retinarods

Retinal rods are one of the two main classes of photoreceptor cells in the retina and are essential for vision in low-light conditions. They are highly sensitive to light and enable scotopic vision, offering high sensitivity at the cost of reduced spatial resolution and no color discrimination. In humans, rods outnumber cones by a wide margin and are predominantly located in the peripheral retina, whereas the central retina (the fovea) is densely packed with cones.

Anatomy and structure: Each rod has a rod outer segment filled with stacked membranous discs that contain the photopigment rhodopsin. The outer segment is connected to a slender inner segment housing cellular organelles and the nucleus, and a synaptic terminal forms connections with rod bipolar cells.

Physiology and phototransduction: In darkness, cGMP keeps a cGMP-gated Na+/Ca2+ channel open, depolarizing the rod and supporting continuous glutamate release. When light is absorbed, rhodopsin undergoes conformational change (11-cis-retinal to all-trans-retinal), activating the G protein transducin and phosphodiesterase. This reduces intracellular cGMP levels, causing the channels to close, hyperpolarizing the cell, and decreasing glutamate release. Rods feed into the rod pathway via rod bipolar cells and AII amacrine cells, contributing to vision under low luminance and to broader retinal processing through subsequent circuits.

Distribution and function: Rods are densest in the peripheral retina and are largely absent from the central fovea. Their high sensitivity allows vision in very dim light, but their signals provide limited acuity and no color information, complementing cone-mediated daylight vision.

Clinical relevance: Rod dysfunction can cause night blindness and contribute to inherited retinal diseases such as retinitis pigmentosa and cone-rod dystrophies. Rod health is also impacted by nutritional factors and genetic mutations affecting rhodopsin and related phototransduction components.