Vision: Phototransduction and Adaptation

Study Notes

Phototransduction process: Cis-retinal transforms into trans-retinal upon absorbing light, initiating a series of chemical changes.
Bleaching: Trans-retinal detaches from opsin within a minute, leading to a decrease in photoreceptor sensitivity.
Enzymatic conversion: Retinal isomerase facilitates the conversion of trans-retinal back to cis-retinal.
Regeneration: Cis-retinal rebinds to opsin, regenerating photopigments crucial for vision.
Light adaptation: The visual system reduces sensitivity rapidly when exposed to bright environments.
Dark adaptation: Sensitivity gradually increases over several minutes in low light conditions, allowing detection of faint stimuli.
Photopigment dynamics: In bright light, rod photopigments bleach faster than they can regenerate; conversely, cone pigments regenerate quickly.
Threshold detection: Full regeneration of cone photopigments occurs in the first 8 minutes of darkness, allowing perception of color; rods gain sensitivity to single photons in low light.

Accessory structures: Include eyelids, eyelashes, eyebrows, lacrimal apparatus, and extrinsic eye muscles (superior/inferior rectus, lateral/medial rectus, superior/inferior oblique).
Components of the eye:
- Fibrous tunic: Comprises the cornea (anterior) and sclera (posterior), with the scleral venous sinus facilitating aqueous humor drainage.
- Vascular tunic: Includes the choroid, ciliary body (supports the lens), iris, and pupil.
- Retina: Contains the optic disc, pigmented layer, and a neural layer with specialized cells for light transduction.

Photoreceptor layer: Contains rods (120 million, responsible for night vision without color) and cones (6 million, enable color vision: blue, red, green).
Neural structure: Comprises layers including outer synaptic, bipolar, inner synaptic, and ganglion cell layers.
Macula lutea and fovea centralis: The macula is the retina's center, providing high visual acuity.

Phototransduction: Rhodopsin in rods regenerates over 30-40 minutes; cone photopigments regenerate much faster at 90 seconds.
Isomerization: Dark conditions see retinal in a bent shape (cis-retinal) that fits opsin; light exposure straightens this structure.

Anatomy of the ear: Essential for hearing and balance; inner ear plays a critical role in equilibrium.
Physiology of hearing: Sound waves converted to nerve impulses via specialized structures in the ear.
Balance perception: Inner ear structures detect positional changes leading to balance adjustments.

Olfactory transduction: Involves odorants binding to receptors, activating G proteins and adenylate cyclase, resulting in depolarization and nerve impulse generation.
Sensitivity and adaptation: Rapid adaptation occurs within seconds of odor exposure, complete insensitivity develops within one minute.

Gustatory transduction: Taste stimuli (tastants) interact with salivary solutions to engage gustatory microvilli, initiating receptor potential and neurotransmitter release.
Thresholds of taste: Bitter substances have the lowest detection threshold due to toxicity; sour tastes are relatively higher, while sweet and salty thresholds are similar but higher compared to bitter and sour.