Module 2: Visual Anatomy, Photoreceptors & Neural Signalling

Primary role of Iris & Pupil

Iris: Muscle that contracts/expands to control light.

Pupil: Opening through which light enters.

Rods vs. Cones (Count, Location, Function)

Rods: ~120 million, periphery, high sensitivity, scotopic (dim light) vision.

Cones: ~6 million, concentrated in fovea, high acuity, photopic (bright light/color) vision.

Dark Adaptation Curve behavior

Cones adapt first (faster, lower threshold capacity). Rods take longer (~20-30 mins) but achieve far greater overall sensitivity.

Purkinje Shift

Shift in peak spectral sensitivity toward shorter (bluer) wavelengths in low-light conditions when shifting from cone-driven to rod-driven vision.

Strategy for threading a needle vs. looking at a faint star

Needle: Focus directly on fovea (uses cones for high acuity).

Star: Look slightly off-center (uses peripheral rods for high sensitivity).

Cause of the Visual Blind Spot

Absence of photoreceptors where optic nerve axons exit the eye; brain fills in missing visual details via context.

Neuron Potentials (Resting vs. Action)

Resting: -70mV

Action Potential: +40mV (follows the All or None Law)

Spatial Convergence: Rods vs. Cones to Ganglion Cells

~120 rods converge onto 1 ganglion cell (high convergence = high sensitivity, low acuity).

~6 cones converge onto 1 ganglion cell (low convergence = low sensitivity, high acuity).