ANATOMY OF THE EAR - 52
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- What is the neuromast organ?
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It is the sensory receptor used for hearing and balance. It is a highly sensitive mechanoreceptive organ that transforms mechanical stimuli into nervous impulses. - Describe the role of each part of a neuromast organ?
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The blue rectangle at the bottom of the figure (not labeled) is the membrane of the neuromast.
All neuromast organs (all organs for hearing and balance) are in a cavity filled with fluid called endolymph.
Cranial nerve VIII always carries sensory information from all neuromast-related organs to the brain. CN VIII is only attached to neuromast organs.
The neuromast organ has no homology with the organs of touch, taste and smell.
The neuromast functions as follows:
A pressure wave (ex. sound) enters the endolymph, and proceeds to the cupula.
The sound wave pushes on the cupula.
Movement of the cupula stimulates hair cells.
The hair cells project an impulse to cranial nerve VIII.
Keep in mind that the neuromast organ is just a “blueprint†for the functions of the ear – it is not identified as an actual organ in humans. - Describe the location of the ear?
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The ear is housed mostly within the petrous portion of temporal bone. The petrous portion of temporal bone is solid inside – it is not spongy in there! - Discuss the 3 layers of the ear?
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There are three layers of the ear: The outer ear, middle ear, and inner ear.
1. The outer ear contains the pinna and the external auditory meatus.
2. The middle ear contains the tympanic cavity and tympanic membrane, which are filled with air.
3. In humans, all neuromast organs are found in the inner ear. - What is the role of the outer ear?
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The pinna gathers sound waves and passes them to the external auditory meatus.
The external auditory meatus leads to the tympanic membrane. - Describe the middle ear?
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The middle ear contains the tympanic cavity and tympanic membrane, which are filled with air.
The eardrum can only vibrate if its pressure is equalized with the pressure around it.
If pressure is not equal, your ears “pop†and you lose hearing sensitivity.
The Eustachian tube (pharyngotympanic tube) helps to keep this pressure equal.
If your ears “pop†you can equalize pressure by swallowing. The action of swallowing opens the Eustachian tube, which leads to the pharynx.
The external surface of the tympanic membrane is supplied by the auriculotermporal nerve (CN V3 branch)
The middle ear contains three bones (auditory ossicles); malleus, incus and stapes. They transfer vibrations to the inner ear. - Where are the 3 bones of the middle ear derived from?
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- Malleus and Incus come from 1st branchial arch;
- Stapes comes from 2nd branchial arch. - How is the middle ear connected to the inner ear?
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The open window is the gateway to the inner ear form the middle ear. - What is located in the inner ear?
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In humans, all neuromast organs are found in the inner ear. - What supplies the external surface of the tympanic membrane?
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The external surface of the tympanic membrane is supplied by the auriculotermporal nerve (CN V3 branch) - Describe the role of the mastoid process in the ear?
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In the mastoid process, there are holes in the anterior and posterior walls.
-The anterior hole leads to the Eustachian tube.
-The posterior hole leads to mastoid air cells, which are great targets for stubborn middle ear infections. - Describe the different boundaries of the tympanic cavity?
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Suppose the ear were visualized as a box:
1. The roof of the ear is called the tegmen tympani; it separates the dura from the floor of the middle cranial fossa.
2. The medial wall has two openings:
a. the oval window and
b. the round window.
The two windows have plugs that keep liquid from flowing out. The round window is sealed by the second tympanic membrane.
Note the bulge on the medial wall in slide 6; this is the promontory.
The nerves on the promontory form the tympanic plexus.
There are two important muscles on the ear: the tensor tympani and stapedius.
The tensor tympani is supplied by the mandibular nerve.
This muscle pulls on the handle of the malleus to reduce its oscillations – this, in turn, protects the inner ear from damage due to loud sounds.
The stapedius is supplied by a branch of the facial nerve.
This muscle pulls on the stapes to reduce its movements – this also protects the inner ear from damage when loud noises occur. - What is hyperacusia?
- Excessive acuteness of hearing due to paralysis of the stapedius muscle (causing uninhibited movement of the stapes), resulting from a lesion of the facial nerve.
- Discus the role of the annular ligament in the stapes?
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The base (footplate) of the stapes is attached by the annular ligament to the margin of the oval window - Discuss the 2 types of labyrinth in the inner ear?
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1. The bony labyrinth is a solid portion of the upper petrous portion of the temporal bone.
The white area is a continuous cavity or the hollow space of the temporal bone, while the black space is a membranous structure.
2. The membranous labyrinth is inside the black area; it is filled with endolymph. Perilymph fills the space between the bony and membranous labyrinths. - Discuss the 3 parts of the bony labyrinth?
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The 3 parts of the bony labyrinth are:
1. The Vestibule: Cavity of the bony labyrinth that communicates with:
--a.) cochlea anteriorly
--b.) semicircular canal posteriorly
2. The semicircular canal: There are 3 of them
3. The Cochlea: Has 2 adjacent ducts:
a.) upper scala vestibuli
b.) lower scala tympani
NOTES:
The bony labyrinth contains the scala tympani, scala vestibuli, vestibule and semicircular canal.
The vestibular membrane separates the scala vestibuli from the scala tympani.
The vestible contains not one, but three semicircular canals (seen on slide 8, too).
The front of the vestibule there is the scala vestibule, which is a “staircase†leading off the vestibule.
The cochlear is made up of the scala media, scala tympani and scala vestibuli. - Discuss the parts of the membranous labyrinth?
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The membranous labyrinth is inside the black area; it is filled with endolymph.
The membranous labyrinth contains the scala media (cochlear duct), saccule, ampulla, utricle and semicircular duct.
Perilymph fills the space between the bony and membranous labyrinths. - Discuss the cavities of the cochlea?
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The Cochlea has numerous cavities; each cavity is called the spiral lamina.
The basilar membrane is seen in all four cavities (visualized easily by the red line in slide 11)
Each cavity has a vestibular membrane, which divides the cochlear into the scala media, scala tympani, and scala vestibule. - Discuss the organ of corti?
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The spiral organ of Corti sits on the basilar membrane in each cavity.
This organ is a neuromast organ, so it has a gelatinous membrane (this time, called the tectorial membrane) that sits on hair cells.
CN VIII (cochlear nerve) comes from the hair cells, and it exits out of the spiral lamina. - Describe the width of the basilar membrane?
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The basilar membrane is widest in the lowermost cavity; it picks up low tones (bass).
The basilar membrane is narrowest in the uppermost cavity; it picks up high tones. - Illustrate the pathway of sound from the external ear to one spiral lamina?
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The sound progresses as follows:
External acoustic meatus Tympanic membrane
Malleus Incus Stapes Oval window
Scala vestibule (received by hair cells all around the cavity)
Round window (sound exits here; excess energy from sound is removed here)