Anatomy and Function of the Ear

Characteristics of Sound Waves

• Frequency determines the pitch of a sound, measured in Hertz (Hz), typically ranging from 16-20,000 Hz for human hearing.
• Amplitude determines the loudness of a sound, measured in decibels (dB), ranging from 0-120 dB.

Peripheral Organ of the Hearing System

• Outer ear transmits sound waves to the middle ear
• Middle ear amplifies sound vibrations.
• Inner ear senses the amplified vibrations and converts them into electrical signals.
• Ng et al. is a reference to research or a publication

Functions of the Outer Ear

• Collects sound waves from the environment.
• Transmits sound waves to the middle ear.
• Provides a defense function for the ear.
• Regulates humidity and temperature of the tympanic membrane.

Swimmer's Ear (Otitis Externa)

• Swimmer's ear is an inflammation or infection of the external auditory canal.
• Also known as otitis externa.

Middle Ear Functions

• Transmits sound from the outer to the inner ear.
• Amplifies sound through:
  • Differences in tympanic and oval window membrane size.
  • Lever system of ossicles.
• Provides a defence function for the ear.
• The Eustachian tube drains fluid and equilibrates air pressure in the middle ear.

Additional Notes on Ossicles and Middle Ear Muscles

• Ossicles work as levers, providing a mechanical advantage of approximately ×1.3.
• The tympanic membrane area is greater than the footplate of the stapes by a factor of ×17.
• Muscles of ossicles partly protect from excessive vibration by loud sound, but the reflex time is too long

Inner Ear Function

• Receives sound waves (cochlea).
• Responsible for the reception of equilibrium (semi-circular canals and maculae).

Cochlea Details

• Bony labyrinth is filled with liquid.
• Contains the organ of Corti
• Responsible for secondary reception of sound.
• SV is the scala vestibuli; SM is the scala media; RM is the Reissner's membrane; and ST is the scala tympani

Components and Function of the Organ of Corti

• Inner hair cells are responsible for sound reception.
• Outer hair cells contribute to the modification of sound.
• Afferent fibers constitute 95% of the cochlear nerve and project to inner hair cells.
• Efferent fibers make up 5% of the cochlear nerve and project to outer hair cells.

Electrical and Chemical Gradients in the Cochlea

• A higher concentration of K+ ions (150 mmol/l) exists in the endolymph (scala media) than in the cells (130 mmol/l).
• The endolymph potential is +85 mV relative to perilymph (scala vestibuli).
• The intracellular potential of the outer hair cells is -70 mV, so the total difference is 155 mV.
• These two factors force the K+ ions to move into the cell.

Mechanism of Activation of Hearing Receptors

• Bending of hairs of receptor cells opens mechanically-gated K+ channels.
• K+ influx leads to depolarization.

Types of Hearing Neurons

• Type I neurons:
  • Build up a cochlear nerve
  • 88% of ganglionic cells
  • Bipolar and myelinated cells
  • Innervates just inner hair cells (exists even one-one relationships)
• Type II neurons:
  • 12% of ganglionic cells
  • Unipolar and unmyelinated cells
  • Innervate just outer hair cells (one neuron – many hair cells)
  • Have reciprocal connectivity (afferent and efferent role).

Outer Hair Cells

• The lateral wall contains motor protein prestin, which shortens the cell during depolarization and increases the bending of the basilar membrane.
• Inner hair cells do not contain prestin.

Communication in Hair Cells

• Communication takes place between Inner and outer hair cells.
• Accomplished via afferent and efferent fibers.

Comparison of Hair Cells

Equal Loudness Contour

• Isophones are lines on a graph or map that connect points of equal loudness levels across different frequencies.
• Illustrates how the perceived loudness of a sound changes with its frequency.
• Sensitivity to frequencies is between 1,000 Hz and 5,000 Hz.
• Higher and lower frequencies require higher sound pressure levels to be perceived as equally loud.
• Phon Scale:
  • Is a unit of measurement for perceived loudness.
  • One phon equals the loudness of a 1,000 Hz tone at a given dB SPL.

Rinne Test

• Designed to test air conduction versus bone conduction.
• Equipment: A tuning fork.
• Instructions: Ensure the test person understands what they need to hear and how the test is conducted.

Weber's Test

• Tests both ears to determine if hearing loss is present in the right or left ear.
• The test person must indicate in which ear they perceive the sound longer and better or if they perceive it at all in cases of total hearing loss in one or both ears.
• If the best sound is perceived in the middle, the hearing sensation is equal in both ears.

Audiometry Exam

• Tests ability to hear sounds

Audiogram

• Shows a graph of the softest sounds a person can hear at different pitches and frequencies
• Thresholds of normal hearing typically are 25 dB HL or better (lower).
• An audiometric notch indicates hearing loss at 3 to 6 kHz compared with higher and lower frequencies.

Presbyacusis

• Age-related hearing loss is in high frequencies.

Sound Coding

• Volley theory: AP frequency codes sound frequency to 3 kHz.
• Tonotopic organization: cochlear location codes sound frequency after the labeled line principle.

Encoding of Frequency

• Each nerve fiber responds to the sound of a certain frequency characteristic frequency.
• Sound elicits a phase-locked response at a low frequency.
• At high frequencies, the response cannot appear on every wave and does not have a fixed phase - relationship.

Intensity and Direction Encoding

• The intensity of a sound is encoded
• Actions increase:
  • More action potentials are generated in the nerve fiber
  • More nerve fibers are excited at the same hair cell
  • More hair cells are activated and more nerve fibers are excited
• Direction is encoded:
• Time difference (for lower frequencies with phase locked response < 2 kHz)
• Pressure difference (for higher frequencies > 2 kHz)

Sound Waves and the Basilar Membrane

• A sound evokes a traveling wave along the basilar membrane

Physical Characteristics of the Basilar Membrane

• The base of the basilar membrane is narrow and stiff.
• The apex is wide and floppy.

Tinnitus

• A ringing in the ears, but the cause is not fully understood
• Tinnitus is linked with some variety of causes:
  • Damage to cilia in the inner ear
  • Injuries or trauma
  • Earwax blockage
  • Ear infections
  • Medications

Acoustic Neuroma

• Condition involves cancer that causes physical compression of nerves.

Meniere's Disease

• A disorder is caused by fluid build-up in chambers in the inner ear.

Vestibulocochlear Nerve (VIII n.)

• Connects the CNS in the pontomedullary junction.

Auditory Pathways

• Start from the pons and medulla oblongata region and go till the superior temporal cortex (A1).

Complex of Cochlear Nuclei

• Rostral medulla oblongata ipsilateral via VIII cranial nerve switches to 2nd order neurons

Nucleus of Superior Olive

• Contains the in mid-pons hearing information where it divides ipsilateral and contralateral.

Binaural Hearing

• Interaural time difference (ITD) and intensity difference (ILD).

Sound Localization

• Azimuth is the direction right to left.
• Elevation is the direction up to down.
• Distance is how far the sound source is.

Interaural Intensity Difference (ILD)

• Opposite sides of the lateral superior olive (LSO) are modulated (inhibited).
• Efferent innervation to outer hair cells.

Head Related Transfer Function (HRTF)

• Heads and ears form influences sound wave traveling till outer ear.
• "First notch" frequencies changes with elevation.

Nucleus of Lateral Lemniscus

• The nucleus has a monaural information pathway from the contralateral ear.

Inferior Colliculus

• The integral centre is in the midbrain
• it is involved in binaural and monaural integration

The Inferior Colliculi

• "Calculates" topographic map of sounds location in space.
• Adjusts to visual map.
• Auditory oriental reflex.

Tonotopic Organisation

• Frequency coding (to species specific sounds).
• Coding of loudness.

Auditory Thalamus

• Relays nuclei to cortex from:
  • Inferior colliculi;
  • Brain stem nuclei.
• Functions:
  • Generates response to combining sound frequencies (harmonies);
  • Generates response to time differences between different frequencies;
  • Detects spectral and time differences in monaural sounds (needed for speech recognition).

Auditory Cortex

• Core area,
• Belt area,
• Parabelt area.

Primary Auditory Cortex (A1)

• Contains a tonotopic map of frequencies.
• Has an orthogonal distribution of neurons processing binaural signals:
  • EE neurons: process signals from both ears.
  • El neurons: selectively process signals from one ear.

Planum Temporale

• The belt areas of the auditory cortex
• Wernicke's area divides into two functional parts:
  • "Word recognition area" is a part of the ventral stream (word semantic meaning).
  • "Internal speech area" is a part of the dorsal stream (word phonetic meaning).

Ventral Stream

• Involved in auditory-object processing ("Which kind of sound"?)
• Meaning of sounds

Dorsal Stream

• Analyses space and motion ("From where and how"?)
• Is involved in audiomotor processing.

Equilibrium System

• Structures of inner ear:
  • Semi-circular chanals;
  • Vestibule (utriculus, sacculus).
• Functions - integrate other sensory modalities:
  • Concentration of eyesight at time of movement;
  • Position and balance (muscles of neck and extremities) at time of self-movement and movement of surrounding;
  • Complex and conscious movement (navigation, reaction).

Semicircular Channels

• Detects angular acceleration in 3D (head's rotation).

Vestibule (Ottolite Organ)

• Detects linear acceleration in 3D (body's movement in space according the effect of gravity or other force).

Distribution of Receptors (Utricle and Saccule)

• Gravity pulls the otoliths, and the cilia bend (gravity receptors)
• Otoliths (statoconia): heavy crystals of CaCO3

Bending of the Cupula in the Semicircular Duct

• At the beginning of head rotation, the endolymph lags behind (due to inertia) and bends the cilia in the opposite direction.
• When the head stops, the endolymph continues moving and bends the cilia in the same direction (not shown).
• Note: no otoliths here

Vestibular Hair Cell

• Kinocilium – largest stereocilium (not present in the cochlea)
• Stereocilia – hairs of hair cells

Directional Sensitivity

• Depolarization and Hyperpolarization in reaction to direction stimuli

Type I and II Hair Cells

• Type I hair cells:
  • Characteristically in amniotes (amphibians, reptiles and mammals)
  • Calyx type connection (synapse) with neuron.
  • Has a large amount of stereocilia
  • Irregular discharge causes irregular activation on afferent neuron
• Type II hair cells:
  • In all vertebrates.
  • Regular synaptic connection with vestibular neurons.
  • Smaller amount of stereocilia compared with type I hair cells.
  • Regular discharge and generation of action potentials in neurons.

Types of Vestibular Afferents

• B type cells - regular afferent’s bouton ending contacting a type II hair cell
• C type cells - irregular afferent’s calyx ending around a type I hair cell
• D type cells - irregular afferent contacting both types of hair cells

Processing of Sensor Input

• Information provided is processed and used for various functions, with sensor input going to Vestibular Nuclei

• estimation of self-motion
  

• gaze stabilization (VOR)
• posture and balance (VSR)

Vestibular Nerve

• Part of VIII n

Vestibular Nuclei

• Four bilaterally located nuclei exist in the level of medulla and pons
• These are known as lateral and superior

Oculomotor Reactions

• Rotation of the head is sensed by the semi-circular canals.
• These reactions regulate eyesight fixation during movements.
• Regulation is executed by the superior and medial vestibular nuclei.
• Involves cranial nerves:
  • Motor nucleus of N. abducens
  • Motor nucleus of N. trochlearis
  • Motor nucleus of N. oculomotorius

Estimation of Self-Motion

• The relay nuclei in the thalamus projects to the somatosensory cortex and posterior parietal cortex.
• It Exists as higher integration of vestibular information in the cortex.