Ch. 17 - Special Senses Flashcards
What are the 4 steps in the process of sensation?
- stimulation of receptor
- transduction into graded potential
- generation of nerve impulse
- integration of sensory input
Where are receptors for smell found?
olfactory epithelium (that lines the inf surface of the cribiform plate of the ethmoid bone)
What are the 4 cells of the olfactory membrane?
olfactory receptors, supporting cells, basal (stem) cells, olfactory glands
Describe the structure of olfactory receptors
- bipolar neurons with olfactory hairs (cilia
- dendrite extends into nasal cavity
- axon goes through cribiform plate and joins other axons to form CN I
What is the function of non-motile cilia? Where are they found?
contain olf receptor proteins and act as sites of transduction; on dendrite side
What are the 5 steps in the physiology of olfaction?
- odorants bind to olf receptor cells
- Na+/Ca+ channels open and ions enter
- depolarization occurs, generates epsp
- triggers nerve impulse
- rapid adaptation
What do olf receptor cells (first-order neurons) synapse with and where?
mitral cells (second-order) in the olfactory bulb
What do second-order neurons form?
olfactory tract that synapses on primary olfactory area of temporal lobe
What is a distinct feature of the olfactory pathway?
only special sense where sensory info does not relay in thalamus, and goes directly to cortex
Aside from the primary olfactory area, where else do the axons reach?
- limbic system and hypothalamus (emotional/memory-evoked responses)
- orbitofrontal cortex (identify/differentiate odours)
What are the 5 classes of tastants?
sour, sweet, bitter, salty, umami (savoury)
Where are taste buds located?
papillae on tongue
What are the 3 types of papillae that contain taste buds? Where are they located?
- vallate papillate (back of tongue)
- fungiform papillae (over entire tongue)
- foliate papillae (in lateral trenches, degenerate during early childhood)
What is the 4th type of papillae and what are its features?
filiform papillae (entire tongue surface)
- no taste buds; contain tactile receptors
- increase friction to facilitate movement of food within mouth
What are the 3 types of cells in taste buds?
- gustatory receptor cells (contain microvilli)
- supporting cells
- basal stem cells
What is the physiology of gustation?
dissolved tastants bind to receptors on gustatory microvilli (site of transduction)
receptor potential in gustatory cell –> NT release –> AP in first-order neuron
How does transduction occur for salty tastes?
high in NaCl –> stimulates opening of Na+ to trigger depolarization
How does transduction occur for sour tastes?
high in H+ (acidic) –> causes H+ channels to open to trigger depolarization
In which cranial nerves are first-order gustatory fibers located?
CN VII, IX, X
Where do taste impulses from cranial nerves synapse?
gustatory nucleus in medulla
Aside from the medulla, where else do taste impulses travel to?
- limbic system and hypothalamus
- thalamus –> primary gustatory area of cerebral cortex
What is the function of eyelids?
- lacrimal caruncle contains sebaceous and sudoriferous glands
- protect and lubricate eye
- contains levator palpebrae superioris muscle to elevate upper eyelid
What are other accessory structures of the eye?
extrinsic eye muscles, conjunctiva, tarsal plate
What are the 3 tunics (coats) of the eyeball?
- fibrous tunic (cornea, sclera)
- vascular tunic (choroid, ciliary body, iris)
- inner tunic (retina)
What is the function of the cornea?
helps focus light to retina, receives O2
Where are the 3 layers of the cornea superficial to deep?
- nonK stratified squamous epithelium
- collagen fibers + fibroblasts
- simple squamous epithelium
What is the function of the sclera?
dense irregular CT that gives shape to eyeball
- site of attachment for extrinsic eye muscles
What is the function of the choroid?
highly vascular, provides nutrients to retina
- pigments in melanocytes absorb scattered light
What is the function of the ciliary processes?
secrete aqueous humour
What is the function of the ciliary muscle?
alters shape of lens for near/far vision; holds lens in place so light must pass through before reaching retina
Where is the iris suspended between?
between cornea and lens; attached to ciliary processes at outer margin
What is the function of the iris?
regulates amount of light entering eye
Which pupillary muscles constrict/dilate?
constrict - circular muscles (parasym)
dilate - radial muscles (sym)
What is the macula lutea?
center of retina where light hits
What is the central fovea?
small depression in center of macula lutea, where most cone receptors lie
What is the blind spot?
in optic disc, where there are no cones/rods (photoreceptors)
What are the 3 layers of retinal cells?
- photoreceptors
- bipolar cell layer
- ganglion cell layer (first-order)
Describe features of anterior cavity
- anterior chamber bt cornea and iris
- posterior chamber bt iris and lens
- both chambers are filled with aqueous humpur
What are the 3 interior cavities of the eyeball?
- anterior cavity
- vitreous chamber
- lens
What is the function of the vitreous chamber? Where is it?
filled with vitreous body (jelly) and holds retina against choroid; contains phagocytic cells
- betweens lens and retina
What is the function of the lens?
focuses light on retina
Describe features of the lens
held in place by zonular fibers and attach to ciliary processes; avascular; enclosed in transparent CT capsule
What are the 3 major processes of image formation?
- refraction of light
- accommodation of lens
- constriction of pupil
What is refraction?
bending of light as it passes from one substance (e.g. air) to another substance with different density (e.g. cornea)
What occurs in refraction?
bends lights rays to focus on retina; 75% done by cornea; 25% done by lens
Describe refraction of light from distant objects
light rays are nearly parallel, only need to be bent to focus on central fovea
Describe refraction of light from nearby objects
light rays are more divergent, need to be more refracted to be focused via accommodation
What is accommodation?
curving of lens for near vision caused by contraction of ciliary muscle
How does contraction of the ciliary muscle lead to accommodation?
–> decreases tension of zonular fibers pulling on lens –> lens becomes more spherical –> increases refraction
If eyeballs/lens are too long/thick…
image focuses before retina –> myopia
If eyeballs/lens are too short/thin…
image focuses behind retina -> hyperopia
How is pupillary constriction involved with accommodation?
prevents light rays from entering eye through edge lens; sharpens vision
Describe features of rods
- for b&w vision in dim light
- detects shapes and movement
- along periphery of retina
Describe features of cones
- for sharp colour vision
- detect blue, green, red light
- fovea of macual lutea –> visual acuity
What does the proximal end of photoreceptors do?
synapse with bipolar cells, where NT are released and detected by bipolar cells
What are photopigments?
integral membrane proteins in plasma membrane of photoreceptor cells consisting of opsin and retinal
What is the function of photopigments?
absorb light; undergoes structural change when light is absorbed and leads to receptor potential
What are the steps in responding to light?
- isomerization
- bleaching
- retinal isomerase
- regeneration
What occurs in isomerization?
light causes retinal to change shape; leads to graded potential
What occurs in bleaching?
retinal separates from opsin; photopigment is now non-responsive to light
What occurs in retinal isomerase?
bends retinal back to original shape
What occurs in regeneration?
retinal binds to photopigment again
Describe daytime vision
- rods contribute little due to slow regeneration
- cones regenerate quickly
Describe dark adaptation
- rod photopigments regenerate and respond to light after exposure to darkness
- more sensitive than cones
- can perceive dim light
Describe formation of receptor potentials in complete darkness
- Na+ channels are always open; photoreceptor is always partially depolarized (-30 mV)
- continuous release of inhibitory NT onto bipolar cells
- IPSPs hyperpolarize bipolar cells and prevent signalling to ganglion cells
Describe formation of receptor potentials in light
- retinal changes shape –> Na+ channels close
- hyperpolarizing receptor potential (-65 mV)
- stops release of inhibitory NT onto bipolar cells
- bipolar cells become excited and activate first-order neurons
7 steps of processing visual input
- light penetrates retina
- rods and cones transduce light into receptor potentials
- rods and cones excite bipolar cells
- bipolar cells excite ganglion cells
- ganglion cells form optic nerve, leave eyeball (blind spot)
- optic nerves cross at optic chiasm
- to thalamus –> prim visual area of cortex
How is convergence related to light sensitivity?
600 rods synapse on a single bipolar cell, increasing light sensitivity, but producing a slightly blurry image
How is convergence related to visual acuity?
one cone synapses onto one bipolar cell, produces visual acuity (not as sensitive)
Aside from the thalamus/cortex, where else does visual info in the optic nerve travel to?
- hypothalamus (circadian rhythms)
- brainstem (pupil size, coordination of head/eye movements)
What is a visual field and how is it divided?
what an eye sees; nasal and temporal halves
How do the two visual field halves relate to each other?
- light rays from object in nasal half of visual field land on retina’s temporal half (and vice versa)
i. e. crossing over of rays when they enter eye - fibers from nasal half of each retina cross in optic chiasm
- info from R half of each visual field goes to L side of brain (and vice versa)
Describe pathway for the left eye from nasal half of each retina
- fibers from nasal half cross in optic chiasm
- temporal half of VF
- nasal half of retina
- cross
- R occipital lobe
Describe pathway for left eye from temporal half of each retina
- do not cross
1. nasal half of VF
2. temporal half of retina
3. L occipital lobe
Describe pathway for right eye from nasal half of each retina
- fibers from nasal half cross in optic chiasm
- temporal half of VF
- nasal half of retina
- cross
- L occipital lobe
Describe pathway for right eye from temporal half of each retina
- do not cross
1. nasal half of VF
2. temporal half of retina
3. R occipital lobe
In summary, describe crossing stimuli at visual field
- peripheral stimuli cross at optic chiasm
- central stimuli do not cross at optic chiasm
What is retinal detachment and what can it cause?
- detachment of neural portion of retina from pigmented epithelium; fluid accumulates bt layers
- causes distorted vision and blindness in corresponding field of vision
What is a cataract and what can it lead to?
loss of transparency of lens; blindness
What is glaucoma?
abnormally high intraocular pressure from buildup of aqueous humour inside eyeball, destroying neurons of retina
What does the external ear contain?
auricle, external auditory canal, tympanic membrane
What is the function of the auricle?
elastic cartilage that captures soundwaves
What is the function of the external auditory canal?
transmits sound to the eardrum
What is the function of the tympanic membrane?
vibrates in response to sound waves; transmits sound to middle ear
What are the 3 auditory ossicles and what are their function?
malleus, incus, and stapes; transmit vibrations from eardrum to oval window
What is the function of the auditory tube?
leads to nasopharynx; helps equalize pressure on both sides of eardrum
What are 2 muscles of the middle ear that help protect the ear from loud noises?
- stapedius muscle
2. tensor tympani muscle
What are the 2 divisions of the inner ear (labyrinth) ? What is each division filled with?
- outer bony labyrinth; perilymph
2. inner membranous labyrinth; endolymph
How is the inner ear divided based on shape? What do each of these 3 areas function?
- semicircular canals; contain equilibrium receptors
- vestibule; contains equilibrium receptors
- cochlea; contains hearing receptors
What are the 3 divisions of the cochlea? (fluid-filled channels)
- scala vestibuli (contains perilymph)
- scala timpani (contains perilymph)
- cochlear duct (contains endolymph)
What are the membranes that separate the 3 fluid-filled channels in the cochlea?
- vestibular membrane: separates cochlear duct from scala vestibuli
- basilar membrane: separates cochlear duct from scala tympani
What is the spiral organ and where does it lie?
contains receptors for hearing (hair cells); rests on basilar membrane
- microvilli contact the tectorial membrane
- basal sides of inner hair cells synapse with CN VIII
What is the tectorial membrane?
flexible and gelatinous membrane over spiral organ
What do frequency and amplitude respectively determine?
F - pitch of sound
A - volume/intensity of sound
What are the 8 steps in the physiology of hearing?
- auricle directs sound waves to ext auditory canal
- eardrum vibrates
- vibrations of eardrum are transmitted to ossicles
- stapes makes oval window vibrate in and out of inner ear
- fluid pressure waves develop in perilymph of scala vestibuli in cochlea
- pressure waves transmitted from scala vestibuli to scala tympani through helicotrema, to round window
- waves push vest membrane back and forth, creating pressure waves in endolymph inside cochlear duct
- waves in endolymph cause basilar membrane to vibrate –> hair cells of spiral organ move against tec membrane –> stereocilia and cause K+ influx –> receptor potential generated –> APs in CN VIII
How do hair cells function in hearing?
- bending of stereocilia convert mech deformation to electrical signals
- stretching causes mech-gated K+ channels to open and K+ enter
- hair cells depolarize
- voltage-gated Ca2+ channels open
- NT released onto first-order neuron
Where do the axons of CN VIIi travel to?
medulla, pons, inferior colliculi of midbrain, thalamus –> primary auditory area of cerebral cortex
What are the 2 sets of equilibrium receptors?
- macula receptors within saccule and utricle
2. crista receptors within ampulla of semicircular ducts
What is the function of macula receptors?
maintain position of the body/head relative to force of gravity; detect linear acceleration/deceleration
What is the function of crista receptors?
maintain position of the body/head during rotational acceleration or deceleration
What are the utricle and succule?
sacs of membranous labyrinth
What is the macula?
specialized region in walls of utricle and saccule to sense head tilts and linear movement
What are the cell types within the macula?
- hair cells with stereocilia
- supporting cells that secrete otolithic membrane (secrete gelatinous layer)
What does the otolithic membrane do and what does it contain?
- covers hair cells
- contains crystals that move when head moves
What is the pathway behind the vestibular apparatus?
- head movement causes movement of otolithic membrane
- stereocilia bend
- depolarization of hair cells
- release NT to vest branch of CN VIII
What is the function of semicircular ducts?
positioning permits detection of rotational acceleration and deceleration
What is the ampulla?
dilated portion of each semicircular duct; contains elevated portion for sensing rotational movement (crista)
What are the hair cells within crista covered by?
cupula (gelatinous material)
Describe the pathway behind semicircular ducts
- as head moves, semicircular ducts and hair cells move with it
- cupula bends –> hair cells bend
- depolarization
- NT release onto first-order neurons forming vest branch of CN VIII
Where do axons travel to in the equilibrium pathway?
medulla, pons, cerebellum, CN III/IV/VI/XI, motor areas of cerebral cortex
