W8 Visual ystem ( eye retine, phototransduction) Auditory system Flashcards
What does Vision allow us to do?
Detect pray/predator/mates/communicate
What is involved in analysing the visual word
1/3 of the human neocortex
What does light has?
a wavelength: distance between peaks or troughs.
a frequency: number of waves per second
an amplitude: difference between wave peak and trough
Refraction (light and the environment, optics)
Refraction occurs becasue the speed of light differs between mediums (slow through water than air)
The greater difference in speed in the two media the greater the angle of refaction.
Electromagnetic light
Travels in straight lines, known as rays, until it interats with atoms and molecules.
3 ways of interaction: Reflection, Absoption and refraction
Example of Absoprtion (electromagnetic light)
Photoreceptors and pigmented epithelium of retina
Example of Refraction (electromagnetic light)
Used by comea to form images on the retina
Eye position can vary with species
Monocular vs binocular vision: field of view/depth of perception/predator vs prey
Pupil and Retina (light)
appear black due to heavy pigment at back of the eye. Reflected light appears bright red hwen light is shined due to blod vessels on surface of the retina.
Pupil - Function
Lets light inside the eye
Iris - Function
contains muscles which ocntrol the amount of light entering the eye.C
Cornea - Function
glassy, transparent covering of the pupil and iris that refracts light.
Sclera - Function
Continuous with cornea, forms the tough, pretective wall of th eeyeball to give it its shape.
Extraocular muscles - Function
Move the eyeball, controled by oculomotor nerve
Optic nerve - Function
carries axons from retina to brain
Optic disk
origin of blood vessels and optic nerve, cannot sense light (blind spot)
Macula
Region of retina for central vision, devoid of large blood vessels to improve vision quality
Fovea
Retina is thinnes there and is the area of highest visual acuity
Zonal fibers
Suspensory ligaments which are attached to the ciliary muscle, enabling stretching of the lens
Lens
Spherical obejct, it is strenghts as it is attached to zonal fibers, they can be tighter or not depending on the ciliary muscle
Two diffferent solution - Eye
Viterous humor
Aqueous humor
What is Image formation
Light rays must be focused onto the retina (fovea)
Refraction occurs at the cornea (80%) and less (20%)
Degree of refraction is determined by
Difference in refractive indices betweween the two media. The angle at which light hits the interface between these two media.
Refractive index
essentially a measure of speed of light within it – so light moves quicker through air (1.0003) than the cornea (1.376) due to the increased density of the cornea.
Refraction by the cornea
occurs when light hits at an angle, light arrives at the cornea through air, but the cornea is mainly water.
Refraction occurs, light travels more slowly through water than air due to higher density.
Focal distance
Distance from refractive surface to vonvergence of parallel light rays.
Accommodation by the lens
Distant objects: almost parallel light rays, cornea provides sufficient refraction to focus them on the retina.
Closer Objects: light rays are not parallel, require additional refraction to focus them on the retina, rpovided by the fatterning of the lens.
Len - elasticity
Len has natural elasticity so if not stretched it will become more spherical.
Problem with focusing
eye is emmetropic when less is flat and we are focussing on a distant object
Laminar organisation of the retina
Layers organized in reference to center of the eye i.e. photoreceptors are in the outer layer. Outer layer of the retina (is closest to the inside of your head).
Light - neural activity
Light focused on the retina must now be converted into neural activity. Light mus tpass through ganglion cels and bipolar cells before it reaches the photoreeptors.
Light absorbtion through the retina
Light that asses throught the retina is absorbed by the pigmented epithalami.
Nuclear layers contain cell bodies, plexiform layers are regions where synaptic connections occur.
Retina - Ganglion cells
output from retina
Retina - Amarcrine cells
Modulate iformation transfer between GCs adn BCs
Retina - Bipolar cells
connect photoreceptors to ganglion cells.
Retina - Horizontal cells
Modulate information transfer between photoreceptors and BCs
Retina - Photoreceptors [Short definition]
Sensory transducers both rods and cones
Photoreceptors
Both have membraneous disk, inner segments: lots of mitochondria. Only graded potentials in photoreceptors.
Membranous disks contain light-sensitive photopigments that absorb light
Duplicity theory
Can’t have high sensitivity and high resolution in single receptor. Seperate systems for monocrome and color
Rod photoreceptors
Greater number of disks.
Higher photopigment concentration. 1000 times more sensitive to light than cones.
Enable vision in low light (scotopic) conditions i.e. at night-time. Low visual acuity/resolution
Cone photoreceptors
Fewer disks.
used during daylight (photopic) conditions. Enable color vision, high visual acuity/resolution.
Low sensitivity
Where are the cones and rods located
Mainly in the fovea
Central retina
low convergence, low sensitivity, high resolution
Peripheral retina
high convergence, high sensitivity, low resolution
Scotpopic condition (wavelenght/visual)
night time. Rode photopigments (rhodopsin) most sensitive
Photopic condition (wavelenght/visual)
Day-time. Cone photopigments:
3 cones: defined by what type of opsins they have Short (S): blue spectrum of light. Medium (M) cones, yellow, Long (L) cones.
High sensitivity to ligh tin middle and long cones => photopic ocndition is closer to 550m wavelenghts.
Retinal ganglion photopigment
Melanopsin, sensitive to blue light
Unusual photoreceptor cell property
Unusual cell property: photoreceptors are hyperpolarized by light. Photoreceptors are depolarized at rest.
Rods (phototransduction)
dark/night-time.
cGMP-gated non-selective cation channels are open in the dark allowing a Na+ influx known as the dark current to depolarizes photoreceptors.
To maintain the Na gradeint, Na/K pump are found in the inner segment that removes Na in exchange for K.
light’s affect on cGMP
Light dereases cGMP levels, closing the channels and preventing Na+ inflx, hypoerlarizing photoreceptors.
Phototransduction (molecular level - rods)
Rhodopsin is activated by light. Stimulates the G-protein, transducin, to become transducin GTP.
The α subunit activates the enzyme phosphodiesterase (PDE). PDE reduces cGMP levels, closing Na+ channels
Signal amplification occurs as this is an enzyme cascade
Saturation of resposne in bright light (differences in rods adn cones)
Rods cannot process bright light, they become saturated.Rhodopsin is bleached. cGMP levels are so low that noa dditional hyperpolarisation can occur.
Cones are not saturated as easily, so they are used in bright light.
Ligth adaptation
Ohotoreceptors initially hyperpolarise greatly. Photoreceptors gradually depolarise with continued bright light.
Does light adaptation require calcium?
In the dark: Ca2+ normally enters cells and blocks guanylyl cyclase. This reduces cGMP production, so closes some ion channels.
The CGMP keep broken down in the light, so you are getting less calcium.
In the light: Channels are shut so Ca2+ cannot enter cells. Block on guanylyl cyclase is released. More cGMP produced = more channels open.
Why does calicuim mediates light adaptation?
to continued light stimulation
Classification of bipolar cells
Based on bipolar response to glutamat.
Hyperpolarise: Off bipolar cells.
Depolarises: On biplar cells.
Photoreceptors hyperpolarise to whaat?
Light, reduce glutamate release
Receptive field - Retinal Ganglion cells
Will only fire action potentials when specific areas of retina are illuminated
Biplar cells are organised how?
They have a center surround organization.
Take the information from multipole photoreceptors but not only they receive information from center organization.
What is sound?
Particles of air moving back and forwards, hitting eachother.
Rarefied air
low density
Compressed air
High density
What are the properties of sound?
Intenisty: Air pressure increases when there are more particles around and decreases when less air.
Air is compressed when pressure increase.
Frequency: Number of compressed or rarefied patches of air that pass by our ear each second, expressed as Hz.
Phsyiological ranges of requency and intensity for humans
The Louder the sound is the air pressure difference between the compression and rarefied air is bigger
Human hairing range
20Hz to 20,000 Hz
Outer ear -
Helps know where the sound comes from. (behind/infront or below/above)
Ossicles
Air filled with 3 ossicles (little bones)
Malleus, incus, stapes
How does the middle ear transfer sound?
The oval would barely move if it was moved directly by sound due to the air fluid interface, as fluid has a greater inertia (impedance).
Impedance matching – air and water have different impedances: tendency of each medium to oppose movement brought about by a pressure wave.
Ossicles amplify sounds
Exerts ~20 times more pressure on the oval window than on the tympanic membrane. Overcomes the greater impedance of cochlear fluid.
What are hair bundles
are made of stereocilia and are located at the top (apical) part of the hair cells. Stereocilia connected by tip links.
Fluid moves in the air, the stereocilia move around.
Lechanoelectrical transducer channel, opens and depolareised hair cells: positive going in.
Stapes moves outwards
Basilar membrane moves upwards, hair cells depolarise
Stapes move inwards
Basilar membrane moves downards, hair cells hyperpolarise
IHCs
Primary sensory receptors
