PHSL231 - special senses module

What is different about the special senses?

specific organ exists for their detection

What is signal transduction?

stimulus turned into a change in membrane potential, action potentials

What part of the electromagnetic spectrum is visible light?

wavelengths of around 400-750 nm

What is the function of the visual system?

detect visible radiation/information

Why can we not detect other frequencies of radiation on the electromagnetic spectrum?

don't have receptors that respond to other frequencies eg microwaves, x-rays

What is infrared radiation?

radiation given off as heat, invisible to the human eye

What are the features of the wavelengths of red light?

longer wavelengths (750nm)

Why can we see light?

have receptors for light

What is the function of the eye?

organ that detects light and processes visual information to create visual perceptions/guide behaviour

What are the two components of the eye?

optical and neural component

What is the optical component of the eye?

collects and focuses light on the plane of the retina

What is the neural component of the eye?

retina, located in the brain, converts light energy into action potentials

Where does signal transduction occur in the eye?

in the retina

What are the major structures of the eye?

cornea, ciliary body, iris, lens, anterior chamber, posterior chamber, aqueous humor, sclera, zonule fibres, choroid, retina, vitreous humour, fovea, optic nerve, optic disc, nasal retina, temporal retina

What is the pupil?

hole in iris, where light enters the eye

What is the sclera?

fibrous capsule surrounding eye

What is the optical part of the eye?

cornea and lens

What are the features of the optical part of the eye?

needs to be transparent, avascular, other cells with no pigment, gets nutrients from diffusion, living tissue

What is the anterior chamber filled with?

aqueous humor

What is the iris?

pigmented structure in front of the lens

What is the lens?

3D structure, avascular, focuses light on the retina

What are the zonule fibres?

attaches lens to a muscle around perimeter of lens

What is the retina?

neural component of the eye, where signal transduction occurs, richly innervated with blood

What is the pigment epithelium?

back of the eye, black so that light is not reflected

What is the optic nerve?

bundle of axons carrying visual information from the eye

What is the optic disc?

blindspot in our eye, no light sensitive cells

What is vitreous humor?

transparent, protein solution like collagen, in the eye behind the lens

What is the fovea?

highest visual acuity, two point discrimination, small receptive fields in fovea

What is refraction?

bending of light, results in change of direction of light and is related to the difference between the refractive indices and curvature of surface

What does a convex lens do?

converges light behind the lens onto the retina

When does refraction occur?

when light travels from one medium into another with a different refractive index, eg air to water

How does refraction occur in the eye?

light moves through the eye from air to vitreous humor, cornea converges light rays so they are in focus on the retina

What is auto-focus?

lens changes shape to focus the image

What is the refractive index of air?

1

What is the refractive index of the cornea?

1.376

What is the refractive index of aqueous humor?

1.336

What is the refractive index of the lens?

1.386

What is the refractive index of vitreous humor?

1.336

What is refractive power?

ability of lens to bend light, measured in diopters, reciprocal of focal length in meters eg a lens that converges light to a focal point 1 meter beyond the lens has a power of 1 diopter

How far behind the lens would a 2 diopter lens converge light?

0.5m

How far behind the lens would a 10 diopter lens converge light?

0.1m

What is the refractive power of the relaxed eye?

60 diopters, focal point of around 1.7cm

What is the near response?

ability to focus on something from far away to close

What happens in the near response?

accommodation, constriction of pupil, convergence of eyes

What happens to the ciliary muscle when looking at a close object?

muscle contracts, lens changes shape, pupil gets smaller, increased refractive power

What happens to the ciliary muscle when looking at a far object?

muscle relaxes, pupil gets bigger, decreased refractive power

What is accommodation?

contraction/relaxation of ciliary muscle to alter lens shape and change refractive power

What happens in constriction of the pupil?

improved depth of focus, less light enters the eye

What happens in convergence of the eyes?

objects remain in register on corresponding parts of the two retina, go cross-eyed

What happens in accommodation when looking at far objects?

ciliary muscle relaxed due to low parasympathetic activity, zonule fibres are taut and lens is flattened, not a great refractive power

What happens in accommodation when looking at close objects?

parasympathetic activation of ciliary muscle increases, ciliary muscle contracts, tension removed from zonular fibres, lens becomes more spherical due to natural elasticity, refractive power increases

What is emmetropia?

normal vision

What is myopia?

short sightedness, eyeball that is elongated retina pushed further back, image out of focus/focused in front of the retina, fix this with a concave lens brings image into focus on retina

What is hyperopia?

far sightedness, focal point falls behind the retina, need extra refractive power, convex lens brings focal point of mage further forward

Why is the incidence of myopia increasing?

normal growth of eye requires sunlight, people spending more time inside on screens

What is astigmatism?

irregular curvature of cornea or lens, light rays from different planes brought into focus at different points, ground does not look flat, walls look like they're sloping, corrected with cylindrical lens

What is presbyopia?

age related loss of accommodation, lens loses elasticity, accommodation falls from 15 diopters in children to 2 diopters around 45-50 years and around 0 at 70 years, near point is further away, reading glasses (convex lens), needed to bring near point closer/restore near vision

What are cataracts?

clouding of the lens/lens becomes opaque, especially with age, lens is living tissue but doesn't have blood supply, death of cells in lens, similar to frosted glass - light does not get transmitted clearly, no formation of coherent image, can still see light and dark, in surgery - lens broken up and replaced with plastic lens, lose ability to accommodate

What is the function of photoreceptors in the retina?

where signal transduction occurs

What are the two light sensitive cell types in the retina?

rods and cones (photoreceptors)

What are the photoreceptors?

rods and cones

How is the image projected onto the retina?

upside down and backwards/back to front

What is the pathway of light through the retina?

nerve fiber layer -\> ganglion cell layer -\> inner plexiform layer -\> inner nuclear layer -\> outer plexiform layer -\> outer nuclear layer -\> photo receptor outer segment -\> pigment epithelium

What is the cellular pathway of light through the retina?

ganglion cells -\> amacrine cells -\> bipolar cells -\> horizontal cells -\> rods and cones

What is the function of amacrine cells?

interneurons, modulate information from ganglion cells to bipolar cells

What is the function of ganglion cells?

synapse with bipolar cells and transmit APs to the brain via axons in the optic nerve.

What is the function of horizontal cells?

horizontal dendrites, transfer information in horizontal direction

What is the function of bipolar cells?

important link in chain of transmission

Where are rods and cones located?

at the back of retina, furthest from incoming light, light has to pass through axons to reach them

What is the structure of rods?

outer segment of rods - stack of discs, disc shape increases membrane, surface area and light sensitivity

What is the structure of cones?

cone shape, less surface area than rods, invaginations of the membrane

Are rods or cones more light sensitive?

rods, more surface area for light sensitive pigments in rods than cones

What is the simplified pathway of light through the retina?

ganglion cells -\> interneurons (nuclei) -\> photoreceptors (nuclei)

What happens to light as it passes through the retina?

experiences refraction

What cells in the retina send action potentials?

ganglion cells

How do other cells in the retina communicate?

local/graded potentials

Why does the fovea have the highest visual acuity?

less tissues to pass through to reach retina

Are there more rods or cones?

more rods

What is the function of rods?

function in low light (night light), 120 million per retina, few in fovea, don't report colour information

What is the function of cones?

8 million per retina, many in fovea, vision during day and twilight (require relatively high light levels), 3 types - most sensitive to red, blue, green

How does the output from rods differ to the output from cones?

goes through visual processing that doesn't involve colour

Why do cones need more light?

have less surface area

What are photopigments?

pigments in photoreceptors that allow them to respond to light

What are the two components of photopigments?

opsin and retinal

What is opsin?

a membrane spanning protein

What type of opsin do rods have?

rhodopsin

What type of opsin do cones have?

photopsin

What are the different types of photopsin?

s photopsin, m photopsin, l photopsin

What is s photopsin?

short wavelengths, sensitive to blue light

What is m photopsin?

medium wavelengths, sensitive to green light

What is l photopsin?

long wavelengths, sensitive to red light

What is retinal?

a chromophore, vitamin A derivative, eg carrots contain vitamin A, good for eyesight, can't make retinal if you have a diet deficient in vitamin A

What happens in phototransduction in the dark?

no light, retinal non-active (inactive 11-cis isoform), lots of cGMP, cGMP channels open, high Na+ influx, photoreceptor depolarised to around -35mV, lots of glutamate released onto bipolar cells (more neurotransmitter)

What happens in phototransduction in light?

light energy, retinal changed to active (trans isoform), G protein activates cGMP phosphodiesterase, cGMP phosphodiesterase breaks down cGMP, less cGMP, cGMP gated channels close, less Na+ influx (more outflux), photoreceptor hyper polarised (-60mV), less glutamate released onto bipolar cells (less neurotransmitter)

Why is phototransduction counter-intuitive?

cells (photoreceptors) depolarised by a lack of light

What are the main things that occur in phototransduction?

amount of glutamate released goes up and down, fluctuations in neurotransmitter release converted into a signal

What is the only thing that light does?

convert retinal from cis to trans, everything else about vision is due to chemical reactions and brain processing

What happens when cGMP phosphodiesterase is activated?

broken down into GMP, cGMP channels close

What happens when cGMP binds to cGMP gated ion channels?

cell becomes depolarised, cation flows in, non-selective ion channel

Where is retinal located?

on opsin