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Structure of the Ear and hearing
Terms in this set (16)
The temporal bone shell of the inner ear is one of the hardest bones in the body. It is lined with periosteum and is filled with perilymph, a fluid closely resembling cerebral spinal fluid in its chemical composition.
Perilymph resembles in its chemical composition other extracellular fluids that are characterized by high Na+ concentration and low K+. Osmolarity of perilymph is similar to that of plasma, hence it is in osmotic equilibrium with blood. Continuous with subarachnoid space.
Is a closed system within the bony labyrinth. Like a "tube within a tube". Has a diffusion barrier of tight junctions, is filled with
. Its fluid composition is more similar to intracellular fluid composition with a high K+ and low Na+.
The amount of fluid in the system is maintained by a balance between the secretion and reabsorption. If secretion were to exceed the rate of reabsorption the pressure inside the membranous labyrinth would increase and this could cause malfunctioning of the sensory organs
Endolymphatic hydrops may result. This will be discussed in the Vertigo lecture
Three tubular canals:
1. Scala vestibuli
2. Scala Tympani
3. Scala media: contains endolymph (high K+). The scala media is bordered by the basilar membrane, which is the site of the organ corti.
Where does sound enter in the cochlea?
The Scala Vestibuli is where the sound enters. Think about when you enter a building, the first room you enter is often the vestibule. The vibrations of the fluid in this chamber is created by vibrations of the Stapes against the Oval Window. At the distal part of the cochlea there is an opening called the Helicotrema which allows the Scala Vestibuli to be continuous with the Scala Tympani. These chambers are filled with Perilymph. The Scala media is between these two chambers and is filled with Endolymph
Organ of Corti
The organ of corti is located on the basilar membrane. It contains the receptor cells (inner and outer hair cells) for auditory stimuli. Cilia protrude from the hair cells and are embedded in the tectorial membrane.
Inner hair cells: are arranged in single rows and few in number
Outer hair cells: are arranged in parallel rows and are greater in number than the inner hair cells.
The spiral ganglion: contains cell bodies of the auditory nerve (cranial nerve 8) which synapse on the hair cells.
Steps in auditory transduction by the organ of corti
1. The cell bodies of hair cells contact the basilar membrane. The cilia of hair cells are embedded in the tectorial membrane.
2. Sound waves cause vibration of the organ of corti. Because the basilar membrane is more elastic than the tectorial membrane, vibration of the basilar membrane causes the hair cells to bend by a shearing force as they push against the tectorial membrane.
3. Bending of the cilia causes changes in K+ conductance of the hair cell membrane. Bending in one diraction causes depolarization; bending in the other direction causes hyperpolarization. The oscillating potential that results is the cochlear microphone potential.
4. The oscillating potential of the hair cells causes intermittent firing of the cochlear nerves.
The up and down shaking of the basilar Membrane translates into a side to side motion of the stereocilia on top of the hair cells
Embedded in the membrane of each of these hairs are transmembrane channels. The channel on one stereocilia is connected to a channel on the neighboring stereocilia - these are called TipLinks. When a TipLink is tensed it pulls the channels open allowing ions to flow in. The ion concentrations of the various chambers thus become important because the hairs project into the Scala Media. The Scala Media region is high in Potassium similar to the ECF, however it is held at a different electrical potential. The higher electrical potential requires that the entire membranous labyrinth be intact, if there were a leak the electrical potential would change and no longer be its ideal +80 mV. It is important that the Scala Media be at this positive electrical potential because the Tip links operate on channels that are selective for Potassium. So when the tip links open the Potassium flows from its +80mV in the Scala Media through the channel towards the -40 mV area of the interior of the Hair Cell, depolarizing it and leading to increased Glutamate release at the synapse. When the tip links are relaxed the channel is closed and repolarization occurs and the glutamate concentration thus decreases. It is this alternating increase and decrease of glutamate that ultimately leads to the electrochemical signal that is transduced to the CN VIII nerve afferent.
How is sound encoded?
The frequency that activates a particular hair cell depends on the location of the hair cell along the basilar membrane.
The base of the basilar membrane: (near the oval window and round windows) is narrow and stiff. It responds best to high frequencies
The apex of the basilar membrane: (near the helicotrema) is wide and compliant. It responds best to low frequencies.
Low frequency sounds tend to excite the Basilar Membrane at the point furthest from the point of entry of the sound waves. High frequency sounds travel a relatively shorter distance and excite the Basilar Membrane proximal to the point of entry which is the Oval Window. Complex sounds such as music can contain sounds with simultaneous frequencies that are different. Several parts of the Basilar Membrane will be vibrating at the same time
Tonotrophic Organization refers to the fact that each area of the Basiliar Membrane is preferentially excited by sounds of different frequencies.
This tonotopic organization is maintained throughout the Primary Auditory Cortex. On the transverse temporal gyrus (primary auditory) the low frequencies tend be more lateral and the high frequency sounds more medial.
The Sound Pressure Level Scale
Expressed in decibels. Known as 'The Bel" named after Alexander Graham Bell
20 dB of sound pressure is equal to:
10 dB sound energy
1 bel sound energy
10 times the sound energy relative to hearing threshold
140 dB sound pressure = 10,000,000 times the sound energy relative to hearing threshold. This is the threshold of pain
Sensi-neural hearing loss at the receptor level can occur when the stereocilia begin to become damaged. Physical breakdown of stereocilia tufts on hair cells. Prolonged exposure to 90+ dB SPL. Cochlear implant may be necessary over time
Central auditory pathways
Fibers ascend through the lateral lemniscus to the inferior colliculus to the medial geniculate nucleus of the thalamus to the auditory cortex. Fibers may be crossed or uncrossed. As a result, a mixture of ascending auditory fibers represents both ears at all higher levels. Therefore, lesions of the cochlea of one ear cause unilateral deafness, but more central unilateral lesions do not.
There is tonotropic organization of frequencies at all levels of the central auditory pathway. Discrimination of complex features (recognizing a patterned sequence) is a property of the cerebral cortex.
In general Wernicke's and Broca's Areas are in the Left Brain. The left brain is perceived as being a cold, calculating literal part of the system, dealing with math, calculations and the face value meaning of words. The right brain is perceived as being more artistic and can determine variations in meaning based upon the musical qualities of sound - the tonal properties. For example a person can perceive different meanings to the phrase "I had a great time in lecture today" based upon the tone of voice. This is called Sensory Aprosodia and it is when a patient has difficulty understanding meaning conveyed by a tone of voice - they cannot detect sarcasm, etc.
Motor Aprosodia is when a patient has difficulty conveying meaning through tone of voice
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