Vision and Light

Vision

• The retina contains photoreceptors that detect different wavelengths of light

Light: Frequency & Wavelengths

• Electromagnetic radiation is emitted by light waves
• Wavelength is how electromagnetic radiation is measured
• Longer wavelengths mean a lower frequency and less energy
• The visible light spectrum is 400-750nm

Basic Eye Review

• Light first hits the cornea
• The cornea refracts the light onto the lens
• The lens refracts the light onto the retina

Refraction

• The lens bends and refracts light onto the retina
• Images are inverted on the retina
• Ciliary muscles change the shape of the lens to focus the image on the retina

Focal Point

• The focal point is where waves of light meet/converge following refraction via the lens
• The focal point should occur on the retina
• Ciliary muscles adjust the shape of the lens to bring an image into focus on the retina
• Contracting the ciliary muscles, creating a rounder lens, focuses nearby light
• Relaxing the ciliary muscles, creating a tapered lens, focuses light that is further away

Corrective Lenses

• In some individuals, the focal point may occur before or after the retina
• This results in an unclear image
• An unclear image occurs via the misshaping of the cornea or the lens
• Corrective lenses adjust how light is refracted onto the retina, making the focal point occur at the retina

Retina

• Photoreceptors of the retina are found posterior to many layers of nervous cells
• Light must pass through the cell layers to get to photoreceptors
• There are 2 classes of photoreceptors:
  • Rods detect the presence/absence of light
  • Cones are responsible for color vision

Photoreceptors

• Each photoreceptor detects a specific range of wavelengths and contains a "peak" wavelength
• Rods detect the presence/absence of light with a peak wavelength of 500nm
• Cones include:
  • Red cones with a peak of 558nm
  • Green cones with a peak of 531nm
  • Blue cones with a peak of 420nm

Color Blindness

• Color blindness occurs when one or more cone cell types does not function properly or is absent
• An example is Deuteranomaly
• Red-green colorblindness is typically found in males
• Red-green colorblindness is caused by weak green cones

Neural Pathways of Vision

• Photoreceptors can only produce graded potentials
• The neurotransmitter released is glutamate
• Bipolar or ganglion cells can either excite or inhibit signals
• Horizontal cells and amacrine cells carry information between adjacent areas of the retina
• Axons of the ganglion cells form the optic nerves
• Optic chiasm is where some of the axons cross and provide information to the opposing side of the brain
• The optic chiasm allows for each hemisphere to receive information from each eye

Monocular vs. Binocular Vision

• Monocular vision is the outer region of our visual field that can only be perceived by one eye
• Binocular vision is the central region of our visual field that can be observed by both eyes
• Binocular vision is important for depth perception

Hearing

• Hearing is also known as audition
• Hearing is the detection of vibrations/disturbances of air molecules
• Pitch is the frequency of sound waves
• Amplitude is the volume/loudness of the sound

Hearing Process

• Sound waves hit the tympanic membrane
• The tympanic membrane vibrates
• Ear ossicles oscillate with the tympanic membrane
• The stapes vibrates against the oval window
• Waves form in the perilymph of the cochlea
• The cochlear duct detects the vibrations
• The round window dampens waves

Cochlear Duct

• Vibrations produced within the scala vestibuli travel through the cochlear duct and into the scala tympani
• The hair cells within the cochlear duct are responsible for detecting vibrations and sending information back to the CNS
• The more "proximal" portion of the cochlear duct detects higher frequencies
• The "distal" portion detects lower frequencies

Hair Cells

• “Hairs" are called stereocilia
• Stereocilia contain mechanically gated cation channels
• Stereocilia bend
• Bending of stereocilia causes the cell to either repolarize or depolarize
• Depolarization occurs when the stereocilia move in the direction of the "longest hair"
• In depolarization, neurotransmitter is released
• Repolarization occurs when the stereocilia move in the opposing direction of the "longest hair"

Detection of the Rotation of the Head

• The inner ear contains 3 semicircular canals that are responsible for detecting movement of the head in their corresponding axis

Semicircular Canals

• Each semicircular canal is filled with fluid and contains receptors with stereocilia
• Fluid moves in opposing direction of head movements
• Stereocilia are encapsulated in a gelatinous mass called the cupula
• Fluids "collide" with the cupula following head movements, resulting in the stereocilia bending
• Receptor cells continuously (and steadily) fire action potentials
• Depolarization occurs following bending to the "preferred" side, increasing the frequency of action potentials produced
• Hyperpolarization occurs following bending to the opposing side, decreasing the frequency of action potentials

Utricle and Saccule

• The utricle detects linear acceleration
• The saccule detects gravity
• Both the utricle and saccule contain hair cells (receptors that contain stereocilia) covered in a gelatinous like fluid
• Otoliths are calcium carbonate crystals found embedded within the gelatinous substance

Gustation

• There are 5 different classes of chemoreceptors:
  • Bitter
  • Sweet
  • Salty
  • Sour
  • Umami
• Taste receptors are organized into taste buds
• Molecules must enter the taste bud via the taste pore and must bind to their corresponding receptor

Olfaction

• The olfactory epithelium contains many olfactory receptors that detect specific odorants
• Odorants are molecules of an odorous substance that diffuse into the air and enter the olfactory epithelium

Olfactory Receptor Cells

• Olfactory receptor cells contain nonmotile cilia that project into the mucus that lines the olfactory epithelium
• Cilia contain the receptor proteins that the odorants bind to
• Olfactory receptor cells only have a lifespan of ~ 2 months