Ear/ Visual Pathway & Phototransduction

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