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Visual, Auditory and Vestibular Systems Max MattaTILO'S COVERED I. Special senses: Describe II. Nervous system disorders: the structure and function of Summarise the pathology the visual, hearing and and pathophysiology of balance systems and explain central and peripheral their clinical assessment. nervous system disorders.LECTURE TIMELINE 1. Visual System 2. AuditorySystem 3. Vestibular SystemEye AnatomyLayers of the Eye Conjunctiva – thin transparent tissue covering outer surface ● Covers visible part of the eye and lines inner eyelids ● Nourished by tiny blood vessels Sclera – white of the eye ● Hard, opaque protective coat ● High water content Layers of the Eye Cornea – transparent dome-shaped window at the front of the eye Uvea - vascular coat of the eye ● 3 parts: ○ Iris – controls light levels in eye, opening in centre (pupil) ○ Ciliary body ○ Choroid – blood vessels supplying the back of the eye ● Intimately connected meaning disease affects all partsLayers of the Eye Lens ● 1/3 of the eye’s refractive power ● Accommodation ● Structure ○ Outer acellular capsule ○ Inner elongated cell fibres (regular → transparent) ○ Can become less transparent with age = cataract ● Absence of lens = aphakiaLayers of the Eye Retina ● Thin layer of tissue forming the innermost lining of the eye ● Captures light rays, sends them to the brain via the optic nerve Optic nerve ● Transmits electrical impulses from the retina to the brain ● Connects to the back of the eye near the macula ○ This is the blind spot (no light sensitive cells) ● Optic disc – visible portion of the optic nerveLayers of the Eye Macula ● Centre of the retina, temporal to optic nerve ● Highly sensitive, responsible for central vision ● Fovea – centre of the macula ● Highest concentration of cones, low concentration of rodsQuestions 1. Where does the refractive power of the eye come from? 2/3 from the cornea and 1/3 from the lens 2. Which of the following is not part of the uvea? a. Choroid b. Sclera c. Iris d. Ciliary bodyVision Central vision – mediated by cones ● Detailed vision during the day , colour vision (photopic vision) ● Reading and facial recognition ● Assessed via visual acuity assessment ● Loss of foveal vision → poor visual acuity Peripheral vision – mediated by rods ● Shape, movement, navigation, night vision (scotopic vision) ● Assessed via visual field assessment ● Loss of peripheral vision → inability to navigate surroundings, may need a stick even with 20/20 visionRetinal Structure Outer layer ● Photoreceptors (1’ neurons) Middle layer ● Bipolar cells (2’ neurons) ● Modulate sensitivity Inner layer ● Retinal ganglion cells (3’ neurons) ● Transmit signals to brainPhotoreceptors Rods – 120 million rods Cones Colour ● Longer outer segment ● 100x more sensitive to light (night vision) Central ● Slow response Cones – 6 million ● Shorter outer segment ● Less sensitive to light (day vision, colour vision) ● Faster response Different cones for different colours ● S cones – blue ● M cones – green ● L cones - redColour Blindness Test via Ishihara test Deuteranomaly aka Daltonism is the most common type ● Inability to perceive the colour red Achromatopsia = full colour blindness, very rareOptics Emmetropia: axial length matches refractive power ● Parallel rays fall on the retina Ametropia: mismatch between axial length and refractive power ● Parallel rays do not fall on the retina Myopia – near sightedness ● Parallel light rays converge anterior to the retina ● Treatment ● Axial myopia – excessive axial length ○ Diverging lenses (negative ● Refractive myopia – excessive refractive power lenses) ● Symptoms ○ Remove physiological lens ○ Blurry distance vision to reduce refractive ○ Squint power ○ HeadacheOptics Hyperopia – far sightedness ● Parallel light rays converge posterior to the retina ● Axial hyperopia – insufficient axial length ● Refractive hyperopia – insufficient refractive power ● Symptoms ○ Blurry near vision ○ Symptoms – eye pain, headache ● Treatment ○ Converging lenses (positive lenses) ○ Insert intraocular lens to increase refractive power ● If untreated, can lead to amblyopia (lazy eye)Presbyopia Natural loss of accommodation with age ● Begins around age 40 ● Distance vision unaffected Treatment ● Reading glasses with convex lensesNear Response Triad 3 separate, simultaneous pathways that mediate near vision ● Pupillary miosis ● Convergence ● Accommodation Pupillary miosis ● Sphincter pupillae contracts to constrict pupil in bright light Convergence ● Both eyes adduct medially to align with near objects ● Medial rectus muscle Accommodation ● Lens accommodates to increase refractive power to focus on near objects ● Ciliary muscles Visual Pathway Optic nerve Optic chiasm – ½ of fibres decussate Optic tract Lateral geniculate nucleus (thalamus) - synapse Optic radiation – quaternary neuron Primary visual cortex/striate cortex Optic chiasm ● Nasal retinal fibres (temporal visual field) cross ● Lesions before optic chiasm affect 1 eye only ● Lesions after optic chiasm affect both eyes ● Lesions at the optic chiasm e.g. pituitary tumour ○ Damage crossed fibres from both eyes ○ Bitemporal hemianopiaVisual Pathway Vertical field loss – neurological Horizontal field loss – ophthalmic Lesions posterior to chiasm e.g. stroke ● Right sided lesion – left homonymous hemianopia ● Left sided lesion – right homonymous hemianopia Optic tract ● Contains fibres from the same side of the retina ● Therefore contralateral part of the visual field ● L optic tract lesion → R homonymous hemianopia Lesion at primary visual cortex – macular sparing as macular part has dual blood supplyVisual PathwayQuestions 1.What happens for a lesion in front of the optic chiasm? 2/3 from the cornea and 1/3 from the lens 2. What happens for a lesion after the optic chiasm?Pupillary Reflex Regulates light input to the eye Bright – pupil constricts ● Circular muscles contract ● Decreased glare, increased depth of field, decreased bleaching of photopigments ● Mediated by CNIII (parasympathetic) Dark – pupil dilates ● Radial muscles contract ● Increased sensitivity to light ● Mediated by sympathetic nervePupillary Reflex Afferent ● Rods and cones synapse on bipolar cells ● Bipolar cells synapse on retinal ganglion cells ● Ganglion cells specific to the pupils exit at posterior 1/3 of optic tract ● Enter lateral geniculate nucleus ● Afferent pathway from each eye synapses on Edinger-Westphal nucleus (brainstem) Efferent ● Oculomotor nerve efferents from Edinger-Westphal nucleus ● Synapses at ciliary ganglion ● Short posterior ciliary nerve supplies pupillary sphincter Direct vs consensual reflex ● Direct light reflex: constriction of pupil in light-stimulated eye ● Consensual light reflex: constriction of pupil in other eye ● Afferent pathway on either side stimulates the efferent pathway on both sidesPupillary Reflex Swinging torch test ● Light shone into one eye and then swung to the other side etc. ● Relative afferent pupillary defect (RAPD) ● Both pupils constrict when the normal eye is stimulated ● Both pupils paradoxically dilate when the damaged side is stimulatedEye Movements 7 extraocular muscles ● Levator palpebrae superioris – elevates upper eyelid ● Superior rectus – elevates the eye ● Inferior rectus – depresses the eye ● Lateral rectus – abducts the eye ● Medial rectus – adducts the eye ● Superior oblique – depresses and abducts the eye ● Inferior oblique – elevates and abducts the eye Obliques do the opposite of what their name suggestsEye Movements Superior oblique ● Attaches high on the nasal side of the orbit ● Passes through trochlea and under superior rectus ● Inserts onto the posterolateral aspect of the eye Inferior oblique ● Attaches low on the nasal side of the orbit ● Passes over inferior rectus ● Inserts on the inferolateral aspect of the eye Innervation CNIII (oculomotor) ● Superior rectus CNIV (trochlear) ● Levator palpebrae superioris ● Superior oblique ● Inferior rectus ● Medial rectus CNVI (abducens) CN3, LR6, SO4 ● Inferior oblique ● Lateral rectus Abducens abductsEye Movements Testing eye movements – isolate muscles by looking up/down and to the side not just straight up/down ● Abduction aligns the eye with the insertion of vertical rectus muscles Why don’t we test the obliques by looking laterally? ● You can look down and out using the lateral and inferior rectus even if superior oblique isn’t working ● Looking inwards prevents action of the inferior rectus so only superior oblique can depress the eyeEye Movements Third nerve palsy ● Affected eye down and out ● As superior oblique and lateral rectus are unopposed ● Droopy eyelid due to loss of levator palpebrae superioris Sixth nerve palsy ● Affected eye deviates inwards (cannot abduct) ● Double vision worsens when looking towards affected eyeAuditory System The Ear Vestibular organ – captures low frequency motion (movement) Hearing organ – captures high frequency motion (sound) Human range of hearing: ● 20 – 20,000 Hz, 0 – 120 dB/SPL Pinna – visible part of the outer ear ● Modest amplification of some frequencies ● Capture sound and focus it to tympanic membrane ● Protect ear from external threats Tympanic membrane (eardrum) ● Converts air vibrations to mechanical movements Middle ear (tympanic membrane to oval and round windows) ● Mechanical amplificationInner Ear Inner ear/cochlea ● Transduces vibration to nervous impulses ● Captures intensity and frequency of sounds Parts of the cochlea ● Scala vestibuli and tympani ○ Bony structures containing perilymph (high in Na+) ● Scala media ○ Membranous structures containing endolymph (high in K+) Basilar membrane ● Arranged tonotopically ● Thin, stiff base – sensitive to high frequency ● Wide, loose apex – sensitive to low frequency ● Contains Organ of Corti (hearing organ)Organ of Corti Contains inner hair cells and outer hair cells, which both have stereocilia ● 1 column of IHCs, 3 columns of OHCs ● Tectorial membrane lies above hairs IHCs: afferent signals to auditory nerve ● Transduce sound OHCs: efferent signals from auditory nerve ● Modulate sensitivity of response Organ of Corti Organ of Corti Tectorial membrane allows deflection of hair cells → depolarization Longest stereocilium = kinocilium ● Deflection towards kinocilium opens K+ channels → depolarisation of afferent nerve ● Louder sound → greater deflection → greater depolarisationAuditory Pathway Auditory information crosses at the superior olive level ● From superior olive onwards all connections are bilateral Cochlea → Spiral ganglia of vestibulocochlear nerve → Cochlear nucleus (pons) → The neurons goes to both L & R superior olive → Inferior colliculus (brainstem) → Medial geniculate body (thalamus) → Auditory cortex (temporal lobe)Hearing Loss Location ● Conductive: outer or middle ear ● Sensorineural: cochlea or auditory nerve ○ Most common ● Central: brain/brainstem ○ Very rare ● Mixed: problems in areas affecting conduction and transduction Onset ● Sudden: minutes to days ● Progressive: months to years Degree ● Mild, moderate, severe, profoundCauses of Hearing Loss Conductive ● Outer ear: wax, foreign body ● Middle ear: otitis, otosclerosis Sensorineural ● Inner ear: noise, presbycusis (loss of OHCs with age), ototoxicity ● Auditory nerve: acoustic neuroma/vestibular Schwannoma ○ Usually unilateral Examination ● Whisper in ear while rubbing fingers in contralateral ear ● Rinne and Weber testsAudiometry Audiometer produces sound of varying intensity and frequency Audiograms – hearing thresholds plotted to identify hearing loss Normal threshold 0-20 dB ● Red line = Right ear Otoacoustic emissions ● bLue line = Left ear Conductive Sensorineural • Low intensity sounds produced by normal cochlea • Expansion and contraction of OHCs • Often tested in newborn hearing screening and as part of hearing loss monitoringHearing Loss Treatment Underlying cause ● Remove wax or foreign bodies ● Remove tumour ● Treat infection Conductive ● Hearing aids Sensorineural ● Cochlear implants (requires functional auditory nerve) Central ● Brainstem implantsVestibular SystemVestibular Organ Functions ● Detect and inform about head movements ● Keep images fixed in the retina during head movements ● Balance Vestibulo-ocular reflex ● Keep images fixed in the retina ● Connection between oculomotor nuclei and vestibular nuclei ● Eye movement is opposite direction to head movement ● Same velocity and amplitude Vestibulo-spinal reflex ● Maintains posture and balance Vestibular Organ Utricle and saccule (otolith organs) joined by a conduit ● Saccule – horizontal hair cells detect vertical movement ● Utricle – vertical hair cells detect horizontal movement ● Contain otoliths ● Carbonate crystals that help deflection of hairs ● Linear acceleration ● Maculae – contain hair cells, gelatinous matrix and otoliths Semicircular canals (anterior , posterior, lateral) ● Angular acceleration ● Have ampullae containing hair cells Saccule = Sitting and StandingVestibular Organ Canals ● Hair cells located within cristae of ampullae ● Rest of canals contains endolymph (high K+) ● Hairs surrounded by gelatinous substance called cupula ● Aids deflection Hair cells have a resting potential → basal discharge to nerve ● Deflection towards kinocilium ● Depolarisation and increased nerve discharge ● Deflection away from kinocilium ● Hyperpolarisation and reduced nerve dischargeVestibular Organ Head rotation ● Endolymph moves ● Displaces cupula ● Cilia deflect ● Hyper/depolarisation of hair cells ● Velocity signal to CNS Pathway Vestibular nerve → vestibular ganglion → joins auditory nerve → vestibulocochlear nerve → vestibular nuclei (pons)Vestibular Disorders Main complaints of acute unilateral ● Imbalance ● Dizziness Vertigo ● Vertigo ● Feeling that themselves or the world ● Nausea is spinning around Main complaints of Slow unilateral or ● Nystagmus should be present any bilateral ● Imbalance ● Nausea Main diagnoses ● No vertigo ● BPPV (pathological presence of Red flags otoliths in semicircular canals) ● Headache or hearing loss ● Vestibular neuritis ● Gait problems ● Vestibular migraine ● Hyperacute onset ● Stroke ● Symptoms > 4 daysVestibular Disorders Peripheral (most common) HINTS exam for acute ● Vestibular organ or CNVIII Acute ● Vestibular neuritis dizziness -> differentiates ● Vestibular neuritis between stroke or vestibular ● BPPV ● Stroke Intermittent neuritis ● Menière’s disease ● BPPV • Head Impulse Recurrent • Nystagmus Central ● Migraine • Test Skew ● CNS – brainstem/cerebellum ● Menière’s disease ● Stroke Progressive ● Multiple sclerosis ● Tumours ● Vestibular Schwannoma ● Degenerative conditions e.g. MSQuestions 1. What are the planes of the semi-circular canals? 2. Which of the following is not a red flag symptom of vestibular disorders? a. Hearing loss b. Long-lasting symptoms c. Vision loss d. HeadacheQuestions?Please provide any feedback using the QR code