Sense organs

he sense organs — eyes, ears, tongue, skin, and nose — help to protect the body. The human sense organs contain receptors that relay information through sensory neurons to the appropriate places within the nervous system.

Each sense organ contains different receptors.

General receptors are found throughout the body because they are present in skin, visceral organs (visceral meaning in the abdominal cavity), muscles, and joints.
Special receptors include chemoreceptors (chemical receptors) found in the mouth and nose, photoreceptors (light receptors) found in the eyes, and mechanoreceptors found in the ears.

1. SENSE ORGANS Prof. Amol B Deore MVPs Institute of Pharmaceutical Sciences, Adgaon , Nashik ( Maharashstra )

2. SENSE OF VISION: EYES

3. Eyelids The eyelids are two movable folds of skeletal muscles situated above and below the front of each eye. The free edges of eyelids composed of short curved hairs called eyelashes. Conjunctiva is a fine transparent membrane which lines the eyelids and the front of eyeball. It consists of columnar epithelium. It protects the cornea and the front of the eyeball. Eyelashes Eyelashes along the border of each eyelid help keep dust out of the eyes. Eyelids and eyelashes protect the eye from foreign objects. Blinking of the eyelids lubricates the surface of the eye by spreading tears that are produced by the lacrimal gland.

4. Eyebrows The eyebrows help shade the eye and keep perspiration from getting into the eye and causing an irritation to the eye.

5. LACRIMAL APPARATUS The lacrimal apparatus consists of an almond shaped lacrimal glands located in upper, outer corner of the eyeball, within the orbital cavity. The lacrimal glands produce and drains the lacrimal fluid i.e. tears. The lacrimal ducts take tears to the anterior of the eyeball, and blinking spreads the tears and washes the surface of the eye.

6. Secretion of tears occurs constantly, but is increased by the presence of irritating chemicals (for example, onion vapours) or dust, and in certain emotional situations (sad or happy). Composition : Daily tears secretion is about 1 mL. Tears are composed of water, mucus, sodium chloride (about 1%), mineral salts, bactericidal enzyme lysozyme and gamma globulin. Functions : the tears permit cleaning, lubrication and moistening of the eyeball.

7. The wall of the eye is composed of three layers of tissue. The outer fibrous layer : sclera and cornea The middle layer : choroid, ciliary body and iris The inner nervous tissue layer : retina LAYERS OF EYE BALL

8. Sclera : The outermost layer of eyeball is the sclera. It is white of the eye and made up of fibrous connective tissue. We see it as the white of the eye when looking in a mirror. The cornea is the transparent part of this outermost layer that permits light to enter the eye. Choroid : The middle layer is the choroid. It contains blood vessels and dark blue pigment cells. It is black in colour due to melanin pigment which absorbs light rays and prevents reflection and scattering of light within the eyeball. Retina : The innermost layer of the eye is the retina. It is gray in colour and contains the photosensitive cells: the rod and cone cells.

9. The lens is made of a biconvex, transparent and flexible elastic protein and has no blood capillary network. The lens is located behind the pupil. The ciliary body is a circular muscle that that hold the biconvex lens in place. The ciliary body is connected to the lens by suspensory ligaments. The lens bends (refracts) light rays focusing from objects in front of eye. The shape of the lens is changed by the ciliary muscle, which permits the eye to focus light from objects at varying distances for clear vision. LENS

10. For near objects : the refractive power to be increased by contraction of ciliary muscle thereby increasing convexity of lens (thickness) For distant objects: the refractive power is reduced by relaxation of ciliary muscle thereby making the lens thinner.

11. IRIS AND PUPIL The iris is the circular and coloured part (melanin) of the eyeball. It is suspended between the cornea and the lens and is attached to the ciliary body. What we call “eye colour” is the colour of the iris and is a genetic characteristic. The iris consists of two smooth muscles: circular muscles and radial muscles. The hole (opening) in the center of the iris is called pupil. The iris regulates the intensity of light that enters through the pupil into the eyeball.

12. In a strong sunlight ( bright light ), the parasympathetic stimulation leads to contraction of circular iris muscles and pupil size decreases (pupil constriction). Pupil constriction permits fewer light rays. In a dark room (dim light ), the sympathetic stimulation leads to contraction of radial iris muscles and pupil size increases (pupil dilation). Pupil dilation permits more light rays.

13. The retina is the innermost layer of the eyeball which detects light and colours. Retina is composed of two types of photosensitive cells: the rods and cones. Rods Rod cells (around 120 million) are situated toward the periphery, or edge, of the retina. Rod cells detect black & white colours in dim light (low intensity light). They are not sensitive to other colours. Rod cells consist of a photosensitive pigment Rhodopsin ( Scotopsin + Retinal) . Retinal is a derivative of Vitamin A synthesized from carotenoids. RETINA

14. Cones Cone cells (around 6-7 million) are found in the center of the retina called the macula lutea and a small depression called fovea centralis . There are three kinds of cones; each is sensitive to a different colour: red, green, or blue. These three types of cones allow us to distinguish between different colours. Cone cells consist of a photosensitive pigment Iodopsin ( Photopsin+Retinal ) which permit light absorption.

15. The rod and cone cells synapse with the bipolar cells of the retina. The bipolar neurons synapse with ganglionic cells whose axons form the optic nerve. Eventually the fibers of the optic nerve reach the thalamus of the brain and synapse at its posterior portion and enter to the visual cortex of the occipital lobe of the cerebrum for interpretation. The yellowish spot in the center of the retina is called the macula lutea . In its center is a depression called the fovea centralis . This region produces the sharpest vision, like when we look directly at an object. Medial to the fovea centralis is the optic disk. It is here that nerve fibers leave the eye as the optic nerve. Because the optic disk has no receptor cells, it is called the blind spot.

16. Aqueous humor and vitreous humor

17. For the vision, light rays must be focused on the retina and the resulting nerve impulses must be transmitted to the visual areas of the cerebral cortex in the brain. Refraction is the bending of a light ray as it passes through one medium and into another medium. The refraction of light within the eye takes place in the following pathway of structures: the cornea, aqueous humor , pupil, lens, and vitreous humor . The lens is the only adjustable part of the refraction system. PHYSIOLOGY OF VISION

18. When light rays strike the retina, they stimulate chemical reactions in the rods and cones. In rods, on colour absorption the photosensitive pigment Rhodopsin breaks down to form scotopsin and trans-retinal . In cone cell, on colour absorption the photosensitive pigment Iodopsin is breaks down to form photopsin and trans-retinal. This chemical reaction generates nerve impulse . The nerve impulses are transmitted to bipolar cells then to the ganglion cells. These ganglionic neurons entered at the optic disc and become the optic nerve. The images focused on retina are upside down (inverted).

19. The optic nerves from both eyes come together at the optic chiasma in the brain. Here, the fibers of optic nerve cross to each other. This crossing permits each visual area to receive impulses from both eyes, which is important for binocular vision. The impulses are further transmitted to visual areas of cerebrum. The visual areas integrate them, to make a single image that has depth and three dimensions. This is called binocular vision. The visual areas also right the image, because the image on the retina is upside down.

20. Disorders

21. 1) Conjunctivitis : Inflammation of conjunctiva membrane is referred as conjunctivitis, may be caused by allergies or by certain bacteria or viruses, and makes the eyes red, itchy, and watery. 2) Hypermetropia (far-sightedness): In hypermetropia , the patient can see far distant objects well but fails to see near vision. The far-sighted eye focuses light from near objects “behind” the retina due to the flattening of the lens. This condition can be corrected by convex eye glasses or lenses.

22. 3) Myopia (near-sightedness): In myopia, the patient can see near objects well but fail to see distant (far objects). The near-sighted eye can see only if the object is brought to 20 feet away. The near-sighted eye focuses images from distant objects in front of the retina, because the eyeball is too long or the lens too thick. Correction requires a concave lens to spread out light rays before they strike the eye.

23. 4) Presbyopia : The term presbyopia means “old eye” and is a vision condition characterised by inability of eye to focus on closed objects. It is associated with aging and loss of elasticity of the lens. Hence lens is unable to recoil and thicken for near vision, and glasses for reading are often necessary. 5) Astigmatism : astigmatism is an error of refraction, caused by an irregular curvature of the cornea or lens that scatters light rays and blurs the image on the retina. Correction requires a lens ground specifically for the curvature of the individual eye.

24. 6) Glaucoma : Glaucoma defined as a group of disorders characterised by damage the optic nerve and cause loss of vision due to increase in intraocular pressure. Increased pressure in the anterior cavity is transmitted to the lens, the vitreous humor , and the retina and optic nerve. As pressure on the retina increases, halos may be seen around bright lights, and peripheral vision is lost. Other risk factors include high blood pressure and diabetes.

25. 7) Night blindness : Night blindness is the inability of the patient to see in dim light (darkness). Night blindness is due to disorder of rod cells in the retina that are responsible for vision in dim light. Its causes include: vitamin A deficiency, glaucoma, cataract and near-sightedness. Night blindness affects the ability to drive in night . 8) Cataract : A cataract is a clouding (opacity) of the lens of the eyeball leading to a decrease in vision. Cataracts are most commonly due to aging and develop slowly. Symptoms may include: faded colours, blurred vision, halos around light and trouble with bright lights and at night.

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27. SENSE OF HEARING: EARS

28. The ear consists of three areas: the outer ear, the middle ear, and the inner ear. The ear contains the receptors for two senses: hearing and equilibrium . These receptors are all found in the inner ear.

29. The outer ear consists of the auricle and the ear canal . The auricle (or pinna) is made up of elastic cartilage. This auricle connected with ear canal known as the external auditory canal. The auricle allows sound waves to enter the ear canal, which then directs those waves to the delicate eardrum (or tympanic membrane) . The ear canal is lined with hairs and ceruminous glands that produce earwax (or cerumen ). The hairs and earwax protect the eardrum from foreign matters. Outer ear

30. The middle ear is the air-filled cavity that contains the three auditory ossicles: malleus, the incus, and the stapes. These bones transmit the sound vibrations from the eardrum to the oval window. The middle ear also consists of the auditory tube. This auditory tube opens into the pharynx and permits air pressure to be equalized between the middle ear and the outside air, thus ensuring that hearing is not distorted. Middle ear

31. The inner ear is made of the cochlea, the vestibule and the semicircular canals. The cochlea involved hearing whereas semicircular canals involved in body posture, balance and equilibrium. INNER EAR

32. AUDITORY OSSICLES Auditory ossicles are the three small bones of middle ear that extend from tympanic membrane to the oval window. These bones transmit the sound vibrations from the eardrum to the oval window. They are connected to each other by synovial joints. They are named according to their shapes.

33. The malleus : the malleus is hammer shaped bone. The handle is in contact with the tympanic membrane and the head forms a synovial joint with the incus. The incus : the incus is anvil shaped bone. Its body articulates with malleus and its other end articulates with stapes. The stapes : the stapes is stirrup shaped bone. Its head articulates with the incus and its base attached to oval window.

34. A cross section of cochlea contains three compartments: Scala vestibuli Cochlear duct Scala tympani The scala vestibule originates from the oval window and consists of perilymph fluid. The scala tympani ends at the round window and consists of perilymph fluid. These two compartments are continuous to each other. The cochlear duct is composed of membranous canals filled with endolymph fluid. The cochlear duct is consists of basilar membrane and hair cells with auditory receptors. These hair cells respond to sound vibrations and generate nerve impulses. The nerve impulses are transmitted by vestibuloochlear nerve to the auditory area of cerebrum COCHLEA

35. The process of hearing involves the transmission of vibrations and the generation of nerve impulses. When sound waves enter the ear canal, vibrations are transmitted by the following sequence of structures. PHYSIOLOGY OF HEARING

36. High-intensity sound waves cause greater vibration of basilar membrane, which leads to a higher frequency of nerve impulses conducted to brain.

37. 1) Motion sickness: Motion sickness is characterized by cold sweating, hyperventilation, nausea, and vomiting when the person is exposed to repetitive motion that is unexpected or unfamiliar, or that cannot be controlled . 2) Deafness : Deafness is the inability to hear properly. Deafness may be caused by punctured eardrum, deterioration of the hair cells in the cochlea, auditory cranial nerve damaged, and damage to the auditory areas in the cerebrum . 3) Otitis media: Otitis media or middle ear infection is quite common in young children. It can result in a temporary loss of hearing due to fluid buildup near the tympanic membrane. Symptoms include fever and irritability, and on examination, a red eardrum. DISORDERS

38. SKIN

39. Epidermis

40. The epidermis is made of stratified squamous epithelial tissue and is thickest on the palms and soles. There are no capillaries present between them. The epidermis may be further subdivided into four or five sublayers .

41. The stratum corneum consists on average of 25 to 30 layers of flattened dead keratinocytes. Keratinocytes produce the protein keratin is a tough, fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemicals. These cells are continuously shed and replaced by cells from the deeper strata. The interior of the cells contains mostly keratin. These cells are also covered and surrounded with lipids to prevent any passage of fluids through this layer. This layer acts as a physical barrier to light and heat waves, microorganisms (e.g., like bacteria, fungi, protozoa, and viruses), and most chemicals. 1) Stratum corneum

42. The stratum lucidum is present only in the thick skin of areas such as the fingertips, palms, and soles. It consists of three to five layers of flattened clear, dead keratinocytes that contain large amounts of keratin and thickened plasma membranes. 2) Stratum lucidum

43. 3) Stratum granulosum The stratum granulosum consists of two or three layers of flattened cells. 4 ) Stratum spinosum The stratum spinosum consists of 8 to 10 layers of polyhedron shaped cells.

44. It is the deepest layer of the epidermis is composed of a single row of keratinocytes. Some cells in this layer are stem cells that undergo cell division to continually produce new keratinocytes . It also contains melanocytes , which are responsible for producing skin color . They produce a pigment called melanin ; which is responsible for variations in skin pigmentation. Melanocytes are activated to produce melanin by exposure to sunlight. We darken when we expose ourselves to the sun. All races get darker after exposure to the sun over a period of time. We call this getting a suntan. 5) Stratum basale

45. THE DERMIS The second, deeper part of the skin, the dermis, is composed of a strong connective tissue containing collagen and elastic fibers . The few cells present in the dermis include predominantly fibroblasts, with some macrophages, and a few adipocytes near its boundary with the subcutaneous layer. Blood vessels, nerves, glands, and hair follicles are embedded in the dermal layer. Based on its tissue structure, the dermis can be divided into a superficial papillary region and a deeper reticular region.

46. Papillary region Reticular region

47. The papillary region consists of connective tissue containing thin collagen and fine elastic fibers. Dermis contain tactile receptors called Meissner corpuscles (touch receptors). Different sensory receptors give rise to sensations of warmth, coolness, pain, tickling, and itching. The reticular region , which is attached to the subcutaneous layer, it consists of few adipose cells, hair follicles, nerves, sebaceous (oil) glands, and sweat glands occupy the spaces between fibers.

48. The combination of collagen and elastic fibers in the reticular region provides the skin with strength, extensibility (ability to stretch), and elasticity (ability to return to original shape after stretching)

49. SKIN GLANDS

50. Sebaceous glands (oil glands) are connected to hair follicles with few exceptions. The secreting portion of a sebaceous gland lies in the dermis and usually opens into the neck of a hair follicle. Locations : Sebaceous glands are present in the skin of the breasts, face, neck, and superior chest the lips, glans penis, labia minora , and glands of the eyelids. Sebaceous glands are absent in the palms and soles. Sebaceous glands

51. Functions : Sebaceous glands secrete an oily substance called sebum (a mixture of triglycerides, cholesterol, proteins, and inorganic salts). Sebum covers the surface of hairs and helps keep them from drying and becoming hard. Sebum also prevents excessive evaporation of water from the skin, keeps the skin soft and pliable, and inhibits the growth of some (but not all) bacteria.

52. Sweat glands release sweat onto the skin surface through pores. Sweat glands are divided into two main types, eccrine and apocrine, based on their structure, location, and type of secretion . Eccrine sweat glands Apocrine sweat glands Sweat Glands

53. A. Eccrine sweat glands: They are distributed in the skin of the forehead, palms, and soles. The sweat produced by eccrine sweat glands (about 600 mL per day) consists of water, minerals, urea, uric acid, ammonia, amino acids, glucose, and lactic acid. The main function of eccrine sweat glands is to help regulate body temperature through evaporation. Eccrine sweat glands also release sweat in response to an emotional stress such as fear or embarrassment. This type of sweating is referred to as emotional sweating or a cold sweat.

54. Apocrine sweat glands: They are found mainly in the skin of the underarm (armpit), around the nipples of the breasts, and bearded regions of the face in adult males. Apocrine sweat is slightly viscous and appears milky or yellowish in color.

55. Normal body temperature is maintained at approximately 98.6°F (37°C). Temperature regulation is critical to our survival because changes in temperature affect the functioning of enzymes. The presence of enzymes is critical for normal chemical reactions to occur in our cells. When people get high fevers they can die because the heat of a fever destroys the enzymes by breaking up their chemical structure. Without enzymes, chemical reactions cannot occur and our cellular machinery breaks down and death results. The skin contributes to thermoregulation in two ways: THERMOREGULATION

56. In response to high environmental temperature , autonomic nervous system initiates vasodilation (dilation of blood vessels) in the dermis to increase blood flow to the dermis of skin, which increases the amount of heat loss from the body. The heat is then lost by convection, conduction, and evaporation. When we sweat, the water in sweat evaporates, which requires energy and thus carries away heat to reduce body temperature.

57. In response to low environmental temperature , production of sweat from eccrine sweat glands is decreased, which helps conserve heat. Also, vasoconstriction (constriction blood vessels) in the dermis of the skin decreases blood flow through the skin and reduces heat loss from the body.

58. Sensation: Sensory receptors in the skin produce the sensations of external environment for temperature, pressure, pain, and touch. These receptor sites allow us to react to external stimuli and to interpret what is occurring in the outside world. Protection: The skin acts an elastic, resistant covering. It prevents passage of harmful physical and chemical agents. The melanin produced by the melanocytes protects us from the damaging ultraviolet rays of sunlight. The lipid content of the skin inhibits the excessive loss of water and electrolytes through the skin. Functions of skin

59. Thermoregulation: Normal body temperature is maintained at approximately 98.6°F (37°C). In response to high environmental temperature evaporation of sweat from skin helps to lower elevated body temperature. In response to low environmental temperature, sweating is decreased to conserve heat. Excretion : The skin produces two secretions: sebum and sweat. Sweating helps in excretion of toxins. Sebum is secreted by the sebaceous glands. Sebum helps for moisturizing of skin. Sebum has both antifungal and antibacterial properties.

60. Synthesis of vitamin D: The skin is involved in the production of vitamin D. Exposure to the ultraviolet rays of the sun stimulates our skin to produce a precursor molecule of vitamin D to form calcitriol (vitamin D) . Calcitriol helps for absorption of calcium. Calcium is necessary for muscle contraction and bone development. Immunity : Skin kills most bacteria and other microorganisms that make contact with our skin.

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