15 The Special Senses.

15 The Special Senses

Section 1: Olfaction and Gustation
Learning Outcomes 15.1 Describe the sensory organs of smell, trace the olfactory pathways to their destinations in the cerebrum, and explain how olfactory perception occurs. 15.2 Describe the sensory organs of gustation. 15.3 Describe gustatory reception, briefly describe the physiologic processes involved in taste, and trace the gustatory pathway.

Section 1: Olfaction and Gustation
Special senses introduction Special sense organs provide us with information about external environment Two types of receptors used Dendrites of specialized neurons Bind chemicals producing a depolarization of the cell or generator potential Example: olfactory (smell) receptors Specialized receptors that synapse with sensory neurons Stimulated receptor releases chemical transmitters that depolarize sensory neuron (generator potential) Small delay due to synapse Examples: vision, hearing, taste, equilibrium

The function of olfactory receptors
Stimulus removed Action potentials Stimulus Dendrites Threshold Generator potential Stimulus to CNS Figure 15 Section 1 An Introduction to the Special Senses: Olfaction and Gustation Specialized olfactory neuron Figure 15 Section 4

The function of receptors for the senses of
taste, vision, equilibrium, and hearing Receptor cell Stimulus removed Stimulus Threshold Receptor depolarization Axon Action potentials Stimulus Synaptic delay Stimulus Figure 15 Section 1 An Introduction to the Special Senses: Olfaction and Gustation to CNS Receptor cell Synapse Axon of sensory neuron Generator potential Figure 15 Section 5

Module 15.1: Olfaction Olfaction Provided by olfactory organs
Located in nasal cavity, either side of nasal septum Cover: Inferior surface of cribiform plate Superior portion of perpendicular plate Superior nasal conchae of ethmoid

Module 15.1: Olfaction Olfactory pathway
Sensory neurons in olfactory organ stimulated by chemicals Olfactory epithelium axons collect into 20 or more bundles penetrating cribiform plate of ethmoid bone Synapse with olfactory bulb Axons leaving bulb travel along olfactory tract to olfactory cortex, hypothalamus, and portions of limbic system Explains why smells can produce profound emotional and behavioral responses

Olfactory Pathway to the Cerebrum
The sensory neurons within the olfactory organ are stimulated by chemicals in the air. Axons leaving the olfactory epithelium collect into 20 or more bundles that penetrate the cribriform plate of the ethmoid. The first synapse occurs in the olfactory bulb, which is located just superior to the cribriform plate. Axons leaving the olfactory bulb travel along the olfactory tract to reach the olfactory cortex, the hypothalamus, and portions of the limbic system. The distribution of olfactory information to the limbic system and hypothalamus explains the profound emotional and behavioral responses, as well as the memories, that can be triggered by certain smells. Olfactory organ Cribriform plate of ethmoid Olfactory epithelium Figure Olfaction involves specialized chemoreceptive neurons and delivers sensations directly to the cerebrum Superior nasal concha Figure 8

Module 15.1: Olfaction Olfactory organ composition Two layers
Olfactory epithelium Olfactory receptor cells Each cell produces knob (base of 20 cilia) 10–20 million receptors in 5 cm2 area Supporting cells Basal (stem) cells Replace worn-out receptors One of the few examples of neuronal replacement Lamina propria Contains olfactory glands that produce mucus

A portion of an olfactory organ, which consists of the
olfactory epithelium and the lamina propria Olfactory (Bowman) gland To olfactory bulb Olfactory nerve fibers Lamina propria Basal cell: divides to replace worn-out olfactory receptor cells Developing olfactory receptor cell Figure Olfaction involves specialized chemoreceptive neurons and delivers sensations directly to the cerebrum Olfactory epithelium Olfactory receptor cell Supporting cell Mucous layer Knob Olfactory cilia: surfaces contain receptor proteins Figure 10

Module 15.1: Olfaction Steps of olfactory reception
Binding of odorant (dissolved chemical) to receptor protein Activates adenylyl cyclase (enzyme converting ATP to cAMP) cAMP opens sodium channels, depolarizing membrane With sufficient depolarization, an action potential may be generated and relayed to CNS

Module 15.1: Olfaction Odorants Generally small organic molecules
Strongest smells associated with molecules with either high water or lipid solubilities As few as four odorant molecules can activate receptor

Step 1: The binding of an odorant
to its receptor protein leads to the activation of adenylyl cyclase, the enzyme that converts ATP to cyclic-AMP (cAMP). Step 2: The cAMP then opens sodium channels in the plasma membrane, which, as a result, begins to depolarize. Step 3: If sufficient depolarization occurs, an action potential is triggered in the axon, and the information is relayed to the CNS. RECEPTOR CELL Sodium ions enter Inactive enzyme Active enzyme Depolarized membrane Figure Olfaction involves specialized chemoreceptive neurons and delivers sensations directly to the cerebrum Closed sodium channel Odorant molecule MUCOUS LAYER The process of olfactory reception on the surface membranes of the olfactory cilia Figure 13

Module 15.1 Review a. Describe olfaction.
b. Which neurons associated with olfaction are capable of regenerating? c. Trace the olfactory pathway, beginning at the olfactory epithelium.

Module 15.2: Gustation Gustation or taste provides information about consumed food and liquids Taste (gustatory) receptors Found mainly on superior surface of tongue within taste buds Also some located in pharynx and larynx but decrease in importance and abundance with age

Module 15.2: Gustation Taste bud structure Gustatory cells Basal cells
Each has slender microvilli into surrounding fluids through narrow opening (taste pore) of taste bud Each only survives ~10 days Approximately 40–100 receptor cells/bud Basal cells Stem cells that divide and mature to produce more gustatory cells

Transitional cell Taste hairs (microvilli) The structure of taste buds Gustatory cell Basal cell Taste pore Diagrammatic view of a taste bud Taste buds Figure Gustation involves epithelial chemoreceptor cells located in taste buds Taste bud LM x 650 Taste buds LM x 280 Figure – 4 17

Module 15.2: Gustation Taste bud location
Recessed along epithelium lining tongue projections (lingual papillae; papilla, nipple-shaped mound) Papillae types Circumvallate (circum-, around + vallate, wall) papillae Large with deep folds containing ~100 taste buds Located in V-shape on tongue posterior Fungiform (fungus, mushroom) papillae Shaped like small buttons with shallow depressions Each contains ~5 taste buds Filiform (filum, thread) papillae Provide friction but contain no taste buds

Circumvallate Papillae
Are relatively large and are surrounded by deep epithelial folds; each contains as many as 100 taste buds The lingual papillae on the superior surface of the tongue Water receptors (pharynx) Taste buds Umami Circumvallate papillae Fungiform Papillae Sour Bitter Salty Sweet Contain about five taste buds each Figure Gustation involves epithelial chemoreceptor cells located in taste buds Filiform Papillae Provide friction that helps the tongue move objects around in the mouth but do not contain taste buds Figure – 2 19

Module 15.2: Gustation Taste sensations
Four primary sensations: sweet, salty, sour, and bitter Found in taste buds all over tongue Two other sensations Umami Meaty or savory Receptor binds amino acids Discovered in Japan Water receptors Demonstrated in human pharynx Information sent to hypothalamus to manage thirst

Module 15.2: Gustation Taste receptor sensitivity
More sensitive to unpleasant stimuli 100,000× more sensitive to bitter, 1000× more sensitive to sour (acids) compared to sweet and salty May have survival value Toxic compounds are often bitter Acids can create chemical burns Overall sensitivity declines with age Number of taste receptors declines Number of olfactory receptors declines

Module 15.2 Review a. Define gustation. b. Describe filiform papillae.
c. Relate the adaptive sensitivity of taste receptors for bitter and sour sensations, to sweet and salty sensations.

Module 15.3: Gustatory receptors and pathways
Mechanism of gustatory reception Two types Chemically gated ion channels whose stimulation produces depolarization of the cell and release of neurotransmitters Salt and sour receptors Taste receptor activates G-proteins (gustducins) that activate 2nd messenger system to release neurotransmitters Sweet, bitter, and umami receptors

The mechanisms involved in gustatory reception
Salt and Sour Receptors Sweet, Bitter, and Umami Receptors Salt receptors and sour receptors are chemically gated ion channels whose stimulation produces depolarization of the cell. Receptors responding to stimuli that produce sweet, bitter, and umami sensations are linked to G proteins called gustducins (GUST-doos-inz)—protein complexes that use second messengers to produce their effects. Receptor cells Sweet, bitter, or umami Sour, salt Gated ion channel Membrane receptor Resting plasma membrane Inactive G protein Active G protein Channel opens Plasma membrane depolarizing Plasma membrane depolarizing Figure Gustatory reception relies on membrane receptors and channels, and sensations are carried by facial, glossopharyngeal, and vagus nerves Active G protein Active 2nd messenger Inactive 2nd messenger Depolarization of membrane stimulates release of chemical neurotransmitters. Activation of second messengers stimulates release of chemical neurotransmitters. Figure 24

Gustatory information is relayed to the cerebral cortex along three different cranial nerves dependent on the location of the receptor Facial nerve (VII) – anterior 2/3 of tongue to line of circumvallate papillae Glossopharyngeal nerve (IX) – circumvallate papillae and posterior 1/3 of tongue Vagus nerve (X) – surface of epiglottis

Gustatory pathway Receptors respond to stimulation Relay information to appropriate cranial nerve Sensory afferents synapse in solitary nucleus of medulla oblongata Postsynaptic neuron axons cross over at medial lemniscus with other somatic sensory information and relay to thalamus After synapse in thalamus, impulse is routed to appropriate area of primary sensory cortex

The components of the gustatory pathway
After another synapse in the thalamus, the information is projected to the appropriate portions of the gustatory cortex of the insula. The axons of the postsynaptic neurons cross over and enter the medial lemniscus of the medulla oblongata. Cranial Nerves Carrying Gustatory Information The sensory afferents carried by these three cranial nerves synapse in the solitary nucleus of the medulla oblongata. The facial nerve (VII) innervates all the taste buds located on the anterior two-thirds of the tongue, from the tip to the line of circumvallate papillae. The glossopharyngeal nerve (IX) innervates the circumvallate papillae and the posterior one-third of the tongue. Figure Gustatory reception relies on membrane receptors and channels, and sensations are carried by facial, glossopharyngeal, and vagus nerves The vagus nerve (X) innervates taste buds scattered on the surface of the epiglottis. Start Receptors respond to stimulation. Figure 27

Central processing of gustatory sensations Conscious perception of taste occurs at the primary sensory cortex Taste sensation is analyzed with taste-related sensations “Peppery” or “burning hot” from afferents in trigeminal nerve (V) Olfactory stimulation significantly contributes to taste perception Central adaptation quickly reduces sensitivity to new tastes

Module 15.3 Review a. What are gustducins?
b. Identify the cranial nerves that carry gustatory information. c. Trace the gustatory pathway from the taste receptors to the cerebral cortex.

Section 2: Equilibrium and Hearing
Learning Outcomes 15.4 Describe the structures of the external, middle, and inner ear, and explain how they function. 15.5 Describe the structures and functions of the bony labyrinth and membranous labyrinth. 15.6 Describe the functions of hair cells in the semicircular ducts, utricle, and saccule.

Learning Outcomes 15.7 Describe the structure and functions of the organ of Corti. 15.8 Explain the anatomical and physiological basis for pitch and volume sensations for hearing. 15.9 Trace the pathways for the sensations of equilibrium and hearing to their respective destinations in the brain.

Chemoreceptors compared to mechanoreceptors Olfactory and gustatory receptors are located in epithelia exposed to the external environment Olfactory receptors are modified neurons Gustatory receptors communicate with sensory neurons Equilibrium and hearing receptors are isolated and protected from external environment Located in inner ear Information is integrated and organized locally before forwarding to CNS

Sensory receptors that are located within epithelia exposed to the
external environment Gustatory receptor Olfactory receptor Figure 15 Section 2 Equilibrium and Hearing Figure 15 Section 33

Hair cell receptors of the inner ear Free surfaces covered with specialized processes 80–100 stereocilia (like long microvilli) May contain single large kinocilium Hair cells are mechanoreceptors that are not actively moved External forces push against processes causing distortion of cell membrane and neurotransmitter release Provide information about direction and strength of mechanical stimuli Complex inner ear structure determines what stimuli can reach different hair cells

Figure 15 Section 2 Equilibrium and Hearing
Inner ear Location of the receptors for equilibrium and hearing Displacement in this direction stimulates hair cell Displacement in this direction inhibits hair cell Kinocilium Stereocilia Receptors for equilibrium and hearing, which are isolated and protected from the external environment Hair cell Figure 15 Section 2 Equilibrium and Hearing Dendrite of sensory neuron Supporting cell A hair cell, the receptor located in the inner ear Figure 15 Section 35

Module 15.4: Ear regions and structures
Three anatomical regions of the ear External ear – visible portion that collects and directs sound waves toward middle ear Auricle External acoustic meatus (passageway in temporal bone) Lined with Ceruminous glands (secrete waxy cerumen) Hairs Has some protection against entering foreign objects, insects, and bacteria

The ear’s three anatomical regions: the external ear, the middle ear, and the inner ear
The visible portion of the ear; collects and directs sound waves toward the middle ear An air-filled chamber; is connected to the nasopharynx by the auditory tube Site of sensory organs for hearing and equilibrium; receives amplified sound waves from the middle ear Elastic cartilages Auditory ossicles Semicircular canals Petrous part of temporal bone Auricle Facial nerve (VII) Vestibulocochlear nerve (VIII) Bony labyrinth Figure The ear is divided into the external ear, the middle ear, and the inner ear Tympanic cavity To nasopharynx External acoustic meatus Tympanic membrane (tympanum or eardrum) Auditory tube (pharyngotympanic tube or Eustachian tube) Figure 37

Three anatomical regions of the ear (continued) Middle ear Tympanic membrane (also tympanum or eardrum) Border between external and middle ear Thin, transparent sheet Auditory ossicles (3) Bones that connect tympanic membrane to receptor complexes of inner ear (smallest bones and synovial joints of body) Malleus (attached at three points to tympanum) Incus (middle ossicle) Stapes (bound to oval window of cochlea)

Three anatomical regions of the ear (continued) Middle ear (continued) Auditory tube (pharyngotympanic or Eustachian tube) Connects middle ear to pharynx to equalize pressure on either side of tympanic membrane Also can lead to bacterial infection (otitis media) Middle ear muscles Tensor tympani muscle Connects to malleus and can dampen tympanic membrane vibrations Stapedius muscle (smallest muscle in body) Connects to stapes and reduces its movement at oval window

The structures of the middle ear
Auditory Ossicles Malleus Incus Stapes Temporal bone (petrous part) Stabilizing ligament Oval window Branch of facial nerve VII (cut) Muscles of the Middle Ear Tensor tympani muscle External acoustic meatus Figure The ear is divided into the external ear, the middle ear, and the inner ear Stapedius muscle Tympanic cavity (middle ear) Auditory tube Round window Tympanic membrane Figure 40

Sound impulse pathway Sound waves vibrate tympanic membrane Tympanic membrane and ossicles amplify and conduct vibrations to oval window of inner ear Vibrations can be dampened by actions of middle ear muscles Animation: The Ear: Balance Animation: The Ear: Ear Anatomy

Module 15.4 Review a. Name the three tiny bones located in the middle ear. b. What is the function of the auditory tube? c. Why are external ear infections relatively uncommon?

Module 15.5: Labyrinths of the inner ear
Bony labyrinth Shell of dense bone Surrounds and protects membranous labyrinth Filled with perilymph (similar to CSF) Consists of three parts Semicircular canals Vestibule Cochlea

Membranous labyrinth Collection of fluid-filled tubes and chambers Houses receptors for hearing and equilibrium Filled with endolymph Receptors only function when exposed to unique ionic composition

Membranous labyrinth (continued) Consists of three parts Semicircular ducts (within semicircular canals) Receptors stimulated by rotation of head Utricle and saccule (within vestibule) Provide sensations of gravity and linear acceleration Cochlear duct (within cochlea) Sandwiched between pair of perilymph-filled chambers Resembles snail shell Receptors stimulated by sound

Figure 15.5.1-2 The bony labyrinth protects the membranous labyrinth
The structures of the inner ear Bony Labyrinth Surrounds and protects the membranous labyrinth and contains a fluid called perilymph Semicircular canals Vestibule Cochlea Receptor areas Membranous Labyrinth Houses the receptors for equilibrium and hearing and contains a fluid called endolymph Semicircular duct Utricle and saccule Cochlear duct Figure The bony labyrinth protects the membranous labyrinth Bony labyrinth Perilymph Membranous labyrinth KEY Endolymph Membranous labyrinth Bony labyrinth A cross section of a semicircular canal Figure – 2 46

Module 15.5 Review a. Identify the components of the bony labyrinth.
b. Describe hair cells. c. Explain the regional differences among the receptor complexes in the membranous labyrinth.

Module 15.6: Receptors for equilibrium
Semicircular ducts Structure Three ducts continuous with utricle and filled with endolymph Anterior Posterior Lateral Each contains an enlarged region (ampulla) with area (crista) housing receptors Hair cells with kinocilia and stereocilia embedded within gelatinous matrix

The location and structure of an ampulla, which
contains receptors that respond to rotation The location of the ampullae within the inner ear Semicircular Ducts Ampulla Anterior Posterior Lateral Utricle Cupula Ampulla filled with endolymph Figure Hair cells in the semicircular ducts respond to rotation, while those in the utricle and saccule respond to gravity and linear acceleration Hair cells Crista Supporting cells Sensory nerve The structure of an ampulla Figure 49

Semicircular ducts Function Head rotating in plane of one duct causes endolymph movement and cupula bends, causing distortion of hair cells Movement one way causes stimulation Opposite movement causes inhibition Horizontal rotation (“no”) stimulates lateral duct receptors Nodding (“yes”) stimulates anterior duct receptors Tilting head stimulates posterior duct receptors

The response when the head rotates in the plane of a semicircular duct
Direction of duct rotation Direction of relative endolymph movement Direction of duct rotation Semicircular duct Ampulla At rest Figure Hair cells in the semicircular ducts respond to rotation, while those in the utricle and saccule respond to gravity and linear acceleration The response when the head rotates in the plane of a semicircular duct Figure 51

The analysis of complex movements, which involves the response of each
Anterior semicircular duct for “yes” Lateral semicircular duct for “no” Posterior semicircular duct for tilting head to the side Figure Hair cells in the semicircular ducts respond to rotation, while those in the utricle and saccule respond to gravity and linear acceleration The analysis of complex movements, which involves the response of each semicircular duct to rotational movements Figure 52

Utricle and saccule Provide equilibrium sensations, whether body is stationary or moving Connected by slender passageway continuous with endolymphatic duct ending in endolymphatic sac After being secreted in cochlear duct, endolymph returns to general circulation in endolymphatic sac

Utricle and saccule (continued) Contain hair cells clustered in maculae Processes embedded in gelatinous mass with calcium carbonate crystals (statoconia; statos, standing + conia, dust) Whole complex called otolith (“earstone”) With head in upright position, statoconia sit on hair cells compressing, but not bending, them With tilted position or with linear movement, otolith movement bends hair cell processes stimulating macular receptors

The anatomy of equilibrium receptors in the utricle and saccule
Endolymphatic sac The locations of the utricle and saccule in the inner ear Endolymphatic duct Saccule Otolith Gelatinous material An otolith, which consists of a gelatinous matrix and statoconia Statoconia Figure Hair cells in the semicircular ducts respond to rotation, while those in the utricle and saccule respond to gravity and linear acceleration Maculae Nerve fibers Figure – 5 55

The effects of gravity and linear acceleration on the hair cell processes in the maculae
When your head is in the normal, upright position, the statoconia sit atop the macula. Their weight presses on the macular surface, pushing the hair cell processes down rather than to one side or another. When your head is tilted, the pull of gravity on the statoconia shifts them to the side, thereby distorting the hair cell processes and stimulating the macular receptors. A similar mechanism accounts for your perception of linear acceleration, as when your car speeds up sud- denly. The statoconia lag behind, and the effects on the hair cells is comparable to tilting your head back. Gravity Gravity Otolith moves “downhill,” distorting hair cell processes Figure Hair cells in the semicircular ducts respond to rotation, while those in the utricle and saccule respond to gravity and linear acceleration Receptor output increases Figure 56

Module 15.6 Review a. Define statoconia.
b. Cite the function of receptors in the saccule and utricle. c. Damage to the cupula of the lateral semicircular duct would interfere with what perception?

Module 15.7: Receptors for hearing
Cochlear duct Lies between pair of perilymphatic chambers Vestibular duct (separated by vestibular membrane) Tympanic duct (separated by basilar membrane) Both ducts connect at tip of cochlear spiral creating one long chamber Begins at oval window Ends at round window Hair cells for hearing located in organ of Corti on basilar membrane Animation: The Ear: Receptor Complexes

The location of the cochlea in the inner ear
Round window Stapes at oval window Vestibular duct Cochlear duct Tympanic duct Cochlear branch Vestibular branch Figure The cochlear duct contains the hair cells of the organ of Corti Vestibulocochlear nerve (VIII) Semicircular canals KEY From oval window to tip of spiral From tip of spiral to round window Figure 59

A cross section of the cochlea
From oval window Vestibular membrane Basilar membrane Vestibular duct Organ of Corti Cochlear duct Tympanic duct Figure The cochlear duct contains the hair cells of the organ of Corti Temporal bone (petrous part) Cochlear nerve Vestibulocochlear nerve (VIII) To round window Figure 60

Sectional view of the cochlear spiral LM x 200
Figure The cochlear duct contains the hair cells of the organ of Corti Sectional view of the cochlear spiral LM x 200 Figure 61

Module 15.7: Receptors for hearing
Organ of Corti Hair cells arranged in longitudinal rows Lack kinocilia In contact with overlying tectorial membrane Sound waves cause pressure waves within perilymph, vibrating the basilar membrane and organ of Corti Hair cells press into tectorial membrane and are distorted/stimulated Sensory neurons relay the message through the spiral ganglion and cochlear branch of vestibulocochlear nerve (VIII)

A sectional view showing a single turn of the cochlea
Bony cochlear wall Vestibular duct Spiral ganglion Vestibular membrane Cochlear duct Basilar membrane Figure The cochlear duct contains the hair cells of the organ of Corti Tympanic duct Cochlear branch of the vestibulocochlear nerve (VIII) Organ of Corti Figure 63

Tectorial membrane The structure of the organ of Corti Outer hair cell
Figure The cochlear duct contains the hair cells of the organ of Corti Basilar membrane Inner hair cell Nerve fibers Figure 64

perilymph triggered by sound waves arriving at the tympanic membrane
Shear At rest Pressure wave in perilymph The distortion of hair cells in response to pressure changes within the perilymph triggered by sound waves arriving at the tympanic membrane Figure The cochlear duct contains the hair cells of the organ of Corti Figure 65

Module 15.7 Review a. Where is the organ of Corti located?
b. Name the fluids found within the vestibular duct, tympanic duct, and cochlear duct. c. Identify the features visible in the LM sectional view of the cochlear spiral.

Module 15.8: Sensations of pitch and volume
Physical characteristics of sound Consists of waves of pressure conducted through a medium In air, a pressure wave consists of alternating regions of compressed and separated molecules Wavelength of sound Distance between two adjacent wave crests (peaks) or between two adjacent wave troughs

Sounds, which consist of pressure waves
conducted through a medium such as air Wavelength of a sound Figure The organ of Corti provides sensations of pitch and volume Air molecules Tympanic membrane Tuning fork Figure 68

Sensory response to wave frequency Frequency = number of waves passing a fixed point in a given time Often measured in waves or cycles/sec called hertz (Hz) Pitch and wave frequency are directly related Example: high-pitched sound might be 15,000 Hz while low-pitched sound might be 100 Hz or less All sound travels at same speed so as frequency increases, wavelength must become shorter

Sound energy arriving at
A graph showing the relationships among the characteristics of sound waves. Wavelength, which is inversely related to frequency Amplitude of a sound 1 wavelength Sound energy arriving at tympanic membrane Figure The organ of Corti provides sensations of pitch and volume Time (sec) Figure 70

Volume or intensity (energy in sound waves) Energy variation in sound is represented by changes in wave height or amplitude Wave amplitude = wave energy = perceived loudness Directly related Sound energy reported in decibels (dB)

Figure 15.8.3 The organ of Corti provides sensations of pitch and volume
72

Sound wave energy causes movements of flexible structures in the ear At a particular frequency and amplitude, object will vibrate at same frequency = Resonance Examples: tympanic membrane, basilar membrane

Basilar membrane flexibility changes along length Different sound frequencies vibrate different areas of basilar membrane Location = pitch Number of hair cells stimulated: volume As stapes pushes on oval window, basilar membrane distorts toward round window Opposite action when stapes retracts

Basilar membrane distorts Basilar membrane distorts
The movement of the flexible basilar membrane in response to sound waves with a frequency of 6000 Hz Stapes at oval window Cochlea 16,000 Hz 6000 Hz 1000 Hz Round window Basilar membrane Stapes moves inward Basilar membrane distorts toward round window Round window pushed outward Figure The organ of Corti provides sensations of pitch and volume Stapes moves outward Round window pulled inward Basilar membrane distorts toward oval window Figure 75

Events Involved in Hearing
Sound waves arrive at the tympanic membrane. Movement of the tympanic membrane causes displacement of the auditory ossicles. Movement of the stapes at the oval window establishes pressure waves in the perilymph of the vestibular duct. The pressure waves distort the basilar membrane on their way to the round window of the tympanic duct. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane. Information about the region and the intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII. Tympanic duct Basilar membrane Cochlear duct Vestibular membrane Movement of sound waves Vestibular duct Figure The organ of Corti provides sensations of pitch and volume Tympanic membrane Round window Figure 76

Module 15.8 Review a. Define decibel.
b. Beginning at the external acoustic meatus, list, in order, the structures involved in hearing. c. How would sound perception be affected if the round window could not bulge out as a result of increased perilymph pressure?

Module 15.9: Sensory pathways for equilibrium and hearing
Sensory pathway for equilibrium Hair cells in vestibule and semicircular canals become stimulated Sensory neurons take information through vestibular branch of vestibulocochlear nerve (VIII) Vestibular nuclei integrate information from both ears and relay to cerebral cortex, cerebellum, and motor nuclei of brain stem and spinal cord

Motor responses for equilibrium Automatic movements of eyes Directed by superior colliculi Keep gaze focused on point despite movement Distribution of motor commands to motor nuclei for cranial nerves controlling eye, head, neck movements (CN III, IV, VI, XI) Vestibular nuclei relay information to cerebellum Information relayed down vestibulospinal tracts of spinal cord to adjust peripheral muscle tone and coordinate with head and neck movements

The path of equilibrium information from receptors to the brain stem to muscular effectors
Hair cells of the vestibule and semicircular ducts monitor body position and motion. Sensory neurons located in adjacent vestibular ganglia carry information from the hair cells. These sensory fibers form the vestibular branch of the vestibulocochlear nerve (VIII). The vestibular nuclei in the medulla oblongata integrate sensory information from both ears, and relay that information to the cerebral cortex, cerebellum, and motor nuclei in the brain stem and spinal cord. The automatic movements of the eyes that occur in response to sensations of motion are directed by the superior colliculi. These movements attempt to keep your gaze focused on a specific point in space, despite changes in body position and orientation. The reflexive motor commands issued by the vestibular nuclei are distributed to the motor nuclei for the cranial verves involved with eye, head, and neck movements (N III, N IV, N VI, and N XI). Vestibular ganglion The vestibular nuclei relay information about position and balance to the cerebellum. Figure The vestibulocochlear nerve carries equilibrium and hearing sensations to the brain stem Semicircular canals Vestibule Instructions descending in the vestibulospinal tracts of the spinal cord adjust peripheral muscle tone and complement the reflexive movements of the head or neck. Cochlear branch Vestibulocochlear nerve (VIII) Figure 80

Figure The vestibulocochlear nerve carries equilibrium and hearing sensations to the brain stem Figure 81

Sensory pathway for hearing Hair cells in specific area of basilar membrane become stimulated Sensory neurons relay information through cell bodies in spiral ganglion to cochlear branch of vestibulocochlear nerve (VIII) Information reaches cochlear nuclei of medulla oblongata and ascends to midbrain From inferior colliculi of midbrain, auditory sensations synapse at medial geniculate nucleus of thalamus Projection fibers relay information to different areas of auditory cortex in temporal lobe

Sensory pathway for hearing Most auditory information from one cochlea is projected to the auditory cortex on the opposite side Some information from cochlea on the same side also is received by auditory cortex These interconnections aid in localizing sounds and reduce functional impact of damage to a cochlea or ascending pathway

Motor responses for hearing Inferior colliculi coordinate a number of reflexive responses involving skeletal muscles of head, neck, and trunk Reflexes automatically change position of head in response to a noise (usually toward source)

Module 15.9 Review a. Where are the hair cells for equilibrium located? b. Which cranial nerves are involved with eye, head, and neck movements? c. What is your reflexive response to hearing a loud noise, such as a firecracker?

Section 3: Vision Learning Outcomes
Identify the accessory structures of the eye and explain their functions. Describe the layers of the wall of the eye and the anterior and posterior cavities of the eye. Explain how light is directed to the fovea of the retina. Describe the process by which images are focused on the retina.

Describe the structure and function of the retina’s layers of cells, and the distribution of rods and cones and their relation to visual acuity. Explain photoreception, describe the structure of the photoreceptors, explain how visual pigments are activated, and describe how we are able to distinguish colors. Explain how the visual pathways distribute information to their destinations in the brain.

15.17 CLINICAL MODULE Describe various accommodation problems associated with the cornea, lens, or shape of the eye. 15.18 CLINICAL MODULE Describe age-related disorders of olfaction, gustation, vision, equilibrium, and hearing.

Section 3: Vision Eye development
Optic vesicles form in prosencephalon lateral walls Contain cavity continuous with neurocoel Optic cups form as lateral bulges become indented Remain connected to diencephalon by slender stalks Overlying epidermis pinches off and becomes lens Retina develops Ependymal cells on optic cup outer wall become photo receptors Ependymal cells on optic cup inner wall become pigmented cells Neural tissue on optic cup outer wall becomes neurons, ganglion cells, and specialized glial cells

Section 3: Vision Eye development (continued)
Supporting layers of connective tissue develop from aggregating mesoderm around optic cup Fluid-filled interior chambers develop

vesicles in the lateral walls of the prosencephalon
The formation of optic vesicles in the lateral walls of the prosencephalon Optic vesicle Figure 15 Section 3 Vision Figure 15 Section 91

Layers of Developing Retina
The ependymal cells on the outer wall of the optic cup develop into the photoreceptors. The formation of optic cups that remain connected to the diencephalon by slender stalks Optic cup The ependymal cells on the inner wall of the optic cup develop into pigment cells that absorb light that has passed through the photoreceptor layer. The separation gradually decreases until the photoreceptors and pigment cell layers are in contact. Developing lens The neural tissue of the outer wall of the optic cup forms layers of neurons, ganglion cells, and specialized glial cells that are responsible for preliminary processing and integration of visual information. Figure 15 Section 3 Vision Week 5 Figure 15 Section 92

Week 6 The development of interior chambers filled with fluid
that is continuously generated and reabsorbed Optic nerve, N II (follows path of original connecting stalk) Lens Eyelids Fluid-filled chambers Connective tissue layers Figure 15 Section 3 Vision Week 6 Figure 15 Section 93

Module 15.10: Eye accessory structures
Eyelids (palpebrae) Eyelashes (hairs that help prevent foreign particles from reaching the eye) Medial canthus (medial connection) Lateral canthus (lateral connection) Tarsal glands (along inner margin of lids, secretion keeps eyelids from sticking together)

Eyelids (continued) Palpebral fissure (space between eyelids) Eye structures seen within: Cornea (transparent anterior surface) Iris (colored part of eye) Pupil (hole that light passes through in center of iris) Lacrimal caruncle (produces thick secretion during sleep)

The accessory structures of the eye
Cornea Eyelids and Eyelashes Eyelashes Lateral canthus Palpebra (eyelid) Medial canthus Pupil within iris Palpebral fissure Figure The accessory structures of the eye provide protection while allowing light to reach the interior of the eye Lacrimal caruncle Figure 96

Conjunctiva Epithelium lining surface of eye and inner eyelids Ocular conjunctiva Continuous with thin corneal conjunctiva Palpebral conjunctiva Fornix (pocket where conjunctivae meet) Mucous membrane covered by stratified squamous epithelium Conjunctivitis Swelling associated with conjunctiva due to damage to/irritation of conjunctiva May be caused by infection, physical, allergic, or chemical irritation

The conjunctiva, the specialized epithelium
covering the inner surfaces of the eyelids and the outer surface of the eye Tarsal glands (Meibomian glands) Conjunctiva Palpebral conjunctiva Ocular conjunctiva Fornix Figure The accessory structures of the eye provide protection while allowing light to reach the interior of the eye Cornea Figure 98

Conjunctivitis, or pinkeye
Figure The accessory structures of the eye provide protection while allowing light to reach the interior of the eye Conjunctivitis, or pinkeye Figure 99

Lacrimal apparatus components Lacrimal gland Almond-shaped gland that produces tears (~1 mL/day) Tear ducts (10–12) Deliver tears from gland to under upper eyelid Lacrimal puncta Two small pores that drain lacrimal lake

Lacrimal apparatus components (continued) Lacrimal canaliculi Small canals connecting puncta to lacrimal sac Lacrimal sac Chamber in lacrimal sulcus of orbit Nasolacrimal duct Delivers tears from lacrimal sac into nasal cavity inferior and lateral to inferior nasal concha Animation: The Eye: Accessory Structures

The lacrimal apparatus
Superior rectus muscle Components of the Lacrimal Apparatus Lacrimal gland Tear ducts Upper eyelid Lacrimal puncta Lower eyelid Lacrimal canaliculi Orbital fat Lacrimal sac Nasolacrimal duct Figure The accessory structures of the eye provide protection while allowing light to reach the interior of the eye Inferior rectus muscle Drainage of the nasolacrimal duct into the inferior meatus Inferior oblique muscle Figure 102

Function of tears Reduce friction Remove debris Prevent bacterial infection with lysozyme and antibodies Provide nutrients and oxygen to portions of conjunctiva

Module Review a. List the accessory structures associated with the eye. b. Explain conjunctivitis. c. Which layer of the eye would be the first affected by inadequate tear production?

Module 15.11: Eye layers and cavities
Three layers of the eye (tunics) Fibrous tunic Outermost layer of eye Consists of cornea (clear) and sclera (white) Joined at corneal limbus Functions Provides mechanical support and some physical protection Attachment site for extrinsic eye muscles Contains structures assisting in focusing process

Three layers of the eye (continued) Vascular tunic (uvea) Iris (colored part of eye that controls size of pupil) Anterior: incomplete layer of fibroblasts and melanocytes Posterior: pigmented epithelium of neural tunic Color determined by: Genes influencing density and distribution of melanocytes Density of pigmented epithelium Blue: less melanin, light reaches pigmented layer Green, brown, black: increasing melanin

Three layers of the eye (continued) Vascular tunic (continued) Ciliary body (thickened region connecting to lens) Ciliary muscle (smooth-muscle ring) Ciliary processes (epithelial folds covering muscle) Suspensory ligaments (attach to lens) Choroid (vascular layer covered by sclera) Contains extensive capillary network delivering oxygen and nutrients to neural tissue in neural tunic Animation: The Eye: Uvea Parts

Three layers of the eye (continued) Neural tunic (retina) Innermost layer of the eye Two layers Pigmented layer (outer) Absorbs light Ora serrata (jagged anterior edge) Neural layer (inner) Contains photoreceptors sensitive to light Animation: The Eye

The three tunics of the wall of the eye
Fibrous Tunic The outermost layer of the eye, consisting of the cornea and the sclera Sclera Corneal limbus Cornea Vascular Tunic The middle layer of the eye; also called the uvea; contains numerous blood vessels, lymphatic vessels, and the intrinsic (smooth) muscles of the eye Iris Lens Optic nerve Ciliary body Figure The eye has a layered wall; it is hollow, with fluid-filled anterior and posterior cavities Choroid Neural Tunic The innermost layer of the eye; also known as the retina; contains photoreceptors Figure 109

Anterior Cavity Posterior Cavity A sectional view showing that the
ciliary body and the lens divide the interior of the eye into a small anterior cavity and a large posterior cavity Anterior Cavity Cornea Anterior chamber Iris Ciliary body Lens Posterior chamber Posterior Cavity Contains the vitreous body Figure The eye has a layered wall; it is hollow, with fluid-filled anterior and posterior cavities Optic nerve Figure 110

Eye cavities Anterior cavity (cornea to lens) Filled with aqueous humor Two chambers Anterior chamber (cornea to iris) Posterior chamber (iris to ciliary body and lens) Posterior cavity (main volume of eye) Filled with gelatinous vitreous body (humor)

Aqueous humor Secreted by epithelia of ciliary processes Rate of 1–2 µL/min Similar to CSF Circulates between anterior and posterior chambers Distributes nutrients and wastes Acts as fluid cushion Helps to retain eye shape Reabsorbed at canal of Schlemm (at corneal limbus) Into veins in sclera Reabsorption rate is approximately the same as production Tonometry (measures intraocular pressure)

Posterior cavity (vitreous chamber)
A view of the anterior and posterior cavities of the eye Site of secretion of aqueous humor Iris Cornea Ciliary muscle Anterior chamber Canal of Schlemm Lens Conjunctiva Ciliary processes Figure The eye has a layered wall; it is hollow, with fluid-filled anterior and posterior cavities Posterior cavity (vitreous chamber) Ora serrata Suspensory ligaments Figure – 4 113

Module 15.11 Review a. Name the three tunics of the eye.
b. What give eyes their characteristic color? c. Where in the eye is aqueous humor located?

Module 15.12: Visual axis structures
Imaginary line drawn from object in view through structures of the eye to retina Structures Cornea Dense matrix of collagen fibers organized to permit light Has no blood vessels Must get oxygen and nutrients from tears Animation: The Eye: Light Path

Visual axis (continued) Structures (continued) Pupil (size controlled by two sets of muscles) Pupillary dilator muscles Extend radially from pupil edge Contraction enlarges pupil Stimulated by sympathetic nervous system Pupillary constrictor muscles Concentrically arranged around pupil Contraction constricts pupil Stimulated by parasympathetic nervous system Animation: The Eye: Interior Parts of Eye

Visual axis (continued) Structures (continued) Lens Concentric layers of cells Cells filled with transparent proteins (crystallins) Give clarity and focusing power Dense fibrous capsule Connects to suspensory ligaments Animation: The Eye: Lens and Retina

Visual axis (continued) Structures (continued) Retina Macula lutea (highest photoreceptor concentration) Contains fovea (shallow depression) that is the site of sharpest vision

A sectional view showing aspects of eye anatomy associated with
positioning the eye and allowing light to reach the photoreceptors of the retina Cornea: transparent and lacks blood vessels Lens: consists of concentric layers of cells filled with transparent proteins called crystallins Suspensory ligaments: resist the tendency of the lens to assume a spherical shape Ciliary body: supports the lens and controls its shape. Nose Retina: contains the photoreceptors, pigment cells, supporting cells, and neurons Blood vessels of the choroid: directly or indirectly provide nutrients to all structures within the eye. Sclera (“white of the eye”): stabilizes the shape of the eye during eye movements and is site of insertion of the six extrinsic eye muscles Figure The eye is highly organized and has a consistent visual axis that directs light to the fovea of the retina Optic nerve (N II): carries visual information to the brain Figure 119

Pupillary constrictor
How the two layers of the pupillary muscles of the iris control the amount of light entering the eye Pupillary constrictor (sphinctor) Pupillary dilator (radial) The pupillary dilator muscles extend radially away from the edge of the pupil. Contraction of these muscles enlarges the pupil. The pupillary constrictor muscles form a series of concentric circles around the pupil. When these sphincter muscles contract, the diameter of the pupil decreases. Figure The eye is highly organized and has a consistent visual axis that directs light to the fovea of the retina Decreased light intensity Increased sympathetic stimulation Increased light intensity Increased parasympathetic stimulation Figure 120

How light passing along the visual axis of the eye
strikes a specific location that contains the highest density of photoreceptors Iris Visual axis of the eye Nose Lens Neural portion of the retina: site of the photoreceptors Choroid Sclera Fovea in center of macula lutea: site of sharpest vision Figure The eye is highly organized and has a consistent visual axis that directs light to the fovea of the retina Orbital fat Figure 121

Module Review a. Which eye structure does not contain blood vessels? b. List the structures and fluids that light passes through from the cornea to the retina. c. What happens to the pupils when light intensity decreases?

Module 15.13: Focusing at the retina
Creating a focused image of an object involves bending (refracting) light rays together to create a focal point on the retina Two refracting structures of eye Cornea Lens Focal distance (distance between center of lens and focal point) Determined by distance from object to lens and shape of lens Accommodation (changing lens shape to keep focal distance constant)

Focal distance, which is determines by the
shape of the lens and the distance between the lens and the object being viewed Focal distance Light from distant source (object) Focal distance Close source Focal point Lens The closer the light source, the longer the focal distance Figure Focusing produces a sharply defined image at the retina Focal distance The rounder the lens, the shorter the focal distance Figure 124

Accommodation Lens shape changed by ciliary muscle action For close vision: Ciliary muscle contracts, moving toward lens  suspensory ligaments’ tension relaxes lens  lens shape becomes more spherical  lens increases refractive power Near point of vision (inner limit of clear vision) Increases with age as lens becomes inflexible

Accommodation (continued) Lens shape changed by ciliary muscle action (continued) For distance vision: Ciliary muscle relaxes  suspensory ligament tension increases  lens becomes flatter  lens decreases refractive power Animation: The Eye: Cilliary Muscles

keep the focal distance constant
Accommodation, the process by which the eye focuses images on the retina by changing the shape of the lens to keep the focal distance constant Accommodation when objects viewed are far from the eye Accommodation when objects viewed are near the eye For Close Vision: Ciliary Muscle Contracted, Lens Rounded For Distant Vision: Ciliary Muscle Relaxed, Lens Flattened Focal point on fovea When the ciliary muscle contracts, the ciliary body moves toward the lens, thereby reducing the tension in the suspensory ligaments. The elastic capsule of the lens then pulls it into a more spherical shape that increases the refractive power of the lens, enabling it to bring light from nearby objects into focus on the retina. When the ciliary muscle relaxes, the suspensory ligaments pull at the circumference of the lens, making the lens flatter and bringing the image of a distant object into focus on the retina. Figure Focusing produces a sharply defined image at the retina Figure 127

Image formation Most objects are more than just one point Full images invert and reverse passing through lens Brain compensates during processing

The inversion and reversal of images projected onto the retina
A sagittal section through an eye showing the inversion of the image of a viewed object Figure Focusing produces a sharply defined image at the retina A sagittal section through an eye showing the reversal of the image of a viewed object Figure 129

Module 15.13 Review a. Define focal point.
b. When the ciliary muscles are relaxed, are you viewing something close up or something in the distance? c. Why does the near point of vision typically increase with age?

Module 15.14: Retinal layers and cells
Pigmented part Absorbs light passing through neural part Keeps light from bouncing around retina Has important biochemical interactions with neural part Neural part Ganglion cells (innermost layer) Axons converge on optic disc Also called blind spot since lacking photoreceptors Axons exit eye through optic nerve Animation: The Eye: Blind Spot

Neural Part of the Retina Pigmented Part of the Retina
A diagrammatic sectional view through the eye showing the retina near the origin of the optic nerve Neural Part of the Retina Pigmented Part of the Retina Contains photoreceptors, supporting cells, and neurons that perform preliminary processing and integration of visual information Absorbs light that passes through the neural part, preventing light from bouncing back and producing visual “echoes” Layer closest to the pigmented part of the retina; contains the photoreceptors Ganglion cells Optic disc (blind spot) Central retinal vein Central retinal artery Figure The neural tissue of the retina contains multiple layers of specialized photoreceptors, neurons, and supporting cells Optic nerve Blood vessels entering and leaving the interior of the eye within the optic nerve Sclera Choroid Figure 132

A photograph of the retinal surface, taken through the cornea, pupil, and lens of the right eye
Optic disc (blind spot) Fovea Figure The neural tissue of the retina contains multiple layers of specialized photoreceptors, neurons, and supporting cells Macula lutea Central retinal artery and vein emerging from center of optic disc Figure 133

Retinal layers (continued) Neural part (continued) Bipolar cells Synapse with photoreceptors and ganglion cells Amacrine cells Facilitate or inhibit communication between ganglion cells and bipolar cells

Retinal layers (continued) Neural part (continued) Horizontal cells Facilitate or inhibit communication between photoreceptors and bipolar cells Photoreceptors (outermost layer) Rods (low light, monochromatic vision) Cones (high light, color vision) Animation: The Eye: The Retina

A sectional view showing the retina’s multiple
layers of specialized cells Pigmented part of retina Photoreceptors of the Retina Rods (for vision under dimly lit conditions) Horizontal and Amacrine Cells Cones (provide color vision under brightly lit conditions) Facilitate or inhibit communication between photoreceptors and ganglion cells, thereby altering the sensitivity of the retina Horizontal cells Figure The neural tissue of the retina contains multiple layers of specialized photoreceptors, neurons, and supporting cells Amacrine cells Bipolar cells Ganglion cells LIGHT Figure 136

Photoreceptor distribution in retina Cones Maximum density at fovea (no rods) Has highest visual acuity (sharp vision) ~6 million in retina overall Rods Maximum density at retina periphery ~125 million in retina overall

The relative densities of cones and rods on either
The retina of each eye contains approximately 6 million cones. The density of cones reaches its maximum at the fovea of the macula lutea, where there are no rods. Low Density of Cones High Density of Cones Fovea Optic disc The retina contains approximately 125 million rods. The density of rods is highest at the periphery of the retina, where there are very few cones. Low Density of Rods High Density of Rods Visual acuity Plot of sharpness of vision (visual acuity) along the horizontal line; note the direct correlation between visual acuity and cone density Lateral border Figure The neural tissue of the retina contains multiple layers of specialized photoreceptors, neurons, and supporting cells Fovea Nasal border The relative densities of cones and rods on either side of a horizontal line passing through the fovea and optic disc of the right eye Figure 138

Module 15.14 Review a. Define rods and cones.
b. If you enter a dimly lit room, will you be able to see clearly? Why or why not? c. If you had been born without cones in your eyes, explain why you would or would not be able to see.

Module 15.15: Photoreception
Photoreceptors Detect photons of light Light energy also occurs as a wave Our visible spectrum of light is 400–700 nm Contain visual pigments that transduce light Are derivatives of rhodopsin (pigment in rods) Consist of: Retinal (pigment synthesized from vitamin A) Opsin (protein that determines wavelength absorption of pigment)

The structure of rhodopsin (visual purple), the visual pigment found
molecule Retinal Opsin Figure Photoreception, which occurs in the outer segment of rods and cones, involves the activation of visual pigments The structure of rhodopsin (visual purple), the visual pigment found in rods Figure 141

Photoreceptor structure Outer segment Contains flattened membranous plates or discs Contain visual pigment In cones, are plasma membrane infoldings In rods, each disc is separate entity In cones, tapered In rods, elongate cylinder Inner segment Contains organelles for maintaining cell

Structure of Cones Structure of Rods Pigment Epithelium Discs of cones: are infoldings of the plasma membrane and taper to a blunt point The pigment epithelium absorbs photons that are not absorbed by visual pigments. Melanin granules Discs of rods: are independent entities and form an elongated cylinder Outer Segment The outer segment of a photoreceptor contains flattened membranous plates, or discs, that contain the visual pigments. Discs Connecting stalks Inner Segment Mitochondria The inner segment contains the photoreceptor’s major organelles and is responsible for all cell functions other than photoreception. Golgi apparatus Figure Photoreception, which occurs in the outer segment of rods and cones, involves the activation of visual pigments Nuclei Cone Rods Each photoreceptor synapses with a bipolar cell. Bipolar cell LIGHT The major structural features of rods and cones, and the adjacent pigment epithelium and bipolar cells Figure 143

Steps of phototransduction Light absorption makes retinal molecule more linear Opsin activity changes Na+ outer segment permeability and changes neurotransmitter release to bipolar cell Bipolar cell activity changes are relayed to ganglion cells Steps of photopigment regeneration Pigment breaks down into retinal and opsin (= bleaching) Retinal converted to original shape (requires ATP) Converted retinal can recombine with opsin Animation: Photoreception

Color vision Three types of cones Blue cones (16% of all cones) Green cones (10%) Red cones (74%) Combined differential stimulation allows brain to discern colors All stimulated equally = white

The range of wavelength sensitivities for the three
types of cones, each of which contains a different form of opsin Rods Blue cones Red cones Green cones (percent of maximum) Light absorption Figure Photoreception, which occurs in the outer segment of rods and cones, involves the activation of visual pigments Violet Blue Green Yellow Orange Red Figure 146

Module 15.15 Review a. Identify the three types of cones.
b. Compare rods with cones. c. How could a diet deficient in vitamin A affect vision?

Module 15.16: Visual pathways
Receptors  bipolar cells  ganglion cells ~1 million ganglion cells converge at optic disc Optic nerves converge at optic chiasm to reach thalamus At thalamus Half of the fibers to lateral geniculate nucleus on same side Half of the fibers to lateral geniculate nucleus on opposite side Fibers radiate to visual cortex of occipital lobe = Optic radiation

How the visual pathways transmit information
from both eyes to the visual cortex of each cerebral hemisphere Combined Visual Field Left side Right side Left eye only Binocular vision Right eye only The Visual Pathways The visual pathways begin at the photoreceptors in the retina. Each photoreceptor monitors a specific receptive field, and when stimulated, passes the information through a bipolar cell and to a ganglion cell. Axons from the approximately 1 million ganglion cells converge on the optic disc, penetrate the wall of the eye, and proceed toward the diencephalon as the optic nerve (II). Retina Optic disc The two optic nerves, one from each eye, reach the diencphalon at the optic chiasm. From that point, approximately half the fibers proceed toward the lateral geniculate nucleus of the same side of the brain, whereas the other half cross over to reach the lateral geniculate nucleus of the opposite side. Diencephalon and brain stem Collaterals from fibers synapsing in the lateral geniculate nuclei Optic tract From each lateral geniculate nucleus, visual information travels to the occipital cortex of the cerebral hemisphere on that side. The bundle of projection fibers linking each lateral geniculate nucleus with the visual cortex is known as the optic radiation. Figure The visual pathways distribute visual information from each eye to both cerebral hemispheres Lateral geniculate nucleus Superior colliculus The perception of a visual image reflects the integration of information that arrives at the visual cortex of the occipital lobes. Each eye receives a slightly different visual image, because (1) the foveae are 5–7.5 cm (2–3.0 in.) apart, and (2) the nose and eye socket block the view of the opposite side. Projection fibers (optic radiation) Left cerebral hemisphere Right cerebral hemisphere Figure 15.16 149

Module 15.16: Visual pathways
Depth perception Interpretation of 3-D relationships of objects in view Obtained by comparing relative positions of objects between images from both eyes Perception varies due to position of each eye Each visual cortex receives overlapping visual field information for both eyes

Module 15.16 Review a. Define optic radiation.
b. Where are visual images perceived? c. Trace the visual pathway, beginning at the photoreceptors in the retina.

CLINICAL MODULE 15.17: Accommodation problems
Emmatropia (emmetro-, proper + opia, vision) Normal vision Distant image focused on retinal surface Ciliary muscle relaxed and lens flattened Myopia (myein, to shut + ops, eye) Nearsightedness Focal distance too short (in front of retina) Cause may be: Eyeball too deep Resting curvature of lens too great Corrected with diverging (concave) lens in front of eye

Hyperopia Farsightedness Focal distance too long (in back of retina) Cause may be: Eyeball is too shallow Lens is too flat Corrected with converging (convex) lens in front of eye

The shape of the eye and the site at which light is focused for three conditions
Enmetropia, or normal vision Myopia, or nearsighted vision Emmetropia Myopia Diverging lens In the normal healthy eye, when the ciliary muscle is relaxed and the lens is flattened, the image of a distant object will be focused on the retina’s surface. This condition is called emmetropia (emmetro-, proper + opia, vision), or normal vision. If the eyeball is too deep or the resting curvature of the lens is too great, the image of a distant object is projected in front of the retina. Such individuals are said to be nearsighted because vision at close range is clear but distant objects are blurry and out of focus. Their condition is more formally termed myopia (myein, to shut + ops, eye). Myopia can be treated by placing a diverging lens in front of the eye. Diverging lenses have at least one concave surface and spread the light rays apart as if the object were closer to the viewer. Hyperopia, or farsighted vision Hyperopia Figure Accommodation problems result from abnormalities in the cornea or lens, or in the shape of the eye If the eyeball is too shallow or the lens is too flat, hyperopia results. The ciliary muscle must contract to focus even a distant object on the retina, and at close range the lens cannot provide enough refraction to focus an image on the retina. Individuals with this problem are said to be farsighted, because they can see distant objects most clearly. Hyperopia can be corrected by placing a converging lens in front of the eye. Converging lenses have at least one convex surface and provide the additional refraction needed to bring nearby objects into focus. Figure 15.17 154

Surgical correction Photorefractive keratectomy (PRK) Laser shapes cornea Removes 10–20 µm (<10%) of cornea Laser-assisted in-situ keratomileusis (LASIK) Interior corneal layers reshaped and covered by normal corneal epithelium ~70% of LASIK patients achieve normal vision ~10 million people have had corrective procedure Immediate and long-term visual problems can occur

Myopia, or nearsighted vision
Diverging lens If the eyeball is too deep or the resting curvature of the lens is too great, the image of a distant object is projected in front of the retina. Such individuals are said to be nearsighted because vision at close range is clear but distant objects are blurry and out of focus. Their condition is more formally termed myopia (myein, to shut + ops, eye). Myopia can be treated by placing a diverging lens in front of the eye. Diverging lenses have at least one concave surface and spread the light rays apart as if the object were closer to the viewer. Figure Accommodation problems result from abnormalities in the cornea or lens, or in the shape of the eye Figure 15.17 156

CLINICAL MODULE 15.17 Review
a. Define emmetropia. b. Discuss two surgical procedures for correcting myopia and hyperopia. c. Which type of lens would correct hyperopia?

CLINICAL MODULE 15.18: Disorders of the special senses
Olfaction disorders May relate to nerve (N I) or receptor damage Aging issues Number of receptors decline with age Remaining receptors are less sensitive Gustation disorders Damage to taste buds (mouth infection, inflammation) Damage to cranial nerves (N VII, IX, X) Problems with olfactory receptors

Figure Aging is associated with many disorders of the special senses; trauma, infection, and abnormal stimuli may cause problems at any age Figure 159

Figure Aging is associated with many disorders of the special senses; trauma, infection, and abnormal stimuli may cause problems at any age Figure 160

Vision disorders Many mentioned previously Cataracts (opaque lens) Can result from: Injury Radiation Reaction to drugs Aging (senile cataracts) Most common Damaged lens can be replaced by synthetic lens

Normal eye Eye with cataract
Figure Aging is associated with many disorders of the special senses; trauma, infection, and abnormal stimuli may cause problems at any age Eye with cataract Figure 162

Equilibrium disorders Vertigo (illusion of movement) Caused by conditions that alter function of: Inner ear receptor complex Usually affect endolymph Vestibular branch of N VII Sensory nuclei and CNS pathways Motion sickness (most common cause)

Hearing disorders Conductive deafness (issue between tympanic membrane and oval window) Causes include: Excess wax or trapped water in external ear Scarring or perforation of tympanic membrane Immobility of ear ossicles (fluid or tumor)

Hearing disorders (continued) Nerve deafness (issue with cochlea or along auditory pathway) Damage to receptors 20–20,000 Hz in young children but hearing loss later Neural damage to cochlear branch of N VIII Caused by loud noises or pathogenic infections

CLINICAL MODULE 15.18 Review
a. Which cranial nerves provide taste sensations from the tongue? b. Indentify two common classes of hearing-related disorders. c. What causes vertigo?

15 The Special Senses.

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