Sense organ note

  SUBJECT: BIOLOGY 

 TOPIC: SENSE ORGANS

 PERFORMANCE OBJECTIVE: AT THE END OF THE LESSON TE STUDENT SHOULD BE ABLE TO: 1. List the major sense organs and infer their location 

2. Describe the organ of taste and identify different parts of the tongue associated with sweetness, bitterness, salty taste and sore taste.

 3. Describe the structure of the mammalian ear and explain the functions of the ear-hearing, balance.

 4. Describe the mammalian eye and explain the functions of its various parts.

 5. Explain the function of the eye: - Image formation - Accommodation, 6. Explain the term myopia, hypermetropia, astigmatism cataracts, night blindness and name the kind of lens that can be used to correct myopic and hyper-metropia defects. 

                            DEFFINITION AND TYPES OF SENSE ORGANS

 Definition: A sense organ is defined as group of specialised cells tissues or receptors which is able to receive, perceive or detect stimulus and transmit the information, impulse or message to the central nervous system. Types of sense organs: there are five sense organs in human body. These sense organs and their functions are shown below. Sense organs Function (i) Eyes Sense of sight or vision (ii) Nose Sense of smell (iii) Tongue Sense of taste (iv) Ear Sense of hearing and balance (v) Skin Sense of touch, pain, heat, or cold and pressure ORGANS OF TASTE (TONGUE): The tongue is the organ responsible for the sense of taste buds located on the tiny swellings on the exposed surface of the tongue. They are the receptors for chemicals in solution from food, drinks, medicine or whatever is placed on the tongue. They are connected by fine sensory nerves to the brain which gives information about what is tasted. Human tongue showing the four primary area of taste The tongue is sensitive to four primary tastes: Sweet, sour, salty and bitter. The back of the tongue is sensitive to bitter stimuli, the sides of the tongue are sensitive to salty and sour stimuli while the tip is sensitive to sweet sensation. Apart from the above tastes, it is an establish fact that the tongue can also detect alkaline taste, texture of food as well as its temperature. Certain factors which affect taste by the tongue include: (i) The temperature of the food being tasted (ii) The state of health of the taster (iii) Whether or not the nostrils are closed when chewing the food (iv) The number of food being chewed at the same time. How Bitter Taste is Detected in Human - Taste buds are unevenly distributed on the surface of the tongue; - Taste buds for detecting bitterness are at the back or end of the tongue, - For a substance to be tasted it must be dissolved in a solution or saliva; - When bitter substance is dissolved in saliva, sensory cells at end or back of the tongue are stimulated - Which pass impulses to the brain; - Which interprets the taste as bitter. 

 ORGAN OF SIGHT: The organ of sight is the eye found in all vertebrate. The mammalian eye is almost spherical in shape. The protective structures of the eye include eye socket, eyelashes, tear glands, and conjunctiva. The eye is housed in a protective cavity in the skull called eye socket. The eye is attached to the socket by six muscles which serve to the hold it firmly in place and to enable it rotate freely upward, downward and sideways without moving the head. The upper and lower eyelids which are muscular folds of skin, the eyelids during blinking helps to moisten the eye with the aid of tear gland. The eyelashes which are rows of long bristle hair on the eyelids protect the eyeballs from dust, excessive light and shield the eye against sweat and water. There are also tear gland (lacrimal glands) at the meeting points of the eyelids on the outer part of the socket and above the eyeball. The secretion of the tear gland, a saline or salty fluid called tears, Washes away dust that have settled on eye, moistens the conjuctiva and have the ability to destroy most bacteria because of the chemical substance called Lysozyme. By so doing, the tear has antiseptic effect. Vertical section of the human eye Structure of the Eyes The above diagram shows the eyeball which is a fluid –filled hollow structure. The wall of the eyeball consists of three layers which are the 

 1. Outermost Sclera or Sclerotic layer 

 2. Middle choroids Layer 

 3. Inner retina. 

1. Outermost Sclera or Sclerotic layer: the sclera or sclerotic layer, also called the White eye, is the outer of the eye. It is a thick fibrous connective tissue forming the white of the eye.

 Functions: (i) It gives shape and Firmness to the eye. (ii) It protects and supports the inner parts of the eye. The cornea, conjunctiva and optic nerve are all part of the sclerotic layer. 

a. Cornea: the sclerotic layer bulges out in front the eye to form the transparent cornea. In the other words, it is a continuation of the sclera in front of the eye. 

 Functions: (i) It admits the light into the eyes. (ii) It bends the light rays to bring them to a focus on the retina. (iii) It protects the eyes externally.

 b. Conjunctiva: It is a thin tough transparent membrane which lines the inside of eyelids and covers the cornea protectively. It has almost similar function with cornea. c. Optic nerve: Optic nerve is found at the back of the sclerotic layer. It also penetrates the choroid and retina at a point knows as the Blid spot (part of retina). Which is devoid of light sensitive cells.

 FUCTIONS: (i) Optic nerve transmits sensory impulses to the brain. (ii) It also transmits sensory impulses from the brain.

 

2. Middle choroids Layer: the choroids layer is highly vascularised; Pigmented and rich in blood capillaries may make the layer brownish or reddish. It contains a black pigment.

 FUCTIONS: (i) It provides oxygen and food to the cells of the eye. (ii) The pigment in the layer helps to absorb light rays and prevent light reflection into the eye. The choroid layer consists of some parts as following: Iris: Controls the amount of light passing through the eye, with radial and circular muscle fibers. Pupil: the pupil found below the choroid layer in front of the eye. And it performs also most similar function with iris. Ciliary muscle: the ciliary muscle alters the focal length and it brings about proper accommodation of the eye. Lens: the lens is transparent biconvex elastic structure which is held in position by the suspensory ligaments. And it has two major functions, it helps to refract light rays that enter the eye and it makes fine adjustment to focus the image of an object on the retina. 3. Retina: the retina is the innermost layer of the eyeball. It is the part of the eye that is sensitive to light. This layer is also vascularised, pigmented and elastic. Images formed on it are always inverted and smaller than real object. Retina has two types of cells. These are Cones (sensitive to colour and high light insensitive) and Rods (sensitive to all light intensives, bright and dim). Some other parts of Eye are: Yellow spot Blind Spot Aqueous humour Vitreous humour Function of eye: the two major functions of eyes are: Image formation and accommodation. Image formation: To form the image rays from the object get in the eye through the cornea. These rays then pass through the aqueous humor, the lens and the vitreous humor and finally fall on the retina to form the images. The images that form on the retina are real and inverted due to the converging optical system of the eye. The inverted images is corrected and identified in the optical center of the brain. A B Accommodation: Accommodation is defined as the ability of the eye to focus (both near and distant) objects on the retina. In the words accommodation is the ability to see clearly (far and near) objects or adjustments of the focal length of the eye to see clearly. Adaptation for near vision: It also means to see near object clearly. i) The ciliary muscles contract. ii) The suspensory ligaments relax their tension on the lens. iii) The lens assumes a more convex shape iv) The focal length of the lens is reduced. Under these conditions, the eye is able to focus the in image of near objects on the retina. Adaptation for Distant vision In order to see a distant object clearly (i) The ciliary muscles relaxed (ii) The suspensory muscles pull tightly on the lens (iii) The focal of the lens is increased (iv) The convex lens assumes a near flat shape. Eye Defects and their correction Eye will have a defect whenever an image cannot be formed or formed properly on retina the major defects of the eye include: Short-sightedness (myopia) Long-sightedness (Hypermetropia) Presbyopia Astigmatism 1. Short-sightedness (myopia): is a condition in which parallel light rays from a distant object are brought to a focus in front of the retina, i.e., the image of a distant object is formed before the retina. Myopia may be caused by the eyeball being too long or lens being too convex (i.e. too thick) such person tends to move the object near the eye. Correction for myopia: the defect can be corrected by wearing concave lenses diverge the light rays from a distant object before they enter the eye so that eye now bring the rays to focus right on retina. The corrective lens used in the above it called: concave lens which diverges light and cause image to form on the retina. 2. Long-sightedness (Hypermetropia): Long –sightedness is a condition in which parallel light rays from near objects are brought to a focus behind the retina. The eye can see distant objects like clearly but not near objects. This may be cause by the eyeball being too short or the lens being too flat. Diagram for long-sightedness and its correction Correction for Hypermetropia: Hypermetropia is corrected by using spectacles fitted with suitable convex or converging lenses. The convex lenses help to converge the rays of light so that they come to a focus direction on retina. 

 3. Presbyopia: Presbyopia is the loss, with age of the elasticity of the lens and ciliary muscles. The lens can no longer easily adjust its shape to focus the images of near and distant objects clearly on retina. This leads to loss of accommodation. Correction of Presbyopia is corrected by wearing bi-focal lenses (a combination of convex and concave lenses). 4. Astigmatism: Astigmatism is caused as a result of unequal curvature of the cornea or lens. The rays light are brought to focus at different points on the retina causing the image to be blurred. Correction for Astigmatism: Astigmatism can be corrected by using cylindrical lenses. Other eye defects include: (i) Colour blindness (ii) Night blindness (iii) Cataract and (iv) Conjunctivitis Care Of The Eye i. Use clean water to wash face always. ii. Avoid staying in area containing fumes and or smoke that can irritate the eye. iii. Use clean handkerchief for cleaning the eyes. Iv. Use antibiotic eye drops e.g. chloranphenicol in case it is inflamed or reddish. v. Avoid rubbing the eyes with dirty fingers. Why a person in a dark room is dazzled for some seconds when he is suddenly exposed to bright sunlight: In a dark room, the iris contracts, the pupil dilates or is widely opened to let in more light. On sudden exposure to bring sunlight, the pupil which is still wide opened, lets in too much light which bleaches rhodopsin or pigment in the rod cells. This cause the dazzling sensation experienced when someone in dark room is suddenly exposed to bright light. The iris muscle gradually relaxes within a few seconds and the pupil gradually constricts to regulate the entry of the requited amount of light for vision to return to normal. 

 ORANS OF HEARING: The ear is the organs of hearing as well as the organ of balance in mammals. A mammal has two ears, one on each side of the head. The greater part of each ear is embedded in the skull for protection. Structure of the Ear: the mammalian ear is divided into three regions namely, the outer ear, the middle ear and the inner ear (labyrinth) Structure of the ear Outer Ear: the outer ear consists of the pinna, auditory meatus or ear tube, and tympanic membrane (ear drum). (i) Pinna: The pinna made of soft cartilage covered by skin. It is flexible and only found in mammals. Functions: (a) Pinna collects sound waves and directs them into the auditory meatus (auditory canal or Ear tube). (b) It detects the direction of sound waves. (ii) Auditory meatus or Ear tube: The auditory meatus is narrow passage which contains wax-producing glands. It also contains fine hairs. Hence the auditory meatus prevents the entry of tiny insects, germs and dust. (iii) Eardrum or Tympanic membrane: It is found at the end of the auditory meatus. It is a thin membrane, like drum-skin which vibrates when sound waves impinge on it. The eardrum is a delicate but firm membrane. Hence it separates the external ear from the middle ear and helps to transmit sound wave from the outer ear to the middle ear. Note: The pinna, auditory canal and Eardrum form the outer ear.

 Description of middle ear: The middle ear is an oval and air filled cavity which connects the ear drum to the oval window (fenestra ovalis). It contains the three small bones, namely, hammer (malleus), anvil (incus) and stapes (stirrup), collectively known as ossicles, which amplify sound. The malleus is attached to tympanic membrane or ear drum. Each of this bone has direct contact with the next one to it. The round window connects the middle ear to the inner ear. The eustachain tube is found at the base of middle ear and links ear to the back of throat or pharynx. It helps to equalise the air pressure on both side of the drum. Inner Ear: The inner ear contains a complex of bony passageways called bony labyrinth filled with a fluid called perilymph. Within these passageways are membranous sacs and tubes (membranous labyrinth) filled with another fluid called endolymph. These complex of fluid- filled passageways, sacs and tubes form two