10.25.11+-+Anatomy+of+the+Ear

Ear


 * 1a. Describe the topographic features of the external ear, and explain how these features contribute to their function.**


 * auricle** - The flap of the ear (soft cartilage covered with skin) with the concha (inner hemisphere), helix (fold around rim), antihelix (ridge opposing helix), tragus (central squared), antitragus (ridge opposing tragus).
 * external acoustic meatus** - The canal/hole of the ear. Thin-skinned. External 1/3, lined with skin that produces cerumen, cartilaginous. Internal 2/3, lined with thin skin, boney.
 * outer leaflet of the tympanic membrane** - 1 cm in diameter. Outer surface thin skin and inner surface mucous membrane. The handle of the malleus is attached to the internal surface. Features include: umbo (central depression), cone of light (antero-inferior reflection), pars flaccid (superior), and pars tensa (inferior)


 * 1b. Name the nerves that provide sensory innervation for these various topographic features.**

Auriculotemporal (V3 branch) innervates anterosuperior 1/3 of auricle and part of external tympanic membrane. Great Auricular Nerve (cervical branch) innervates the other 2/3 of auricle. Auricular branch (vagus) innervates concha and other part of external tympanic membrane.


 * 2a. Describe the location and shape of the middle ear (i.e., tympanic cavity).**

The tympanic cavity is a parallelepiped (A 3d parallelogram. It's a thing.). 3 walls are simple (roof, floor, lateral); 3 walls are complexish (anterior, posterior, and lateral)


 * 2b. Name the nerve that provides sensory innervation for the middle ear.**

The middle ear is innervated by the tympanic plexus overlying the promontory. The tympanic plexus is formed by the tympanic nerve from CN IX and sympathetic fibers entering with blood vessels. This includes the internal tympanic membrane, the internal surface, and much of the pharyngotympanic tube.


 * 2c. Name the spaces and tubes that are connected with the middle ear****.**

Pharyngotympanic tube Facial nerve canal Epitympanic recess Tympanic cavity proper Mastoid air cells


 * 3. Identify the three ossicles of the middle ear. Describe their relative positions, functions and pertinent relationships.**

A --> P: Malleus - Incus - Stapes

Continuous pathway lateral to medial: Tympanic membrane - handle and 2 processes of malleus - head of malleus - body of incus - long limb of incus - lenticular process of incus - head of stapes - limbs, base of stapes - oval window

Functions: Functionally and structurally connect outer ear to inner ear. Increase force of sound waves by decreasing displacement.

Pertinent relationships: Chordae tympani (special sensory branch to tongue from facial nerve CN VII) passes medial to neck of malleus and along tympanic membrane (P --> A) to eventually join with lingual nerve.


 * 4. Identify the two skeletal muscles of the middle ear and the ossicles to which they attach. Identify which cranial nerve innervates which muscle.**

Their functions are the same, they are protective by reducing excessive oscillation. The tensor tympani attaches to the handle of the malleus while the stapedius attaches to the head of the stapes. Tensor Tympani is innervated by a branch of the CN V: Trigeminal nerve (TTT) called the nerve to tensor tympani. Stapedius is innervated by a branch of CN Seven (SS): Facial nerve called the nerve to stapedius.


 * 5. Describe the walls of the middle ear and the special features associated with each wall:**

Roof: Tegmental: petrous temporal bone Floor: Jugular: petrous temporal bone and tympanic canaliculus (hole for tympanic nerve) Lateral Wall: Membranous: petrous temporal bone and tympanic membrane Anterior Wall: Carotid: petrous temporal bone with two holes for tensor tympani tendon and pharyngotympanic tube Medial Wall: Labyrinthine: 5 features in order from anteroinferior to posterosuperior (RPOFL): Round window, Promontory, Oval window, Facial nerve canal, Lateral semicircular canal Posterior Wall: Mastoid: Continuation of the facial nerve canal and the aditus (entrance) to mastoid antrum (cavity)


 * 6. Identify the types of fibers found in each of the following middle ear nerves and what functions these different fibers subserve:**


 * chorda tympani** - See Q3
 * tympanic plexus** - See Q2b
 * tympanic nerve (of Jacobson)** - See Q2b


 * 7. Describe the structure and function of the pharyngotympanic tube, as well as its sensory innervation.**

Exits the anterior wall of the middle ear heading anteroinferiorly to connect with the nasopharynx. The posterior 1/3 is bony (innervated by CN IX via tympanic nerve/tympanic plexus), the anterior 2/3 is cartilaginous (innervated by CN V2). Normally the tube is collapsed, but opens to equalize pressure between tympanic cavity and atmosphere.


 * 8. Describe the significance of the aditus, mastoid antrium and mastoid air cells.**

This is such a poorly phrased question, but the aditus is the opening to the mastoid antrium, which is the entrance to the mastoid air cells. The significance is that it is posterior and superior to the tympanic cavity. Other significance?


 * 9. Describe the otic capsule and bony labyrinth.**

The otic capsule is the densest portion of the temporal bone. The bony labyrinth is housed within the otic capsule. It is a series of hollowings within the otic capsule. The cochlea is connected to the vestibule, which is connected to the semicircular canals.

The cochlea is a spiral that goes around 2.5 times and ends in the center at the modeolus. The cochlea is the name of the shell, the spiral canal is the spiral within. The vestibule is actually pretty simple. The oval window opens into it. The semicircular canals occupy (WE ARE THE 99%) three planes in space at right angles to each other.


 * 10. Describe the structure and functions of the various portions of the membranous labyrinth.**

Vestibular labyrinth (Semicircular labyrinth --> Ampullae of semicircular labyrinth --> Saccule --> Utricle) --> Cochlear labyrinth (aka cochlear duct)

The perilymph within the bony labyrinth is the standard high [Na+]/low [K+]. The endolymph within the membranous labyrinth is high [K+]/low [Na+]. Mimics intra/extracellular fluid.

Both the saccule and utricle have small patches on their inner surface called macula. The macula are regions of dense specialized hair cell growth (elaborate cilia) covered by an otolithic membrane embedded with otoliths (little crystal precipitates). Depending on the position of the head, the hair cells are bent or not bent by gravity affecting the position of the otolithic membrane. Once bent, the hair cells activate action potentials that are relayed to the brain signaling static head position. The utricle macula is horizontal (UH). The saccule macula is vertical (SV).

The ampullae of the semicircular labyrinths are also covered with hair cells (elaborate cilia) that respond to bending by triggering action potentials. When the head undergoes dynamic rotational movements, the membranous endolymph flows past the hair cells bending them.

The spiral organ is the inner surface of the cochlear duct. It is (again) a collection of hair cells that respond to flow of endolymph. This time the flow is initiated by oscillation of the base of stapes in the oval window. The frequency and amplitude triggers action potentials sent via CN VIII. The cochlear duct is connected at both ends to the lateral surface of the cochlea nd to the modeolus. These connections divide the spiral canal into two perilymph-filled channels (called the scala vestibule and the scale tympani) that are continuous at the helicotrema (the apex of the modiolus).


 * 11. Describe the functions of the cochlear and the vestibular portions of CN VIII.**

Innervation of the cochlea and vestibule is accomplished by the vestibulocochlear nerve. Post-ganglionic neurons exit the vestibular ganglion via the vestibular branch of the vestibulocochlear nerve to innervate specialized hair cells of the saccule, utricle, and ampulae of the semicircular labyrinths. Post-ganglionic neurons exit the cochlear ganglion via the cochlear branch of the vestibulocochlear nerve to innervate specialized hair cells of the spiral organ.