Phototransduction Mechanisms

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.