Scenario 27: Rodney's Hearing Loss and Harry's Scotoma Flashcards
What constitutes the vestibular system?
Integration of balance, posture, eye movements. 2 organs of equilibrium- semicircular canals and otolith organs (both found in labyrinth of inner ear)
What is the external ear formed of?
Elastic fibrocartilaginous structure with helix, lobule and tragus
What is the middle ear formed of?
Malleous, incus and stapes bones lying in tympanic cavity, Eustachian tube joins this to nasal cavity to equalise pressure
What is the inner ear formed of?
The oval window is the beginning of the inner ear. The bony labyrinth of the inner ear divides it into 3: vestibule, semicircular canals and the cochlea. There is the membraneous labyrinth lying inside the bony one. There is also a round window
Which bone is the auditory system embedded in?
Deep in the petrous part of the temporal bone
What are the fenestra vestibuli and fenestra cochlae?
Fenestra vestibuli- oval window
Fenestra cochlae- round window
What are the semicircular canals?
3 interconnected tubes: posterior, lateral and anterior bony structures with an osseous ampulla filled with many hair cells
What are the two otolith organs?
The utricle and saccule which lie in the vestibule of the inner ear
What is the ductus reuniens?
It connects the saccule to the cochlear duct
What is the composition of endolymph?
High K+ and low Na+
What is the composition of perilymph?
High Na+ and low K+
Where are the endolymph and perilymph?
Endolymph fills the membraneous labyrinth, perilymph fills the bony labyrinth. There is a voltage difference between them
What are stereocilia?
Tall, actin rich, pyramidal structures on hair cells which come to a point called the kinocillium. They form a staircase like structure where the axis going up to the tip is the axis of polarity. They are connected at the tips to one another by cadherin bridges, tip links.
What happens if a stereocilia tilts away from it’s axis of polarity?
It causes channels on tip links to close (as if pushed closer together) and receptor hyperpolarisation. This causes a decrease in firing in the afferent neurone
What happens if a stereocilia tilts towards it’s axis of polarity?
It causes channels on tip links to open (as if stretched) and receptor depolarisation due to influx of positive ions. This causes an increase in firing in the afferent neurone
How is an AP generated in the VIII cranial nerve by stereocilia?
Depolarisation by ion channel opening causes glutamate release from the hair cell to the afferent fibre of the vestibulocochlear nerve. If sufficient- AP
How do hair cells help our body know what direction we are moving?
The fluid surrounding them will either depolarise or hyperpolarise them by pushing them one way or the other. If we turn our head left, fluid in the horizontal semicircular duct flows right and because the hair cells are all highly ordered in the ampulla, it turns all the hair cells on the left side toward their axis (DP) and on the right away from their axis (HP) There are hair cells pointing in every direction so each dimension of movement is covered
What are the ampullary crests?
Where hair cells stick up into endolymph embedded in the cupula
How can static head position be detected?
When upright the macula is roughly horizontal so otoliths rest directly on it. If the head is tilted, gravity will act on the otolithic mass so it sags in the direction of tilt and bends hair cells.
How can complex signals of linear movement and head position be generated?
The cilia of hair cells in the utricle macula don’t all face in one direction, they face toward the striola (a curving landmark) any tilt or movement will depolarise some cells and hyperpolarise others so that complex signals can be generated to get an accurate measure of head position
How are the hair cells in the saccule arranged and how does this relate to function?
They are arranged pointing away from the striola so that vertically orientated force (e.g. up and down in lift) can be felt
What is the vestibulo-ocular reflex?
Projection of nuceli of extraocular nerves and to cervical spinal cord to coordinate head movements to eye movements. When you turn your head but keep your vision fixed, the eyes move the opposite way to the head but to the same degree (doll’s eyes)
What is Meniere’s disease?
Excess endolymph, distends membraneous labyrinth and causes vertigo, nausea, nystagmus, hearing loss andd tinnitus and even deafness
What is benign positional vertigo?
When otoconia become dislodged from the utricle and migrate into the semicircular ducts so that when the head moves, gravity-dependant movement of otoconia causes abnormal fluid displacement and vertigo
What are otoconia?
Small crystals of calcium carbonate in otoliths which stimulate hair movement when the head moves
What is an acoustic neuroma?
A benign tumour of the myelin forming cells of the vestibulocochlear system (vestibular Schwann cells) located at the CPA
What is pitch?
Cycles of vibrations per second is the frequency of the sound wave in Hz corresponding to pitch
What is loudness?
A higher pressure amplitude measured in dB
What is the function of the external ear?
To collect the sound waves and somewhat change them via their nobules
What is the external auditory meatus?
S shaped curve leading to tympanic membrane which is concave and lies at an oblique angle. The outer 1/3 is membraneous whilst the inner 2/3 is formed from temporal bone
Which nerve fibres supply the external auditory meatus and tympanic membrane?
Sensory fibres from vagus and trigeminal
How does sound transverse the middle ear?
Sounds cause the tympanic membrane to vibrate which is conveyed through the middle ear by the 3 ossicles then to the cochlea via the oval window
What amplifies the sound signal as it moves from the external ear to the inner ear?
The ossicles amplify the signal as does the concept that the area of the tympanic membrane is greater than that of the oval window so as the sound wave is forced through a smaller space, the pressure of it increases
Why do we need to amplify the sound signal as it goes from middle to inner ear?
The middle ear is air filled and the inner ear is fluid filled. Fluid takes more energy to vibrate than air does so the signal needs to be made greater to ensure it is not lost
How does the middle ear communicate with the pharynx?
Via the pharyngotympanic tube, the Eustachian tube, which is opened by swallowing and yawning. It opens into the nasopharynx at the level of the inferior cochae
How is the cochlea divided?
Into scala vestibuli, scala media and scala tympani
What is the location of the oval window in the cochlea?
Pushed up against the scala vestibular
What is the function of the round window?
It moves when the oval window transmits movement to the inner ear giving the walls of the cochlea yield so that the pressure can enter
How are cochlea arranged?
In cochlea tree with auditory nerve branches supplying each
What is the organ of Corti?
It is found at the basilar surface of the cochlea where the hair cells are found. There is an inner and outer layer of hair cells. The inner hair cells are the sensory receptors of the auditory system
What is the helicotrema?
Part of cochlear labyrinth where scala tympani and vestibuli meet, cochlear apex
What the scala tympani, media and vestibuli filled with?
Scala media: endolymph
Scala tympani and vestibuli: perilymph
How do the hair cells transmit sound information to the afferent nerve fibre?
The vibrations of sound pass in a wave over the basilar membrane and cause it to vibrate. This causes hair cells to move backwards and forwards with the vibrations causing an association voltage of alternating depolarisation and hyperpolarisation which is transmitted to the afferent nerve fibre
How are the difference between high and low frequency sounds picked up by the cochlea?
Because the basilar membrane is stiffer near the oval window, at the base, high frequency sounds will vibrate the membrane here, they are ‘strong’ enough and this means they don’t have to travel as far.
For the low frequency sounds, they will have maximum vibration at the apex where the membranes is wide and flexible (helicotrema). This means that the location of a hair cell along the cochlea codes for a frequency.
What is the function of outer hair cells?
Amplify basilar membrane motion and enhance frequency sensitivity by responding to electrical stimulation by changing their length generating receptor potentials in response to sound, this amplifies sounds especially weak ones
How do outer hair cells respond to depolarisation and hyperpolarisation?
DP: contract
HP: elongate
What allows outer hair cells to change their length?
Motor protein: prestin
What are otoacoustic emissions?
Sounds emitted by outer hair cells which propagate out of tympanic membrane and can be measured by clincians
How do we code for amplitude?
Louder sound- more vibrations of basilar membrane + bigger depolarisation/hyperpolarisation causing more neurotransmitter release and therefore a higher frequency of action potentials in the afferent fibre. AP frequency codes for loudness.
What is the spiral ganglion?
Bipolar neurones with one axon to hair cell and one to cochlear nerve, cell bodies wrapped up in cochlear tree
Where on the ventral and dorsal cochlear nuclei is high frequency sound found?
Dorsal parts of both nuclei
Where on the ventral and dorsal cochlear nuclei is low frequency sound found?
Ventrolateral parts of both nuclei
How is auditory information passed from sensory receptor to the auditory cortex?
Projections cross the midline to the inferior colliculus in the mid brain, then to the thalamic medial geniculate nucleus then to auditory cortex
Why is the auditory system bilateral?
Though the midline is crossed, there are also ipslateral connections and other crossings so the brain can understand what is coming into both ears at the same time
How is sound fine tuned by efferent fibres?
Superior olivary nucleus has efferent fibres which project back towards the cochlear hair cells for fine tuning of sound
What are some conductive causes of deafness?
Earwax buildup, eardrum damage, otosclerosis of middle ear, trauma, middle ear infection, genetic
What are some sensorineural causes of deafness?
Cochlea infection, trauma, noise damage, old age, ototoxic drugs, genetic, tumours
What are some central causes of deafness?
MS, vascular incident, trauma, infection, tumour, neonatal disease
What is a reflex?
A stereotyped response to a stimulus not subject to conscious control
What are autonomic reflexes?
E.g. pupillary light reflex, mediated by autonomic nervous system and activated by cardiac/smooth muscle and glands
What are somatic reflexes?
Mediated by somatic nervous system activated by stimulation of skeletal muscle
What are the 5 components of a reflex?
1) receptor 2) sensory neurone afferent to CNS 3) interpretation centre 4) motor neurone efferent from CNS 5) effector organ/gland
What is a monosynaptic reflex?
1 synapse, 1 quick twitch e.g. control muscles in legs to maintain upright posture
Why are polysynaptic reflexes slower?
Synaptic delay of several synapse
What are proprioceptors?
Receptors located in skeletal muscle at tendon-muscle junction, joints and ligaments. Carry info on joint location to CNS along sensory afferents
How do muscle spindles lie in relation to extrafusal fibres fibres?
In parallel with extrafusal fibres, lie embedded in muscle (intrafusal)
What are the proprioceptors in muscles and tendons?
Muscle spindles and Golgi tendon bodies
How do Golgi tendon bodies lie in relation to extrafusal fibres?
Lie with musclotendon fibres in series with extrafusal fibres between collagen fibres
Why do muscle spindles have a contractile element?
Due to presence of y motor neurones
What is the function of muscle spindles?
Monitor muscle length and rate of changing of length to prevent overstretching (safety device)
What do the tonically active sensory neurones do?
Measure the tone of the muscle fibres
What is alpha-gamma coactivation?
The concept that alpha motor neurones fire to contract extrafusal fibres and gamma motor neurones are coactivated to fire with alphas to contract intrafusal fibres simutaneously to ensure both fibres are the same length
What will prevent overstretching of muscles past limits e.g. by holding a load which is too large?
Inhibitory interneurones prevent overtensing e.g. by dropping load
What are cross cord reflexes?
E.g. when we flex one limb we need to stabilise the other to prevent falling over by extension
What are spinal pattern generators?
Rhythm generators built into circuit to allow coordinated movement e.g. of stepping, timing of flexion and extension
What are central pattern generators?
Rhythm generators for more complex movements such as turning (need intersegmental control from body)
When can a reflex be overridden?
By descending pathways from higher levels e.g. if you’re holding your nan’s best plate, you won’t drop it, even if red hot
What are Renshaw cells?
Inhibitory interneurons which regulate spinal motor neurones by receiving excitatory collateral from alpha neurone and sending an inhibitory axon back
What is the supraspinal reflex?
Labyrinthine righting reflex/ vestibular reflex. Stimulation of semicircular canals when you lean off balance, motor response in neck and limbs to maintain upright posture,
What can a weak or absence reflex response indicate?
Damage to nerves outside the spinal cord, peripheral neuropathy, damage to motor neurones- MND, damage to NMJ- myasthenia gravis, or muscle disease- myopathy
What can an excessive response in a reflex indicate?
Spinal cord damage above the level controlling hyperactive response, higher CNS damage
What can an asymmetric response indicate?
Early onset of progressive disease, localised nerve damage e.g. trauma
How can reflex tests be useful for testing for a spinal cord injury?
Determine area of injury: if the injury is above the reflex the reflex will be unaffected, if below then the reflex will be absent
What is Babinski’s sign?
In anyone older than 2 years, if the lateral sole of the foot is stroke then the big toe should flex (curl down), if there is an upper motor lesion or if it’s a neonate the big toe will extend (spring up)
How is cerebral palsy characterised?
Random, uncontrolled movements due to failed development of higher control, spectra of disorder retain primitive reflexes. Affects sensory perception of movemetn.
What forms the lateral wall of the orbit?
Thick zygomatic bone and greater wing of sphenoid
What forms the medial wall of the orbit?
Thin, ethmoid bone, lacrimal, frontal and lesser wing of sphenoid
What forms the inferior wall of the orbit?
Thin, maxilla, palantine, zygomatic
What forms the superior wall of the orbit?
Orbital part of frontal bone, lesser wing of sphenoid
Why is the orbit surrounded by air? Why is this clinically important?
Due to presence of frontal, ethmoid and maxillary sinuses. Due to blowout fractures of the orbit which show black on x-rays
What happens when the suspensory ligament of the eye ball is torn?
The eye will sink down
What is the function of eyelids?
Protect from light, dirt and irritation as well as spreading tears across the eyeball
What are eyelids formed of?
Bands of connective tissue, tarsi
What do tarsal glands secrete?
An oily fluid to prevent tears evaporating
What is conjunctiva?
Lining of the inside of the eyelids and covering for the sclera
What is the purpose of conjunctiva?
Produces mucus and tears
What stimulates lacrimal fluid production?
Parasympathetic impulses from CN VII
How is lacrimal fluid secreted and spread across the eye?
Through lacrimal ducts to surface of conjunctiva, a blink pushes it medially
How does lacrimal fluid drain?
Through lacrimal canaliculi into lacrimal sac where it is conveyed to nasal cavity by the nasolacrimal duct
What is lacrimal fluid formed of?
A plasma ultrafiltrate with enzymes for protection
What is conjunctivitis?
A pink or red eye due to inflammation of highly vascularised conjunctiva
Which muscles raise the eyelid? What are the innervation?
Levator palpebrae superioris, CN III
Superior tarsal muscle, sympathetic fibres from superior cervical ganglion
What is the clinical consequence of either levator palpebrae superioris or superior tarsal muscle paralysis?
Ptosis
What is the function and innervation of the orbicularis oris muscle?
Palpable part closes eyes during sleep and blinking, the rest screws eyes up tightly. Facial nerve.
What is the function of the lens?
Accomodation
Where is the aqueous humour?
Between lens and cornea
Where is the vitreous humour?
In the vitreous chamber
What is the scelera?
White, avascular tough, opaque layer for muscle insertion continuous with cornea
What is the choroid?
Pigmented and highly vascular continuous with ciliary body and iris
What is the retina?
The light sensitive neural part of eye
What is the function of the ciliary process?
Secretion of aqueous humor
Which muscles dilate the pupil?
The radial one under sympathetic control
Which muscles constrict the pupil?
The circular one under parasympathetic control
Which muscle controls shape of lens and therefore focus?
Ciliary muscle
How do we focus on near objects?
Ciliary muscle contracts and releases tension in zonal fibres, the lens becomes rounder so the focal point shifts closer to your eye
How does age change focus ability?
Older, stiffer lens has trouble focussing on near things
How do we focus on far objects?
At rest Zonules of Zinn are tight and the lens is relaxed so the focal point is further away
How does fluid move through the eye?
Ciliary process secrete aqueous humour into posterior chamber, it flows through the pupil to the anterior chamber and the drains though the trabecular network into the canal of Schelemm and sceral veins
How can we get glaucoma?
Fluid outflow is blocked so pressure builds up in anterior and posterior chambers causing compression of retina and retinal arteries
How can we treat glaucoma?
Tested for routinely at opticians and treated with eye drops to reduce humour and increase drainage or surgery
What is the venous drainage of the eye?
Supraorbital vein drains into superior or cavernous sinus sinus
Which muscle abducts the eye?
Lateral rectus
Which muscle adducts the eye?
Medial rectus
Which muscle elevates the eye?
Superior rectus
Which muscle depresses the eye?
Inferior rectus
Which muscle intorses the eye?
Superior oblique
Which muscle extorses the eye?
Inferior oblique
What runs through the superior orbital fissure?
ophthalmic V1, trochlear IV, oculomotor III, abducens VI
What runs through the optic canal?
Optic nerve and central artery of the retina
What is the course of the superior oblique muscle?
Passes through trochlea, a loop of connective tissue on the medial orbit and attaches at the superior lateral surface
What is the primary action of the superior rectus muscle change when the eye is adducted?
Intorsion
How does the action of the superior rectus muscle change when the eye is abducted?
It doesn’t, it still elevates
What is the primary action of the inferior rectus muscle change when the eye is adducted?
Extorsion
What is the primary action of the superior oblique muscle change when the eye is adducted?
Depression
What is the primary action of the inferior oblique muscle change when the eye is adducted?
Elevation
What are saccades?
Rapid point to point movements with brief pauses to allow foveal inspection (fixation points). Voluntary and can be guided by stimuli
What are smooth pursuit eye movements?
Slower than saccades, these conjugate eye movements are used to track slow, moving predictable targets but cannot be generated in the absence of visual stimulus (e.g. following a finger)
What are vergence movements?
Eye movements which adjust to distance e.g. shifting gaze from far to near
How do we shift gaze from far to near?
Disconjugate, changes angle between eyes coupled with changes in lens shape and increased pupil diameter- triad
How do we shift gaze from near to far?
Eyes converge by media rectus muscles, curvature of lens increases to bring objects to focus in retina, pupil constricts to increase the depth of field
How do we stabilise our gaze?
By the vestibulo-ocular reflex and optokinetic response
What is the optokinetic response?
If following something with our eyes in smooth pursuit (e.g. telephone poles as we drive by them) then when it is out of our vision there will be a saccade movement to bring the eyes back to where they first saw the object
What is the function of the optokinetic response?
Uses movement of visual field across retina (retinal slip) to determine the direction and speed of head/body movement
What detects the optokinetic response?
Wide field retinal ganglion cells that are sensitive to slow movements
Which muscles of the eye does the occulomotor nerve control?
Superior and inferior rectus, medial rectus and inferior oblique
Which muscles of the eye does the abducens nerve control?
Lateral rectus
Which muscles of the eye does the trochlear nerve control?
Superior oblique
How are motor nuclei of the eyes connected to ensure coordinated eye movement?
By the medial longitudinal fasciculus
What are the pulse-step changes in eye activity?
The pulse is a transient large increase in firing of motor neurones which rapidly shifts the eye to a new position.
The step is a prolonged smaller increase in firing rate which holds the eye at a new position
What will happen to motor neurone activity if, for example, the right eye turns right?
There will be an increase in firing in the abducens nerve due to the contract of the lateral rectus muscle, there will be a decrease in firing of the oculomotor nerve as the medial rectus relaxes
What is the central pathway for smooth pursuit movements?
Information from the fovea is sent to area 17 in the visual cortex via the dorsal lateral geniculate nucleus of the thalamus which sends information back to the pontine nucleus to have motor effect
What is the central pathway for the optokinetic reflex?
Information from the retina is sent to the nucleus accessory optic and the dorsal lateral geniculate nucleus of the thalamus
What does the dorsal pathway of vision allow you to do?
Find an object in space, orientation, motion, direction, binocular disparity, colour, contrast, spatial frequency, ocular dominance
What does the ventral pathway of vision allow you to do?
Recognise an object via detailed information at high spatial frequency
What is strabismus?
Squint, loss of conjugate gaze resulting in diplopia because the two foveas are looking at different objects
What can cause strabismus?
Damage to abducens, trochlear or oculomotor nerves, damage to extra-ocular muscles, damage to system linking eye nuclei on either side
What is the consensual response in the pupillary light reflex?
Iris responses are yoked, one eye will have the same response as the opposite even if different stimuli
Describe the pathway of pupil constriction
1) Classes of retinal ganglion cells which respond to luminance project to midbrain, half of these decussate at chiasm (underlies consensual response)
2) RGCs terminate in olivary pretectal nucelus
3) Olivary nucleus projects to Edinger-Westphal nucelus for neurones which control ciliary muscles for lens accomodation
4) Pupil constricts
What will happens to the pupillary light reflex response if the right optic nerve is damaged?
Right eye has no pupillary response and there is no consensual response in the left eye.
Left eye has pupillary response and the consensual response will happen in the right eye.
What will happens to the pupillary light reflex response if the right optic tract is damaged?
Optic tract carries information from both eyes but only about one half of the visual field. As a result a lesion of the optic tract neither the direct or consensual response is lost, may be weaker
What will happens to the pupillary light reflex response if the right oculomotor never is damaged?
Commands for pupil dilation/constriction do not reach the right eye, therefore direct and consensual responses are lost
What do the axons of RGCs form?
The optic nerve
What are the types of photoreceptors in the retina?
1 rod type, 3 cone types
What are the 3 cell layers of the retina?
Photoreceptors, bipolar cells and retinal ganglion cells
What are the characteristics of rod cells?
Very sensitive in low light levels, no colour vision (scotopic vision)
What are the characteristics of cone cells?
Less sensitive, 3 types varying in wavelength sensitivity (red blue and green), colour vision, mediate vision in bright light (photopic vision)
Where are cones most concentrated?
In the fovea for high acuity vision
How do receptive fields of RGC and photoreceptors compare and why is this?
The receptive field of a RGC is much larger than that of a photoreceptor because a single RGC is pooling information from many bipolar cells which in turn pool info from photoreceptors
What makes RCGs sensitive to contrast?
Lateral connections of amacrine and horizontal cells
What are P cells and what do they recognise?
Parvocellular cells which recognise the fine details of a visual scene, have a small receptive field, fine grain, high resolution vision and colour detection. Have a tightly packed cell body mosaic. Sustained response
What are M cells and what do they recognise?
Magnocellular cells which recognise motion information, coarse grain detection, larger receptive field, low resolution, sensitive to movement and flicker. Have a broadly spaced cell body mosaic. Transient response
What is the monocular visual field?
The edge bits on either side which are only viewed by one eye
What is the binocular visual field?
The central visual field viewed by both eyes
How can the visual field be divided in respect to the fovea?
Into quadrants for both eyes. The nasal field is the medial edges for both eyes, the temporal field is the lateral edge for both eyes. The upper half is the superior visual field, the lower half the inferior visual field
Which bits of the eyes view the left visual field?
The temporal retina of the right eye and the nasal retina of the left eye
Which bits of the eyes view the right visual field?
The temporal retina of the left eye and the nasal retina of the right eye
Where does the information from the left visual field project to?
The right half of the cortex via the lateral geniculate nucelus
Where does the information from the right visual field project to?
The left half of the cortex via the lateral geniculate nucelus
Which way do nasal retinal axons project?
Contralaterally
Which way do temporal retinal axons project?
Ipslaterally
What will a lesion of the right optic nerve cause?
Blindness in the right eye
What will a lesion of the right optic tract cause?
Contralateral homogenous hemianopia- blindness in the left half of the vision field in both eyes
What will a lesion of the optic chiasm cause?
Bitemporal hemianopia so can’t see periphery in both eyes
What is a quadrantonopia?
A lesion to optic radiations from the lateral geniculate nucleus to the cortex meaning there is a lesion in quarter of the field
Describe the central connections of the visual pathways
From LGN to area 17 of V1 (visual cortex) which projects to V2 (area 18) which then diverges into dorsal (V5) and ventral (V4) streams
What is rhodopsin?
A g-protein coupled receptor which responds to light (retinal in the ligand, attached to opsin).
What is the g-protein of rhodopsin? What does it do?
The g-protein is Gt (transducin), the a subunit activates PDE6 which converts cGMP to GMP in photoreceptors
What is the function of cGMP in photoreceptors?
Keeps open the nucleotide gated cation channels and allows positive ions into the cell
What does a light stimulus do to the cation channels in photoreceptors?
Switches them off
What is happening in photoreceptors in the dark?
1) The optic disc membrane has rhodopsin bound to 11cis retina in inactive betagamma state (inverse agonism)
2) GTP is being converted into cGMP by GC so there are high levels of cGMP
3) The GC is activated by Ca2+ dependant GCAP so cyclic nucleotide gated ion channel (CNG) is open, allowing Na+ and Ca2+ in (dark current)
4 )Na+ ions cause depolarisation allowing even more VGCC to open so glutamate is released
What is happening in photoreceptors in the light?
1) Photon hits rhodopsin and makes retinal trans, allows it to couple with G-protein and activate it
2) G protein alpha subunit dissociates and activates PDE6
3) PDE6 converts cGMP into GMP so conc of cGMP decreases
4) CNG closes so no entry of Na+ and Ca2+ (no dark current)
5) No Ca2+ entry decreases conc of Ca2+, inhibiting GC and further reducing cGMP levels
6) The cell hyperpolarises and VGCCs close
7) No glutamte release
How will a rod bipolar neurone and RGC respond to a light stimulus?
Rods have ON bipolar neurones. The lack of glutamate release triggers a depolarisation in the bipolar neurone, which is connected to the photoreceptor by an invaginating synapse. The BP cell will release glutamate onto the RGC which will depolarise and fire action potentials down the optic nerve [LIGHT DETECTION PATHWAY]
What are the two pathways for light stimulus in cones?
There is an ON bipolar pathway (invaginating synapse) and an OFF bipolar pathway (flat synpase)
What is the purpose of the two pathways for a light stimulus in cones?
Contrast detection and high light levels in the retina
What is the pathway in a cone OFF bipolar neurone?
When light hits the cone, glutamate release stops. The bipolar cell stops responding and hyperpolarises. It is connected to an off RGC which does not DP or send AP
How can the same stimulus (light) cause different effects in ON and OFF bipolar neurones?
ON bipolar neurones are metabotropic so that in the dark, glutamate activates the betagamma subunit which blocks the TRPM1 channel and prevents ion entry therefore the cell HP and no glut is sent to RCG
OFF bipolar neurones are ionotropic so that in the dark, the AMPA receptor is open due to glutamate release, allowing ion entry causing the BP to DP and send glutamate to the RCG
Then, in the light, the opposite will happen in each cell
Where are horizontal cells found?
Cell bodies in the inner nuclear layer and synapse with photoreceptors and bipolar in the outer plexiform layer
What is the function of horizontal cells?
Radial inhibition of cones (GABA)
Where are amacrine cells found?
Cell bodies in the inner nuclear layer and synapse in the inner plexiform layer. Interact with bipolar neurones and RGCs
What is the function of amacrine cells?
Some are inhibitory, some excitatory to BP or RGCs variety of neurotransmitters
What will a stimulus in the centre of ON RGC cause?
Stimulation
What will a stimulus in the periphery of ON RGC cause?
Inhibition
What will a stimulus in the centre of OFF RGC cause?
Inhibition
What will a stimulus in the periphery of OFF RGC cause?
Stimulation
Where is the bipolar: RGC synapse in the ON cone pathway?
Outer inner plexiform layer
Where is the bipolar: RGC synapse in the OFF cone pathway?
Inner inner plexiform layer
What do simple cells of V1 respond to?
A neurone with responses that display orientation and positional selectivity. This means that a simple cell fires at an optimal orientation orientation.
What do complex cells of V1 respond to?
Like simple, complex cells receptive fields are orientation selective. Complex cells do not respond to stationary stimuli. To produce a sustained response, the stimulus must be moving across the receptive field. They display directional selectivity, such that movement in only one direction produces an optimal response
What do hypercomplex cells of V1 respond to?
Hypercomplex cells are selective to orientation, motion, and direction and are a measure of length of stimuli (stop firing if too long)
What does area V4 respond to?
Colour
What does area V5 respond to?
External motion
What does area V6 respond to?
Self motion
What is blind sight?
Damage to V1 so that there is a loss of awareness in part of the visual stream however may still have a reflex response to things happening in that part of the visual field. Due to info processing by many areas which all communicate with each other
What are agnosias?
Damage to higher level cortical areas causing high level visual agnosias. In occipital temporal (ventral) region
