b223 special senses 16

olfaction and gustation

located in specific sense organs

chemoreceptors

• exteroceptors - respond to chemical stimuli from external environment

• receptor areas have chemically gated ion channels

receptors (cells) deteriorate rapidly and are continuously replaced with new cells

exteroreceptors

respond to chemical stimuli from external environment

olfactory organs

olfactory (bowman) gland, regenerative basal cell (divides to replace worn out olfactory receptor cells), cribriform plate, lamina propria, olfactory epithelium, olfactory nerve fibers, olfactory bulb, olfactory receptor cell/developing cell, supporting cell, mucous layer, knob, olfactory cilia (surfaces contain receptor proteins)

olfaction

sense of smell

olfactory epithelium - located in superior portion of nasal cavity

• olfactory receptors

• basal stem cells

• supporting cells, glands, blood vessels, etc

epithelium surface coated with mucus secretions from olfactory glands

• to be detectable as a smell, chemical must be volatile and soluble in mucus

• mucus cleans olfactory epithelium, removing chemicals

• voluntary cell must diffuse through mucus

olfactory receptors (cilia) are:

• g-protein coupled receptors located on the dendrites of bipolar neurons

• large area of membrane on which chemically gated channels are located

• humans have approximately 390 functional olfactory receptors

• each neuron type is sensitive to only one chemical

olfactory epithelium

located in superior portion of nasal cavity

• olfactory receptors

• basal stem cells

• supporting cells, glands, blood vessels, etc.

olfactory mucus

epithelium surface coated with mucus secretions from olfactory glands

• to be detectable as a smell, chemical must be volatile and soluble in mucus

• mucus cleans olfactory epithelium, removing chemicals

• voluntary cell must diffuse through mucus

cilia (olfactory receptors)

g-protein coupled receptors located on the dendrites of bipolar neurons

large area of membrane on which chemically gated channels are located

humans have approximately 390 functional olfactory receptors

each neuron type is sensitive to only one chemical

olfactory receptors

1) the binding of an odorant to its receptor protein leads to the activation of adenylate cyclase, the enzyme that converts ATP to cyclic AMP (cAMP)

2) the cAMP opens sodium ion channels in the plasma membrane, which then begins to depolarize

3) if sufficient depolarization occurs, an action potential is triggered in the axon, and the information is relayed to the CNS

olfactory discrimination

humans can discriminate at least 1 trillion different “smells”

CNS interprets different smells on the basis of the overall pattern of activity in the 390 different receptor/neuron types

• example: smell A (popcorn) activates primary smell neurons 1, 3, 4; smell B (mint) activates neurons 4, 27, 32, 41

very sensitive - only a few molecules needed to open enough gates to reach AP threshold in some olfactory neurons

1st order neuron (olfactory pathways)

receptor cell in olfactory epithelium - axon extends into CNS as cranial nerve I

• approximately 20 small bundles of axons (nerves) go through cribriform plate

• instead of one single peripheral nerve

2nd order neuron (olfactory pathways)

located in olfactory bulb

• much processing, including central adaptation

• 2nd order axons form olfactory tract that diverges to multiple sites

3rd order neurons (olfactory pathways)

at various locations

• olfactory cortex - medial surface temporal lobe

• hypothalamus

• limbic system

gustatory discrimination

humans can detect 4 to 6 “primary tastes”

• sweet, salty, sour, bitter, umami, water

CNS interprets different tastes on the basis of the overall pattern of activity in the 4 to 6 different receptor types

to be detectable as a taste, chemical must be dissolved

much less sensitive than olfactory receptors

• least sensitive to sweet and salty (requires more to trigger taste)

• 1000 times more sensitive to acids (sour) than sweet and salty

• most sensitive to bitter (plant secondary metabolites)

taste sensitivity shows significant individual differences, some of which are inherited

• phenylthiocarbamide (PTC) - bitter or tasteless

gustatory reception

involves: transitional cell, gustatory cell (not a nerve), basal cell, taste hairs (microvilli), taste pore

taste buds

contain gustatory receptor cells and basal stem cells

recessed into surface of surrounding epithelium of lingual papillae

gustatory receptor cells

four to six different types of gustatory receptor cells, each sensitive to only one type of chemical

• most taste buds have all 4-6 types

salt and sour channels

the diffusion of sodium ions from salt solutions or hydrogen ions from acids or sour solutions into the gustatory epithelial cells lead to depolarization

sweet, bitter, and umami receptors

receptors responding to stimuli that produce sweet, bitter, and umami that produce sweet, bitter, and umami sensations are linked to g-proteins called gustducins—protein complexes that use second messengers to produce their effects

gustatory pathways

receptor cell in taste bud - gated ion channels open, producing receptor potential

1st order neuron

• dendrite receive synapse from receptor cell

• amount of NT released by receptor cell determines AP frequency in 1st order neuron

• axon extends into CNS in cranial nerves

  • CN VII (facial) - anterior 2/3 of tongue

  • CN IX (glossopharyngeal - posterior 1/3

olfactory and gustation parallels

level of stimulation of olfactory receptors has major role in taste perception

• olfactory receptors much more sensitive than gustatory receptors

aging reduces olfactory and gustatory sensitivity

• number of receptors declines with age as fewer new cells are produced

• receptor sensitivity declines

inner ear

bony labyrinth

• surrounds and protects membranous labyrinth

• contain perilymph

membranous labyrinth

• contains endolymph

labyrinths are divided into 3 areas

• vestibule with utricle and saccule

• semicochlear canals with semicircular ducts

• cochlea with cochlear duct

bony labyrinth

surrounds and protects membranous labyrinth

contains perilymph

membranous labyrinth

contains endolymph

3 labyrinth areas

vestibule with utricle and saccule

semicircular canals with semicircular ducts

cochlea with cochlear duct

equilibrium and hearing

sensory organs located in the inner ear

specialized receptor cells termed hair cells are mechanoreceptors

• hair cells respond to physical distortion of their cilia

• amount of neurotransmitter released by hair cells controls AP frequency in 1st order neurons

cranial nerve VIII - vestibulocochlear

• contain 1st order neurons that conduct APs into CNS

organization of sensory pathways (ear)

hair cell components

kinocilium, gelatinous material, stereocilia, hair cell, sensory nerve ending, supporting cell

location of hair cells

semicircular ducts

• in cristae - small patch of hair cells in ampullarf region at one end of each of the 3 ducts

utricle and saccule

• in maculae - small patch of hair cells - one inside utricle, one inside saccule

cochlear duct

• in organ of corti on basilar membrane - long strip of hair cells extending the length of the coiled duct

equilibrium (vestibular sensation)

semicircular ducts - cristae hair cells

• sense rotational movement

• activated only during movement

• cupula - floating gelatinous mass in which “hairs” are embedded

utricle and saccule - maculae hair cells

• sense head position and change in movement

• active at all times

• otolith - “hairs” embedded in gelatinous mass containing calcium carbonate crystals

vestibular neural pathway

hair cells synapse onto 1st order sensory neurons, which enter CNS as vestibular branch of CN VIII

2nd order neurons located in vestibular nuclei of the brainstem

• integrate input from left and right inner ears

• send information to the somatosensory cortex

• send information to the cerebellum

• send information to motor nuclei in BS and SC for reflex control of eye, head, neck

motion sickness

disconnect between equilibrium and vision

nystagmus

involuntary eye movement

• postrotatory and opticokinetic (induced) (spinning in chair)

• pathological from damage to the vestibular system

hearing - auditory sense

external and middle ear assist in getting stimulus (sound waves) to receptors in the cochlea of the inner ear

external ear (auricle, external acoustic canal)

• collects and directs sound waves toward middle ear

middle ear (tympanic membrane and auditory ossicles)

• conducts and applies vibrations from tympanic membrane to inner ear

pathway of sound

1) sound waves arriving at tympanic membrane cause it to vibrate

2) auditory ossicles conduct and amplify the vibration onto the oval window of the inner ear

• tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive

3) in/out movement at the oval window creates pressure wave in the perilymph of the cochlea

4) pressure waves vibrate the basilar membrane area of the cochlear duct

5) hair cells of the organ of corti are pushed against the tectorial membrane

sound frequency and intensity discrimination

basilar membrane flexibility varies along length of cochlear duct

• highest frequency sounds (20000 hertz) vibrates areas closest to oval window

• lowest frequency sounds (20 hertz) vibrates areas furthest from oval window

higher intensity (louder) sounds cause larger vibration and stimulate more hair cells (intensity is measured in decibels)

frequency discrimination

auditory neural pathway

hair cells synapse onto 1st order sensory neurons, which enter CNS as cochlear branch of CN VIII

2nd order neurons located in cochlear nuclei of the brainstem

2nd order axons decussate and ascend to 3rd order neurons in inferior colliculi of mesencephalon

• inferior colliculus coordinates reflex movement of head and neck to sounds

3rd order axons ascend to thalamus

conscious awareness and interpretation of sound

• primary auditory cortex - superior surface of temporal lobe

• auditory association cortex in surrounding areas of temporal lobe

  • left hemisphere interprets language

eye

sensory organ that collects and focuses light onto the photoreceptors (rods and cones)

cornea and lens focus light rays onto retina located on the inner surface of eye ball

light passes through layers of retinal neurons to reach photoreceptors

retina

outer pigmented portion

• absorbs excess light

• transports nutrients to neural part

inner neural part

• photoreceptors: rods and cones

• 1st order neurons: bipolar cells

• 2nd order neurons: ganglion cells, axons form the optic nerve and tract

• embryologic origin - outgrowth from brain

organization of sensory pathways (eye)

cones

provide color vision

• require more light to be activated than rods

• located in macula lutea portion of retina

• in highest concentration in fovea portion of macula

re

rods

do not distinguish colors

• highly sensitive to light, allow for vision in dim light conditions

• found in retinal areas peripheral to macula

inner segment (rods and cones)

the inner segment contains the photoreceptor’s major organelles and is responsible for all cell functions other than photoreceptor, it also releases neurotransmitters

outer segment (rods and cones)

the outer segment of a photoreceptor contains flattened membranous plates, or discs, that contain the visual pigments

pigmented epithelium (rods and cones)

the pigmented epithelium absorbs photons that are not absorbed by visual pigments. it also phagocytizes old discs shed from the tip o the outer segment

photoreceptors (rods and cones)

convert light signals into electrical signals

gated ion channels

• respond to amount of light absorbed by pigment molecules in the photoreceptor

current flowing through gated ion channels changes membrane potential, controls amount of neurotransmitter released onto 1st order neurons

photoreceptor physiology

gated ion channels are indirectly controlled by light

light absorption activates enzyme pathway that controls 2nd messenger (cGMP) levels that open gated channels

light is absorbed by retinal molecule embedded in an opsin protein molecule

• retinal (photo reactive molecule) - synthesized from vitamin A

• opsin differ in which light frequencies can reach the retinal molecule

rods (color vision)

retinal and rhodopsin

respond to wide range of light wavelengths, responds to all the different wavelengths of the different colors

cones (color vision)

retinal and one of the 3 different opsin (red, blue, and green sensitive cones)

respond to specific, narrower ranges of wavelengths, ranges overlap

color discrimination

integration of information from red, blue and green cones

• white perceived when all cones equally stimulated

• yellow perceived when green cones stimulated strongly, red cones are stimulated moderately, and blue cones are not stimulated

• variable stimulation of the three cones leads to the ability to discriminate >10 million colors

colorblindness

inability to detect certain colors due to lack of one or more color opsin

• inherited genetic trait - genes for red and green opsin are located on X chromosome

• 10% of males, 0.67% of females

visual pathway

each ganglion cell receives input from a specific receptive field of the retina, a group of photoreceptors

image of retina surface is mapped onto visual cortex

receptive fields are very small in fovea area of retina

• highest visual acuity

• lowest area in visual cortex

photoreceptors to bipolar cells to ganglion cells

axons of ganglion cells form the optic nerve and optic tract

• synapse in superior colliculus for visual reflex pathways

• synapse on 3rd order neurons in the thalamus for conscious processing pathway

conscious processing of visual input (visual pathway)

primary visual cortex and visual association cortex are in occipital lobe of brain

right visual field projects to left visual cortex

• left half of both retinas view right visual field

visual accommodation

nearsighted and farsighted