NMH; Lecture 9, 10 and 11 - Sound conduction and transduction, Vestibular systems, Structure and function of eye Flashcards
What is the anatomy of the ear?
What is the function of the outer ear?
Focuses sound on the tympanic membrane and boosts sound pressure
What is the function of the middle ear?
- Increases pressure of vibrations by -> focusing vibrations from large SA (tympanic membrane) to smaller SA (oval window) and by changing SA, the pressure is increased;
- the incus has a flexible joint with staples (ossicles use leverage to increase force on oval window)
What is the function of the inner ear?
Cochlea -> tranduces vibration into nervous impulses and whilst doing so produces frequency (pitch) and intensity (loudness) analysis of the sound
What are the 3 compartments of the inner ear?
Scala vestibuli, scala tympani (which both contain perilymph) and scala media (contains endolymph fluid)
How is the basilar membrane of the inner ear arranged and how does this help with its function?
Using same principle as xylophone -> membrane is sensitive to different frequencies at different points along the length;
high frequencies at base which is narrow and short
How does depolarisation in the inner ear work?
- Opens K+ channels - upward movement of basilar membrane displaces stereocilia away from modiolus ->
- K+ channels open ->
- K+ enters from endolymph ->
- hair cell depolarises
How does hyper-polarisation in the inner ear work?
- Closes K+ channels ->
- Downward movement of the basilar membrane displaces stereocilia towards modiolus ->
- K+ channels close and hair cell hyperpolarises
What are the central auditory pathways?
Spiral ganglions from each cochlea project via auditory vestibular nerve to ipsilateral cochlear nuclei (monoaural neurones), and after this point all connections are biateral so deafness in one ear due to central causes must only be affecting cochlear nucleus or VIII nerve (rare)
How is hearing organised in the brain?
In the primary auditory cortex it is arranged tonotopically
What is loudness?
Loudness is the characteristic of a sound that is primarily a psycho-physiological correlate of physical strength (amplitude). More formally, it is defined as “that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud
What are the characteristics of sound?
Compressed and rarefied air, frequency/pitch (Hz), amplitude/loudness -> humans hear 20-20000Hz and 0-120 dB sound pressure level
At what frequency does human speech sound and what is an issue with this?
2-5kHz where hearing loss increases with age and particularly at this frequency
How do you evaluate ears?
Otoscopy of the tympanic membrane; tuning fork tests (Rinne and Weber tests)
How is the tympanic membrane examined?
Checking for light reflection, differentiation of its parts and mobility (ask patient to block nose and breathe); see cone of light
How is the tuning fork test carried out?
- Used to differentiate between conductive and sonsorineural haring loss -> for common practice 256, 512 and 1025 Hz are used;
- larger forks vibrate at slower frequencies, activated by striking against examiners elbow and placed 2cm away from EAC for air conduction and on mastoid for bone conduction;
- where chochlea is stimulated directly by the vibrations conducted through the skull
How do you assess hearing?
Audiometry (speech audiometry and pure tone audiometry), tympanometry and otoacustic emission
What is audiometry and how is it measured?
Science of measuring hearing acuity for variations in sound intensity and frequency - audiometer used to produce sound of varying intensity and fequency
What is tympanometry?
Examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in ear canal
What is spontaneous otoacoustic emission?
Normal cochlear outer hair cells expand and contract producing low intensity sounds called OAEs -> often part of newborn hearing screening program
What are the types of hearing loss?
Conductive hearing loss, sensorineural hearing loss and mixed hearing loss
What are the causes of conductive hearing loss?
Outer ear: congenital malformations, impacted wax, foreign bodies, external otitis, exostosis; Middle ear: Acute otitis media, otitis media with effusion, chronic otitis media, otos clerosis
What kind of congenital malformations occur in the outer ear?
Congenital atresia -> collapse or closure of the ear canal, may occur in isolation but typically associated with congenital malformations of the middle ear
What can impacted wax cause?
Mild hearing loss, easily treated by removing the wax
Which foreign bodies can cause hearing loss and how do you treat it?
More common in children which may need removal under general anaesthesia -> insects can be removed using alcoholic/oily solution or lidocaine
What is external otitis?
Swelling and redness of EAC, otorrhea, pain on mobilisation of ear and tragus, systemic symptoms in severe cases
What is exostosis?
Benign bone growth, usually in people with a history of exposure to repeated cold water - usually multiple, bilateral (possible extension to middle ear)
What are the symptoms of exostosis?
Hearing loss, external repeat ear infections and accumulation of ear wax
What is acute otitis media?
Inflammation of the middle ear, by far the most common health problem in children
What is otitis media with effusion?
Otitis media with accumulation of fluid, history of flu -> causes hearing loss, ear fullness and autophonia
What are the kinds of chronic otitis media?
What is a cholesteatoma?
Destructive and expanding growth consisting of keratinising squamous epithelium in middle ear/mastoid process
What is otosclerosis?
Begins as soft, spongy growth of new bone anywhere in ME but often near oval window, where in 10% of cases growth reduces mobility of stapes, causing conductive HL
How do you treat otosclerosis?
Stapedectomy (remove stapes and replace with artificial ones)
What is presbyacusis?
Starts in adolescence, but is associated with ageing; gradual, symmetric, affects frequencies of speech for 5th decade of life; men are 2x affected and they start earlier with more significant losses -> often associated with tinnitus
What is sudden hearing loss?
Occurs unexplained, rapid loss of hearing usually in one ear; defined as greater than 30 dB hearing reduction, over at least 3 contiguous frequencies all over a period of 72hrs or less
What are ototoxic drugs?
Toxicity effects vary from mild and temp to severe and perm with 30% of elderly people with hearing loss potentially taking ototoxic meds
Which infections cause Sensorineural hearing loss?
- Bacterial/viral infections that invade inner ear and can disrupt vestibuar function;
- infectious diseases (MMR, meningitis, encephalitis, chicken pox, influenza and syphilis) can also invade inner ear and
- cause sensorineural hearing loss or vestibular symptoms
What is noise-induced hearing loss due to acoustic trauma?
Injury due to brief exposure to very intense sounds, like gun shots, artillery fire, explosions, where H may be perm and severe but substantial recovery is common
What is noise-induced hearing loss due to long term noise exposure?
Damage results from long term exposure to high levels of noise, common in some occupational settings
What is the function of the hair cells - both inner and outer?
Hair cells are displaced; inner cell have connections to brain (afferent), outer hair cells move in response to information in brain
What is sound amplitude measured by?
Decibel scale -> log scale useful as sensitivity is very large
What is the dizziness triad?
x
What are the 3 semi circular canals stimulated by and what do they give a signal about?
Angular acceleration -> give signal of angular velocity
What are the 2 otolith organs stimulated by and what do they give a signal about?
Linear acceleration and gravito-intertial force; give signal of head acceleration and tilt
What are the signals from the semi circular canals and the otolith organs used for?
Control balance reactions, provide compensatory reflexes, provide spatial reference for other sensory motor coordinations, tune cardio-vascular function for re-orientations, serve perception of motion in space
What happens when control of balance reactions doesn’t work?
Ataxia
What happens when compensatory ocular reflexes aren’t provided?
Total loss- oscillopsia; unilateral loss - nystagmus
What happens when CV function for reorientations aren’t tuned?
Hypotension
What happens when perception of motion in space isn’t served?
Dizziness
What happens when there is unusual stimulation of balance organs?
Motion sickness is provoked
What are the functional causes of vestibular disorders?
Misinterpretation of sensory input, maladaptaion, loss of rules of correspondence between senses, over awareness/magnification of sensory input
What are the structural causes of vestibular disorder?
Destructive or irritative disease
What are the both structural and functional causes of vestibular disorder?
Structural disorder provoking chronic dysfunction
What is nystagmus?
Flick back of the eye when looking at something -> optokinetic nystagmus is due to watching out the window in the train
What are the common causes of vertigo for a few seconds?
Benign positional vertigo - due to debris in canals; intense vertigo and nausea -> paroxysmia which responds to Tegretol
What are the common causes of vertigo for a few minutes?
Vertebrobasilar insufficiency, migraine (mild vertigo in repeated attacks occasionally strong vertigo
What are the common causes of vertigo for a few hours?
Meniere’s syndrome -> intense vertigo and nausea, hearing disturbance, pressure in repeated attacks
What are the common causes of vertigo for a few days?
Vestibular neuritis -> intense vertigo+ nausea, hearing disturbance in isolated attack; herpes scarpa’s ganglion, infarction of labyrinth
What are the common causes of fluctuating/continuous vertigo?
Uncompensated vestibular lesion (mild vertigo+nausea), functional (mild vertigo+disproportionate disability
What are the common causes of silent vertigo?
Acoustic neuroma: mild imbalance, tinnitus, hearing loss
What is the use of the vestibular ocular reflex?
Only eye movement control that can stabilise the eyes during high frequency oscillation -> intensity can be shown by rapid head shaking
What is the head rotation test?
Rapid head rotation separately to right and left whilst patient views examiner’s eye; during rotation towards the intact labyrinth the patient maintains fixation; during rotation towards lesioned labyrinth the patient looses fixation, eyes go with the head and after the movement the patient makes saccades back to the fixation point
What are the vestibulo-ocular pathways?
Sup and medial vestibular neurons project to motor nuclei supplying extraocular muscles; axons ascend in the MLF and excite the ipsilateral oculomotor nucleus and contralateral abducens nucleus
What is the vestibulo-ocular reflex?
When the head rotates to the left, eyes rotate to the right and saccade to the left;
VOR operates to maintain the gaze on a selected target
What is the anatomy of the bones of the orbit?
x
What is the anatomy of the outer eye?
What is the function of the lacrimal system?
Tear production ->
- basal tears (constant level of production),
- reflex tears in response to irritation (afferent - cornea CNV1 and efferent - PSNS with ACh),
- emotional tears
How does the lacrimal system work?
Tear produced by lacrimal gland, tear drains through the two puncta, opening on medial lid margin; lid flows through the superior and inferior cannaliculi; tear gathers in the tear sac and exits through the tear duct into the nose cavity
What is the function of the tear film?
Maintains smooth cornea air-surface -> helps O2 supply to cornea as there are no blood vessels on the cornea; Removes debris (tear film and blinking), bactericide
What are the layers of the tear film?
- Superficial oily layer to reduce tear film evaporation (produced by row of meibomian glands along the lid margins),
- aqueous tear film (tear gland),
- mucinous layer on the corneal surface to maintain surface wetting
What is the conjunctiva?
Thin transparent tissue that covers the outer surface of the eye -> begins at outer edge of cornea, covers visible part of eye and lines the inside of eyelids
What is the supply of the conjunctiva?
Nourished by tiny blood vessels that are nearly invisible to naked eye
What is the inner eye anatomy?
What are the layers that coat the eye?
Eye measures 24mm in adults (anteroposterior diameter) -> 3 layers: Sclera = hard and opaque; Choroid = pigmented and vascular; Retina = neurosensory tissue
What is the sclera?
White of the eye -> tough, opaque tissue that serves as eye’s protective outer coat with a high water content
What is the cornea?
Transparent, dome shaped window covering the front of the eye -> powerful refracting surface, providing 2/3 of eye’s focusing power (convex curvature with higher refractive index than air) with low water content; continuous with sclera; acts as a physical and infection barrier
What is the structure of the cornea?
5 layers -> Epithelium, Bowman’s membrane, Stroma, Descemet’s membrane, endothelium
What is the function of the stroma (corneal layer)?
Regularity contributes toward transparency, corneal nerve endings provides sensation and nutrients for healthy tissue, normal cornea has no blood vessels
What is the function of the endothelium (corneal layer)?
Pumps fluid out of the corneal and and prevents corneal oedema -> only 1 layer of endothelial cells, no regeneration power, endothelial cell density decreases with age, endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
What is the uvea?
Vascular coat of eye ball that lies between sclera and retina; composed of 3 parts: Iris, ciliary body and choroid -> intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree
What is the choroid?
Lies between the retina and sclera, composed of layers of blood vessels that nourish the back of the eye
What is the iris?
Coloured part of eye, controls light levels inside the eye using the imbedded muscles that dilate and constrict the pupil size. The aperture of the iris is called the pupil
What is the structure of the lens?
Outer acellular capsule, regular inner elongated cell fibres - transparency but may lose it with age (cataracts)
What is the function of the lens?
Transparency (regular structure), refractive power (1/3 power with higher refractive index than aqueous fluid and vitreous), accommodation (elasticity)
What are the lens zonules?
Lens is suspended by a fibrous ring known as lens zonules which is passive connective tissue
What is the retina?
Very thin layer of tissue that lines the inner part of the eye -> responsible for capturing light rays that enter the eye -> which are then sent to brain for processing via optic nerve
What is the optic nerve?
Transmits electrical impulses from the retina to the brain, connects to the back of the eye near the macula -> visual art of optic nerve is called optic disc
What is the macula?
Located roughly in the centre of the retina, temporal to the optic nerve; small highly sensitive part of the retina, responsible for detailed central vision, 6mm in diameter -> fovea is very centre of macula, pit due to absence of ganglion cell layer
What is the function of the macula?
Allows us to appreciate detail and perform tasks that require central vision such as reading
What are the 2 segments of the eye?
Anterior (Ocular structure anterior to the lens) and posterior (Ocular structure posterior to the lens)
What is the anterior chamber of the eye?
Between cornea and lens, filled with clear aqueous fluid, supplies nutrients
What is the structure of the ciliary body?
What is the function of the ciliary body?
Secretes aqueous fluid in the eye; intraocular Aqueous fluid flows anteriorly into anterior chamber; aqueous fluid supplies nutrients
What drains the fluid out of the eye?
Trabecular meshwork
What is the normal intraocular pressure?
12-21mmHg
What is glaucoma?
Medical condition of sustained raised IOP; retinal ganglion cell death and enlarge optic disc cupping
What are the symptoms of glaucoma?
Visual field loss, blindness
What are the types of glaucoma?
Primary Open Angle Glaucoma (common - trabecular meshwork dysfunction), closed angle glaucoma (acute/chronic - increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork causing a vicious cycle)
What are the risk factors for Closed Angle Glaucoma?
Small eye (hypermetropia), narrow angle at trabecular meshwork
How does closed angle glaucoma present?
Sudden painful red eye with acute drop in vision
How can closed angle glaucoma be treated?
Peripheral laser iridotomy to create drainage hole on the iris
What does a retina look like?
x
Where is the blind spot of the optic nerve?
Where optic nerve meets retina has no light-sensitive cells which is a blind spot
How do you test the fovea?
Optical coherence tomography; Fovea is most sensitive part of retina with the highest conc of cones but a low conc of rods -> stars are brighter out of the corner of your eye; but only fovea has conc of cones to perceive detail
What is central vision?
Detail day vision, colour vision (Fovea), reading, facial recognition, assessed by visual acuity assssment; loss of foveal vision leads to poor visual acuity
What is peripheral vision?
Shape, movement, night vision, navigation vision, assessed by visual field assessment; extensive loss of visual field unable to navigate in environment, patient may need white stick even with perfect visual acuity
What parts of the ear are classified as the outer ear?
External auditory meatus and pinna
What is the external auditory meatus?
Cone at start with focuses the noise and it increases pressure at the tympanic membrane
What is the function of the shape of the pinna?
Gives idea of elevation of sound = floor level/ceiling
What is the middle ear?
Ossicles (smallest bones in body), tympanic cavity (filled with air); middle ear between cochlea and tympanic membrane
What occurs to the middle ear when sound occurs?
Vibrates due to air waves and ossicles allow improvement of signal
How do the ossicles increase the pressure of vibrations?
Focusing vibrations from large SA of tympanic membrane to smaller SA of oval window increasing pressure - convert movement of tympanic membrane into movement of foot plate; incus has flexible joint with stapes which ossicles use as leverage to increase force on oval window
How does impedance affect sound transduction?
Sound waves can travel through air easily but inside the cochlea you have fluid which requires more energy to move through
What 2 muscles control ossicle vibrations?
Tensor tympani and stapedius, which contract and educe movement of ossicles -> part of auditory reflex which has 50-100ms latency
When do the tensor tympani and stapedius work?
When loud noises occur, contracting to reduce vibration; also work when chewing/talking to not hear internally produced noises
What is hyperacusis?
Painful sensitivity to low intensity sounds -> can occur in conditions leading to flaccid paralysis of the auditory reflex muscles (Bell’s palsy)
What is the Weber test?
Tuning fork placed on scalp, testing if hearing loss is conductive, wax, ossification or tympanic membrane damage or if problem with inner ear (sensorineuronal hearing loss)
How does sound move through the inner ear?
Cochlea connects with ossicles through stapes which with the footplate vibrate the oval window (cochlear membrane part) distorting the internal membrane; round window is a pressure release window below oval window, as fluid is in the cochlea, so when stapes pushes the oval window in, the round window moves outwards to equalise the pressure. Vibration passes through scala vestibuli and then it starts distorting the membranee of scala media, which also causes basilar membrane to ripple
What is contained in the inner ear?
Basilar membrane which vibrates in response to pressure waves moving through the cochlea -> 3 compartments: scala vestibuli, scala tympani, scala media (endolymph)
What are the 3 compartments of the inner ear and how are they connected?
Scala vestibuli, scala tympani, scala media (endolymph) -> SV/T are connected via helicotrema which allows fluid to mix and both contain perilymph fluid
What is the function of the inner ear?
Stapes vibrates and generates pressure wave in perilymph in scala vestibuli, which vibrates basilar membrane -> cochlear is made of bone and fluid is incompressible so round window vibrates outwards when oval window vibrates inward
How is a tonotopic map formed in the brain?
Different parts of basilar membrane are sensitive to different frequencie, so specific tone is associated with spatial position on membrane -> mirrored in higher levels of auditory processing. Higher freq = base of basilar membrane vibrate more; lower freq = apex of basilar membrane vibrate more
What is the helicotrema?
Small hole allowing fluid to pass between scala vestibuli and scala tympani
Where is the scala media?
Between scala vestibuli and scala tympani; full of endolymph containing basilar membrane
What is the organ of corti?
Sense organ of cochlea of inner ear which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Where does the organ of corti lie and what does it consist of?
On top of basilar membrane and beneath tectorial membrane; consists of inner hair cells and outer hair cells which group together forming the auditory nerve and cell bodies found in spiral ganglion
What is the function of the organ of corti?
Sensor for when basilar membrane is deflected by a pressure wave
What are the inner hair cells of the organ of corti?
Found on their own which send connections back to the brain; have stereocilia which move in response to movement of endolymph -> DO NOT touch tectorial membrane
What are the outer hair cells of the organ of corti?
Found in groups of 3; contact with tectorial membrane; receiving efferent connections; are electromotile which can amplify vibration amounts (basis of cochlear amplifier) -> damage can lead to sensorineural hearing loss
What is the function of outer and inner hair cells?
Stereocilia stick out into endolymph of scala media and base is in perilymph -> need to de/repolarise rapidly; OHC = responsible for otoacoustic emissions (noises ear makes itself - can be responsible for tinnitus); stereocilia are connected by tip links -> IHC moves due to endolymph movement causing depolarisation (due to K channels opening)
What occurs when the basilar membrane moves upward?
Displaces stereocilia away from modiolus which opens K channels, K enters from endolymph, hair cell depolarises
What is the modiolus?
Bony conical structure at centre of cochlea
What occurs when the basilar membrane moves downward?
Displaces stereocilia towards modiolus; K channels close, hair cell hyperpolarises
What are the different K/Na concentrations in endolymph and perilymph in each section of inner ear?
Scala media (endo) = high K/low Na; scala tympani (peri) = high Na/low K; scala vascularis maintains conc grad -> producing endocochlear potential across membrane of stereocilia
What is the function of central auditory pathways?
Important in localising sound -> fibres from organ of Corti project out of cochlea via spiral ganglion to form auditory vestibular nerve -. ipsilateral cochlear nucleus and after this point everything is bilateral so representation is on both sides
How does the central auditory pathway travel?
Fibres from OoC project out of cochlea via spiral ganglion to cochlear nucleus to superior olivary nucleus and converge on inferior colliculus (caudal midbrain), with input from both cochlea (pathways are bidirectional); then pathway continues to medial geniculate nucleus and then to auditory cortex -> also have collateral pathways to reticular formation and cerebellum
What is the function of the inferior colliculus?
Reflex associations -> turning head towards loud sounds
How does lateral inhibition occur and what is its purpose?
Neurons don’t just fire and excite next neurone, they inhibit neighbouring neurones, sharpening the signal from the middle (strongest stimulus intensity) neurone, so it stands out more
Where is the primary auditory cortex located and how is it arranged?
A1 is in the temporal lobe -> arranged tonotopically, same as cochlea, basilar membrane and auditory pathways; neurones start specialising = have a favourite sound
What does the secondary auditory cortex respond to sounds and how?
May responds to sounds coming on and off; may respond to duration of sound
Which neurones project to the primary auditory cortex?
Medial geniculate nucleus axons project via acoustic radiations via the internal capsule
How do you localise sounds in the vertical plane?
Shape of the pinna
How do you localise sounds in the horizontal plane?
Interaural time difference and interaural intensity difference (head casts sound shadow over other ear so other ear hears it quieter)
What are the causes of sensory deafness?
Presbyacusis, exposure to loud noise, Ménière’s disease, toxicity (some Ab)
What are the causes of neural deafness?
Acoustic neuroma, viral infection
What are the causes of central sensorineural deafness?
Demyelination in MS, injury to central auditory pathway (unlikely to cause severe deafness unless both auditory cortices affected)
What is conductive hearing loss?
When diseases of middle ear destroy ossicles or stiffen their joints -> amplification system elimination (heavily waxy ear can also block sound waves)
What is sensorineural hearing loss?
Cochlea or cochlea nerve damaged, reducing/losing signal transmitted to auditory cortex -> typically seen in acoustic schwannoma (tumour of cochlea nerve) or cerebellar tumours (impinging cochlea nerve)
What is the latency of the auditory reflex?
50-100ms but auditory reflex might not be fast enough to protect ossicles from very loud/high frequency sounds
What is the structure of the lens?
Outer acellular capsule, inner elongated cell fibres
What is the function of the lens?
Transparency (regular structure), refractive power and accommodation (allows focus on near/distant targets by changing their shape - elasticity)
How does the eye work when light enters?
Light is refracted by cornea and lens to focus the incoming light rays onto retina to form a clear image -> pupil and pigmented uvea (absorb scatter light once entered the eye) regulate light entry
How is the shape of the eye maintained?
Scleral coat, maintenance of IOP by ciliary body production and drainage via trabecular meshwork
Which parts of the eye carry out visual information processing?
Retina and optic nerve
What is emmetropia?
Perfect focusing -> parallel rays (from distance) converge exactly on fovea forming a clear image on the retina; can see at long distances clearly without glasses
What is the refractive power of the eye?
Ability to focus light to form an image on the retina -> Cornea 2/3, lens 1/3
What is hypermetropia?
Long sighted -> parallel rays focus behind retina, so blurred vision without glasses -> eye doesn’t have enough focusing power, so can be corrected with CONVEX lens to increase converging power. Exacerbated by near vision
What is the cause of hypermetropia?
Short eyeball (common) and rarely, a flat corneal surface
What is myopia?
Short sightedness -> light rays focus in front of retinal surface, so have excessive refractive power -> near objects can be seen clearly without glasses; need CONCAVE lenses for far away objects
What are the causes of myopia?
Long eyeball or highly curved cornea
What is astigmatism?
Cornea is oval (not spherical), so refractive power varies along different planes, meaning some objects will focus before/behind/on the retina, meaning glasses required to correct different planes
How does accommodation occur?
Contraction of circular ciliary muscle within ciliary body, relaxing the zonules (passive fibres), so lens returns to original convex shape due to innate elasticity, increasing refractive power of lens -> CNIII (efferent)
What is the near response triad?
Pupillary miosis (sphincter pupillae) to increase depth of field; convergence (medial recti from both eyes) to align both eyes to a near object; accommodation (circular ciliary muscle) to increase refractive power of lens for near vision
What is presbyopia?
Naturally occuring loss of accommodation with age (unable to focus on near objects); onset from 40y, distant vision is intact, so reading glasses (convex) given to increase refractive power
What is the optic disc?
Retinal ganglion cells which exit via the optic nerve (blind spot)
What is the macula?
Highest concentration of photoreceptors = vital for fine vision (macula lutea = yellow patch)
What are the 4 branches of vessel arcades radiating from optic disc?
Superior/inferior temporal; superior/inferior nasal -> provide circulation to inner 2/3 of retina, outer 1/3 by choroidal vasculature
How can you differentiate veins from arteries in the eye?
Veins are darker and thicker than arteries
What is the retina?
Innermost layer of coat of the eye in posterior segment, consisting of outer layer of retinal pigment epithelium and inner thicker layer called neuroretina (photoreceptors and neurone)
What is the retinal pigment epithelium?
Nutrient support to retina, removes metabolic debris from photoreceptors
What are the divisions of the neuroretina?
Outer: photoreceptors (rods and cones); middle: bipolar cells; inner: retinal ganglion cells (axons run to optic nerve)
What is the fovea?
Anatomical dip is characteristic due to absence of overlying ganglion cell layer; has highest conc of photoreceptors for fine vision (cones); can be assessed by OCT
What are the types of photoreceptors?
- Rods:
- Longer outer segment with photosensitive pigment,
- 100x more sensitive to light with slow response to light and are responsible for night vision (scotopic),
- 120million rods;
- Cones:
- less sensitive to light,
- faster response to light,
- responsible for daylight fine vision and colour vision (photopic vision)
- 6 million cones.
What is the journey of photopigments - synthesised, destroyed?
Synth in inner photo-receptor segment, transported to outer segment discs. Distal discs with deactivated photopigments are shredded from the tips and phagocytosed by the retinal pigment epithelial cells.. Deactivated pigments are regenerated inside the retinal pigment epithelial cells and transported back to the photoreceptors
What is ametropia?
Mismatch between axial length and refractive power, don’t fall on retina -> myopia, hyperopia, astigmatism and presbyopia
What are the symptoms of myopia?
Blurred distance vision, squint in attempt to improve uncorrected visual acuity when gazing far, headache
What are the symptoms of hyperopia?
Visual acuity at near tends to blur relatively early; asthenopic symptoms = eyepain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis; amblyopia (uncorrected hyperopia)
What are the symptoms of astigmatism?
Asthenopic symptoms (headache, eyepain); blurred vision, distortion of vision, head tilting and turning
How do you treat presbyopia?
Convex lenses in near vision -> reading/bifocal/trifocal/progressive power glasses
What are the types of optical correction?
Monofocal lenses, spherical lenses, cylindrical lenses, multifocal lenses; contact lenses; interocular lenses; surgical correction (keratorefractive surgery, intraocular surgery)
What are the complications and disadvantages for contact lenses?
Disadv: careful cleaning and disinfection, expense. Complication: infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis
What are the photopigments for rod photoreceptors?
Rhodopsin (max reaction at 498nm) -> opsin is transmembrane protein with co-factor 11-cis retinal (vit A derived); cofactors react to photons causing conformational change to rhodopsin, activating G-protein pathway, resulting in nerve action potential
What are the photopigments for cone photoreceptors?
3 different subtypes of photopsin and react maximally 3 different light frequencies; S-cone (blue - 420-440nm), M-cone (534-545nm - green), L-cone (red - 564-580nm); form basis of colour vision
How are photoreceptors distributed in the eye?
Scotopic vision is due to rods which are widely distributed around the retina, highest density just outside macula; completely absent within macula; density reduces toward periphery. Cones = photopic, and are only found in macula
What is the most common form of colour vision deficiency?
Deuteranomaly (red-green colour blindness), caused by shifting of M-cone sensitivity towards L-cone peak
What are some less common versions of colour vision deficiency?
Caused by a shift in the photopigment peak sensitivity -> caused by shifted peak = anomalous trichromatism; also caused by absence of one or more of the three cone photopigment subtypes: dichromatism (only 2 cone subtypes are present) and monochromatism (complete absence of colour vision)
What is the Isihara test?
Colour perception test, with the isochromatic plates testing only red-green deficiencies, consists of plates of circle of dots appearing randomly in size -> normal vision IDs correct pattern in form of 2-digit number, colour vision deficiency IDs abnormal/no pattern
How do our eyes adapt to the dark?
Increase in light sensitivity; biphasic process -> cone adaptation 7 min and rod adaptation 30 min (regeneration of rhodopsin takes a long time) -> rods take longer to adapt as they are greatly suppressed during day/light so need to be regenerated. Pupil dilates in the dark
How do our eyes adapt to the light?
Occurs over 5 minutes; Bright light bleaches the photo pigments and neuroadaptation occurs, inhibiting rod/cone function -> cones take over from suppressed rods in a minute -> pupil also constricts with light
