Homework #2-Exam 2
What is the difference between decussations and commissures? What station (s) do not have any decussations?
Commissures is a nerve fiber bundle that cross the midline at the same level that they originate (Ex: Left SOC to Right SOC). Decussations are crossover points in the CANS that also cross the midline but they cross at a level other than where they originated (Ex: Left SOC to Right LL). So the projection is not going to the same structure on the contralateral side but it travels farther up the CANS. The MGB does not have any decussations because there are no commissural neurons at that level.
Discuss how intensity and frequency are encoded in the cochlea
Frequency: Traveling wave theory: For each inward and outward movement of the footplate of the stapes, there is a downward and upward movement of the basilar membrane, produced by disturbance of the endolymph. The wave moves down the cochlear duct from base to apex, with the maximum amplitude of the wave occurring at the basal end of the cochlea for high frequency tones and at the apical end of the cochlea for the low frequencies. The OHCs function is to sharpen the wave for greater frequency discrimination. Although high frequencies excite only the fibers in the basal turn of the cochlea, the low frequencies excite fibers all along the basilar membrane, the input frequency, then, determines not only the distance the traveling wave moves before it peaks, but also the rate of basilar membrane vibration. The frequency of basilar membrane vibration is directly related to the perceived frequency. Intensity: The place theories, including the traveling wave theory, explain loudness in terms of the amplitude of movement of the basilar membrane; that is, a louder sound creates greater amplitude of basilar membrane movement than a softer sound. This greater amplitude increases the number of impulses transmitted by the nerve fibers. The frequency theory theories explain loudness in terms of the amount of spread along the basilar membrane. The greater the amplitude of the input signal, the larger the surface area of the basilar membrane that is stimulated, and the greater the number of nerve fibers that are firing, both at the peak of the traveling wave and on both sides of it. Although intensity coding is extremely complicated and poorly understood, it is generally agreed that as the intensity of the signal is increased, the neurons in the brainstem fire at high rates, resulting in greater loudness of the signal.
Discuss the propagation of sound through the auditory nerve and the central auditory pathway using the correct anatomical terminology
The nerve fibers begin at the base of the hair cells, pass through the osseus spiral lamina, through the modiolus, and join with the vestibular nerve fibers. Together the cochlear and vestibular nerve fibers form the VIIIth cranial nerve. The VIIIth cranial nerve travels through the internal auditory canal along with the VIIth CN and terminates at the brainstem at the level of the pontomedullary junction (junction of the cerebellum, medulla oblongata, and pons). The VIIIth cranial nerve is organized so that nerve fibers from the base of the cochlea (high frequencies) form the center and nerve fibers from the apex (low frequencies) form the outer portion. The first station in the central auditory nervous system (CANS) is the cochlear nucleus. The VIIIth auditory nerve splits and one section descends to the dorsal cochlear nucleus, and the other ascends to the ventral cochlear nucleus. Next the signal is sent to both the ipsilateral and contralateral superior olivary complex (SOC). The SOC analyzes differences in the time and intensity of the signal arriving at either ear. The lateral lemniscus (LL) a nerve fiber tract, provides a pathway for sound to be transmitted from the SOC to the ipsilateral and contralateral LL, and the inferior colliculus (IC). The IC receives ascending information from both the ipsilateral and contralateral SOC. The next relay station in the central auditory nervous system is the medial geniculate body which is located in the thalamus. From the MGB, auditory radiations extend and transmit the auditory signal to the auditory cortex. It is important to note that the tonotopic organization of the cochlea (base - high frequencies, apex- low frequencies) is continued in the auditory cortex to a lesser degree.
Discuss the two main functions of the outer hair cells.
The three rows of outer hair cells are responsible for "sharpening" the fluid borne wave within the cochlea, thereby enhancing frequency discrimination. OHC damage, which occurs in many cochlear hearing losses, reduces this sharpening, with a resultant decrease in speech recognition abilities, especially in background noise. The OHCs also enhance the reception of sound by the IHC by shortening themselves, thereby bringing the IHC into contact with the tectorial membrane. Without this assistance from the OCH, the IHCs are limited to stimulation from the motion of the endolymphatic fluid within the scala media and thus respond only to sounds above about 40 to 60 dB SPL. Hearing loss below those levels would be attributed primarily to OHC damage. Greater degrees of hearing loss would imply damage to both the outer and inner hair cells (Martin and Clark, 280)
Discuss the propagation of sound through the inner ear using the correct anatomical terminology
When the oval window is moved in by the stapes, the annular ligament around the footplate stretches and displaces the perilympth at the basal end of the cochlea, propagating a wave toward the apex of the cochlea. Because the fluids of the inner ear are non-compressible, when these fluids are inwardly displaced at the oval window the round window membrane must yield by moving into the middle ear. Sound vibrations that are introduced to the scala vestibule are conducted into the cochlea duct by the yielding of the Reissner's membrane. The endolymph is thereby disturbed so that vibrations continue and the basilar membrane is similarly displaced, resulting in the release of the round window membrane. Therefore, sounds introduced to the inner ear cause a wave like motion, which always moves from the base of the cochlea to the apex. Tones of low frequency with longer wave lengths show maximum displacement near the apical end. Whereas tones of high frequency with shorter wavelengths show maximum displacement near the basal end. The basilar membrane is more sensitive to vibrations of the inner ear than do most of the other structures. Since the organ of Corti rests upon the basilar membrane, when the basilar membrane moves, the organ of corti moves. The stereocilia on the tips of the OHCs are embedded in the tectorial membrane. Then the basilar membrane moves up and down in the response to fluid displacement caused by the in and out wared movement of the stapes, the hair cells are sheared or twisted in a complex manner. When the cilia are sheared, a chemical is released at the base of the hair cell. The nerve fibers attach to the base of the hair cell. The nerve fibers exit the cochlea and extend centrally toward to modiolus, where their cell bodies group together to form the spiral ganglion. The nerve fibers pass form the modiolus to form the cochlear branch of the auditory nerve.
