Biology – Know Your Sense Organs

BIOLOGY

Topic: Sense Organs

Introduction

A sense organ is defined as a group of specialized cells, tissues or receptors which are able to receive, perceive or detect stimulus and transmit the information, impulse or message to the central nervous system. The human senses contain receptors that relay information through sensory neurons to the appropriate places within the nervous system. The receptors can be classified into two parts, the general and special receptors. The former is present throughout the body while the latter includes chemo receptors, photo-receptors and mechano-receptors.

There are five sense organs in human body. These sense organs are eye, ear, nose, tongue and skin.

The Skin

The skin is the outer layer of vertebrate animals. Its major functions are protection, temperature regulation and to act as a sense organ. The skin contains general receptors which can detect touch, pain, pressure and temperature. Throughout your skin, they are present. Skin receptors generate an impulse when activated, which is carried to the spinal cord and then to the brain.

There are two major layers of the skin: epidermis and dermis.

Epidermis  

The epidermis is the outer renewable layer of the skin. As it wears away at the surface it grows at the base.

Malpighian Layer: This is the base layer, which is constantly producing new cells by mitosis. The new cells are pushed towards the surface. The dark pigment melanin is produced here.

Granular Layer: The protein keratin accumulates in the cells giving them a granular appearance. The cells finally die.

Cornified Layer: This is the surface layer of dead keratinized cells, which is constantly being eroded.

Dermis 

The dermis is the inner layer of the skin, above it is the epidermis and below it is adipose tissue.

Many different structures are present in the dermis: hair follicles, sebaceous glands, sweat glands, blood vessels, sensory nerve endings all of which are embedded in a dense matrix of connective tissue.

Functions of the Skin

• Prevents excessive loss of water — the cornified layer of the epidermis is waterproof.
• Prevents the entry of pathogens — the cornified layer is made of dead cells.
• Sebum from the sebaceous glands contains anti-microbial chemicals.
• Sebum oil’ keeps the skin intact preventing it from ‘cracking’.
• Melanin gives protection against the damaging UV rays of sunlight.
• The dermis and adipose tissue protect against mechanical injury.
• Temperature Regulation – maintains blood at 37°C

The Skin as a Sense Organ

The skin contains receptors for touch, pressure, pain, temperature rise and temperature decrease. The skin supplies information about a variety of external environment condition. The sensory nerve endings are unevenly located within the skin. Those that are sensitive to pressure (pacinia corpuscles) are found deepest in the skin and therefore require a stronger stimulation to register in the brain. Those that are sensitive to touch (meissner’s corpuscles) are largely distributed and they are closest to the surface of the skin especially in areas without hairs like tongue, lips, forehead, fingers, etc. They need a mild and gentle stimulus to register in the brain. Found between the receptors of pressure and touch is the ones which detect cold, heat and pain.

The Eye (The Organ of Sight)

The organ of sight is the eye found in all vertebrates. The mammalian eye is almost spherical in shape. The protective structures of the eye include eye socket, eyelids, eye lashes, tear glands, and conjunctiva. The eye is housed in a protective cavity in the skull called the eye socket. The eye is attached to the socket by six muscles which serve to hold it firmly in place and enable it rotate free upward, downward and sideways without moving the head.The iris in the eye is the colored part that controls the size of the eye. Behind the lens of the eye one will find the vitreous body, which is filled with a gelatinous material called the vitreous humor. This substance gives shape to the eyeball and also transmits light to the very back of the eyeball, where the retina  This retina contains photoreceptors, which detect light.Two types of sensors detect light: Rods which detect motion work harder in low light and cones which detect fine detail and color, work best in bright light. There are three types of cones: one that detects blue, one that detects red and one that detects green. Color blindness occurs when one type of cone is lacking.

The eye has a complex structure consisting of a transparent lens that focuses light on the retina. The retina is covered with two basic types of light-sensitive cells-rods and cones. The cone cells are sensitive to color and are located in the part of the retina called the fovea, where the light is focused by the lens. The rod cells are not sensitive to color, but have greater sensitivity to light than the cone cells. These cells are located around the fovea and are responsible for peripheral vision and night vision. The eye is connected to the brain through the optic nerve. The point of this connection is called the “blind spot” because it is insensitive to light. Experiments have shown that the back of the brain maps the visual input from the eyes.

The brain combines the input of our two eyes into a single three-dimensional image. In addition, even though the image on the retina is upside-down because of the focusing action of the lens, the brain compensates and provides the right-side-up perception. Experiments have been done with subjects fitted with prisms that invert the images. The subjects go through an initial period of great confusion, but subsequently they perceive the images as right side up.

Structure of the Eye

• Sclera or Sclerotic layer: tough outer white layer of the wall of the eye. It gives shape and firmness to the eye.
• Cornea: transparent ‘window’ of the eye, focusing of light on the retina.
• Iris: the coloured sheet of muscle, controls the pupil size so controls entry of light.
• Pupil: a hole in the iris letting light into the back of the eye.
• Ciliary Body: a ring of muscle controlling the shape of the lens.
• Suspensory Ligaments: transfer the pull of the ciliary body to the lens.
• Lens: accommodation — the fine adjustment to the focusing of light onto the retina.
• Retina: light sensitive layer of rods and cones converting light into nerve impulses.
• Fovea or Yellow Spot: a tiny area of densely packed cones for detailed and coloured vision.
• Choroid: a black-pigmented layer preventing internal reflection of light.
• Blind Spot: exit point of the optic nerve cutting through the retina so no rods or cones here.
• Optic Nerve: carries the impulses from the rods and cones to the visual centre of the brain.
• Aqueous Humour: a clear liquid in front of the lens maintaining the shape of the cornea.
• Vitreous Humour: a clear jelly offering support and shape to the back of the eye.

Functions of the Eyes

The eyes perform two major functions. These are: image formation and accommodation.

Image Formation: The ability to see involves many processes. Light enters a denser medium like the cornea and lens and is made to refract. This causes the rays to be focused at a point to the retina. The image formed is upside down, but is corrected by the optic centre of the brain (cerebral cortex).

Accommodation: Accommodation is the ability of the eye to focus an object at varying lengths. In other words, accommodation is the ability to see clearly (far and near) objects.

Adaptation for rear vision:

• The ciliary muscles contract.
• The suspensory ligament relaxes.
• The lens also relaxes (assumes normal round shape).
• The focal length shortens.

Adaptation for distant vision:

• Ciliary muscles relax and are pushed outward by the pressure of the vitreous humour.
• The suspensory ligaments contract due to the tension created.
• The lens is stretched into an elongated shape.
• The focal length is increased.

Eye Defects and Correction

• Short Sight (myopia):Condition: can view close object clearly but distant objects are out of focus. Light rays are focused short of the retina. It is caused as a result of the eyeball being too long or the focusing elements of the eye are too strong.

Correction: use a concave (divergent) lens to widen the angle over which the light rays have to be refracted.

• Long Sight (hypermetropia): can view distant object clearly but close objects are out of focus, the focal point is long of the retina i.e. is behind retina. It is caused as a result of eyeball being too short or the focusing elements of the eye are too weak.

Correction: Use a convex (convergent) lens to reduce the angle over which the light rays have to be refracted.

• Astigmatism: It is caused by irregularities in the curvature of the cornea and the lens leading to the blurring of image.

Correction: the use of cylindrical lenses.

• Night Blindness: Night blindness is a deficiency disease caused by lack of vitamin A.

Correction: Use of vitamin A.

Care of the Eyes

1. Bath with salt solution
2. Use a clean handkerchief for cleaning the eyes.
3. Consult a medical doctor in case of suspected infection
4. Use antibiotic eye drops e.g. chloramphenicol and visine in case it is inflammed (or turns reddish).

The Ear (Organ of Hearing)

The ear is the organ of hearing.  It’s not only responsible for hearing but also for maintaining balance or equilibrium. For this the ear must detect motion, hence they respond to mechanical stimulation by sound waves. The eardrum sets the mechanics in motion. The brain then interprets them. The outer ear protrudes away from the head and is shaped like a cup to direct sounds toward the tympanic membrane, which transmits vibrations to the inner ear through a series of small bones in the middle ear called the malleus, incus and stapes. The inner ear, or cochlea, is a spiral-shaped chamber covered internally by nerve fibers that react to the vibrations and transmit impulses to the brain via the auditory nerve. The brain combines the input of our two ears to determine the direction and distance of sounds.

The inner ear has a vestibular system formed by three semicircular canals that are approximately at right angles to each other and which are responsible for the sense of balance and spatial orientation. The inner ear has chambers filled with a viscous fluid and small particles (otoliths) containing calcium carbonate. The movement of these particles over small hair cells in the inner ear sends signals to the brain that are interpreted as motion and acceleration.

The human ear can perceive frequencies from 16 cycles per second, which is a very deep bass, to 28,000 cycles per second, which is a very high pitch. Bats and dolphins can detect frequencies higher than 100,000 cycles per second. The human ear can detect pitch changes as small as 3 hundredths of one percent of the original frequency in some frequency ranges. Some people have “perfect pitch”, which is the ability to map a tone precisely on the musical scale without reference to an external standard.

Structure of the Ear

To hear sound, the ear has to do three basic things:

• Direct the sound waves into the hearing part of the ear.
• Sense the fluctuations in air pressure.
• Translate these fluctuations into an electrical signal that the brain can understand.

The outer ear

The external part of the ear consists of the pinna and ear lobe.

The pinna or ear shell is the shell-like part of the external ear, and it is made of cartilage and skin. The pinna directs sound waves from the outside into the external auditory canal (ear canal), which in turn channels sound waves to the tympanic membrane (known as the eardrum), causing it to vibrate. The tympanic membrane is a thin, semi-transparent, flexible membrane that separates the outer and middle ear.

The outer ear functions to collect sound (acoustic energy), and funnel it to the eardrum (tympanic membrane).

The middle ear

The middle ear is an air-filled space that contains three tiny bones known as ossicles which transmit sound. The bones are known individually (according to their shapes) as the:

• Malleus (hammer).
• Incus (anvil).
• Stapes (stirrup).

The Inner ear

The delicate membranous inner ear (labyrinth) is enclosed and protected by a bony chamber that is referred to as the bony labyrinth. The inner ear contains two main structures:

• The cochlea, in the shape of a snail, which is involved in hearing. The round window (fenestra cochlea) is a membrane that connects the cochlea to the middle ear. It helps dampen the vibrations in the cochlea.
• The vestibular system (consisting of the semicircular canals, saccule and utricle), which is responsible for maintaining balance and a sense of position.

Function of the Ear

The major task of the ear is to detect, transmit and transduce sound. Another very important function of the ear is to maintain our sense of balance.

The Tongue (Organ of Taste)

The receptors for taste, called taste buds, are situated chiefly in the tongue, but they are also located in the roof of the mouth and near the pharynx. The senses, smell and taste tend to work closely together. The sense organs is tongue. If one cannot smell something, they can’t taste it either. Taste buds on your tongue contain chemo receptors that work similar to the chemo receptors in the nasal cavity. But, the chemo receptors in the nose will detect any kind of smell, whereas there are four different types of taste buds and each detect different types of tastes: sweet, sour, bitter, and salty.  The tongue also can detect a sensation called “umami” from taste receptors sensitive to amino acids. Generally, the taste buds close to the tip of the tongue are sensitive to sweet tastes, whereas those in the back of the tongue are sensitive to bitter tastes. The taste buds on top and on the side of the tongue are sensitive to salty and sour tastes. At the base of each taste bud there is a nerve that sends the sensations to the brain. The sense of taste functions in coordination with the sense of smell. The number of taste buds varies substantially from individual to individual, but greater numbers increase sensitivity. Women, in general, have a greater number of taste buds than men. As in the case of color blindness, some people are insensitive to some tastes.

Structure of the Tongue

Functions of the Tongue

The functions of the tongue are licking, breathing, tasting, swallowing and articulating speech.

1. Speech – Short tongue or tongue tie patients will have difficulty in pronouncing words. The sound that is produced from the voice box needs the help of the tongue to make a word more audible and apprehensive.
2. Tongue helps to identify the thermal nature of food i.e. the hotness/coldness of food.
3. Tongue helps to keep the oral cavity clean. The food particles attached to the teeth are cleared by tongue.
4. Assimilation of Food – Tongue in assimilation of food: due to its muscular nature, tongue can manipulate in any direction, this facilitates the food to be properly mixed with saliva. Tongue can now turn the chewed food into a bolus and push it into the esophagus, from where the food will proceed further into the stomach through the peristaltic movement.
5. Mastication – The tongue is an important accessory organ in the digestive system. The tongue is used for crushing food against the hard palate, during mastication and manipulation of food for softening prior to swallowing. The epithelium on the tongue’s upper, or dorsal surface is keratinised. Consequently, the tongue can grind against the hard palate without being itself damaged or irritated.

Tongue Diseases/Conditions

• Thrush (candidiasis): Candida albicans (a yeast) grows over the surface of the mouth and tongue. Thrush can occur in almost anyone, but it occurs more often in people taking steroids or with suppressed immune systems, the very young, and the elderly.
• Macroglossia (big tongue): This can be broken down into various categories based on the cause. These include congenital, inflammatory, traumatic, cancerous, and metabolic causes. Thyroid disease, lymphangiomas, and congenital abnormalities are among some of the causes of an enlarged tongue.
• Canker sores (aphthous ulcers): Small, painful ulcers appear periodically on the tongue or mouth. A relatively common condition, the cause of canker sores is unknown; they are unrelated to the cold sores caused by herpes viruses. Canker sores are not contagious.
• Geographic tongue: Ridges and colored spots migrate over the surface of the tongue, periodically changing its appearance. Geographic tongue is a harmless condition.
• Oral cancer: A growth or ulcer appears on the tongue and grows steadily. Oral cancer is more common in people who smoke and/or drink alcohol heavily.
• Oral leukoplakia: White patches appear on the tongue that can’t be scraped off. Leukoplakia may be benign, or it can progress to oral cancer.
• Burning mouth/burning tongue syndrome: a relatively common problem. The tongue feels burned or scalded, or strange tastes or sensations develop. Apparently harmless, burning mouth syndrome may be caused by a mild nerve problem.
• Atrophic glossitis (bald tongue): The tongue loses its bumpy texture, becoming smooth. Sometimes this is due to anemia or a B vitamin deficiency.
• Hairy tongue: Papillae can overgrow the surface of the tongue, giving it a white or black appearance. Scraping off the papillae corrects this harmless condition.
• Herpes stomatitis: The herpes virus can uncommonly cause cold sores on the tongue. Herpes virus cold sores are usually on the lip.
• Lichen planus: A harmless condition that can affect the skin or the mouth. The cause is unknown; however, it is believed to be caused by the immune system attacking the skin and lining of the mouth.

The Nose (Organ of Smell)

The nose is the organ responsible for the sense of smell.  The nose is the body’s primary organ of smell and also functions as part of the body’s respiratory system. Air comes into the body through the nose. As it passes over the specialized cells of the olfactory system, the brain recognizes and identifies smells. Hairs in the nose clean the air of foreign particles. The olfactory cells tend to line the top of your nasal cavity. On one end, olfactory cells have cilia that project into the nasal cavity and on the other end of the cell, are olfactory nerve fibers. As one breathes, anything that is in the air enters your nasal cavity. The olfactory cells are chemo-receptors, which goes to mean that the olfactory cells have protein receptors that can detect subtle differences in chemicals. The chemicals bind to the cilia, which generate a nerve impulse that is carried to your brain. Your brain then determines what you have smelled. The cavity of the nose is lined with mucous membranes that have smell receptors connected to the olfactory nerve. The smells themselves consist of vapors of various substances. The smell receptors interact with the molecules of these vapors and transmit the sensations to the brain. The nose also has a structure called the vomeronasal organ whose function has not been determined, but which is suspected of being sensitive to pheromones that influence the reproductive cycle. The smell receptors are sensitive to seven types of sensations that can be characterized as camphor, musk, flower, mint, ether, acrid, or putrid. The sense of smell is sometimes temporarily lost when a person has a cold. Dogs have a sense of smell that is many times more sensitive than man’s.

Structure of the Nose

Functions of the Nose

1. Breathing – Air flows in and out during normal breathing. The inner lining of the nose has many blood vessels at the surface. Blood flowing through your nose warms the air as your breathe in. Your nose also humidifies the air before it gets to your lungs.
2. Smell – This is one of the most important function of the nose. The sense of smell is not completely understood. Smell is a key component of memory, physical attraction and emotional connections. The olfactory nerves are cranial nerves that allow communication between your nose and brain. Conditions, such as a cold, will decrease your sense of smell.
3. Taste – Although taste is a completely separate sense than smell, the nose plays a role in the way the tongue perceives taste. The aroma of the food plays a role in the taste.
4. Cleansing and Filtering – The nose has many small hairs inside the nostrils. These hairs act as a filter, removing dirt and particles before the air enters the lungs. Sneezing and nose blowing also help remove the particles out of your body.
5. Voice/Sound – Air resonating in your nose affects the sound of your voice. The shape of your septum also plays a role in the sound of your voice. As a result, surgery on your nose may also alter the sound of your voice.

Care of the Nose

Secretions can usually be removed from the nose by having the patient blow into a soft tissue. With a cotton swab and warm, mildly salty water you can clean your nose without risk of scratching or cutting and you will not damage the sensitive membranes. Cleaning your nose not only helps to keep it comfortable but it also helps to improve its ability to filter particles. Cleaner noses tend to develop fewer allergic reactions and help to reduce snoring as well.