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Special Senses
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The Senses General senses of touch Temperature Pressure Pain
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The Senses Special senses Smell Taste Sight Hearing Equilibrium
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The Eye and Vision 70% of all sensory receptors are in the eyes
Each eye has over a million nerve fibers Protection for the eye Most of the eye is enclosed in a bony orbit A cushion of fat surrounds most of the eye
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Accessory Structures of the Eye
Eyelids and eyelashes Conjunctiva Lacrimal apparatus Extrinsic eye muscles
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Accessory Structures of the Eye
Eyelids and eyelashes Tarsal glands lubricate the eye Ciliary glands are located between the eyelashes
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Accessory Structures of the Eye
Conjunctiva Membrane that lines the eyelids Connects to the surface of the eye Secretes mucus to lubricate the eye
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Accessory Structures of the Eye
Lacrimal apparatus Lacrimal gland—produces lacrimal fluid Lacrimal canals—drain lacrimal fluid from eyes
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Accessory Structures of the Eye
Lacrimal apparatus (ctd) Lacrimal sac—provides passage of lacrimal fluid towards nasal cavity Nasolacrimal duct—empties lacrimal fluid into the nasal cavity
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Accessory Structures of the Eye
Function of the lacrimal apparatus Protects, moistens, and lubricates the eye Empties into the nasal cavity Properties of lacrimal fluid Dilute salt solution (tears) Contains antibodies and lysozyme
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Accessory Structures of the Eye
Extrinsic eye muscles Six muscles attach to the outer surface of the eye Produce eye movements
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Accessory Structures of the Eye
Figure 8.3a–b
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Accessory Structures of the Eye
Figure 8.3c
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Structure of the Eye Layers forming the wall of the eyeball
Fibrous layer Outside layer Vascular layer Middle layer Sensory layer Inside layer
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Structure of the Eye Figure 8.4a
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Structure of the Eye Figure 8.4b
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Structure of the Eye: The Fibrous Layer
Sclera White connective tissue layer Seen anteriorly as the “white of the eye” Cornea Transparent, central anterior portion Allows for light to pass through Repairs itself easily The only human tissue that can be transplanted without fear of rejection
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Structure of the Eye: Vascular Layer
Choroid is a blood-rich nutritive layer in the posterior of the eye Pigment prevents light from scattering Modified anteriorly into two structures Ciliary body—smooth muscle attached to lens Iris—regulates amount of light entering eye Pigmented layer that gives eye color Pupil—rounded opening in the iris
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Structure of the Eye: Sensory Layer
Retina contains two layers Outer pigmented layer Inner neural layer Contains receptor cells (photoreceptors) Rods Cones
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Structure of the Eye: Sensory Layer
Signals pass from photoreceptors via a two-neuron chain Bipolar neurons Ganglion cells Signals leave the retina toward the brain through the optic nerve Optic disc (blind spot) is where the optic nerve leaves the eyeball Cannot see images focused on the optic disc
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Structure of the Eye: Sensory Layer
Figure 8.5a
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Structure of the Eye: Sensory Layer
Figure 8.5b
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Structure of the Eye: Sensory Layer
Neurons of the retina and vision Rods Most are found towards the edges of the retina Allow dim light vision and peripheral vision All perception is in gray tones
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Structure of the Eye: Sensory Layer
Neurons of the retina and vision Cones Allow for detailed color vision Densest in the center of the retina Fovea centralis—area of the retina with only cones No photoreceptor cells are at the optic disc, or blind spot
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Structure of the Eye: Sensory Layer
Cone sensitivity Three types of cones Different cones are sensitive to different wavelengths Color blindness is the result of the lack of one cone type
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Sensitivities of Cones to Different Wavelengths
Figure 8.6
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Lens Biconvex crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body
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Lens Cataracts result when the lens becomes hard and opaque with age
Vision becomes hazy and distorted Eventually causes blindness in affected eye
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Lens Figure 8.7
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Two Segments, or Chambers, of the Eye
Anterior (aqueous) segment Anterior to the lens Contains aqueous humor Posterior (vitreous) segment Posterior to the lens Contains vitreous humor
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Anterior Segment Aqueous humor
Watery fluid found between lens and cornea Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm
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Posterior Segment Vitreous humor
Gel-like substance posterior to the lens Prevents the eye from collapsing Helps maintain intraocular pressure
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Ophthalmoscope Instrument used to illuminate the interior of the eyeball Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina Posterior Wall of Retina as Seen with Ophthalmoscope
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Pathway of Light Through the Eye
Light must be focused to a point on the retina for optimal vision The eye is set for distance vision (over 20 feet away) Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away)
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Pathway of Light Through the Eye
Figure 8.9
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Pathway of Light Through the Eye
Image formed on the retina is a real image Real images are Reversed from left to right Upside down Smaller than the object
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Visual Fields and Visual Pathways
Optic chiasma Location where the optic nerves cross Fibers from the medial side of each eye cross over to the opposite side of the brain Optic tracts Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye
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Visual Fields and Visual Pathways
Figure 8.11
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Eye Reflexes Internal muscles are controlled by the autonomic nervous system Bright light causes pupils to constrict through action of radial, circular, and ciliary muscles Viewing close objects causes accommodation External muscles control eye movement to follow objects Viewing close objects causes convergence (eyes moving medially)
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Eye Reflexes
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Pupillary dilation and constriction
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
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A Closer Look Emmetropia—eye focuses images correctly on the retina
Myopia (nearsighted) Distant objects appear blurry Light from those objects fails to reach the retina and are focused in front of it Results from an eyeball that is too long
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A Closer Look Hyperopia (farsighted)
Near objects are blurry while distant objects are clear Distant objects are focused behind the retina Results from an eyeball that is too short or from a “lazy lens”
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A Closer Look Astigmatism Images are blurry
Results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens
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Homeostatic Imbalances of the Eyes
Night blindness—inhibited rod function that hinders the ability to see at night Color blindness—genetic conditions that result in the inability to see certain colors Due to the lack of one type of cone (partial color blindness) Cataracts—when lens becomes hard and opaque, our vision becomes hazy and distorted Glaucoma—can cause blindness due to increasing pressure within the eye
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The Ear Houses two senses Hearing Equilibrium (balance)
Receptors are mechanoreceptors Different organs house receptors for each sense
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Anatomy of the Ear The ear is divided into three areas
External (outer) ear Middle ear (tympanic cavity) Inner ear (bony labyrinth)
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Anatomy of the Ear Figure 8.12
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The External Ear Involved in hearing only
Structures of the external ear Auricle (pinna) External acoustic meatus (auditory canal) Narrow chamber in the temporal bone Lined with skin and ceruminous (wax) glands Ends at the tympanic membrane
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The Middle Ear (Tympanic Cavity)
Air-filled cavity within the temporal bone Only involved in the sense of hearing
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The Middle Ear (Tympanic Cavity)
Two tubes are associated with the inner ear The opening from the auditory canal is covered by the tympanic membrane The auditory tube connecting the middle ear with the throat Allows for equalizing pressure during yawning or swallowing This tube is otherwise collapsed
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Bones of the Middle Ear (Tympanic Cavity)
Three bones (ossicles) span the cavity Malleus (hammer) Incus (anvil) Stapes (stirrip) Function Vibrations from eardrum move the malleus anvil stirrup inner ear
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Anatomy of the Ear Figure 8.12
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Inner Ear or Bony Labyrinth
Includes sense organs for hearing and balance Filled with perilymph A maze of bony chambers within the temporal bone Cochlea Vestibule Semicircular canals
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Anatomy of the Ear Figure 8.12
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Organs of Equilibrium Equilibrium receptors of the inner ear are called the vestibular apparatus Vestibular apparatus has two functional parts Static equilibrium Dynamic equilibrium
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Organs of Equilibrium Figure 8.14a–b
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Static Equilibrium Maculae—receptors in the vestibule
Report on the position of the head Send information via the vestibular nerve Anatomy of the maculae Hair cells are embedded in the otolithic membrane Otoliths (tiny stones) float in a gel around the hair cells Movements cause otoliths to bend the hair cells
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Structure and Function of Maculae
Figure 8.13a
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Structure and Function of Maculae
Figure 8.13b
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Dynamic Equilibrium Crista ampullaris—receptors in the semicircular canals Tuft of hair cells Cupula (gelatinous cap) covers the hair cells
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Dynamic Equilibrium Figure 8.14c
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Dynamic Equilibrium Action of angular head movements
The cupula stimulates the hair cells An impulse is sent via the vestibular nerve to the cerebellum
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Organs of Hearing Organ of Corti Located within the cochlea
Receptors = hair cells on the basilar membrane Gel-like tectorial membrane is capable of bending hair cells Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe
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Organs of Hearing Figure 8.15a
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Organs of Hearing Figure 8.15b
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Mechanism of Hearing Vibrations from sound waves move tectorial membrane Hair cells are bent by the membrane An action potential starts in the cochlear nerve Continued stimulation can lead to adaptation
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Mechanism of Hearing Figure 8.16a
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Mechanism of Hearing Figure 8.16b–c
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Olfaction—The Sense of Smell
Olfactory receptors are in the roof of the nasal cavity Neurons with long cilia Chemicals must be dissolved in mucus for detection Impulses are transmitted via the olfactory nerve Interpretation of smells is made in the cortex
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Olfactory Epithelium Figure 8.17
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The Sense of Taste Taste buds house the receptor organs
Location of taste buds Most are on the tongue Soft palate Cheeks
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The Tongue and Taste The tongue is covered with projections called papillae Filiform papillae—sharp with no taste buds Fungifiorm papillae—rounded with taste buds Circumvallate papillae—large papillae with taste buds Taste buds are found on the sides of papillae
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Taste Buds Figure 8.18
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Structure of Taste Buds
Gustatory cells are the receptors Have gustatory hairs (long microvilli) Hairs are stimulated by chemicals dissolved in saliva
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Taste Buds Figure 8.18
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Structure of Taste Buds
Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas Facial nerve Glossopharyngeal nerve Vagus nerve
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Taste Sensations Sweet receptors (sugars) Saccharine Some amino acids
Sour receptors Acids Bitter receptors Alkaloids Salty receptors Metal ions Umami (savory) receptors
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Chemical Senses: Taste and Smell
Both senses use chemoreceptors Stimulated by chemicals in solution Taste has five types of receptors Smell can differentiate a large range of chemicals Both senses complement each other and respond to many of the same stimuli ~80% of what you “taste” is actually determined by your smell!
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Developmental Aspects of the Special Senses
Formed early in embryonic development Eyes are outgrowths of the brain All special senses are functional at birth
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Developmental Aspects of the Special Senses
Eye problems Strabismus—“crossed eyes” results from unequal pulls by the external eye muscles in babies Ophthalmia neonatorum—conjunctivitis resulting from mother having gonorrhea. Baby’s eyelids are swollen and pus is produced Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging
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Developmental Aspects of the Special Senses
Ear problems Presbycusis—type of sensorineural deafness Otosclerosis—ear ossicles fuse
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Sensorineural deafness-damage to auditory pathway
Conduction deafness - Sound vibrations cannot be conducted to the inner ear Severe- SN Normal tympanic membrane Ruptured tympanic Membrane- conduction Mild- SN
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