Ear/ Visual Pathway & Phototransduction

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Questions and Answers

What initiates visual transduction in the photoreceptors?

Inhibition of guanylate cyclase

Chemical change in sodium ions

Absence of light

Light absorption resulting in retinal isomerization (correct)

What form of retinal is generated after it absorbs light energy?

Active-retinal

Photon-retinal

Cis-retinal

Trans-retinal (correct)

What must happen to trans-retinal before it can absorb another photon?

It must bind to GTP

It must activate phosphodiesterase

It must be converted back to cis form (correct)

It must undergo conformational change

Which enzyme is responsible for producing cGMP from GTP during phototransduction?

Guanylate cyclase Signup and view all the answers

What occurs after the activation of photopigment by a photon?

Trans-retinal exits the photopigment Signup and view all the answers

What happens to sodium ion channels when cGMP is present in high concentrations?

They open and allow Na+ to enter the cell Signup and view all the answers

Which molecule replaces GDP with GTP in the phototransduction pathway?

Transducin Signup and view all the answers

How is trans-retinal converted back to cis-retinal in the retina?

Through an energy-dependent process in the pigmented epithelium Signup and view all the answers

What is the role of Guanylate Cyclase in phototransduction?

It produces cGMP from GTP. Signup and view all the answers

What happens to cGMP when phosphodiesterase (PDE) is activated?

It is converted into GMP, decreasing its intracellular amount. Signup and view all the answers

Which event occurs first during phototransduction after light activation?

GTP-bound α subunit separates from βγ subunit. Signup and view all the answers

What initiates the spreading of the depolarization in photoreceptor cells?

Movement of Na+ into the cell. Signup and view all the answers

What is the effect of decreased intracellular cGMP levels?

It leads to hyperpolarization of the photoreceptor. Signup and view all the answers

Which component is responsible for the recycling of retinal in phototransduction?

Photopigment Signup and view all the answers

What is the primary function of cyclic nucleotide-gated (CNG) channels in phototransduction?

To allow Na+ influx into photoreceptor cells. Signup and view all the answers

In the phototransduction pathway, what is the state of the outer segment upon activation by light?

It depolarizes. Signup and view all the answers

What occurs when the GTP-bound alpha subunit separates from the beta-gamma subunit?

It activates phosphodiesterase (PDE). Signup and view all the answers

What happens to cGMP levels during the process of photoreception in light?

cGMP levels decrease significantly. Signup and view all the answers

In the dark, how does the state of the sodium (Na+) channels differ from that in the light?

Na+ channels open in the dark, leading to depolarization. Signup and view all the answers

What is the result of less cGMP in the photoreceptor cells?

The closure of the cyclic nucleotide-gated (CNG) channels. Signup and view all the answers

Which form of retinal is present in rhodopsin during light exposure?

All-trans retinal Signup and view all the answers

What key process occurs during retinal recycling?

All-trans retinal is removed from opsin. Signup and view all the answers

How does the closure of Ca++ channels affect neurotransmitter release in light conditions?

Inhibitory neurotransmitter is not released. Signup and view all the answers

What is the rate-limiting step in the visual cycle?

Transport of all-trans retinal to RPE cells. Signup and view all the answers

What happens when the basilar membrane moves in response to sound waves?

It generates a pressure wave in the cochlear duct. Signup and view all the answers

Which part of the hair cells causes depolarization when bent toward the kinocilium?

Stereocilia Signup and view all the answers

What role do the top links between stereocilia play?

They facilitate the bending of the stereocilia. Signup and view all the answers

How does amplitude discrimination affect sound perception?

It affects the intensity of vibrations on the basilar membrane. Signup and view all the answers

Where do the axons from the hair cells converge?

Cochlear nucleus of the vestibulocochlear nerve. Signup and view all the answers

What occurs at the round window during the auditory process?

It allows pressure waves to dissipate. Signup and view all the answers

What primarily affects frequency discrimination in sound perception?

The region of the spiral organ that responds to specific frequencies. Signup and view all the answers

What condition is created when the hair cells are stimulated away from the kinocilium?

Ion channel closure leading to hyperpolarization. Signup and view all the answers

What is the primary function of the inferior colliculus in the auditory pathway?

Localizing sounds Signup and view all the answers

Which part of the auditory pathway is responsible for the perception of sound?

Auditory cortex Signup and view all the answers

How is the direction of high-frequency sounds determined by the auditory system?

By different intensities received by both ears Signup and view all the answers

What does the vestibular apparatus primarily monitor?

Head position and movement Signup and view all the answers

The otolith organs are key structures involved in detecting what kind of changes?

Static equilibrium and linear acceleration Signup and view all the answers

What is the role of the superior olivary nuclei in the auditory system?

To localize sounds and reflex movements Signup and view all the answers

Which aspect of sound localization requires input from only one ear?

Timing of sound reception Signup and view all the answers

What type of acceleration do the semicircular canals detect?

Angular acceleration Signup and view all the answers

What is primarily contained within the otolith organs?

Hair cells and gelatinous layer Signup and view all the answers

What type of crystals are found within the otolithic membrane?

CaCO4 crystals Signup and view all the answers

Which acceleration does the utricle primarily detect?

Horizontal acceleration Signup and view all the answers

How do hair cells react when the stereocilia bend toward the kinocilium?

They yield stronger depolarizations Signup and view all the answers

What happens to the otolithic membrane when the head is bent backward?

The otolithic membrane becomes disturbed Signup and view all the answers

What is the primary function of the vestibular nerve branches?

Sending signals related to head position Signup and view all the answers

What occurs when the head is held erect concerning hair cell pressure?

Hair cells exhibit constant pressure Signup and view all the answers

What is the arrangement of stereocilia in the saccule?

Stereocilia are arranged in a horizontal position Signup and view all the answers

Study Notes

Phototransduction: The Players

Guanylate Cyclase produces cGMP from GTP
Photopigment is stimulated by light absorption
Transducin is a G-protein
When activated, Transducin activates Phosphodiesterase (PDE)
Phosphodiesterase (PDE) converts cGMP into GMP
Cyclic nucleotide-gated (CNG) channel is activated by cGMP
CNG channel allows Na+ into the cell when open

Phototransduction: Darkness

Guanylate Cyclase produces cGMP from GTP
Photopigment is inactive
Transducin is inactive
Phosphodiesterase (PDE) is inactive
Cyclic nucleotide-gated (CNG) channel is activated by cGMP
Sodium moves into the cell down the concentration gradient
Outer segment of the photoreceptor depolarizes
Inhibitory neurotransmitter is released

The Dark Current

In darkness, photoreceptor cells are depolarized
CNG channels open in the outer segment, allowing Na+ influx
Some calcium influx also occurs
Open K+ channels in the inner segment, leading to K+ efflux
Na+-K+ pumps in the inner segment function to pump Na+ out and K+ in

Phototransduction: Initiation

Guanylate Cyclase produces cGMP from GTP
Cis-retinal absorbs photon energy, converting to trans-retinal
Transducin is inactive
Phosphodiesterase (PDE) is inactive
Cyclic nucleotide-gated (CNG) channel is activated by cGMP
Outer segment depolarizes, sending signal to inner segment
Inhibitory neurotransmitter is released

Retinal Shape Initiates Visual Transduction

Light absorption causes retinal isomerization
Retinal converts from cis to trans-form
Trans-retinal converts back to cis-form before it can absorb another photon
Converted in the pigmented epithelium
Energy-dependent process

Phototransduction: Activation

Guanylate Cyclase produces cGMP from GTP
Cis-retinal is activated by a photon
Undergoes conformational change, and trans-retinal leaves the photopigment
Transducin is activated by photopigment
GDP is replaced by GTP
Phosphodiesterase (PDE) is inactive
Cyclic nucleotide-gated (CNG) channel is activated by cGMP
Outer segment depolarizes, sending signal to inner segment
Inhibitory neurotransmitter is released

Visual Cycle

Process of retinal recycling
Rate-limiting step in retinal recycling
All-trans retinal is removed from opsin
Transported to retinal pigment epithelium (RPE) cells, then back to the photoreceptor cells
Rods take about 10 minutes for full adaptation; cones take about 3 minutes

Visual Adaptation

Ability of photoreceptor cells to sense very low to very high light levels
Function of how long it takes photoreceptor cells to respond to changes 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 retinal ganglion cells converge to form optic nerves
Optic chiasm: optic nerves converge; medial fibers cross to other tract
Optic tract: contains axons from both eyes; project to thalamus or midbrain
Lateral geniculate nucleus of the thalamus: majority of optic tract axons project here
Primary visual cortex of occipital lobe: receives processed information from the thalamus

Visual Cortex (Do Not Memorize)

V1: visual map relating to the visual field of each rod and cone; sensitivity to small changes in visual field
V2: visual memory; responds to object orientation, spatial position, size, color, and shape
V3: processing of motion
V4: large patterns within the visual field; object orientation, spatial position, and color; best sensitivity to intermediate complexity of objects
V5: perception of motion and guidance of eye movements

The Ear

Responsible for hearing and equilibrium
Three parts: external, middle, and inner ear
External ear: transmits and amplifies airborne sound waves to the inner ear
Middle ear: fluid-filled; two sensory apparatuses: cochlea (converts sound into nerve impulses) and vestibular apparatus (responsible for equilibrium)
Inner ear: fluid-filled

External Ear

Auricle (pinna): skin-covered flap of cartilage; collects sound and directs it to the ear canal
External acoustic meatus (ear canal): possesses fine hairs and ceruminous glands; creates a barrier to capture airborne particles

Middle Ear

Tympanic Cavity: separated from external and inner ear by tympanic and oval/round windows
Auditory Ossicles (Malleus, Incus, Stapes): transmit sound vibrations from tympanic membrane to oval window and fluid of inner ear. Amplify sounds.; two muscles (Tensor Tympani and Stapedius) reflexively contract to diminish the strength of incoming sound waves.
Auditory Tube (Eustachian tube): Opens to the nasopharynx, equalizes pressure within tympanic cavity with atmospheric pressure

Inner Ear

Bony Labyrinth: bony structure within the temporal bone, similar in composition to IF, contains cavities and spaces filled with fluid called perilymph, and supports and suspends membranous labyrinth
Membranous Labyrinth: is located within the bony labyrinth, filled with endolymph, Houses the cochlea, vestibule, semicircular canals, each containing the organs for specific senses. Contains receptors for hearing (cochlear duct) and balance (utricle and saccule of the vestibule and semicircular ducts).

The Cochlea

Snail-shaped, spiral chamber; organ of hearing
Bony labyrinth partitioned into three chambers by two membranes
Cochlear Duct: membranous labyrinth; middle chamber,; houses the spiral organ; roof = vestibular membrane; floor = basilar membrane
Scala Vestibuli: superior chamber; floor = vestibular membrane; proximal end houses oval window
Scala Tympani: inferior chamber; roof = basilar membrane; distal end houses round window;
Helicotrema: point where scala vestibuli becomes scala tympani; located at the apex of the cochlea

The Spiral Organ

Organ of Corti; sensory receptors for hearing; arranged over basilar membrane
Hair cells: sensory receptors for hearing; arranged over basilar membrane; tectorial membrane (stiff/gelatinous membrane) above them
Spiral Ganglion: Possesses afferent fibers from hair cells; form cochlear branch of CN VIII (vestibulocochlear nerve)

Hair Cells

Sensory receptors for hearing
Mechanoreceptors, possess actin-stiffened stereocilia
One row of inner hair cells, act as sensory receptors
Three rows of outer hair cells, modulate activity within spiral organ

Sound

Sound waves are traveling vibrations of molecules
Alternating high and low pressure due to compression and rarefaction
sound energy dissipates as it travels
Sound is characterized by frequency (number of waves per second measured in Hertz, interpreted as pitch) and intensity (amplitude of the wave, measured in decibels, interpreted as loudness)

The Hearing Pathway

Sound waves collected by auricle, directed to tympanic membrane
Tympanic membrane vibrates, responding to pressure waves and transferring energy to auditory ossicles
Auditory ossicles amplify vibrations; transfer energy to oval window
Oval window vibrates at same frequency as the incoming sound waves
Oval window generates pressure waves in perilymph within scala vestibuli
Pressure waves cause vibrations of vestibular and basilar membrane.
Hair cells are distorted, initiating nerve signals in the cochlear branch of CN VIII
Pressure waves transferred to scala tympani and exit inner ear via round window

Equilibrium

Awareness and monitoring of head position
Regulated by the vestibular apparatus
Vestibule:
- Utricle and saccule (otolith organs): Senses head orientation and linear acceleration
Semicircular canals: detect angular acceleration

The Otolith Organs

Macula: contains receptor cells; consists of receptor cells, supporting cells, and gelatinous layers; hair cells with stereocilia arranged with and connected to a single kinocilium
Otoliths: calcium carbonate crystals within the gelatinous layer; provide mass and inertia to the otolithic membrane
Vestibular nerve branches: attach to hair cells; send a steady rate of nerve signals to the CNS to indicate head position

The Otolith Organs (continued)

Movement of the head affects otolithic membrane, altering sterocilia position. Bending toward the kinocilium results in stronger depolarizations; bending away, weaker depolarizations
Utricle: hair cells in vertical position, senses horizontal acceleration
Saccule: hair cells in horizontal position, senses vertical acceleration

Semicircular Canals

Three canals in differing planes at right angles to one another
Ampulla: broad end nearest the utricle; contains hair cells that detect angular motion
Hair cells embedded in the cupula (gelatinous dome)
Hair cells have kinocilia and stereocilia
No otoliths

Semicircular Canals (continued)

Turning the head causes endolymph to exert pressure on the cupula
Results from fluid inertia, causing hair cells to bend
Bending of hair cells increases or decreases the rate of nerve signals

Vestibular Sensation Pathways

Equilibrium stimuli transmitted as nerve signals along the vestibular branch of CN VIII
Vestibular branch axons project to the vestibular nuclei and cerebellum
Equilibrium information projected through vestibulospinal tracts to maintain muscle tone and balance
Vestibular nuclei transmit nerve signals to cranial nerve nuclei to control reflexive eye movements
Nerve signals transmitted to the thalamus, then to the cerebral cortex for processing and awareness of body position

Temporal Mapping for Sound: Vertical Plane

Understanding sound location depends on the timing of reflected sound from the pinna; requires only one ear
Sounds reflected off structures of the pinna and their relative delay are used for vertical plane location

Temporal Mapping for Sound: Horizontal Plane

Dependent on both ears
High-frequency sounds have different intensities received by both ears, indicating direction
Low-frequency sounds indicate the direction based on the delay between sound reception at both ears

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