rods

Meaning: Rods are photoreceptor cells in the retina that contain the pigment rhodopsin. They respond to very low light, so they drive vision in dim conditions, but they signal only light intensity and give no color information. Many rods converge onto one ganglion cell pathway, which raises sensitivity but lowers acuity. Rods far outnumber cones and are concentrated in the peripheral retina; the fovea has essentially none. In the biology sense, a rod is depolarized in the dark and hyperpolarizes in light.

On the MCAT: Expect dark adaptation. After moving from bright light to darkness, rhodopsin regenerates and sensitivity rises over many minutes, with rods taking over from cones. Expect peripheral-versus-central questions too: a faint star is easier to see when you look slightly to the side of it. Loss of rod function predicts poor night vision and reduced peripheral vision, while loss of cone function or damage to the fovea predicts poor color vision and blurred central vision.

Common confusion: Students swap rods and cones. Rods mean dim light, periphery, black-and-white, high sensitivity and low acuity. Cones mean bright light, color, fine detail and the fovea. In bright light the rods are not the main contributors to vision because their pigment is bleached, so cones dominate.