Lecture 10:Anatomy and Functions of the Ear

Functions of the Ear

• The ear has two main functions: hearing (detection of sound) and equilibrium (maintenance of body balance)
• Hearing involves the detection of sound waves by the cochlea
• Equilibrium involves the maintenance of body balance by the vestibule and semicircular canals

Parts of the Ear

• The ear consists of three main regions: external ear, middle ear, and inner ear
• The external ear (outer ear) collects sound waves and channels them down the ear canal
• The middle ear transmits sound waves to the inner ear, amplifying sound energy
• The inner ear consists of two sensory systems: cochlea (hearing) and vestibular apparatus (balance)

External Ear

• Plays a role in sound localization
• Amplifies sound by approximately 5-6 dB
• Pinna (auricle) collects sound waves and channels them down the ear canal
• Ceruminous (wax) glands produce ear wax, trapping foreign particles and preventing them from entering the ear
• Ends at the tympanic membrane (eardrum), which vibrates when sound waves hit it and transmits the sound waves to the middle ear

Middle Ear

• Contains three ossicles (smallest bones): malleus (hammer), incus (anvil), and stapes (stirrup)
• Ossicles are connected to each other and transmit sound waves from the tympanic membrane to the inner ear
• Amplifies sound waves by 22 times
• The auditory tube (Eustachian tube) connects the middle ear with the throat (nasopharynx), helping to equalize air pressure on both sides of the tympanic membrane during yawning or swallowing

Muscles of the Middle Ear

• Two muscles: tensor tympani and stapedius
• Tensor tympani muscle:
  • Contracts in response to loud sounds, pulling the tympanic membrane inward and restricting its freedom of movement
  • Protects the ear from loud noise or trauma
• Stapedius muscle:
  • Smallest muscle in the body
  • Contracts in response to loud sounds, damping the ossicular chain vibration and limiting potential damage

Inner Ear

• Most complex portion of the ear
• A maze of bony chambers within the temporal bone
• Consists of three parts: semicircular canals (balance), vestibule (balance), and cochlea (hearing)

Hearing (Audition)

• Hearing is the transduction of sound waves into electrical signals
• Spiral organ of Corti is housed within the cochlear duct and contains hair cells (hearing receptors)
• Hair cells convert sound waves into nerve impulses, transmitted via the auditory nerves to the brain for perception and interpretation as sound
• Cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz

Mechanism of Hearing

• Sound waves in the ear canal strike the tympanic membrane (eardrum) and become vibrations
• Vibrations of the stapes against the oval window are converted into fluid waves within the vestibular duct
• Fluid waves push on the membranes of the cochlear duct, activating the sensory hair cell receptors
• Activated hair cells stimulate the auditory nerve, transmitting the signal to the brain for perception and interpretation as sound

Auditory Pathway

• The nerve impulse travels along the vestibulocochlear nerve, synapsing with neurons in the cochlear nuclei of the medulla
• From the cochlear nucleus, some fibers synapse at the ipsilateral superior olivary nuclei, while most fibers decussate to the contralateral superior olivary nuclei
• Fibers from superior olivary nuclei project upwards through the lateral lemniscus to reach the inferior colliculus nucleus
• Axons carrying auditory information project to the medial geniculate nucleus of the thalamus, then to the auditory cortex in the temporal lobe of the cerebral cortex for interpretation as hearing

Physiology of Balance: Vestibular Apparatus

• The vestibular apparatus consists of the vestibule (monitoring static equilibrium) and the semicircular canals (monitoring dynamic equilibrium)
• The vestibule is responsible for monitoring static equilibrium, detecting changes in position of the head in space with respect to the pull of gravity
• The semicircular canals are responsible for monitoring dynamic equilibrium, detecting angular or rotatory movements of the head

Vestibule: Utricle and Saccule

• Maculae (static equilibrium receptors) are located within the membranous sacs of the vestibule (utricle and saccule)
• Each macula is a patch of hair cells with stereocilia embedded in the otolithic membrane (a viscous gel containing calcium crystals called otoliths)
• When the head tilts, otoliths roll in response to the pull of gravity, creating a pull on the otolithic membrane and bending of macula hair cells
• The hair cells become activated and send impulses along the vestibulocochlear nerve to the brain stem and cerebellum, informing it of the head position in space