Anatomy and Physiology of Speech and Hearing Mechanism RHS 247 Lecture 7: The middle ear Dr. Aseel Alkhamees

Middle ear cavity Tympanum/tympanic cavity. Narrow air-filled cavity between outer and inner ear. Within the petrous portion temporal bone of the skull. Irregular, laterally compressed space. Narrowest at Umbo ~2mm across (~6 mm top to bottom). The middle ear contains a chain of movable bones, which connect its lateral to its medial wall, and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear.

Fig 1: Schematic diagram of tympanum.

Tympanic cavity Walls of the tympanum Vertical and anteroposterior diameters each ~15mm. Superior portion- epitympanic recess or epitympanum or attic. Communicates posterior with mastoid cavity (antrum). Walls of the tympanum Medial bony wall of labyrinth of cochlea. -Oval window (fenestra vestibuli)  Scala vestibuli. -Round window (fenestra cochlea)  Scala tympani. Promontory (bony prominence in wall of cochlea anterior to windows)- created by first turn of cochlea. Bony prominence – due to lateral semi-circular canal. Canal of facial nerve (also curves down into posterior wall of cavity).

Anterior  Contains opening of channels that opens into nasopharynx Anterior  Contains opening of channels that opens into nasopharynx ?! Why .. (travels anteromedially and downwards). Also opening of canal of tensor tympani muscles. Posterior Contains opening into mastoid cavity –aditus. Pyramidal eminence – houses stapedius muscle. Superior  Thin plate of bone tegmen tympani. Separates middle ear from cranial nerves- temporal lobe of brain. Inferior  Thin plate of bone-Its also known as the jugular wall and it separates the middle ear from the jugular bulb underneath.

Tympanic membrane- Previous lecture. Mastoid cavity Air-filled space in mastoid part of temporal bone. Connects anteriorly with epitympanic recess. Posterior with mastoid cells -Small interconnecting air-filled opening within mastoid process -Vary considerably in number As increase effective volume of middle ear cavity influences impedance, transmission of sound through ME.

Tympanum lining Middle ear ossicles The wall of the tympanum are lined with a mucous membrane. This is continuous with the pharynx. Tympanum- pale thin, slightly vascular and covered with ciliated epithelium. Provides a warm moist environment within the cavity. More mucous glands in cartilaginous portion of Eustachian tube. Middle ear ossicles There are three tiny bones in the middle ear known as ossicles. Malleus (8-9 mm long). Incus (5 x 7 mm). Stapes (3.5 x 1.4 mm): Smallest bone of the human body (size as grain of rice). Join tympanic membrane to oval window of cochlea. Not attached to any other bones. Suspended by series of ligaments, tendons and at ends of ossicular chain to tympanic membrane and oval window to labyrinth.

Malleus Divided to: Manubrium, neck, head. Head in epitympanic recess- superior ligament. Short anterior and lateral processes near top of manubrium. Firmly attached to TM (along manubrium, lateral process). Anterior malleal ligament. Tensor tympani tendon – medial manubrium. End of manubrium is Umbo. Articular facet- joint with boy of incus. Hammer/Mace.

Incus Divided to: body, long and short processes. Body articulates with malleus, malleolar-incudal joint  fairly rigid. Saddle joint  moves as one unit. Short process projects posteriorly- posterior incudis ligament. Superior ligament joins body to tegmen tympani. Long process (parallel to manubrium)- inferiorly bends into lenticular process. Ends in round projection. Form classic ball and socket joint with head of stapes. Anvil- bicuspid tooth with two roots.

Stapes Divided to: head, neck, curae, footplate. Incudo-stapedial joint at head. From neck arises anterior (straighter thinner and shorter) and posterior curae. End of curae is a flattened oval shaped footplate. Attached to oval window- annular ligament. Stapedius tendon inserts into posterior surface of neck. Stirrup shape.

Fig 2: Middle ear ossicles schematic

Middle ear muscles Tensor tympani Tendon connected to upper part of manubrium of the malleus (heads medially, then bends around cochleariform process into the muscle in the anterior wall of the middle ear cavity). Supplied by tensor tympani branch of the trigeminal nerve (Vth cranial nerve). Function to pull malleus medially. Stapedius Smallest muscle in body approx. 6.3 mm. Tendon (from posterior wall of middle ear cavity) the pyramidal eminence connected to neck of stapes. Supplies by stapedial branch of the cranial nerve (VIIth cranial nerve). Pulls on neck of stapes- stiffens ossicular chain.

Eustachian tube Also called auditory/ or pharyngotympanic tube. Connects middle ear cavity to nasopharynx. The Eustachian tube originates from the nasopharynx at the point called the tubal elevation a mass of lymphoid tissue. Approx. 2.5 – 3 cm long. Lined by mucous membrane. Lateral third (toward middle ear cavity) is bone. Middle two-thirds cartilage. Normally closed. Opens by reflex action of tensor veli palatini muscle. Swallowing, yawning, sneezing. Innervated by trigeminal (Vth cranial nerve). Thought to work with action of tensor tympani.

Function of the Eustachian tube Drainage Ventilation Pressure equalisation- e.g: air flight

Summary Middle ear cavity. Walls of the tympanum (Medial, anterior, posterior, superior, and inferior) Mastoid cavity: air-filled space in mastoid part of temporal bone. Middle ear ossicles (Malleus, Incus, Stapes). Middle ear muscles (Tensor tympani, Stapedius). Eustachian tube.