Histology 9f The Ear

Question 1

What are the 3 divisions of the ear?

Answer 1

1. External Ear
• Auricle (Pinna)
• Collects & amplifies sound
• External acoustic meatus
• Conducts sound to tympanic membrane
• Clinical correlation: Conductive hearing loss 

2. Middle Ear
   • Tympanic membrane
   • Separates external meatus from the middle ear
   • Auditory ossicles
   
   1. Malleus (hammer)
   2. Incus (anvil)
   3. Stapes (stirrup)
      • Auditory tube
      • Connects middle ear to the nasopharynx
      • Clinical correlation: Conductive hearing loss
3. Internal Ear
   • Bony labyrinth
   • Membranous labyrinth
   • Clinical correlations: Vertigo, Sensorineural hearing loss

Question 2

What are the compartments of the internal ear?

Answer 2

2 labyrinthine compartments

1. Bony labyrinth
• 3 connected spaces
      1. Semicircular canals
         • Anterior, posterior & lateral
       2. Vestibule
        3. Cochlea
• Perilymph 

2. Membranous labyrinth
   • Communicating sacs & ducts suspended within the bony labyrinth

• 2 divisions
     1. Cochlear labyrinth
        • Cochlear duct
2. Vestibular labyrinth
• Semicircular ducts
• Utricle & Saccule
• Endolymph

Question 3

Contrast the perilymphatic and endolymphatic spaces

Answer 3

• Perilymphatic space (white)
• Betweenwallofbonylabyrinth
& wall of membranous labyrinth • Perilymph
• Similar in composition to extracellular & cerebral spinal fluid
• Low K+ / High Na+ concentration

• Endolymphatic spaces (blue)
• Contained within the membranous labyrinth
• Endolymph
• Similar in composition to
intracellular fluid
• High K+ / Low Na+ concentration

Question 4

Whats the purpose of hair cells?

Answer 4

Hair cells
• Mechanoreceptorsofthevestibular&
cochlear labyrinth
Hair bundle
• Sensory hairs = Rows of stereocilia
• Mechanoelectric transducer (MET) channel
proteins at the distal ends
• Ribbon synapses
• Communication with afferent nerve fibers

Kinocilium
• True cilium found in the vestibular system
• Defines the polarity of the hair bundle
• Positioned behind the longest row of stereocilia

Question 5

Describe the vestibular labyrinth has 2 types of hair cells

Answer 5

Vestibular labyrinth has 2 types of hair cells
1. Type I
• Flask-shaped
• Surrounded by an afferent nerve chalice &
a few efferent fibers

2. Type II
   • Cylindrical shape
   • Afferent & efferent bouton nerve ending

Question 6

Describe the vestibular labyrinth as a sensiry organ

Answer 6

Vestibular labyrinth:

1. Semi-circular ducts
   • 3 ampullae each contain a crista ampullaris
   • Sensors of angular movement of the head
   • Type I & II cells; supporting cells
2. Saccule & utricle
   • Saccule and utricle will have maculae
   • Maculae oriented at right angles to each other
   • Sensors of gravity & linear acceleration
   • Type I & II cells ;supporting cells

Clinical correlations: vertigo

Question 7

Describe the cochlear labyrinth of the sensory organ

Answer 7

Cochlear labyrinth:
• Spiral organ of Corti
• Sensor of sound vibrations
• Inner&outerhaircells;supporting cell’s
 • Spiral ganglion

• Clinical correlation: Sensorineural hearing loss

Question 8

Describe the Crista Ampullaris

Answer 8

• Sensory receptor for angular movement of the head
• Cupula
• Gelatinous protein-
polysaccharide mass

• Projects into endolymph

• Attached to Type I & II hair cells
1. Rotational movement of Hair
   head
2. Movement of endolymph
3. Swaying of cupula
4. Deflection of stereocilia
5. Activation of MET channels
6. Generation of nerve impulse

Question 9

Describe the structure of the Macula of Utricle

Answer 9

• Sensors of gravity & linear acceleration

• Otolithic membrane
• Gelatinous polysaccharide mass
• Projects into endolymph
• Outer surface covered in otoliths/otoconia
• Crystalline bodies of calcium carbonate
• Attached to Type I & II hair cells
• Hair cells are polarized to the striola
Sensitive to multiple directions of linear accelerations

1. Linear movement of head
2. Movement of endolymph
3. Movement of otolithic membrane
4. Deflection of stereocilia
5. Activation of MET channels
6. Generation of nerve impulse

Question 10

Describe the cochlea

Answer 10

Cone-shaped helix
2 3⁄4 turns around a central core of
bone called the modiolus
Spiral ganglion lie in the modiolus

Cochlea has 3 compartments
1. Scala media (cochlear duct)
• Endolymph space
• Contains the spiral organ of
Corti
2. Scala vestibuli→Perilymph space
3. Scala tympani→Perilymph space
• Scala vestibuli & scala tympani connect at the apex of the cochlea via a small channel called the helicotrema

Question 11

Describe the cochlear duct

Answer 11

• Scala media or Cochlear duct
• Stria vascularis
  * Production & maintenance of endolymph
• Vestibular (Reissner’s) membrane
• Basilar membrane
• SpiralorganofCorti

Spiral organ of Corti
• Sensor of sound vibrations
• Inner&outerhaircells

Supportingcells
• Phalangeal & pillar cells

Tectorial membrane (rigid)
• Gel-like structure
• Bundles of collagen II, V & IX;
glycoproteins
• Attached medially to modiolus
• Lateral edge projects over & attaches to stereocilia of hair cells

Question 12

Describe the stria vasvularis

Answer 12

• Production & maintenance of endolymph
• Encloses a capillary network

• 3 types of cells
1. Marginal cells (M)
• Line endolymphatic space (E)
• Involved in K+ transport

2. Intermediate cells (I)
   • Scattered among the capillaries
   • Contain pigment
3. Basal cells
   • Separate stria vascularis from spiral ligament (SpL)

Question 13

Describe the hair cells of the spiral organ of Corti

Answer 13

• Inner hair cells
• Form a single row of cells along the length of
the cochlear duct
• Characteristic straight line of stereocilia
• Primary sensory cells of hearing
• ~90% dendritic processes
• Notcapableofregeneration

• Outer hair cells
• Variablenumberofrows
• 3 rows in the basilar part of the duct
• Up to 5 rows possible at the apex
• Characteristic “W” of stereocilia
• ~10% dendritic processes
• Likely role in amplification

Question 14

Describe the structure of outer hair cells

Answer 14

• Outer hair cells
• Structure of outer phalangeal cells
• Apical ends bound to each other & hair cells by tight
junctions
• Junctions form the reticular lamina
• Seals the endolymphatic space from the intercellular spaces of the organ of Corti
• Extracellular fluid is called cortilymph

Question 15

Explain sound perception

Answer 15

• Mechanical vibrations converted to fluid vibrations at the oval window
• Fluid vibrations cause displacement of the basilar membrane (traveling wave)
• Different frequencies cause maximum displacement of certain segments along the length of the basilar membrane
• Basilar membrane is relatively narrow & thick at the base
  * High frequency sounds
• Basilar membrane is relatively wide and thin at the apex
  * Low frequency sounds

Shearing effect between basilar membrane & tectorial membrane causes deflection of stereocilia
Activation of MET channels
Generation of nerve impulse
Action potentials conveyed to brain via cochlear nerve

Question 16

What causes conductive hearing loss?

Answer 16

Conductive hearing loss
• Sound waves are mechanically impeded from reaching the auditory sensory receptors of the internal ear
• Causes
• Ear infection (otitis media)
• Excess ear wax
• Foreign bodies 
• Otosclerosis

Question 17

What causes sensorineural hearing loss?

Answer 17

• Sensorineural hearing loss
• Injury to the sensory receptors, nerves or
auditory cortex

• Causes
• Infections of membranous labyrinth
• Fracture of temporal bone
• Acoustic trauma
• Prolonged exposure to excessive noise • Aging
• Presbycusis
• Loss of sensory cells
• Begins at basal end of cochlea
• High frequency hearing los