVl.4), is localized to rod terminals. Mutations in this channel produce a congenital stationary night blindness (Bech-Hansen et al., 1998). 

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