2 The Nature of SoundSound is generated by mechanical vibrations that generate pressure waves in the air, or some other medium, such as water.Sound is characterized byits frequency, which is perceived as a high or low tone or pitch, its intensity oramplitude, which we perceive as loudness.The unit for the frequency of sound is cycles per second (abbreviation: cps) orHertz (abbreviation: Hz).Humans are sensitive to sound frequencies ranging from about 20 to about 20,000 Hz.The unit for soundintensities is decibel
3 Functional Anatomy of the Ear Structures of the Outer, Middle and Inner EarFirst you should review the structure of the human ear.The outer ear, filled withair, focuses sound into theexternal auditory meatus.Pressure waves generatedby sound producevibrations of the tympanicmembrane, whichseparates the (air filled)external auditory meatusfrom the (air filled) middleear.The tympanic membraneseparates the outer earfrom the inner ear, which is also filled with air, and connected with thenasopharynx through the Eustachian (pharyngotympanic) tube. It contains achain of ossicles (malleus, incus and stapes).After amplification of the signal through the chain of ossicles, the vibration of thestapes, the last ossicle in the chain, produces vibratory waves inner ear. Thestapes sits on the oval window of the inner ear, which is filled with perilymphand endolymph, located inside a bony compartment, the cochlea.The signal transduction takes place in the organ of Corti of the cochlea, whichcontains inner and outer hair cells. We will discuss the auditory transductionprocess in the following lecture. The cochlea is the site of an additionalamplification mechanism.Hair cells transmit the signal on the afferent fibers of the neurons of the spiralganglion, which form the axons of the auditory nerve (CN VIII), the first elementof 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 areaForce
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.
9 Fluid Compartments in the Inner Ear Perilymph in vestibular and tympanic duct Similar to plasmaEndolymph in cochlear duct Secreted by epithelial cells,Similar to intracellular fluidThe cochlea of the inner earconsists of three fluidcompartments, the scalavestibuli, the scala mediaand the scala tympani.Scala vestibuli and scalatympani are continuousthrough the helicotrema atthe apex of the cochlea.They are filled withperilymph, which has asimilar ionic composition tothe extracellular fluid.The scala media is filled with endolymph, which is high in potassium ions, due toan active secretion process of the stria vascularis, which forms the lateral wall ofthe 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 andsupports the organ of Corti, which contains the auditory receptor cells, the haircells. In this anatomical arrangement, which forms one of the prerequisites ofsignal transduction in the inner ear, the transduction site of the hair cells isbathed in the endolymph compartment (scala media), whereas the base of thehair 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 thesignal transduction sitewhich contains cilia (mostlystereocilia).This area is surrounded by endolymph,which has a higher concentration of potassium ions, compared to the usual extracellular fluid (or theperilymph of the inner ear), is essential for the signal transduction process.At the base of a hair cellwe find a synaptic terminal, with vesiclescontaining excitatory transmitter. Synaptictransmission 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 CellsMechanical force producedby a “tip link” between neighboring stereociliadirectly opens the cation channels during deflection of the cilia towards thetallest cilium(kinocilium). Thefollowing inward current of potassium ions depolarizes the hair cells.Depolarization of Hair Cells increases Intracellular Calciumand induces Transmitter Release
15 The Primary Auditory Cortex (A1) The primary auditory cortex is localized in areas 41 and 42, according toBrodmann’s classification.Anatomically, these areas comprise the transverse temporal gyrus (gyri) ofHeschl on the superior surface of the temporal lobe.The Essence of the Auditory Pathwaysthe 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, orcochlear nuclei, there are no lesions that produce unilateral hearing loss.
16 Neurological Examination of Auditory Function: Weber’s and Rinne’s Tuning Fork TestsIn case the basic tests indicate that hearing might be diminished, tuning forktests can be performed to localize the side of the lesion (left or right), and toclassify the type of the lesion (sensorineural hearing loss or conductive hearingloss).
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.