Notes on Vision

Vision

Sensory Systems

Our senses include:

• Eye (vision)

• Ear (hearing, balance)

• Nose (smell)

• Tongue (taste)

• Skin (touch, temperature, pain, pressure, proprioception)

Each sensory system contains specialized receptors to detect and transduce specific stimuli from the environment into electrical signals that the nervous system can process. For example:

• Mechanoreceptors detect mechanical stimuli such as touch, pressure, and vibration.

• Thermoreceptors detect temperature changes.

• Nociceptors detect pain.

• Photoreceptors detect light.

• Chemoreceptors detect chemical stimuli such as odorants and tastants.

Sensory Transduction

Sensory transduction involves the transformation of the outside world into neuronal activity. Here's how different stimuli are transduced:

• Sound: Hair cells in the inner ear transduce sound vibrations.

• Taste: Taste receptors transduce chemical stimuli.

• Vision: Cones and rods in the retina transduce light.

• Touch: Pacinian corpuscles, Merkel's disks, and other mechanoreceptors transduce pressure and touch.

• Smell: Olfactory receptors in the olfactory epithelium transduce chemical stimuli.

• Pain: Free nerve endings transduce pain signals.

Inner and Outer Hair Cells

• Inner hair cells transduce vibrations into neural signals.

• Outer hair cells amplify or attenuate sounds by converting sound to movement; they enhance the sensitivity and frequency selectivity of the inner ear.

Transducing Temperature and Touch

Temperature

Temperature transduction involves various receptors:

• TRPV1: Activated by heat pain and temperatures above 43°C. It's also sensitive to capsaicin, the active component in spicy peppers.

• TRPM3: Another heat-sensitive receptor.

• TRPA1: Activated by noxious cold and certain irritants.

• TRPM2: Involved in temperature sensing.

• TRPM8: Activated by cold temperatures and menthol.

These receptors are transient receptor potential (TRP) channels. They are ion channels that open in response to specific temperature ranges, allowing ions to flow across the cell membrane and generate electrical signals. For example:

• TRPV1 is a nonselective cation channel that allows calcium and sodium ions to enter the cell.

• TRPM8 is also a nonselective cation channel but is more permeable to calcium ions.

The Nobel Prize in 2021 was awarded for the discovery of receptors for temperature and touch.

Touch

• PIEZO1 and PIEZO2 are mechanically-activated ion channels involved in touch and proprioception.

These channels are essential for:

• Touch perception: Allowing us to feel textures and shapes.

• Proprioception: Providing information about body position and movement.

• Blood pressure regulation: PIEZO1 is involved in sensing mechanical forces in blood vessels.

Electromagnetic Spectrum

We only see a tiny sliver of the electromagnetic spectrum.

• The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma waves.

• Different wavelengths correspond to different types of electromagnetic radiation.

Visible light ranges from approximately 400 nm (violet) to 700 nm (red).

Important relationships:

• Wavelength and frequency are inversely proportional. Higher frequency means shorter wavelength, and vice versa.

• c = \lambda f where c is the speed of light (3.0 \times 10^8 m/s), \\lambda is the wavelength, and f is the frequency.

• Energy of a photon: E = hf where E is energy, h is Planck's constant (6.626 \times 10^{-34} J·s), and f is the frequency.

The atmosphere is opaque to many wavelengths, but there are