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Hearing
Uni of Notts, fundamentals of neuroscience, first year
| Term | Definition |
|---|---|
| How sound is generated | Compression & rarefaction (reducing density) of molecules in the air. The molecules aren't necessarily moving, rather it's the wave of air pressure that moves |
| Waveform of sound | Comprised of factors such as peaks & troughs; compression forms a peak, rarefaction forms a trough. Frequency is number of waves past a point per second, amplitude is difference in pressure between peak & trough |
| Difference between frequency & pitch | Frequency is a quantifiable physical characteristic of a sound, pitch is the subjective sensation from detecting a particular sound |
| Different measurements of sound (2) | Pascal - pressure exerted by a sound wave over a given area, ranging from 20-200,000 Decibel - Logarithmic scale measuring sound intensity by comparing it a reference, ranging from 0 (subthreshold) to 140 (pain threshold) |
| Anatomy of the outer ear | Contains pinna (folded cartilage) surrounded by the helix to filter sound with the flange, concha, & meatus funnelling different sound frequencies to the tympanic membrane at the temporal bone of the skull |
| Anatomy of the middle ear | Tympanic membrane (eardrum) has large surface area connecting to ossicle bones (malleus, incus, stapes) with small surface areas to amplify the force & strike the oval window to pass pressure to the inner ear |
| Anatomy of the inner ear | In bony cavity of temporal bone called the bony labyrinth. The cochlea is spiralised tubes filled with endolymph & microscopic hairs that detect sound as vibrations in the liquid. This sends an action potential through cranial nerve XIII to the brain |
| Organ of Corti | Sensory hearing organ in the cochlea containing hair cells which amplify sound vibrations & convert them into electrical signals by bending open ion channels |
| Anatomy of the Scala media | Cochlear duct between a fluid filled chamber above (scala vestibuli) & below (scala tympani) with a basilar membrane below containing organs of Corti & tectorial membrane above to stimulate them |
| How the Scala media transduces sound signals | Sound waves cause pressure waves in perilymph, basilar membrane moves up & down, shearing force between basilar & tectorial membranes bends hair cells causing action potential generation |
| Oval window | Membrane covered opening at the entrance, attached to the stapes which pushes on it from sound energy causing pressure waves in the perilymph of the Scala Vesibuli |
| Round window | Membrane covered opening at the exit, since the cochlear fluid is incompressible the window bulges out, relieving pressure & allowing the fluid to move from the Scala tympani |
| Frequency tuning | Ability to detect differences in frequency to send different signals to the CNS, different frequencies cause different sections of the basilar membrane to vibrate, higher at the base close to the oval window & lower near the apex |
| Travelling wave theory | Frequencies of sound waves create mechanical waves in the perilymph that travel along the basilar membrane & peak at a point along the membrane matching its frequency |
| Helicotrema | opening near the apex of the cochlea connecting the Scala Vesibuli & Scala Tympani allowing flow of perilymph to decrease pressure & aid lower frequency detection |
| Signals formed from bending of hair cells | If cells bend toward the apex, stretch-mediated VGSCs in stereocilia open causing an action potential propagated to bipolar cells by VGCCs. If they bend towards the windows then they hyperpolarise |
| How hair cells are bent | Vibrations through the Scala Media & basilar membrane move hair cells against the tectorial membrane causing stereocilia to open using crosslinks |
| Stereocilia | Flexible actin-containing projections on the tips of hair cells in increasing height order which detect vibrations to transduce the signal |
| Tip links | Fine protein bridges connecting the tips of each stereocilium to their adjacent & mechanically distort them in response to vibration |
| Lateral link connectors | Additional protein bridges connecting adjacent stereocilia at the top, shaft, & ankle to hold the bundles together during movement & allow them to maintain staircase conformation |
| Type 1 fibres | Myelinated axons connecting to 1 inner hair cell to carry majority of sound information to auditory nerves such as pitch & volume. 90% are type 1 |
| Type 2 fibres | Unmyelinated axons of spiral ganglion neurones connecting to over 20 outer hair cells used for amplification & feedback as well as nociception & cochlear protection. 10% are type 2 |
| Inner hair cells (IHCs) | Pear-shaped slanting afferent (sensory) cells arranged in 1 row along the cochlea which transduce vibrations using stereocilia to transfer information to the brain |
| Outer hair cells (OHCs) | Cylindrical upright efferent (motor) cells arranged in 3 or 4 rows to amplify & finetune vibrations before they reach inner hair cells while doing nociception. Use voltage sensitive prestin motor proteins in cochlear amplification |
| Cochlear amplification | Stereocilia on OCHs detect quiet or low pitch stimuli & elongate using prestin to increase vibration magnitude by up to 50db so IHCs can detect them but switches off when sound gets louder |
| Specific chemical properties of endolymph | Very high K+ & low Na+ making it one of the post positive fluids in the body. Doubles amplification of action potentials & uses K+ to depolarise rather than Na+. All sound (including cochlear amplification) would be halved without this |
Created by:
Beech47
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