Visual, Hearing, Vestibular system Flashcards
What protects the eye?
- Bony socket in skull
- Eyelids: shutters to protect from environmental hazards
- Eyelashes: trap fine, airborne debris before falling into the eye
- Tears: lacrimal glands and conjunctiva lubricate, cleanse and act as bacteriocide.
What are the three tissue layers that enclose the eye?
- Sclera/cornea,
- choroid/ciliarybody/iris
- Retina
What is the sclera?
Tough outer layer of connective tissue, forms visible white part of the eye
What is the cornea?
Anterior, transparent layer continuous with sclera that allows passage of light rays
What is the choroid?
Middle layer underneath sclera which contains blood vessels that nourish retina and contain melanin under retina for absorption of light
What is the ciliary body?
Continuous tissue of muscle from choroid layer that controls the curving and flattening of the lens.
what is the retina?
Innermost layer under choroid consisting of outer pigment cells, rods and cones, and axons of visual nerve fibres.
What are the two fluid filled cavities separated by the lens?
- Vitreous humor: gelatinous, between lens and retina
- Aqueous humor: fluid, between cornea and lens
What is the iris?
Contains circular and radial smooth muscle that control the amount of light entering the eye.
What is the pupil?
Opening through which light enters the eye
What is the lens?
Part of eye that bends light to focus it on the retina.
What is the tapetum lucidum?
Additional reflective layer within the choroid that reflects light back towards retina. It improves sensitivity of vision under low light but causes blurriness in high light.
Why do we change pupil size?
Control of light for optimization of light and dark conditions and providing a wide dynamic range.
What is the relationship between depth of field and pupil size?
Small aperture (pupil size) = large depth of field
Large aperture (pupil size) = small depth of field
What causes pupil size to change?
Parasympathetic stimulation contracts circular muscle, constricting pupil
Sympathetic stimulation contracts radial muscle, dilating pupil
How does the lens work?
Light from distant source must enter parallel for the lens to focus the image on the retina. If the image is close, the light is not parallel and the lens must become strong and round to keep it in focus.
What is accomodation?
Change in strength of lens to focus on an image, regulated by effect of parasympathetic branch of autonomic nervous system on ciliary muscle.
What is the shape of the lens as it acommodates near and far images?
When ciliary muscle is relaxed, ligaments pull on and flatten the lens. When ciliary muscle contracts, ligaments release tension and brings lens back to its relaxed rounded shape.
What is myopia, hyperopia, and amblyopia?
Myopia: Near sightedness when the focal point falls in front of the retina.
Hyperopia: Far sightedness when the focal point falls behind the retina
Amblyopia: weakness of ocular muscles despite normal development of visual cortex (adult) or lack of development (children)
What is the order of organization of the retina from closest to the lens to furthest?
- Fibers of optic nerve + ganglion cell: travel to thalamus
- Amacrine cell: interneuron
- Bipolar cell: connect retinal cell (rod/cone) to to ganglia
- horizontal cell: interneuron
- Cones and Rods
What is the difference between rods and cones in terms of amount, type of vision, sensitivity, convergence and location?
Rods:
1. more
2. grayscale
3. high sensitivity (collect more light), low acuity
4. more convergence
5. peripheral retina
Cones:
1. Fewer
2. colour vision
3. low sensitivity (collect less light), high acuity
4. Less convergence
5. fovea
What is the macula?
Area of high concentration of cones where images are focused to ensure highest resolution
What is the fovea?
Area where light strikes photoreceptors directly, containing the highest density of cones. Single photoreceptors synapse with bipolar neurons that synapse onto single ganglion cells.
What are the steps of phototransduction?
- Light bleaches rhodopsin, turning cis-retinal inside opsin to trans-retinal, activating opsin.
- Activated rhodopsin activates G protein transducin
- Transducin activates phosphodiesterase (PD) that degrades cGMP
- Decrease in cGMP causes Cyclic nucleotide gated Na+/Ca++ channels to close as it would have acted as the ligand.
- Closing Na+/Ca++ and continually leaking K+ hyperpolarizes the cell
- Hyperpolarization causes less release of neurotransmitter onto bipolar cells
What is the information flow in the visual pathway?
- In the retina, photoreceptors synapse with bipolar cells which sends signals to ganglion cells and optic nerves.
- Optic nerves on the right side of each eye bring light from the left side of each eye to the optic chiasm where they join and synapse at the lateral geniculate nucleus of thalamus
- Neurons form lateral geniculate nucleus of thalamus synapses at the occipital lobe of cortex as well as the midbrian which controls pupilary constriction
What is hearing?
Conversion of mechanical energy of soundwaves to electrical energy. Includes interpretation of frequency (pitch) and amplitude (loudness) of sound waves, as well as distance, direction and movement.
What are the 3 parts of the ear?
External ear, middle ear and inner ear.
What are the parts of the external ear?
Pinna/Auricle: Elastic cartilage covered with skin
Meatus: external auditory canal
Tympanic membrane
What are the parts in the middle ear?
Eustachian tube
Ossicles: malleus (against tympanic membrane), incus, stapes (against oval window)
What are the parts of the inner ear?
Bony labyrinth: tunnels in temporal bone
Membranous labyrinth: fluid filled organ
What are the three parts of the membranous labyrinth?
- Cochlear duct + organ of corti
- Three Semicircular ducts, each with one crista ampullaris
- Utricle/Saccule inside vestibule with one macula each
What are the steps of sound transmisison?
- Sound wave strikes tympanic membrane and becomes vibrations
- Tympanic membrane vibrates the three bones in the middle ear
- Stapes vibrates to attached oval window membrane, creating fluid waves within the cochlea
- Fluid waves push on flexible membranes of cochlear duct, bending hair cells and opening mechanically gated ion channels
- Opening of channels alter neurotransmitter release onto sensory neurons, which creates action potentials travelling through the cochlear nerve to the brain
- Energy from waves transfers across cochlear duct into the tympanic duct and is dissipated back into the middle ear via round window
What are the parts of the cochlea?
Oval window: where vibrations from stirrup originate
Vestibular duct: contain perilymph that creates waves when in contact with vibrations; surrounded by vestibular membrane
Cochlear duct: contains endolymph secreted by epithelial cells; contains hair cells on organ of corti
Tectorial membrane: flap that covers hair cells of organ of corti
Tympanic duct: contains perilymph that passes waves from vestibular duct to round window, lined with basilar membrane
What is the structure of a hair cell?
Modified neuron with stereocilia connected by tiplinks. Contain special K+ channel proteins that are mechanically gated.
What is the pathway of excitation of a hair cell?
- At sound vibrations, hair cells bend in towards kinocilium (tallest stereocilium)
- Bending opens mechanically gated K+ channels
- Endolymph [K+] > Hair cell [K+], K+ flows inside and depolarizes cell, increasing action potential frequency in the associated sensory neuron
What is the pathway of inhibition in a hair cell?
- Hair cells bend away from kinocilium (tallest stereocilium)
- Bending closes mechanically gated K+ channels
- Cell hyperpolarizes and sensory neuron signalling decreases
What is basilar membrane displacement relationship with pitch?
Basilar membrane will be more sensitive to low pitch near the round window, and high pitch near the helicotrema.
How does each ear synapse in the brain?
- Primary sensory neurons send axons via vestibulocochlear nerve to cochlear nucleus in the medulla
- Cochlear nucleus goes to both sides of the superior olivary nucleus in the medulla for sound processing and to cerebellum for sense of balance
- From the olivary nucleus in the medulla, it synapses to inferior colliculus in the midbrain
- Inferior colliculus to thalamus
- Thalamus to auditory cortex
What are the three types of hearing loss?
Conductive: easily reversible, damage to eardrum or ear infection
Sensorineural: damage to hair cells and primary neurons
central: not easily reversible, damage to auditory nerve or auditory cortex
What are the parts of the vestibular apparatus?
Otolith organs in vestibula: includes macula (sensor for balance) inside saccula and utricle (linear acceleration + head position)
Three semicircular canals: sensitive to rotational acceleration, include crista ampullaris.
How does the crista ampullaris work?
Movement of endolymph in cochlear duct pushes on gelatinous cupula, activating hair cells, providing info about rotation of head.
How does the macula in saccule and utricle work?
In neutral position, otoliths within macula creates gravitational pressure onto gelatinous otolith membrane, moving along with head/gravity and stimulating hair cells.
where do hair cells in vestibular apparatus synapse?
- Hair cells synapse with neurons of vestibular nerve
- Vestibular nerve synapses with cerebellum or to vestibular nuclei in medulla to control eye movement
- From medulla it synapses with midbrain, then thalamus then cortex
