Special Sense Lecture 3 - Hearing/Vision

Similarities between On and OFF pathway of vision

Steps 1-3 are the same - Photoreceptor is depolarized in absence of light rays, Light hyperpolarizes photoreceptor, decreased glutamate release on bipolar cell

Differences between ON and OFF pathway of vision

Pathways respond opposite to light

In ON pathway - reduce inhibition causes ON bipolar cell to depolarize and release glutamate to ganglion cell

In OFF pathway - decreased glutamate from bipolar cell causes inhibition, preventing action potentials

Where is the visual cortex located

In the occipital lobe

Information pathway of lateral vision

1. Lateral information hits nasal region of retina
2. Information travels via retinal ganglion axons towards the optic chiasm (where optic nerve meets brain base)
3. Information from nasal retina crosses to the contralateral side at the optic chiasm
4. Information synapses on to the lateral geniculate nucleus
5. Neurons take information to the visual cortex

Information pathway of medial vision

1. Medial information hits the temporal region of the retina
2. Information travels via retinal ganglion axons towards optic chiasm
3. Information stays on the ipsilateral side of the optic chiasm (same side)
4. Information travels through the medial geniculate nucleus to the visual cortex

Use of the coexistence of the ON and OFF pathways

Improves image resolution by increasing the brain's ability to perceive contrast at edges or borders

How does hearing work

Based on the physics of sound, physiology of the ear, nerves in the brain, and the brain processing of acoustic information

Pinna

The physical/external ear that resides outside the head

Temporal lobe

Region of brain containing the auditory cortex

Most common medium in which we hear sound energy

Air

How is sound created

Movement of an object displaces air particles

Zones of compression

Regions where air molecules that are tightly packed or close together

Zone of rarefaction

Regions with low amount of air molecules

How is amplitude (volume) determined

Number of air molecules located within zone of compression

How is frequency (pitch) determined

The distance between zones of compression/rarefaction in a given time

The faster the vibration...

The higher the pitch

How does our ear hear sounds

Zones of compression and rarefaction hit the ear

Role of the outer ear and external auditory canal

Funnel the zones of compression and rarefaction in towards the middle and inner ear

Tympanic membrane

Vibrates in and out at the same amplitude and frequency consistent with the features of the sound you are listening to

Cochlea

Inner ear

Three bones of the middle ear

Malleus, incus, stapes

Role of the ear bones

Amplify sound 15-20X

Attachment of the bones

Tympanic membrane --> Malleus --> Incus --> Stapes

Why is amplification via the ear bones necessary

Outer/middle ear are filled with air, the inner ear is filled with fluid

Role of the ear skeletal muscles

Contract to dampen movement of the bones, reducing impact on the inner ear in response to loud, consistent sounds

Muscle attached to the malleus

Tensor tympani muscle

Muscle attached to the stapes

Stapedius muscle

Role of the stapes

Terminates on the inner ear onto the oval window. Pushes fluid forward into the middle ear (causing movement)

3 components of the cochelea

Scala vestibuli, scala tympani, cochlear duct

Scala vestibuli

Top of the cochlea - contains perilymph (same composition of CSF)

Scala tympani

Middle of the cochlea - contains perilymph

Cochlear duct

Bottom of the cochlea - contains endolymph and contains the sensory receptors of the auditory system

Pathway of sound from the external auditory canal

1. Sound waves come through he external auditory canal
2. Soundwaves move tympanic membrane
3. Tympanic membrane moves bones of the middle ear
4. Stapes pushes against oval window
5. Oval window movement causes movement of perilymph down across the cochlear duct, moving from scala vestibuli towards scala tympany
6. Movement of fluid causes activation of sensory receptors of the cochlear duct

Hair cells

Specialized auditory receptor of the cochlear duct that allow for sound transduction of vibrations into neural signals. Takes place on the organ of corti

2 groups of hair cells

Single row of inner hair cells and three rows of outer hair cells

Role of the single row of inner hair cells

Stereocilia extend into endolymph and transduce pressure waves caused by fluid movement into receptor potentials

Role of the 3 rows of outer cell hairs

Attach to the basilar membrane - different regions of the basilar membrane vibrate at different frequencies

How sound reaches the CNS

Hair cells in the region of the most vibration undergo the most mechanical deformation. This information is sent to the CNS, interpreting the hair cell stimulation as a particular frequency

Does the organ of corti detect different sound frequencies

Yes. Different parts of the basilar membrane correspond to different frequencies

Vestibucoclear nerve

Takes auditory information from the ear towards the brain

Pathway of sound from the eardrum to the hair cells

1. Tympanic membrane vibrates based on the characteristics of the sound
2. activates the malleus, incus and stapes in the middle ear → pushes against the oval window and amplifies the movements
3. pushes the perilymph down across the cochlear duct
4. when fluid is pushed down, the basilar membrane moves up and down based on the amplitude and the frequency (volume and the pitch) of the sounds
5. the hair cells move back and forth when basilar membrane moves up and down

Receptor pathway activation of hair cell movement

1. Stereocilia are bent by vibrations
2. Mechanically-gated cation channels open, allowing potassium to flow into the cell (down the concentration gradient)
3. Hair cell depolarizes, generated graded potentials
4. Glutamate is released from hair cell onto the afferent neurons (the vestibulocochlear nerve)
5. Signal travels from the afferent neurons to the brain
6. Hair cells are bent in opposite direction closes channels, allowing cells to repolarize

Two ways to restore hearing

Hearing aids, cochlear implants

When are hearing aids used

When the auditory of the ear is not as sensitive

Hearing aid function

Amplifier placed in auditory canal to amplify existing sounds

When are cochlear implants used

When someone has permanent damage to the outer, middle, or inner ear (machinery does not work)

Cochlear implant function

Speaker put on the outside of the head converts sound to electrical impulses. Impulses are sent to the vestibulocochlear nerve, bypassing the ear machinery