Title | CHAP8 Special Senses |
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Author | Anonymous User |
Course | Theoretical Foundations of Nursing |
Institution | Iloilo Doctors' College |
Pages | 5 |
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Chapter 8 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
Cleanse, protect, moisten, lubricate the eye Extrinsic eye muscles Six muscles attach to the outer surface of the eye Produce gross eye movements
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 Fibrous layer = sclera + cornea Sclera White connective tissue layer Seen anteriorly as the “white of the eye”
Anatomy of the Eye
Accessory structures include the: Extrinsic eye muscles Eyelids Conjunctiva Lacrimal apparatus
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
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 keep it moist 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
Cornea
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
Sensory layer Retina contains two layers Outer pigmented layer absorbs light and prevents it from scattering Inner neural layer contains receptor cells (photoreceptors) Rods Cones
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
Rods Most are found toward the edges of the retina Allow vision in dim light and peripheral vision
All perception is in gray tones
No photoreceptor cells are at the optic disc, or blind spot Cone sensitivity
2. Posterior (vitreous) 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 Vitreous humor Gel-like substance posterior to the lens Prevents the eye from collapsing Helps maintain intraocular pressure 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
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) Image formed on the retina is a real image
Optic radiation Axons from the thalamus run to the occipital lobe Synapse with cortical cells, and vision interpretation (seeing) occurs
Summary of the pathway of impulses from the retina to the point of visual interpretation
1. 2. 3. 4. 5. 6.
Optic nerve Optic chiasma Optic tract Thalamus
Optic radiation Optic cortex in occipital lobe of brain Visual fields Each eye “sees” a slightly different view Field of view overlaps for each eye Binocular vision results and provides: Depth perception (three-dimensional vision)
A Closer Look
Physiology of Vision
Optic tracts Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye Synapse with neurons in the thalamus
Posterior to the lens Contains vitreous humor, a gel-like substance
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
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
Visual fields and visual pathways to the brain Optic nerve Bundle of axons that exit the back of the eye carrying impulses from the retina
Three types of cones Each cone type is sensitive to different wavelengths of visible light
Real images are: Reversed from left to right Upside down Smaller than the object
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
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” 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
Anatomy of the Ear
The ear is divided into three areas 1. External (outer) ear 2. Middle ear
Cochlea Vestibule Semicircular canals
External (outer) ear Auricle (pinna) External acoustic meatus (auditory canal)
Equilibrium
Static Equilibrium
Links middle ear cavity with the throat Equalizes pressure in the middle ear cavity so the eardrum can vibrate
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
Equilibrium receptors of the inner ear are called the vestibular apparatus Vestibular apparatus has two functional parts 1. Static equilibrium
2. Dynamic equilibrium
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 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)
Bony labyrinth is filled with perilymph Membranous labyrinth is suspended in perilymph and contains endolymph
3. Internal (inner) ear
Internal (inner) ear Includes sense organs for hearing and balance Bony labyrinth (osseous labyrinth) consists of:
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 cells
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
Hearing
Olfactory Receptors and the Sense of Smell
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 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 impulse travels to the temporal lobe 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
Hearing and Equilibrium Deficits
Deafness is any degree of hearing loss Conduction deafness results when the transmission of sound vibrations through the external and middle ears is hindered Sensorineural deafness results from damage to the nervous system structures involved in hearing Ménière’s syndrome affects the inner ear and causes progressive deafness and perhaps vertigo (sensation of spinning)
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 cortex
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 Filiform papillae 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
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 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 amino acid glutamate or the beefy taste of meat +
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
Part IV: Developmental Aspects of the Special Senses
Special sense organs are formed early in embryonic development Maternal infections during the first 5 or 6 weeks of pregnancy may cause visual abnormalities as well as sensorineural deafness in the developing child Vision requires the most learning The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth The eye continues to grow and mature until age 8 or 9 Age-related eye issues Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging Causes difficulty to focus for close vision
Lacrimal glands become less active Lens becomes discolored Dilator muscles of iris become less efficient, causing pupils to remain constricted
The newborn infant can hear sounds, but initial responses are reflexive
By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins Age-related ear problems Presbycusis—type of sensorineural deafness that may result from otosclerosis Otosclerosis—ear ossicles fuse
Congenital ear problems usually result from missing pinnas and closed or missing external acoustic meatuses Taste and smell are most acute at birth and decrease in sensitivity after age 40 as the number of olfactory and gustatory receptors decreases
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