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 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 is divided into three chambers:
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.
Sound depends on elastic medium for its transition (can not be transmitted in vacuum).
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
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
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
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).
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
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
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.
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
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
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
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.
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
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