HLTH1004 W7 L7.3.3

Photoreception Process
- Photoreception begins with the absorption of photons, transitioning energy from light to action potentials.
- This process is a mechanism of transduction.
Visual Pigments
- Visual pigments are derivatives of rhodopsin (opsin protein + retinal pigment).
- Retinal is synthesized from Vitamin A.
- There are three types of photoreceptor cells (cones): blue, green, and red, each with different opsins responsive to specific wavelengths:
- Blue cones: ~450 nm
- Green cones: ~550 nm
- Red cones: ~620-630 nm
- Loss of any cone type can result in color blindness, which is more prevalent in males.

Activation of Photoreceptors
- When a photon hits the retinal molecule, it changes from the cis format to the trans format, triggering opsin activation.
- Activated opsin activates transducin (a G protein), which then activates phosphodiesterase (PDE).
- PDE decreases cyclic GMP levels, affecting sodium ion channels—leading to a reduction in neurotransmitter release (dark current).
Neurotransmitter Release
- In darkness, neurotransmitter release occurs continuously.
- In light, reduced neurotransmitter signals the activation of bipolar cells, which communicate visual information further.
Cyclic GMP and Ion Channels
- As cyclic GMP levels drop, sodium ion channels close, hyperpolarizing the membrane (potential drops to -70 mV), combining to decrease neurotransmitter release.
- This reduced release activates adjacent bipolar cells, transmitting visual signals.
Bleaching Process
- After photon absorption, retinal does not revert immediately but undergoes a process called bleaching, breaking down into retinol and opsin.
- Bleaching requires ATP to reassemble from trans-retinal back to cis-retinal and to combine with opsin again for future activation.

Output from Retinal Ganglion Cells
- The retinal ganglion cells are the output nodes carrying visual information through the optic nerve.
- Visual field mapping occurs, allowing integration from both eyes for depth perception (binocular vision).
- Visual information must cross at least two synapses:
- From photoreceptor to bipolar cell
- From bipolar cell to retinal ganglion cell.
Retinal Mapping
- Each retinal ganglion cell has a receptive field, which responds to specific photons reflecting stimuli in the visual field.

Optic Nerve to Visual Cortex
- Axons from retinal ganglion cells converge at the optic disc (blind spot) and exit as the optic nerve (Cranial Nerve II).
- Optic nerves partially cross at the optic chiasm, sending signals to the visual cortex in the occipital lobe for processing.
Visual Field Integration
- The brain integrates images from both eyes, enhancing depth perception through comparative analysis of stimuli to each eye.
- This occurs through pathways crossing at the optic chiasm and pathways projecting to the lateral geniculate nucleus (LGN) where synapses occur before continuing to the visual cortex.

Lateral Geniculate Body (LGN)
- Acts as a relay and processing station before information goes to the visual cortex,
- Visual information is sorted for shapes, colors, and motion.
Superior Colliculus
- Receives direct input about movement from rods, aiding in motion detection and orientating spatial awareness.
- Integrates visual and auditory information, assisting in tracking movement and coordination of extraocular muscles for eye movement.

Binocular Vision
- The visual field created by two eyes allows for better three-dimensional understanding of surroundings.
- Depth perception comes from slight timing differences in light perception between eyes.
Circadian Rhythm
- Photons also regulate other physiological functions like circadian rhythms, influencing behavior depending on light exposure, which can affect mental health, metabolism, and blood pressure.