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15. The Special Senses: Part C. The Ear: Hearing and Balance. The three parts of the ear are the inner , outer , and middle ear The outer and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium Receptors for hearing and balance :

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The Special Senses: Part C

the ear hearing and balance
The Ear: Hearing and Balance

The three parts of the ear are the inner, outer, and middle ear

The outer and middle ear are involved with hearing

The inner ear functions in both hearing and equilibrium

Receptors for hearing and balance:

Respond to separate stimuli

Are activated independently

the cochlea
The Cochlea

The cochlea is divided into three chambers:

Scala vestibuli

Scala media

Scala tympani

the cochlea1
The Cochlea

The scala tympani terminates at the round window

The scalas tympani and vestibuli:

Are filled with perilymph

Are continuous with each other via the helicotrema*

The scala media is filled with endolymph

The helicotrema is the part of the cochlear labyrinth where the scala typmani and the scala vestibuli meet.


properties of sound
Properties of Sound

Sound depends on elastic medium for its transition (can not be transmitted in vacuum).

Sound is:

A pressure disturbance (alternating areas of high and low pressure) originating from a vibrating object

Composed of areas of rarefaction (less molecules) and compression (more /compressed molecules)

Represented by a sine wave in wavelength, frequency, and amplitude

properties of sound1
Properties of Sound

Frequency – the number of waves that pass a given point in a given time

Wavelength – the distance between 2 consecutive crests; it is constant for a particular tone

Pitch – perception of different frequencies (we hear from 20–20,000 Hz)

The higher the frequency – the higher the pitch

Amplitude – Height of the wave - loudness

Loudness – subjective interpretation of sound intensity

transmission of sound to the inner ear
Transmission of Sound to the Inner Ear

The route of sound to the inner ear follows this pathway:

Outer ear – pinna, auditory canal, eardrum

Middle ear – malleus, incus, and stapes to the oval window

Inner ear – scalas vestibuli and tympani to the cochlear duct

Stimulation of the organ of Corti

Generation of impulses in the cochlear nerve

transmission of sound to the inner ear1
Transmission of Sound to the Inner Ear

Figure 15.31

resonance of the basilar membrane
Resonance of the Basilar Membrane

As the stapes rocks back and forth against the oval window, it moves the perilymph in the scala vestibuli into a similar back-and-forth motion

A pressure wave travels through the perilymph from the basal end toward the helicotrema.

Sounds of very low frequency (below 20 Hz) create pressure waves that take the complete route through the cochlea toward the round window through the scala tympani.

Such sounds do not activate the spiral organ (are below the threshold of hearing).

resonance of the basilar membrane1
Resonance of the Basilar Membrane

Sound waves of low frequency (inaudible):

Travel around the helicotrema

Do not excite hair cells

Audible sound waves:

Penetrate through the cochlear duct

Vibrate the basilar membrane

Excite specific hair cells according to frequency of the sound

the organ of corti
The Organ of Corti

Is composed of supporting cells and outer and inner hair cells

The hair cells are arranged in one row of inner hair cells and three rows of outer hair cells - sandwiched between the tectorial and basilar membranes.

Afferent fibers of the cochlear nerve are in contact with the bases of the hair cells.

The hair cells have numerous stereocilia (actually long microvilli) and a single kinocilium (a true cilium) project from their apices.

The “hairs” (stereocilia) of the hair cells are stiffened by actin filaments and linked together by fine fibers called tip-links

They project into the K+-rich endolymph, and the longest of them are embedded in the overlying tectorial membrane

excitation of hair cells in the organ of corti
Excitation of Hair Cells in the Organ of Corti

Transduction of sound stimuli occurs after the stereocilia of the hair cells are turn aside by movements of the basilar membrane.

Bending the cilia toward the kinocilium puts tension on the tip-links, which in turn opens cation channels in the adjacent shorter stereocilia.

This results in an inward K+ (and Ca2+) current and a graded depolarization

Depolarization increases intracellular Ca2+ and so increases the hair cells’ release of neurotransmitter (glutamate), which causes the afferent cochlear fibers to transmit a faster stream of impulses to the brain for auditory interpretation.

Bending the cilia away from the kinocilium relaxes the tip-links, closes the mechanically gated ion channels, and allows repolarization and even a graded hyperpolarization.


excitation of hair cells in the organ of corti1
Excitation of Hair Cells in the Organ of Corti

The outer hair cells send little information to the brain. Instead, they act on the basilar membrane itself.

Most (90-95%) nerve fibers around the OHC are efferent (from the brain to the ear)

In response to sound, the OHC send signals to the medulla and the pons sends immediately signals back

In response, The OHC contract by about 15% of their height

Because the OHC are attached to the basiliar membrane and the tectorial membrane, contraction decrease the ability of the basiliar membrane to vibrate.

As a result, some areas of the duct send less signals to the brain which allow the brain to distinguish between more and less active hair cell.

Give a more precise perception of different pitches

mechanisms of equilibrium and orientation
Mechanisms of Equilibrium and Orientation

Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibule

Maintains our orientation and balance in space

The position of the body with respect to gravity (static equilibrium) – the vestibule

The motion of the body (dynamic equilibrium) – the semicircular canals

static equilibrium
Static equilibrium
  • The receptors for static equilibrium are the maculae – one in the urticle and one in the saccule
  • The utricle is sensitive to a change in horizontal movement,
  • The saccule gives information about vertical movement

anatomy of maculae
Anatomy of Maculae

Maculae are the sensory receptors for static equilibrium

Contain supporting cells and hair cells

Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane

Otolithic membrane – jellylike mass covered with tiny CaCO3 stones called otoliths

Utricular hairs respond to horizontal movement

Saccular hairs respond to vertical movement

effect of gravity on receptor cells
Effect of Gravity on Receptor Cells

When the head starts or stops moving in a linear direction, the otolithic membrane slides backward or forward like a plate over the hair cells, bending the hairs.

The hair cells release neurotransmitter continuously but movement of their hairs modifies the amount they release.

When the hairs are bent toward the kinocilium, the hair cells depolarize, increasing their pace of neurotransmitter release, and a faster stream of impulses travels up the vestibular nerve to the brain

When the hairs are bent in the opposite direction, the receptors hyperpolarize, and neurotransmitter release and impulse generation decline.

In either case, the brain is informed of the changing position of the head in space.

crista ampullaris and dynamic equilibrium
Crista Ampullaris and Dynamic Equilibrium

The crista ampullaris (or crista):

Is the receptor for dynamic equilibrium

Is located in the ampulla of each semicircular canal

Responds to angular movements

Each crista has support cells and hair cells that extend into a gel-like mass called the cupula

Dendrites of vestibular nerve fibers encircle the base of the hair cells

activating crista ampullaris receptors
Activating Crista Ampullaris Receptors

The cristae respond to changes in the velocity of rotation movements of the head.

the endolymph in the semicircular ducts moves briefly in the direction opposite the body’s rotation, deforming the crista in the duct.

As the hairs are bent, the hair cells depolarize and impulses reach the brain at a faster rate.

Bending the cilia in the opposite direction causes hyperpolarization and reduces impulse generation.

Because the axes of the hair cells in the complementary semicircular ducts are opposite, rotation in a given direction causes depolarization of the receptors in one ampulla of the pair, and hyperpolarization of the receptors in the other

rotary head movement
Rotary Head Movement

Figure 15.37d

equilibrium pathway to the brain
Equilibrium Pathway to the Brain
  • Pathways are complex and poorly traced
  • Impulses travel to the vestibular nuclei in the brain stem or the cerebellum, both of which receive other input
  • Three modes of input for balance and orientation
    • Vestibular receptors
    • Visual receptors
    • Somatic receptors