Hearing and Equilibrium PDF - University of Toronto

Hearing and Equilibrium PSL 300 University of Toronto  Question of the day Why did Beethoven have a bite bar on his piano?  Outline n Hearing n n Sound n Anatomy of the ear n n Cochlea n n Auditory Processing n n Hearing Loss n Equilibrium n n Sem...

Hearing and Equilibrium PSL 300 University of Toronto  Question of the day Why did Beethoven have a bite bar on his piano?  Outline n Hearing n n Sound n Anatomy of the ear n n Cochlea n n Auditory Processing n n Hearing Loss n Equilibrium n n Semicircular canals n Utricle and saccule  The ear is the organ of hearing and equilibrium Pinna Vestibular apparatus Canal equilibrium Eardrum Cochlea hearing Middle ear Eustachian tube n The external ear consists of the pinna and the ear canal, sealed at its end by the tympanic membrane, or eardrum. n n Beyond the eardrum is the middle ear, an air-filled space connec- ted to the pharynx by the Eustachian tube. n n The inner ear contains the sensors: cochlea for hearing and the vestibular apparatus for equilibrium.  Hearing Ultrasound is used to heal sports injuries. Cranked up, it can heat small spaces almost as hot as the sun. Acoustical levitation uses ultrasound as intense as a jet engine. Ultrasonic horns repel teenagers and scare dogs and deer off the roads. Pet collars, so high-pitched even cats and dogs can’t hear them, repel fleas. Crickets communicate ultrasonically; the chirping we hear from them is just a by- product. Different species have their own frequency bands, like radio stations. Frogs, and some snakes and lizards, hear through their lungs. Porpoises and dolphins may hear through an oil-filled lower jaw. Sperm whales and bottlenose dolphins may use sound as a weapon, emitting bangs that stun large prey and cause small fish to hemorrhage internally. The deaf can enjoy music: Helen Keller wrote about holding a radio to feel a concert.  Sound is pressure waves n At the peaks of the waves, the molecules are crowded together and the pressure is high; at the troughs the molecules are far apart and the pressure is low.  Frequency is the number of wave peaks per second Pressure Time Middle-aged hear up to ~14 kHz extuba n We perceive frequency as pitch: low frequencies as low-pitched sounds, high frequencies as high-pitched. expiccolo n n Frequency is measured in waves per second, i.e. in hertz (Hz). n strink over years. ~ He wage dose n Humans hear sounds in the range 16–20,000 Hz — ~10 octaves. Acuity is highest 1000–3000 Hz. Some bats and dolphins can hear 200 kHz. Elephants and crocodiles hear infrasonics. Middle C is ~261.6 Hz, adult male voices ~100 Hz, female ~150 Hz.  Amplitude is the pressure difference between peak and trough Wavelength } } Pressure Amplitude Time n Amplitude is the main factor that determines our perception of loudness: the larger the amplitude, the louder the sound (for any one sound frequency). n n Loudness depends on frequency as well, e.g. a sound of 30,000 Hz is beyond the range of human hearing, so it won’t be loud no matter how large its amplitude. Powerful ultrasonics don’t damage hearing unless they make objects resonate in the audible range.  Sound waves vibrate the eardrum Pinna Canal Eardrum Middle ear 7 : 26. - n The eardrum separates the outer ear from the middle ear.  & A chain of small bones conveys vibrations through the middle ear Incus Malleus Stapes Oval window Eardrum n The eardrum vibrates the malleus (hammer) bone, which moves the incus (anvil), which moves the stapes (stirrup), which pushes like a piston against the oval window, a membrane between middle and inner ear. n n These 3 bones, called the ossicles, are the smallest in the body. They act as a lever system carrying vibrations from the eardrum to the much smaller oval window. The oval window leads into the cochlea, which contains the receptor cells Oval window Cochlea Cochlea  Uncoiling the cochlea makes its anatomy clearer Saccule Vestibular duct Oval Cochlear duct window Round window Helicotrema Tympanic duct n The vestibular duct (or scala vestibuli) and tympanic duct (scala tympani) contain perilymph (a fluid similar to plasma). These 2 ducts communicate at the helicotrema. n n The cochlear duct (scala media) contains endolymph (similar to intracellular fluid).  The oval window vibrates, setting up waves in the perilymph Auditory nerve Round Perilymph window Cochlear duct n Wave energy enters the cochlea at the oval window and exits, eventually, back into the middle ear through another membrane called the round window. n n En route, the waves shake the cochlear duct, which contains the auditory receptor cells (hair cells), though to see those cells we have to zoom in... A cross section shows that the cochlear duct contains the organ of Corti Organ of Corti Vestibular duct Tympanic duct  The organ of Corti sits on the basilar membrane and under the tectorial membrane Tectorial membrane Hair cell Fibers of Basilar auditory membrane nerve n The organ of Corti contains the auditory receptor cells — mech- anoreceptors called hair cells. They are epithelial cells, not neurons, and number ~20,000 per cochlea. n n Each hair cell has 50–100 stiff “hairs” called stereocilia, which extend into the tectorial membrane. They bend when waves in the perilymph deform the basilar and tectorial membranes.  When its cilia bend toward the longest cilium, the hair cell excites its neuron Hair cell Neuron n The hair cell depolarizes and releases transmitter, activating a primary sensory neuron. n n Axons of these neurons form the auditory nerve (also called the cochlear nerve), a branch of cranial nerve VIII.  When its cilia bend away, the hair cell releases less transmitter Hair cell Neuron n The hair cell hyperpolarizes, so it releases less transmitter and doesn’t excite its neuron as much. The basilar membrane responds to different frequencies at different points High Low frequencies frequencies Stiff near Basilar Flexible near oval window membrane helicotrema n The membrane is narrow and stiff near the round and oval win- dows, wider and more flexible at its other end. n n High-frequency waves maximally displace the membrane at the oval-window end; low-frequency waves maximally displace the other end. So the brain can deduce the frequency by noting which hair cells are most active. The pattern of membrane motion reveals pitch to the brain 3 100 Hz Membrane motion (:m) 0 3 400 Hz 0 3 1600 Hz 0 0 10 20 30 Distance from oval window (mm) Auditory Processing  Auditory signals pass from each ear to both sides of the brain Auditory cortex Thalamus Medial geniculate nucleus Midbrain To cerebellum Cochlear nuclei Auditory nerve in medulla These nuclei are tonotopic, i.e. neighboring cells in them prefer similar frequencies. they project to the inferior colliculus (IC), directly and via the superior olive (SO). Primary auditory cortex (A1) is in the temporal lobe A1 mcb.berkeley.edu  The brain localizes sounds based on loudness and timing n If a sound is louder in the right ear than in the left then it is coming from the right side of the head. Loudness is conveyed by firing frequency, i.e. louder sounds make auditory sensory neurons fire at a faster rate. n n If the sound reaches the right ear before the left then it is coming from the right side of the head. Hearing Loss  There are 3 kinds of hearing loss n In conductive hearing loss, sound can't be transmitted through the external or middle ear. n n In sensorineural hearing loss, there is damage to the hair cells or elsewhere in the inner ear. Mammals can't replace dead hair cells, though birds can. 90% of hearing loss in the elderly (presbycusis) is sensorineural. n n In central hearing loss, there is damage to the cortex or the path- ways from cochlea to cortex. Typically the patient’s trouble is in recognizing and interpreting sounds, rather than in detecting them.  Clinical tests distinguish conductive from sensorineural loss n In the Rinne test you hold a tuning fork against the mastoid bone and then beside the ear, and ask when the sound is louder. Normally it is louder through the ear canal. If it is louder through the bone, there is conductive loss. n n In the Weber test you hold the tuning fork to the patient’s forehead, in the midline, and ask in which ear the sound is louder. With sensorineural loss, it is louder in the good ear. With conductive loss, it is louder in the bad ear, because it doesn't have to compete with sounds heard through that ear canal. Remember finger-in-ear test. Heinrich Adolf Rinne (1819--1868) was not a Frenchman; he was German. So was Ernst Heinrich Weber (1795--1878). Equilibrium  Different parts of the vestibular apparatus sense head position and motion Semicircular canals: Utricle Superior Saccule Posterior Horizontal n The utricle and saccule contain hair cells that are activated when the head tilts relative to gravity. n n The semicircular canals are fluid-filled hoops that detect head rotation, e.g. when your head turns rightward, the fluid in the tubes sloshes leftward, activating hair cells.  Equilibrium pathways project mainly to the cerebellum n Vestibular hair cells activate primary sensory neurons of the vesti- bular nerve, which is a branch of cranial nerve VIII. n n These neurons may either pass directly to cerebellum or synapse in the medulla, whence they proceed to the cerebellum or up through thalamus to cortex. n n Your brain uses vestibular information to infer your own position and motion, and keep you upright. Reading in Silverthorn’s Human Physiology n 8th edition: Sections 10.4 “The Ear: Hearing” and 10.5 “The Ear: Equilibrium” (pages 328–337), and “Location of the Stimulus” (page 313). n 7th edition: “The Ear: Hearing” and “The Ear: Equilibrium” (pages 329–340), and “Location of the Stimulus” (page 315). n 6th edition: “The Ear: Hearing” and “The Ear: Equilibrium” (pages 346–356) and Figure 10.23, and “Location of the Stimulus” (pages 331–332).