Makindo Medical Notes

Anatomy of the Ear

The ear is a complex organ responsible for hearing and maintaining balance. It is divided into three major sections: the external ear, middle ear, and inner ear. Each section plays a unique role in transmitting sound waves and processing auditory information, as well as maintaining equilibrium.

External Ear

The external ear consists of the auricle (pinna) and the external auditory canal, and it terminates at the tympanic membrane (eardrum).

• Auricle (Pinna):
  • The auricle is the visible part of the ear that collects sound waves and funnels them into the external auditory canal. It is composed of cartilage covered by skin.
  • Its structure includes the helix, antihelix, tragus, antitragus, concha, and lobule. These features help in determining the direction of sound.
• External Auditory Canal:
  • The external auditory canal (meatus) is approximately 2.5 cm in length. It channels sound waves from the auricle to the tympanic membrane.
  • The canal is lined with skin containing ceruminous and sebaceous glands that produce cerumen (earwax), which protects the ear by trapping dust and other particles.
• Tympanic Membrane (Eardrum):
  • The tympanic membrane is a thin, cone-shaped membrane that separates the external ear from the middle ear. It vibrates in response to sound waves and transmits these vibrations to the ossicles in the middle ear.

Middle Ear

The middle ear is an air-filled cavity that lies between the tympanic membrane and the inner ear. It contains three ossicles (tiny bones) that are essential for sound conduction, as well as the Eustachian tube, which regulates pressure.

• Ossicles: The three ossicles of the middle ear amplify sound vibrations and transmit them to the inner ear.
  • Malleus (Hammer): The malleus is attached to the tympanic membrane and articulates with the incus.
  • Incus (Anvil): The incus connects the malleus to the stapes and transmits vibrations between them.
  • Stapes (Stirrup): The stapes is the smallest bone in the human body and is attached to the oval window of the inner ear. It transmits vibrations to the fluid-filled inner ear.
• Eustachian Tube:
  • The Eustachian tube connects the middle ear to the nasopharynx. It helps equalize air pressure on both sides of the tympanic membrane, which is crucial for proper hearing.
  • This tube opens during activities such as swallowing and yawning to maintain balance between atmospheric pressure and ear pressure.

Inner Ear

The inner ear, also known as the labyrinth, is composed of the cochlea (involved in hearing) and the vestibular system (involved in balance). The inner ear is filled with fluid, and the mechanical vibrations from the middle ear are converted into electrical impulses, which are sent to the brain.

• Cochlea:
  • The cochlea is a spiral-shaped structure that contains the organ of Corti, the sensory organ for hearing. The cochlea is divided into three fluid-filled chambers: the scala vestibuli, scala tympani, and scala media (cochlear duct).
  • Within the scala media, hair cells in the organ of Corti detect sound vibrations and convert them into electrical signals. These signals are transmitted via the cochlear nerve to the brain.
• Vestibular System:
  • The vestibular system is responsible for maintaining balance and equilibrium. It includes the semicircular canals, utricle, and saccule.
  • Semicircular Canals: These three fluid-filled canals (anterior, posterior, and lateral) are oriented in different planes and detect rotational movements of the head (angular acceleration). The canals are filled with endolymph and contain hair cells that respond to head movement.
  • Utricle and Saccule: These two otolithic organs detect linear acceleration and head position relative to gravity. They contain hair cells embedded in a gelatinous layer with calcium carbonate crystals (otoconia) that move with changes in head position.

Nerve Supply of the Ear

• Cochlear Nerve (Part of CN VIII): Transmits auditory information from the cochlea to the brainstem, where it is further processed by the auditory pathways in the brain.
• Vestibular Nerve (Part of CN VIII): Transmits balance-related information from the semicircular canals, utricle, and saccule to the brain, aiding in the control of balance and spatial orientation.
• Facial Nerve (CN VII): The facial nerve runs in close proximity to the middle ear and provides motor innervation to the muscles of facial expression. It also gives off a branch called the chorda tympani, which conveys taste sensation from the anterior two-thirds of the tongue.
• Glossopharyngeal Nerve (CN IX): Provides sensory innervation to parts of the middle ear and the Eustachian tube.

Vascular Supply of the Ear

• External Ear: The external ear receives blood supply from the posterior auricular artery (branch of the external carotid artery) and the superficial temporal artery.
• Middle Ear: The middle ear is primarily supplied by branches of the maxillary artery (anterior tympanic artery) and the ascending pharyngeal artery.
• Inner Ear: The inner ear is supplied by the labyrinthine artery, which is a branch of either the anterior inferior cerebellar artery (AICA) or the basilar artery.

Functions of the Ear

• Hearing: The ear transmits sound waves from the external ear to the inner ear, where they are converted into electrical impulses and sent to the brain. This allows the brain to interpret sound based on frequency (pitch) and amplitude (loudness).
• Balance: The vestibular system of the inner ear detects changes in head position and motion, helping maintain posture and balance. Information from the semicircular canals and otolithic organs (utricle and saccule) is integrated in the brain to coordinate balance.
• Pressure Regulation: The Eustachian tube maintains pressure equilibrium between the middle ear and the external environment. Proper pressure regulation is essential for optimal hearing and preventing ear discomfort during changes in atmospheric pressure.