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Auditory System.

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Presentation on theme: "Auditory System."— Presentation transcript:

1 Auditory System

2 The Nature of Sound Sound is generated by mechanical vibrations that generate pressure waves in the air, or some other medium, such as water. Sound is characterized by its frequency, which is perceived as a high or low tone or pitch, its intensity or amplitude, which we perceive as loudness. The unit for the frequency of sound is cycles per second (abbreviation: cps) or Hertz (abbreviation: Hz). Humans are sensitive to sound frequencies ranging from about 20 to about 20,000 Hz. The unit for sound intensities is decibel

3 Functional Anatomy of the Ear
Structures of the Outer, Middle and Inner Ear First you should review the structure of the human ear. The outer ear, filled with air, focuses sound into the external auditory meatus. Pressure waves generated by sound produce vibrations of the tympanic membrane, which separates the (air filled) external auditory meatus from the (air filled) middle ear. The tympanic membrane separates the outer ear from the inner ear, which is also filled with air, and connected with the nasopharynx through the Eustachian (pharyngotympanic) tube. It contains a chain of ossicles (malleus, incus and stapes). After amplification of the signal through the chain of ossicles, the vibration of the stapes, the last ossicle in the chain, produces vibratory waves inner ear. The stapes sits on the oval window of the inner ear, which is filled with perilymph and endolymph, located inside a bony compartment, the cochlea. The signal transduction takes place in the organ of Corti of the cochlea, which contains inner and outer hair cells. We will discuss the auditory transduction process in the following lecture. The cochlea is the site of an additional amplification mechanism. Hair cells transmit the signal on the afferent fibers of the neurons of the spiral ganglion, which form the axons of the auditory nerve (CN VIII), the first element of the auditory pathways.

4 Middle Ear Amplification: Increase of Pressure
Mechanical transformation of the sound signal (pressure waves) in the middle ear leads to a fold amplification of the pressure force. Two mechanisms contribute to this pressure amplification. Pressure = Surface area Force

5 Middle Ear Amplification: Mechanism 1
Tympanic membrane: large area Oval window: small area Picture detail from: Siegel and Sapru, Essential Neuroscience, 1st edition, © Lippincott, Williams & Wilkins (2006) Size Difference Due to the small size of the oval window, compared to the size of the tympanic membrane, which is 20 times larger, the force at the oval window becomes about 20 times greater than at the tympanic membrane. 5

6 Middle Ear Amplification: Mechanism 2
Tympanic membrane: little force Oval window: big force Picture detail from: Siegel and Sapru, Essential Neuroscience, 1st edition, © Lippincott, Williams & Wilkins (2006) Lever Ratio of the Ossicular Chain The force at the oval window is further increased because the ossicles act like levers of a mechanical scale. Large movements (with little force) of the tympanic membrane are transformed into little movements (with greater force) at the oval window 6

7 Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles.
Hyperacusis (also spelled hyperacousis) is a condition characterized by an over-sensitivity to certain frequency ranges of sound (a collapsed tolerance to normal environmental sound). A person with severe hyperacusis has difficulty tolerating everyday sounds, some of which may seem unpleasantly loud to that person but not to others.

8 Anatomy Summary: The Cochlea

9 Fluid Compartments in the Inner Ear
Perilymph in vestibular and tympanic duct Similar to plasma Endolymph in cochlear duct Secreted by epithelial cells, Similar to intracellular fluid The cochlea of the inner ear consists of three fluid compartments, the scala vestibuli, the scala media and the scala tympani. Scala vestibuli and scala tympani are continuous through the helicotrema at the apex of the cochlea. They are filled with perilymph, which has a similar ionic composition to the extracellular fluid. The scala media is filled with endolymph, which is high in potassium ions, due to an active secretion process of the stria vascularis, which forms the lateral wall of the scala media.

10 Sensory Coding for Pitch
Near the oval window, at its base, the basilar membrane is narrow and stiff, and therefore is activated most effectively deflected by high frequencies. At the tip (apex) of the cochlea (helicotrema), the basilar membrane is wide and floppy, and therefore most effectively deflected by low frequencies.

11 Anatomy Summary: The Cochlea
The basilar membrane separates the scala media and the scala tympani and supports the organ of Corti, which contains the auditory receptor cells, the hair cells. In this anatomical arrangement, which forms one of the prerequisites of signal transduction in the inner ear, the transduction site of the hair cells is bathed in the endolymph compartment (scala media), whereas the base of the hair cells is oriented towards the perilymph compartment (scala tympani).

12 Auditory Signal Transduction by Inner Hair Cells
The first step in the sound transduction process is the deflection of hair cell stereocilia. This is caused by traveling waves through the cochlea, which deflect the basilar membrane relative to the tectorial membrane.

13 Hair Cells are the Transducers of the Inner Ear
The apex of a hair cell is the signal transduction site which contains cilia (mostly stereocilia). This area is surrounded by endolymph, which has a higher concentration of potassium ions, compared to the usual extracellular fluid (or the perilymph of the inner ear), is essential for the signal transduction process. At the base of a hair cell we find a synaptic terminal, with vesicles containing excitatory transmitter. Synaptic transmission between hair cells stimulates the afferent fibers of the vestibular and cochlear portions of the vestibulocochlear nerve (CN VIII).

14 Depolarization of Hair Cells increases Intracellular Calcium
Opening of mechanically gated Potassium Channels causes Depolarization of the Hair Cells Mechanical force produced by a “tip link” between neighboring stereocilia directly opens the cation channels during deflection of the cilia towards the tallest cilium(kinocilium). The following inward current of potassium ions depolarizes the hair cells. Depolarization of Hair Cells increases Intracellular Calcium and induces Transmitter Release

15 The Primary Auditory Cortex (A1)
The primary auditory cortex is localized in areas 41 and 42, according to Brodmann’s classification. Anatomically, these areas comprise the transverse temporal gyrus (gyri) of Heschl on the superior surface of the temporal lobe. The Essence of the Auditory Pathways the auditory pathways are characterized by extensive crossing fiber connections at each level of the auditory system. For this reason, except for lesions affecting the structures of the ear, the eighth nerve, or cochlear nuclei, there are no lesions that produce unilateral hearing loss.

16 Neurological Examination of Auditory Function: Weber’s and
Rinne’s Tuning Fork Tests In case the basic tests indicate that hearing might be diminished, tuning fork tests can be performed to localize the side of the lesion (left or right), and to classify the type of the lesion (sensorineural hearing loss or conductive hearing loss).

17 Chief Complaint: Unilateral hearing loss
History: A 53 year old left handed university professor realized that he has been changing the hand in which he holds his telephone receiver to hear more clearly. This has been going on for the last 3 months. Recently he also developed headache and dizziness. Since yesterday, he has visual difficulties, which he cannot explain very clearly, and is “not able to walk like he used to.” General Examination: Normal vital signs. Mildly obese, otherwise unremarkable. Neurological examination: Ocular movements were full in all directions of primary gaze. A pathological nystagmus was seen during the H-Test. The patient could distinguish between known standard odors. Snellen chart testing reveals 20/20 vision bilaterally. On the left side of the face, the nasolabial fold was flattened and the angle of the mouth drooped downwards. Wrinkling of the forehead was also diminished on the left side. Corneal reflex on the left side was significantly reduced. Weber test lateralized to the right, and the Rinne test showed the following: Right: AC > BC; Left: AC> BC (total times for both AC and BC much less than on the right.) Gag reflex, trapezius / sternocleidomastoid motor strength, and tongue protrusion were normal.


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