Ear

Lecture Notes on Human Structure II: Ear

Introduction

  • Course: MEDU3300 Human Structure II

  • Institution: The Chinese University of Hong Kong, Faculty of Medicine

  • Lecturer: Prof WH Kwong

  • Date: 23 Sep 2025

Objectives

  • To study the compositions and relations of the external and middle ear.

  • Internal ear will be covered in the auditory system focused in neuroscience.

Learning Outcomes

  • Locate and Describe: Identify and explain the important structures of the middle ear and their relations.

  • Explain Relations: Understand how the middle ear relates to nearby anatomical structures.

  • Anatomical Background: Provide simple explanations on common ear problems in children and adults.

Overview of the Ear

  • Three Divisions:

    • External Ear: Consists of the auricle and external acoustic meatus (ear canal) leading to the tympanic membrane.

    • Middle Ear: Contains key structures like the tympanic membrane and ear ossicles (malleus, incus, stapes).

    • Internal Ear: Includes the cochlea, vestibule, and semicircular canals.

  • Major Components:

    • Temporal bone (petrous part), dura mater, oval window, ear ossicles (malleus, incus, stapes).

External Ear

  • Anatomy:

    • Auricle:

    • Structure: Composed primarily of elastic cartilage except for the lobule (earlobe), which contains fibroareolar tissue.

    • Function: Directs sound waves into the external acoustic meatus.

    • External Acoustic Meatus (Ear Canal):

    • Length: Approximately 2.5 cm in adults, S-shaped, with a lateral third being cartilaginous and medial two-thirds bony.

    • Internal Lining: Thin skin that becomes continuous with the outer surface of the tympanic membrane, supplied by auriculotemporal nerve (CN V3) and auricular branch of the vagus nerve (CN X).

    • Contains hairs and ceruminous glands which contribute to earwax (cerumen).

    • Clinical Note: Cleaning with cotton swabs can cause injury and lead to otitis externa.

  • Relations with Neighbouring Structures:

    • Anterior:

    • Temporomandibular (TM) Joint

    • Parotid Gland

    • Posterior:

    • Mastoid Process (with sigmoid and transverse sinuses located medially).

  • Supplied by:

    • Auriculotemporal nerve (CN V3), superficial temporal artery, and relevant branches of facial nerve (CN VII).

Middle Ear

  • Anatomy of Middle Ear: Rectangular cavity with a roof, floor, anterior, posterior, medial, and lateral walls. Contains:

    • Tympanic membrane.

    • Mastoid with antrum and air cells.

    • Epitympanic recess (part of the tympanic cavity above the tympanic membrane).

    • Auditory Tube (Eustachian Tube): Connects middle ear to the nasopharynx, facilitating air pressure balance.

  • Auditory Tube Details:

    • Orientation: Runs forward, downward, and medially; consists of bony (1/3) and cartilaginous (2/3) parts, with an isthmus (narrow junction).

    • Muscles: Tensor veli palatini and levator veli palatini accompany the tube, aiding in opening during swallowing/yawning.

  • Clinical Note: Dysfunction of the auditory tube can lead to:

    • Negative pressure in middle ear, pain, and hearing difficulties.

    • Serous otitis media due to impaired fluid drainage.

Tympanic Membrane**

  • Structure:

    • Composed of two parts: pars flaccida (less taut) and pars tensa (more taut).

    • The handle of the malleus is attached to the inner surface, pulling it inward.

  • Blood Supply and Innervation:

    • Blood supply from branches of the maxillary and posterior auricular arteries.

    • Nervous supply: External aspects from CN V3 and CN X, internal aspect from CN IX.

Middle Ear: Auditory Ossicles

  • Three bones: malleus, incus, stapes that connect to transduce sound vibrations from the tympanic membrane to the oval window.

  • Muscles Responding to Sound:

    • Tensor Tympani: Reduces tympanic membrane vibration.

    • Stapedius: Reduces stapes vibration; paralysis leads to hyperacusis.

  • Innervation: Tensor tympani supplied by CN V3; stapedius by CN VII.

Nerves and Their Functions in the Middle Ear

  • Facial Nerve (CN VII): Passes through facial canal and supplies motor fibers to stapedius and sensory taste fibers via chorda tympani.

  • Glossopharyngeal Nerve (CN IX): Supplies sensation to middle ear and innervates parotid gland.

  • Vagus Nerve (CN X): Provides sensation to parts of the middle ear.

Clinical Considerations

  • Conditions affecting middle ear can lead to serious complications:

    • Mastoiditis: Infection can spread to mastoid air cells, leading to inflammation.

    • Facial nerve complications:

    • Hyperacusis due to stapedius paralysis.

    • Loss of taste, dry eyes, and dry mouth due to impaired innervation.

  • Myringotomy: Procedure to drain pus in the tympanic cavity; typically performed in the inferior quadrants to avoid critical structures.

Conclusion

  • The ear's structures collectively contribute to hearing and balance, with various clinical implications arising from dysfunction or infection. Understanding these anatomical relationships and functions is crucial for diagnosing and treating ear-related ailments.