Biol 216 - Topics 7 + 8

special senses

receptors strategically placed in unique organs (taste, vision, sight, smell)

sensory receptors

gather info about internal/external environment - formed by specialized cells or terminals of afferent neurons

how do sensory receptors response to stimuli

change conductance to ions → change in membrane potential

sensory transduction

conversion of stimulus to a change in membrane potential

mechanoreceptors

detect changes in touch, body position, pressure, acceleration - auditory receptors of the ear

photoreceptors

detect light - located in the eye

chemoreceptors

detect specific molecules or conditions - acidity in taste buds

thermoreceptors

temperature

nociceptors

detect tissue damage/poisonous chemicals → pain

touch and pressure receptors in human skin can be…

free neuronal endings or encapsulated neuronal endings of sensory neurons

pacinian corpuscle

encapsulated neuronal ending - touch receptors detects deep pressure and vibrations → tool use

ruffini endings

touch receptors detects deep pressure → hand shape/finger position

meissner’s corpuscle

encapsulated neuronal ending - touch receptors detects light touch and surface vibrations → grip control

proprioception

sense of relative position of neighboring parts of body - composed of sensory neurons in inner ear + stretch receptors in muscles (mechanoreceptors)

propriceptors

mechanoreceptors in muscles/tendons/joins that detect changes in pressure/tension of body parts → detect stimuli used by CNS to monitor and maintain body/limb positions

stretch receptor

type of proprioceptor found in muscles and tendons - detect how much/how fast muscle is stretched → position and movement in limbs

golgi tendon organ (GTO)

nerve fiber with collagen strands - connect muscle to tendon

stretch receptor process

muscle generates force → sensory terminals compressed → stretch-sensitive cation channels on afferent axon open → axon depolarizes → fires AP spreads to spinal cord

vestibular apparatus

3 semicircular canals and 2 fluid filled chambers (utricle and saccule) - perceives position and motion of head using mechanoreceptors

semicircular canals

filled with endolymph (rich in K+) - detect rotational motion

ampulla

region at base of semicircular canal with sensory hair cells - detects rotational movement of head/body

ampulla process

detects rotational movement → endolymph moves → fluid displaces cupula + bends sensory hair cell (mechanoreceptor) → generates AP in afferent neurons

utricle and saccule

fluid filled chambers contian sensory hair cells w/ stereocilia - give info about head position (up/down) and changes in rate of linear motion of body

otoliths

calcium carbonate crystals found in membrane of hair cells - similar to cupula

head tilting/non-linear body movement process

otolithic membrane moves → bends hair cells → NT release → AP → brain perceives movement

overview of balance process

gravity/movement → gelatinous mass moves → AP initiates → translated by brain into specific info about head position/acceleration

sound waves

exists as variations of pressure in a medium created by vibration of an object

wave amplitude

volume/loudness of a sound

wave frequency

pitch of a sound

eardrum

aka tympanic membrane

malleus

hammer

incus

anvil

stapes

stirrup

eustachian tube

leads to throat - tube opens when we swallow so air can flow in/out of ear to equalize pressure

ossicles

contain malleus, incus, and stapes

hearing in inner ear process

vibrations transmitted from eardrum through fluid in inner ear → basilar membrane vibrates → hair cells bend against tectorial membrane → generates AP to auditory regions in brain

cochlea

spiraled hollow conical chamber of bone containing scala vestibuli, scala media, scala tympani

scala vestibula

lies superior to cochlear duct on outer side of cochlea, contains perilymph (rich in Na+)

scala media

membranous cochlear duct containing endolymph (rich in K+) and organ of corti

scala tympani

lies inferior to scala media, outer side of cochlea, contains perilymph (rich in Na+)

basilar membrane

forms part of the floor of cochlear duct

anchors sensory hair cells in organ of corti

organ of corti

region in cochlear duct distributed along partition separating fluid chambers in cochlea

contains sensory hair cells that detect sound vibrations transmitted to inner ear

basilar membrane/hair cell process

vibrates in response to vibrations transmitted through inner ear → synapse to afferent neurons → auditory nerve → synapse to temporal lobe

hearing overall process

vibrations at different frequencies/intensities along basilar membrane of cochlea → specific AP generated → transmitted to auditory centers in brain

detection of sound vibrations process

hairs cells arrange in rows of inner and outer hair cells → tip link attaches tip of stereocilium in hair bundle side of next one → bending of stereocilia open K+ gates → depolarizes cell → NT relese

affect of pitch on basilar membrane

high pitched sounds vibrate basilar membrane at narrow, stiff beginning end

low pitched sounds vibrate near wider/less stiff end → low frequencies travel down tube

detailed hearing process

1. sound waves strike tympanic membrane → vibrates

2. malleus, incus, and stapes vibrate

3. foot plate of stapes vibrate → perilymph in scala vestibuli vibrate

4. vestibular membrane vibrates → endolymph vibrates

5. basilar membrane displaces based on pitch

6. hair cells detect basilar membrane movement

7. vibrations transferred to scala tympani → transferred to round window and dampen

neuronal process of hearing

1. sensory axons from cochlear ganglion terminate in cochlear nucleus of brainstem

2. axons from neurons in cochlear nucleus project to thalamus → auditory cortex

blind spot

area where optic nerve passes through optic disc - no light detecting photoreceptive cells → part of field of vision not perceived

cornea

covers iris - admits and refracts light

iris

behind cornea, around pupil - controls diameter of pupil + regulates amount of light that strikes the lens

sclera

whiter outer layer of eyeball - protective layer

choroid

between sclera and retina - vascular layer of the eye

lens

focuses image on the retina

retina

layer of neural cells that line back of the eye - has photoreceptor cells sensitive to light and neurons that integrate info detected by photoreceptors

macula

pigmented area in retina (contains fovea) - involved in high acuity vision

fovea

region of macula in the retina - has high density of cone cells for color + needed for sharp vision/detail

anterior chamber

lies between cornea and iris - filled with aqueous humor

posterior chamber

located behind iris and front of lens - filled with aqueous humor

vitreous chamber

located behind the lens - filled with vitreous humor

aqueous humor

thin watery fluid found in anterior/posterior chambers of the eye - maintains intraocular pressure, supplies nutrients to avascular parts of the eye, removes waste

vitreous humor

thick viscous fluid behind lens - helps maintain shape of eye and absorb shocks

ciliary body

part of eye that includes ciliary muscle and ciliary epithelium

ciliary muscle

ring of smooth muscle fibers - controls shape of lens to achieve accomodation

ciliary epithelium

produces aqueous humor

accomodation

change of shape of lens depending if object is near/far

accommodation of distant object

ciliary muscles relax → supporting ligaments tighten → flatten lens

accommodation of near object

ciliary muscles contract → loosening ligaments → lens becomes rounder

types of photoreceptors in retina

rods and cones

pathway of vision

photoreceptors → bipolar cells → ganglion cells

horizontal cells

receive info from photoreceptors → transmit it to surrounding bipolar neurons

amacrine cells

receive inputs from bipolar cells → activate ganglion neurons that are in their vicinity by secreting dopamine

impact of horizontal cells on eyes

allow eyes to adjust to see well under both bright and dim light conditions - involved with lateral inhibition

amacrine cells and retinal dopamine

releases dopamine into extracellular medium during daylight → enhances activity of cone cells in retina + suppresses rod cells → increased sensitivity to color during bright light conditions

structure of rods and cones

outer segment containing light-absorbing photopigment, inner segment with cell’s metabolic machinery, synaptic terminal

rods

specialized for detection of low-intensity light

cones

specialized for detecting light of different wavelengths → colors

photoreceptive molecules

absorb photons and generate changes in membrane potential

rhodopsin

photopigment for rods - retinal + opsin protein (GPCR)

iodopsin

photopigment for cones - contains protein complexes photopsin I, II, III

when light hits rhodopsin…

cis-retinal absorbs light → converted to trans-retinal → opsin activates triggering activation of G protein transducin → activates phosphodiesterase → breaks down cGMP into 5’ -GMP → detaches from Na+ channel and closes → rod cell hyperpolarized → less glutamate released

glutamate in dark/light conditions

dark: rod cell = depolarized → releases glutamate constantly

light: rod cell = hyperpolarized → glutamate release is diminished → bipolar cell NO LONGER inhibited → stimulate the ganglion cells to send a signal to the brain

photopsin

protein complexes in iodopsin - respond to blue, red, and green light

cones mainly found…

in fovea and macula lutea - fewer over rest of retina

receptive fields

specific group of photoreceptor cells - smaller receptive fields → sharper images

receptive fields structure

circular area of the retina in bipolar cells that contain center and surround

receptive field center

provides direct input from photoreceptors to bipolar cells

receptive fields surround

provides indirect input from photoreceptors to bipolar cells via horizontal cells

types of bipolar cells

on center and off center - exhibit graded potentials

bipolar cell synapsis

on-center bipolar cell → synapses w/ on-center ganglion cell

off-center bipolar cell → synapses w/ off-center ganglion cell

on center pathway when receptive field center is dark

glutamate constantly released from photoreceptor → stimulates metabotropic glutamate receptors on the on-center bipolar cells → K+ channels open → efflux of K+ hyperpolarizes cells → decrease of NT release → decrease in firing of on-center ganglion cell

off center pathway when receptive field center is dark

glutamate released from photoreceptor terminals → stimulates ionotropic glumate receptors on off-center bipolar cells → Na+ channel open → Na+ influx depolarizes off-center bipolar cell → release of glutamate increases → increase in firing of corresponding off-center ganglion cell

on center pathway when receptive field center has light

rod is hyperpolarized → glutamate release decrease → depolarization of on-center bipolar cell → incr in NT release → incr firing of corresponding on-center ganglion cell

off center pathway when receptive field center has light

reduction in release of glutamate → hyperpolarization of off-center bipolar cell → decr NT release → decr in firing of off-center ganglion cell

ratio of bipolar cells to photoreceptive cells

one bipolar cell → synapses with a set of photoreceptive cells

importance of retina processing by on-center and off-center ganglion cells

enhances differences in relative brightness → defines contours

light adaptation

more rhodopsin broken down → rods less sensitive to light

pupils constrict

dark adaptation

more rhodopsin produces → rods more sensitive to light

pupils dilate - open to get more light

visual stimulus encoding in nervous system

compares input from two eyes - left/right, depth, up/down → relays map-like projection from retina to cortex