Auditoy System Flashcards
What are the mechanisms of pressure amplification in the middle ear?
. Two Mechanisms of Pressure Amplification in the Middle Ear
Mechanical transformation of the sound signal (pressure waves) in the middle ear leads to a 70-100-fold amplification of the pressure force. Two mechanisms contribute to this pressure amplification:
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.
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 windo
Middle ear can amplify low intensity sounds by two mechanisms…
• The middle ear can amplify low intensity sounds by two mechanisms
Pressure= Force/ Area
1. Decrease Area (decrease area by 20–> increase pressure by 20)
2. Increase Force
How can two middle ear muscles limit excessive pressure?
Two Middle Ear Muscles Limit Excessive Pressure
The movement of the ossicular chain can be limited by the contraction of two small middle ear muscles: activation of the m. tensor tympani, which is attached to the malleus and is innervated by the trigeminal nerve (CN V), limits the movement of the tympanic membrane; activation of the m. stapedius, innervated by the facial nerve (CN VII), limits the movement of the stapedius. They both protect the inner ear from damage.
High intensity sound activates the tensor tympani and stapedius muscles through the attenuation reflex. Due to the delay of this reflex from the onset of sound, this reflex cannot prevent damage from sudden increases in sound intensity (for example, caused by explosions).
Describe sound attenuation
Contraction of m. tensor tympani and m. stapedius occurs in response to high intensity sound – attenuation reflex
- Contraction restricts the movement of the tympanic membrane and stapes against the oval window
- The chain of ossicles becomes more rigid
- Deleterious effects of sustained loud noises on the inner ear are reduced
- Reflex does not offer protection against sudden loud sounds - acquired hearing loss is possible
There is a place code for sound frequencies…..
There is a Place Code for Sound Frequencies
The mechanical properties of the basilar membrane differ at different places and the amount of deflection of the basilar membrane is based on its mechanical properties.
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.
The sum of deflection of the basilar membrane by a certain frequency of sound is described by the “envelope of waves”, which covers the deflections of the basilar membrane at different stages of the travel of the waves.
Maximum deflection of the basilar membrane causes maximum activation of the hair cells at this part of the basilar membrane and produces the highest signal in the afferent neurons.
This concept, where the place (or location) of a nerve cell encodes for a specific stimulus feature (frequency, in this instance) is called a place code.
The envelopes of waves for high frequencies have their maxima closer to the oval window, where the basilar membrane is narrow and stiff. Low frequencies are represented closer to the apex of the cochlea near the helicotrema, where the basilar membrane is wide and floppy.
There is a place code for sound frequencies on the basilar membrane…
Maximum deflection = maximum hair cell activation
Base narrow and stiff, high frequencies —> apex wide and floppy, low frequencies
Summarize encoding sound frequency
Encoding Sound Frequency
Place Code for Sound Frequencies
• Any small region of the basilar membrane undergoes its largest oscillation for only a narrow range of frequencies
• Since the auditory neurons are connected to a small number of hair cells, near each other, each neuron is most sensitive over a narrow range of frequencies
• The most sensitive frequency for an auditory nerve fiber is that neuron’s characteristic frequency
What does the cochlea do?
The Cochlea Decomposes Sound Stimuli into Component Frequencies Using a Place Code
What are the 2 kinds of hair cells?
Outer hair cells act as signal amplifiers in the inner ear (remember, there are also two signal amplification mechanisms in the middle ear earlier). Similar to muscle cells, motor proteins cause shortenings of the outer hair cells when they are depolarized and elongation when they are at rest.
What’s the impact of basilar membrane for inner ear amplification?
Basilar membrane deflected upwards by travelling waves - hair cells depolarised - outer hair cells compressed - basilar membrane pulled further upwards
Basilar membrane deflected downwards by travelling waves - hair cells hyperpolarised - outer hair cells expanded - basilar membrane pushed further
downwards
Describe the essence of the auditory pathways
The Essence of the Auditory Pathways
The auditory pathways, as outlined in your Haines Atlas, 9th edition, Figure 8-49, appear much more complicated than those of other sensory systems. While it is not necessary to know the pathway in full detail, it is important for you to at least understand the basics, such as the elements involved and the different levels within the central nervous system. The major elements of the auditory pathways are listed on the slide overleaf.
The pathways involve the peripheral nervous system (PNS), in essence the auditory portion of the vestibulo-cochlear nerve (CN VIII) and all levels of the brain, starting in the upper portion of the medulla and ending in the primary auditory cortex (A1) of the cerebral cortex. The auditory nerve enters the brainstem at the level of the ponto-medullary junction, and the fibers synapse in the anterior and posterior cochlear nuclei.
From there onwards, 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. Unilateral lesions affecting higher structures, such as the primary auditory cortex, can disrupt the ability to localize sound, however.
What are the elements of the auditory pathways?
Cerebral cortex • Transversetemporalgyrus Thalamus • Medial geniculate nucleus Midbrain • Inferior colliculus Pons • Lateral lemniscus nucleus • Superior olive • Trapezoid nucleus Medulla • Cochlear nuclei (A & P)
Above are CNS
Auditory nerve (CN VIII)
• Spiral ganglion
Above are PNS
What are the anatomy of the primary auditory cortex?
The Primary Auditory Cortex (A1)
Localization of the Primary Auditory Cortex
Primary auditory cortex is localized in Brodmann areas 41 and 42 at the transverse temporal gyrus (gyri) of Heschl on the superior surface of the temporal lobe.
Organization of the Primary Auditory Cortex
Similar to other primary sensory cortices, primary auditory cortex has:
Tonotopic Organization Instead of a spatial map, as we saw in primary somatosensory and visual cortices, the organization of the primary auditory cortex is tonotopic, which means that the sound frequencies are mapped and distributed along a rostro-caudal axis. Low frequencies are represented more rostrally (and laterally), whereas high frequencies are represented more caudally (and medially).
What is conductive hearing loss?
Conductive hearing loss is caused by anything that impedes the conduction of sound vibrations through the external auditory canal or middle ear. Conversely, a lesion of the organ of Corti or the auditory nerve leads to sensorineural hearing loss
How does otosclerosis cause hearing loss?
Otosclerosis causes Conductive Hearing Loss
In otosclerosis there is a gradual replacement of normal bone of the bony labyrinth and the stapes footplate by lamellar new bone. This leads to a fusion of the stapes with the borders of the oval window and, as a consequence, a conductive hearing loss, which may be up to an extent of 40 dB sound pressure level.
