Phototransduction, Audition & Equilibrium PDF

Phototransduction: the players Guanylate Cyclase Produces cGMP frm GTP Photopigment β...

Phototransduction: the players Guanylate Cyclase Produces cGMP frm GTP Photopigment β Stimulated by light absorption γ α GDP Transducin Na+ G-protein When activated, activates PDE Guanylate Phosphodiesterase Cyclase Transducin cGMP Phosphodiesterase (PDE) Photopigment Enzyme that converts cGMP into GMP (Opsin + cis-retinal) CNG channel Cyclic nucleotide-gated (CNG) channel Activated by cGMP Allows Na+ into cell when open Phototransduction: Darkness Guanylate Cyclase Produces cGMP from GTP Photopigment Inactive Na+ Transducin β Na+ Inactive γ α GDP Na+ Phosphodiesterase (PDE) GTP cGMP Inactive cGMP Na+ Na+ Cyclic nucleotide-gated (CNG) channel cGMP Activated by cGMP Na+ cGMP cGMP Channel is open Na+ Na+ cGMP cGMP Na+ moves into cell down concentration gradient cGMP Na+ Na+ cGMP Photoreceptor Outer segment depolarizes Spreads to inner segment Inhibitory neurotransmitter is released The dark current In the dark, photoreceptor cell is depolarized CNG channels open in outer segment Na+ influx Also some Ca2+ influx Open K+ channels in inner segment K + efflux Na+-K+ pumps in inner segment Na + pumped out K + pumped in  Phototransduction: Initiation photon Guanylate Cyclase Produces cGMP from GTP Photopigment Cis-retinal absorbs energy of photon Na+ Converted to trans-retinal β α Na+ Transducin γ GDP Inactive Na+ GTP cGMP Phosphodiesterase (PDE) cGMP Na+ Inactive Na+ cGMP Cyclic nucleotide-gated (CNG) channel Na+ cGMP cGMP Activated by cGMP Na+ Na+ cGMP cGMP Channel is open cGMP Na+ Na+ moves into cell down concentration gradient Na+ cGMP Photoreceptor Outer segment depolarizes Spreads to inner segment Inhibitory neurotransmitter is released Retinal shape initiates visual transduction Light absorption results in retinal isomerization Absence of light retinal in cis form After light energy absorption, retinal converted to trans” form Trans form must be converted back to cis form before it can absorb another photon Converted in pigmented epithelium Energy dependent process Phototransduction: Activation Trans-Retinal Guanylate Cyclase Produces cGMP from GTP Photopigment Cis-retinal activated by photon Na+ Undergoes conformational change Trans-retinal leaves photopigment β α Na+ Transducin γ GTP Activated by photopigment Na+ GDP is replaced by GTP GTP cGMP GDP cGMP Na+ Phosphodiesterase (PDE) Na+ Inactive cGMP Na+ cGMP cGMP Cyclic nucleotide-gated (CNG) channel Na+ cGMP Na+ Activated by cGMP cGMP cGMP Channel is open Na+ Na+ Na+ moves into cell down concentration gradient cGMP Photoreceptor Outer segment depolarizes Spreads to inner segment Inhibitory neurotransmitter is released Phototransduction: Activation Cis-Retinal Guanylate Cyclase Produces cGMP from GTP Photopigment Undergoing retinal recycling Na+ Transducin β GTP-bound α subunit separates from βγ subunit α Na+ Activates PDE γ GTP Na+ Phosphodiesterase (PDE) GTP cGMP Converts cGMP into GMP cGMP GMP cGMP Na+ Decreasing amount of intracellular cGMP Cyclic nucleotide-gated (CNG) channel Na+ GMP cGMP Activated by cGMP Na+ Na+ Channel is open cGMP GMP Na+ moves into cell down concentration gradient Na+ Na+ Photoreceptor Outer segment depolarizes Na+ Spreads to inner segment Inhibitory neurotransmitter is released Phototransduction : Light Cis-Retinal Guanylate Cyclase Produces cGMP from GTP Photopigment Undergoing retinal recycling Na+ Transducin β GTP-bound α subunit separates from βγ subunit α Na+ Activates PDE γ GTP Na+ Phosphodiesterase (PDE) GTP cGMP Converts cGMP into GMP GMP cGMP Na+ Decreasing amount of intracellular cGMP cGMP Cyclic nucleotide-gated (CNG) channel Na+ GMP Less cGMP to open channel Na+ Channel is closed Na+ remains outside cell GMP Na+ GMP Na+ Photoreceptor Outer segment hyperpolarizes Spreads to inner segment Inhibitory neurotransmitter is not released  Initiation of an action potential in the visual pathway In the Light In the Dark Trans-retinal in rhodopsin Cis-retinal in rhodopsin Na+ channels closed Na+ channels open membrane hyperpolarized membrane depolarized (spreads to synaptic terminal) (spreads to synaptic terminal) Ca++ channels closed Ca++ channels open No Inhibitory transmitter released Inhibitory transmitter released Inhibitory Synapse Inhibitory Synapse (Removal of Inhibition (Inhibition of bipolar cell) of bipolar cell) Visual cycle Process of retinal recycling Rate limiting step Basic process All-trans retinal removed from opsin Transported to RPE cells (or Mueller cells) Multiple steps to 11-cis retinal Transported back to photoreceptor cell Returned to opsin Appears cones can recycle their own retinal as well Rods take about 10 minutes for full adaptation Cones take about 3 minutes for full adaptation Visual adaptation Ability of the photoreceptor cells to sense very low levels of light (rods) to very high levels of light (cones) Function of how long it takes your photoreceptor cells to respond to the change in light intensity Dark adaptation Process of adjusting to low light intensity Light adaptation Process of adjusting to high light intensity Visual pathway to the brain Optic nerve Axons of ganglion cells converge to form Optic chiasma Optic nerves converge Medial fibers cross to other tract Allows each hemisphere of the visual cortex in the brain to be informed by both eyes Provides for binocular vision and improves depth perception Thalamus Lateral geniculate nucleus Primary visual cortex Visual cortex (do not memorize) V1 A visual map that relates to the visual field of each rod and cone Sensitivity to small changes within the visual field V2 Visual memory Responds to object orientation, spatial position, size, color and shape V3 Processing of motion Large patterns within the visual field V4 Object orientation, spatial position, and color Best sensitive to intermediate complexity of objects V5 Perception of motion and guidance of eye movements The Ear Responsible for hearing and equilibrium Three parts External ear Transmits & amplifies airborne Middle ear sound waves to the inner ear Inner ear – fluid filled Two sensory apparatuses: Cochlea – converts sound waves into nerve impulses Vestibular apparatus – responsible for equilibrium External Ear Auricle aka pinna Skin covered flap of cartilage Collect sound & direct it to the ear canal External acoustic meatus Aka ear canal Possesses fine hairs and ceruminous glands Creates a barrier to capture airborne particles Directs sound to the tympanic membrane Tympanic membrane Membrane spanning across the entrance to the middle ear Vibrates when struck by sound waves Middle Ear Tympanic Cavity Separated from external ear by tympanic membrane Separated from inner ear by oval and round windows Three bones (auditory ossicles) Malleus, Incus, & Stapes Responsible for transmitting sound vibrations from tympanic membrane to oval window (and fluid of inner ear) Amplify sound waves so they can travel through fluid medium of inner ear Two muscles reflexively contract to diminish the strength of incoming waves & protect the inner ear from loud sounds Tensor tympani & stapedius Auditory tube Aka Eustachian tube Opens to the nasopharynx Equalizes pressure within tympanic cavity with atmospheric pressure Inner Ear Located within the Petrous part of temporal bone Bony labyrinth Bony structure Contains cavities and spaces filled with fluid called perilymph Similar in content as IF Supports, protects and suspends the membranous labyrinth Forms three structures Cochlea Vestibule Semicircular canals Membranous labyrinth Located within the bony labyrinth Contains the receptors for hearing and balance Filled with endolymph Similar in content as intracellular fluid Membrane lined, fluid filled tubes Cochlear duct in the cochlea possesses the organ for hearing, the spiral organ Vestibule houses utricle and saccule, organs for balance Semicircular canals possess semicircular ducts, organ for balance The Cochlea Snail-shaped, spiral chamber Houses the spiral organ, the organ of hearing Bony labyrinth is partitioned into three chambers by two membranes Scala vestibuli Superior chamber Floor is the vestibular membrane Proximal end houses the oval window which connects to the middle ear Scala media (cochlear duct) Membranous labyrinth Middle chamber Houses the spiral organ Roof is vestibular membrane Floor is basilar membrane Scala tympani Inferior chamber Roof is basilar membrane Distal end houses round window which is sealed from the middle ear Helicotrema Point where the scala vestibuli becomes the scala tympani Located at the apex of the cochlea The Spiral Organ aka the Organ of Corti Located within the scala media (cochlear duct) Organ of hearing Structures Hair cells Sensory receptors for hearing Arranged over the basilar membrane Tectorial membrane Stiff, gelatinous membrane Overlies and contacts hair cells Spiral ganglion Possesses afferent fibers from hair cells Join to form cochlear branch of CN VIII (Vestibulocochlear nerve) Hair cells Hair cells are the sensory receptors for hearing Mechanoreceptors Possess actin-stiffened stereocilia One row of inner hair cells Act as the sensory receptor Three rows of outer hair cells Modulate activity within the spiral organ Sound Sound waves Traveling vibrations of molecule Alternating high & low pressure caused by the compression and rarefaction of air molecules Sound energy dissipates as it travels from the source Sound is characterized by Frequency The number of waves that pass a given point in a period of time Measured in Hertz (Hz) Interpreted as pitch Intensity Represented by the amplitude of the wave Measured in decibels (dB) Loudness The Hearing Pathway Sound waves are collected by the auricle and directed to the tympanic membrane Tympanic membrane vibrates in response to the pressure wave and transfers energy to the auditory ossicles Tympanic membrane vibrates at the same frequency as the incoming sound waves Auditory ossicles amplify the vibrations and transfer the energy to the oval window Oval window vibrates at same frequency as the incoming sound wave Oval window transfer the energy to the perilymph of the scala vestibuli, creating a pressure wave The Hearing Pathway Pressure wave displaces the vestibular membrane at a specific location for that particular frequency Displacement of the vestibular membrane causes a pressure wave within the perilymph in the cochlear duct which displaces the basilar membrane The Hearing Pathway Movement of the basilar membrane forces the hair cells against the tectorial membrane Causes the stereocilia to bend Results in depolarizations and hyperpolarizations Nerve signals are sent to CNS Pressure wave caused by displacement of the basilar membrane progresses to round window Round window bulges and absorbs remaining energy Hair cells Stereocilia V like arrangement from larger to smaller stereocilium Largest is kinocilum Interconnected by top links Ion channels located at base Bending of the hair cells leads to depolarization and hyperpolarization Toward kinocilium opens channels Leads to depolarization K+ moves in Away from kinocilium closes channels Leads to hyperpolarization No K+ movement Frequency and amplitude discrimination Frequency discrimination Ability to distinguish between various frequencies of incoming sound waves Sound waves travel to the region within the spiral organ that responds maximally to that frequency The energy is dissipated so that the wave dies out in that region Amplitude discrimination Dependent on the amplitude of the vibrations Causes the basilar membrane to vibrate more vigorously Auditory Pathway Movement of basilar membrane produces nerve signals Axons converge to form cochlear branch of vestibulocochlear nerve (CN VIII) Terminate in the cochlear nucleus of the medulla Secondary neurons project along two pathways To superior olivary nuclei Localize sounds Reflexes to loud sounds Sends signals to middle ear muscles to prevent excessive ossicle vibration Project to inferior colliculus To inferior colliculus in midbrain Reflexes to loud sounds Sends signals to skeletal muscles (causing us to jump, turn head, etc.) Neurons project from inferior colliculus to medial geniculate nucleus of thalamus Initial processing Filtering of auditory sensory input Tertiary neurons project to auditory cortex in temporal lobe Nerve signals are perceived as sound Auditory cortex is tonotypically organized Temporal mapping for sound, vertical plane Understanding where sound comes from is dependent on the timing of auditory reception Requires only one ear Dependent on how sounds are reflected off the structures of the pinnae (and their relative delay) Temporal mapping for sound, horizontal plane Dependent on both ears High frequency sounds are received by both ears but at different intensities to indicate direction Low frequency sounds are received and the delay between reception at both ears is the indicator of direction Equilibrium Awareness and monitoring of our head position Regulated by the vestibular apparatus Vestibule Utricle and saccule, aka otolith organs Detect head position during static equilibrium Detect linear acceleration changes of the head Semicircular canals Three canals in three planes Detect angular acceleration The otolith organs Macula Region within the otolith organs containing the receptor cells Consists of receptor cells, support cells and gelatinous layer Hair cells Receptor cells Have stereocilia arranged with and connected to a single kinocilium Otoliths CaCO4 crystals Located within the gelatinous layer Provide mass and inertia to the gelatinous layer Otoliths and gelatinous layer are together called otolithic membrane Vestibular nerve branches Attached to hair cells Sends a steady rate of nerve signals to CNS to indicate position of head The otolith organs Movement of the head influences the position of the otolithic membrane which alters the position of the sterocilia of the hair cells Bending toward the kinocilium yield stronger depolarizations Bending away from kinocilum yield weaker depolarizations Utricle Hair cells possess sterocilia in the vertical position Detects horizontal acceleration Saccule Hair cells possess sterocilia in the horizontal position Detects vertical acceleration The otolith organs Head is held erect Both utricle and saccule exhibit no changes in the pressure on the hair cells Head is bent forward (downward) or backward (upward) Otolith membrane is disturbed which disturbs hair cells in both utricle and saccule Acceleration of the head in the horizontal plane Hair cells are disturbed in utricle Acceleration of the head in the vertical plane Hair cells are disturbed in saccule Semicircular canals Three canals that lie in differing three separate planes at right angles from one another At their broad end nearest the utricle, the ampula contains the hair cells to detect angular motion Hair cells are embedded in a gelatinous dome called the cupula Hair cells have a kinocilium and stereocilia No otoliths Extends across the length of the semicircular canal up to the roof of the ampulla Semicircular canals Turning head causes endolymph to exert pressure on cupula Results from inertia Capula sways with the fluid inertia causing hair cells to bend As hair cells are bent the rate of nerve signals increase or decrease Vestibular sensation pathways