Vision Lecture Flashcards

Vision

Sensory Receptors

The human body uses various sensory receptors to interact with the environment:

  • Eye: Vision

  • Ear: Hearing and balance

  • Nose: Smell

  • Tongue: Taste

  • Skin: Touch, etc.

Sensory Transduction

Sensory transduction is the process by which sensory neurons convert external stimuli into neuronal activity.

  • PNS (Peripheral Nervous System): Contains sensory receptors like hair cells, taste receptors, cones, Pacinian corpuscles, olfactory receptors, and free nerve endings.

  • CNS (Central Nervous System): Receives signals from the PNS.

  • Examples of transduction:

    • Sound to neuronal activity via hair cells.

    • Taste to neuronal activity via taste receptors.

    • Vision to neuronal activity via cones.

    • Touch to neuronal activity via Pacinian corpuscles.

    • Smell to neuronal activity via olfactory receptors.

    • Pain to neuronal activity via free nerve endings.

Inner Hair Cells

Inner hair cells transduce vibrations into neural signals.

Outer Hair Cells

Outer hair cells amplify or attenuate sounds. They convert sound to movement rather than directly to neural signals.

  • Components of the inner ear:

    • Cells of Hensen

    • Cells of Claudius

    • Tectorial membrane

    • Outer hair cell

    • Pillar cells

    • Tunnel of Corti

    • Cells of Deiters

    • Nerve fibers

    • Basilar membrane

    • Hair bundle

    • Reticular lamina

    • Inner hair cell

    • Cells of Boettcher

Transducing Temperature and Touch

David Julius and Ardem Patapoutian are notable figures in the study of temperature and touch transduction.

Transducing Temperature

Different receptors are activated at different temperatures:

  • TRPV1: Activated by heat pain (>43°C>43°C) and capsaicin (spicy pepper extract).

  • TRPM3

  • TRPA1

  • TRPM2

  • TRPM8: Activated by menthol.

These receptors are sensory neurons that open ion channels in response to temperature changes.

Nobel Prize 2021

The 2021 Nobel Prize was awarded for the discovery of the mechanisms of sensing heat and touch. Gunther Hollopeter contributed to the study of menthol's effects (giving cells menthol).

Gene Silencing and Mechanical Force

Gene silencing is used to identify genes responsible for mechanical force transduction. Candidate genes are tested to measure their response to mechanical force. PIEZO1 and PIEZO2 are examples of such genes.

  • PIEZO1

  • PIEZO2: Involved in touch and proprioception. They are ion channels that open in response to mechanical force.

Electromagnetic Spectrum and Vision

We only see a tiny sliver of the electromagnetic spectrum.

  • Radio Waves: 10610^6 Hz, 10210^2 meters (length of a football field). Includes AM radio.

  • Microwaves: 10910^9 Hz, 10−110^{-1} meters to 10−210^{-2} meters (width of a baseball). Includes FM Radio, cell phones, microwave ovens and Wi-Fi.

  • Infrared: 101210^{12} Hz, 10−310^{-3} meters to 10−610^{-6} meters (thickness of paper). Human bodies radiate heat, remotes.

  • Visible Light: 101510^{15} Hz. The optical window.

  • Ultraviolet: 101810^{18} Hz, 10−710^{-7} meters to 10−810^{-8} meters. Sunblock.

  • X-rays: 102110^{21} Hz, 10−910^{-9} meters to 10−1010^{-10} meters (width of a water molecule). Medical X-rays.

  • Gamma Waves: 10−1110^{-11} meters to 10−1210^{-12} meters (size of atomic nuclei). Nuclear Power.

The atmosphere is opaque to many wavelengths except for radio waves (Radio Window) and visible light (Optical Window).

Light and Objects

Light with some combination of wavelengths is emitted by a source. Objects absorb some wavelengths and reflect others. We see the wavelengths in the visible spectrum. What we see depends on light sources and absorptive/reflective objects.

The Eye

The eye consists of several key components:

  • Cornea: The transparent outer layer.

  • Iris: Controls the size of the pupil.

  • Pupil: The opening through which light enters.

  • Lens: Focuses light onto the retina.

  • Ciliary muscle: Controls the shape of the lens.

  • Sclera: The white part of the eye.

  • Retina: Contains photoreceptor cells.

  • Fovea: The central focal point in the retina.

  • Optic nerve: Transmits signals to the brain.

  • Blind spot: Where the optic nerve leaves the eye.

  • Ligament

  • Eye muscle

Cell Types in the Retina

The retina contains several layers and cell types:

  • Photoreceptors: Rods and cones.

  • Outer nuclear layer: Contains the cell bodies of rods and cones.

  • Outer plexiform layer: Where photoreceptors synapse with bipolar and horizontal cells.

  • Bipolar cells: Transmit signals from photoreceptors to ganglion cells.

  • Horizontal cells: Mediate lateral interactions in the outer plexiform layer.

  • Inner nuclear layer: Contains the cell bodies of bipolar, horizontal, and amacrine cells.

  • Amacrine cells: Mediate lateral interactions in the inner plexiform layer.

  • Inner plexiform layer: Where bipolar cells synapse with ganglion and amacrine cells.

  • Ganglion cells: Transmit signals to the brain via the optic nerve.

  • Ganglion cell layer: Contains the cell bodies of ganglion cells.

Photoreceptors: Rods and Cones

  • Rods:

    • Sensitive to low light levels.

    • Mediate vision in the dark.

  • Cones:

    • Active at high light levels.

    • Mediate color vision.

The Photoreceptors

  • Opsins are proteins that transduce photons

    • work similarly to metatropic receptors

  • Retinal changes shape when it absorbs a photon

  • Light transduction hyperpolarizes the photoreceptor

  • Photoreceptor Density determines visual acuity

The fovea

  • cone density is highest in the fovea

  • visual acuity is only sharp here

  • Eye muscles scan the fovea across the visual scene

  • Eye movements depend on what information youre after

Receptive field - the location in the visual scene in which a stimulus can influence the activity of a given cell

  • There are no photoreceptors in the blind spot

Electrophysiological Recording

  • metal electrode is inserted into the brain

  • when it is close to a neuron it can record that neurons action potentials

  • Stronger stimulus = more action potentials

Center-Surround Receptive Fields

neural integration creates a “center-surround” receptive field that emphasize edges

  • On-center - fires action potentials at the highest rate if light is presented tp its center, reduces its firing rate if light is presented to the surround, firing rate is unaffected if light is presented center & surround at the same time

  • Off-center - fires APs at the highest rate if light is presented to its surround, reduces its firing rate if light is presented to the center, firing rate is unaffected if light is presented center & surround at the same time

  • Retinal ganglion cells respond to edges

Color Vision

Trichromatic theory - Hermann von Helmholtz

  • color matching experiments - can match all of the colors in the visible spectrum by mixing of three primary colors

  • proposed that there were three types of receptor

    • Electromagnetic Spectrum

    • wavelength = hue/color

    • amplitude = brightness

  • light reflected from an object depends on the light source

  • Color Constancy - for a given object, different illuminants present a different color spectrum to the eyes, the brain corrects this

  • Retinal circuits contributes to color constancy

    • cones

    • bipolars

    • B/Y ganglion

Color Afterimages

  • Off response caused by sudden release of inhibition from neighboring cells

  • Y+B cell responds to removal of Blue inhibition by firing rapidly for a short period