Question

Briefly summarize the major steps in phototransduction in rods.

Short answer

Q: Explain the process of phototransduction in rod cells. A: Phototransduction in rod cells starts with the absorption of light by the photopigment rhodopsin in the outer segment of the rod cell. The light causes a change in the retinal molecule's configuration, which activates the opsin protein and the G-protein transducin. This triggers a cascade of enzymatic reactions involving the activation of phosphodiesterase (PDE) enzyme and the breakdown of cGMP to GMP. The decrease in intracellular cGMP levels leads to the closure of CNG ion channels, causing a reduction in Na+ and Ca2+ ion influx, and hyperpolarization of the rod cell. This hyperpolarization reduces neurotransmitter release at the synaptic region, ultimately leading to the transmission of visual signals to the brain via bipolar cells and the optic nerve.

Step-by-step solution

Understand the concept of phototransduction

Phototransduction is the process by which light energy is converted into electrical signals in the rod cells, cone cells, and photosensitive ganglion cells of the retina. In this exercise, we will focus on phototransduction in rod cells, which are responsible for vision in dim light conditions.

Anatomy of a rod cell

A rod cell consists of an outer segment, an inner segment, and a synaptic region. The outer segment contains a series of stacked membranes called discs that contain the photopigment rhodopsin. The inner segment houses the energy-producing organelles and other cellular components, while the synaptic region is responsible for transmitting electrical signals to bipolar cells and ultimately to the brain.

Activation of photoreceptor cells by light

When light photons enter the eye, they are absorbed by the photopigment rhodopsin in the outer segment of the rod cells. This absorption of light energy causes a conformational change in the protein structure, which triggers the next steps in the phototransduction pathway.

Role of rhodopsin

Rhodopsin consists of two components: opsin (a protein) and retinal (a light-sensitive molecule derived from vitamin A). When light is absorbed by retinal, it switches from an 11-cis configuration to an all-trans configuration. This change in retinal's configuration leads to the activation of opsin, which in turn activates a G-protein called transducin.

Activation of the cascade

The activated transducin triggers a cascade of enzymatic reactions. This process involves the activation of a phosphodiesterase (PDE) enzyme, which breaks down cyclic guanosine monophosphate (cGMP) into guanosine monophosphate (GMP). The decrease in intracellular cGMP concentration leads to the closure of cyclic nucleotide-gated (CNG) ion channels in the rod cell membrane.

Hyperpolarization and signal transmission

When CNG channels close, the influx of Na+ and Ca2+ ions into the rod cell decreases. This results in a hyperpolarization (a more negative membrane potential) of the rod cell. Hyperpolarization causes a reduction in the release of neurotransmitters at the synaptic region, which in turn leads to changes in the electrical activity of bipolar cells and ultimately the transmission of visual signals to the brain via the optic nerve.