Transcript Hearing part III

‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
• Two types of cochlear potentials can be recorded in
the cochlea;
• a) Endocochlear potential:
• b) Cochlear microphonic potential:
• A resting electrical potential of about +80 mV ( ) endolymph in
scala media and perilymph in scala vestibuli and scala tympani
• It is due to difference in chemical composition ( ) endolymph and
perilymph
• It is maintained by a K+ pump (in stria vascularis) that transports
K+ from the perilymph to the endolymph
• Tight junctions between hair cells and the adjacent supporting
cells which prevent the endolymph to reach the bases of hair
cells.
• So, hair cells have –ve intracellular potential of -60 mV with
respect to the perilymph, but -140 mV with respect to the
endolymph, [-60-(+80)] = [-140] mV surrounding its upper
surfaces.
• This high electrical potential at the tips of the stereocilia greatly
sensitizes the cell, thereby increasing its ability to respond to
the slightest sound stimulus
+ 80 mv
Tight Junctions
- 140 mv
Endolymph
Hair
cells
Hair
cells
- 60 mv
Basilar
Membrane
Perilymph
Organ of Corti
+ 0 mv
• These are receptor potentials that can be recorded from
most parts of the cochlea when the ear is exposed to
sound.
• They are recorded from an electrode placed at or near the
round window.
• They represent the sum of potentials generated by a large
population of hair cells mainly that produced by lateral hair
cells.
• So, its detection in surface recordings has been
considered a distinctive sign of outer hair cell integrity.
• Its magnitude is dependent on the proximity of the
recording electrodes to the hair cells and proportional with
the intensity of sound, hence the degree of displacement
of basilar membrane.
• Cochlear microphonic potential is maintained so long the
basilar membrane is vibrating and it corresponds closely to
the sound stimulus regarding: the frequency, the wave
form and the amplitude
• They have the same characteristics of receptor potentials
i.e. can summate, proportional with the intensity of sound
(graded), and not obey all or none rule.
• The auditor pathway from the cochlea to the cerebral
cortex consists of at least 4 neurons and may increase
to 6 neurons
• formed of 1ry and 2ry areas;
A) Primary auditory area
(areas 41 & 42):
• located in the upper part of
temporal lobe
B) Secondary (associated)
auditory area (area 22):
• Surrounds the primary area
and covers the insular
cortex
• Receive auditory impulses from both ears from
ipsilateral MGB.
• Anterolateral part receives impulses from the apex of
the cochlea (low pitched sounds)
• Posteromedial part receives impulses from the base
of the cochlea (high pitched sounds)
Functions:
1. Conscious perception of pitch, amplitude, and
sound pattern without understanding its meaning.
2. Perception of the source of the sound.
Lesion:
1. Bilateral damage of the primary auditory areas
greatly reduces the person's capacity for hearing
2. Unilateral damage slightly reduces hearing of the
opposite ear ?
• It receives impulses from the primary auditory area.
Functions:
• Its function is to associate sound information with
afferent information from other sensory areas of the
cortex for interpreting and understanding the meaning
of sounds.
Lesion:
• The person will be unable to interpret the meaning of
the heard sound (auditory aphasia or word
deafness).
1. Signals from one ear are transmitted to both sides of the
brain.
2. At least 3 crossing-over occur between the right and left
pathways in the brain stem:
3. Many fibers from the auditory tracts pass directly into the
RAS of brain stem which projects upward to cerebral cortex
and downward to spinal cord → activates the entire nervous
system in response to a loud sound.
4. Some fibers also go to the vermis of the cerebellum which is
also activated in response to sudden noise.
5. Inferior colliculi represent the center of spinal reflexes of
hearing.
6. A high degree of spatial arrangement is maintained in the
fiber tracts from the cochlea all the way to the cortex.
• Human ear can recognize frequencies from 20-20.000 Hz with
maximum sensitivity of ear occur at frequencies from 1,000 to
4,000 HZ.
• Discrimination of sound pitch can be explained by 2 theories;
• a) The place principle or theory:
• b) The frequency principle or theory:
• It is the most accepted theory.
• Basilar membrane fibers are short thick stiff fibers at the base
→ maximally activated by the high frequency sounds and long
thin lax fibers at the apex → maximally activated by low
frequency sounds
• So each frequency causes vibration of its own particular
"place" on the basilar membrane.
• So, the basilar membrane serves as a frequency analyzer and
have tonotopic map
• There is a 2nd tonotopic map in cochlear nuclei
• However, the place principal cannot explain discrimination of
frequencies -from 200-20 HZ occurring at the apex of the
cochlea.
• It was the first theory suggested to explain frequency
discrimination.
• It postulates that, for low frequency sounds, the basilar
membrane vibrates in the same frequency and the auditory
nerve fibers can fire at the same frequency of the sound.
• While at high frequency sounds the nerve cannot discharge at
the same rate due to the absolute refractory period (the
nerve fibers cannot transmit impulses at rate greater than 1000
impulses/sec).
• It is determined by the number of impulses discharged from the
hair cells to the auditory cortex.
• This occurs by means of 3 different ways:
• 1. Temporal summation: ↑ sound intensity →↑ movement the
basilar membrane →↑ firing of hair cells
• 2. Spatial summation: ↑ sound intensity →↑ movement the
basilar membrane →↑ number of hair cells stimulated →↑
neurons stimulated in auditory cortex.
• 3. Certain hair cells do not become stimulated until the
vibration of the basilar membrane reaches a relatively high
intensity (in very loud sounds).
• It depends on the binaural hearing.
• Superior olivary nuclei and auditory cortex have a role
• Sound localization depends on:
1. Time-lag ( ) entries of sound in both ears.
2. Difference ( ) sound intensities in the two ears.
• Ears are roughly 20 cm apart and the speed of sound is
342 m/sec, so the time delay between the arrival of sound
wave to one ear and the opposite side is about 0.06 msec.
• SON divided into 2 parts:
1. Medial superior olivary nucleus →detect time-lag.
2. Lateral superior olivary nucleus → detect intensity
differences
• The folds and bulges of the ear pinna produce different
reflections of sounds based on their angle of entry along
the vertical plane.
• Role of auditory cortex is proved to be important
because destruction of auditory cortex on both
sides of the brain causes loss of the ability to detect
the direction from which the sound comes
THANKS