Auditory System Lecture 2 PDF

Auditory System Biol2051/52 – Lecture 2 James Dillon ([email protected])  Lecture structure & Learning Outcomes Mechanotransduction Auditory system – Anatomy of the ear – Molecular, cellular and structural features underlying hearing – Integration and encoding of auditory information by the...

Auditory System Biol2051/52 – Lecture 2 James Dillon ([email protected])  Lecture structure & Learning Outcomes Mechanotransduction Auditory system – Anatomy of the ear – Molecular, cellular and structural features underlying hearing – Integration and encoding of auditory information by the brainstem Learning outcomes: Be able to describe the specialized sense organs involved in hearing at a molecular and cellular level Be able to describe the circuitry that processes this type of sensory information in the CNS Hearing: the ability to detect and interpret sound Sound is generated by the movement of air molecules Atmospheric pressure  Properties of sound Sound is characterised by its PITCH(tone), INTENSITY (loudness) Tone -> frequency of the sound wave (Humans 20- 20,000Hz, Bats 20-200KHz) Loudness -> amplitude  The Human Ear OUTER EAR: MIDDLE EAR: Amplifies sound Important for pressure for amplifying the sound frequencies 2- wave 5KhZ Localization of a sound source (elevation) hree regions: outer, middle and inner ear pecialised sensory cells (hair cells) are located within the inner ear  Signal Amplification The sound wave is moving from air, which has a low impedance (low resistance) to an aqueous environment in the inner ear which has a high impedance (high resistance) OUTER INNER OUTER MIDDLE INNER AIR FLUID AIR AMPLIFY FLUID (Cochlea) (Cochlea) 1. Focuses the force of large tympanic membrane down onto the much smaller oval window 2. Lever action of the malleus and incus Structural Features of the Inner Ear Endolymph Organ of corti Perilymph 16,000 hair cells 4,000 inner 12,000 outer  Topographical Mapping of the Cochlea Basilar membrane: tapered structure – narrow at one end and gets progressively wider along its length Tonotopic organisation of frequency in the basilar membrane allows us to distinguish between different frequencies/pitches Spectral analysis  Sound induced vibration activates hair cells How does this vibrational energy in the basilar membrane get transduced into an electrical signal? Shear force The mechanical movement of specialised sensory cells: hair cells Shear force Fluid movements in the cochlea are caused by pressure waves generated by the incoming sound wave cause the basilar membrane to undulate up and down  The Hair Cell Apical Surface Basolateral Surface Outer Hair Inner Hair Cells have efferent inputs Cell has afferent inputs  The Hair Cell: Outer and Inner Hair cells Inner hair cells send signals back to the CNS Outer hair cells receive signals from the CNS Hair Cells have TIP LINKS  Distortion of Stereocilia Opens Ion Channels http://147.162.36.50/cochlea/cochleapages/theory/hcells/hcells.htm  Distortion of Stereocilia Opens Ion Channels http://147.162.36.50/cochlea/cochleapages/theory/hcells/hcells.htm  Ionic basis of hair cell activity Endocochlear potential: stereo cilia are bathed in endolymph (high K+); the base is bathed in perilymph (low K+) Endolymph: high K+ +80mV -45mV Tonic release Perilymph: Low K+ at rest AT REST DEPOLARISATION HYPERPOLARISATION Both depolarisation and hyperpolarisation K+ dependent = Biphasic response This organisation means that the movement of the stereocilia of the hair cells will create a graded response (= generator potential)  Encoding properties of the sound wave atterns of electrical activity = ncoding properties of the sound ave  Labelled Line Coding The tonotopic organization of the basilar membrane is an example of labelled line coding A single neurone/nerve fiber responds maximally to a very specific stimulus Outer hair cells (oHC) 2 types of hair cells (inner – outer) Inner = sensory receptors (transduction) oHC contract and expand in response to electrical currents (active process) Electromotility Depolarisation = Contraction Hyperpolarisation = Relaxation Amplifies motion of basilar membrane enhances the responsiveness of inner hair cells Outer hair cells (oHC) https://www.youtube.com/ watch?v=wKEhjpqpzCE Outer hair cells (oHC): Mechanism of signal amplification  The major auditory pathways – Signal Integration Tonotopic Map -> Auditory nerve cells are bipolar -> Cell bodies of auditory nerves are in the spiral ganglion -> information of each ear reaches both sides of the system at the superior olive Integration How is sound localization encoded? Lloyd Jefress (1948) suggested that neural circuits could encode short time intervals by acting as coincidence detectors i.e. they only respond when two or more signals occur simultaneously – or coincidentally. In example (A) he showed that there was little or no time delay for a sound to arrive from the left side to the left ear, but that the same sound occurred at the right ear with a finite time delay – in this case 30μs. Due to this delay to the right ear there is no coincidence of the left and right input pathways on the common output neuron (C) which shows 2 distinct small responses. The model proposed by Jeffress incorporated a time delay line on the left side that added approx. 30μs to the left pathway. Now the sounds arrive at the coincidence detector C at exactly the same time. This  Integration and Encoding Time Differences The Jeffress model was further developed to incorporate an anatomical method of producing a time delay. Consider neurons converging from the left and right sides. Nerve impulses take a finite time to conduct along a nerve so that by increasing the length of a neural path you can generate a delay. In this model five neurons are arranged in an array and each is a coincidence detector. Neurons from the left and right sides connect with the coincidence detectors sequentially from opposite end of the  Take a sound source in front of the animal. Sound will reach the left and right ears at the same time and the lengths of neural pathways to the left and right will be the same so the neural response will arrive at the coincidence detector at the same time to give a response. If the sound is to the right, the sound arrives at the right ear first, so a longer neural pathway is introduced to the right so that the signal arrives at the coincidence detector simultaneously with the signal from the left side This arrangement means each detector is tuned to a different time delay. sound information is converted to a place code. Each neuron has a specific unique location in the network and the population as a  Anatomical Location Where does this take place? MSO: Medial Superior Olive  Summary - Hearing  The human ear has 3 key compartments: outer, middle and inner ear  The specialised sensor is the HAIR CELL located in the inner ear  Mechanical activation of the tip links opens K+ channels which depolarise the hair cell and triggers neurotransmitter release (Transduction)  Specific movements of the stereocilia and the activation of different populations of hair cells can create different patterns of electrical activity and signalling (Encoding)  Inner hair cells conduct signals back to the brainstem and outer hair cells amplify signals  Auditory circuits are organized for the integration and