Vision pt.1

Sensation vs. Perception

• Sensation is how the nervous system detects stimuli and converts them into signals.
• Perception is the conscious experience and interpretation of sensory information.

Sensory Neurons

• Specialized cells that detect specific physical events.
• Sensory neurons can detect:
  • Molecules (smell, taste, nausea, pain)
  • Physical pressure (touch, stretch, vibration, acceleration, gravity, balance, hearing, thirst, pain)
  • Temperature (heat, cold, pain)
  • pH (sour taste, suffocation, pain)
  • Electromagnetic radiation (light)
• Some animals have additional senses like detecting electric and magnetic fields, humidity, and water pressure.

Sensory Transduction

• Sensory neurons have specialized receptors that convert sensory stimuli into changes in membrane potential.
• Sensory neurons can take many shapes and sizes.
• Many sensory neurons do not have axons or action potentials.
• These neurons release neurotransmitter in a graded fashion based on their membrane potential.

Photoreceptors

• Photoreceptor cells are responsible for vision.
• These cells convert light energy into changes in membrane potential, affecting neurotransmitter release.
• They lack action potentials.

Opsins

• Light-sensitive proteins.
• Opsins in photoreceptor cells are metabotropic receptors.
• They bind a molecule of retinal which changes shape in response to light.
• This shape change activates the metabotropic receptor.

Retinal

• A small molecule synthesized from vitamin A.
• It attaches to opsin proteins.
• Retinal absorbs electromagnetic energy of light.
• The two configurations of retinal:
  • cis-retinal (inactive)
  • trans-retinal (active)

Neural Transduction of Light

• light + retinal molecule + opsin protein = activation
• Activation launches a g protein signaling cascade, changing the photoreceptor cell's membrane potential and affecting neurotransmitter release.

Types of Photoreceptor Cells

• 4 types contribute to vision:
  • Red cone cells (red cone opsin)
  • Green cone cells (green cone opsin)
  • Blue cone cells (blue cone opsin)
  • Rod cells (rhodopsin opsin)
• Each opsin is sensitive to different wavelengths of light.
• Rod cells, the last to evolve, are 100 times more sensitive to light than cone cells.

Visible Light

• Electromagnetic radiation with wavelengths between 380-760 nm.
• Detected by 4 photoreceptor cell types: 1 rod cell and 3 cone cells.

Cone Photoreceptors/Trichromatic Coding

• Blue cone opsins are most sensitive to short wavelengths.
• Green cone opsins are most sensitive to medium wavelengths.
• Red cone opsins are most sensitive to long wavelengths.
• These opsins allow for color vision.

Color Vision Deficiency

• Protanopia: Absence of the red cone opsin (1% of males), difficulty distinguishing colors in the green-yellow-red spectrum, visual acuity is normal.
• Deuteranopia: Absence of the green cone opsin (1% of males), difficulty distinguishing colors in the green-yellow-red spectrum, visual acuity is normal.
• Tritanopia: Absence of the blue cone opsin (1% of the population), blue cone cells don't compensate, visual acuity is not affected.

Achromatopsia

• True color blindness.
• Caused by mutations in the g protein signaling cascade used by all cone opsins.

Anatomy of the Eye

• Conjunctiva: Mucous membrane lining the eyelid.
• Cornea: Outer, front layer of the eye, focuses incoming light.
• Sclera: Opaque, does not permit light entry.
• Iris: Ring of muscle, controls pupil size and light entering the eye.
• Lens: Transparent layers, changes shape for focusing near and far objects (accommodation).
• Retina: Inner lining of the eye containing photoreceptor cells.
• Vitreous humor: Clear, gelatinous fluid behind the lens.
• Fovea: Central region of the retina, primarily contains cone cells, sharpest visual acuity (20/20 vision).
• Periphery: Outer area of the retina, contains only rod cells.
• Optic disk: Where blood vessels enter/exit the eye and the optic nerve exits, contains no photoreceptors, creating a blind spot.

Eye Movements

• Orbits: Bony sockets in the skull that hold the eyes.
• Saccadic eye movements: Rapid, jerky shifts in gaze.
• Pursuit movements: Smooth, slow eye movements when focusing on a moving object.
• Extraocular muscles: Muscles attached to the sclera, rotate the eye and hold it in place.

Organization of the Retina

• Visual information travels: Photoreceptor cells -> Bipolar cells -> Retinal ganglion cells -> Brain.
• Light passes through all retinal layers to reach the photoreceptor cells.

Retina Fovea vs. Periphery

• Fovea: Equal number of photoreceptor cells, bipolar cells, and retinal ganglion cells, no information compression, high visual acuity.
• Periphery: Information compression, lower visual acuity.