Anatomy of the External & Middle Ears

List the three components of the external ear and describe their functions.

-Auricle: The auricle concentrates sound waves and directs them to the opening of the external auditory canal. -External auditory canal: this canal conducts sound waves from the auricle to the tympanic membrane -Tympanic membrane: forms the medial boundary of the external ear and the lateral wall of the tympanic cavity of the middle ear, transmits vibrations

List the three subdivisions of the ear and describe the function of each.

-Inner ear houses receptors for the special sense of hearing in its cochlear portion (consisting of the snail shellshaped cochlea), and receptors for the special "vestibular sense", or sense of balance -External and middle ears are concerned with conducting sound waves to the inner ear.

Describe the relationship of the following structures to the tympanic (middle ear) cavity: mastoid antrum/air cells

Filled with Mastoid Air Cells similar in character to Ethmoidal Sinuses, the Mastoid Antrum lies posterior to the Tympanic Cavity, within the Mastoid Process of the Temporal bone; the Antrum communicates with the Tympanic Cavity through a small opening called the Aditus. The roof of the Mastoid Antrum is the Tegmen Mastoideum

Describe the relationship of the following structures to the tympanic (middle ear) cavity: middle cranial fossa

Has a direct relationship with the Tympanic Cavity as the Tympanic Cavity (Middle Ear) is a narrow air-filled chamber of the Petrous portion of the Temporal bone (which is part of the Middle Cranial Fossa). The lateral surface of the Petrous part of the Temporal bone is formed by the Tegmen Tympani, a thin osseous lamina that separates the Tympanic Cavity (Middle Ear) from the Middle Fossa.

Describe the relationship of the following structures to the tympanic (middle ear) cavity: chorda tympani

Just prior to reaching the Stylomastoid Foramen the Chorda Tympani leaves the motor portion of the Facial Nerve and the Facial Canal and passes through the air-filled Tympanic Cavity (hence its name). The Chorda Tympani exits the Tympanic Cavity by passing through a small fissure in its floor.

Distinguish the functions of CNs VII in the ear

Provides skeletal motor innervation to the stapedius muscle of the middle ear. The "motor branch" also carries a few somatic afferent fibers; these fibers carry sensory information from a very small section of the skin of the external ear.

Specify the functions of the tensor tympani muscle in hearing and specify their motor innervation of each.

Pulls the handle of the Malleus medially to tightens the Tympanic Membrane. The increased tension reduces the amplitude of the Tympanic Membrane oscillations and thereby reduces sound transmission to the Vestibular Window. Mandibular Nerve V3

Specify the functions of the stapedius in hearing and specify their motor innervation of each.

Reduces the vibrations of the stapes during loud noises so as to protect the sensory receptors of the inner ear from injury. Motor Portion of NC VII Facial Nerve

Distinguish the functions of CNs X in the ear

Somatic afferent fibers of this nerve carry sensory information from portions of the skin of the external auditory meatus and a small portion of the external surface of the tympanic membrane

Distinguish the functions of CNs IX in the ear

Somatic afferent fibers of this nerve carry sensory information from the uvula and opening of the auditory tube, mucosa lining the tympanic cavity of the ear, & internal surface of the tympanic membrane

Describe the relationship of the following structures to the tympanic (middle ear) cavity: nasopharynx

The Auditory Tube connects the Tympanic Cavity to the Nasopharynx - the posterolateral third of the tube is bony, the remainder of the tube is cartilaginous.

Describe the course of the facial nerve and its branches within the petrous temporal bone.

The Petrous Temporal Bone houses the Facial Canal. The Facial Canal begins at the Internal Acoustic Meatus and crosses superior to the structures of the Inner Ear. Once beyond the Inner Ear the canal makes a sharp turn (where the facial nerve gives off its Greater Petrosal Branch) and then passes adjacent to the posterior wall of the Tympanic Cavity of the Middle Ear as it descends towards its opening at the Stylomastoid Foramen.

Describe the relationship of the following structures to the tympanic (middle ear) cavity: auditory tube

The function of the Auditory Tube is to equalize pressure in the Middle Ear with the atmospheric pressure by allowing air to enter and leave the Tympanic Cavity.

Describe the function of the auditory (pharyngotympanic; Eustachian) tube.

The function of the Auditory Tube is to equalize pressure in the Middle Ear with the atmospheric pressure by allowing air to enter and leave the Tympanic Cavity. Normally "closed" due to the apposition of its cartilaginous walls, the tube is opened by contraction of the muscles of the soft palate (which explains why "popping the eardrums" is associated with yawning and swallowing)

Distinguish the functions of CNs VIII in the ear

Two nerves in one - the vestibular portion carries information concerned with equilibrium and head movements; the cochlear portion carries information relating to the sense of hearing.

Distinguish the functions of CNs V in the ear

V3 - Sensory innervation of the skin of the Auricle, External Auditory Canal and external surface of the Tympanic Membrane is via branches of the Mandibular branch of the Trigeminal Nerve (CN V3). Also innervates the Tensor Tympani Muscle.