Vision and Hearing Flashcards
What are the features of the eyes surface anatomy?
- Eyelids (palpebra)
- Sclera (white part)
- Sclerocorneal junction (where sclera meets the cornea)
- Pupil
- Iris
What are the muscles involved in the eye?
-Levator palpebrae superioris (lifts up the upper eyelid)
-Recti muscles:
Superior (above the eye)
Lateral (further away from the body
Medial (closer to the body)
Inferior (underneath)
-Obliques:
Superior (comes out the back and hooks through a bone hook above eye)
Inferior (Comes from underneath the eye)
What are saccades?
Intentional eye movements
How are each of the muscles innervated to cause eye movement?
3 Cranial nerves cause innervation
CN III (oculomotor nerve):
- Superior rectus
- Inferior rectus
- Medial rectus
- Inferior Oblique
- Levator Palpebrae superioris
CN IV (trochlear): -Superior oblique
CN VI (abducens): -Lateral rectus
What are the types of fixational eye movements (FEM’s)?
- Microsaccades (snap it back to the centre to keep image at highest part of sensitivity in the eye)
- Drifts (move image across the retina)
- Tremors (Ignore)
FEM’s prevent fading of the field of vision
What is the anatomy of the anterior chamber of the eye? (front of the eye)
- Cornea (transparent membrane allowing light to enter)
- Anterior chamber filled with aqueous humor to bring nutrients into the eye
- Iris (coloured opaque muscle that regulates light entry)
- Ciliary body (changes lens shape)
What is the anatomy of the posterior chamber? (the back of the eye from lens back)
- Posterior chamber is filled with vitreous humor
- Fovea (point at which there is the highest sensitivity of light due to highest concentration of photoreceptors)
- Retina (photoreceptor layer)
- Choroid layer (light-absobing pigment prevents glare)
- Sclera (protection)
- Optic disc (where the optic nerve meets the eye and there is a blind spot due to no photoreceptors)
- Optic nerve (axons projecting to visual cortex)
Where is the most concentrated area of cones in the eye?
Fovea (also where lowest concentration of rod cells are present except blind spot)
What is the macula?
Large area surrounding where the fovea is located
What is acommodation?
Where the shape of the lens changes in order to better focus on an object
How does the lens adjust when the object is far away? (greater than 6m)
- The ciliary muscles are relaxed causeing the suspensory ligaments to be tightened.
- This causes the lens shape to be flat so light refraction is small
-If an eye can’t do this then they are near-sighted (myopia) which can be corrected with a biconcave lens to move rays further apart
How does the lens adjust when the object is near the eye? (less than 6m away)
- The ciliary body is pulled forward and inward which causes the suspensory ligaments to relax allowing the lens to become more round which causes large light refraction
- If you are unable to do this you are far sighted (hyperopia) and it can be fixed with a boconvex lens to move the rays closer together
How do photoreceptors create an impulse?
When its dark the Na+ channels are open and the Na+/K+ ATP-ase pump is active. This balances out the molecules and keeps the cell at baseline depolarised (-40mV)
When light is present, the pigment changes shape and blocks the Na+ channels so they close. This causes a build up of positive ions in the synapse so the cell becomes hyperpolarised (more -ve). This prevents glutamate from being released
What are the photosensitive proteins called?
Opsins (rhodopsin/conopsin)
-Contain Vitamin A
How do rhodopsin cause a hyperpolarisation?
- Rhodopsin is sensitive to approx500nm wavelength (takes the lowest intensity of green light to be activated)
- The rhodopsin is turned into metarhodopsin which creates a shape change which closes the Na+ channel
How is hyperpolarisation acheived in cone cells?
three different cones have three opsins:
- S Cones (420nm so blue light)
- M Cones (534nm so green light)
- L Cones (564nm so red light)
What are the features of cone cells?
- 6 million per eye
- located mostly in Fovea
- Can see Colour
- Higher resolution due to 1-to-1 with ganglion cells
What are the features of rod cells?
- 120million per eye
- Periphery
- Monochrome
- Low resolution due to many to 1 with ganglion cells
What occurs to the image before reaching the optic nerve?
- Horizontal Cells that inhibit adjacent cells with GABA (lateral inhibtion)
- Bipolar cells form a modified 2nd image
- Amacrine cells form a modified third image
- Ganglion cells then produce action potentials that project along the optic nerve to the CNS
What are common problems that can occur in the eye?
Glaucoma - fluid outflow is blocked causing pressure to build which constricts the optic nerve and artery which causes the loss of the peripheral visual field
Macular degeneration - If the eye stops clearing debris properly, drusen can build up that can block of damage photoreceptors in the fovea causing loss of central visual field
What happens after the AP has left the eye through the optic nerve?
- The projections from each eye switch sides at the optic chiasm
- The optic tract projects to the LGN in the thalamus with some projections to the hypothalamus
- The optic radiations then project to the visual cortex at the back of the brain
What is retinotopy?
Mapping the visual field onto the cortex so by seeing if an area of the visual cortex is damaged it can be directly linked to an area on the eye that is damaged
How is the visual system adapted to light conditions?
When in the dark:
- cones adapt in less than 10minutes but not suited to dark so low sensitivity
- rods adapt in about 30minutes but will increase hugely in sensitivity by increasing the amount of rhodopsin produced
The reverse happens in light but its much quickers with both taking around 5mins to adapt
How is sound transmitted?
Disturbance in the air through compression and rarefraction
How does the human ear detect changes in the pressure?
Use of mechanoreceptors that can detect frequencys between 20Hz-20,000Hz
-trained ears can hear around 400,000 sounds
What is amplitude?
The size of the peak and trough. Directly proportional to the loudness
What is the frequency?
The amount of waves per second. Directly proportional to the pitch
What is the regularity?
The amount of sound waves together at one time. Low regularity gives a greater tonal quality
What is the doppler effect?
As the car moves towards you, the sound waves become compressed so the pitch increases.
As the car moves away from you, the sound waves become stretched so the pitch decreases.
What are the components of the anatomy of the outer ear?
Pinna - funnels sound into the ear
External auditory meatus - technical term for ear canal
Temporal bone - the sound travels through the bone and into the tympanic membrane
Tympanic membrane- seperation between the outer ear and the middle ear (also known as the ear drum)
What are the components of the anatomy of the middle ear?
Auditory ossicles - 3 bones in the ear that act as a lever (Malleus, Incus, Stapes). As the tympanic membrane extends, it causes the malleus to move causing the oval window to be hit
Tensor tympani & stapedius - middle ear muscles that contract to prevent damage by loud noise to dampen the sound
Eustacian tube - equalises pressure between the outer and middle ear (shown with a pop in pressure on airplanes)
What are the components of the anatomy of the inner ear?
Semicircular canals - sense equilibrium
Cochlea - contains mechanoreceptors for hearing
Round window - membrane that descends back into the middle ear
Vestibulocochlear nerve - sends action potentials along to the CNS
What are the sections of the cochlea?
Vestibular duct - where the oval window presses against, contains a liquid called perilymph. The vibrations will travel along the whole duct and then follow into the tympanic duct
Tympanic duct - where the vibrations will follow along to the round window where the sound is absorbed to prevent an echo through a reflection
Cochlear duct - where the organ of corti lies
What is the relationship between the frequency of the waves and the distance travelled in the cochlea?
Inversely proportional so the higher the frequency, the shorter the distance travelled in the cochlea so the sound will enter the cochlea duct earlier on.
What are the two membranes between the 3 sections of the cochlea?
Vestibular membrane (Vestibular duct & Cochlea duct) Basilar membrane which is shaped like a divers flipper (Tympanic duct and Cochlea duct), the organ of corti are on the basilar membrane where the mechanoreceptors are present
How is the pitch determined?
Which hair cells are affected along the basilar membrane
How is the loudness determined?
It is proportional to the frequency of action potentials in hair cells
How is sound location determined?
Time difference in reaching the two ears (low pitch sound) and the difference in intensity (high pitch sound)
What are the different types of hair cells?
Inner (free) hair cells: 3,500 cells with 95% of nerve endings so are most important for hearing
Outer (attached) hair cells:
20,000cells with 5% of nerve endings to improve tonal quality and amplify sound
How do the hair cells work?
Operate through mechanotransduction (fluid movement):
- Fluid vibrates
- Hairs bend
- K+ influx from endolymph
- Cell depolarisation
- Receptor potential
- Voltage Gated Calcium Channels open
- Neurotransmitter release towards synapse
- Action potentials continue along the afferent auditory nerve
What happens to the AP after leaving the ear?
- Travel along the afferent auditory nerve
- Join other nerves from the vestibular system to form Vestibulocochlear nerve
- To the brainstem where the some information switches sides
- Through the inferiour colliculus and into the thalamus
- Thalamus directs impulse to auditory cortex
Lateral inhibition is always occurring to sharpen the sound
Where is the auditory cortex in the brain?
Located on the each side of the brain equidistant the centre
Damage to this cortex can result in hearing loss if only one side is damaged
Damage to both sides can lead to cortical deafness
What are the three major types of deafness?
- Cortical deafness (damage to auditory cortex)
- Conduction deafness (impaired transmission in the outer/middle ear)
- Nerve deafness (damage to hair cells or nerves in inner ear)
What is tinnitus?
When you hear a sound but there is no sound present
Subjective tinnitus - damage to hair cells
Objective tinnitus - caused by myoclonus (muscles in the ear twitching)
