CHAP8 Special Senses PDF

Title CHAP8 Special Senses
Author Anonymous User
Course Theoretical Foundations of Nursing
Institution Iloilo Doctors' College
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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

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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



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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

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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

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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.

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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

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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)

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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

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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)

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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

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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

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 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

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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

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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

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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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