Lecture 47

Question 1

Auditory and vestibular system

Answer 1

• External ear: collects and conducts sound
• Middle ear: amplifies and transduces sound waves into mechanical force
• Internal ear: converts mechanical force into neuronal signal
• Auditory system: detects sound
• Vestibular system: responds to gravity and head movements

Question 2

External ear

Answer 2

• Auricle (pinna), made of cartilage and skin
• External meatus to eardrum (tympanic membrane), contains ceruminous glands that secrete earwax – impedes the entry of particles

Question 3

Middle ear

Answer 3

• Air filled space (tympanic cavity)
• Contains the ossicles: hammer (malleus), anvil (incus), and stirrup (stapes), connected by two joints and to two muscles (attenuation reflex). They connect the eardrum to the oval
  window and convert the movement of the tympanic membrane into mechanical force.
• Eustachian tube connects middle ear to nasopharynx, is normally collapsed, open upon yawning or swallowing

Question 4

Internal ear: Cochlea

Answer 4

• Behind oval window – perilymph filled ducts (Scala vestibuli and Scala tympani), lead to round window, endolymph filled duct (Scala media)
• Fluid vibrations in the duct make basilar membrane between Scala media and Scala tympani move – leads to stimulation of hair cells in organ of Corti through shearing forces of tectorial membrane and deflection of stereocilia

Organ of Corti:

• Sits on basilar membrane
• One row of inner and 3-5 rows of outer hair cells, supporting cells (e.g. phalangeal cells, pillar cells)
• Covered by tectorial membrane (collagen bundles and glycoproteins)
• Innervated by afferent and efferent nerve fibres of the auditory nerve

Hair cells:

• Sterocilia (60-100 per cell, supportedby actin containing cuticular plate), One Kinocilium (gets lost in auditory hair cells during development)
• Deflection of stereocilia activate mechanically activated ion channels, mechanism is not fully understood, tip links probably play a role

Question 5

Internal ear general or inner ear

Answer 5

Bony labyrinth contains membranous labyrinth suspended in perilymph

Three spaces (interconnected):

• Three semicircular canals, at a right angle to each other, each with an ampulla
• Vestibule: utricle and saccule
• Cochlea

Membraneous labyrinth contains endolymph

Three sensory regions:

• 3x ampullary crests (Crista ampullaris) in the three ampullae (detects head rotations)
• 2x maculae: in utricle and saccule (they detect head position and linear acceleration)
• 1x Organ of Corti

Question 6

Inner Ear

Answer 6

Collectively called the membranous labyrinth

Vestibular System: 2 sacs (utricle and saccule) + 3 semi circular canals

Auditory System: Cochlea containing the Organ of Corti

Inner ear consists of osseous bony labyrinth and membraneous labyrinth that is suspended in perilymph in the bony labyrinth. The bony labyrinth is the outer shell of the inner ear that is filled with perilymph (perilymph is extracellular fluid)
- Inside of membraneous labyrinth is endolymph fluid (intracellular fluid)

Scala media aka cochlear duct is filled with endolymph
- cochlear duct is continuous with 2 structures that are filled with endolymph and are sensitive organs: macula saccule and macula utricle. These organs are spaces.

3 archs are semicircular ducts filled with endolymph.

• Superior semicircular duct, posterior semicircular duct, horizontal semicircular duct
• they are organized in 3 dimensions, lying perpendicular to each other.
• at beginning of each duct, there is ampulla, which is thickening of semicircular ducts.
• within ampulla, there are also sensitive organs

Question 7

Vestibular system

Answer 7

Semicircular canals (ducts)
– respond to rotational movement of the head and body
– all connect to the utricle
– dilations of the semicircular canals are ampullae
– ampullae contain a prominent ridge called the crista ampullaris

Saccule and utricle
– respond to linear acceleration
– contain epithelia thickening: maculae

Question 8

Vestibular nerve and cochlear nerve

Answer 8

Both contain afferent and efferent
fibers. So, they contain axons of spiral ganglion cells, and also axons whose cell bodies lie in the brain and project back to the hair cells

Question 9

Vestibulo-ocular reflex

Answer 9

Eyes can fix gaze although body or face is moving
- the reflex keeps the image on the retina steady while the head moves

As soon as vestibular system senses body and head move, it sends respective signals to muscles that move eyes to compensate for the movements.

It is fast and effortless

It is mediated by a short brainstem pathway

Does not require vision

If you spin someone very fast, at some point, there is no more inertia of endolymph, so the membranous canal will all spin in the same manner.
- BUT, if you stop spinning, the endolymph keeps going but the head is stable, so
the eyes will keep spinning.

Question 10

Crista Ampullaris

Answer 10

Summary:

• Detect head rotations, angular accelerations
• Thickened transverse epithelial ridge in ampullae of the semicircle canals
• Hair cells and supporting cells
• Attached cupula (gelatinous)
• When head rotates, semicircular canals move, but endolymph lags behind because of its inertia – deflects cupula and stereocilia
• Innervated by afferent and efferent fibres of the vestibular nerve

Bottom to top: nerve fibers, supporting cells amongst type I and type II hair cells, stereo cilia, cilium, cupula

Crista
- ampulla has cells called crista
- crista is mostly supporting cells
- has hair cells embedded within
- hair cells on the top portion of crista has
cupula

Cupula
- Gelatinous cap surrounded by endolymph

Sensory Epithelium

• Supporting cells
• Hair cells

If i move my head up and down, the endolymph in the semicircular canal
will flow.
- the liquid flows against bony labyrinth or membranous labyrinth, meaning that it bends the gelatinous cap located on top of the crista.
- this bends the hair cells bc gelatinous cap is dislocated.
- This will create depolarization, which leads to nt release, causing an AP in the innervating spiral ganglion sensory neuron

Question 11

Ampullary crest

Answer 11

crista (of Crista Ampullaris of ampulla) = ampullary crest (crest is 1 line of supporting cells and hair cells that are innervated)

Question 12

Macula

Answer 12

Summary:
- Detect gravity and linear head movements
- Sensory thickening of epithelium
- The two maculae are in right angle to each other (utricle is horizontal, saccule vertical)
- Covered by otolithic membrane (gelatinous) that contains otoliths (calcium carbonate
crystals)
- Otoliths are heavier than the endolymphs – deflect stereocilia

From bottom to top: nerve fiber, sensory dendrites, supporting cell within type I (goblet) and type II (columnar) hair cells, stereo cilia, cilium, glycoprotein layer, otoliths

Macula: Saccule and Utricle
- Responds to linear acceleration and gravitational pull

Macula is 1 line of supporting cells and hair cells.

• covered with gelatinous layer, but it does not reach into canal
• contains stones called otoliths (calcium carbonate structures that sit on gelatinous cap)

When there is linear acceleration (i.e. moving up and down), the saccule will respond
- stones aka otoliths will be heavy and will bend gelatinous cap, therefore bending the stereocilia of hair cells

When there is horizontal acceleration, the utricle will respond
- the otoliths will lag behind and bend
gelatinous cap backwards

Main difference of macula from crista ampullaris is the presence of otoliths in the macula