NROS 418: Auditory System (EX 1)
Of what use to the bat is the ability to move its external ear?
By moving its external ear it has the ability to hear from every angle and use this for echo-location in flight. They also have the ability to deform their outer ear and change its shape to perform different hearing tasks. (Virginia Tech. "Bats show ability to instantly change their ear shapes, making their hearing more flexible."
Explain how a cochlear implant works. You should draw an extended basilar membrane with hair cells above it and with the cochlear implant electrodes below it. Show a sample of the output of the frequency analyzer for given sound X, and then indicate which parts of the electrode might be activated (keeping in mind the variation in frequency sensitivity along the length of the basilar membrane.)
Cochlear implants bypass the normal acoustic hearing process and replaces sound vibration signals picked up by an external device and transforms them into electrical signals to directly stimulate the auditory nerve. A person with a cochlear implant receiving intensive auditory training may learn to interpret those signals as sound and speech. The CI is fed into the cochlea and each area of stimulation from the implant that is on the cochlea itself corresponds to the frequency (to some degree of error) that that area of the cochlea is responsible for. The person with the CI will need to either newly learn or relearn sounds and adjustments to the CI will need to be made by the doctor.
In the horizontal plane, humans use both ITD and ILD. Draw the medial superior olive circuit for ITD and the lateral superior olive circuit for ILD, showing both sides. Trace the response to sound from one side and then from a more medial position. Explain the concept of coincidence detection. Explain why axonal diameter and the large synapses in the MNTB matter for the ILD circuit.
Coincidence Detection - a process by which a neuron or a neural circuit encodes information by detecting stimuli that are temporally close together but separated by spatially distributed input signals. Coincidence detectors reduce temporal noise, spontaneous activity, and also forms associations between separate neural events. Large axonal diameter size of the bushy cell axons allows the inhibitory signal produced by the MNTB neurons to reach the Superior Olivary Complex almost immediately after initial excitation of the cochlea. This rapid measurement is important for comparing the contralateral (opposite side) and ipsilateral (same side) stimulation necessary in sound localization in the horizontal plane, and is key in distinguishing the location of low frequency sounds. The large synapse promotes fast, efficient information transfer using glutamate as the NT.
The barn owl overlays its auditory and visual maps so that they are precisely aligned, obviously a very good thing for precise targeting of prey. Using prism glasses, you can shift the visual map, putting it out of alignment with the auditory map. A) What would be the consequence of this on the owl's prey targeting? B) Do you think it would matter if the owl is young or an adult? C) What happens to produce re-alignment?
Having precisely aligned auditory and visual maps allows the owls to locate precisely where their target is in space. When you use prism glasses the visual input does not match the visual map that the animal has of its space. The animal will eventually learn to adjust the visual map to account for the distorted visual input but in the meantime the owl will have a hard time obtaining its prey because the sounds it hears vs. what is interprets as its location in space are no longer precisely mapped. Neural Plasticity and changing inputs result in cortical remapping of visual cortex which will then be re-aligned to the auditory map.
Why is it useful to have the mechanosensitive channels associated with the tip links be slightly open at rest?
Tonic release of transmitter allows the hair cells to quickly respond to mechanical stimuli and more effectively transduce the mechano signal into an electrical one.
What is tonotopy?
Tonotopy is the spatial arrangement of sounds of different frequency in the precise locations that they are processed in the brain. Tones close to each other in terms of frequency are represented in topologically in neighboring regions in the brain.
Delineate the pathways from the sensory receptors to their primary cortex. Be sure you think of these pathways bilaterally; in fact it would be good to make yourself diagrams of the pathway extending from the sensory receptors in both inner ears. You do NOT need to explain how they work, or even their transmitters.
Draw it out
Describe the physical structure of the sensory receptors and their physical environment as well as any accessory structures (e.g., the external ear, tympanum, ossicles and cochlea and of course hair cells (both kinds) and basilar membrane). Include a brief (a sentence), statement of the function of each part.
External Ear: determines the location of vertical information Tympanum: (eardrum) the tympanic membrane is a thin membrane that separates the ear canal (part that is open to the outside) from the middle ear. -->Receives sound vibrations from the outer air and transmits them to the auditory ossicles. Ossicles: are the tiny bones bones inside the middle of the ear that are attached to the tympanic membrane. -->It consists of the malleus, incus, and stapes. The transduction of sound is facilitated through the middle ear to the cochlea by transferring movements of the tympanic membrane into the second membrane that cover the oval window. The pressure at the oval window becomes greater than the pressure at the tympanic membrane if the force on the oval window membrane is greater than the force on the tympanic membrane. The ossicles provide the necessary amplification in pressure, facilitating the transduction of sound through the middle ear to the cochlea. Cochlea: (Greek for snail) the spiral cavity of the inner ear containing the organ of Corti. The Cochlea is made up of three canals wrapped around the modiolus. The canals are: scala tympani, scala vestibuli, and scala media (cochlear duct). Pressure waves received at the eardrum (tympanic membrane) are carried to the oval window, which leads to the cochlea, via the amplifying ossicles. Movement of the oval window moves fluid in the cochlea, which is made possible because of the elasticity of the membrane of the round window. The average cochlea is only 31.5 mm long and only about 10 mm in diameter at its widest point. Hair cells: The basilar membrane moves, which will push on the adjacent tissue which will move the hair cells against the tectorial membrane. --> Inner Hair Cells: Signal transduction; 3,000 - 3,500 in humans --> Outer Hair Cells:Outer hair cells function as tiny motors that amplify the movement of the basilar membrane during low-intensity sound stimuli, and are often referred to as the cochlear amplifier. The key to this function is the motor proteins that are found in the membranes of outer hair cells. --> Sound amplification: 10,000 - 12,000 in humans Both the inner hair cells and the outer hair cells are found within the organ of corti, more specifically superior to the basilar membrane as the tip links are inferior to the inferior aspect of the tectorial membrane. Basilar Membrane: found within the cochlea of the inner ear which separates two liquid filled tubes that run along the coil of the cochlea, the scala media, and scala tympani. --> When waves reach the tympanic membrane, they cause the membrane and the attached chain of auditory ossicles to vibrate. The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate.
What's the difference between the inner and outer hair cells in terms of their functions? If the outer hair cells are hyperpolarized by efferent input, what would be the effect on their length?
Inner hair cells: - communicates mostly w/the 8th nerve fibers and ends at the lowest brainstem - mostly afferent--> theuy send info to the brain (sensory fibers) - when they are closer to the tectorial membrane, the inner hair cells can be bent and can send sound to the brain - damage often results in difficulty understanding speech in quiet and noise due to sound from the ear to the brain Outer hair cells: - makes contact w/ the bottom of the tectorial membrane - communicates mostly through the olivocochlear bundle and ends at the outer hair cells - mostly efferent--> take info from the brain back to the cochlea (motor fibers) - stimulated by soft sounds Hyperpolarization will lengthen the outer hair cells and depolarization will shorten them
Explain how transduction works in the inner hair cells.
Mechanoelectrical transduction is mediated by hair cells. When the hair bundle is deflected toward the tallest stereocilium, cation-selective channels open near the tips of the stereocilia, allowing K+ ions to flow into the hair cell down their electrochemical gradient. The resulting depolarization of the hair cell opens voltage-gated Ca2+ channels in the cell soma, allowing calcium entry and release of neurotransmitter onto the nerve endings of the auditory nerve. An unusual adaptation of the hair cell in this regard is that K+ serves both to depolarize and repolarize the cell, enabling the hair cell's K+ gradient to be largely maintained by passive ion movement alone. As with other epithelial cells, the basal and apical surfaces of the hair cell are separated by tight junctions, allowing separate extracellular ionic environments at these two surfaces. The apical end is exposed to the K+-rich, Na+-poor endolymph, which is produced by dedicated ion pumping cells in the stria vascularis. The basal end is bathed in the same fluid that fills the scala tympani, known as perilymph, which resembles other extracellular fluids in that it is K+-poor and Na+-rich. In addition, the compartment containing endolymph is about 80 mV more positive than the perilymph compartment (this difference is known as the endo-cochlear 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) 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 leads to depolarization of the hair cell, which in turn opens voltage-gated Ca2+ and K+ channels located in the membrane of the hair cell. The opening of somatic K+ channels favors K+ efflux, and thus repolarization; the efflux occurs because the perilymph surrounding the basal end is low in K+ relative to the cytosol, and because the equilibrium potential for K+ is more negative than the hair cell's resting potential. Repolarization of the hair cell via K+ efflux is also facilitated by Ca2+ entry. 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.
Carefully review the anatomical organization of the outer, middle and inner ear.
Outer ear: has two divisions (auricle and external acoustic meatus) Pinna: located in the external ear → flaps to capture sound/direct it into ear canal External auditory canal: ear canal transmits sound waves from the pinna to the tympanic membrane of the middle ear Tympanic membrane: the eardrum → a structure that separates the outer ear from the middle ear and vibrates in response to sound waves Middle ear: the chamber between the eardrum and cochlea containing three tiny bones (malleus, incus, and stapes) that concentrate the vibrations of the eardrum on the cochlea's oval window 3 ossicles: malleus, incus, and stapes Malleus: hammer, first of the three auditory ossicles of the middle ear Incus: anvil, middle of the three auditory ossicles of the middle ear Stapes: stirrup, last of the three auditory ossicles of the middle ear Eustachian tube: tube that connects the middle ear to the nasopharynx, also auditory tube Inner ear: the innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs Osseous labyrinth: bony tunnels that house membranous labyrinth; gives the membrane structure; of organ of corti Cochlea: a coiled, bony, fluid-filled tube in the inner ear through which sound waves trigger nerve impulses; sense of gravity and acceleration Semicircular canals: three loop-like structures in the inner ear that contain sensory receptors to monitor balance and sense of rotation Perilymph: fills space between the bony labyrinth and membranous labyrinth, very similar to cerebrospinal fluid Membranous labyrinth: continuous series of membranous sac found within bony labyrinth, follows contours of bony labyrinth, filled with endolymph Endolymph: the fluid within the membranous labyrinth Vestibulocochlear nerve: cranial nerve, the nerve that takes information about balance and hearing to the brain
What is the nature of the physical stimulus for hair cells in the inner ear? Your goal here is to explain the physical stimulus sufficiently well for the physiology of the response mechanism in the sensory cell to make sense, so it's important, for example, to know the portion of the sound spectrum to which the cells respond.
Sound waves are vibrations that enter the ear and are converted from mechanical to electrical impulses. The mechanical vibrations of the stapes footplate at the oval window caused by these sound waves, creates a pressure wave in the perilymph fluid of the scala vestibuli in the cochlea. These waves travel around the tip of the cochlea through the helicotrema into the scala tympani and dissipate as they hit the round window and the motions is transmitted to the endolymph inside the cochlear duct. This results in vibrations of the the basilar membrane causing the organ of Corti to move against the tectoral membrane generating sheering force and stimulating generation of nerve impulses to the brain. Within the cochlea the different frequencies of complex sounds are sorted out, and the physical energy of these sound vibrations is converted, into electrical impulses that are transmitted to the brainstem by the cochlear nerve to the auditory cortex. The cochlea analyzes sound frequencies by means of the basilar membrane, which exhibits different degrees of stiffness, or resonance, along its length with the apex being sensitive to 20Hz and the base around 20,000 Hz.
How is frequency of incoming sound translated to movement of the basilar membrane?
The basilar membrane is tapered and stiffer the thinner tip than the fatter base. Sound waves travelling to the thinner tip at the end of the basilar membrane need to travel through a longer fluid column than sound waves travelling to the nearer, stiffer end. high stiffness and low mass, means high resonant frequencies at the near (base) end, and low stiffness and high mass, means low resonant frequencies, at the far (apex) end. This causes sound input of a certain frequency to vibrate some locations of the membrane more than other locations. The distribution of frequencies to places is called the tonotopic organization of cochlea. Along the basilar membrane lie 3,500 inner hair cells spaced in a single row. Each cell is attached to a tiny triangular frame. The 'hairs' (hair bundles/stereocilla) are minute processes on the end of the cell, which are very sensitive to movement. When the vibration of the membrane rocks the triangular frames, the hairs on the cells are repeatedly displaced, producing streams of corresponding pulses in the nerve fibers, which are transmitted to the auditory pathway. The outer hair cells feed-back to amplify the traveling wave. In the membrane of outer hair cells, there are motor proteins associated with the membrane. Those proteins are activated by sound-induced receptor potentials generated as the basilar membrane moves up and down. These motor proteins can amplify the movement, causing the basilar membrane to move a little bit more, amplifying the traveling wave. Consequently, the inner hair cells get more displacement of their cilia and move a little bit more and get more information than they would in a passive cochlea.
Humans use spectral filtering as a way to localize sound in the vertical plane. What is spectral filtering? How is the external ear involved?
The external ear forms direction-selective filters that are unique to the shape of the persons individual ear. Depending on the sound input direction in the vertical plane, different filter resonances (frequencies) are captured by the external ear and filtered into the ear canal. These resonances implant direction-specific patterns into the frequency responses of the ears, which are used for vertical sound localization. Sound localization in the vertical plane can be compromised if the shape of the external ear is altered or if the patterns are changed via headphones.
How does sensory adaptation work in inner hair cells? This requires you to explain how tension of the tip links are adjusted by action of the myosin motor. Try drawing it.
The motor protein myosin-1c allows for slow adaptation and provides the tension needed to sensitize transduction channels and also participate in signal transduction. The calcium-sensitive binding of calmodulin to myosin-1c potentially modulates the interaction between the motor and other transduction components. Slow adaptation occurs when myosin-1c slides down the stereocilium in response to elevated tension when the hair bundles are moving. The resulting decrease in tension at the tip links permits the bundle to move farther in the opposite direction. As tension decreases, channels close, producing the decline in transduction current.
Explain what each part of the ear does: the tympanum, the ossicles, the oval and round windows, the basilar membrane and the Organ of Corti. Include how each contributes to the characteristics of sound that we perceive.
Tympanic membrane: also called eardrum, thin layer of tissue in the human ear that receives sound vibrations from the outer air and transmits them to the auditory ossicles (malleus, incus, and stapes), which are tiny bones in the tympanic (middle-ear) cavity. Round Window: allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated and that audition will occur. Oval Window: the sound waves travel via the hammer and anvil to the stirrup and then on to the oval window which acts as an acoustic transformer Basilar Membrane: functions are Endolymph/perilymph separation, A base for the sensory cells, Frequency dispersion, and Generating receptor potentials. Transduction occurs through vibrations of structures in the inner ear causing displacement of cochlear fluid and movement of hair cells at the Organ of Corti to produce electrochemical signals
