Premed · Premed · Anatomy Physiology 1
Lecture 22: Special Senses — Hearing and Equilibrium
Anatomy and Physiology I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the anatomy of the external, middle, and inner ear
- Explain the process of sound transmission from the external ear to the cochlea
- Describe the structure of the organ of Corti and the mechanism of auditory transduction
- Trace the auditory pathway from the cochlea to the auditory cortex
- Distinguish between conductive and sensorineural hearing loss
- Describe the vestibular apparatus and the receptors for static and dynamic equilibrium
- Explain the physiology of the vestibular system and the vestibulo-ocular reflex
Lecture Content
I. Anatomy of the Ear
The ear consists of three regions: the external ear, the middle ear, and the inner ear. Together, these structures serve two distinct functions: hearing and equilibrium (balance).
A. External (Outer) Ear
The auricle (pinna) is the visible, cartilaginous external structure that funnels sound waves into the ear canal. Its key landmarks include the helix (outer rim), lobule (earlobe), and tragus (the small projection anterior to the canal opening). The external acoustic meatus (ear canal) is an approximately 2.5 cm tube extending from the auricle to the tympanic membrane. Its lateral third is composed of elastic cartilage and contains ceruminous glands that produce cerumen (earwax) and hairs that trap debris, while the medial two-thirds is formed by the temporal bone. Together, cerumen and hairs protect the delicate tympanic membrane.
The tympanic membrane (eardrum) is a thin, semitransparent membrane separating the external and middle ear. It is cone-shaped and composed of three layers: an outer skin epithelium, a middle layer of connective tissue, and an inner mucous membrane. The external surface is innervated by CN V3, while the internal surface receives innervation from CN IX and X. The tympanic membrane vibrates in response to incoming sound waves, converting airborne sound into mechanical vibration.
B. Middle Ear (Tympanic Cavity)
The middle ear is a small, air-filled cavity within the temporal bone. It connects to the nasopharynx via the pharyngotympanic (auditory/Eustachian) tube, which equalizes pressure on both sides of the tympanic membrane by opening during swallowing and yawning. When the tube is blocked, as occurs with upper respiratory infections or allergies, the resulting pressure differences can cause ear pain and hearing impairment. This tube also provides a route for infection to spread from the throat to the middle ear, causing otitis media, which is especially common in children because their tube is shorter and more horizontal.
The middle ear contains the three auditory ossicles, the smallest bones in the body. The malleus (hammer) is attached to the tympanic membrane, the incus (anvil) articulates with the malleus and stapes, and the stapes (stirrup) has a footplate that fits into the oval window of the inner ear. The ossicles transmit and amplify vibrations from the tympanic membrane to the oval window through two amplification mechanisms. The lever action of the ossicle chain provides approximately 1.3-fold amplification, and the area ratio between the tympanic membrane (approximately 17-20 times larger than the oval window) concentrates force onto the smaller area, providing approximately 17-20-fold amplification. The combined amplification of approximately 22 times is essential for transmitting sound from air (low impedance) to fluid (high impedance) in the inner ear, a process known as impedance matching.
Two small muscles protect the inner ear from damage by loud sounds. The stapedius muscle (innervated by CN VII) and the tensor tympani (innervated by CN V3) reflexively contract to dampen ossicle movement in the acoustic reflex.
C. Inner Ear (Labyrinth)
The inner ear is located within the petrous part of the temporal bone and consists of two divisions. The bony labyrinth is a system of interconnected cavities carved in bone, filled with perilymph (which is similar to extracellular fluid, being high in sodium and low in potassium). Its components are the vestibule, semicircular canals, and cochlea. Within the bony labyrinth lies the membranous labyrinth, a system of membrane-walled ducts and sacs filled with endolymph (which is similar to intracellular fluid, being high in potassium and low in sodium). Its components are the utricle and saccule within the vestibule, the semicircular ducts within the semicircular canals, and the cochlear duct within the cochlea.
II. The Cochlea and Hearing
Cochlear Anatomy
The cochlea is a coiled, snail-shaped structure making approximately 2.5 turns. In cross-section, it is divided into three fluid-filled chambers called scalae. The scala vestibuli is the superior chamber, beginning at the oval window and containing perilymph. The scala media (cochlear duct) is the middle chamber, representing the membranous labyrinth portion and containing endolymph. The scala tympani is the inferior chamber, ending at the round window and containing perilymph. The scala vestibuli and scala tympani connect at the apex of the cochlea through the helicotrema and both contain perilymph.
Two membranes separate these chambers. The vestibular membrane (Reissner membrane) separates the scala vestibuli from the scala media. The basilar membrane separates the scala media from the scala tympani and supports the organ of Corti. Critically, the basilar membrane varies in its physical properties along its length: it is narrow and stiff at the base near the oval window, where it responds to high-frequency sounds, and wide and flexible at the apex near the helicotrema, where it responds to low-frequency sounds. This gradient is the basis of tonotopic organization, whereby different frequencies of sound maximally stimulate different regions of the basilar membrane.
Organ of Corti (Spiral Organ)
The organ of Corti is the receptor organ for hearing, resting on the basilar membrane within the scala media. It contains hair cells, the mechanoreceptors responsible for auditory transduction. Inner hair cells (IHCs) form a single row of approximately 3,500 cells and serve as the primary auditory receptors, with approximately 95% of auditory nerve fibers synapsing on them. Outer hair cells (OHCs) form three rows totaling approximately 12,000 cells and function to amplify and fine-tune the cochlear response through active mechanical amplification, contracting and lengthening to enhance basilar membrane vibration.
Hair cells possess stereocilia (modified microvilli) projecting from their apical surface. The tallest stereocilia of the outer hair cells are embedded in the tectorial membrane, a gelatinous shelf overhanging the organ of Corti. Adjacent stereocilia are connected by tip links, filaments that are central to the transduction mechanism.
<image>A cross-section through one turn of the cochlea showing the three scalae. The scala vestibuli (top, filled with perilymph) is separated from the scala media (middle, filled with endolymph) by the vestibular membrane (Reissner membrane). The scala media is separated from the scala tympani (bottom, filled with perilymph) by the basilar membrane. The organ of Corti sits on the basilar membrane and contains one row of inner hair cells and three rows of outer hair cells with stereocilia projecting upward. The tectorial membrane overhangs the organ of Corti, with the tallest stereocilia of the outer hair cells embedded in it. Afferent nerve fibers (spiral ganglion neurons forming CN VIII) are shown synapsing on the base of the hair cells. The stria vascularis on the lateral wall of the scala media is labeled as the source of endolymph production. An inset shows a close-up of a hair cell with stereocilia arranged in rows of increasing height, connected by tip links, with arrows showing that deflection toward the tallest stereocilia opens mechanically gated K+ channels (depolarization) and deflection away closes them (hyperpolarization).</image>
III. Mechanism of Hearing (Auditory Transduction)
Sound Transmission
Sound transmission through the ear proceeds in a stepwise fashion. Sound waves enter the external acoustic meatus and vibrate the tympanic membrane. These vibrations are transmitted through the ossicle chain (malleus, incus, stapes) with amplification. The stapes footplate pushes on the oval window, creating pressure waves in the perilymph of the scala vestibuli. These pressure waves travel through the scala vestibuli, across the vestibular membrane (or through the helicotrema for low frequencies), and down through the scala tympani to the round window, which bulges outward to relieve pressure. The pressure wave causes the basilar membrane to vibrate at the location corresponding to the frequency of the sound, maintaining tonotopic organization.
Hair Cell Transduction
The transduction process in hair cells converts mechanical vibration into electrical signals. Basilar membrane vibration causes the organ of Corti to move relative to the tectorial membrane, shearing the stereocilia of the hair cells. When stereocilia are deflected toward the tallest stereocilia, the tip links stretch and open mechanically gated potassium channels. Potassium enters the hair cell from the endolymph, which has an unusually high potassium concentration of approximately 150 mM. The difference between the endolymph potential (+80 mV) and the hair cell resting potential (-60 mV) creates a large driving force of approximately 140 mV known as the endocochlear potential. This potassium influx depolarizes the hair cell, opening voltage-gated calcium channels at the base of the cell. Calcium triggers exocytosis of glutamate-containing vesicles, which excites the afferent terminals of spiral ganglion neurons (the first-order neurons of the cochlear division of CN VIII). Action potentials are then generated and travel via the cochlear nerve to the brainstem.
Encoding of Sound
The properties of sound are encoded in distinct ways. Pitch (frequency) is encoded by the location on the basilar membrane that vibrates maximally, following tonotopic or place coding principles, with high frequencies at the base and low frequencies at the apex. Loudness (amplitude) is encoded by the frequency of action potentials in auditory nerve fibers and the number of hair cells and nerve fibers activated. Timbre is encoded by the pattern of harmonics activating different regions of the basilar membrane simultaneously.
IV. Auditory Pathway
The auditory pathway involves multiple relay stations. Spiral ganglion neurons (first-order), with cell bodies in the modiolus of the cochlea, send axons forming the cochlear nerve (part of CN VIII). These synapse in the cochlear nuclei (second-order) in the medulla, from which most fibers cross to the opposite side, meaning each hemisphere processes sound primarily from the contralateral ear. The superior olivary nucleus in the pons is the first site of binaural processing, comparing input from both ears for sound localization based on interaural time and intensity differences. The inferior colliculus in the midbrain serves as an auditory reflex center for the startle reflex and head turning toward sound. The medial geniculate nucleus (MGN) in the thalamus relays auditory information to the cortex. The primary auditory cortex (A1) on the superior temporal gyrus (Brodmann areas 41 and 42) is tonotopically organized. The auditory association cortex, including Wernicke area and surrounding regions, interprets sounds and enables speech recognition.
V. Hearing Loss
Conductive Hearing Loss
Conductive hearing loss results from impaired transmission of sound through the external or middle ear. Common causes include cerumen impaction, otitis media with fluid accumulation, otosclerosis (abnormal bone growth fusing the stapes to the oval window), and perforated tympanic membrane. On the Rinne test, bone conduction is greater than or equal to air conduction on the affected side (normally air conduction is greater). On the Weber test, sound lateralizes to the affected ear because bone conduction bypasses the middle ear and directly stimulates the cochlea.
Sensorineural Hearing Loss
Sensorineural hearing loss results from damage to the cochlea (hair cells) or the cochlear nerve. Causes include noise-induced damage from chronic exposure to loud sounds (which destroys hair cells irreversibly), presbycusis (age-related progressive loss of high-frequency hearing), ototoxic drugs (aminoglycosides, cisplatin, loop diuretics), Meniere disease, and acoustic neuroma. On the Rinne test, air conduction remains greater than bone conduction, but both are reduced. On the Weber test, sound lateralizes to the unaffected ear. Cochlear implants can bypass damaged hair cells by directly stimulating the cochlear nerve.
VI. The Vestibular System — Equilibrium
The vestibular apparatus is located in the inner ear and detects head position and movement. It mediates two types of equilibrium: static equilibrium, which senses head position relative to gravity and linear acceleration (detected by the utricle and saccule), and dynamic equilibrium, which senses rotational (angular) acceleration of the head (detected by the semicircular canals).
A. Maculae — Receptors for Static Equilibrium
The maculae are located in the utricle (oriented horizontally, detecting horizontal linear acceleration and head tilt) and the saccule (oriented vertically, detecting vertical linear acceleration). Their structure consists of hair cells with stereocilia and a single tall kinocilium that project into a gelatinous otolithic membrane. Embedded within this membrane are otoliths (otoconia), calcium carbonate crystals that add mass and make the membrane sensitive to gravity and linear acceleration.
The mechanism works through gravitational or inertial displacement. When the head tilts or accelerates linearly, gravity or inertia causes the otolithic membrane to slide over the hair cells, bending the stereocilia. Bending toward the kinocilium produces depolarization (excitation), while bending away from the kinocilium produces hyperpolarization (inhibition). The resulting changes in firing rates in vestibular nerve fibers inform the brain about head position and linear motion.
B. Cristae Ampullares — Receptors for Dynamic Equilibrium
The cristae ampullares are located in the ampullae, the expanded bases of the three semicircular canals. The three canals are oriented in three perpendicular planes to detect rotation in any direction. The anterior (superior) canal detects rotation in the sagittal plane (nodding "yes"), the posterior canal detects rotation in the coronal plane (tilting the head to the shoulder), and the lateral (horizontal) canal detects rotation in the transverse plane (shaking the head "no").
Each ampulla contains a crista ampullaris, a ridge of hair cells whose stereocilia and kinocilia project into a gelatinous cap called the cupula. The cupula spans the width of the ampulla, forming a fluid barrier. When the head rotates, the endolymph initially lags behind due to inertia, pushing against the cupula. The cupula bends, deflecting the stereocilia of the hair cells. Deflection toward the kinocilium produces depolarization and increased firing, while deflection away produces hyperpolarization and decreased firing. When rotation stops, the endolymph eventually catches up and the cupula returns to its neutral position. Each canal works as a push-pull pair with its counterpart on the opposite side; for example, turning the head right increases firing from the right lateral canal and simultaneously decreases firing from the left lateral canal.
<image>A detailed diagram of the vestibular apparatus. Panel A shows the bony labyrinth with the three semicircular canals (anterior, posterior, lateral) oriented in three perpendicular planes, the vestibule containing the utricle and saccule, and the cochlea. Ampullae are labeled at the base of each semicircular canal. Panel B shows a cross-section through an ampulla: the crista ampullaris (ridge of hair cells) with stereocilia and kinocilia projecting into the gelatinous cupula that spans the ampulla. Arrows show endolymph flow during head rotation deflecting the cupula. Bending toward the kinocilium is labeled as depolarization (excitation), and bending away is labeled as hyperpolarization (inhibition). Panel C shows a cross-section through the macula of the utricle: hair cells with stereocilia and kinocilia embedded in the otolithic membrane studded with otoliths (otoconia). When the head tilts, gravity pulls the otolithic membrane, bending the stereocilia. The utricle macula is shown in horizontal orientation and the saccule macula in vertical orientation. Afferent fibers of the vestibular nerve (CN VIII) are shown at the base of hair cells in both panels.</image>
VII. Vestibular Pathways and Reflexes
Vestibular Pathway
Hair cells in the maculae and cristae synapse on vestibular ganglion neurons in Scarpa ganglion, which serve as first-order neurons. Their axons form the vestibular nerve (part of CN VIII). Most fibers terminate in the vestibular nuclei (four nuclei in the medulla and pons), while some proceed directly to the cerebellum (flocculonodular lobe). The vestibular nuclei project to multiple destinations: the cerebellum for coordination of balance and posture adjustments; the spinal cord via vestibulospinal tracts for maintaining posture and balance; cranial nerve nuclei III, IV, and VI for controlling eye movements through the vestibulo-ocular reflex; the thalamus and vestibular cortex for conscious perception of position and movement; and the reticular formation for autonomic responses such as nausea and vomiting in motion sickness.
Vestibulo-Ocular Reflex (VOR)
The vestibulo-ocular reflex stabilizes the visual image on the retina during head movements. When the head turns in one direction, the eyes reflexively move in the opposite direction at the same speed, keeping the gaze fixed. This reflex is mediated by connections from the vestibular nuclei to the oculomotor nuclei (CN III, IV, and VI). Nystagmus is an involuntary, rhythmic oscillation of the eyes consisting of a slow phase (the VOR-driven compensatory movement) and a fast phase (a resetting saccade). While nystagmus is normal during and briefly after rotation, pathological nystagmus at rest indicates vestibular or CNS dysfunction.
VIII. Clinical Correlations
Vertigo
Vertigo is a sensation of spinning or movement when stationary. Peripheral vertigo originates from the inner ear and is often severe but self-limiting. Benign paroxysmal positional vertigo (BPPV) is the most common cause, resulting from dislodged otoconia that migrate into a semicircular canal and cause inappropriate stimulation. It is treated with the Epley maneuver for repositioning. Meniere disease involves excess endolymph (endolymphatic hydrops) producing episodic vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural fullness. Vestibular neuritis and labyrinthitis result from viral inflammation of the vestibular nerve or labyrinth. Central vertigo originates from the brainstem or cerebellum and may indicate stroke, multiple sclerosis, or tumor.
Motion Sickness
Motion sickness arises from conflict between vestibular, visual, and proprioceptive inputs. The vestibular system signals movement (for example, in a car or boat) while visual input suggests the environment is stationary, or vice versa. This mismatch results in nausea, vomiting, and dizziness mediated by vestibular connections to the reticular formation and vomiting center.
Clinical Tests
The Romberg test asks the patient to stand with feet together and eyes closed; swaying or falling indicates proprioceptive or vestibular dysfunction. The Dix-Hallpike maneuver provokes vertigo and nystagmus in BPPV. Caloric testing uses warm or cold water irrigation of the ear canal to induce endolymph convection currents and is used to assess vestibular function in unconscious patients.
