How the eye works

Rod Cells in dark...

• opsin + cis-retinal ----> rhodopsin
• causes sodium channels to open
• membrane depolarised
• neurotransmitter released from rod cell to bipolar cell
• bipolar neurone hyperpolarised
• no neurotransmitter released into synapse between bipolar cell and ganglion
• no generator potential
• no action potential

Rod cells in the light...

• Rhodopsin ----> opsin and trans-retinal
• causes sodium channels to close
• membrane hyperpolarised
• no neurotransmitter released into synapse
• bipolar neurone depolarised
• neurotransmitter released into synapse between bipolar cell and ganglion
• generator potenrials add together to form action potential
• action potential travels along ganglion neurone

Changes the shape of the opsin protein in a process called beaching

the reverse reaction trans to cis requires an enzyme reaction and is very slow

this explains why you are initially blind when you walk from sunlight to a dark room; in the light almost all your retinal was in the trans form and it takes time to form enough cis retinal to respond to the lower level of light indoors

the light intensity is also too low to stimulate the cones and takes about 20 minutes for enough rhodopsin to reform for us to see properly.