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 • The adult eye is a sphere with a diameter of about 2.5 cm (1 inch). • 70 percent of all sensory receptors of the body 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(only 1/6 of the eye surface is seen). • A cushion of fat surrounds most of the eye
Accessory structures of the eye The accessory structures of the eye include: • eyelids, • conjunctiva, • lacrimal apparatus, • and extrinsic eye muscles.
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. • Projecting from the free margin of each eyelid are the eyelashes. The follicles of the eyelash hairs are richly innervated by nerve endings (hair follicle receptors), and anything that touches the eyelashes (even a puff of air) triggers reflex blinking.
Several types of glands are associated with the eyelids: • The tarsal glands (Meibomian glands; mi-bo′me-an) ,their ducts open at the eyelid edge just posterior to the eyelashes. These modified sebaceous glands produce an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together. • Associated with the eyelash follicles are a number of smaller, more typical sebaceous glands, and modified sweat glands called ciliary glands lie between the hair follicles (cilium = eyelash). • Infection of a tarsal gland results in an unsightly cyst called a chalazion (kah-la′ze-on; “swelling”). Inflammation of any of the smaller glands is called a sty.
II-Conjunctiva • Is a transparent mucous membrane. It lines the eyelids and reflects (folds back) over the anterior surface of the eyeball. The latter covers only the white of the eye, not the cornea. The conjunctiva is very thin, and blood vessels are clearly visible beneath it. • The major function of the conjunctiva is to produce a lubricating mucus that prevents the eyes from drying out. • HOMEOSTATIC IMBALANCE Inflammation of the conjunctiva, called conjunctivitis, results in reddened, irritated eyes. Pinkeye, a conjunctival infection caused by bacteria or viruses, is highly contagious.
III_Lacrimal apparatus Consists of : 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: Figure 8.1a
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. • it cleanses and protects the eye surface as it moistens and lubricates it. When lacrimal secretion increases, tears spill over the eyelids and fill the nasal cavities.” This happens when the eyes are irritated and when we are emotionally upset. • HOMEOSTATIC IMBALANCE :Because the nasal cavity mucosa is continuous with that of the lacrimal duct system, a cold or nasal inflammation often causes the lacrimal mucosa to become inflamed and swell. This constricts the ducts and prevents tears from draining from the eye surface, causing “watery”eyes.
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 . • These muscles allow the eyes to follow a moving object, and help to maintain the shape of the eyeball and hold it in the orbit.
Extrinsic Eye Muscles • Muscles attach to the outer surface of the eye • Produce eye movements Figure 8.2
HOMEOSTATIC IMBALANCE 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 wall is composed of three tunics: • Fibrous layer – the outermost layer (Sclera &Cornea) • Vascular layer- (Choroid)the middle layer • Sensory layer – (Retina)the • innermost layer Figure 8.3a
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..
The Vascular Layer (Uvea),Choroid: -Blood-rich nutritive layer -Dark pigment prevents light from scattering -Modified interiorly into two structures • Cilliary body ,a thickened ring of tissue that encircles the lens. The ciliary body consists chiefly of interlacing smooth muscle bundles called ciliary muscles, which are important in controlling lens shape. • Iris ,the visible colored part of the eye, it lies between the cornea and the lens and is continuous with the ciliary body posteriorly. Its round central opening, the pupil, allows light to enter the eye.
The Inner Layer (Retina) : it is formed of two-layeres. 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.
Neurons of the Retina Figure 8.4
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.
Cone Sensitivity • There are three types of cones blue,green, and red. • Different cones are sensitive to different wavelengths. • Color blindness is the result of lack of one cone type. Figure 8.6
HOMEOSTATIC IMBALANCE 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.
Lens • Lens is a biconvex, transparent, flexible structure that can change shape to allow precise focusing of light on the retina. Like the cornea, the lens is avascular; blood vessels interfere with transparency. • Lens is a biconvex, transparent, flexible structure that can change shape to allow precise focusing of light on the retina. Like the cornea, the lens is avascular; blood vessels interfere with • It is held in place • by suspensory • ligament. Figure 8.3a
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
Internal Eye Fluids • Aqueous humor • Watery fluid found in the anterior chamber (between the lens and cornea) • Similar to blood plasma • Helps maintain intraocular pressure • Provides nutrients for the lens and cornea • Reabsorbed into venous blood through the canal of Schlemm which is located at the junction of the cornea and sclera.
Vitreous humor(vitreous body) • Gel-like substance in the posterior chamber (behind the lens). • Keeps the eye from collapsing by maintaining intraocular pressure. • Lasts a lifetime and is not replaced.
HOMEOSTATIC IMBALANCE -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.
Lens Accommodation • Light must be focused to a point on the retina for optimal vision • The eye is set for distance vision (over 20 ft away) • The lens must change shape to focus for closer objects Figure 8.9
Images Formed on the Retina Figure 8.10
Visual Pathway • Photoreceptors of the retina • Optic nerve • Optic nerve crosses at the optic chiasma Figure 8.11
Visual Pathway • Optic tracts • Thalamus (axons form optic radiation) • Visual cortex of the occipital lobe Figure 8.11
Eye Reflexes • Internal muscles are controlled by the autonomic nervous system. • Bright light causes pupils to constrict (photopupillary reflex) through action of radial and circular muscles of iris. • Viewing close objects causes pupils to constrict ( accommodation pupillary reflex) • External muscles control eye movement to follow objects. • Viewing close objects causes the eyes to move medially( convergence reflex).
HOMEOSTATIC IMBALANCES • Myopia (mi-o′pe-ah; “short vision”) occurs when distant objects are focused not on, but in front of, the retina . • Hyperopia (hy′per-o″pe-ah; “far vision”), or farsightedness, occurs when the parallel light rays from distant objects are focused behind the retina. • Astigmatism Unequal curvatures in different parts of the cornea or lens lead to blurry images.
The Ear • Houses two senses • Hearing • Equilibrium (balance) • Receptors of the ear are called mechanoreceptors, as they respond to sound vibration(hearing receptors) and gross movement of the head(balance organs) • Different organs house receptors for each sense.
Anatomy of the Ear Figure 8.12
Anatomy of the Ear External ear • Involved in hearing only • Structures of the external ear -Ear Pinna (auricle) - External auditory canal :Narrow chamber in the temporal bone -Lined with skin -has Ceruminous (wax) glands . -ends at the tympanic membrane Figure 8.12
The Middle Ear or Tympanic Cavity • Air-filled cavity within the temporal bone • Only involved in the sense of hearing • Two tubes are associated with the inner ear • The auditory canal which is closed by the tympanic membrane • The auditory tube connecting the middle ear with the throat • Allows for equalizing pressure (by yawning or swallowing) on both sides of eardrum.