Yuste C7 Flashcards
Sound
Conveys info at a distance about objects and their locations. Series of compression and expansion of waves that move through the air at 300 m/s. Movement of molecules within. medium is determined by the spacing between the molecules. Media particles move in the same direction as the propagation of the wave, compressing them together then spreading them apart, rarefaction.
Characteristics of sound
Frequency, amplitude, phase.
Frequency – pitch
Amplitude – loudness
Phase – location of sound in space
Sinusoidal waves
Our auditory system decomposes sound waves into mixtures of simpler sinusoidal waves.
Impedance mismatch problem
Airborne sound should bounce off our bodies and never penetrate our ears. Our body compensates by amplifying the sound wave.
Solved by the 100 and 200 fold amplification created by the pinna and the 3 bones of the tympanic membrane.
Sensitivity of auditory system
Auditory system is sensitive over 12 orders of magnitude. Our hair cells can detect movements as small as the diameter of an individual atom. Right up to the physical limit of sound.
Division of mammalian ear
Outer, middle and inner ear.
Outer ear
The pinna and the ear canal.
The pinna
Shape is critical for sound localisation; captures sound differently at different angles. The sharp of the pinna funnels the sound to the ear canal and amplifies it by 100 fold.
Middle ear
Contains three bones – malleus, incus, and stapes.
Sound travels through the ear canal and hits the tympanic membrane in the middle ear. These bones act as levers, transmitting mechanical energy from the vibrating tympanic membrane, further amplifying the sound 200 fold.
Inner ear
Fluid filled; sound, previously moving through air (low impedance medium) now moving through liquid medium (higher impedance) which provides faster travelling speed but requires more energy.
The tiny bones of the middle ear press on the oval window in the cochlea – has a thin elastic membrane (that contains hair cells) that runs along its entire length.
Cochlea
Tonotopic organisation. High frequency waves vibrate the narrower base while lower frequency waves vibrate the wider tip. Its scale is logarithmic, can detect a broad range of frequencies.
Has three parallel chambers running all the way through it: Scala tympani, Scala media, Scala vestibuli.
Scala media
Filled with the endolymph that resembles the IC medium in ionic composition and bathes the top of special neurons called hair cells located on its bottom surface, the basilar membrane.
Scala tympani and scala vestibuli
Contain perilymph, rich in sodium and with little potassium.
Hair cells
Have stereo cilia, which are bundles of hair-like projections made from cross-linked actin and are arranged in ascending order of heigh; top of stereo cilia touch the tectorial membrane; when it vibrates due to incoming sound wave, they wiggling which results in hair cell’s depolarisation and subsequent transmission of e- signals to the brain.
Tip link
Each stereocilium is linked by a tip link to its neighbouring cilium. One of end of the tip link is attached to the side of the stereo cilium and the other is attached to the top of a channel on its neighbour. When the stereo cilium moves in one direction, it pulls the tip link taut and opens the channel, depolarising the cell; if it moves in the other direction, the tip link slackens and the channel closes.
Inner hair cells
Stereocilia are bathed in K+ rich endolymph, so when the tip link channels open, K+ comes in and depolarises the hair cell. Opens voltage sensitive Ca channels at the bottom of the hair cell, triggering transmitter release, depolarising the terminal of the afferent nerve (cochlear nerve).
Hair cells in the cochlea can respond to the movements their cilia of 3 Armstrongs.
Hair cells located in different parts of the cochlea also differ in electrical properties, so they match the frequency of the spot along the cochlea where they are located. Achieved by diffs in the densities and kinetics of K+ channels, with faster channels being expressed in hair cells transducing frequencies of sound.
Outer hair cells
Receive efferent axons from the brain; transfusing electrical signals into mechanical movements. Maintain the tension of the tectorial membrane and serve as mechanical levers that can push the membrane up or down.
=> brain actively modulating, amplifying or blocking particular auditory signals that the brain is interested in listening to.
Brain stem
Axons carry sound info as patterns of APs in the cochlear never to the brain; first to the cochlear nucleus and olivary body.
Cochlear nucleus
In the medulla. Has tonotopic maps of sounds.
Olivary body
Sits in the medulla of the brainstem. Helps localise sound in space. Tales advantage of the time and intensity differences in sound arrival to our ears. Interaural time difference is tiny; we detect up to 10msec, when we should only be able to detectt up to 600msec so 60x better.
Does this via two parallel sets of axons; one coming from left ear and from right. These contact the same set of postS neurons arranged from left to right in a line. Neurons that are closest to the left ear will be activated ahead of the neurons farthest away from left year. There will be some neurons in the olivary nucleus that will respond to just the right sets of complimentary delays; each neuron in this row in the olivary nucleus responds to sound sources located at a particular position in the horizon.
From olivary nucleus onwards
Most of the auditory pathway axons then connect to the inferior colliculus. Involved in building more refined spatial maps of sound info; superior colliculus involved in building visual maps of the world – object located in a particular post of the visual field stimulates a neuron of the visual pa in the superior colliculus that is connected to the neuron in the auditory map in the inferior colliculus that responds to sound emanating from that same position.
Auditory thalamus and cortex
Auditory pathway continues to the medial geniculate nucleus, and then to the 1º auditory cortex. In bats, Neurons in the auditory thalamus respond to frequency modulated sounds. Auditory thalamus likely builds receptive fields of increasing complexities, and also serves as an attention filter. Very strong efferent, top down pathway, reaching all the way down to the cochlea, whereas the visual pathway top down control ends in the thalamus.
Primary auditory cortex
In the temporal lobe, arranged in a tonotopic fashion with different frequencies mapped systematically.
Contains binaural maps for audition, with binaural or monaural cells, which keep track of which ear the sound is coming from. Maps for measuring loudness and other sound features likely too.
Sound processing
Given that sound is temporally modulated, a lot of processing in auditory cortex probably involves temporal sequences of stimuli. 1º auditory cortex connects tot he rest of the cortex in a similar way as the visual cortex, with where and what pathways.
