Rods are photoreceptors that work in dim light. They contain the light-sensitive pigment rhodopsin, made of a protein, opsin, joined to a light-absorbing molecule, retinal, which is made from vitamin A.
In the dark:
• Na+ channels in the rod cell membrane are open and Na+ flows in
• the rod cell is depolarised (about −40 mV)
• it continuously releases the neurotransmitter glutamate onto a bipolar neurone
• glutamate inhibits the bipolar neurone, so no impulses are sent along the optic nerveIn the light:
• retinal absorbs light and changes from cis-retinal to trans-retinal
• rhodopsin breaks down into opsin and trans-retinal (bleaching)
• opsin activates a series of reactions that close the Na+ channels
• the pump keeps moving Na+ out, so the rod becomes hyperpolarised (about −70 mV)
• glutamate release stops, so the bipolar neurone is no longer inhibited; it depolarises and stimulates a ganglion cell to send action potentials along the optic nerveRhodopsin is then remade using ATP. In bright light it is bleached faster than it is remade, which is why it takes time to see after entering a dark room.Several rods synapse with one bipolar neurone (retinal convergence). The small effects from many rods add together (summation), so even dim light can trigger a response. This makes rods very sensitive. However, the brain cannot tell which of the rods was stimulated, so rods give low visual acuity. All rods contain the same pigment, so they give only black and white vision.
In the dark:• Na+ channels in the rod cell membrane are open and Na+ flows in
• the rod cell is depolarised (about −40 mV)
• it continuously releases the neurotransmitter glutamate onto a bipolar neurone
• glutamate inhibits the bipolar neurone, so no impulses are sent along the optic nerveIn the light:
• retinal absorbs light and changes from cis-retinal to trans-retinal
• rhodopsin breaks down into opsin and trans-retinal (bleaching)
• opsin activates a series of reactions that close the Na+ channels
• the pump keeps moving Na+ out, so the rod becomes hyperpolarised (about −70 mV)
• glutamate release stops, so the bipolar neurone is no longer inhibited; it depolarises and stimulates a ganglion cell to send action potentials along the optic nerveRhodopsin is then remade using ATP. In bright light it is bleached faster than it is remade, which is why it takes time to see after entering a dark room.Several rods synapse with one bipolar neurone (retinal convergence). The small effects from many rods add together (summation), so even dim light can trigger a response. This makes rods very sensitive. However, the brain cannot tell which of the rods was stimulated, so rods give low visual acuity. All rods contain the same pigment, so they give only black and white vision.