Special Senses: Eye and Vision PDF

Special Senses Special Senses  Special senses include:  Smell  Taste  Sight  Hearing  Equilibrium  Special sense receptors  Large, complex sensory organs  Localized clusters of receptors Part I: The Eye and Vision  70 percent of all sensory receptors are in the eyes  Eac...

Special Senses Special Senses  Special senses include:  Smell  Taste  Sight  Hearing  Equilibrium  Special sense receptors  Large, complex sensory organs  Localized clusters of receptors Part I: The Eye and Vision  70 percent of all sensory receptors are in the eyes  Each eye has over 1 million nerve fibers carrying information to the brain  Accessory structures include the:  Extrinsic eye muscles  Eyelids  Conjunctiva  Lacrimal apparatus Figure 8.1 Surface anatomy of the eye and accessory structures. Site where Eyebrow conjunctiva merges with Eyelid cornea Eyelashes Palpebral Pupil fissure Lacrimal caruncle Lateral Medial commissure commissure (canthus) (canthus) Sclera Iris (covered by Eyelid conjunctiva) External and Accessory Structures  Eyelids  Meet at the medial and lateral commissure (canthus)  Eyelashes  Tarsal glands produce an oily secretion that lubricates the eye  Ciliary glands are located between the eyelashes  Conjunctiva  Membrane that lines the eyelids and eyeball  Connects with the transparent cornea  Secretes mucus to lubricate the eye and External and Accessory Structures  Lacrimal apparatus = lacrimal gland + ducts  Lacrimal gland—produces lacrimal fluid (tears); situated on lateral end of each eye  Tears drain across the eye into the lacrimal canaliculi, then the lacrimal sac, and into the nasolacrimal duct, which empties into the nasal cavity  Tears contain:  Dilute salt solution  Mucus  Antibodies  Lysozyme (enzyme that destroys bacteria)  Function of tears Figure 8.2a Accessory structures of the eye. Lacrimal Excretory duct gland of lacrimal gland Conjunctiva Anterior aspect Eyelid Eyelashes Tarsal glands (a) Eyelid Figure 8.2b Accessory structures of the eye. Lacrimal Lacrimal sac gland Excretory ducts of lacrimal gland Lacrimal canaliculus Nasolacrimal duct Inferior meatus of nasal cavity Nostril (b)  External and Accessory Structures  Extrinsic eye muscles  Six muscles attach to the outer surface of the eye  Produce gross eye movements © 2018 Pearson Education, Figure 8.3a Extrinsic muscles of the eye. Superior oblique muscle Superior oblique tendon Superior rectus muscle Conjunctiva Lateral rectus muscle Optic Inferior Inferior nerve rectus oblique (a) muscle muscle Figure 8.3b Extrinsic muscles of the eye. Trochlea Superior oblique muscle Superior oblique tendon Axis at Superior center of rectus muscle eye Inferior rectus muscle Medial rectus muscle Lateral rectus muscle Common (b) tendinous ring Internal Structures: The Eyeball  Three layers, or tunics, form the wall of the eyeball  Fibrous layer: outside layer  Vascular layer: middle layer  Sensory layer: inside layer  Humors are fluids that fill the interior of the eyeball  Lens divides the eye into two chambers Figure 8.4a Internal anatomy of the eye (sagittal section). Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis Iris Pupil Optic nerve Aqueous humor (in anterior segment) Lens Scleral venous sinus Central artery (canal of Schlemm) and vein of Vitreous humor the retina (in posterior Optic disc segment) (blind spot) (a) Figure 8.4b Internal anatomy of the eye (sagittal section). Ciliary body Vitreous humor in posterior Iris segment Margin Retina of pupil Choroid Sclera Aqueous humor Fovea centralis (in anterior segment) Optic disc Lens Optic nerve Cornea Ciliary zonule (b) Internal Structures: The Eyeball  Fibrous layer = sclera + cornea  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 Internal Structures: The Eyeball  Vascular layer  Choroid is a blood-rich nutritive layer that contains a pigment (prevents light from scattering)  Choroid is modified anteriorly into two smooth muscle structures  Ciliary body  Iris—regulates amount of light entering eye  Pigmented layer that gives eye color  Pupil—rounded opening in the iris Internal Structures: The Eyeball  Sensory layer  Retina contains two layers 1. Outer pigmented layer absorbs light and prevents it from scattering 2. Inner neural layer contains receptor cells (photoreceptors)  Rods  Cones Figure 8.5a The three major types of neurons composing the retina. Pigmented layer of retina Rod Cone Bipolar cells Ganglion Pathway cells of light (a) Figure 8.5b The three major types of neurons composing the retina. Pigmented layer of Neural layer retina of retina Central artery and vein of retina Optic disc Sclera Optic (b) nerve Choroid Internal Structures: The Eyeball  Sensory layer (continued)  Electrical 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 Internal Structures: The Eyeball  Sensory layer (continued)  Rods  Most are found toward the edges of the retina  Allow vision in dim light and peripheral vision  All perception is in gray tones  Cones  Allow for detailed color vision  Densest in the center of the retina  Fovea centralis–lateral to blind spot  Area of the retina with only cones  Visual acuity (sharpest vision) is here  No photoreceptor cells are at the optic disc, or blind spot Internal Structures: The Eyeball  Sensory layer (continued )  Cone sensitivit y  Three types of cones  Each cone type is sensitive to Internal Structures: The Eyeball  Lens  Flexible, biconvex crystal-like structure  Held in place by a suspensory ligament attached to the ciliary body  Lens divides the eye into two chambers 1. Anterior (aqueous) segment  Anterior to the lens  Contains aqueous humor, a clear, watery fluid 2. Posterior (vitreous) segment  Posterior to the lens Internal Structures: The Eyeball  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  Vitreous humor  Gel-like substance posterior to the lens  Prevents the eye from collapsing  Helps maintain intraocular pressure Internal Structures: The Eyeball  Ophthalmoscope  Instrument used to illuminate the interior of the eyeball and fundus (posterior wall)  Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina Physiology of Vision  Pathway of light through the eye and light refraction  Light must be focused to a point on the retina for optimal vision  Light is bent, or refracted, by the cornea, aqueous humor, lens, and vitreous humor  The eye is set for distant vision (over 20 feet away)  Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away) Figure 8.8 Relative convexity of the lens during focusing for distant and close vision. Retina Light from distant source Focal point (a) Light from near source Focal point Retina (b) Physiology of Vision  Pathway of light through the eye and light refraction (continued)  Image formed on the retina is a real image  Real images are:  Reversed from left to right  Upside down  Smaller than the object Physiology of Vision  Summary of the pathway of impulses from the retina to the point of visual interpretation 1. Optic nerve 2. Optic chiasma 3. Optic tract 4. Thalamus 5. Optic radiation 6. Optic cortex in occipital lobe of A Closer Look  Emmetropia – eye focuses images correctly on the retina  Myopia (nearsightedness)  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  Hyperopia (farsightedness)  Near objects are blurry, whereas distant objects are clear  Distant objects are focused behind the retina  Results from an eyeball that is too short or from a ―lazy lens‖ A Closer Look  Astigmatism  Images are blurry  Results from light focusing as lines, not points, on the retina because of unequal curvatures of the cornea or lens Physiology of Vision  Eye reflexes  Convergence: reflexive movement of the eyes medially when we focus on a close object  Photopupillary reflex: bright light causes pupils to constrict  Accommodation pupillary reflex: viewing close objects causes pupils to constrict Part II: The Ear: Hearing and Balance  Ear houses two senses 1. Hearing 2. Equilibrium (balance)  Receptors are mechanoreceptors  Different organs house receptors for each sense  The ear is divided into three areas 1. External (outer) ear 2. Middle ear 3. Internal (inner) ear Figure 8.11 Anatomy of the ear. External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear nerve Auricle (pinna) Semicircular canals Oval window Cochlea Vestibule Round window Pharyngotympanic (auditory) tube Tympanic membrane (eardrum) Hammer Anvil Stirrup (malleus) (incus) (stapes) External acoustic meatus Auditory ossicles (auditory canal) Anatomy of the Ear  External (outer) ear  Auricle (pinna)  External acoustic meatus (auditory canal)  Narrow chamber in the temporal bone  Lined with skin and ceruminous (earwax) glands  Ends at the tympanic membrane (eardrum)  External ear is involved only in collecting sound waves Anatomy of the Ear  Middle ear cavity (tympanic cavity)  Air-filled, mucosa-lined cavity within the temporal bone  Involved only in the sense of hearing  Located between tympanic membrane and oval window and round window  Pharyngotympanic tube (auditory tube)  Links middle ear cavity with the throat  Equalizes pressure in the middle ear cavity so the eardrum can vibrate Anatomy of the Ear  Middle ear cavity (tympanic cavity) (continued)  Three bones (ossicles) span the cavity 1. Malleus (hammer) 2. Incus (anvil) 3. Stapes (stirrup)  Function  Transmit vibrations from tympanic membrane to the fluids of the inner ear  Vibrations travel from the hammer → anvil → stirrup → oval window of inner ear Anatomy of the Ear  Internal (inner) ear  Includes sense organs for hearing and balance  Bony labyrinth (osseous labyrinth) consists of:  Cochlea  Vestibule  Semicircular canals  Bony labyrinth is filled with perilymph  Membranous labyrinth is suspended in perilymph and contains endolymph Equilibrium  Equilibrium receptors of the inner ear are called the vestibular apparatus  Vestibular apparatus has two functional parts 1. Static equilibrium 2. Dynamic equilibrium Static Equilibrium  Maculae—receptors in the vestibule  Report on the position of the head  Help us keep our head erect  Send information via the vestibular nerve (division of cranial nerve VIII) to the cerebellum of the brain  Anatomy of the maculae  Hair cells are embedded in the otolithic membrane  Otoliths (tiny stones) float in a gel around hair cells  Movements cause otoliths to roll and bend hair Figure 8.12a Structure and function of maculae (static equilibrium receptors). Membranes in vestibule Otoliths Otolithic membrane Hair tuft Hair cell Supporting cell Nerve fibers of vestibular division (a) of cranial nerve VIII Figure 8.12b Structure and function of maculae (static equilibrium receptors). Force of Otolithic Otoliths gravity membrane Hair cell Head upright Head tilted (b) Dynamic Equilibrium  Crista ampullaris  Responds to angular or rotational movements of the head  Located in the ampulla of each semicircular canal  Tuft of hair cells covered with cupula (gelatinous cap)  If the head moves, the cupula drags against the endolymph  Hair cells are stimulated, and the impulse travels the vestibular nerve to the cerebellum Figure 8.13a Structure and function of the crista ampullaris (dynamic equilibrium receptor region). Semicircular canals Ampulla Vestibular nerve Vestibule (a) Figure 8.13b Structure and function of the crista ampullaris (dynamic equilibrium receptor region). Ampulla Endolymph Cupula of crista (b) ampullaris Figure 8.13c Structure and function of the crista ampullaris (dynamic equilibrium receptor region). Flow of endolymph Cupula Nerve fibers Direction of body (c) movement Hearing  Spiral organ of Corti  Located within the cochlear duct  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 Figure 8.14a Anatomy of the cochlea. Temporal Perilymph in scala vestibuli bone Spiral Vestibular organ of membrane Corti Afferent fibers of the cochlear nerve Temporal bone Cochlear duct (contains Perilymph in endolymph) scala tympani (a) Figure 8.14b Anatomy of the cochlea. Vestibular Hair (receptor) Tectorial membrane cells of spiral membrane organ of Corti Fibers of Basilar Supporting the cochlear membrane cells nerve (b) Hearing  Pathway of vibrations from sound waves  Move by the ossicles from the eardrum to the oval window  Sound is amplified by the ossicles  Pressure waves cause vibrations in the basilar membrane in the spiral organ of Corti  Hair cells of the tectorial membrane are bent when the basilar membrane vibrates against it  An action potential starts in the cochlear nerve (cranial nerve VIII), and the Hearing  High-pitched sounds disturb the short, stiff fibers of the basilar membrane  Receptor cells close to the oval window are stimulated  Low-pitched sounds disturb the long, floppy fibers of the basilar membrane  Specific hair cells further along the cochlea are affected Part III: Chemical Senses: Smell and Taste  Chemoreceptors  Stimulated by chemicals in solution  Taste has five types of receptors  Smell can differentiate a wider range of chemicals  Both senses complement each other and respond to many of the same stimuli Olfactory Receptors and the Sense of Smell  Olfactory receptors are in roof of nasal cavity  Olfactory receptor cells (neurons) with long cilia known as olfactory hairs detect chemicals  Chemicals must be dissolved in mucus for detection by chemoreceptors called olfactory receptors  Impulses are transmitted via the olfactory filaments to the olfactory nerve (cranial nerve I)  Smells are interpreted in the olfactory Figure 8.17 Location and cellular makeup of the olfactory epithelium. Olfactory bulb Cribriform plate of ethmoid bone Olfactory tract Olfactory filaments of the olfactory nerve Supporting cell Olfactory Olfactory receptor mucosa cell Olfactory hairs Mucus layer (a) (cilia) Route of inhaled air containing odor molecules (b) Taste Buds and the Sense of Taste  Taste buds house the receptor organs  Locations of taste buds  Most are on the tongue  Soft palate  Superior part of the pharynx  Cheeks  The tongue is covered with projections called papillae that contain taste buds  Vallate (circumvallate) papillae  Fungiform papillae Figure 8.18a Location and structure of taste buds. Epiglottis Palatine tonsil Lingual tonsil Foliate papillae Fungiform papillae (a) Figure 8.18b Location and structure of taste buds. Vallate papilla Taste buds (b) Taste Buds and the Sense of Taste  Gustatory cells are the taste receptors  Possess gustatory hairs (long microvilli)  Gustatory hairs protrude through a taste pore  Hairs are stimulated by chemicals dissolved in saliva Figure 8.18c Location and structure of taste buds. Epithelium of tongue Taste bud Connective tissue Surface of the tongue Gustatory (taste) cell Basal cell Sensory nerve fiber Gustatory hairs (microvilli) emerging from a taste pore (c) Taste Buds and the Sense of Taste  Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas  Facial nerve (cranial nerve VII)  Glossopharyngeal nerve (cranial nerve IX)  Vagus nerve (cranial nerve X)  Taste buds are replaced frequently by basal cells Taste Buds and the Sense of Taste  Five basic taste sensations  Sweet receptors respond to sugars, saccharine, some amino acids  Sour receptors respond to H+ ions or acids  Bitter receptors respond to alkaloids  Salty receptors respond to metal ions  Umami receptors respond to the