Hearing

Pressure waves in the air.

What is sound?

Sound waves are extremely low energy so specialised adaptations are required to perceive them.

Why is the ear so heavily adapted for its function?

Water is better at conducting pressure waves than air meaning that it is easier to perceive sound underwater with less specialised adaptations.

Why don't fish have as well-developed ears as terrestrial animals?

-Travels down the auditory canal to reach the tympanic membrane

-Causes vibration of the tympanic membrane which oscillates the malleus, incus and stapes

Describe the pathway sound waves take to reach the tympanic membrane and what occurs when they hit it

-Sound is amplified

-This vibrates the fluid-filled cochlea

What occurs when auditory ossicles are vibrated?

A = Oval window (tympanic membrane)

B = Basilar membrane

C = Scala vestibuli

D = Scala tympani

E = Scala media

F = Round window

G = Stirrup

Label this section of an uncoiled cochlea

Basement membrane.

What is an alternate name for the basilar membrane of the cochlea?

Causes a standing wave in the basement membrane

What does vibration of the fluid in the cochlea cause in the basement membrane?

Pressure equalisation of the cochlea with environmental pressure to prevent tympanic membrane (oval window) rupturing.

What is the function of the round window?

-A wave travels along the basement membrane

-This causes hair cells to get squished against the non-moving tectorial membrane

-This causes the generation of electrical signals which can be perceived by the brain

Describe what occurs when the oval window (tympanic membrane) is vibrated

-Thicker and more pliable at the apex (end) of the cochlea

--Moves mostly in response to low frequencies of sound

-Thinner and less pliable at the start of the cochlea

--Moves mostly in response to high frequencies of sound

Describe the structure of the basement membrane in the cochlea

-Scala vestibuli

-Scala media

Scala tympani

What are the 3 chambers of the cochlea?

Scala vestibuli and scala tympani.

Which chamber of the cochlea contains perilymph?

Scala media.

Which chamber of the cochlea contains endolymph?

Between the tectorial and basement membrane.

Where are the sensory hair cells of the cochlea found?

Tectorial membrane.

Which membrane of the cochlea remains stationary regardless of sound wave conduction?

Endolymph.

Which fluid found in the cochlea has unusually high potassium content?

Stria vascularis.

What produces endolymph in the scala media?

-Sound waves cause vibration of the tympanic membrane

-This vibration is amplified by transmission through the auditory ossicles

-This causes the basilar (basement) membrane in the cochlea to vibrate, causing the sensory hairs to bend when pushed against the tectorial membrane

-This bending of the sensory hairs causes ion channels to open which causes depolarisation of the hair cells and hence a sound signal action potential is generated

-This action potential can then be interpreted by the brain as sound

Describe how sound is converted to electrical signals that the brain can interpret

Organ of cortii.

What is the name of the structure in the cochlea that allows sound conduction into action potentials?

High frequencies.

What frequency of sound do older animals struggle to perceive?

Lower frequencies.

What frequency of sound is generally suppressive to animal behaviour?

Higher frequencies.

What frequency of sound is generally excitatory to animal behaviour?

The start of the cochlea, decreasing as you reach the centre.

In the cochlea, which region typically picks up the higher frequency sound waves?

The frequency at which that neuron fires most easily (at the lowest tone volume).

What is the characteristic frequency of an auditory neurone?

-Phase locking

-Frequency coding

What are the main ways of frequency perception in the ear?

-Action potentials are generated in phase with the frequency of the incoming sound wave

-Each neuron here has a characteristic frequency

-Therefore generally, it only works for low-frequency sound waves

What is phase locking? What frequencies of sound does it typically work for?

The membrane may be vibrating at thousands of Hz and action potentials can not be fired that rapidly.

Why is phase locking rarely used to perceive high frequency sound waves?

-Particular neurons are programmed to fire upon the vibration of the basilar membrane at a particular frequency

-These neurons send their action potentials to particular centres in the brain that are programmed to perceive the action potential as a signal that the basilar membrane is vibrating at a particular frequency

-This is typically used for high frequencies of sound as action potentials can't be fired to the same frequency

What is frequency coding (tonotopy)? What frequencies of sound does it typically work for? Why?

1) Different frequencies of sound are detected at different points along the length of the cochlea

2) The highest frequencies of sound are detected by cells around the thinnest part of the basilar membrane in the base of the cochlea

3) Low frequencies are detected around the thick part of the basilar membrane at the (thin end) apex of the cochlea

4) Afferent nerves along the length of the cochlea therefore have their own characteristic frequency (i.e. the frequency at which they fire the most readily)

5) These afferent neurones are tuned to particular frequencies either by using phase locking or tonotopy

How is the frequency of a sound determined by the brain and ear?

Brainstem Auditory Evoked Response (BAER).

What diagnostic tool can you use to measure the perception of sound in an animal?

-Where sound cannot pass into the ear

Causes:

-Tumours

-Perforation of the ear drum

-Outer and middle ear infections

-Wax build-up in the ear canal

-Ear mites

What is conductive deafness? List some causes

Yes, by treating the root cause.

Is conductive deafness generally reversible?

-Where nerves associated with the ear do not function properly

Causes:

-Genetics (congenital)

--e.g. blue-eyed white cats, white bull terriers, dalmations; albino animals prone

-Inner ear infections

-Drug toxicity (antibiotics)

-Noise trauma

-Age-related degeneration

What is nerve deafness? List some causes

No, they are generally irreversible.

Is nerve deafness generally reversible?

-Type 1 = large, fat, myelinated and fast-conducting; from inner hair cells

-Type 2 = thin, unmyelinated and slow; from outer hair cells

What are the main types of neuron in the organ of corti? Describe their structure

Type 1 - 95% of afferents are T1.

Which type of neuron is most common in the ear?

Auditory nerve.

What do the axons of type 1 afferents form?

Type 1.

Which type of neuron is sound mostly carried by?

-To amplify or dampen sounds as necessary

-Allows hearing modulation and focussing

-Otoacoustic sounds

What are the outer hair cells thought to be used for?

1) Cochlea to dorsal and ventral cochlear nuclei via auditory nerve

2) Cochlear nuclei to synapse in the superior olivary complex (and decussate)

3) Superior olivary complex to lateral lemniscus

4) Lateral lemniscus to inferior/caudal colliculus then to medial geniculate nucleus of the thalamus via CN VIII

5) Medial geniculate nucleus to auditory radiation (in auditory centre (in temporal lobe of cerebral cortex))

Describe the auditory neural pathway

Perceives:

-Location of sound

-Pitch

Controls:

-Startle reflex

-Ocular reflex

What is the function of the inferior (caudal) colliculus with regards to sound perception?

Time delay:

-Which ear perceives the sound first

Volume or level differences:

-Which ear perceives the sound louder

What are the two ways an animal can detect the source of a sound?

-Sends signals to the motor cortex of the brain to control the body's response to sound

-Also sends signals to the primary auditory cortex and the auditory association cortex for further processing of sound

What is the function of the medial geniculate nucleus of the thalamus with regards to sound?

-Accepts auditory input from the medial geniculate nucleus

-Processes sounds (e.g. for recognition and association)

What is the function of the auditory cortex of the cerebral cortex with regards to sound?