Human Anatomy and Physiology Tenth Edition Chapter 15 Part D The Special Senses Copyright © 2016 Pearson Education, Inc. All Rights Reserved

Hearing

15.8 Sound Detection Hearing is the reception of an air sound wave that is converted to a fluid wave that ultimately stimulates mechanosensitive cochlear hair cells that send impulses to the brain for interpretation Properties of Sound Sound is a pressure disturbance (alternating areas of high and low pressure) produced by a vibrating object and propagated by molecules of the medium (air)

Properties of Sound (1 of 6) Sound waves are created when an object moves: Air molecules that are displaced by object movement are pushed forward into adjacent area, adding to air molecules already there Creates an area of high pressure due to compression of molecules together As object returns to original position, the area it leaves now has fewer air molecules Creates an area of low pressure due to presence of fewer air molecules Referred to as rarefaction

Properties of Sound (2 of 6) Sound waves are alternating areas (waves) of compressions and rarefactions Object vibrating causes waves to move outward in all directions as air all around it is compressed and rarefied Kinetic energy of object is transferred to air molecules, which then transfer it to other air molecules Wave energy declines with time and distance

Properties of Sound (3 of 6) Illustrated as an S-shaped curve, or sine wave Compressions shown as crests, rarefactions as troughs

Figure 15.28a Sound: Source and Propagation

Figure 15.28b Sound: Source and Propagation

Properties of Sound (4 of 6) Sound can be described by two physical properties: frequency and amplitude Frequency Number of waves that pass given point in a given time Pure tone has repeating crests and troughs Wavelength Distance between two consecutive crests Shorter wavelength = higher frequency of sound Wavelength is consistent for a particular sound

Properties of Sound (5 of 6) ​​Frequency Frequency range of human hearing is 20–20,000 hertz (Hz = waves per second), but most sensitive between 1500 and 4000 Hz Pitch: perception of different frequencies Higher the frequency, higher the pitch Quality: characteristic of sounds Most sounds are mixtures of different frequencies Tone: one frequency (ex: tuning fork) Sound quality provides richness and complexity of sounds (music)

Figure 15.29a Frequency and Amplitude of Sound Waves

Properties of Sound (6 of 6) Amplitude Height of crests Amplitude perceived as loudness: subjective interpretation of sound intensity Measured in decibels (dB) Normal range is 0–120 decibels (dB) Normal conversation is around 50 dB Threshold of pain is 120 dB Severe hearing loss can occur with prolonged exposure above 90 dB Amplified rock music is 120 dB or more

Figure 15.29b Frequency and Amplitude of Sound Waves

Transmission of Sound to Internal Ear (1 of 2) Pathway of sound ​​Tympanic membrane: sound waves enter external acoustic meatus and strike tympanic membrane, causing it to vibrate The higher the intensity, the more vibration​ ​Auditory ossicles: transfer vibration of eardrum to oval window Tympanic membrane is about 20× larger than oval window, so vibration transferred to oval window is amplified about 20×

Transmission of Sound to Internal Ear (2 of 2) ​​Scala vestibuli: stapes rocks back and forth on oval window with each vibration, causing wave motions in perilymph Wave ends at round window, causing it to bulge outward into middle ear cavity 4a. Helicotrema path: waves with frequencies below threshold of hearing travel through helicotrema and scali tympani to round window 4b. Basilar membrane path: sounds in hearing range go through cochlear duct, vibrating basilar membrane at specific location, according to frequency of sound

Figure 15.30 Pathway of Sound Waves (1 of 5)

Figure 15.30 Pathway of Sound Waves (2 of 5)

Figure 15.30 Pathway of Sound Waves (3 of 5)

Figure 15.30 Pathway of Sound Waves (4 of 5)

Figure 15.30 Pathway of Sound Waves (5 of 5)

Resonance of the Basilar Membrane Resonance: movement of different areas of basilar membrane in response to a particular frequency Basilar membrane changes along its length: Fibers near oval window are short and stiff Resonate with high-frequency waves Fibers near cochlear apex are longer, floppier Resonate with lower-frequency waves So basilar membrane mechanically processes sound even before signals reach receptors

Figure 15.31-1 Basilar Membrane Function

Figure 15.31-2 Basilar Membrane Function

Sound Transduction (1 of 3) Excitation of inner hair cells Movement of basilar membrane deflects hairs of inner hair cells Cochlear hair cells have microvilli that contain many stereocilia (hairs) that bend at their base Longest hair cells are connected to shortest hair cells via tip links Tip links, when pulled on, open ion channels they are connected to Stereocilia project into K+-rich endolymph, with longest hairs enmeshed in gel-like tectorial membrane

Figure 15.27c Anatomy of the Cochlea

Sound Transduction (2 of 3) Excitation of inner hair cells Bending of stereocilia toward tallest ones pull on tip links, causing ion channels in shorter stereocilia to open flow into cell, causing receptor potential that can lead to release of neurotransmitter (glutamate) Can trigger AP in afferent neurons of cochlear nerve Bending of stereocilia toward shorter ones causes tip links to relax Ion channels close, leading to repolarization (and even hyperpolarization)

Figure 15.32 Bending of Stereocilia Opens or Closes Mechanically Gated Ion Channels in Hair Cells

Sound Transduction (3 of 3) Role of outer hair cells Nerve fibers coiled around hair cells of outer row are efferent neurons that convey messages from brain to ear Outer hair cells can contract and stretch, which changes stiffness of basilar membrane This ability serves two functions: Increase “fine-tuning” responsiveness of inner hair cells by amplifying motion of basilar membrane Protect inner hair cells from loud noises by decreasing motion of basilar membrane

15.9 Auditory Pathways to Brain (1 of 2) Neural impulses from cochlear bipolar cells reach auditory cortex via following pathway: Spiral ganglion → Cochlear nuclei (medulla) → Superior olivary nucleus (pons-medulla) → Lateral lemniscus (tract) → Inferior colliculus (midbrain auditory reflex center → Medial geniculate nucleus (thalamus) → Primary auditory cortex

15.9 Auditory Pathways to Brain (2 of 2) Some fibers cross over, some do not; so both auditory cortices receive input from both ears

Figure 15.33 The Auditory Pathway

Auditory Processing (1 of 2) Perception of pitch: impulses from hair cells in different positions along basilar membrane are interpreted by brain as specific pitches Detection of loudness is determined by brain as an increase in the number of action potentials (frequency) that result when hair cells experience larger deflections

Auditory Processing (2 of 2) Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears If timing is increased on one side, brain interprets sound as coming from that side

Equilibrium

15.10 Maintenance of Equilibrium Equilibrium is response to various movements of head that rely on input from inner ear, eyes, and stretch receptors Vestibular apparatus: equilibrium receptors in semicircular canals and vestibule Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium

Figure 15.24b Structure of the Ear

The Maculae (1 of 6) Maculae: sensory receptor organs that monitor static equilibrium One organ located in each saccule wall and one in each utricle wall Monitor the position of head in space Play a key role in control of posture Respond to linear acceleration forces, but not rotation

Figure 15.26 Membranous Labyrinth of the Internal Ear

The Maculae (2 of 6) Anatomy of a macula Each is a flat epithelium patch containing hair cells with supporting cells Hair cells have stereocilia and special “true stereocilium” called kinocilium Located next to tallest stereocilia Stereocilia are embedded in otolith membrane, jelly- like mass studded with otoliths (tiny CaCO3 stones) Otoliths increase membrane’s weight and increase its inertia (resistance to changes in motion)

The Maculae (3 of 6) Anatomy of a macula Utricle maculae are horizontal with vertical hairs Respond to change along a horizontal plane, such as tilting head Forward/backward movements stimulate utricle Saccule maculae are vertical with horizontal hairs Respond to change along a vertical plane Up/down movements stimulate saccule (Example: acceleration of an elevator)

The Maculae (4 of 6) Anatomy of a macula Hair cells synapse with fibers of vestibular nerve whose cell bodies are located in superior and inferior vestibular ganglia Part of vestibulocochlear cranial nerve (VIII)

Figure 15.34a Structure and Function of a Macula

The Maculae (5 of 6) Activating receptors of a macula Hair cells release neurotransmitter continuously Acceleration/deceleration causes a change in amount of neurotransmitter released Leads to change in AP frequency to brain Density of otolith membrane causes it to lag behind movement of hair cells when head changes positions Base of stereocilia moves at same rate as head, but tips embedded in otolith are pulled by lagging membrane, causing hair to bend Ion channels open, and depolarization occurs

The Maculae (6 of 6) Activating receptors of a macula Bending of hairs in direction of kinocilia: Depolarizes hair cells Increases amount of neurotransmitter release More impulses travel up vestibular nerve to brain Bending of hairs away from kinocilia: Hyperpolarizes receptors Less neurotransmitter released Reduces rate of impulse generation Thus brain is informed of changing position of head

Figure 15.34b Structure and Function of a Macula

The Cristae Ampullares (1 of 5) Receptor for rotational acceleration is crista ampullaris (crista) Small elevation in ampulla of each semicircular canal Cristae are excited by acceleration and deceleration of head Major stimuli are rotational (angular) movements, such as twirling of the body Semicircular canals are located in all three planes of space, so cristae can pick up on all rotational movements of head

The Cristae Ampullares (2 of 5) Anatomy of a crista ampullaris Each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula Dendrites of vestibular nerve fibers encircle base of hair cells Activating receptors of crista ampullaris Cristae respond to changes in velocity of rotational movements of head Inertia in ampullary cupula causes endolymph in semicircular ducts to move in direction opposite body’s rotation, causing hair cells to bend

Figure 15.35a Location, Structure, and Function of a Crista Ampullaris in the Internal Ear

Figure 15.35b Location, Structure, and Function of a Crista Ampullaris in the Internal Ear (1 of 2)

The Cristae Ampullares (3 of 5) Activating receptors of crista ampullaris Bending hairs in cristae causes depolarization Rapid impulses reach brain at faster rate Bending of hairs in opposite direction causes hyperpolarizations Fewer impulses reach brain Thus brain is informed of head rotations

The Cristae Ampullares (4 of 5) Activating receptors of crista ampullaris Axes of hair cells in complementary semicircular ducts are opposite Depolarization occurs in one ear, while hyperpolarization occurs in other ear Endolymph will come to rest after a while, so this system detects only changes in movements

Figure 15.35b Location, Structure, and Function of a Crista Ampullaris in the Internal Ear (2 of 2)

The Cristae Ampullares (5 of 5) Vestibular nystagmus Semicircular canal impulses are linked to reflex movements of eyes Nystagmus is strange eye movements during and immediately after rotation Often accompanied by vertigo As rotation begins, eyes drift in direction opposite to rotation; then CNS compensation causes rapid jump toward direction of rotation As rotation ends, eyes continue in direction of spin, then jerk rapidly in opposite direction

Equilibrium Pathway to the Brain Equilibrium information goes to reflex centers in brain stem Allows fast, reflexive responses to imbalance so we don’t fall down Impulses from activated vestibular receptors travel to either vestibular nuclei in brain stem or to cerebellum Three modes of input for balance and orientation: Vestibular receptors Visual receptors Somatic receptors

Figure 15.36 Neural Pathways of the Balance and Orientation System

Clinical – Homeostatic Imbalance 15.15 (1 of 2) Equilibrium problems are usually unpleasant and can cause nausea, dizziness, and loss of balance Nystagmus in the absence of rotational stimuli may be present

Clinical – Homeostatic Imbalance 15.15 (2 of 2) Motion sickness: sensory inputs are mismatched Visual input differs from equilibrium input Conflicting information causes motion sickness Warning signs are excess salivation, pallor, rapid deep breathing, profuse sweating Treatment with antimotion drugs that depress vestibular input, such as meclizine and scopolamine

15.11 Homeostatic Imbalances of Hearing (1 of 2) Deafness Conduction deafness Blocked sound conduction to fluids of internal ear Causes include impacted earwax, perforated eardrum, otitis media, otosclerosis of the ossicles Sensorineural deafness Damage to neural structures at any point from cochlear hair cells to auditory cortical cells Typically from gradual hair cell loss

15.11 Homeostatic Imbalances of Hearing (2 of 2) Sensorineural deafness research is under way to prod supporting cells to differentiate into hair cells Cochlear implants that convert sound energy into electrical signals are effective for congenital or age/noise cochlear damage Inserted into drilled recess in temporal bone So effective that deaf children can learn to speak

Figure 15.37 Boy with a Cochlear Implant

Tinnitus Ringing, buzzing, or clicking sound in ears in absence of auditory stimuli Due to cochlear nerve degeneration, inflammation of middle or internal ears, side effects of aspirin

Ménière’s Syndrome Labyrinth disorder that affects cochlea and semicircular canals Causes vertigo, nausea, and vomiting Treatment: anti–motion sickness drugs in mild cases or surgical removal of labyrinth in severe cases

Developmental Aspects of the Special Senses (1 of 7) Taste and Smell All special senses are functional at birth Chemical senses: few problems occur until fourth decade, when these senses begin to decline Odor and taste detection is poor after 65

Developmental Aspects of the Special Senses (2 of 7) Vision Optic vesicles protrude from diencephalon during week 4 of development Vesicles indent to form optic cups Stalks form optic nerves Later, lens forms from ectoderm Vision is not fully functional at birth; babies are hyperopic because eyes are shortened See only gray tones Eye movements are uncoordinated Tearless for about 2 weeks

Developmental Aspects of the Special Senses (3 of 7) Vision By 5 months of age, infants can follow objects, but acuity is still poor Depth perception and color vision develop by age 3 Adult eye size reached around 8–9 years of age Around year 40, lenses start to lose elasticity, resulting in presbyopia

Developmental Aspects of the Special Senses (4 of 7) Vision With age, lens loses clarity, dilator muscles are less efficient; visual acuity is drastically decreased by age 70 Lacrimal glands less active, so eyes are dry, more prone to infection

Developmental Aspects of the Special Senses (5 of 7) Hearing and Balance Ear development begins in 3-week embryo Inner ears develop from thickening of ectoderm called otic placodes, which invaginate into otic pit and otic vesicle Otic vesicle becomes membranous labyrinth, and surrounding mesenchyme becomes bony labyrinth

Developmental Aspects of the Special Senses (6 of 7) Hearing and Balance Middle ear structures develop from endodermal pharyngeal pouches, ossicles from neural crest cells, and pharyngeal cleft (branchial groove) develops into outer ear structures Newborns can hear, but early responses are reflexive in nature By month 4, infants can turn head toward voices of family members

Developmental Aspects of the Special Senses (7 of 7) Hearing and Balance Language skills tied to ability to hear well Few ear problems until 60s, when deterioration of spiral organ becomes noticeable Hair cell numbers decline with age Presbycusis: loss of high-pitch perception occurs first Type of sensorineural deafness

Clinical – Homeostatic Imbalance 15.16 Congenital problems of eyes are relatively uncommon, but incidence is increased by certain maternal infections Rubella (German measles) is dangerous, especially during critical first 3 months of pregnancy Common problems associated with rubella infections are blindness and cataracts

Clinical – Homeostatic Imbalance 15.17 Congenital abnormalities are common Missing pinnae, closed or absent external acoustic meatuses Maternal rubella causes sensorineural deafness