Presentation on theme: "Auditory System. The Nature of Sound Sound is generated by mechanical vibrations that generate pressure waves in the air, or some other medium, such as."— Presentation transcript:
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
Functional Anatomy of the Ear
Middle Ear Amplification: Increase of Pressure Pressure = Surface area Force 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.
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.
Anatomy Summary: The Cochlea
Perilymph in vestibular and tympanic duct Similar to plasma Endolymph in cochlear duct Secreted by epithelial cells, Similar to intracellular fluid Fluid Compartments in the Inner Ear
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.
Anatomy Summary: The Cochlea
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.
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).
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
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 Primary Auditory Cortex (A1) 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.
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).
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.