Cones Pathways

Two sections of cones

Inner segment and outer segment

Inner segment of cones

Contains organelles and metabolic operations

Outer segment of cones

Membrane discs

Where are discs located?

Inside the cytoplasm of rods

Rhodopsin

Retinal and opsin

Where is rhodopsin in the disc?

embedded in membrane

Retinal

Vitamin A derivative, changes the conformation of the rhodopsin molecule

Opsin

Protein

Transducin

G-protein, activates PDE, activated by changing conformation of retinal

Phosphodiesterase (PDE)

Converts cGMP to GMP

Guanylate cyclase

Converts GTP to cGMP, always on

cGMP

Internal signaling molecule, binds to Na+ channels to open

Trans retinal

On, conformation changes and activates transducin

Cis retinal

Off, no changing conformation and no activation of transducin

What conformation is retinal in the dark?

Cis, off

What conformation is retinal in the light?

Trans, on

Rods signaling pathway in the dark

Retinal is cis and off, rhodopsin is inactive, transducin is inactive, PDE is inactive, guanlyate cyclase is covering GTP to cGMP, cGMP is binding Na+ channels, Na+ in, cell depolarizes, exocytose glutamate

What happens when the rod exocytosis glutamate?

Glutamate inhibits bipolar neuron, which inhibits ganglion, which reduces AP frequency of the ganglion cell to the brain, which indicates darkness

Rods signaling pathway in dim light

Some cis retinal converted to trans, activates transducin, transducin activates PDE, PDE converts cGMP to GMP, guanylate cyclase converts GTP to cGMP, less cGMP, only some cGMP binds intracellularly to NA+ channel, less depolarization, less glutamate released, less inhibition

Rods signaling pathway in bright light

Most retinal is trans, rhodopsin activated and bleaches, transducin activated and activates PDE, PDE degrades cGMP to GMP, guanlyate cyclase turns GTP to cGMP, no cGMP to bind to Na+ channels as PDE degrades it all, no Na+ open, no depolarization, no glutamate released, no inhibition

Rod bleaching

Trans retinal ejected out of rhodopsin when there are too many photons, rod becomes inactivated

Rod saturation

All Na+ channels closed as so much cGMP is being degraded that no channels are open