Special Senses. The Senses. General senses: -Cutaneous sensory organs Temperature (cold &heat) Pressure Fine touch Pain -Proprioceptors of muscles and joints Special senses Sight - Hearing Equilibrium -Smell - Taste. The Eye and Vision.
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-Cutaneous sensory organs
-Proprioceptors of muscles and joints
The accessory structures of the eye include:
I-EyelidsAnteriorly, the eyes are protected by the mobile eyelids or palpebrae (pal′pĕ-bre). The eyelids are separated by the palpebral fissure (“eyelid slit”) and meet at the medial and lateral angles of the eye—the medial and lateral commissures (canthi),respectively.
1- the lacrimal gland :
It lies in the orbit above the lateral end of the eye and is visible through the conjunctiva when the lid is everted. It continually releases a dilute saline solution called lacrimal secretion—or, more commonly, tears—into the superior part of the conjunctival sac through several small excretory ducts. Blinking spreads the tears downward and across the eyeball to the medial commissure, where they enter:
2-the paired lacrimal canaliculi via two tiny openings called lacrimal puncta, visible as tiny red dots on the medial margin of each eyelid. From the lacrimal canaliculi, the tears drain into the:
3- lacrimal sac and then into:
4- the nasolacrimal duct, which empties into the nasal cavity at the inferior nasal meatus.
Lacrimal fluid contains mucus,antibodies, and lysozyme, an enzyme that destroys bacteria.
IV-Extrinsic Eye MusclesThe movement of each eyeball is controlled by six straplike extrinsic eye muscles, which originate from the bony orbit and insert into the outer surface of the eyeball .
Congenital weakness of the external eye muscles may cause strabismus (strah-biz′mus; “cross-eyed”, squint), a condition in which the affected eye rotates medially or laterally.
Internal Structure of the EyeballThe eye itself, commonly called the eyeball, is a slightly irregular hollow sphere .
-Its wall is composed of three layers (formerly called tunics): the fibrous, vascular, and sensory layers.
-Its internal cavity is filled with fluids called humors that help to maintain its shape.
-The lens, the adjustable focusing apparatus of the eye, is supported vertically within the internal cavity, dividing it into anterior and posterior segments, or cavities.
The Fibrous Layer The outermost coat of the eye, and is composed of dense avascular connective tissue. It has two obviously different regions: the sclera and the cornea.
a-The sclera (skle′rah), forming the posterior portion and the bulk of the fibrous layer, is glistening white and opaque. Seen anteriorly as the “white of the eye,” it protects and shapes the eyeball and provides attachment for the extrinsic eye muscles. Posteriorly, where the sclera is pierced by the optic nerve, it is continuous with the dura mater of the brain.
b- the cornea The anterior one-sixth of the fibrous layer is modified to form the transparent ,crystal-clear cornea . It forms a window that lets light enter the eye, and is a major part of the light-bending apparatus of the eye.
The cornea is well supplied with nerve endings, most of which are pain receptors. When the cornea is touched, blinking and increased tearing occur reflexively. Luckily, its capacity for regeneration and repair is extraordinary.
Because it has no blood vessels, it is beyond the reach of the immune system, so the cornea is the only tissue in the body that can be transplanted from one person to another with little or no possibility of rejection..
-Blood-rich nutritive layer
-Dark pigment prevents light from scattering
-Modified interiorly into two structures
a- Its outer pigmented layer, a single-cell-thick lining the choroid .
-These pigmented epithelial cells, like those of the choroid, absorb light and prevent it from scattering in the eye.
-They also act as phagocytes to remove dead or damaged photoreceptor cells, and store vitamin A needed by the photoreceptor cells.
b-The transparent inner neural layer extends anteriorly to the posterior margin of the ciliary body. Originating as an outpocketing of the brain, the retina contains millions of photoreceptors that transduce light energy.
-Although the pigmented and neural layers are very close together, they are not fused.
- Only the neural layer of the retina plays a direct role in vision.
From posterior to anterior, the neural layer is composed of three main types of neurons: photoreceptors, bipolar cells, and ganglion cells. Signals are produced in response to light and spread from the photoreceptors to the bipolar neurons and then to the innermost ganglion cells, where action potentials are generated. The ganglion cell axons leave the posterior aspect of the eye as the thick optic nerve. The optic disc, is a weak spot in the fundus (posterior wall) of the eye because it is not reinforced by the sclera. The optic disc is also called the blind spot because it lacks photoreceptors, so light focused on it cannot be seen.
The photoreceptors found in the neural retinas are of two types: rods and cones.
-The more numerous rods are our dim-light and peripheral vision receptors. They are far more sensitive to light than cones are, but they do not provide either sharp images or color vision. Most are found towards the edges of the retina
-Cones, by contrast, operate in bright light and provide high-acuity color vision. Densest in the center of the retina. Lateral to the blind spot of each eye, a minute (0.4 mm) pit called the fovea centralis .It contains only cones; so it is the area of greatest visual acuity.
Color blindness is due to a congenital lack of one or more of the cone types.
-Inherited as an X-linked condition, it is far more common in males than in females. As many as 8–10% of males have some form of color blindness.
-The most common type is red-green color blindness, resulting from a deficit or absolute absence of either red or green cones. Red and green are seen as the same color—either red or green, depending on the cone type present.
-Many color-blind people are unaware of that.
HOMEOSTATIC IMBALANCE:A cataract is a clouding of the lens as if seen through frosted glass .Some cataracts are congenital, but most result from age-related hardening and thickening of the lens or are a secondary consequence of diabetes mellitus.
- Heavy smoking and frequent exposure to intense sunlight increase the risk for cataracts.
-whereas long-term dietary supplementation with vitamin C may decrease the risk.
-Fortunately, the lens can be surgically removed and an artificial lens implanted to save the patient’s sight
-If the drainage of aqueous humor is blocked, pressure within the eye may increase to dangerous levels and compress the retina and optic nerve—a condition called glaucoma (glaw-ko′mah). The eventual result is blindness.
-Unfortunately, many forms of glaucoma can not be realized until the damage is done. Late signs include seeing halos around lights and blurred vision. The intraocular pressure is determined by Tonometer. This exam should be done yearly after the age of 40.
-Ear Pinna (auricle)
- External auditory canal :Narrow chamber in the temporal bone
-Lined with skin
-has Ceruminous (wax) glands .
-ends at the tympanic membrane
Otitis media (me′de-ah), or middle ear inflammation, is a fairly common result of a sore throat, especially in children, whose auditory tubes are shorter and run more horizontally.
Otitis media is the most frequent cause of hearing loss in children.
In acute forms, the eardrum bulges and becomes inflamed and red. Most cases of otitis media are treated with antibiotics. When large amounts of fluid or pus accumulate in the cavity, an emergency myringotomy (lancing of the eardrum) may be required to relieve the pressure, and a tiny tube implanted in the eardrum permits pus to drain into the external ear. The tube falls out by itself within the year.
N,B.,Continued stimulation can lead to adaptation.
-Semicircular canals(Dynamic equilibrium)
or rotatory movement
of the head rather than
to straight-line movements.
vestibular nerve to the
Homeostatic Imbalances of Hearing-DeafnessAny hearing loss is called deafness.Two types:1 - Conduction deafness occurs when something hampers sound conduction to the fluids of the internal ear as impacted earwax or a perforated (ruptured) eardrum. But the most common causes of conduction deafness are middle ear inflammations (otitis media) and otosclerosis (o″to-sklĕ-ro′sis) of the ossicles. Otosclerosis (“hardening of the ear”) occurs when overgrowth of bony tissue fuses the ossicles to one another.
2-Sensorineural deafness results from damage to neural structures at any point from the cochlear hair cells to the auditory cortical cells as in:
a- gradual loss of the hair cells throughout life.
b- hair cells can also be destroyed at an earlier age by a single explosively loud noise or
c-prolonged exposure to high-intensity sounds which tears off their cilia.
D- degeneration of the cochlear nerve, cerebral infarcts, and tumors in the auditory cortex are other causes.
-TinnitusTinnitus (tĭ-ni′tus) is a ringing or clicking sound in the ears in the absence of auditory stimuli.
It is more a symptom of pathology than a disease. For example, tinnitus is:
have gustatory hairs (long microvilli)
Taste maps that put sweet receptors to the tip of the tongue, salty and sour receptors to the sides, bitter receptors to the back, and umami receptors to the pharynx are common in the literature. However, researchers have known for years that these mapped areas are misrepresentations.
- As a rule, vision is the only special sense not fully functional at birth.
-Most babies are hyperopic. The newborn eye movements are uncoordinated, and often only one eye at a time is used.
-The lacrimal glands are not completely developed until about two weeks after birth, so babies are tearless for this period.
- By 5 months, infants can follow moving objects with their eyes.
- By the age of 5 years color vision is well developed.
- By school age, the initial hyperopia has usually been replaced by emmetropia, and the eye reaches its adult size at 8–9 years of age.
- Emmetropia usually continues until presbyopia begins to set in around age 40 owing to decreasing lens elasticity.
Taste and SmellSmell and taste are sharp, and infants relish food that adults consider tasteless.