Sense Organs; Hearing

Question 1

Ear

Answer 1

The ear is the organ of hearing.
Anatomically the ear is divided into three parts;
1. Outer ear
2. Middle ear
3. Inner ear
The middle ear is connected to the nasopharynx by the eustachian tube (aka ‘pharyngotympanic’ tube)

Pharyngo = pharynx, tympanic = ear drum

Question 2

Ear; Nerve supply

Answer 2

The nerve supply is the 8th cranial nerve – vestibulocochlear nerve

Question 3

Ear; Outer ear

Answer 3

The outer ear consists of:
1. Auricle / Pinna
2. External auditory canal
3. Tympanic membrane (ear drum)

Question 4

Outer ear;
Auricle / Pinna

Answer 4

Flap of elastic cartilage covered by skin that traps and directs sound waves into the external auditory canal

Question 5

Outer ear;
Tympanic membrane

Answer 5

AKA ear drum
A thin semi-transparent partition between the external auditory canal and middle ear
* Lined with simple cuboidal epithelium, elastic and collagen fibres
* Transmits sounds from the outer ear (outside) to the auditory ossicles. Converts sound waves into mechanical vibration

Question 6

Middle ear

Answer 6

A small air filled cavity in the temporal bone.
Contains the auditory ossicles:
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)

• Transmits sound in the form of vibration from the tympanic membrane to cochlea
• Stapes attaches to base of stapes ‘oval window’, which connects to the cochlea
• A small muscle called ‘stapedius’ dampens large vibrations and is innervated by the facial nerve (protects the oval window) = hence auditory symptoms in Bell’s palsy

Question 7

Inner ear

Answer 7

The inner ear provides information on balance and hearing.
* It is also called the labyrinth because of its complicated series of canals.
* It consists of an outer bony labyrinth that encloses an inner membranous labyrinth
* The bony labyrinth consists of a series of cavities in the temporal bone, divided into;
1. 3 semicircular canals and vestibule (receptors for balance)
2. Cochlea (receptors for hearing)
* The vestibule is the oval central portion

Question 8

Inner ear; Fluid

Answer 8

The labyrinth contains perilymph and endolymph (fluid).

Question 9

Cochlea

Answer 9

• Anterior to the vestibule – a spiralled hollow chamber that makes almost three turns around a central bony core
• Contains epithelial cells lined with hair cells (sensory cells topped with 40-80 cilia each, called ‘stereocilia’)
• Stereocilia are the receptors for hearing that extend into the endolymph
• Fluid movement (vibration) causes stereocilia movement and initiates an electrical impulse

Question 10

Vestibulocochlear Nerve

Answer 10

The vestibulocochlear nerve (cranial nerve VIII) provides sensory information for hearing and balance.
* The nerve contains two parts;
1. Vestivular fibres; The vestibular fibres consist of nerve branches that synapse with receptors for balance
2. Cochlear fibres. The cochlear nerve provides ‘hearing’
* The vestibularcochlear nerve ganglia is located within the brainstem (pons / medulla oblongata)
* The nerve can be damaged with a basilar skull fracture

Question 11

Sound Sensation - waves / impulses

Answer 11

1. Sound waves (pressure waves) 
2. Mechanical vibration (sound makes the eardrum vibrate, the eardrum makes the bones vibrate) 
3. Fluid waves (the bones make the fluid move and the hair cells bend inside the cochlea 
4. Nerve impulse (the auditory nerve sends the message to the brain)

Question 12

Sound Physiology

Answer 12

Sound waves are vibrations in the air.
1. The auricle (pinna) concentrates the waves, directing them along the external auditory canal causing the tympanic membrane to vibrate.
2. Vibrations are transmitted through the middle ear by movement of the three auditory ossicles
3. The footplate of the stapes (stirrup) rocks at the oval window setting up fluid waves in the cochlear perilymph
4. Pressure waves are transmitted into the cochlea, which causes the round window to bulge into the middle ear
5. This pressure wave is transmitted into the endolymph
6. This results in vibration of the basilar membrane and hair cells
7. The bending of hair cell stereocilia creates a receptor potential – generating an action potential
8. The nerve impulses which have been generated, pass to the brain via the vestibulocochlear nerve
9. The vestibulocochlear nerve transmits the impulses to the hearing area in the cerebrum where sound is perceived

Question 13

Properties of Sound; Pitch

Answer 13

Pitch;
* Frequency of sound waves, measured in Hz (Hertz)
* The higher the frequency of vibration, the higher the pitch

Question 14

Properties of Sound; Volume

Answer 14

Volume;
* Amplitude of sound waves measured in decibels (dB)

Question 15

volume; Decibel scale

Answer 15

Decibel scale:
* Smallest audible sound near silence is 0 dB
* A sound 10 x more powerful is 10 dB
* A sound 100 x more powerful is 20 dB
* A sound 1000 x more powerful is 30 dB

Question 16

Common sounds decibel ratings

Answer 16

• Near total silence – 0 dB
• A whisper – 15 dB
• Normal conversation – 60 dB
• A lawnmower – 90 dB (prolonged above 90 dB = hearing loss)
• A car horn – 110 dB
• A rock concert or a jet engine – 120 dB
• A gunshot or firecracker – 140 dB (causes damage and pain)

Question 17

White Noise

Answer 17

A constant noise that contains all the different frequencies of sound (a mixture of different sound waves)
* A background noise that the brain ignores
* Used to mask other sounds – privacy enhancers, sleep aids, tinnitus masking
* White noise CDs, when used with headphones, can aid concentration by blocking out irritating or distracting noises in a person’s environment

Question 18

Eustachian Tube (‘Auditory Tube’)

Answer 18

The Eustachian tube connects the nasopharynx to the middle ear
* The tube is closed but can open for air to equalize pressure between the middle ear and atmosphere – ears popping on aircraft.
* Yawning and swallowing contracts the neck muscles, opening the tube
* It is essential that air can escape the middle ear, or damage would occur with pressure changes
* The Eustachian tube also functions to drain mucus and is a common route for infection to spread between the pharynx and middle ear (more horizontal in children)

Question 19

Balance; semi-circular canals and vestibular

Answer 19

The semi-circular canals and vestibular provide information about head position
* The walls of the vestibule and semi-circular canals contain specialised hair cells with stereocilia
* A dense layer of calcium carbonate crystals extends over and rests on the hairs called ‘otolithic membrane’
* Any change in head position causes movement in the perilymph and endolymph – bending hair cells and stimulating the sensory nerve endings
* This action potential (via CN VIII) is transmitted to the cerebellum

Question 20

Balance; cerebellum

Answer 20

The cerebellum makes postural adjustments in order to maintain balance. This occurs through its input of;
1. Vestibular feedback (from inner ear)
2. Visual feedback (from eyes)
3. Proprioceptors in the skeletal muscles, joints and surrounding ligaments
The three sources of information are coordinated and efferent impulses pass to the cerebrum and skeletal muscles