Physiology of Special Senses: Hearing & Balance

Question 1

The ear is divided into three major areas:

Answer 1

❑ External ear
❑ Middle ear
❑ Internal ear

• The external and middle ear structures are involved with hearing only and are rather simply engineered.
• The internal ear functions in both equilibrium and hearing and is extremely complex.

Question 2

The external (outer) ear consists of:

Answer 2

• the auricle,
• the external acoustic meatus,
• eardrum.

Question 3

Describe the auricle (pinna)

Answer 3

• Composed of elastic cartilage
• Has a Thick helix
• Dangling lobule

Question 4

Describe the external acoustic meatus

Answer 4

• Extends from the auricle to the eardrum
• Lined with skin bearing hairs, sebaceous glands, and cereminious glans (secrete cerumen)

Question 5

Describe the tympanic membrane

Answer 5

• Connective tissue membrane
• covered by skin and mucosa
• sound waves make it vibrate which turns sound energy to the bones of the middle ear

Question 6

Describe the middle ear (tympanic cavity)

Answer 6

• Flanked laterally by the tympanic membrane
• Has an oval and round windown
• Has an auditory tube (normally flattened and closed but swallowing or yawing opens it - important bc vibration only occurs when pressure is the same in both ears)

Question 7

What are the Three bones in the middle ear

Answer 7

• malleus
• incus
• stapes

Question 8

What opens the pharyngotympanic tube. Why?

Answer 8

swallowing or yawning opens it briefly to equalize pressure in the middle ear cavity with external air pressure

Question 9

What transmit the vibratory motion of the eardrum to the oval window

Answer 9

The ossicle

Question 10

What are the muscles associated with the ossicle? Describe them

Answer 10

• The tensor tympani arises from the wall of the pharyngotympanic tube and inserts on the malleus.
• The stapedius runs from the posterior wall of the middle ear to the stapes.

Question 11

What happens to the tensor tympani and stapedius we hear loud sounds?

Answer 11

the tensor tympani and stapedius contract reflexively to limit the ossicles’ vibration and minimize damage to the hearing receptors.

Question 12

The internal ear has two major divisions:

Answer 12

• The bony labyrinth is a system of channels worming through the bone. It’s filled with perilymph
• The membranous labyrinth is a continuous series of membranous sacs and ducts contained within the bony labyrinth following its contours. It contains endolymph, which is chemically similar to K+ rich intracellular fluid.

Question 13

The bony labyrinth has three parts

Answer 13

Vestibule, Semicircular canals, and Cochlea

Question 14

Explain human hearing:

Answer 14

1. Sounds set up vibrations in air, which Beat against the eardrum
2. It pushes a chain of tiny bones and Press fluid in the internal ear against membranes
3. Set up shearing forces that pull on the tiny hair cells
4. Stimulate nearby neurons and Give rise to action potentials
5. Travel to the brain, interprets them, and you hear

Question 15

The route of sound to the inner ear follows which pathway:

Answer 15

• Outer ear – pinna, auditory canal, eardrum
• Middle ear – malleus, incus, and stapes to the oval window
• Inner ear – scalae vestibuli and tympani to the cochlear duct - Stimulation of the organ of Corti and Generation of impulses in the cochlear nerve

Question 16

What is Step 1 of the pathway of sound waves?

Answer 16

Tympanic membrane.

• Sound waves entering the external acoustic meatus strike the tympanic membrane and set it vibrating at the same frequency.
• The greater the intensity, the farther the membrane is displaced in its vibratory motion.

Question 17

What is Step 2 of the pathway of sound waves?

Answer 17

Auditory ossicles.

• The motion of the tympanic membrane is amplified and transferred to the oval window by the ossicle lever system.
  ** increased pressure overcomes the stiffness and inertia of cochlear fluid and sets it into wave motion.

Question 18

What is Step 3 of the pathway of sound waves?

Answer 18

Scala vestibuli.

• As the stapes rocks back and forth against the oval window, it sets the perilymph in the scala vestibuli into a similar motion.

Question 19

What is Step 4a of the pathway of sound waves?
low frequency sounds

Answer 19

Helicotrema path.

• Sounds of very low frequency (below 20 Hz) create pressure waves that take the complete route through the cochlea, up the scala vestibuli, around the helicotrema, and back toward the round window through the scala tympani.
• These low- frequency sounds do not activate the spiral organ and so are below the range of hearing.

Question 20

What is Step 4b of the pathway of sound waves?
high frequency sounds

Answer 20

Basilar membrane path.

• pressure waves are transmitted through the cochlear duct, into the perilymph of the scala tympani and vibrates the basilar membrane.
• This vibration activates hair cells through the basilar membrane, causing action potentials to be sent to the brain.

Question 21

Sound waves make what vibrate.

Answer 21

the basilar membrane

Question 22

Describe inner hair cells

Answer 22

• sensory cells
• nerves extending from them send acoustic information to the brain

Question 23

Describe outer hair cells

Answer 23

• motile
• amplify and modify the movement of the basilar membrane.
• improve hearing sensitivity

Question 24

Describe stereocilia

Answer 24

• protrude into the K+ endolymph
• the longest are enmeshed in the overlying stiff gel-like tectorial membrane.

Question 25

What happens when there is Bending cilia of inner hair cells

Answer 25

• Opens mechanically gated ion channels, Ca and K of inner hair cells
• Causes a graded potential and the release of a neurotransmitter (probably glutamate)

Question 26

Outer hair cell motility serves two functions:

Answer 26

• Increases the responsiveness of the inner hair cells by amplifying the motion of the basilar membrane (to increases the cochlea’s ability to distinguish small differences in frequency)
• help protect the inner hair cells from damage.

Question 27

Describe the auditory pathway

Answer 27

1. AP in the cochlear receptors pass through the spiral ganglion & the afferent fibers of the cochlear nerve to the cochlear nuclei of the medulla
2. neurons project to the superior olivary nucleus.
3. axons ascend in the lateral lemniscus to the inferior colliculus (midbrain), which projects to the medial geniculate nucleus of the thalamus
4. Axons of the thalamic neurons then project to the primary auditory cortex

Question 28

The equilibrium receptors of the vestibular apparatus can be divided into two functional arms:

Answer 28

• The receptors in the vestibule monitor linear acceleration and the position of the head with respect to gravity. Because gravity is constant, called static equilibrium.
• The semicircular canals monitor changes in head rotation, called dynamic equilibrium.

Question 29

Describe the macula

Answer 29

• Sensory receptor organ
• Monitor the position of the head in space
• Play a key role in controlling posture
• Respond to changes in straight-line speed and direction, but not rotation

Question 30

Describe the utricle

Answer 30

• the macula is horizontal, and the hairs are vertically oriented when the head is upright.
• the utricular maculae respond to horizontal movements and tilting the head, because

Question 31

Describe the saccule

Answer 31

• the macula is nearly vertical, and the hairs protrude horizontally into the otolith membrane.
• The saccular maculae respond best to vertical movements, such as the sudden acceleration of an elevator.

Question 32

What happens to hairs cells when they bend toward the kinocilium or away from the kinocilium

Answer 32

• Toward - the hair cells depolarize, stepping up their pace of NT release, and more impulses travel up the vestibular nerve to the
  brain.
• Away - the receptors hyperpolarize and release less NT, generating fewer impulses.

Question 33

Where are The cell bodies of the sensory neurons located

Answer 33

superior and inferior vestibular ganglia

Question 34

The receptor for rotational acceleration, called the …

Answer 34

crista ampullaris

Question 35

Each crista ampullaris is composed of:

Answer 35

• Supporting cells
• Hair cells whose structure and function are the same as the hair cells of the cochlea and maculae.
• Ampullary cupula, the gelled mass which resembles a pointed cap.

Question 36

Describe The Equilibrium Pathway to the Brain

Answer 36

• The transmission sequence begins when the hair cells in the vestibular apparatus are activated.
• Impulses travel initially to one of two destinations: the vestibular nuclei in the brain stem or the cerebellum.
• the information is integrated and then send commands to brain stem motor centers that control the extrinsic eye muscles and reflex movements of the neck, limb, and trunk muscles
  *

Question 37

What is the vestibular nuclei

Answer 37

• major integrative center for balance,
• receive inputs from the visual and somatic receptors, particularly from proprioceptors in neck muscles that report on the position of the head.