special senses

the eye is composed of two components

optical and neural component

cornea

thin, transparent epithelium without blood vessels

aqueous humor

liquid that flows between the iris and the anterior surface of the lens (in the anterior chamber); slightly pressurized to keep the eyes shape

lens

a translucent and elastic structure composed of cells with high concentration of a-crystallins, proteins that increases its density and focusing power

position and shape of lens is controlled by

cilliary muscles

vitreous humor

kept in the posterior chamber and kept pressurized by the production of aqueous humor

retina information process

output signals travel to the brain via axons of retinal ganglion cells that form the optic nerves (one for each eye). optic nerves cross partially at chiasm so that input from one half of visual field is conveyed to the contralateral side of the brain. visual pathway continues to thalamus and then to the visual cortex in the occipital lobe

light is perceived in

the retina

which area of the eye has the finest vision

fovea; our eyes move to focus the image of interest on the fovea and the retinal layers are pushed aside to the surrounding macula so light can reach fovea photoreceptors with minimal image distortion

area of the retina involved in central vision

macula

optic disc

blind spot where ganglion axons leave the retina and form the optic nerve; lacks photoreceptors

why are we unaware of blind spots in our eyes

that part of the visual field is seen by the contralateral eye and the image is filled by neural processing

fovea has the highest concentration of

light receptors

macular degeneration =

loss of fine vision

the focusing of an image on the retina is dependent on

the lens and the iris

the iris regulates the amount of light entering the eye by adjusting

pupil size; dilator muscles activated by sympathetic nervous system, constrictor muscles activated by parasympathetic system

when the pupil constricts, light coming in ___ and depth of field ___

decreases, increases

pupillary light reflex

optic nerve fibers synapse to brainstem nuclei that stimulate PREganglionic parasympathetic neurons that go to cililary ganglia, which stimulate POSTganglionic parasympathetic neurons that constrict pupils in both eyes

direct light response

shining a light into one eye will cause that eye’s pupil to constrict

consensual light response

pupil constricts in response to shining a light into the other eye

all output from one eye comes out its

optic nerve

where are photoreceptors located

on the outer surface of the retina, pointing AWAY from incoming light

retinal interneurons include

bipolar, horizontal, amacrine, and ganglion cells

retinal interneurons function

to form signal processing circuits that modify light signals before it leaves the eye

retinal photoreceptors are also known as

rods and cones

photoreceptor structure is composed on

cell body containing the nucleus, an inner segment containing organelles, an outer segment, and a synaptic terminal at the end of a short axon

difference between the outer segments of cones and rods

cones have shorter conical outer segments and contain stacks of membraneous discs to increase surface area whereas rods are longer and the stacks of discs float freely inside

outer segments of photoreceptors connect to inner segment via

modified cilium

where are photopigments located

in the photoreceptor outer segment discs

which type of photoreceptor is more sensitive to light?

rods; partially due to having greater density of photopigments; can respond to single photon whereas cones require hundreds

photopigments consist of

opsin proteins that bind to light-sensitive chromophore retinal (vitamin A)

visual pigment specific for rods

rhodopsin

visual pigment for cones

photopsins; 3 different types based on the type of opsin they contain; each sensitive to a color

L, M, and S types for long, medium, and short wavelength responses

photoreceptors are what kind of receptors

G-protein coupled receptors

in light stimulation, photoreceptors are (hyper/de) polarized and cGMP-gated cation channels are (open/closed)

hyperpolarized, closed

in darkness, photoreceptors are (hyper/de) polarized because the cGMP channels in the outer segments are (open/closed)

depolarized, open

depolarizing current is AKA dark current

constant depolarization of photoreceptors induces

tonic release of neurotransmitter glutamate

the number of cGMP cation channels that close in response to light depends on

how many photons are absorbed (light intensity)

in contrast to other sensory systems, visual neurons ___ in response to stimulus and release ___ neurotransmitter

hyperpolarize, less

phototransduction process

light is absorbed (photoisomerization) by rhodopsin and activates a G protein called transducin that increases cGMP phosphodiesterase activity, lowering cGMP concentration and closing the channel. hyperpolarization occurs and decreases neurotransmitter glutamate release

site of transduction

outer segments of rods and cones

the short distance between the site of transduction and the synapse allows the response to be a (graded potential/action synapse)

graded response

after light hyperpolarizes the receptor, the bipolar receiving cell can either be

on or off

ON bipolar cell pathway

photoreceptor is depolarized in the dark, releasing neurotransmitter glutamate onto bipolar cells. incoming light rays hyperpolarize the cell and decrease glutamate release. bipolar cell DEPOLARIZES when photoreceptor hyperpolarizes, releasing more neurotransmitter and generating AP’s in the ganglion cells

OFF bipolar cell pathway

photoreceptors are depolarized in the dark, releasing glutamate onto bipolar cells. incoming light rays hyperpolarize the cell and glutamate is released. bipolar cell HYPERPOLARIZE when photoreceptor hyperpolarizes and releases less neurotransmitter. ganglion cell hyperpolarizes and generates less AP’s

ON bipolar cells use (metaboglutamate/ionoglutamate) receptor

metaboglutamate receptor

ON ganglion cells trigger AP in response to

light

OFF bipolar cell uses (metaboglutamate/ionoglutamate) receptors

ionoglutamate receptors

OFF ganglion cells trigger AP in response to

absence of light

the vestibular and auditory hair cells reside in

the membraneous labyrinth in the inner ear canal

vestibular system 5 sensory structures

2 otolithic organs: saccule and utricle and 3 semicircular canal

the otolithic organs of the vestibular system detect which type of head movement

head position and linear head movement

the semicircular canal detect which type of head movement

head rotation

both the vestibular system and the auditory system transmit information to the CNS via the

vestibulocochlear nerve (8th cranial nerve)

which part of the inner ear canal gets clogged during sickness?

eustachian tube; usually drains the middle ear and equalizes its pressure

hearing frequency is measured in

hertz Hz

hearing intensity is measured in

decibels (db)

>100 db = damage

>120 db = acute pain and permanent damage

young hearing range

20-20,000 Hz

speech range

300-3500 Hz

auditory structure is broken down into what 3 compartments

external, middle, and inner ear

external ear is separated from the middle ear by

the tympanic membrane (eardrum)

3 small bones of the inner ear

malleus, incus, stapes

how does sound transduction take place

sound makes the tympanic membrane vibrate which shakes the inner ear bones and pushes the stapes footplate into the oval window which displaces fluid inside the cochlea

what part of the cochlea houses the cells that detect sounds?

scala media (cochlear duct)

vestibule

space in the cochlea facing the oval window

inner hair cells do what

transduce sound

mechanoreceptors that detect movement along one axis

endolymph

surrounds the top of hair cells and is produced by stria vascularis; high K+ and low Na+ which creates large potential gradient in hair cell cilia

scala media has 3 membranous walls

basilar membrane, Reissner’s membrane, stria vascularis

scala vestibuli and scala tympani are filled with

perilymph

scala media is filled with

endolymph

organ of Corti

neural structure in scala media that transduces sound

3 rows of outer hair cells, 1 row of inner hair cells, tectorial membrane, and supporting cells

the organ of corti is innervated by nerve cells of

the cochlear division of the vestibulocochlear nerve, most innervation going to inner hair cells

sterocilia

nonmotile cilia on the apical surface of inner hair cells

tip links

fine strands that connect taller stereocilia to shorter stereocilia

even though hair cells are not neurons, they have

Ca2+ storage at the base of the cell where synapse occurs

auditory inner hair cells use what neurotransmitter

glutamate

mechanical deformation of stereocilia towards the tallest stereocillium causes K+ channels to

open; depolarizes the hair cell and releases neurotransmitter

mechanical deformation of stereocilia away from the tallest stereocillium causes K+ channels to

close; hyperpolarize the hair cell and does not release neurotransmitter

mechanosensitive channels of the inner hair cell are (selective/nonselective) ion channels

nonselective; since the apical side is exposed to endolymph, K+ is the main ion flowing in

process of sound transduction

sound waves make tympanic membrane vibrate, transmitting to the scala vestibuli via oval window. pressure waves in the perilymph causes the basilar membrane and organ of Corti to move up and down, bending the stereocilia tallest cilium which depolarizes the cell, releasing neurotransmitter, and produces excitatory post-synaptic potential in the afferent nerve fibers.

high frequency sounds are transduced by the hair cells at

their base

low frequency sounds are transduced by the hair cells at

the apex

the vestibular system detects ___ accelerations of the head

angular and linear

vestibular inputs go to the brainstem and from there they are processed in

the medulla, the pons, and the cerebellum (most important)

vestibular dysfunction can make it hard to

stabilize the image on the retina, causing vertigo

semicircular canal detects which type of head acceleration

angular acceleration (head movement along 3 axis)

where do vestibular hair cells reside in the semicircular canals

the ampullary crest on each canal

cupula

gelatinous structure inside the semicircular canal where stereocilia of hair cells are embedded

how is angular acceleration of the head detected

head rotation causes pressure waves in the endolymph which displaces the cupula and bend the stereocilia hair cells

where do vestibular hair cells reside in the otolith organs

inside a region called macula

where are stereocilia and crystals embedded in the otolithic organs

in the otolithic membrane

how is linear acceleration of the head detected

movement of the head produces inertia on otoliths that make the otolothic membrane move deflecting the sterecilia

taste receptor cells

modified epithelial cells that synapse to sensory neurons

olfactory receptor cells

neurons with axons that travel to the brain

where do taste receptors reside

in papillae on the dorsal surface of the tongue

each taste cell lives for

2 weeks; turnover depends on innervation; if sensory nerve is cut, taste buds dissapear

five different taste senses

bitter, salty, sweet, sour, umami

flavors are combinations of these 5

salty taste transduction

Na+ channels are open at rest. Na+ molecule enters the cell via the channel and depolarize the cell (receptor potential)

sour taste transduction

cells contain acid sensitive, non-selective cation channel. Free H+ ions activate these channels and depolarize the cell

sweet, bitter, and umami taste transduction

taste molecules bind to membrane G-protein coupled receptors that initiate a second messenger system that opens non-selective cation channel that depolarizes the cell. if depolarization magnitude is sufficient, Ca2+ channels open and release into the cytoplasm and neurotransmitter is released onto afferent axon. primary afferent neurons travel to CNS