Biology: The Ear - Audition and Balance Flashcards

Overview of the Ear: Audition and Balance

  • Conceptual Focus: The ear is a specialized organ responsible for two primary sensory functions: audition (hearing) and equilibrium (balance).

  • Structural Division: The ear is divided into three distinct regions, each with specific anatomical features:

    • External Ear: Responsible for collecting and transmitting sound waves.

    • Middle Ear: An air-filled cavity that amplifies sound vibrations.

    • Inner Ear: A fluid-filled system that houses the sensory receptors for both hearing and balance.

Anatomy and Function of the External Ear

  • Pinna (Auricle):

    • Structure: Composed of cartilage and thick skin. Key features include the Helix (superior rim) and the Lobule (earlobe).

    • Function: Collects and transmits sound waves toward the middle ear.

  • Auditory Canal (External Acoustic Meatus):

    • Structure: A short tube (approximately 2 cm2\,cm long) carved into the temporal bone.

    • Inclusions: Contains ceruminous glands (produce wax) and fine hairs.

    • Function: Traps and prevents foreign materials from entering the deeper ear structures.

  • Tympanic Membrane (Eardrum):

    • Structure: A thin, semi-transparent connective tissue membrane that serves as the boundary between the external and middle ears.

    • Function: Vibrates in response to incoming sound waves and transfers that energy to the auditory ossicles in the middle ear.

Detailed Structure and Openings of the Middle Ear

  • General Characteristics: An air-filled cavity located within the temporal bone, bounded by the tympanic membrane laterally and the oval and round windows medially.

  • Auditory Ossicles: Three tiny bones that form a chain across the middle ear to transmit vibrations from the tympanic membrane to the cochlea fluid.

    • Malleus (Hammer): Attached directly to the tympanic membrane.

    • Incus (Anvil): The middle bone connecting the malleus to the stapes.

    • Stapes (Stirrup): The smallest bone in the human body; its base sits in the oval window.

  • The Five Openings of the Middle Ear:

    1. Connection to External Ear: Via the tympanic membrane.

    2. Oval Window: Connects to the scala vestibuli of the inner ear. It is membrane-covered and serves as the entry point for vibrations into the fluid system.

    3. Round Window: Connects to the scala tympani of the inner ear. It is membrane-covered and acts as a pressure relief valve for fluid vibrations.

    4. Mastoid Antrum: A canal leading to the air spaces (mastoid air cells) within the mastoid process of the temporal bone.

    5. Pharyngotympanic (Auditory) Tube: Connects the middle ear to the nasopharynx.

The Pharyngotympanic Tube and Clinical Anatomy

  • Function of the Tube:

    • Pressure Equalization: Equalizes air pressure on both sides of the tympanic membrane to prevent it from rupturing.

    • Vibration Capability: Ensure pressure is equal so the membrane can vibrate freely without distortion.

    • Movement: The tube is normally closed but opens during activities like swallowing or yawning.

  • Pathology: Throat to Ear Infections:

    • Otitis Media: Inflammation/infection of the middle ear. Pathogens (e.g., from strep throat) travel from the nasopharynx up the pharyngotympanic tube into the middle ear.

    • Mastoiditis: If a middle ear infection spreads, it can move through the mastoid antrum into the mastoid air cells, leading to potentially serious infections of the temporal bone.

  • Route of Pathogens: Nasopharynx $\rightarrow$ Pharyngotympanic Tube $\rightarrow$ Middle Ear $\rightarrow$ Mastoid Antrum $\rightarrow$ Mastoid Air Cells.

Anatomy of the Inner Ear: Labyrinths and Fluids

  • Bony Labyrinth:

    • Description: A system of tortuous channels running through the temporal bone.

    • Components: Semicircular canals, Vestibule, and Cochlea.

    • Fluid: Filled with perilymph, which is chemically similar to Cerebrospinal Fluid (CSF).

  • Membranous Labyrinth:

    • Description: A series of membranous sacs and ducts contained within the bony labyrinth, mirroring its shape.

    • Fluid: Filled with endolymph, an intracellular-like fluid rich in potassium (K+K^+).

  • Inner Ear Nerves: The sensory information is carried by the Vestibulocochlear Nerve (Cranial Nerve VIII), which splits into the Vestibular Nerve (equilibrium) and the Cochlear Nerve (hearing).

The Cochlea: Architecture of Hearing

  • Physical Properties: A spiral, snail-shaped, conical bony chamber approximately the size of a split pea. It extends from the anterior portion of the vestibule.

  • Chambers of the Cochlea:

    1. Scala Vestibuli: Begins at the oval window; contains perilymph.

    2. Scala Media (Cochlear Duct): The central chamber containing endolymph and the Organ of Corti.

      • Roof: The vestibular membrane, which separates it from the scala vestibuli.

      • Floor: The fibrous basilar membrane.

    3. Scala Tympani: Ends at the round window; contains perilymph.

  • Organ of Corti (Spiral Organ):

    • Components: Hearing receptor cells known as hair cells (inner and outer) and supporting cells.

    • Stereocilia (Hairs): Protrude from the hair cells into the endolymph and are embedded in the gel-like tectorial membrane.

    • Innervation: Afferent fibers of the cochlear nerve attach to the base of the hair cells.

Physiology of Sound Perception and Transduction

  • Properties of Sound:

    • Amplitude: Represented by wave height; correlates to loudness (intensity).

    • Wavelength/Frequency: Measured in Hertz (HzHz); correlates to pitch. High frequency equals high pitch.

  • Resonance of the Basilar Membrane:

    • High-Frequency Sounds (20,000 Hz20,000\,Hz): Vibrate the basilar membrane near its base (where it is short and stiff).

    • Low-Frequency Sounds (20 Hz20\,Hz): Vibrate the basilar membrane near the apex (where it is long and floppy).

    • Out of Range Sounds: Frequencies below the human hearing range travel through the helicotrema (the apex) and do not stimulate hair cells.

  • Mechanism of Signal Transduction:

    1. Basilar membrane vibrates, causing hair cells to move.

    2. Hairs (stereocilia) bend toward the tallest stereocilium in the array.

    3. Tension in protein tethers opens ion channels.

    4. K+K^+ enters the cell from the endolymph, causing depolarization.

    5. Neurotransmitter is released, increasing the frequency of action potentials in the sensory neurons (CN VIII).

    6. Bending toward the shortest stereocilia closes channels, leading to hyperpolarization and decreased signaling.

The Pathway of Sound: Step-by-Step

  1. Sound waves arrive at the tympanic membrane.

  2. Vibration of the tympanic membrane displaces the auditory ossicles (Malleus $\rightarrow$ Incus $\rightarrow$ Stapes).

  3. The stapes acts like a piston ("STOMP STOMP"), pushing on the oval window.

  4. This establishes pressure waves in the perilymph of the scala vestibuli.

  5. Pressure waves take a "short cut" and distort the basilar membrane as they travel toward the round window (whichbulgesinresponsewhich bulges in response).

  6. The vibrating basilar membrane moves hair cells against the tectorial membrane.

  7. Information is relayed to the brain via the cochlear branch of CN VIII.

Physiology of Static Balance (Equilibrium)

  • Definition: Maintaining head/body position relative to gravity; detecting linear acceleration/deceleration (e.g., in an elevator or car).

  • Receptor Organ: The Macula, located in the walls of the Utricle and Saccule within the vestibule.

  • Anatomy of the Macula:

    • Hair Cells: Possess stereocilia embedded in the Otolithic Membrane.

    • Otolithic Membrane: A jellylike mass containing otoliths (calcium carbonate (CaCO3CaCO_3) stones).

  • Functional Differences:

    • Utricular Macula: Oriented horizontally; responds to head tilting and horizontal acceleration.

    • Saccular Macula: Oriented vertically; responds to vertical movements (e.g., gravity, elevators).

  • Mechanism: Gravity or linear movement causes the heavy otolithic membrane to slide, bending the stereocilia. This triggers depolarization/hyperpolarization, sending signals via the vestibular nerve to the brainstem and cerebellum.

Physiology of Dynamic Balance (Equilibrium)

  • Definition: Maintaining position in response to rotation (angular movement/spinning).

  • Receptor Organ: The Crista Ampullaris, located in the ampulla (swollen end) of each of the three semicircular canals.

  • Anatomy of the Crista Ampullaris:

    • Structure: Consists of hair cells and supporting cells.

    • Cupula: A gelled matrix (no otoliths) where the hair bundles are embedded.

  • Mechanism:

    1. The head rotates; the semicircular canals move with the head.

    2. The fluid (endolymph) inside the canals lags behind due to inertia.

    3. This lagging fluid pushes against the cupula, bending the stereocilia inside.

    4. Excitation: Bending during rotational acceleration excites the hair cells.

    5. Inhibition: Slowing down allows the fluid to keep moving (inertia), bending the cupula in the opposite direction.

Integration of Sensory Information

  • The Integrative Center: The Central Nervous System (specifically the brainstem/cerebellum) balances information from three distinct sources to maintain equilibrium:

    1. Vestibular Receptors: From the inner ear (maculae and cristae).

    2. Visual Receptors: From the eyes (visual cues of position).

    3. Somatic Receptors: From the body (proprioceptors in muscles and joints).

Auditory Pathway to the Brain

  • Transduction: Organ of Corti translates mechanical waves to electrical signals.

  • Transmission: Vesticulocochlear Nerve (CN VIII).

  • Processing Stages:

    1. Medulla Oblongata.

    2. Thalamus (the sensory relay station).

    3. Primary Auditory Cortex: Located in the temporal lobe, where signals are interpreted as sound.