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The Special Senses

The Special Senses of the Human Body
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Special senses
-have receptors strategically placed in unique organs
-include: olfaction, taste, visual system, hearing & balance
Olfaction key points
-7 primary odors now recognized, but average person can recognize 4000 different odors. perceived by olfactory epithelium.
-dendrites of olfactory neurons have enlarged ends (olfactory vesicles)
-cilia (olfactory hairs) of olfactory neuron embedded in mucus. odorants dissolve in mucus.
Olfaction key points 2
-odorants attach to receptors, cilia depolarize & initiate action potentials in olfactory neurons. one receptor may respond to more than one type of odor.
-olfactory epithelium replaced as it wears down. olfactory neurons replaced by basal cells every 2 months. unique: most neurons are permanent cells (aren't replaced if they die).
Neuronal Pathways of Olfaction
-olfactory sensory pathway: olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs & synapse w/ tufted cells or mitral cells
-these extend to olfactory tract & synapse w/ association neurons
-association neurons also receive input from brain, so info can be modified before it reaches the brain.
Neuronal Pathways of Olfaction 2
-info goes to olfactory cortex of frontal lobe w/out going through thalamus (only major sense that does not go through thalamus).
-3 regions in frontal lobe affect conscious perception of smell & interact w/ limbic system
-lateral, medial, & intermediate olfactory areas
Lateral olfactory area
conscious perception of smell
Medial olfactory area
visceral & emotional reactions to odors
Intermediate olfactory area
effect modification of incoming info
Taste
Gustatory
Taste buds
-the sensory structures that detect taste
-includes supporting cells surrounding taste (gustatory) cells
-taste cells have microvilli (gustatory hairs) extending into taste pores
-replaced about every 10 days
Papillae
-specialized regions on the tongue
-types: filiform, vallate, fungiform, & foliate
Filiform
filament-shaped; provide a rough surface for food manipulation
Vallate
largest, least numerous. 8-12 in V along border between anterior & posterior parts of tongue. have taste buds.
Fungiform
mushroom-shaped. scattered irregularly over the superior surface of tongue. look like small red dots interspersed among the filiform. have taste buds.
Foliate
leaf-shaped. in folds on the sides of the tongue. contain most sensitive taste buds. decrease in number w/ age.
Tastants
-substances which are dissolved in saliva, enter the taste pores.
-by various mechanism (depending on the taste), tastants cause the taste cells to depolarize.
Taste types
sour, salty, bitter, sweet, umami
Sour
most sensitive receptors on lateral aspects of the tongue. H+ ion of acids cause depolarization.
Salty
most sensitive receptors on tip of tongue. shares lowest sensitivity w/ sweet. anything w/ Na+ causes depolarization.
Bitter
most sensitive receptors on posterior aspect. highest sensitivity. sensation produced by alkaloids, which are toxic.
Sweet
most sensitive receptors on tip of tongue. shares lowest sensitivity w/ salty. sugars, some carbohydrates, & some proteins (NutraSweet: aspartame).
Umami
"Savory"; scattered sensitivity. caused by amino acids such as glutamate, binding to receptors
Taste 2
-texture affects the perception of taste
-temperature affects taste perception
-very rapid adaptation, both at level of taste bud & within CNS
-taste influenced by olfaction
-different tastes have different thresholds w/ bitter being the taste to which we are most sensitive. many alkaloids (bitter) are poisonous
The Vertebrate Eye
cornea, choroid, iris, aqueous humor, vitreous humor, lens, retina, macula lutea, fovea, conjunctiva
cornea
transparent, admits light
choroid
is the vascular layer of the eye, provides oxygen & nourishment to the outer layers of the retina
iris
-behind the cornea
-controls diameter of pupil
-regulates amount of light that strikes the lens
aqueous humor
a clear, gelatinous fluid in front chamber of eye. maintains the intraocular pressure & inflates the globe of the eye.
vitreous humor
clear gel that fills the space between the lens & the retina of the eyeball; helps hold the eye in place
lens
focuses image on the retina
retina
-lines the back of the eye
-photoreceptors and neurons integrate info detected by photoreceptors
macula lutea
is an oval-shaped highly pigmented yellow spot near the center of the retina
fovea
found in the macula lutea; provides highest resolution vision; contains a lot of cone cells (for color vision)
conjunctiva
thin transparent mucous membrane
three tunics
(tunic=layer):
fibrous: sclera & cornea (outer layer)
vascular: choroid, ciliary body, iris (middle)
nervous: retina
fibrous tunic
contains sclera & cornea
sclera
white outer layer. maintains shape, protects internal structures, provides muscle attachment point, continuous w/ cornea
cornea
connective tissue matrix containing collagen, elastic fibers, & proteoglycans
-avascular, transparent, allows light to enter eye; bends & refracts light
vascular tunic
middle layer. contains most of blood vessels of eye: branches off the internal carotid arteries. contains melanin.
-contains iris, ciliary body, & choroid
iris
colored part of eye. controls light entering pupil. smooth muscle determines size of pupil.
-sphincter pupillae: circular smooth muscle; contraction=decrease pupil size (or constricts)
-dilator pupillae: radial smooth muscle; contraction=dilation of pupil
ciliary body
produces aqueous humor that fills anterior chamber
-ciliary muscles: smooth muscle that controls lens shape
choroid
associated w/ sclera. very thin, pigmented. is vascularized.
nervous tunic
2 layers: pigmented & sensory retina
pigmented retina
outer, pigmented layer. pigment of this layer & choroid help to separate sensory cells & reduce light scattering.
sensory retina
inner layer of rod & cone cells sensitive to light
anterior compartment
-anterior to lens; filled w/ aqueous humor
-contains 3 chambers of eye
3 chambers of eye
anterior, posterior & vitreous
anterior chamber
between cornea & iris
posterior chamber
-between iris & lens
-helps maintain intraocular pressure; supplies nutrients to structures bathed by it; contributes to refraction of light
glaucoma
abnormal increase in intraocular pressure
vitreous chamber
posterior to lens. filled w/ jelly-like vitreous humor. helps maintain intraocular pressure, holds lens & retina in place, refracts light.
the lens
-held by suspensory ligaments attached to ciliary muscles. changes shape as ciliary muscles contract & relax
-transparent, biconvex
visible light
portion of electromagnetic spectrum detected by human eye
refraction
bending of light
convergence
light striking a convex surface
focal point
-point where light rays converge & cross
-lens changes shape causing adjustment of focal point on the retina
focusing
causing light to converge
near point of vision
closer than 20 feet. changes occur in lens, size of pupil, & distance between pupils.
accommodation
ciliary muscles contract due to parasympathetic input via cranial nerve III. pulls choroid toward lens reducing tension on suspensory ligaments. lens becomes more spherical, greater refraction of light.
pupil constriction
varies depth of focus
convergence
as objects move close to the eye, eyes are rotated medially. reflex contraction of the medial rectus muscles.
how the eye processes light
involves several cells, especially photoreceptor cells called rods & cones
Amacrine & horizontal cells
help integrate & regulate the input from multiple photoreceptor cells (eg. horizontal cells are responsible for allowing eyes to adjust to see well under both bright & dim light conditions)
Rods
specialized for detection of low-intensity light
Cones
specialized for detecting light of different wavelengths (colors)
The Photoreceptor cells of the Eye
(called rods & cones)...
-contain photoreceptive molecules that
-absorb energy of light
-generate changes in membrane potential
-in rods, photopigment is rhodopsin
-in cones, photopigment is iodopsin
Photoreceptive Molecules
(such as rhodopsin)
-absorb light in photoreceptor cells
-consist of retinal combined w/ an opsin protein
-there are 3 types of photopigments in cones (called photopsins)
-photopigments are found in the discs of photoreceptor cells (that is, the rods & cones)
Rhodopsin=
retinal+opsin=photoreceptive molecule found in rods; is a G-protein coupled receptor
In the rod photoreceptor cell: steps 1-4
1. cis-Retinal absorbs light; is converted to trans-retinal
2. the protein segment (opsin) of rhodopsin is activated, triggering activation of G protein transducin
3. the activated G protein activates phosphodiesterase
4. activated phosphodiesterase breaks down cGMP to 5'-GMP, which then detaches from the Na+ channel
In rod photoreceptor cell: steps 5-7
5. loss of cGMP closes Na channel
6. membrane hyperpolarizes & reduces neurotransmitter release
7. when glutamate is not released (or reduced) from the rod cell, the bipolar cell is no longer inhibited & can now stimulate ganglion cells, produce APs, send signal to brain
Processing visual info
-in dark, rods & cones are depolarized & continually release glutamate
-some bipolar cells depolarize in response to glutamate, others hyperpolarize (depending on type of receptor present on bipolar cell)
-when light hits rods & cones, they hyperpolarize (b/c Na+ channel that was originally open in dark, that allowed Na+ to enter & depolarize cell, has been closed)
-this shuts off release of glutamate
-some bipolar cells will now be no longer inhibited by glutamate. they release neurotransmitters which stimulate ganglionic cells to generate APs to the brain
G-protein-linked receptors
-rhodopsin is a G-protein linked receptor found on membrane of rod photoreceptor cells
-utilizes cyclic GMP:
-concentration of GMP controlled by:
-synthesis by guanylyl cyclase,
-degradation by cyclic GMP phosphodiesterase
-light activates rhodopsin, causing phosphodiesterase to convert to cGMP to 5' GMP
-this closes Na+ channel on rod cell
-glutamate is no longer released from rod cell
-bipolar cell no longer inhibited & can send APs to ganglionic cells to brain
Rods light & dark adaptation
-in bright light, more rhodopsin broken down to Vitamin A, protecting eye & making it less sensitive to light
-in darker conditions, more rhodopsin produced so eye is more sensitive to light
-takes eyes a while to accommodate when going from dark to light & vice versa b/c of these chemical changes that must occur
Pupils light & dark adaptation
constriction in bright light; dilation in dim light
Cones
responsible for color vision & visual acuity
-numerous in fovea & macula lutea; fewer over rest of retina
-visual pigment is iodopsin: 3 types that respond to blue, red & green light
-overlap in response to light, thus interpretations of gradation of color possible: several millions
Receptive Fields
-area from which a ganglion cell receives input
-roughly circular w/ receptive field center
-2 types of receptive fields: on-center & off-center cells
-interneurons present in inner layers & modify signal b/f signal leaves retina. enhance borders & contours, increasing intensity at borders
on-center ganglion cells
generate more action potentials when light is directed onto the receptive field. respond to intensity of light
off-center cells
more action potentials when light is off or when light does not hit center of field. respond to contrasts in light.
How is location of a visual stimulus encoded in nervous system?
I. Comparing input:
-from 2 eyes gives: left/right location, & distance (or depth perception).
-when eyes move up & down gives: up/down location
II. Map-like projection from retina to cortex relays the left/right, up/down location, & distance (depth perception) info
Visual Fields
-close 1 eye. everything you can see w/ your open eye=the visual field for that eye
-visual fields of each eye partially overlap
-region of overlap=area of binocular vision
-see same object w/ both eyes. thus image of object reaches retina of one eye at a slightly different angle from that of the other- results in depth perception.
vision
info in a visual field (eg, "right" visual field or "left" visual field) is processed in the opposite side of the brain
Processed signal...
is sent via optic nerve through lateral geniculate nuclei to visual cortex.
Note: visual field to left is viewed by right side of both eyes & is processed in right visual cortex (& vice versa)
Myopia
nearsightedness
-focal point too near lens, image focused in front of retina
Hyperopia
farsightedness
-image focused behind retina
Presbyopia
degeneration of accommodation, corrected by reading glasses
Astigmatism
cornea or lens not uniformly curved
Strabismus
lack of parallelism of light paths through eyes
Retinal detachment
can result in complete blindness
Glaucoma
increased intraocular pressure by aqueous humor buildup
Cataract
clouding of lens
Macular degeneration
common in older people, loss in acute vision
Diabetes
dysfunction of peripheral circulation
3 divisions of ear
external, middle, & inner ear
external ear
hearing. terminates at eardrum (tympanic membrane). includes auricle & external auditory canal
middle ear
hearing. air-filled space containing auditory ossicles.
inner ear
hearing & balance. interconnecting fluid-filled tunnels & chambers within the temporal bone
external ear 2
-auricle or pinna: elastic cartilage covered w/ skin
-external auditory canal: lined w/ hairs & ceruminous glands. produce cerumen.
-tympanic membrane:
-thin membrane of 2 layers of epithelium w/ connective tissue btwn
-sound waves cause it to vibrate
-border btwn external & middle ear
middle ear 2
-separated from inner air by oval & round windows
-2 passages for air
-auditory or eustachian tube: opens into pharynx, equalizes pressure
-passage to mastoid air cells in mastoid process
-ossicles: malleus, incus, stapes: transmit vibrations from eardrum to oval window
-oval window: connection btwn middle & inner ear. foot of the stapes rests here & is held in place by annular ligament
hearing
hearing relies on sensory hair cells in organs (the ear) that respond to the vibrations of sound waves
sound waves
-exist as variations of pressure in a medium such as air.
-are created by the vibration of an object, which causes the air surrounding it to vibrate
-the vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound
vibrations produce...
soundwaves
volume or loudness
function of wave amplitude
pitch
function of wave frequency
timbre
resonance quality or overtones of sound
eardrum
tympanic membrane
In the cochlea...
vibrations transmitted from the eardrum through the fluid in the inner ear make the basilar membrane vibrate, bending the hair cells against the tectorial membrane and generating action potentials in afferent neurons that lead to auditory regions of the brain
cochlea
a spiraled, hollow, conical chamber of bone
-structures include: scala vestibuli, scala tympani, and scala media
scala vestibuli
(containing perilymph), lies superior to the cochlear duct; abuts the oval window (is on outer side of cochlea)
scala tympani
(containing perilymph), lies inferior to the scala media; terminates at the round window (is on outer side of cochlea)
scala media
(containing endolymph), which is the membranous cochlear duct containing the organ of Corti
Endolymph & perilymph
both contain electrolytes & proteins
Endolymph
is rich in potassium (which produces an ionic, electrical potential)
Perilymph
is rich in Na+
Basilar Membrane
-forms part of floor of cochlear duct
-anchors sensory hair cells in Organ of Corti
-vibrates in response to vibrations transmitted through inner ear
-15,000 hair cells located along basilar membrane
-synapse to afferent neurons
-afferent neurons=auditory nerve -> auditory center in temporal lobe of brain
Organ of Corti
-located in the cochlear duct & contains sensory hair cells that detect the sound vibrations transmitted to the inner ear.
-is the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea
-is the sensory epithelium, a cellular layer on the basilar membrane
Detection of sound vibrations
-hair cells arranged in rows. of inner hair cells (responsible for hearing) & outer hair cells (regulate tension on basilar membrane)
-hair bundle: stereocilia of one inner hair cell
-tip link (gating spring) attaches tip of each stereocilium in a hair bundle to the side of the next longer stereocilium.
-as stereocilia bend, they open K+ gates (mechanically gated ion channel)
Basilar membrane
-narrow near the oval window; wider at the outer end of cochlear duct
-high pitched sounds (which are comprised of high frequency vibrations) vibrate the basilar membrane at its narrow, beginning end
-low pitched sounds vibrate the basilar membrane near the wider, farther end
Basilar membrane 2
-hair cells in the organ of Corti are "tuned" to certain sound frequencies by way of their location in the cochlea due to the degree of stiffness in the basilar membrane
-stiffness is due to the thickness & width of the basilar membrane
-basilar membrane is stiffest & narrowest near its beginning (at the oval window, where the ossicle bones transmit the vibrations coming from the eardrum)
-here, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells
Basilar membrane 3
-the farther from oval window, the less stiff the basilar membrane is, & thus the less sensitive to high frequencies
-low frequencies travel down the tube, & the less-stiff membrane is moved most easily by them where the reduced stiffness allows
-as the basilar membrane gets less & less stiff, it responds better to lower frequencies
Sounds we hear
-combinations of vibrations at different frequencies & intensities occurring simultaneously w/ different degrees of force along the basilar membrane of the cochlea
-specific action potentials generated in specific neurons are transmitted to auditory centers in the brain, integrated, result in our perception of a specific sound (voice, motor, bird)
Balance
mechanoreceptors in humans: vestibular apparatus
-3 semicircular canals
-2 fluid filled chambers: utricle & saccule
-perceives position & motion of head
Semicircular canals
filled w/ endolymph, detects rotational motion. oriented perpendicular
Ampulla of a semicircular canal
Ampulla (region at base of a semicircular canal) detects rotational movement of the head/body & causes movement of the endolymph which displaces the cupula & bends the sensory hair cell -> generates action potentials in afferent neurons that synapse w/ hair cells
Utricle & Saccule
(fluid filled chambers)
-oriented 30 degrees to each other
-info about head position (up/down) & changes in rate of linear motion of body
-contain sensory hair cells w/ stereocilia
-hair cells have a membrane that contains otoliths: calcium carbonate crystals
-when head tilts or body moves non-linearly, otolithic membrane moves -> bends hair cells -> neurotransmitter release -> action potential -> brain perceives movement
Balance: In sum
-gelatinous mass moves in response to gravity, bending hair cells & initiating action potentials
-otoliths stimulate hair cells w/ varying frequencies
-patterns of stimulation translated by brain into specific info about head position or acceleration