Nervous System Physiology Special Senses By Dr. SHAHAB SHAIKH PhD MD MBBS Lecture: Physiology of Hearing •••••••••••••••••••••••••••••••••• Faculty of Medicine Al Maarefa Colleges of Science & Technology

Introduction Hearing, Auditory Perception, or Audition is the ability to perceive sound by detecting vibrations or changes in the pressure of the surrounding medium through the ear. Hearing is the neural perception of sound energy. Hearing mainly involves two aspects: the identification of the sounds (what) and their localization (where). Hearing is performed primarily by the auditory system: mechanical waves, known as vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain.

Sound Sounds are produced when vibrating objects, such as the plucked tuning fork or guitar strings, produce pressure pulses of vibrating air molecules, better known as sound waves.  Sound waves are often simplified as plane waves, which are characterized by these generic properties: Frequency, Amplitude Direction Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. Humans can detect the difference between two sounds occurring 10micro seconds apart in time

Audible Sound Range

Sound Generally loudness of sound is correlated with the amplitude of sound wave (subjective interpretation of the intensity of a sound) Decibel: logarithmic scale to measure the intensity of sound waves human amplitude range - 0 dB - 120 dB avg sound intensity of human speech is 65 dB Greater than 100 dB can cause damage to auditory apparatus The pitch is correlated with the frequency ( no of waves or cycles per second / Hz) The sound frequencies audible in humans - 20 Hz to 20000 Hz ,with greatest sensitivity from 1000 to 4000 Hz Normal pitch: Male – 120 Hz Female - 250 HZ Pure tone result from sinusoidal waves of single frequency Most of the sound are mixture of pure tones Sound waves which have repeating pattern even though individual waves are complex are perceived as musical sound Pure tone sound is sound wave with single frequency Most of the sound are mixture of pure tones

Sound Terminologies Amplitude/loudness Frequency/Pitch/Tone Impedence Strength of the sound Loudness denotes the appreciation of sound intensity Expressed in decibel (dB) Amplitude/loudness Number of cycles per second Pitch /Tone denotes the appreciation of frequency Expressed in Hertz(Hz) Frequency/Pitch/Tone Resistance offered by a medium to sound waves Impedence Resonance is a phenomenon that occurs when a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency. Resonance Attenuation is a general term that refers to any reduction in the strength of a signal. Attenuation

Functional Anatomy External (Outer) ear: Middle ear.: Inner ear Auricle External acoustic meatus Tympanic membrane Ceruminous glands Middle ear.: Tympanic cavity Osicles Malleus (hammer) Incus (anvil) Staples (stirrup). Eustachian tube. Inner ear Bony and membranous labyrinth Cochlea: hearing Vestible: equilibrium 3 perpendicular semicircular canals: equilibrium

Sound Transmission In the human ear, a sound energy is transmitted through four separate mediums along the auditory system before a sound is perceived: in the outer ear—air, in the middle ear— mechanical, in the inner ear liquid and to the brain—Electrical. SOUNDENERGY MECHANICAL ENERGY ELECTRICAL ENERGY

Sound Transmission

Sound Transmission External Ear: Air transmitted sound waves are directed toward the Tympanic Membrane Functions of External Air: Sound collection Increasing pressure on the tympanic membrane in a frequency sensitive way. Sound localization

Sound Transmission Functions: Middle Ear: When sound wave strikes the tympanic membrane (also called EAR DRUM), the tympanic membrane moves. Motion of the eardrum sets the ossicular chain i.e. Malleus, Incus & Stapes into motion. Functions: Maintains resting air pressure on both sides of TM equal Transfers movements of the tympanic membrane to the inner ear Impedance matching Attenuation

Impedance Matching The Impedance (resistance) for the sound waves to travel in air and fluid is different. The middle ear plays role in matching this impedance as follows: The amplitude of movement of the stapes footplate with sound vibration is only three fourth as much as amplitude of the handle of the malleus This increases the force of movement by 1.3 times The surface area of tympanic membrane is about 55 square mm.& that of stapes is 3.2 sq mm This 17 fold difference times the 1.3 fold ratio of the lever system causes about 22 times as much total force exerted on the fluid of the cochlea

Attenuation Reflex Function: When loud sounds are transmitted through the ossicular system to the central nervous system, a reflex occurs to cause contraction of the stapedius muscle and the tensor tympani muscle. This cause the entire ossicular system to develop increased rigidity, thus greatly reducing the sound conduction This reflex occurs after a latent period of about 40 to 80 ms. Function: To protect the cochlea from damaging vibrations caused by excessive sound To mask background noise in loud environments.

Sound Transmission Inner Ear: The ossicular chain transfers energy from air of external ear to the fluid medium of the inner ear via the stapes attached to the oval window. Movement of the oval window creates motion in the cochlear fluid and along the Basilar membrane. Motion along the basilar membrane excites frequency specific areas of the Organ of Corti, which in turn stimulates a series of nerve endings. Nerve impulses are relayed through the VIII C.N., through various nuclei along the auditory pathway to areas to the brain. It is the brain that interprets the neural impulses.

Inner Ear Organ of Corti

Organ of Corti

Organ of Corti Organ of corti has specialized type of nerve cells called hair cells Single row of inner hair cells numbering about 3500 and 3 to 4 rows of outer hair cells numbering about 12,000 90-95% of cochlear nerve ending synapse on inner hair cells

Sound Transmission Basilar membrane motion causes depolarization of the hair cells. While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials.

Sensory Transduction Kinocilia - Stereocilia are Linked Displacement Opens K+ Channels Depolarization → release of glutamate K+ flows through cell Glutamate → increase spike rate in auditory nerve

Inner Ear Function Thus the inner ear has 2 main functions: Sensory transduction: pressure waves are transformed into neural impulse. Frequency analysis – Place Principle

Resonance of the basilar membrane Stapes Scala vestibuli Cochlear nerve Oval window Perilymph Round window Scala tympani Basilar membrane Cochlear duct (a) Base Apex Basilar membrane 500 Hz Relative lengths of basilar fibers within different regions membrane 4000 Hz Hz 20,000 (High notes) Hz 1500 Hz 500 Hz 20 (Low notes) 24,000 Hz (b) (c)

Place Principle The method used by nervous system to detect different sound frequencies is to determine the position along the basilar membrane that are most stimulated. This is called the “place principle

Hair Cell Innervation Hair cells are the sensory receptors of both the auditory system and the vestibular system They derive their name from the tufts of stereocilia that protrude from the apical surface of the cell, largest one being called the Kinocilium Nerve fiber innervation is much denser for inner hair cells than for outer hair cells. A single inner hair cell is innervated by numerous nerve fibers, whereas a single nerve fiber innervates many outer hair cells.

Function of Inner & Outer Hair cells Inner Hair Cells: They transform the mechanical forces of sound into electrical impulses. Outer Hair cell: Increases the sensitivity of inner hair cells for different frequency and intensity.

From there it passes to medial geniculate nucleus Auditory Pathway Auditory cortex From there it passes to medial geniculate nucleus Pathways passes through lateral lemniscus but many bypass this and travel to inf. colliculus Second order neuron from here cross to opposite side of brain stem and terminate in sup olivary nucleus Fibers from the spiral ganglion enters ventral cochlear nuclei located in the upper part of medulla

Peculiarities of auditory pathway Signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway Many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem A high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex

Perceiving Pitch (Frequency): Appreciation Of Loudness Of Sound: Sound Ananlysis Perceiving Pitch (Frequency): location of vibration on the basilar membrane Appreciation Of Loudness Of Sound: Intensity or loudness of sound correlates with two factors: Rate of discharge from the individual fibers of auditory nerve Total number of nerve fibers discharging.

Sound Ananlysis Localization Of Sound Sound localization is the ability to detect the source from where sound is produced. Source of sound is Localized by comparing sounds from both ear for: relative intensity - the amplitude of sound waves hitting the different ears relative timing - the difference in timing in which a sound reaches both ears

Pathophysiology of hearing Conduction deafness That caused by impairment of the physical structures of the ear that conduct sound to cochlea like outer ear and middle ear. Sensorineural deafness It is the deafness caused from impairment of the Hair cells, auditory nerve etc.

References Human physiology by Lauralee Sherwood, 9th edition Text Book Of Physiology by Guyton & Hall, 12th edition

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