1. Senses

2. General Function of Sensory Receptors
Provide information about external and internal environments
Respond to a stimulus
Each type of receptor responds best to a type of stimulus
E.g., light energy for eye receptors; sound energy for ear receptors
Transducers—convert stimulus energy into electrical energy
Receptors have a resting membrane potential
Receptor membranes have modality gated channels that respond to their type of stimulus
Action potentials are conveyed to CNS for interpretation

3. General Structure of Sensory Receptors
Receptors convey signals to CNS by sensory neurons
Receptive field—the distribution area of the endings of a sensory neuron
Smaller receptive fields allow more precise stimulus localization
Figure 16.1

4. Sensory Information Provided by Sensory Receptors
Sensation—a stimulus we are consciously aware of
To enter consciousness, signals must reach cerebral cortex
Only a fraction of stimuli result in sensations
A lot of sensory input goes to other areas of the brain
E.g., blood pressure signals relayed to brainstem where responses are initiated without conscious awareness
Receptors provide CNS information about stimulus modality, location, intensity, and duration

5. Sensory Information Provided by Sensory Receptors
Modality—type of stimulus based on “labeled line”
E.g., brain interprets optic nerve signals to occipital lobe as visual, cochlear nerve signals as to temporal lobe as auditory
Location of stimulus determined by which receptive field is active
Postcentral gyrus has body map represented by homunculus
Intensity of stimulus determined by frequency of nerve signals to CNS
Stronger stimuli cause more neurons to fire and cause sensitive neurons to fire more frequently

6. Sensory Information Provided by Sensory Receptors
Receptor adaptation helps determine stimulus duration
Adaptation—decreased sensitivity to continuous stimulus
Tonic receptors show limited adaptation: respond continuously
E.g., head position receptors in inner ear; all pain receptors
Phasic receptors adapt rapidly: only respond to new stimuli
E.g., pressure receptors

7. Sensory Receptor Classification
Categorized by distribution, stimulus origin, and stimulus modality
Categorization by receptor distribution
General sense receptors
Simple structures distributed throughout the body
Somatic sensory receptors: tactile receptors of skin and mucous membranes; proprioceptors of joints, muscles, and tendons
Visceral sensory receptors: found in walls of internal organs, they monitor stretch, chemical environment, temperature, pain
Special sense receptors
Specialized receptors in complex sense organs of the head
5 special senses: olfaction, gustation, vision, audition, equilibrium

8. Sensory Receptor Classification
Categorization by stimulus origin
Exteroceptors detect stimuli from external environment
Skin and mucus membranes; special sense receptors
Interoceptors detect stimuli from internal organs
Visceral sensory receptors monitoring internal environment
Proprioceptors detect body and limb movements
Somatosensory receptors of muscles, tendons, and joints

9. Sensory Receptor Classification
Categorization by modality of stimulus
Five types: chemoreceptors, thermoreceptors, photoreceptors, mechanoreceptors, and nociceptors
Chemoreceptors detect chemicals dissolved in fluid
Include receptors for external environment (e.g., smell of food) or internal environment (e.g., oxygen levels in blood)
Thermoreceptors detect changes in temperature
Include receptors in skin, hypothalamus
Photoreceptors detect changes in light intensity, color, movement
In the retina of the eye

10. Sensory Receptor Classification
Categorization by modality of stimulus (continued)
Mechanoreceptors detect distortion of cell membrane
Include touch, pressure, vibration, and stretch receptors
Function as baroreceptors, proprioceptors, tactile receptors, and specialized receptors in the inner ear
Nociceptors detect painful stimuli
Somatic nociceptors detect chemical, heat or mechanical damage to the body surface or skeletal muscles
Visceral nociceptors detect internal organ damage

11. Tactile Receptors
Abundant mechanoreceptors of skin and mucous membranes
Endings can be encapsulated or unencapsulated
Figure 16.2

12. Tactile Receptors Unencapsulated tactile receptors
Dendritic ends of sensory neurons with no protective cover
Free nerve endings—terminal ends of sensory neuron dendrites
Simplest tactile receptors
Reside close to skin surface and in mucous membranes
Mainly for pain and temperature but also light touch and pressure
May be phasic or tonic
Root hair plexuses—wrap around hair follicle
Located in deeper layer of dermis
Detect hair displacement
Phasic receptors

13. Tactile Receptors Unencapsulated tactile receptors (continued)
Tactile discs—flattened endings of sensory neurons extending to tactile cells (Merkel cells)
Tactile cells are specialized epithelial cells in basal layer of epidermis
Respond to light touch
Tonic receptors

14. Unencapsulated Tactile Receptors
Figure 16.2, top

15. Tactile Receptors Encapsulated tactile receptors
Neuron endings wrapped by connective tissue or covered by connective tissue and glial cells (neurolemmocytes)
End (Krause) bulbs are ensheathed in connective tissue
Located in dermis and mucus membranes
Detect pressure and low-frequency vibration
Tonic receptors
Lamellated (Pacinian) corpuscles are wrapped in neurolemmocytes and concentric layers of connective tissue
Located deep in dermis, hypodermis, some organ walls
Detect deep pressure, course touch, high-frequency vibration
Phasic receptors

16. Tactile Receptors Encapsulated tactile receptors (continued)
Bulbous (Ruffini) corpuscles are wrapped in CT
Within dermis and subcutaneous layer
Detect deep pressure and skin distortion
Tonic receptors
Tactile (Meissner) corpuscles are intertwined endings wrapped in modified neurolemmocytes, covered in connective tissue
In dermal papillae (especially in sensitive regions of the body)
Discriminative light touch—allow recognition of texture, shape
Phasic receptors

17. Referred Pain Referred pain—inaccurate localization of sensory signals
Signals from viscera perceived as originating from skin, muscle
Many somatic and visceral sensory neurons send signals via the same ascending tracts within spinal cord
Somatosensory cortex unable to determine true source
Figure 16.3

18. Referred Pain Clinical relevance of referred pain
Heart attack pain may be referred to pectoral region, medial arm
Sympathetic innervation of heart and somatic innervation of those skin regions both come from T1–T5 segments of spinal cord
Kidney and ureter pain may be referred to inferior abdomen
T10–L2 spinal nerves
Visceral pain is often conveyed along sympathetic nerves, but occasionally on parasympathetic nerves
Bladder pain can be conveyed via sacral parasympathetic nerves and referred to the buttocks

19. Common Sites of Referred Pain
Figure 16.4

20. Olfaction: The Sense of Smell
Olfaction—detection of odorants dissolved in the air
Odorants (volatile molecules) dissolved in nasal mucus are detected by chemoreceptors
Provides information about food, people, danger
We can distinguish thousands of different odors
Olfactory epithelium—sensory receptor organ
Located in superior region of nasal cavity; has three types of cells
Olfactory receptor cells detect odors
Supporting cells sustain receptors
Basal cells replace olfactory receptor cells every 40 to 60 days

21. Olfactory Epithelium
Figure 16.5

22. Olfaction: The Sense of Smell
Olfactory receptor cells
Primary neurons in sensory pathway for smell
Bipolar structure: a single dendrite and unmyelinated axon
Olfactory hairs: cilia projecting from receptor cell dendrite
House chemoreceptors for a specific odorant
Perceived smell depends on which cells are stimulated
Olfactory nerves (CN I)
Bundles of olfactory cell axons
Project through skull’s cribriform plate and enter olfactory bulbs

23. Olfaction: The Sense of Smell
Olfactory nerve structures and pathways
Olfactory bulbs (pair)
Ends of olfactory tracts located under brain’s frontal lobes
Olfactory nerve fibers synapse here with mitral cells and tufted cells
Connections form olfactory glomeruli
Olfactory tracts (pair)
Axon bundles of mitral and tufted cells on inferior frontal lobe surface
Project directly to primary olfactory cortex (in temporal lobe), hypothalamus, amygdala, and other regions
Does not project through thalamus like other sensory pathways

24. Olfaction: The Sense of Smell
Detecting smells
Sniff repeatedly or breathe deeply
Mucus contains odorant-binding proteins
Olfactory sensations begin when odorant binds to protein and protein stimulates receptor cell (rapidly adapting receptor)
Action potential is triggered on axon, conducted to glomerulus
Secondary neuron conducts signals to various CNS areas
Cerebral cortex (perceive, identify smell), hypothalamus (visceral reaction to smell), amygdala (smell recognition, emotional reaction)

25. Gustation: The Sense of Taste
Gustation = sense of taste; detection of tastants
Gustatory cells are chemoreceptors within taste buds
Papillae of tongue
Filiform papillae: short and spiked
No taste buds (no role in gustation); help manipulate food
Located on anterior two-thirds of tongue surface
Fungiform papillae: mushroom-shaped
Each contains a few taste buds
Located on tip and sides of tongue

26. Gustation: The Sense of Taste
Papillae of tongue (continued)
Foliate papillae: leaflike ridges
Not well developed
House a few taste buds in early childhood
Located on posterior lateral tongue
Vallate (circumvallate) papillae: largest, least numerous
Contain most of the taste buds
Located in a row of 10–12 along posterior dorsal tongue surface

27. Tongue Papillae and Taste Buds
Figure 16.6a

29. Gustation: The Sense of Taste
Taste buds: onion-shaped organs housing taste receptors
Gustatory cells: receptor cells detect tastants (live 7 to 9 days)
Supporting cells: sustain gustatory cells
Basal cells: neural stem cells that replace gustatory cells
Figure 16.6b,c

30. Gustation: The Sense of Taste
Gustatory cells: neuroepithelial chemoreceptive cells of taste buds
Gustatory microvillus (taste hair) forms dendritic ending
Microvillus often extends through taste pore, to tongue surface
Tastants (tasty molecules) dissolve in saliva and stimulate microvillus

31. Gustation: The Sense of Taste
Gustatory pathways
Sensory neurons connect to multiple gustatory cells in the tongue and project to the medulla
In anterior parts of tongue, sensory neurons are part of the facial nerve (CN VII)
In posterior two-thirds of tongue, sensory neurons are part of the glossopharyngeal nerve (CN IX)
Secondary medullary neurons project to thalamus
Tertiary thalamic neurons project to primary gustatory cortex in insula

32. Gustation: The Sense of Taste
Five basic taste sensations spread over broad regions of the tongue
Sweet
Produced by organic compounds, e.g., sugar or artificial sweeteners
Salt
Produced by metal ions, e.g., Na+ and K+
Sour
Associated with acids, e.g., vinegar
Bitter
Produced by alkaloids, e.g., unsweetened chocolate
Umami
Taste related to amino acids producing meaty flavor

33. Gustation: The Sense of Taste
Transduction in gustatory cells
For sweet, bitter, and umami the tastants are molecules
Tastant binds to specific cell membrane receptor
G protein is activated causing formation of 2nd messenger
Results in cell depolarization
For salt and sour the tastants are ions
The tastant depolarizes the cell directly
Depolarized gustatory cell releases neurotransmitter stimulating primary neuron (in CN VII or CN IX)

34. Gustation: The Sense of Taste
Gustatory pathway
Primary neuron in cranial nerve brings signal to nucleus solitarius within medulla
Medullary activity triggers salivation and stomach secretions
Nauseating stimuli instead trigger gag or vomiting
Signal is relayed to thalamus
Then relayed to primary gustatory cortex for conscious taste
Taste is integrated with temperature, texture, and especially smell
Food has less taste if olfaction is blocked (e.g., having a cold)

35. Gustatory Pathway
Figure 16.7

36. External Anatomy of the Eye and Surrounding Accessory Structures: Anterior View
Figure 16.8a
(a) ©McGraw-Hill Education/JW Ramsey

37. External Anatomy of the Eye and Surrounding Accessory Structures: Sagittal Section
Figure 16.8b

38. Accessory Structures of the Eye 4
Lacrimal apparatus: produces, collects, drains fluid
Lacrimal fluid: water, Na+, antibodies, lysozyme (antibacterial enzyme)
Lubricates, cleanses and moistens eye, reduces eyelid friction, defends against microbes, oxygenates and nourishes cornea
Lacrimal gland: produces fluid and secretes it through ducts
Located in superolateral orbit
Blinks (15–20 per minute) wash fluid over eye

39. Lacrimal Apparatus
Figure 16.9

40. Accessory Structures of the Eye
Lacrimal apparatus (continued)
Excess lacrimal fluid produces tears

41. Eye Structure Eye, almost spherical 2.5 cm diameter
Wall is formed by three tunics
Fibrous (external)
Vascular (middle)
Retina (inner)

42. Tunics of the Eye
Figure 16.10b

43. Internal Eye Structures
Figure 16.10c

44. Eye Structure Fibrous tunic: tough outer layer
Composed of sclera and cornea
Sclera: white of the eye
Composed of dense irregular CT
Provides eye shape
Protects internal components
Attachment site for extrinsic eye muscles
Cornea: anterior convex transparent “window”
No blood vessels
Limbus: corneal scleral junction
Refracts light

45. Eye Structure Vascular tunic (uvea): middle layer with many vessels
Houses blood vessels, lymph vessels, intrinsic muscles
Three regions: choroid, ciliary body, iris
Choroid: extensive, posterior region
Many capillaries nourish retina
Many melanocytes make melanin to absorb extraneous light
Ciliary body: ciliary muscles and processes
Located just anterior to choroid
Ciliary muscles: bands of smooth muscle connected to lens
Muscle contraction loosens suspensory ligaments, altering lens shape
Ciliary processes: contain capillaries secreting aqueous humor

46. Eye Structure Vascular tunic (continued)
Iris: gives eye color; most anterior region of uvea
Contains smooth muscle, melanocytes, vessels, neural structures
Divides the anterior segment into the anterior chamber (between cornea and iris) and posterior chamber (between iris and lens)
Pupil is opening in center of iris connecting the two chambers
Iris controls pupil diameter
Sphincter pupillae muscles: concentrically circular fibers constrict pupil with parasympathetic nervous system activity (CN III)
Dilator pupillae muscle: radially organized smooth muscle dilates pupil with sympathetic nervous system activity
Pupillary reflex alters pupil size in response to light (increased brightness leads to constriction)

47. Iris Control of Pupil Diameter
Figure 16.11

48. Eye Structure Retina: internal or neural tunic Pigmented layer
Attached to choroid (internal to it)
Provides vitamin A for photoreceptors
Absorbs stray light to prevent light scatter
Neural layer
Houses photoreceptors and associated neurons
Receives light and converts it to nerve signals

49. Eye Structure 6 Retina: cells of the neural layer form 3 sublayers
Photoreceptor cell layer: outermost neural layer
Contains rods and cones
Contain pigments that react to light
Bipolar cell layer
Their dendrites receive synaptic input from rods and cones
Ganglion cell layer: innermost neural layer
Their axons gather at optic disc and form optic nerve
Capable of action potentials
Other retinal interneurons
Horizontal cells regulate signals sent between photoreceptors and bipolar cells
Amacrine cells regulate signals between bipolar and ganglion cells

50. Structure and Organization of the Retina
Figure 16.12a,b

51. Eye Structure Components of the retina Optic disc Macula lutea
Contains no photoreceptors—blind spot
Where ganglion axons exit toward brain
Macula lutea
Rounded, yellowish region lateral to optic disc
Contains fovea centralis (central pit)
Highest proportion of cones (hardly any rods)
Area of sharpest vision
Peripheral retina
Contains primarily rods
Functions most effectively in low light

52. Eye Structure Lens: changes shape to focus light on retina
Cells within it have lost organelles and are filled with crystallin protein
Lens enclosed by dense, fibrous elastic capsule
Shape determines degree of light refraction
Shape is determined by ciliary muscle and suspensory ligaments
When viewing objects 20 or more feet away
Muscle relaxes, suspensory ligaments are tense, lens flattened
When viewing objects closer than 20 feet = accommodation
Muscle tenses, suspensory ligaments are less tense, lens more spherical

53. Lens Shape in Far Vision and Near Vision
Figure 16.14

54. Clinical View: Functional Visual Impairments
Emmetropia: normal vision
Parallel light rays focused on retina
Hyperopia: far-sighted
Trouble seeing up close; eyeball too short
Only convergent rays from distant points brought to focus
Corrected with convex lens
Myopia: near-sighted
Trouble seeing faraway objects; eyeball too long
Only rays close to eye focus on retina
Corrected with concave lens

56. Functional Visual Impairments
Astigmatism
Unequal focusing
Unequal curvatures in one or more refractive surfaces
Presbyopia: age-related change in vision
Lens less able to become spherical
Reading close-up words becomes difficult
Corrective convex lens
Can be treated with various surgical techniques

57. Physiology of Vision: Refraction and Focusing of Light
Focusing of light for objects at different distances
Near response for objects closer than 20 feet: eyes converge, lens accommodates, pupil constricts
Convergence of eyes: extrinsic muscles pull eyes medially
Directs image of interest onto both foveas
Weak eye muscle may cause diplopia (double vision)
Accommodation of lens: ciliary muscle contraction thickens lens
Slackened suspensory ligaments allow lens to thicken
Refraction increases
Constriction of pupil: sphincter pupillae contraction shrinks hole
Light passes only through center of lens, avoiding blurriness that would result if thin edges of the lens were used

58. Physiology of Vision: Refraction and Focusing of Light
Focusing of light (continued)
For objects 20 feet away and further
Eyes face forward (not converged)
Lens is flattened (ciliary muscles relaxed, suspensory ligaments taut)
Pupil is relatively dilated
Allows greater amount of light into the eye

59. Physiology of Vision: Phototransduction
Phototransduction: converting light to electrical signals
Performed by photoreceptors (rods and cones)
Photoreceptor parts (from outer to inner)
Outer segment extends into pigmented layer of retina
Hundreds of photopigment-containing discs that absorb light
Discs are continually replaced
Inner segment contains cell organelles
Cell body contains nucleus
Synaptic terminals contain vesicles storing glutamate neurotransmitter

60. Physiology of Vision: Phototransduction
Photoreceptors: rods and cones
Rods are longer and narrower than cones; more numerous
Each rod is highly sensitive: activated by even dim light
The periphery of the retina contains many rods
Many rods converge on fewer bipolar cells, which converge on fewer ganglion cells
Results in sensitivity to dim light but a blurry image
Cones are concentrated at fovea centralis
Activated by high intensity light, allow color vision
Cones have one-to-one relationship with bipolar cells and ganglion cells
Results in a sharp image but only possible in bright light

61. Figure 16.17

62. Physiology of Vision: Phototransduction
Photopigments: light-absorbing molecules
Found within membranes of outer segments of rods and cones
Made of opsin protein and light-absorbing retinal (made from Vitamin A)
Different pigment types have different opsins transducing different wavelengths (colors) of light
Each photoreceptor has only one photopigment type
Rods contain rhodopsin
Three types of cones each containing a type of photopsin with a different sensitivity
Blue cones detect short wavelengths, green cones absorb intermediate wavelengths, and red cones best detect long wavelengths

63. Clinical View: Color Blindness
X-linked recessive condition more common in males
Absence or deficit in one type of cone cell
Red and green most commonly affected
Results in difficulty distinguishing red and green

64. Bleaching Reaction and Regeneration of Rhodopsin
Figure 16.19

65. Physiology of Vision: Phototransduction
Dark adaptation
Return of sensitivity to low light levels after bright light
Bleached rods must regenerate rhodopsin
May take 20 to 30 minutes to see well
Light adaptation
Process of adjusting from low light to bright conditions
Pupils constrict, but cones initially overstimulated
Takes about 5 to 10 minutes for full adjustment

66. Visual Pathways In retina Optic nerves
Photoreceptors  bipolar cells  ganglion cells
Ganglion cell axons bundle at disc to form optic nerve
Optic nerves
Exit backs of eyes and converge at optic chiasm
Medial axons cross to opposite side of brain
Lateral axons remain on same side
Optic tracts (ganglion cell axons from both eyes)
Most axons go to lateral geniculate nucleus of thalamus
Thalamic neurons’ axons project to visual cortex of occipital lobe

67. Visual Pathways Left and right eyes have overlapping visual fields
Allows stereoscopic vision (depth perception)
Some optic tract axons project to the midbrain
Superior colliculi coordinate reflexive eye movements
Pretectal nuclei coordinate pupillary reflex and lens accommodation reflex
Ganglion cells projecting to pretectal nuclei are directly photoresponsive and contain melanopsin pigment

68. Figure 16.21

69. Ear Structure The ear detects sound and head movement External ear
Signals transmitted via vestibulocochlear nerve (CN VIII)
External ear
Auricle: funnel-shaped visible part of ear with elastic cartilage
Protects ear entryway and directs sound waves inward
External acoustic (auditory) meatus: ear canal
Extends to tympanic membrane
Ceruminous glands produce cerumen
Ear wax impedes microorganism growth
Tympanic membrane: eardrum
Funnel-shaped epithelial sheet separating external and middle ear
Vibrates when sound waves hit it

70. Anatomic Regions of the Right Ear
Figure 16.23

71. Ear Structure Middle ear Contains air-filled tympanic cavity
Bony wall separates it from inner ear
Wall has two membrane-covered openings: oval window and round window
Auditory tube (Eustachian tube)
Passage extending from middle ear to nasopharynx (upper throat)
Middle ear infections often result from infections spreading from throat through auditory tube
Usually closed, but yawning allows air movement through the tube
Equalizes pressure on either side of tympanic membrane

72. Ear Structure Auditory ossicles: three tiny bones of middle ear
Malleus
Attached to medial surface of tympanic membrane
Resembles a hammer in shape
Incus
Middle ossicle resembling an anvil
Stapes
Resembles a stirrup of a saddle
Has disclike footplate fitting into oval window

73. Ear Structure Auditory ossicles (continued)
Amplify sound waves and transmit them to oval window
Vibrate along with eardrum, so stapes moves in and out of oval window initiating pressure waves in inner ear fluid
Two small muscles restrict ossicle movement during loud sounds
Tensor tympany attaches to malleus
Stapedius attaches to stapes

74. Middle Ear
Figure 16.24

75. Ear Structure Inner ear: spaces within petrous part of temporal bone
Bony labyrinth: mazelike spaces in temporal bone
Perilymph (interstitial fluid) fills most of this space
Membranous labyrinth: membrane-lined fluid-filled tubes within bony labyrinth
Contains receptors for hearing and equilibrium
Contains endolymph, similar to intracellular fluid, rich in K+

76. Ear Structure Inner ear composed of three main regions: Cochlea
Houses membranous cochlear duct
Vestibule
Contains two saclike, membranous parts: utricle and saccule, interconnected and positioned at right angles
Semicircular canal
Contains membranous semicircular ducts

77. Inner Ear
Figure 16.25

78. Clinical View: Otitis Media
Infection of the middle ear
Most often experienced by young children
Horizontal, short auditory tubes
Causative agent from respiratory infection
May spread from pharynx through auditory tube
Fluid accumulation in middle ear
Pressure, pain, sometimes reduced hearing
Evaluated with otoscope

79. Hearing Structures for hearing
Cochlea: snail-shaped chamber of inner ear
Modiolus: bony axis of the spiral
Cochlear duct: membranous labyrinth in cochlea
Roof formed by vestibular membrane
Floor formed by basilar membrane
Scala vestibuli: chamber of bony labyrinth adjacent to vestibular membrane
Scala tympani: chamber of bony labyrinth adjacent to basilar membrane
Helicotrema: small channel connecting scala vestibuli and scala tympani

80. Sectioned Cochlea
Figure 16.26a

81. Close-Up of Cochlea
Figure 16.26b

82. Spiral Organ
Figure 16.26c

83. Hearing Structures for hearing (continued)
Spiral organ: sensory structure for hearing
Within cochlear duct
Thick sensory epithelium consisting of hair cells and supporting cells on basilar membrane
Hair cells: receptors that release neurotransmitter to sensory neurons
Single row of inner hair cells; three rows of outer hair cells
Hair cells have many stereocilia (long microvilli) and one kinocilium (long cilium) at their apex
Stereocilia and kinocilium embedded in tectorial membrane
Base of hair cells synapse with sensory neurons
Cell bodies of sensory neurons are located in spiral ganglia of modiolus

84. Figure 16.27

85. Hearing Pathway from sound wave to nerve signal
Sound waves vibrate tympanic membrane
Ossicles vibrate and transmit waves to oval window
Fluid pressure waves in scala vestibuli push vestibular membrane causing pressure waves in endolymph of cochlear duct
Specific regions of basilar membrane move (depending on sound wave frequency)
Hair cells distorted, causing changes in neurotransmitter release
Sensory neurons with axons in CN VIII are stimulated to fire
Pressure is transmitted to scala tympani and absorbed by round window

86. Hearing Perception of sound
Sound is the perception of pressure waves established from vibrating objects
Vibration pushes molecules, which transfer the energy from one molecule to the next
Pitch depends on the frequency of the vibrating object
Frequency is rate of vibration in Hertz (Hz; cycles per second)
Humans can hear 20–20,000 Hz
Variations in pitch are detectable due to variations in stiffness of basilar membrane from oval window to cochlear apex
High-frequency sounds excite cells in stiff basilar membrane near oval window
Low-frequency sounds excite cells in flexible basilar membrane near apex

87. Sound Wave Interpretation at the Basilar Membrane
Figure 16.29

88. Hearing Perception of sound (continued)
Loudness depends on wave amplitude (degree of molecular compression)
Louder sounds create larger movements of basilar membrane
Larger movements cause faster rate of nerve signals and a larger number of stimulated cells
Temporal lobe’s auditory cortex interprets this as loudness
Loudness measured in decibels (dB)
Zero dB is threshold for hearing
Energy of sound increases ten times for every 10 dB increase

89. Auditory Pathways
Stimulated hair cells initiate signals in CN VIII fibers ending in cochlear nucleus of medulla oblongata
Secondary neurons transmit signals to inferior colliculus of midbrain or to superior olivary nucleus
Inferior colliculus coordinates head orienting reflexes to sounds
Superior olive localizes sound and initiates reflexive contraction of middle ear muscles (tensor tympani, stapedius)
Superior olivary axons also transmit signals to inferior colliculus
Inferior colliculus transmits signals to medial geniculate nucleus (MGN) of thalamus
MGN filters signals and relays them to primary auditory cortex (temporal lobe) for conscious perception

90. Central Nervous System Pathways for Hearing
Figure 16.30

91. Clinical View: Deafness
Deafness is any hearing loss
Two types
Conductive deafness: interference of wave transmission in external or middle ear
Sensorineural deafness: malfunction in inner ear or cochlear nerve

92. Equilibrium and Head Movement
Awareness and monitoring of head position
Monitored by vestibular apparatus: utricle, saccule, semicircular ducts
Information sent to brain to help keep our balance
Vision and proprioception also help
Utricle and saccule detect static equilibrium and linear acceleration
Semicircular ducts detect angular acceleration

93. Vestibular Complex
Figure 16.32a

94. Equilibrium and Head Movement
Static equilibrium and linear acceleration
Macula is receptor for static equilibrium and linear acceleration
Located in utricle and saccule of vestibule
Composed of a layer of hair cells and supporting cells
Hair cells have stereocilia and one kinocilium projecting into gelatinous otolithic membrane
Membrane is covered with otoliths—calcium carbonate crystals
Head tilt shifts otolithic membrane and bends stereocilia
Bending stereocilia toward kinocilium depolarizes hair cells and increases their transmitter release
This increases impulse frequency on vestibular part of CN VIII
Opposite reaction occurs if bending is away from kinocilium

95. Macula
Figure 16.32b,c

96. Macula in Upright Head Position
Figure 16.33a

97. Macula in Altered Head Position
Figure 16.33b

98. Equilibrium and Head Movement
Angular acceleration
Base of each semicircular canal has region called the ampulla
Ampulla contains crista ampullaris with hair cells and support cells
Stereocilia and kinocilia of hair cells are embedded in gelatinous cupula
When head rotates endolymph pushes against cupula
Cupula bends stereocilia and changes hair cell voltage
If stereocilia bent toward kinocilium, the hair cell depolarizes
If stereocilia bent away from kinocilium, the hair cell hyperpolarizes
Neurotransmitter release from hair cells changes
Firing rate changes on vestibular branch of CN VIII

99. Ampulla
Figure 16.34

100. Function of the Crista Ampullaris
Figure 16.35

101. Equilibrium and Head Movement
Equilibrium pathways
Signals from maculae or crista ampullaris are conveyed by the vestibular branch of CN VIII
These axons terminate in vestibular nuclei or cerebellum
Vestibular nuclei (in superior medulla) help control reflexive eye movements and balance
Cerebellum helps coordinate balance and muscle tone
Vestibular nuclei and cerebellum send signals to thalamus
Thalamus relays information to cerebral cortex for awareness of body position