Anatomy of Middle Ear - Contents

Anatomy of the Middle Ear - Contents

Today, we'll focus on the contents of the Middle ear cavity.

The middle ear contains several crucial structures:

• Auditory Ossicles (Malleus, Incus, Stapes)
• Ossicular Articulations
• Middle Ear Muscles (Tensor Tympani, Stapedius)
• Middle Ear Nerves
• Middle Ear Vessels (Arteries, Veins, Lymphatics)
• Middle Ear Mucosal Folds and Spaces

🦴 Auditory Ossicles

These three tiny bones are named after their resemblance to a hammer, anvil, and stirrup.

They are suspended in the middle ear cavity by ligaments and mucosal folds, and covered by the middle ear mucosa. Their essential function is to transmit sound-induced vibrations from the Tympanic Membrane to the Oval Window.

🔨 Malleus

• Shape: Hammer-shaped.

• Size: Largest ossicle, 8-9 mm length, 20-25 mg weight.

• Parts:

  • Head: Lies in the attic (epitympanum). Has an elongated, saddle-shaped facet postero-medially for articulation with the Incus (Incudomalleal Joint).

  • Neck: Narrow, flattened part below the head.

    • Lateral surface forms the medial wall of Prussak's space.

  • Handle (Manubrium): Forms a 135-140° angle with the head. Runs downwards, medially, and slightly backwards between the mucous and fibrous layers of the TM.

    • Inferior end is flattened and attached to the TM, forming the Umbo.

  • Lateral Process: Small conical eminence (~1mm) projecting laterally to the TM side. Gives attachment to the anterior and posterior Tympanomalleal Ligaments (forming malleal folds).

  • Anterior Process (Processus Gracilis): 3-5 mm thin bony spine from the neck into the Petrotympanic (Glaserian) Fissure.

    • Chorda Tympani nerve runs on its medial aspect.
    • Gives origin to the Anterior Malleal Ligament, which passes through the Petrotympanic fissure to the angular spine of the sphenoid.

Which nerve and muscle is related to the Neck of Malleus?

What are the boundaries of Prussak's space?

🦾 Ligaments of the Malleus:

Stabilized by five ligaments, the Incudomalleal joint, Tensor Tympani tendon, and Tympanic Membrane.

• Suspensory Ligaments (outside axis of rotation, offer support):

  • Anterior Suspensory Ligament
  • Lateral Suspensory Ligament
  • Superior Suspensory Ligament

• Other Ligaments:

  • Anterior Malleal Ligament
  • Posterior Malleal Ligament
  • Superior Malleal Ligament

To learn more in details on Malleus and it's ligaments, get access to full notes by purchasing it.

🔨 Incus

• Shape: Anvil-shaped.

• Size: 5x7 mm, 30 mg weight.

• Parts:

  • Body: Flat, situated in the attic with the malleus head. Anterior surface has an elliptical articular facet for the malleus head.

  • Short Process: Extends posteriorly from the body, thick and triangular. Dorsal end rests in the Incudal Fossa (in the floor of the aditus ad antrum).

  • Long Process: Follows the direction of the malleus handle (postero-medially). Caudal end forms a hook ending in the Lenticular Process.

  • Lenticular Process: Rounded, connects the long process to the head of the Stapes. Has a narrow bony pedicle and flattened distal plate. Articulates with the Stapes head via a facet perpendicular to the long process.

Which part of Incus is most prone to erosion and why?

🦾 Ligaments of the Incus:

Fewer ligaments than the malleus, making it susceptible to traumatic dislocation.

• Posterior Incudal Ligament

• Superior Incudal Ligament

🔨 Stapes

• Shape: Stirrup-shaped.

• Size: Smallest bone in the body, 3.25 mm high, 1.4 mm wide, 3-4 mg weight.

• Location: Situated in an almost horizontal plane between the lenticular process and the Oval Window, below the Facial nerve canal.

• Parts:

  • Head: Cylindrical/discoid. Bears a glenoid cavity (fovea) articulating with the lenticular process of the Incus. Constricts to form the neck. Posterior edge has a rough surface for insertion of the Stapedial Muscle tendon.

  • Neck: Connects the head to the crura.

  • Anterior and Posterior Crura: Connect the head/neck to the footplate. Unequal in size: posterior is longer, thicker, and more curved.

    • Obturator Foramen: Area between the crura, sometimes bridged by mucosa.

  • Footplate: Thin, oval lamella of bone.

    • Lateral (Tympanic) surface: Covered by middle ear mucoperiosteum.
    • Medial (Vestibular) surface: Flat, lined by endosteum of otic capsule. In close relation to the Saccule (1 mm deep from anterior part) and Utricle (1.5 mm deep from posterior part).

Why is Posterior crus cut, and not fractured during Stapedectomy?

🦾 Annular Ligament (Ligamentum Annulare Stapedis):

• Ring of elastic fibers attaching the cartilaginous margin of the footplate to the border of the Oval Window.

Why do we see a low frequency Conductive Hearing loss in early stages of Otosclerosis?

✨ Ossicular Articulations

Synovial joints (except Stapedio-Vestibular) with capsules and synovial membranes.

1. Incudomalleolar Joint

   • Synovial joint, in the epitympanum.

   • Head of malleus articulates with the body of incus.

   • Saddle-shaped surfaces, separated by an articular disc. Mobility restricted to rotation on the axis passing through the short process of incus and anterior process of malleus.

2. Incudostapedial Joint

   • Synovial joint.

   • Lenticular process of incus articulates with the head of stapes.

   • Ball and Socket joint, held by a capsule.

3. Stapedio-Vestibular Joint

   • Junction between stapes footplate and oval window.

   • A half-joint (syndesmosis). The Annular Ligament holds the footplate.

   • Allows piston-like and rocking movements crucial for sound transmission.

💪 Middle Ear Muscles

Two small intratympanic muscles that modify ossicular chain movement, mainly for protection and potentially modulating hearing at different frequencies.

1. Tensor Tympani:

   • Origin: Cartilage of the Eustachian tube, bony semicanal wall, adjacent greater wing of sphenoid.

   • Course: Fibers converge to form a tendon that turns sharply laterally around the Cochleariform Process .

   • Insertion: Medial surface of the junction of the neck and handle of the Malleus.

   • Innervation: Mandibular division of Trigeminal Nerve (via nerve to medial pterygoid).

   • Action: Predominantly a peritubal muscle, assists Eustachian tube opening/closing during speech/swallowing. Within the middle ear, pulls the malleus medially, dampening TM/ossicular vibrations. Prevents autophony during chewing/swallowing. Muscle spindles may have barometric properties.

   • Clinical Importance: Hypercontraction can cause low-frequency hearing loss and decreased middle ear compliance. Its pull is opposed by the TM's elasticity.

2. Stapedius:

   • Origin: Bony cavity in the posterior wall of the tympanic cavity.

   • Course: Tendon emerges from the Pyramidal Eminence.

   • Insertion: Head/posterior crus of the Stapes.

   • Innervation: Stapedial branch of the Facial Nerve.

   • Action: Pulls the stapes posteriorly, tilting the footplate (anterior border moves laterally, posterior medially). Stretches annular ligament, dampening footplate movement. Helps protect inner ear from loud noise.

   • Clinical Importance: Lack of action (due to nerve section or facial palsy) leads to Hyperacusis (sensitivity to loud sounds).

What are the causes of Middle ear Myoclonus?

✨ Middle Ear Nerves

The middle ear transmits and receives branches from cranial nerves and sympathetic plexus.

• Chorda Tympani Nerve:

  • Branch of the Facial Nerve (specifically, the nervus intermedius part).

What is the course of Chorda Tympani Nerve in Middle ear?

What is the function of Chorda Tympani nerve?

• Tympanic Plexus: Network of nerves on the surface of the Promontory (medial wall).

What are the nerves forming the Tympanic Plexus?

🩸 Middle Ear Blood Vessels

Arterial supply from branches of both Internal and External Carotid Arteries.

• Arteries:

  • Anterior Tympanic Artery
  • Posterior Tympanic Artery
  • Superior Petrosal Artery
  • Superior Tympanic Artery
  • Caroticotympanic Arteries
  • Inferior Tympanic Artery

• Veins: Drain into the Pterygoid Plexus, Superior Petrosal Sinus, and eventually the External Jugular Vein.

• Lymphatics: Drain into the Parotid and Upper Deep Cervical nodes.

🔬 Middle Ear Mucosa

• Lines the Eustachian tube, middle ear cavity, and mastoid air cell system.

• Eustachian Tube & Most of Middle Ear: Ciliated columnar epithelium with subepithelial connective tissue, goblet cells, and mucous glands. Goblet cells and glands concentrated near Eustachian tube orifice.

• Floor of Tympanum: Epithelium becomes more cuboidal and loses cilia.

• Epitympanum, Antrum, Mastoid Air Cells: Changes to flat pavement epithelium.

• Eustachian Tube Lining: Pseudostratified ciliated columnar epithelium. Goblet cells prominent proximally, producing surfactant-like secretions to keep the tube patent.

• Cilia Function: Cilia density increases towards the nasopharynx in the Eustachian tube, facilitating movement of mucus and drainage from the middle ear.