Case 4 Flashcards
what is the frequency range that humans can detect sound? what is the upper limit in the average adult closer to?
from about 20 Hz to 20 kHz
15-17 kHz
which frequencies is the ear most sensitive to? what does this correspond to?
The ear is most sensitive between 500 and 4000 Hz, which roughly corresponds to the frequency range most important for understanding speech.
external ear
- what does it consist of
- what do these structures do
- what does the configuration of the external auditory meatus do?
- The external ear consists of the pinna (auricle), concha, and external auditory meatus.
- These structures collect sound waves and focus them on the tympanic membrane.
- The configuration of the external auditory meatus selectively boosts the sound pressure (around 30-100 times) for frequencies around 3 kHz via passive resonance effects. This amplification makes humans especially sensitive to frequencies in the range of 2–5 kHz.
how does the ear recognise the elevation of a sound source?
The ear recognises the elevation of a sound source because the vertically asymmetrical convolutions of the pinna are shaped so that the external ear transmits more high-frequency components from an elevated source than from the same source at ear level.
middle ear
- what does it do
- how is it done
• The middle ear converts vibrations in air pressure into movements of the ossicles.
• The middle ear ensures transmission of the sound energy across the air–fluid boundary by boosting the pressure measured at the tympanic membrane greatly by the time it reaches the inner ear.
• Two mechanical processes occur within the middle ear to achieve this large pressure gain:
1. Focusing the force impinging on the relatively large-diameter tympanic membrane on to the much smaller-diameter oval window.
2. A second and related process relies on the mechanical advantage gained by the lever action of the three ossicles (malleus, incus and stapes) which connect the tympanic membrane to the oval window. Sound causes large movements of the tympanic membrane, which are transformed into smaller but stronger vibrations of the oval window.
what is the pressure at the oval window like? why? what does this allow?
• The pressure at the oval window is great because it has a smaller surface area to the tympanic membrane AND because the force on the oval window membrane is greater than that on the tympanic membrane.
The force applied on the oval window is greater than that on the tympanic membrane because behind the tympanic membrane is air, whereas behind the oval window is fluid. Therefore, more force needs to be applied at the oval window.
• The pressure exerted on the oval window is about 20 times greater than at the tympanic membrane, and this increase is sufficient to move the fluid in the inner ear.
what are the two muscles in the middle ear? what are they innervated by?
- The tensor tympani – innervated by mandibular nerve (V3)
2. Stapedius - innervated by facial nerve (CN VII).
what does contraction of the muscles in the middle ear cause? what is it important for? what frequencies is it greater for? when doesn’t it work?
- Contraction of these muscles causes the chain of ossicles to become more rigid, and sound conduction to the inner is greatly diminished.
- The onset of a loud sound triggers a neural response called the attenuation reflex.
- Sound attenuation is much greater at lower frequencies than at high frequencies.
- The automatic contraction of these muscles occurs a few microseconds after a loud noise, and so doesn’t provide protection against sudden loud noises.
describe the anatomy of the cochlea
• At the base of the cochlea are two membrane-covered holes:
1. The oval window – this is below the footplate of the stapes
2. The round window
• In the cross-section of the cochlea, we can see that it is divided into three fluid-filled chambers: the Scala vestibuli, the Scala media and the Scala tympani.
• Reissner’s membrane separates the scala vestibuli and the scala media.
• Basilar membrane separates the scala tympani and the scala media.
• Sitting on the basilar membrane is the organ of Corti, which contains auditory receptor neurons; hanging over this organ is the tectorial membrane.
• At the apex of the cochlea, the scala media is closed off, and the scala tympani becomes continuous with the scala vestibule at a hole in the membranes called the helicotrema. This allows the perilymph in these two scale to mix.
• At the base of the cochlea, the scala vestibuli meets the oval window and the scala tympani meets the round window.
what happens when there is inward movement of the oval window (due to force applied by the ossicles)?
the fluid of the inner ear is displaced, causing the round window to bulge slightly and deforming the cochlear partition
what is the role of the basilar membrane?
The basilar membrane has two structural properties that determine the way it responds to sound:
1. The membrane is wider at the apex than at the base by a factor of about 5.
2. The stiffness of the membrane decreases from the base to apex, the base being about 100 times stiffer.
When sound pushes the footplate of the stapes at the oval window, perilymph is displaced within the scala vestibuli, and endolymph is displaced within the scala media because Reissner’s membrane is very flexible.
Sound can also pull the footplate, reversing the pressure gradient.
Sound causes a continual push-pull motion of the footplate.
The displacement of the endolymph in the scala media makes the basilar membrane bend near its base, starting a wave that propagates toward the apex.
The wave travels up the basilar membrane. The distance this wave travels up the basilar membrane is dependent on the frequency of the sound. The higher the frequency, the shorter the distance it will travel up the basilar membrane. This is because if the frequency is high, the stiffer base of the membrane will vibrate a good deal, dissipating most of the energy, and the wave will not propagate very far.
what is the role of the organ of Corti?
- The auditory receptor cells, which convert mechanical energy into a change in membrane polarisation, are located in the organ of Corti.
- The organ of Corti consists of hair cells (auditory receptors), the rods of Corti, and various supporting cells.
- The auditory receptors are called ‘hair cells’ because each one has about 100 hairy-looking stereocilia extending from its top.
- The critical event in the transduction of sound into a neuronal signal is the bending of these stereocilia.
what is the role of auditory receptors/hair cells?
- These are involved in the mechano-electrical transduction of sound waves.
- The hair cells are sandwiched between the basilar membrane and the reticular lamina.
- The rods of Corti span these two membranes and provide structural support.
- The stereocilia at the tips of the hair cells extend above the reticular lamina into the endolymph, and their tips end either in the tectorial membrane (outer hair cells) or just below the tectorial membrane (inner hair cells).
- Hair cells form synapses (neurotransmitter is glutamate) on neurons whose cell bodies are located in the spiral ganglion.
- Axons from the spiral ganglion enter the auditory nerve, a branch of the auditory-vestibular nerve, which projects to the cochlear nuclei in the medulla.
describe & explain transduction by hair cells
• When the basilar membrane moves in response to the a motion at the stapes, the entire foundation supporting the hair cells moves because the basilar membrane, rods of Corti, reticular lamina, and hair cells are connected.
• Because the tectorial membrane holds the tips of the outer hair cell stereocilia, the lateral motion of the reticular lamina relative to the tectorial membrane bends the stereocilia on the outer hair cells one way or the other.
• The stereocilia are graded in height and are arranged in a bilaterally symmetric fashion:
Displacement of the hair bundle parallel to this plane toward the tallest stereocilia depolarises the hair cell.
While movements parallel to this plane toward the shortest stereocilia cause hyperpolarisation.
Displacements perpendicular to the plane of symmetry do not alter the hair cell’s membrane potential.
- There is a special type of cation channel on the tips of the stereocilia, called the TRPA1 channel.
- When the hair cells bend, the TRPA1 channels open, causing an influx of Na+ and K+ ions. This generates changes in the hair cell receptor potential.
- Each channel is linked with an elastic filament called tip link, to the wall of the adjacent cilium.
- When the cilia are straight, the tension on the tip link holds the channel in a partially opened state, allowing a small leak of K+ from the endolymph into the hair cell.
- Displacement of the cilia in one direction increases tension on the tip link, increasing the inward K+ current.
- Displacement in the opposite direction relieves tension on the tip link, thereby allowing the channel to close completely, preventing inward K+ movement.
- The entry of K+ into the hair cell causes a depolarization, which in turn activates voltage-gated calcium channels.
- The entry of Ca2+ triggers the release of neurotransmitter, probably glutamate, which activates the spiral ganglion fibers lying postsynaptic to the hair cell.
- The hair cell has a resting potential between –45 and –60 mV.
• K+ serves both to depolarise and repolarise the cell, enabling the hair cell’s K+ gradient to be largely maintained by passive ion movement alone.
The apical end (including the stereocilia) protrudes into the scala media and is exposed to the K+ rich, Na+ poor endolymph, which is produced by dedicated ion pumping cells in the stria vascularis (stria vascularis resorbs Na+ and secretes K+).
The basal end of the hair cell body is bathed in perilymph which is K+ poor and Na+ rich.
The compartment containing endolymph is about 80 mV more positive than the perilymph compartment (this difference is known as the endocochlear potential), while the inside of the hair cell is about 45 mV more negative than the perilymph (and 125 mV more negative than the endolymph).
• The resulting electrical gradient across the membrane of the stereocilia (about 125 mV; the difference between the hair cell resting potential and the endocochlear potential) drives K+ through open transduction channels into the hair cell, even though these cells already have a high internal K+ concentration.
K+ entry via the transduction channels depolarizes the hair cell, opening voltage-gated Ca2+ and K+ channels located in the membrane of the hair cell soma.
The opening of somatic K+ channels favours K+ efflux, and thus repolarization. The efflux occurs because the perilymph surrounding the basal end is low in K+ relative to the cytosol.
• In addition to modulating the release of neurotransmitter, Ca2+ entry opens Ca2+-dependent K+ channels, which provide another avenue for K+ to enter the perilymph.
what are the different types of hair cells? how are they arranged? what does each do?
• There are two types of hair cells: Inner hair cells and Outer hair cells.
• There is one row of inner hair cells and three rows of outer hair cells (five at the apex).
• The inner hair cells are the actual sensory receptors. About 95% of the fibres of the auditory nerve that project to the brain arise from the inner hair cells.
One spiral ganglion fibre receives input from only one inner cell.
Each inner cell feeds about 10 spiral ganglion neurites.
• The terminations on the outer hair cells are almost all from efferent axons that arise from cells in the superior olivary complex (this is the first site of convergence of sound from the right and left ears).
- Outer hair cells act like tiny motors that amplify the movement of the basilar membrane during low-intensity sound stimuli.
- Because of this action, the outer hair cells in the basilar membrane are referred to as the cochlear amplifier.
- Cochlear amplification occurs as a result of motor proteins present in the membranes of the outer cells.
- Motor proteins can change the length of outer hair cells.
- Outer hair cells respond to sound with both a receptor potential and a change in length.
- The hair cells’ motor is driven by the receptor potential, and it does not use ATP as an energy source.
- The hair cells’ motor system is extremely fast, as it must be able to keep up with movements induced by high frequency sounds.
- The hair cells’ motor is a protein called prestin. This is tightly packed into the membranes of the outer hair cells, and it is required for outer hair cells to move in response to sound.
- When the outer hair cells amplify the response of the basilar membranes, the stereocilia on the inner hair cells will bend more, and the increased transduction process in the inner hair cells will produce a greater response in the auditory nerve.
describe the auditory pathway
• Afferents from the spiral ganglion enter the brain stem in the vestibulocochlear nerve.
• At the level of the medulla, the axons innervate the dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea where the axons originated.
Each axon branches so that it synapses on neurons in both cochlear nuclei.
• Cells in the ventral cochlear nucleus send out axons that project to the superior olive (superior olivary nucleus) on both sides of the brain stem.
• Axons of the olivary neurons ascend in the lateral lemniscus (a lemniscus is a collection of axons) and innervate the inferior colliculus of the midbrain.
Many efferents of the dorsal cochlear nucleus follow a route similar to the pathway from the ventral cochlear nucleus, but the dorsal path bypasses the superior olive.
• Although there are other routes from the cochlear nuclei to the inferior colliculus, with additional intermediate relays, all ascending auditory pathways converge onto the inferior colliculus.
• The neurons in the inferior colliculus send out axons to the medial geniculate nucleus (MGN) of the thalamus, which in turn projects to auditory cortex.
what are important points to remember with the auditory pathway?
- other pathways
- feedback
- deafness in one ear
• It is important to note that there are many parallel auditory pathways from the cochlear nuclei to the auditory cortex of the brain.
• Projections and brain stem nuclei other than the one described above contribute to the auditory pathways:
For example – the inferior colliculus sends axons not only to the MGN but also to the superior colliculus (where the integration of auditory and visual information occurs) and to the cerebellum.
• There is extensive feedback in the auditory pathways:
For example – brain stem neurons send axons that contact outer hair cells, and auditory cortex send axons to the MGN and inferior colliculus.
• Each cochlear nucleus receives input from just the one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears.
This explains the clinically important fact that the only way by which brain stem damage can produce deafness in one ear is if a cochlear nucleus (or auditory nerve) on one side is destroyed.
The anterior inferior cerebellar artery supplies the cochlear nuclei, and unilateral occlusion can produce deafness in one ear.
what are the monaural pathways? what are they for?
- A second major set of pathways from the cochlear nucleus bypasses the superior olive and terminates in the nuclei of the lateral lemniscus on the contralateral side of the brainstem.
- These particular pathways respond to sound arriving at one ear only and are thus referred to as monaural.
where is the middle geniculate nucleus found? what is it made up of?
in the thalamus
• The MGN has several divisions, including;
The ventral division, which functions as the major thalamocortical relay.
The dorsal and medial divisions, which are organized like a belt around the ventral division.
why is the auditory thalamus/middle geniculate nucleus important? where does input arise from? what happens in the MGN?
- It is an obligatory relay for all ascending auditory information destined for the auditory cortex.
- Most input to the MGC arises from the inferior colliculus, although a few auditory axons from the lower brainstem bypass the inferior colliculus to reach the auditory thalamus directly.
• The MGN is the first station in the auditory pathway where selectivity for combinations of frequencies is found. There is also selectivity for specific time intervals between the two frequencies.
via what do axons leaving the MGN project to the auditory cortex?
via the internal capsule in an array called the acoustic radiation
what does the auditory cortex consist of?
6 layers: Layer I contains few cell bodies, and layers II and III contain mostly small pyramidal cells. Layer IV, where the medial geniculate axons terminate, is composed of densely packed granule cells. Layers V and VI contain mostly pyramidal cells that tend to be larger than those in the superficial layers.
where is the primary auditory cortex (A1) located? in what form does it receive input?
on the superior temporal gyrus in the temporal lobe
- it receives point-to-point input from the ventral division of the MGN; thus, it contains a precise tonotopic map - these are known as isofrequency bands
- the belt areas of the auditory cortex receive more diffuse input from the belt areas of the MGC and therefore are less precise in their tonotopic organization.
what are the ways that information about sound intensity are coded?
• Information about sound intensity is coded in two interrelated ways:
1. The firing rates of neurons in auditory nerve.
2. The number of active neurons in auditory nerve.
• The loudness we perceive is thought to be correlated with the number of active neurons in the auditory nerve (and throughout the auditory pathway) and their firing rates.
• Firing rates of neurons:
As a stimulus gets more intense, the basilar membrane vibrates with greater amplitude, causing the membrane potential of the activated hair cells to be more depolarised or hyperpolarised.
As a result, the nerve fibres with which the hair cells synapse fire action potentials at greater rates.
• Number of activated neurons:
More intense stimuli produce movements of the basilar membrane over a greater distance, which leads to the activation of more hair cells.
In a single auditory nerve fibre, this increase in the number of activated hair cells causes a broadening of the frequency range to which the fibre responds.