The Anatomy and Function of the External and Middle Ear

Introduction

• Importance of understanding the external and middle ear before delving into the inner ear.
• These components play a crucial role in auditory pathways.

External Ear

Anatomy

1. Auricle/Pinna

   • Outer part of the ear, made of elastic cartilage.
   • Covered with stratified squamous epithelial tissue.
   • Elastic cartilage allows it to be stretchable and return to normal shape.

2. External Acoustic Meatus

   • Bony canal leading into the ear.
   • Lined with stratified squamous epithelial tissue.
   • Contains ceruminous glands producing cerumen (earwax).

3. Cerumen (Earwax)

   • Produced by modified apocrine glands called ceruminous glands.
   • Function: Deters insects and protects the ear canal.

Tympanic Membrane (Eardrum)

• Separates the external ear from the middle ear.
• Structure:
  • Pars Tensa: Dense irregular connective tissue.
  • Pars Flaccida: Loose areolar connective tissue.
• Function: Transfers sound waves to the ossicles.

Middle Ear

Ossicles

1. Malleus (Hammer)

   • Vibrates when sound waves hit the tympanic membrane.
   • Connected to the tensor tympani muscle, pulling the malleus medially.

2. Incus (Anvil)

   • Connected to malleus and stapes, transmitting vibrations.

3. Stapes (Stirrup)

   • Taps on the oval window transferring mechanical stimuli to fluid-filled vibrations in the cochlea.

• Tensor Tympani Muscle: Dampens vibrations to prevent excessive movement of the tympanic membrane.

Facial Canal and Nerve Supply

1. Facial Nerve (VII)

   • Motor Function: Innervates stapedius muscle, dampening stapes movement.
   • Parasympathetic Fibers: Affect glands (e.g., salivation).
   • Sensory Fibers: Taste and other sensations.
   • Associated pathways: Chorda Tympani.

2. Glossopharyngeal Nerve (IX)

   • Comes through the tympanic canaliculus forming the tympanic plexus on the promontory.
   • Supplies structures like the parotid gland.

Eustachian (Pharyngotympanic) Tube

• Connects the middle ear to the nasopharynx.
• Equalizes pressure between the middle ear and atmospheric pressure.

Mastoid Sinus

• Located in the posterior wall of the middle ear.
• Sometimes contains mastoid air cells.
• Infection can lead to conditions like mastoiditis.
• Heavily connected to the auditory system and can be surgically managed when infection occurs.

Promontory and Tympanic Plexus

• Promontory: Bony bulge on the medial wall caused by the cochlea.
• Tympanic Plexus: Formed by the glossopharyngeal nerve and carotid tympanic nerves (sympathetic fibers).

Important Considerations

1. Tegmen Tympani (Roof of Middle Ear)

   • Thin bone separating the middle ear from the cranial cavity.
   • Infections (e.g., otitis media) can spread to the cranial cavity causing serious complications.

2. Tensor Veli Palatini and Salpingopharyngeus Muscles

   • Tensor Veli Palatini: Supplied by the trigeminal nerve (V) and tenses the soft palate.
   • Salpingopharyngeus: Elevates the soft palate and helps equalize pressure via the eustachian tube.

Conclusion

• Understanding these foundations is crucial for further study of the inner ear and auditory pathways.
• The middle and external ear structures are vital for hearing function and protecting the auditory system.

Future Topics: Inner ear structures and functions including the cochlea, vestibule, semicircular canals, and disorders like cholesteatoma.