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.
what is tonotopy? where do tonotopic maps exist?
- When auditory axons in the auditory-vestibular nerve synapse in the cochlear nuclei, they do so in an organized pattern based on characteristic frequency.
- Nearby neurons have similar characteristic frequencies, and a systematic relationship exists between position in the cochlear nucleus and characteristic frequency.
- In other words, there is a map of the basilar membrane within the cochlear nuclei.
- Systematic organization of characteristic frequency within an auditory structure is called tonotopy.
- Tonotopic maps exist on the basilar membrane within each of the auditory relay nuclei, the MGN, and auditory cortex.
because of the tonotopy present throughout the auditory system, the location of active neurons in auditory nuclei is one indication of what?
the frequency of the sound
what is phase locking? why is it important?
- When auditory nerve neurons fire action potentials, they tend to respond at times corresponding to a peak in the sound pressure waveform, i.e., when the basilar membrane moves up.
- The result of this is that there are a bunch of neurons firing near the peak of each and every cycle of sound pressure waveform.
- No individual neuron can respond to every cycle of a sound signal, so different neurons fire on successive cycles.
- Nonetheless, when they do respond they tend to fire together.
- The resultant “phase locking” that results provides temporal information from the two ears to neural centres that compare interaural time differences.
- The evaluation of interaural time differences provides a critical cue for sound localization and the perception of auditory “space.”
summarise how different frequencies are represented
At very low frequencies, phase locking is used.
At intermediate frequencies, both phase locking and tonotopy are useful.
At high frequencies, tonotopy must be relied on to indicate sound frequency.
what is the ‘volley principal’ of auditory information transfer?
- When a bunch of neurons fire at a peak of the sound wave, they need time to recover.
- This means that a second bunch of neurons fire action potentials at the second peak; a third bunch at the third peak; until eventually the first bunch of neurons has recovered.
- This cycle is repeated so that all the ‘peaks’ of a low-frequency sound wave (up to about 3-4kHz) are covered.
- This is the “volley principal” of auditory information transfer.
how does sound need to be localised?
in two planes: the horizontal plane and the vertical plane
describe the localisation of sound in the horizontal plane
• Humans use two different strategies to localize the horizontal position of sound sources, depending on the frequencies in the stimulus:
For frequencies below 3 kHz, interaural time differences are used to localize the source.
For frequencies above 3 kHz, interaural intensity differences are used instead.
• Parallel auditory pathways originating from the cochlear nucleus serve each of these strategies for sound localization.
what does the neural circuitry that modulates interaural time differences consist of? how are the ITD compensated for?
Interaural Time Differences (<3kHz):
• The neural circuitry that modulates interaural time differences consists of binaural inputs to the medial superior olive (MSO) that arise from the right and left anteroventral cochlear nuclei.
• The MSO contains cells with bipolar dendrites that extend both medially and laterally.
The lateral dendrites receive input from the ipsilateral anteroventral cochlear nucleus.
The medial dendrites receive input from the contralateral anteroventral cochlear nucleus.
• The MSO cells work as coincidence detectors, responding when both excitatory signals arrive at the same time.
• The axons that project from the anteroventral cochlear nucleus evidently vary systematically in length to create delay lines.
• These anatomical differences compensate for sounds arriving at slightly different times at the two ears, so that the resultant neural impulses arrive at a particular MSO neuron simultaneously, making each cell especially sensitive to sound sources in a particular place.
interaural intensity differences
- why do we have them
- where are the circuits that modulate the position of a sound source by IID found?
- describe what happens
- When high-frequency (> 3kHz) sounds are directed toward one side of the head, an acoustical “shadow” of lower intensity is created at the far ear.
- The circuits that modulate the position of a sound source on this basis are found in the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB).
- Excitatory axons project directly from the ipsilateral anteroventral cochlear nucleus to the LSO.
- The LSO also receives inhibitory input from the contralateral ear, via an inhibitory neuron in the MNTB.
- This excitatory/inhibitory interaction results in a net excitation of the LSO on the same side of the body as the sound source.
- For sounds arising directly lateral to the listener, firing rates will be highest in the LSO on that side; in this circumstance, the excitation via the ipsilateral anteroventral cochlear nucleus will be maximal, and inhibition from the contralateral MNTB minimal.
- In contrast, sounds arising closer to the listener’s midline will elicit lower firing rates in the ipsilateral LSO because of increased inhibition arising from the MNTB.
- For sounds arising at the midline, or from the other side, the increased inhibition arising from the MNTB is powerful enough to completely silence LSO activity.
- Note that each LSO only encodes sounds arising in the ipsilateral hemifield; it therefore takes both LSOs to represent the full range of horizontal positions.
where are ITD and IID processed?
where are the two pathways eventually merged?
• In summary, there are two separate pathways—and two separate mechanisms— for localizing sound in the horizontal pane:
Interaural time differences are processed in the medial superior olive.
Interaural intensity differences are processed in the lateral superior olive.
• These two pathways are eventually merged in the midbrain auditory centers.
describe localisation of sound in the vertical plane
- This is dependent on the curvature and shape of the pinna.
- Specialized neurons within the dorsal cochlear nucleus appear to use this information to determine the elevation of the sound source.
- The ascending projection of the dorsal cochlear nuclei bypasses the superior olivary complex to reach the inferior colliculus directly.
what does the vestibular system use to transduce movements?
hair cells
where are the hair cells (for auditory system) contained?
within sets of interconnected chambers called the vestibular labyrinth
what does the vestibular labyrinth consist of? what is function of each part? why are the functions different?
- otolith organs (saccule & utricle) - detect the force of gravity and tilts of the head
- semicircular canals - sensitive to head rotation
• These structures are sensitive to different kinds of movement not because their hair cells differ, but because of the specialised structures within which the hair cells reside.
to what and how does each hair cell of the vestibular organs transmit impulses?
Each hair cell of the vestibular organs makes an excitatory synapse with the end of a sensatory axon from the vestibular nerve, branch of the vestibulocochlear nerve (CN VIII).
where are the cell bodies of the vestibular nerve axons found?
in Scarpa’s ganglion
what are the membranous sacs within the bone (vestibular system) filled with? what is it collectively called?
endolymph - they are collectively called the membranous labyrinth
what is endolymph high and low in?
high in K+ and low in Na+
where is perilymph found? what’s it high and low in?
between the bony walls (the osseous labyrinth) and the membrnaous labyrinth
low in K+ and high in Na+
where are the vestibular hair cells located? how are they situated?
in the otolith organs and in ampullae, located at the base of the semicircular canals next to the utricle
Within each ampulla, the vestibular hair cells extend their hair bundles into the endolymph of the membranous labyrinth.
what causes neurotransmitter to be released onto the vestibular nerve fibres?
- Movements of the stereocilia toward the kinocilium (this is the largest single cilium in the hair bundle – similar to the auditory system) in the vestibular organs opens mechanically gated transduction channels located at the tips of the stereocilia.
- This causes depolarisation of the hair cell, causing neurotransmitter release onto (and excitation of) the vestibular nerve fibres.
what does the biphasic nature of the receptor potential mean?
The biphasic nature of the receptor potential means that some transduction channels are open in the absence of stimulation, with the result that hair cells tonically release neurotransmitter, thereby generating considerable spontaneous activity in vestibular nerve fibres.
what’s important about the orientations of hair cell bundles? how are they polarised?
• The hair cell bundles in each vestibular organ have specific orientations.
• As a result, the organ as a whole is responsive to displacements in all directions.
In a given semicircular canal, the hair cells in the ampulla are all polarized in the same direction.
In the utricle and saccule, a specialized area called the striola divides the hair cells into two populations with opposing polarities. (otoconia are calcium carbonate crystals that form the striola which in turn rests on the otolithic membrane)
what do the otolith organs detect?
• These detect changes of head angle, as well as linear acceleration of the head, such as tilting the head.
otolith organs form the static system and detect linear acceleration
what do both the otolith organs contain?
sensory epithelium, the macula, which consists of hair cells and associated supporting cells
what surrounds the hair cells in otolith organs?
Overlying the hair cells and their hair bundles is a gelatinous layer; above this layer is a fibrous structure, the otolithic membrane, in which are embedded crystals of calcium carbonate called otoliths.
what happens when the head is tilted? what do the utricle and saccule each repond to?
• When the head tilts, gravity causes the membrane to shift relative to the sensory epithelium. The resulting shearing motion between the otolithic membrane and the macula displaces the hair bundles, which are embedded in the lower, gelatinous surface of the membrane.
• This displacement of the hair bundles generates a receptor potential in the hair cells.
• A shearing motion between the macula and the otolithic membrane also occurs when the head undergoes linear accelerations; the greater relative mass of the otolithic membrane causes it to lag behind the macula temporarily, leading to transient displacement of the hair bundle.
• The striola forms an axis of mirror symmetry such that hair cells on opposite sides of the striola have opposing morphological polarizations.
• A tilt along the axis of the striola will excite the hair cells on one side while inhibiting the hair cells on the other side.
The saccular macula is oriented vertically and the utricular macula horizontally.
• Inspection of the excitatory orientations in the maculae indicates that:
The utricle responds to movements of the head in the horizontal plane, such as sideways head tilts and rapid lateral displacements.
The saccule responds to movements in the vertical plane (up–down and forward–backward movements in the sagittal plane).
• Note that the saccular and utricular maculae on one side of the head are mirror images of those on the other side. Thus, a tilt of the head to one side has opposite effects on corresponding hair cells of the two utricular maculae.
how do otolith neurons sense linear forces?
The structure of the otolith organs enables them to sense both static displacements, as would be caused by tilting the head relative to the gravitational axis, and transient displacements caused by translational movements of the head.
The change in firing rate in response to a given movement can be either sustained or transient, thereby signalling either absolute head position or linear acceleration.
The range of orientations of hair bundles within the otolith organs enables them to transmit information about linear forces in every direction the body moves.
- The utricle, which is concerned with horizontal motion.
- The saccule, which is concerned with vertical motion.
These two combine to effectively gauge the linear forces acting on the head at any instant in three dimensions.
Tilts of the head off the horizontal plane and translational movements of the head in any direction stimulate a distinct subset of hair cells in the saccular and utricular maculae, while simultaneously suppressing the responses of other hair cells in these organs.
what planes do the semicircular canals lie in? what does this mean?
which is each one it?
they lie in orthogonal planes, which means that there is an angle of 90* between any pair of them
Horizontal semicircular canal = horizontal plane = rotation of head
Anterior/superior semicircular canal = sagittal plane = nodding of head
Posterior semicircular canal = coronal plane = head to shoulder
- Sensitive to pitch, roll and yaw (each canal sensitive to its own one – horizontal detects yaw, posterior detects pitch and superior detects roll) – essential for coordination of eye movement
what do the semicircular canals sense?
head rotations
what does each semicircular canal have at its base? what does this contain?
a bulbous expansion called the ampulla, which houses the crista, that contains the hair cells
the hair bundles extend out of the crista into a gelatinous mass called what? forming what?
the cupula, forming a fluid barrier through which endolymph cannot circulate
how does movement of endolymph cause changes in neuronal activity?
• The hair bundles extend out of the crista into a gelatinous mass, the cupula, forming a fluid barrier through which endolymph cannot circulate.
• As a result, the relatively compliant cupula is distorted by movements of the endolymphatic fluid that surrounds the cupula.
• When the head turns in the plane of one of the semicircular canals, the inertia of the endolymph produces a force across the cupula, distending it away from the direction of head movement and causing a displacement of the hair bundles within the crista.
• In contrast, linear accelerations of the head produce equal forces on the two sides of the cupula, so the hair bundles are not displaced.
• Unlike the saccular and utricular maculae, all of the hair cells in the crista within each semicircular canal are organized with their kinocilia pointing in the same direction.
• Thus, when the cupula moves in the appropriate direction, the entire population of hair cells is depolarized and activity in all of the innervating axons increases.
• When the cupula moves in the opposite direction, the population is hyperpolarized and neuronal activity decreases.
Deflections orthogonal to the excitatory–inhibitory direction produce little or no response.
• Each semicircular canal works in concert with the partner located on the other side of the head that has its hair cells aligned oppositely. There are three such pairs:
The two pairs of horizontal canals
The superior canal on each side working with the posterior canal on the other side.
• Head rotation deforms the cupula in opposing directions for the two partners, resulting in opposite changes in their firing rates.
• Thus, the hair cells in the canal towards which the head is turning are depolarized, while those on the other side are hyperpolarized.
how do semicircular canal neurones sense angular accelerations?
• If an experiment has three phases: an initial period of acceleration, then a period of several seconds at constant velocity, and finally a period of sudden deceleration to a stop.
The maximum firing rates observed corresponds to the period of acceleration.
The maximum inhibition corresponds to the period of deceleration.
During the constant-velocity phase, the response adapts so that the firing rate subsides to resting level.
After the movement stops, the neuronal activity decreases transiently before returning to the resting level.
• When rotation is stopped, the inertia of the endolymph causes the cupula to bend in the other direction, generating an opposite response from the hair cells and a temporary sensation of counterrotation.
what are the reflexes associated with vestibular system?
- Vestibulo-ocular reflex
- Vestibulo-cervical reflex
- Vestibulo-spinal reflex
vestibulo-ocular reflex
- what for
- what happens
- what are the three types
• The vestibulo-ocular reflex (VOR) in particular is a mechanism for producing eye movements that counter head movements, thus permitting the gaze to remain fixed on a particular point.
• These eye movements compensate for head movements to produce a stable image on the retina.
• There are three types of vestibulo-ocular reflexes:
1. Rotational VOR – rotational head movement – semicircular canals
2. Translational VOR – linear head movement – otolith organs
3. Ocular counter-rolling response – head tilt – otolith organs
- Prevents retinal slip
- Head rotates left, eyes rotate right!
- Vestibular apparatus detects the angular acceleration of the head to the left, and it compensates using this pathway to counter-rotate the eyes by the same number of degrees to the right
- Left eye: contract medial rectus, relax lateral rectus
- Right eye: contract lateral rectus, relax medial rectus
