AUD 720 exam 2

cochlea

Where is the inner ear located?

in the bony (osseous) portion of the labyrinth of the petrous portion of the temporal bone

Bony (osseous) labyrinth

system of spaces and canals within the temporal bone

Membranous labyrinth

membranous structures lining in the bony labyrinth form the inner ear

consists of a series of ducts which divide the labyrinth into sections

What are within the ducts of the membranous portion of the inner ear?

end organs of hearing and balance

Why are the ducts in the inner ear called the end organs of hearing and balance?

the point where energy that the CNS can understand

brain needs sound pressure wave converted into information it can understand

3 parts of the inner ear

semicircular canals

vestibule

cochlea

The base of the cochlea connects to?

The vestibule

Where are the oval and round window located?

In the vestibule

Each semicircular canal connects into?

The vestibule

What are the names of the 3 semicircular canals?

anterior/superior

posterior/inferior

lateral

Degrees of the angle of the semicircular canals to each other

90 degrees

The base of each semicircular canal widens into?

The ampulla

What is located in the ampulla

Sensory organs for the semicircular canals

The right posterior/inferior canal is paired with?

The left anterior/superior

The left posterior/inferior canal is paired with?

The right anterior/superior

The right horizontal/lateral canal is paired with?

The left horizontal/lateral

Describe the vestibule

Central portion of bony labyrinth

Contains oval window

Contains the utricle and saccule

The oval window is located where?

The lateral wall of the vestibule

What is the bump of bone between the oval and round window?

promontory

Describe the cochlea

Most medial portion of the body labyrinth

snail shaped

Base and apex

How many turns are in the cochlea

2 5/8 turns

How long is the cochlea

35 mm

Describe the base of the cochlea

Widest portion

9 mm wide

oval and round window are located in

Describe the apex of the cochlea

canal ends at the apex

Describe the modulus

Bony central core of the cochlea

turns into internal auditory meatus

Internal auditory meatus

auditory nerve fibers

vestibular nerve

facial nerve

blood vessels

Describe the osseous spiral lamina

shelf-like bony plate winds around the modulus from base to apex

consists of 2 bony plates laying on top of one another

Think 2 staircases connected to each other and spiraling upward

What is the opening in the spiral lamina for the auditory nerve?

Habenula perforata

What are the names of the 3 cochlear ducts?

scala vestibuli

scala media (cochlear duct)

scala tympani

What cochlear ducts meet at the apex (heliocotrema)

scala vestiboli and tympani

Describe the scala vestibuli

Top section of cochlea

Bottom is formed by Reissner’s membrane

Oval window opens into

Contains perilymph

What seals the perilymph into the scala vestibuli at the oval window?

annular ligament (surrounds stapes footplate)

Describe the scala tympani

Bottom section of cochlea

Top border is formed by Basilar membrane and spiral lamina

Round window opens into at basal end

Contains perilymph

How is the perilymph sealed into scala tympani at the round window?

A membrane

Describe the scala media (cochlear duct)

Middle section of cochlea

Top formed by Reissner’s membrane

Bottom formed by Basilar membrane and spiral lamina

Contains endolymph

Stria vascularis located in

Contains organ of corti

What controls the blood supply to the scala media?

Stria vascularis

Describe the basilar membrane

Connects to the spiral laminate at the inner core

Connects to the spiral ligament at the outer bony wall

Width varies along length of cochlea

Can be divided into 2 sections

Width and length of basil membrane

width at base 0.1 mm

width at apex 0.5mm

32 mm long

2 sections of basilar membrane

Zona arcuata

Zona pectinata

Describe the organ of corti

Sits on basilar membrane

End organ of hearing

Consists of hair cells, membranes, and supporting cells

Star of cochlea

What do the cells inside the organ of corti do?

Take the mechanical vibrations of sound and transducer them into electrical signals

What are the most important cells for hearing?

Hair cells

Stars of the organ of corti

What are the 2 types of hair cells?

Inner hair cells

Outer hair cells

Describe the inner hair cells

One row

Closest to the modiolus

3500 within 1 cochlea

Do not make contact with tectorial membrane

Flask/ lightbulb shaped

95% have afferent nerve fibers that innervate/connect to inner hair cells

Describe the outer hair cells

3-5 rows

Clostest to the outer bony wall

12,000 per cochlea

150 stereocilia on each cell

Cylindrical shape

5% of afferent fiber innervates the other hair cells

Eletctromotility

What are the main parts of hair cells?

Cuticular plate

Stereocilia

Basal body of kinocilium

Cross links

Tip links

Describe the cuticular plate

Thickened top of hair cell that contains the cilia

Describe the stereocilia

The hairs on the top of the cell

They are lined up in rows and arranged by height

Basal body of kinocilium

During fetal development a longer stereocilia would have been formed that doesn’t remain after birth

Base of this cilia remains

Cross-links of hair cells

Between 2 stereocilia of the same row

side to side

Tip-links of hair cells

Links from the tips of stereocilia between 2 rows

Can go from side of taller hair cell to tip of shorter hair cell

What do the links of stereocilia do?

Make sure all stereocilia move together

Efferent fibers go?

From CNS to PNS

Afferent fibers go?

Peripheral into central

What is the inner hair cells to nerve fiber ratio?

1.8:1

Where do the efferent fibers connect to the inner hair cells?

Bottom not actual cell

synapse with afferent fibers

Outer hair cell to neuron ratio

5.7:1

Describe the tectorial membrane

Gelatinous membrane that sits on top of the hair cells

Steriocilia of outer hair cells embedded in

Hinged to the spiral lamina by the spiral limbus

Pillars of corti

Inner and outer rods of corti

Provide strength and form to the organ of corti

Between the rods in the tunnel of corti

Deiter’s cells (phalangeal cells)

Each outer hair cell sits in the cupped top of one

Phalangeal process projects between the hair cells and forms part of the top surface of the organ of corti

Hensen’s cells

Supporting cells that border the outside of the organ of corti

Attached to the outer most Dexter’s cells

Claudius’ cells

Lateral to Hensen’s cells

These extend into the spiral ligament

Bottcher’s cells

Lie between the basilar membrane and Claudius’ cells

Decrease in frequency towards the apex, until they are absent

Tunnel of corti

The rods of corti form a tunnel that looks like a triangular cross section

Space of Nuel

Spaces between the outer cells and around all oof the outer hair cells

Contains cortilymph

Inner sulcus

Filled with perilymph

Furrow formed by the edge of the spiral limbus

Medial edge of the tunnel of corti and apically by the tectorial membrane

Recticular lamina

Plate that is formed by the tops of the hair cells and phalangeal process of the Dieter’s cells

Sealed plate doesn’t allow the fluids above these cells (endolymph) to mix with the fluid between these cells (cortilymph)

Describe nerve fibers

Attach to the bottom of hair cells and travel through the habenula perforata

Cell bodies of these nerve fibers are in the modiolus and are called the serial ganglion

What are the 2 aqueducts in the inner ear?

Vestibular (endolymphatic)

Cochlear (perilymph)

Describe the vestibular aqueducts

Bony channel that encompasses endolymphatic duct

Vestibule to endolym[hatic sac

Regulates endolympathic pressure

Endolymphatic hydrops=

Meniere’s disease

What is an endolymphatic shunt?

Open or decompress sac

Describe the cochlear aqueduct

Contains perilmyph

Froms scala tympani to subarchanoid space

Patent in early life

Describe perilymph

Similar in composition to cerebral spinal fluid (CSF)

Dominate cation is Na+ (sodium)

Composition varies slightly throughout cochlea due to a solute exchange process within the inner ear

Ongoing uptake of K+ (potassium) from scala tympani to scala media

How does the base of the scala tympani communicate with the subarachnoid space?

Through the cochlear aqueduct

How does the scala tympani communicate with the scala vestibuli

Through the heliocotrema and then opening into the perilymphatic space of the vestibule

What parts of the body contribute to the pressure of the perilymph?

Cranium (via CFS)

Middle ear space (via oval and round windows)

Blood pressure (via the inner ear vasculature)

Describe the cochlear aqueduct in humans

Thought to be closed

Pressure fluctuations are attenuated

K+ concentration in scala vestibuli compared to scala tympani

K+ concentration twice as high in scala vestibuli

Describe endolymph

Unique ionic composition

K+ (potassium)

Na+ and Ca ²+ are exceptionally low compared with other fluid in the body

What is the endolymphatic space connected to?

The endolymph of the saccule by the ductus reuniens

How is the saccule connected to the endolympahtic sac?

Via the endolymphatic duct

Describe the endolymphatic space

Thought to be an incompressible flexible walled compartment suspended in the perilymph

Electrically polarized due to high level of K+

Are there pressure differences between the perilymph and endolymph?

Normally no

What does the maintenance of constant/ equal pressure difference results in?

An optimal compliance across the basilar membrane

Describe the perilymph in the scala vestibuli and tympani

high sodium/ low potassium concentrations

Low voltage (0 to +5 mV)

Describe endolymph in scala media

High potassium/ low sodium concentrations

High positive voltage (80 mV)

Describe cortilymph

Located in the extracellular spaces of the tunnel of corti (ELS is not the same as the scala media)

Believed to be similar in composition to perilymph

Sensory hair cells are bathed in cortilymph (body of cell) but top of cell is in endolymph (above reticular plate)

The transduction process in the inner ear is dependent on the maintenance of what situation?

The sensory hair cells being bathed in cortilymph and the top of the cell is in endolymph

What are potentials?

Differences in electrical charge between 2 points within the body

What are the 2 of interest for electrical potentials?

Endolymph within the cochlea and a neutral point

When there is a difference in electrical charge between 2 points there is a potential for electric current flow

Describe how positive electrical charges move

Area of positive electrical charge to area of less positive charge

What are the 3 cochlear potentials

Resting potential (endocochlear potential or EP)

Summating potential (SP)

Cochlear microphonic (CM)

Describe how cochlear potentials are generated

Intricate movement and interactions of the cochlear structures and ionic concentrations of the fluids

Describe the resting potential (endocochlear potential EP)

Large electrical potential that can be recorded in the endolymph of the scala media

Thought to be generated by the stria vascular and due to high level of K+ in the endolymph

Always present

What is the resting potential of the EP

+80 - +100 mV

Largest resting potential in the body

What may serve to increase the electrical response of hair cells?

Resting potential EP

What happens if the stria vascularis is damaged?

The resting endocochlear potential is no longer present

The inner ear no longer performs its function of transferring mechanical energy into electrical energy

Describe summating potential

A dc electrical potential

Baseline shift only when an auditory stimulus is present

Thought to be a reception potential of the hair cells

The strength of the summaing potential is directly related to?

The intensity of the stimulus