Auditory System
Of what use to the bat is the ability to move its external ear
Allows it to maximally funnel and amplify sounds coming from a variety of locations. Particularly useful for precise targeting of prey.
The basilar membrane:
An elastic partition runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This partition is called the basilar membrane because it serves as the base on which key hearing structures sit. The basilar membrane varies in stiffness, and is stiffest nearest the oval window, and floppiest near the apex. This allows it to function as a frequency spectrum analyzer.
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. What would be the consequence of this on the owl's prey targeting?
Immediately after prisms are mounted, an owl will strike an auditory target accurately because the prisms do not alter auditory space cues. However, like many other species, owls quickly recalibrate their movements based on vision. Because movements now are calibrated to the prismatically displaced visual field, the owls must also recalibrate their ITD maps to match the visual maps to reestablish an aligned sensorimotor coordinate frame. Young owls do just this, and the recalibration of the auditory map is expressed as a shift of ITD tuning in the OT
Tympanum
(Also known as the eardrum) vibrates from the sound waves incoming from the auricle and ear canal and sends these vibrations to three tiny bones (malleus, incus, and stapes, also known as the Ossicles) in the middle ear.
What happens to produce re-alignment?
Within weeks after being fitted with the prism goggles, the owls are able to change projections in the avian brain in the Optic Tectum and the ICX (inferior colliculus).
Explain how a cochlear implant works.
Cochlear implants bypass the eardrum, the ossicular chain, the basilar membrane, and the (usually dead) hair cells. They stimulate the auditory nerve fibers with electrical pulses delivered from electrodes implanted inside the cochlea
Explain the concept of coincidence detection.
Coincidence detector in a system is a way to amplify a response. For example, in LTP in the hippocampus, ampa receptors open as a result of glutamate binding and allow sodium inside the postsynaptic cell. When there is enough sodium it depolarizes the cell and causes the opening of NMDA receptors which when they open, allow calcium into the cell. NMDA is this case is a coincidence detector because it senses the presence of depolarization and glutamate to bring calcium into the system which amplifies the response. In 1948, Lloyd A. Jeffress proposed that some organisms may have a collection of neurons that receive auditory input from each ear. The neural pathways to these neurons are called delay lines. Jeffress claimed that the neurons that the delay lines link act as coincidence detectors by firing maximally when receiving simultaneous inputs from both ears. When a sound is heard, sound waves may reach the ears at different times. This is referred to as the interaural time difference (ITD). Due to differing lengths and a finite conduction speed within the axons of the delay lines, different coincidence detector neurons will fire when sound comes from different positions along the azimuth. Jeffress' model proposes that two signals even from an asynchronous arrival of sound in the cochlea of each ear will converge synchronously on a coincidence detector in the auditory cortex based on the magnitude of the ITD.
Humans use spectral filtering as a way to localize sound in the vertical plane. How is the external ear involved?
Depending on sound location, its frequencies will be modified by the body (and particularly the outer ears) in specific and characteristic ways, allowing for localization in the vertical plane
If the outer hair cells are hyperpolarized by efferent input, what would be the effect on their length?
Depolarization of the outer hair cells causes shortening of the hair cells. This is known as electromobility. If the outer hair cells are hyperpolarized, this would cause them to not shorten but rather lengthen or retain their original length
Explain why axonal diameter and the large synapses in the MNTB matter for the ILD circuit.
GBC fibers terminating laterally in the MNTB are larger in diameter and have an anomalous internode length to axon diameter ratio that is 35% smaller than the normal value of 100 found in medially terminating axons. This structural specialization, together with the larger diameter of laterally terminating GBC axons, increases speed by ~30% as compared to medially terminating GBC axons. These findings indicate specialized cellular adaptations for low frequency binaural processing,
What's the difference between the inner and outer hair cells in terms of their functions? Inner Hair Cell:
Inner hair cells transduce basilar membrane vibration into electrical activity. Basilar membrane vibration causes stereocilia on the surface of inner hair cells to bend. Tip links that connect adjacent stereocilia stretch, opening cation channels and allowing potassium ions to enter the cells, causing depolarization. Inner hair cells synapse with type I auditory nerve fibers, and depolarization of the inner hair cells increases the probability of action potential generation in these fibers. This provides the main route for transmission of information along the auditory nerve to the central auditory system.
Do you think it would matter if the owl is young or an adult?
Juvenile owls readily acquire alternative maps of auditory space as a result of experience, this plasticity is reduced greatly in adults.
What's the difference between the inner and outer hair cells in terms of their functions? Outer Hair Cell
Outer hair cells amplify basilar membrane motion. The amount of amplification is greatest at low input levels and at frequencies close to the characteristic frequency of the place on the basilar membrane where the hair cell is located. These properties mean that outer hair cells improve hearing sensitivity and lead to a compressive basilar membrane growth function, which is reflected in neural activity in the auditory nerve.
Humans use spectral filtering as a way to localize sound in the vertical plane. What is spectral filtering?
Parts of the human body (head, shoulders, outer ear) cancel out or enhance certain frequencies of an incoming sound depending on where the sound is coming from; this is known as spectral filtering.
What is tonotopy?
Systematic representation of frequency in cochlea/cortex, where certain locations are sensitive to/responsive to input of certain frequency
How is frequency of incoming sound translated to movement of the basilar membrane?
The basilar membrane varies in stiffness, and is stiffest nearest the oval window, and floppiest near the apex. Its motion is like a traveling wave, being the greatest at the point where the frequency of the incoming sound matches that of the movement of the membrane. In the cochlea, low frequency signals will induce oscillations that reach maximum displacements at the apex of the basilar membrane near the helicotrema, while high frequency signals induce oscillations that reach maximum displacement at the base of the basilar membrane near the oval window
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
The filamentous structures that connect tip links directly open cation-selective transduction channels when stretched, allowing K+ ions to flow into the cell. As the linked stereocilia pivot from side to side, the tension on the tip link varies, modulating the ionic flow and resulting in a graded receptor potential that follows the movements of the stereocilia. The tip link model also explains why only deflections along the axis of the hair bundle activate transduction channels, since tip links join adjacent stereocilia along the axis directed toward the tallest stereocilia.
The ossicles:
The malleus (hammer), incus (anvil) and stapes (stirrup) are the tiny bones that make up the ossicles. The stapes bone attaches to the oval window that connects the middle ear to the inner ear. The bones in the middle ear amplify the sound vibrations and send them to the cochlea.
The Organ of Corti:
The organ of Corti is a specialized sensory epithelium that allows for the transduction of sound vibrations into neural signals. It is located on the basilar membrane and contains two types of hair cells: inner hair cells and outer hair cells. Inner hair cells transduce sound from vibrations to neural signals via the shearing action of their stereocilia. Outer hair cells serve a function as acoustic pre-amplifiers which improve frequency selectivity by allowing the organ of Corti to become attuned to specific frequencies, like those of speech or music. The fibrous tectorial membrane rests on top of the stereocilia or the outer hair cells
Carefully review the anatomical organization of the outer, middle and inner ear.
The outer ear: Auricle -> External Auditory Canal The middle ear: Tympanum-> Ossicles-> Basilar Membrane The inner ear: Organ of Corti-> Cochlea
The oval window:
The oval window is a membrane covering the entrance to the cochlea in the inner ear. When the eardrum vibrates, the sound waves travel via the hammer and anvil to the stirrup and then on to the oval window.
The round window:
The round window in the middle ear vibrates in opposite phase to vibrations entering the inner ear through the oval window. In doing so, it allows fluid in the cochlea to move.
Why is it useful to have the mechanosensitive channels associated with the tip links be slightly open at rest?
When channels are slightly open, the hair cells can respond to a stimulus in either direction; hair cells only depolarize when the basilar membrane oscillation moves the stereocilia towards the tallest one; movement in the other direction hyperpolarizes them (and the hyperpolarization is made possible by the slightly open channels that lead to a slightly elevated resting potential). The sensitivity in both directions allows the electrical response to, in some sense, maintain the frequency of the input.
Explain how transduction works in the inner hair cells.
When the basilar membrane is displaced by specific frequency, it causes the inner hair cells to move and since they are mechanoreceptors, they become activated. The apical side of the inner hair cell has K+ channels that open and allow K+ inside which depolarizes it. the depolarization causes voltage gated calcium channels to open and vesicles to be released. The released of vesicles is able to release neurotransmitters that can cause action potentials in the spiral ganglion neuron.
