Special senses - hearing

Question 1

What is the external ear? What can be found within it and what are their functions?

Answer 1

Auricle = elastic fibers; a lobule that lacks cartilage

• the auricle funnels sound waves into the external acoustic meatus

Cerumen = Wax produced in the ear to deter the entrance of dust or small insects

Tympanic membrane = thin layer of CT, skin layer covered in mucosa

• boundary between the external and middle ear
• AKA the eardrum

Question 2

What is the external acoustic meatus?

Answer 2

A pathway from the auricle to the tympanic membrane (eardrum)

Found in the external ear

Question 3

What is the middle ear? What is found within ?

Answer 3

An air filled cavity with eardrum laterally and a bony wall with 2 openings - the oval window and the round window medially

The pharyngotympanic tube links the middle ear with the nasopharynx (otitis media)

Can find the ossicles = the 3 smallest bones in the body; linked by ligaments and mini synovial joints

2 tiny muscles contract to protect hearing receptors (tensor tympani and stapedius) by limiting ossicle vibration

Question 4

What are the two main structures found in the inner ear and what are their functions?

Answer 4

Bony labyrinth;

• tortuous canals worming their way through temporal bones
• contains vestibule, cochlea, semicircular canals
• filled with perilymph and continuous with and similar in composition to CSF

Membranous labyrinth;

• series of membranous sacs within the bony labyrinth
• filled with potassium rich endolymph

Question 5

What do the 2 labyrinths within the inner ear do?

Answer 5

the 2 fluids conduct sound vibrations and respond to mechanical forces linked to changes in body position and accleration

Question 6

What is a vestibule? Where can it be found?

Answer 6

Found in the bony labyrinth of the inner ear

Vestibule = central cavity of bony labyrinth

• contains 2 sacs (utricle and saccule) that are suspended in perilymph
• saccule to cochlea and utricle to semicircular canals
• monitors head position and both contain equilibrium receptors (maculae) that respond to gravity

Question 7

What are the semicircular canals?

Answer 7

3 canals each lying in one of the planes (transverse, coronal/frontal, sagittal)

• membranous semicircular ducts line each canal and links to the utricle
• ampulla = swolen end of each canal which house equilibrium receptors in the crista ampullaris -> these respond to angular movements of the head

Question 8

What is the cochlea? What structures are found within it?

Answer 8

A spiral bony conical chamber

• coils around a bony pillar (modiolus) - a thin elongated bony strcuture
• contains the cochlear duct at the ends of the cochlear apex
• contains the spiral organ (organ or Corti for hearing receptors)

Structures within;

• Scala vestibuli - perilymph filled, continuous with vestibule and begins at oval window
• Scala media - endolymph filled; simply the cochlear duct, contains hearing receptors inside the fluid compartement
• Scala tympani - perilymph filled, links to round window below the cochlear duct

Question 9

What is the helicotrema ?

Answer 9

Allows the 2 perilymph chambers to be continuous

Fluid wise is continuous but still 2 seperate and different compartements

Question 10

What is the spiral organ? Where can it be found?

Answer 10

Found sitting on the basilar membrane

The basilar membrane is narrow and thick near the oval window, gradually widening and thinning closer to the cochlear apex - helps to distinguish between deeper and higher cells

Consists of supporting cells and cochlear hairs between tectorial and basilar membranes

Tectorials = gel-like membrane activated by sound waves

Question 11

What are the two characteristics of soundwaves? What do each of them correlate to physically?

Answer 11

Frequency of a soundwave - pitch (Hz) [how high or low a sound is]

Amplitude - loudness (dB) [sound intensity measured on a logarithmic scale]

Question 12

Describe the process of the transmission of sound

Answer 12

Soundwaves travel through air, membranes, bones and fluid to stimulate receptor cells in the spiral organ

Louder sounds = higher deflection of tympanic membrane

Only sounds within our hearing range can vibrate basilar membrane to activate hair cells

Question 13

What parts of the basilar membrane do high notes and low notes activate?

Answer 13

Note ; fibers near the oval window are short and stiff, they are longer and more floppy closer to the cochlear apex, meaning they are easier to vibrate

High notes activate basilar membrane near the base
Low notes activate basilar membrane closer to the apex

Question 14

Describe the outer hair cells and their functions

Answer 14

• not involved in sound reception; more so have protective and supportive roles
• they help depolarize and repolarize in response to basilar membrane (contracting and stretching)
• increase the responsiveness of inner hair cells by amplifying motion of the basilar membrane
• protects inner hair cells when efferent fibers from the brain tell them to stiffen in response to loud noises to dampen the motion of the basilar membrane

Question 15

Describe auditory pathways

Answer 15

Impulses from cochlea pass via spiral ganglion in periphery to cochlear nuclei in the brain stem

These impulses are then sent to the superior olivary nuclei for localization and to inferior colliculi for the auditory reflex centre to then pass to the auditory cortex

Question 16

Do auditory pathways decussate?

Answer 16

Only some decussate so that both cortices can recieve input from both ears

Question 17

What areas do pitch and loudness stimulate?

Answer 17

Pitch stimulates areas of the basilar membrane and is perceived by the primary auditory cortex (being aware)

Loudness stimulates increased fluid movement which leads to increased deflections of hair cells; both of which further increase amount of graded potentials, increasing NT release and increasing the frequency of APs in the cochlear nerve

Question 18

How do we determine which ear a sound is coming from?

Answer 18

By localization

• Helps by determining the relative sound intensity and the relative timing
• One ear will hear/recieve the sound first (timing) and whichever ear is closer to the sound will hear it more intensely or more loudly (sound intensity)

Question 19

How do balance and hearing correlate?

Answer 19

Balance (equilibrium) receptors are located in the semicircular canas and the vestibule (utricle and saccule) which then form the vestibular apparatus

The semicircular canals monitor head rotation (dynamic equilibrium from receptors lying in each plane)

The vestibule monitors statis equilibrium (linear acceleration and position of the head wrt to gravity) - things such as walking forward or tilting your head

Question 20

What are maculae?

Answer 20

1 each found in the saccule and utricle

consists of supporting cells and hair cells

• each hair cells has sterocilia (microvilli) and one kinocilium (true cilium) embedded in otolithic membrane

Question 21

What is the otolithic membrane?

Answer 21

A jelly like mass studded with tiny calcium carbonate (CaCO3) crystals = otoliths

Question 22

What do the hairs in each maculae respond to?

Answer 22

Utricular hairs respond to horizontal movement (tilting of the head) - hairs are vertical when head is upright

Saccular hairs respond to vertical movement - hairs extend horizontally into the otolith membrane

Question 23

What does the otolithic membrane do? what is the process it undergoes during movement?

Answer 23

The otolithic membrane slides over hair cells when moving, bending the hairs and modifying the amount of NTs

When hairs bend towards kinocilium = depolarizes vestibular nerve fibers; increasing the number of APs

When hair movement is away from the kinocilium = hyperpolarizes vestibular nerve fibers; decreasing the number of APs

Question 24

What is the role of crista ampullaris? Where can they be foud?

Answer 24

1 found in each semicircular canal (located in all 3 planes of space)

• stimulated by rotational type movements such as changes in velocity and if it is changing horizontally or vertically
• support cells and hair cells extend into the cupula; a gel like mass
• the direction of the bending of hairs (endolymph inertia) increases or decreases the rate that impulses reach the brain

Question 25

What is the composition of the axes of hair cells in semicircular ducts?

Answer 25

They are opposite; if one is depolarized the other is hyperpolarized