Phototransduction Mechanisms

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Questions and Answers

What is the key function of rods in the visual system?

Detecting very high levels of light

Sensing very low levels of light (correct)

Facilitating depth perception

Processing color information

How long does it take cones to fully adapt to changes in light intensity?

About 10 minutes

About 3 minutes (correct)

About 1 minute

About 5 minutes

Which structure in the visual pathway allows each hemisphere of the visual cortex to receive input from both eyes?

Optic nerve

Optic chiasma (correct)

Primary visual cortex

Thalamus

What type of adaptation occurs when the eye adjusts to low light intensity?

Dark adaptation Signup and view all the answers

Which area of the visual cortex is primarily responsible for processing motion?

V3 Signup and view all the answers

What is the primary role of the lateral geniculate nucleus in the visual pathway?

Acting as a relay station for visual information Signup and view all the answers

What is the main function of opsin in the visual cycle?

Converting light into electrical signals Signup and view all the answers

What initiates the phototransduction process in photoreceptors?

The binding of light to cis-retinal Signup and view all the answers

What is the role of guanylate cyclase in phototransduction?

To produce cGMP from GTP Signup and view all the answers

How does phosphodiesterase (PDE) contribute to phototransduction?

By converting cGMP into GMP Signup and view all the answers

What happens when the concentration of intracellular cGMP decreases?

Photoreceptors hyperpolarize Signup and view all the answers

What is the first step in the visual pathway to the brain?

Activation of cones and rods in the retina Signup and view all the answers

Which component separates from the βγ subunit during the activation of transducin?

GTP-bound α subunit Signup and view all the answers

What effect does the closure of CNG channels have on photoreceptors?

Photoreceptors hyperpolarize Signup and view all the answers

What is cyclic nucleotide-gated (CNG) channel's primary function in phototransduction?

To allow Na+ influx into the cell Signup and view all the answers

What occurs to cGMP levels when photoreceptors are exposed to light?

cGMP levels decrease, closing Na+ channels. Signup and view all the answers

Which event directly follows the activation of phosphodiesterase (PDE) in the phototransduction pathway?

The α subunit of transducin separates from βγ subunits. Signup and view all the answers

What is the main effect of light on the photoreceptor cell membrane potential?

It causes the membrane to hyperpolarize. Signup and view all the answers

Which structure is primarily responsible for recycling all-trans retinal back to its cis form?

RPE cells (Retinal Pigment Epithelium). Signup and view all the answers

During the dark phase, which statement is true regarding the channels in photoreceptor cells?

Na+ channels are open, Ca++ channels are closed. Signup and view all the answers

What initiates the process of phototransduction in the cells of the retina?

Isomerization of retinal from cis to trans. Signup and view all the answers

How does hyperpolarization of photoreceptors affect bipolar cells during light exposure?

It inhibits bipolar cells by removing inhibition. Signup and view all the answers

What is the role of the Na+ channels in photoreceptors when light is absent?

They open, allowing depolarization to occur. Signup and view all the answers

What length of time is required for rods to achieve full adaptation to changes in light intensity?

Approximately 10 minutes Signup and view all the answers

Which process is characterized by the adjustment to high light intensity?

Light adaptation Signup and view all the answers

Which structure in the visual pathway allows for binocular vision by converging optic nerves?

Optic chiasma Signup and view all the answers

In the visual cortex, which area is primarily involved in responding to object orientation and spatial position?

V4 Signup and view all the answers

During the visual cycle, what component is primarily responsible for changing 11-cis retinal to all-trans retinal?

Opsin Signup and view all the answers

Which visual cortex area is best sensitive to processing large patterns within the visual field?

V3 Signup and view all the answers

What is the first structure in the visual pathway where axons from ganglion cells converge?

Optic nerve Signup and view all the answers

What role does the GTP-bound α subunit of transducin play in the phototransduction process?

It separates from the βγ subunit to activate phosphodiesterase (PDE). Signup and view all the answers

What is the effect of decreased levels of intracellular cGMP in photoreceptors?

It leads to the closure of CNG channels and hyperpolarization. Signup and view all the answers

Which component is responsible for converting cGMP into GMP during phototransduction?

Phosphodiesterase (PDE). Signup and view all the answers

What occurs in the outer segment of photoreceptors when light is present?

The outer segment depolarizes to transmit signals. Signup and view all the answers

Which substance is produced by guanylate cyclase from GTP?

cGMP. Signup and view all the answers

In the context of retinal recycling, what happens to cis-retinal?

It is converted into all-trans retinal upon absorption of light. Signup and view all the answers

How does the release of inhibitory neurotransmitter affect bipolar cells in the phototransduction pathway?

It hyperpolarizes bipolar cells, leading to signal propagation. Signup and view all the answers

What primary function do cyclic nucleotide-gated (CNG) channels serve in phototransduction?

They mediate sodium ion influx into photoreceptor cells. Signup and view all the answers

What occurs to the Na+ channels in photoreceptor cells when trans-retinal is present?

Na+ channels close Signup and view all the answers

What is the primary outcome of reduced intracellular cGMP levels in photoreceptors?

Closure of CNG channels Signup and view all the answers

Which event marks the starting point of the visual cycle?

All-trans retinal removal from opsin Signup and view all the answers

What best describes the role of bipolar cells when photoreceptors are hyperpolarized?

They are inhibited and do not release neurotransmitters Signup and view all the answers

What happens to Ca++ channels in photoreceptors during light exposure?

They close, inhibiting excitatory signaling Signup and view all the answers

What is the predominant state of rhodopsin during darkness?

Cis-retinal configuration Signup and view all the answers

Which steps are crucial in the process of retinal recycling?

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

What initiates the phototransduction signal cascade in response to light?

Separation of the alpha subunit of transducin 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 (Na⁺) moves into the cell down the concentration gradient.
Photoreceptor outer segment depolarizes.
Inhibitory neurotransmitter is released.

The Dark Current

In darkness, photoreceptor cells are depolarized.
CNG channels open in the outer segment, allowing sodium (Na+) influx.
Some calcium (Ca2+) influx also occurs.
Potassium (K+) channels open in the inner segment, causing potassium (K+) efflux.
Sodium-potassium (Na+-K+) pumps in the inner segment pump Na+ out and K+ in.

Phototransduction: Initiation

Guanylate Cyclase produces cGMP from GTP.
Photopigment (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.
Sodium (Na+) moves into the cell down the concentration gradient.
Photoreceptor outer segment depolarizes.
Inhibitory neurotransmitter is released.

Retinal Shape Initiates Visual Transduction

Light absorption results in retinal isomerization.
In the absence of light, retinal is in its cis form.
After light absorption retinal converts to trans form.
Trans form must convert back to cis form before absorbing another photon.
Conversion occurs in the pigmented epithelium.
Energy-dependent process.

Phototransduction: Activation

Guanylate Cyclase produces cGMP from GTP.
Photopigment (cis-retinal) is activated by a photon, undergoing a conformational change, and leaving the photopigment.
Transducin is activated and replaced by GTP.
Phosphodiesterase (PDE) is activated by photopigment.
Cyclic nucleotide-gated (CNG) channels are activated by cGMP.
Sodium (Na+) moves into the cell down the concentration gradient.
Photoreceptor outer segment depolarizes.
Inhibitory neurotransmitter is released.

Visual Cycle

Retinal recycling is a process of returning 11-cis retinal to the photoreceptor.
Rate-limiting step of retinal recycling.
All-trans retinal is removed from opsin.
Transported to RPE cells.
Steps in returning to 11-cis retinal.
Returned to opsin.
Rods take about 10 minutes to adapt, cones about 3 minutes.

Visual Adaptation

Photoreceptor cells sense very low light levels (rods) and high light levels (cones).
Process of adjusting to changes in light intensity.
Dark adaptation adjusts to low light.
Light adaptation adjusts to high light.

Visual Pathway to the Brain

Axons of ganglion cells converge to form optic nerves.
Optic nerves converge at the optic chiasma.
Medial fibers cross to other tracts.
Hemispheres of the visual cortex are informed by both eyes.
Provides binocular vision and improves depth perception.
Thalamus, lateral geniculate nucleus, and primary visual cortex.

Visual Cortex

V1: Visual map, sensitivity to small changes in the visual field.
V2: Visual memory.
V3: Responds to object orientation, spatial position, size, color, and shape.
V4: Processing of motion, large patterns within the visual field, object orientation, spatial position, and color information.
V5: Sensitive to intermediate complexities of objects, 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 sound waves to the inner ear.
Middle ear converts sound waves into nerve impulses.
Inner ear contains two sensory apparatuses: the cochlea and vestibular apparatus.

External Ear

The auricle, aka pinna, is a skin-covered flap of cartilage.
It collects and directs sound waves into the ear canal.
The external acoustic meatus (ear canal) has hairs and ceruminous glands, which create a barrier.
It directs sound to the tympanic membrane (eardrum).
Tympanic membrane is a membrane spanning the entrance to the middle ear, which vibrates when struck by sound waves.

Middle Ear

Tympanic cavity separated the external from inner ear.
Three bones (ossicles): malleus, incus, and stapes.
Transmit sound vibrations from tympanic membrane to the oval window of the inner ear.
Amplify sound waves.
Two muscles (tensor tympani and stapedius) protect the inner ear from loud sounds.
Auditory (Eustachian) tube equalizes pressure between tympanic cavity and atmosphere.

Inner Ear

Located within the petrous part of the temporal bone.
Bony labyrinth contains perilymph.
Membranous labyrinth contains endolymph.
Three structures: cochlea, vestibule, and semicircular canals.
Cochlea, Contains the spiral organ (organ of Corti) for hearing.
Vestibule - Contains utricle and saccule, organs for balance.
Semicircular canals - Possess semicircular ducts, organs for balance.

The Cochlea

Snail-shaped, spiral chamber that houses the spiral organ (organ of Corti).
Bony labyrinth partitioned into three chambers.
Cochlear duct (scala media) houses the spiral organ.
Scala vestibuli and scala tympani are superior and inferior chambers respectively.
Helicotrema is point where scala vestibuli becomes scala tympani.

The Spiral Organ (Organ of Corti)

Located within the cochlear duct (scala media).
Structures for hearing.
Contains hair cells arranged over the basilar membrane.
Tectorial membrane overlies hair cells.
Spiral ganglion, where fibers from the hair cells join with the cochlear branch of the vestibulocochlear nerve.

Hair Cells

Sensory receptors for hearing.
Possess actin-stiffened stereocilia.
Consist of one row of inner hair cells and three rows of outer hair cells that modulate activity in the spiral organ.

Sound

Sound waves are alternating high and low pressure caused by compressions and rarefactions of air molecules.
Sound energy dissipates as it travels from the source.
Sound is characterized by frequency (cycles per second, or Hertz) and intensity (amplitude, measured in decibels, interpreted as loudness).

The Hearing Pathway

Sound waves collect by auricle, directed to tympanic membrane.
Tympanic membrane vibrates, moves auditory ossicles.
Stapes at oval window creates pressure waves in perilymph of scala vestibuli.
Waves cause vestibular membrane to move then endolymph in cochlear duct then basilar membrane displacement.
Hair cells in spiral organ are distorted, which initiates nerve signal.
Remaining pressure waves travel to scala tympani and exit via the round window.

Frequency and Amplitude Discrimination

Frequency discrimination refers to the ability to discern sound frequencies.
Sound waves travel to the region of the spiral organ that responds to that frequency.
Energy dissipates so wave dies out.
Amplitude discrimination is dependent on amplitude of vibrations of the basilar membrane. More vigorous vibrations mean louder sound.

Auditory Pathway

Movement of basilar membrane creates signals.
Axons converge to form cochlear branch of vestibulocochlear nerve.
Terminates in cochlear nucleus of medulla.
Secondary neurons project along two pathways: superior olivary nuclei (localize sound), and inferior colliculi (reflexes to sounds).
Neurons project from inferior colliculus to the medial geniculate nucleus (initial processing) and then to auditory cortex in temporal lobe.

Temporal Mapping for Sound (Vertical Plane)

Understanding sound direction (vertical plane) depends on reflected sound waves from the pinna and their relative timing.
Requires only one ear.

Temporal Mapping for Sound (Horizontal Plane)

Determining horizontal sound location depends on both ears.
High frequencies are identified by differences in intensity between the two ears.
Lower frequencies are identified by the time difference between sounds arriving at the ears.

Equilibrium

Awareness and monitoring of head position is regulated by the vestibular apparatus.
Three structures: utricle, saccule, and semicircular canals, are in three separate planes.
Utricle and saccule, otolith organs, detect head position and linear acceleration changes.
Semicircular canals detect angular acceleration.

The Otolith Organs

Macula region contains receptor cells, support cells, and a gelatinous layer.
Hair cells have stereocilia and one kinocilium.
Otoliths (CaCO₄ crystals) provide mass and inertia to the membrane.
Vestibular nerve branches attached to hair cells deliver signals to the CNS.

The Otolith Organs (continued)

Movement of the head alters the position of the otolithic membrane, affecting the position of stereocilia on hair cells.
Bending towards the kinocilium causes depolarization and increased nerve signal frequency.
Bending away from the kinocilium causes hyperpolarization and decreased nerve signal frequency.
Utricle, detects horizontal acceleration.
Saccule, detects vertical acceleration.

Semicircular Canals

Three canals in different planes.
At their ends, ampullae have hair cells embedded in a cupula of gelatinous substance.
Hair cells have kinocilium and stereocilia.
Movement of the head causes endolymph movement, affecting the cupula and bending the stereocilia on hair cells.
This changes the rate of nerve signals to the brain.

Vestibular Sensation Pathways

Equilibrium stimuli are transmitted along the vestibular branch of CN VIII.
Vestibular branch axons project to the vestibular nuclei and cerebellum.
Vestibular nuclei project to vestibulospinal tracts, cranial nerve nuclei, thalamus, and cerebral cortex.
Conscious awareness of body position occurs in the cerebellum and cerebral cortex.

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Description

Explore the intricate mechanisms of phototransduction including the roles of guanylate cyclase, transducin, and cyclic nucleotide-gated channels. This quiz covers the differences in phototransduction during light and darkness, emphasizing the physiological changes in photoreceptor cells. Test your knowledge on how these processes affect vision.