Histology of the Ear- Heinbockel Flashcards
Outer Ear
Outer ear is the pinna with elastic cartilage covering by skin and the external
auditory canal that consists of elastic cartilage in the outer 1/3 and temporal bone in the inner 2/3 and
covered by stratified squamous epithelium and wax/cerumaneous glands that modify sweat glands in the skin secreting cerumen
ear canal has:
- hair follicles
- sebaceous glands
- cerumen is there to waterproof the skin and protects the canal from whatever goes in there (finger, insects)
TM:
-covered by skin on the external side
and simple squamous or cuboidal epithelium on the inner side
-the covered by simple squamous or cuboidal on
-in the center there is fibroelastic CT
-when sound strikes TM it starts to vibrate and brings us to the middle ear
middle ear:
- air filled space in between the tympanic membrane and structures of the inner ear
- transmits sounds from TM to the fluid structures of the inner ear
- ear ossicles: malleus, incus, stapes touching the cochlear
-Eustachian tube links the middle ear to the nasopharynx
What is the main organ in the inner ear?
Cochlear is the main organ in the inner ear.
- organ in which hearing takes place
- consists of the scala vestibuli and scali tympani
-we have bony structures that cover membranous structures
and thus we talk about the bony AND membranous labyrinth which are both fluid filled
if you have fluid and there’s continuous production of fluid then it needs to go somewhere
Describe the membranous labyrinth.
- it is surrounded by bone
- consists of semicircular ducts, utricle, saccule, and cochlear duct
-fluid in the membranous labyrinth ends up in the narrow endolymphatic sac and the sac projects through the dura mater into the subarachnoid space by capillary network into general circulation
the endolymphatic sac is in the subdural space
What is the function of the vestibular system?
senses motion and the position of our heads
- utricle (horizontal) and saccule (vertical) senses gravity and linear acceleration (whether body is moving or stationary wrt gravity)
- they can be stimulated by sudden acceleration
- semicircular ducts sense head rotation or angular rotation
Where is the endolymphatic sac located?
in the subdural space
Describe the bony labyrinth.
- scala vestibuli and scala tympani of the cochlea
- semicircular canals (bony) with semicircular ducts (membranous)
- vestibule (bony structure): opening or entry hall houses the utricle and saccule (membranous structures)
- all these structures are filled with fluid called perilymph that comes from the perilymphatic duct which connects to the subarachnoid space
What fluid is found in the bony and membranous labyrinth?
endolymph in membranous labyrinth
perilymph in bony labyrinth
What is the difference between endolymph and perilymph?
Endolymph:
-High K+, low Na+
Perilymph:
-High Na+, low K+
What condition is associated with the increase in the volume of the endolymph? What are symptoms related to this condition?
Ménière’s disease
- cause is unknown
- typically involves only one ear
- symptoms include episodic attacks of :
- vertigo
- nausea and vomiting
- nystagmus
- tinnitus
Cochlea
- shape is like that of a snail; has an apex and base
- cochlear duct is the membranous structure surrounded by bone
cross section reveals 3 ducts:
-scala media or the cochlear duct which is lined by vestibular (Reissner’s membrane) on one side and the basilar membrane on the other side
- has bony structures called scala vestibuli and scala tympani
- Scala vestibuli and scala tympani are chambers of the bony labyrinth. Scala media is part of the membranous labyrinth.
Where does hearing occur in the ear?
in the Organ of Corti in the cochlea where hair cells are found and assist in hearing
The medial portion of the cochlea is filled with bony modiolus (part of the petrous bone) through which a nerve runs. What is this nerve?
cochlear nerve
-nerve fibers have cell bodies (spiral ganglion) and run in the cochlea and go to the CNS
What type of neurons are found in the spiral ganglion and what do they innervate to form the cochlear nerve?
- bipolar neurons (remember special senses use bipolar not pseudounipolar)
- innervates the organ of corti to form the cochlear nerve
What are the function of the spiral ligament and stria vascularis?
- spiral ligament it forms the outer wall of the cochlear duct and is a thickening of the periosteum
- stria vascularis is rich in capillaries (epithelium DO NOT typically have BVs); it is the epithelium that makes the endolymph
Stria vascularis
- vascular epithelium
- secretes endolymph
- proper function essential for normal hearing
- contains a few melanocytes, also needed for normal hearing
The organ of Corti sits on the basilar membrane to connect what two structures?
Osseous spiral lamina and spiral ligament
Osseous spiral lamina allows the nerve fibers to fun to the spiral ganglion
How does sound travel for us to hear?
Sound comes into the ear through the acoustic canal and hit the TM the vibration is communicated through the ossicles
the stapes is connected to the cochlea and connects to the oval window
the sound waves or vibration get into the cochlea into the scala vestibuli to the scala media and excite hair cells and then go to the membrane to the scala tympani to the round window
The oval footplate of the stapes contacts the oval window. Sound waves travel AROUND to the round window.
So the sound moves the basilar membrane the stereocilia in the hair cells bend and the tip links are stretched, calcium channels open up triggering glutamate release and and the inner hair cells respond.
The scala vestibuli and scala tympani are connected by what? What separates them
connected by helicotrema
separated by scala media
The organ of Corti
-has a tectorial membrane expands from the spiral limbus and sits on top or forms the top of the organ of Corti (sits on top of the hair cells)
inner tunnel is lined by pillar cells
-has outer and inner phalangeal cells
There are two types of hair cells in the organ of Corti. What are they and which type is involved in sensing sounds?
outer and inner hair cells
inner hair cells are involved in sensing sounds (transduce sound)
both inner and outer hair cells have stereocilia which are long microvilli (which have actin filaments in them)
the inner cells are medial and the outer cells are lateral
What is the function of the outer hair cells?
outer hair cells contract to increase the sensitivity to sound. They move the basilar membrane relative to the tectorial membrane.
They are getting more input from the CNS and can change the sensitivity to sound. They also can lengthen or shorten sound and are believed to more amplify the sound in the associated inner hair cells.
There are 3-4 times more outer hair cells than inner hair cells. There are 3 rows of outer hair cells and 1 row of inner hair cells.
If the outer hair cells are damaged, can we still hear?
no we cannot
Stereocilia of hair cells have several unconventional myosins. What can occur with mutations in some of these myosins?
deafness
What can occur with mutations in ankle links of the stereocilia of hair cells ?
Usher syndrome
deafness as a result of cochlea origin (CNS defect of the nerve fibers which affects the hair cells)
Stereocilia has lateral tip links and ankle links.
