Sense organs

Introduction

You know what is going on around you and inside you because of special receptors in your body. Through them, you are able to see beautiful sights, hear wonderful sounds, taste delicious flavors, and smell appealing aromas. You can touch things and feel a vari­ety of sensations. More importantly, how­ever, these receptors help you survive. You can avoid injuries if you can feel, taste, smell, hear, or see potential danger. Without some of the special sense receptors, you could not move well, balance yourself, or judge your position in space.

Each kind of receptor reacts to a specific type of stimulus. The stimuli are transmitted to the central nervous system, which in turn determines the body's response to conditions in the external environment.

Receptors and Sense Organs

Many receptors that enable the body to obtain information from the environment are located in highly specialized organs called sense organs. The most familiar sense organs are the eyes, ears, nose, mouth, and skin. In addition to these, you have other sense organs that you may not be aware of. For example, recep­tors in your ears enable you to keep your balance. All sense organs have specialized receptors for stimuli. Most sense organs have receptors that pick up stimuli from the body's external environment. Other kinds of receptors pick up stimuli from the body's internal environment.

Types of Receptors

Sense receptors are highly selective. The receptors for taste will not respond to light, no matter how intense it is. The receptors for sight cannot be activated by sound vibrations.

Sense receptors can be classified according to the stimuli that activate them. Photoreceptors detect stimuli generated by light. The receptors for taste and smell are triggered by chemi­cals and are called chemoreceptors. Thermoreceptors respond to heat or cold, either inside or outside the body. Pain receptors generate impulses interpreted as pain. Mechanoreceptors re­spond to mechanical pressures. Such pressures may come from sound vibrations, touch, muscle contractions, or movements of joints. The pressure bends or distorts the part of the sense organ in which the mechanoreceptors are located. Hair cells, which have extremely fine projections like cilia, are the most common type of mechanoreceptors.

Sense Organs

Sense organs act as transducers—that is, they transform one form of energy into another form. For example, when light rays strike the inner lining of the eye, they are changed into im­pulses. These impulses move along a nerve to the brain's visual center where they are interpreted as sight.

Impulses from all sense organs are basically alike. The way the brain interprets impulses from various sense organs dif­fers. Impulses from each sense organ travel to a different part of the brain. The impulses from a particular sense organ are inter­preted in only one way, according to where they are received in the brain. For example, when the eye receives light signals, it produces impulses that the brain interprets as an image. When the ear receives pressure waves, or sound vibrations, it produces impulses that the brain interprets as sound. The brain never interprets impulses from the eye as sound or impulses from the ear as an image. Even if some other type of energy generates an impulse in a receptor cell, the brain will interpret the impulse exactly as it does all other impulses from that receptor. For example, a blow to the eye may cause you to see an image, even though the impulse was generated not by light but by mechani­cal pressure.

Thinking About Biology: The Other Senses

The human body has many special receptors other than those in the familiar sense organs. While some scien­tists claim that all these receptors are "senses," others feel they are more accurately described as "controls." Regardless of the term used, the fact remains that these special receptors react to inter­nal stimuli rather than external.

The internal controls primarily maintain homeo-stasis. For example, thirst is triggered by the hypo-thalamus, which responds to salt concentration in the blood. When the water level in the blood is low, salt be­comes more concentrated. When salt concentration is high, the hypothalamus reacts by generating im­pulses that trigger a thirst sensation. When the water level is high, salt concen­tration is low, and the body eliminates more water. Similarly, chemicals in the cerebrospinal fluid and a low level of glucose in the blood seem to trigger hunger.

Another type of internal control monitors your skeletal muscles. Muscle spindles, a special type of muscle fiber, are part of skeletal muscle. The spin­dles contain two types ofsensory neurons. One type alerts the central nervous system to a change in the stretch or contraction of a muscle. The other type registers how much stretch is involved. This constant monitoring of muscular contraction helps you main­tain posture and keeps your body steady. Joint and tendon receptors work with the muscle spindles. Joint receptors register the angle of ligament move­ment. Tendon receptors indicate the amount of stretch in the tendons.

Vision, Hearing, and Balance

The eyes and ears provide the body with its greatest protection. Because these organs are sensitive to distant stimuli, they can give early warnings about possible dangers.

The Eyes

The eye is often compared to a camera, but it is more compli­cated than the most sophisticated camera. Humans have binocu­lar vision, the ability to view objects with two eyes. People also have stereoscopic vision, the ability to see objects in three di­mensions—height, width, and depth. With stereoscopic vision a person can assess the speed of a moving object and determine the distance of an object in space.

Structure of the Eye

The eye, or eyeball, is an almost per­fect sphere with a diameter of about 2.5 cm (1 in.). The eye is protected in a number of ways. A fatty layer within the orbit, a socket in the skull, cushions the eyeball. Eyelids and eyelashes also provide protection by preventing foreign particles from en­tering the front of the eye. If something touches the eyelashes or moves suddenly in front of the eye, the eyelid closes and reopens rapidly in a blinking reflex.

Each eye is moved by three sets of muscles and is lubricated by mucus and tears. The mucus is secreted by the conjunctiva, a delicate, blood-rich membrane that lines the inner eyelid and covers the front of the eye. Tears are produced by the lacrimal gland near the outer corner of the eye. When the eye closes, the eyelid spreads the mucus and tears, which moisten the eye and help remove foreign particles.

The eye has an outer wall that consists of three layers of tissue: the sclera, the choroid, and the retina. These three layers surround a jellylike substance, the vitreous humor that makes up two-thirds of the eyeball.

The sclera is tough, white connective tissue that forms the outermost layer. About 80 percent of the sclera, including the ''white'' of the eye, is opaque. The remainder is a transparent layer called the cornea at the front of the eye.

The choroid is the middle, darkly pigmented layer of tissue. It absorbs light and so prevents reflection, which would result in fuzzy images. The choroid contains many blood vessels that nourish the eye. Toward the front of the eye, the choroid forms a colored ring, the iris, which gives the eye its color. In the center of the iris is an opening called the pupil. In bright light, one set of muscles in the iris contracts and causes the pupil to become smaller. In dim light, a different set of iris muscles contracts, making the pupil larger.

Behind the pupil is the lens, a transparent, curved structure. By changing shape, the lens helps focus images onto receptor cells at the rear of the eye. The curvature of the lens is con­trolled by ciliary muscles attached to the choroid. A clear, wa­tery fluid called the aqueous humor fills the space between the lens and cornea. The vitreous humor fills the space behind the lens.

The innermost layer of the eye is the light-sensitive retina. The retina contains about 125 million receptors called rods and cones. The rods and cones are stimulated by light to generate nerve impulses. The rods are extremely light-sensitive and can detect various shades of gray even in dim light. However, they cannot distinguish colors, and they produce poorly defined images. The cones detect color, produce sharp images, and are important for seeing in bright light. In a tiny pit at the center of the retina is a concentration of cones. This area, the fovea, produces the sharpest image.

How You See

Light passes through the cornea, aqueous humor, pupil, lens, and vitreous humor on its way to the retina. Impulses generated by the rods and cones travel to the visual center in the occipital lobe of the brain by means of the optic nerve. The optic nerve from each eye consists of about 1 million nerve fibers. No rods or cones exist at the point where the optic nerve enters the retina. This area, called the optic disc, or blind spot, does not transmit impulses. Near the base of the brain, half of the nerve fibers from the left eye cross over and join half of the nerve fibers from the right eye. All these fibers go to the right side of the brain. Likewise, half the nerve fibers from the right eye join half from the left eye and go to the left side of the brain. Each side of the brain thus receives images from both eyes. The point at which the partial crossing-over of the fibers occurs is the optic chiasma.

Thinking About Biology: What Your Eyes Tell About You

A doctor can learn a great deal about the condition of your entire body by looking into your eyes. Using an ophthalmoscope, which has special lenses and a light, he or she can study the optic disc and the blood vessels of the retina. Studying the retina may reveal many disorders that do not directly involve the eyes. For example, high blood pressure can be identified by viewing the blood vessels of the retina. The increased pressure of the blood circulating through these tiny vessels may cause some of them to burst. Diabetes may also cause changes in these blood vessels and in the vitreous humor. Changes in the size and shape of the optic disc may indicate such serious disorders as glaucoma or even brain tumor.

Disorders of the Eye

Eye disorders affect more than 50 percent of the people in the United States. Among the most common of these disorders are myopia and hyperopia. In myo­pia, or nearsightedness, the eyeball is too long from the front to the back. Light focused by the lens falls at a point in front of the retina, resulting in a blurred image of distant objects. In hypero­pia, or farsightedness, the eyeball is too short from front to back. Light is focused at a point behind the retina, resulting in a blurred image of close objects. In astigmatism, irregularities in the curvature of the cornea result in fuzzy images. Prescribed eyeglasses or contact lenses can correct these conditions.

In a condition called glaucoma, the aqueous humor cannot drain into blood vessels around the eye. Because new aqueous humor is constantly produced by the choroid, failure to drain creates excess pressure within the eye. This pressure damages the retina and optic nerve and can lead to blindness.

The Ears

The eyes have only one function—vision. The ears, however, perform two vital functions—hearing and balance.

Structure of the Ear

The ear is divided into three major sections: the outer ear, the middle ear, and the inner ear. Each region has a specific function.

The outer ear consists of a cartilage flap called the pinna and the auditory canal, a tube leading to the middle ear. These structures channel sound to the eardrum, a tightly stretched membrane between the outer ear and the middle ear. The auditory canal is lined with cilia and special cells that secrete ceru­men, or earwax. Together, the cilia and earwax clear foreign particles from the auditory canal.

The middle ear lies within an air-filled space called the tympanic cavity inside the skull bone. A duct called the Eusta-chian tube connects the middle ear to the pharynx. Generally the tube is collapsed, but it opens when you yawn, swallow, cough, or blow your nose. Air pressure be­tween the middle ear and throat is then equalized. Air pressure around you varies with altitude and can change rapidly, as when you ride an elevator or airplane. If the pressure is not equalized, the eardrum can bulge, causing pain and difficulty in hearing. Lying across the middle ear cavity are three tiny bones called the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup. The stapes touches a membrane called the oval window, located between the mid­dle ear and the inner ear.

The inner ear contains the sensory receptors for hearing and balance. It consists of three main parts: the cochlea, the vesti­bule, and the semicircular canals. The organ of hearing is within the cochlea, a bony, coiled tube filled with fluid and lined with hair cells. A second membrane-covered opening is located in the cochlea below the oval window. Called the round window, it maintains a constant pressure within the inner ear. The upper part of the inner ear consists of three semicircu­lar canals, which are fluid-filled tubes positioned alright angles to each other. These canals help maintain balance by responding to head movement. A bony chamber called the vestibule lies between the semicircular canals and the cochlea.

How You Hear

Sound waves are generated when any object vibrates, or moves back and forth, in the air. The human ear can detect sounds between 20 and 20,000 vibrations per second.

The vibrations travel through the auditory canal to the eardrum; to the malleus, which touches the eardrum; and then to the incus and the stapes. The stapes touches the oval window. The oval window sets the fluid in the cochlea in motion. Stimulated by the fluid motion, hair cells in the cochlea generate nerve im­pulses that travel along the auditory nerve to the auditory center in the temporal lobe of the brain. Exactly how vibrations are transformed into impulses is not clear.

Hearing loss due to disease or injury of the auditory nerve or cochlea is called nerve deafness. It is the most common cause of total and permanent hearing loss. Deafness resulting from interference as vibrations pass to the inner ear is called conduc­tive deafness. This condition may be caused by several prob­lems, including excess earwax, infection, swelling and closing of the passage, rupture and inflammation of the eardrum, or immobility of the stapes due to bone overgrowth. Conductive deafness senerally can be treated.

How You Balance Yourself

Fluid in the semicircular canals flows when you change the angle of your head. A different canal in each ear is affected by any particular movement. For example, a movement to the right causes fluid in the right ear to flow toward the hair cells. As a result, many impulses are sent to the cerebellum from the right ear. At the same time, the movement causes fluid in the left ear to flow away from the hair cells. Few impulses are then sent to the brain from the left ear. The cerebellum interprets the two sets of impulses so you know which way your head is turned.

The saccule and utricle, the two sections of the vestibule, also help with balance. They are lined with hair cells covered by a gelatin-like membrane embedded with grains of limestone. Gravity pulls the limestone down onto the hair cells, causing them to generate impulses. The greater the pull on particular grains, the stronger the impulses. The cerebellum interprets the direction of gravity and lets you know the position of your head.

Smell, taste and touch

Smell and taste are closely associated senses. The fact that a stuffy nose makes food seem tasteless demonstrates the close relationship between these senses. Smell and taste seem to oper­ate more simply than sight and hearing, but biologists do not yet know precisely how the receptors for smell and taste discrimi­nate among various chemicals.

The skin, the largest organ of the body, contains several types of receptors. These receptors register touch, pressure, pain, heat, and cold. The receptors for these sensations vary in number and location over the body.

The Nose

The nose, the chief sense organ of smell, contains receptors embedded in mucous membrane. About 50 million of these spe­cial cells, called olfactory receptors, are located in each nasal passage. Airborne substances dissolve in the mucus that covers the olfactory receptors. The receptors produce nerve impulses that travel through olfactory nerves to the olfactory lobe in the cerebral cortex.

Some biologists think that the perception of smell occurs when a specialized molecule on the receptor surface reacts with a specific chemical in inhaled air. The reaction generates an impulse that results in a particular smell. Other scientists believe that the outline, or shape, of a molecule is the cause of its partic­ular odor. They think that a molecule of a specific shape fits into an olfactory receptor that will accept only that shape, just as a lock works with one key. These researchers believe that the thousands of odors humans can distinguish are simply combina­tions of seven basic odors.

The Tongue

The tongue is the major sense organ of taste. The chemical receptors for taste are clusters of sensory hair cells located in the taste buds. Each taste bud consists of about 40 receptor and supporting cells and an opening called the taste pore. The taste buds lie in bunches called papillae, which are visible as the bumps on your tongue. Although most of a person's 10,000 taste buds are on the tongue, a few also exist on the roof of the mouth and in the throat.

Taste buds produce one or a combination of four main taste sensations: sweet, sour, bitter, and salty. A receptor cell may be stimulated by only one taste, but most cells are stimulated by two or more tastes. This combination of different tastes may be what produces the wide variety of flavors you enjoy.

Like smell, taste depends upon chemical reactions that take place only in solution. Saliva constantly bathes taste buds, reaching receptor cells through the taste pores. Food molecules also enter the taste pores. The chemical reactions that take place somehow cause the receptor cells to generate nerve impulses. The impulses travel through three different nerves to the taste center in the cerebral cortex. No one knows precisely how taste receptors function. Some researchers think that sensory cells have sites that accept specific chemical molecules. Other re­searchers think that, as with smell, the shape of a molecule determines its taste. Molecules of a certain shape, they think, activate specific sites on a taste bud to produce one taste.

The Skin

The skin is considered the organ of touch. It actually contains five distinct senses, most with their own type of receptor. These five senses are touch, pressure, pain, heat, and cold. Impulses travel from the various sense receptors to different areas of the sensory cortex.

Touch receptors are the ends of certain nerve fibers .Many touch receptors are located at the base of hairs and generate impulses when the hairs move even slightly. However, touch is most sensitive in the fingertips, palms, lips, and other places where hair is not present. Other receptors react to pressure. Some are sensitive to deep pressure and vibration, while others are sensitive to lighter pressure.

Unlike the other senses of touch, the sense of pain has no specialized receptors. Pain receptors are free ends of unmyeli-nated nerve fibers. Pain appears to stem from a variety of stim­uli. Some parts of the body are almost pain-free. Other parts may sense only one type of pain. Sensitivity to pain may be related to other body conditions, such as mental attitude.

Temperature receptors may be either bare nerve endings or specially shaped cells. Different types of receptors detect heat and cold.

6.8. Endocrine system

Introduction

Activities within the human body are regulated by two systems, the nervous system and endocrine system. Although both systems control body functions, their methods differ.

The nervous system sends its messengers, called impulses, to specific cells, generally muscle or gland cells. The nervous system acts quickly. Its messages travel rapidly and can change instantly. The response is immediate.

The endocrine system uses chemical messengers. They are widely dispersed to every cell throughout the body. However, only specific target cell, equipped with receptors, respond to the messages. The endocrine system generally does not act as quickly as the nervous system. Its messages travel more slowly, but the effect generated by those messages last longer than those from the nervous system.

The Endocrine Glands

The body contains many glands. Glands are cells, groups of cells, or organs that produce and secrete substances. Exocrine glands, such as sweat glands and digestive glands, secrete their products through tubes, or ducts. Endocrine glands, often called ductless glands, release their products directly into the bloodstream. Endocrine glands produce powerful chemicals called hormones, which help regulate the activities of body tissues and organs. Each hormone acts on a specific tissue or organ: that tissue or organ is the hormone’s target.

The Thyroid

The thyroid gland, located on the trachea, secrets thyroxine. Thyroxine controls metabolic activities, including the production of proteins and ATP. Because thyroxine influences protein production, it affects the growth rate of children. This hormone is also necessary for the proper development of the nervous system.

Iodine is necessary for the production of thyroxine. A person needs 1 mg of iodine each week. Eating a moderate amount of iodized salt usually meets that need. Insufficient iodine may cause the thyroid gland to enlarge, a condition called goiter. Frequently a person with goiter also suffers from hypothyroidism, a lack of thyroxine. The result is low metabolic rate. In adults the symptoms are low body temperature, sluggishness, weight gain, and excess fluid in the body. In infants hypothyroidism may cause cretinism. The effects of cretinism include mental retardation and abnormal bone growth. Hyperthyroidism, or an excess of thyroxine, causes a higher-than-normal metabolic rate. The symptoms of hyperthyroidism include weight loss, muscle weakness, excessive sweating, increased heartbeat rate and blood pressure, nervousness, and bulging eyes.

The Parathyroids

On the back of the thryroid gland are four tiny parathyroid glands. They secrete PTH (parathyroid hormone), which regulates the levels of calcium ions and phosphate ions in the blood. These minerals are necessary for proper bone development and for normal functioning of muscles and nerve cells. Too little calcium can make nerve cells so unstable that they send impulses without being stimulated. The result is uncontrollable muscle contractions. If muscles remain contracted, a person may die because breathing stops. The calcium level sometimes is too high. Nerves and muscles then fail to respond to stimuli. Reflexes are slow, and muscle contractions are weak.

The Adrenals

An adrenal gland is located on top of each kidney. Each gland functions as two separate endocrine glands. The inner part of the adrenal gland, called the adrenal medulla, secretes epinephrine and norepinephrine. These hormones produce the same effects as the sympathetic nervous system. They thus help the body respond to stress. For example, they increase blood pressure and heartbeat and breathing rates, dilate the pupils, and inhibit digestion. They also increase metabolism, sometimes as much as 100%.

The outer layer of the adrenal gland is the adrenal cortex, which secretes more than 50 hormones. All belong to a group called corticoids. Among the corticoids are aldosterone: hydrocortisone, also called cortisol; and also androgens. Aldosterone affects water and salt balance by controlling the reabsorption of sodium and potassium ions in the kidneys. Hydrocortisone controls the breakdown of proteins and fats into glucose, inhibits glucose uptake by cells, and aids in healing. Androgens are sex hormones. They regulate development of secondary sex characteristics. A lack of corticoids may result in Addison’s disease. The symptoms of this disease include low blood pressure, darkened skin, dehydration, a low level of sugar and sodium ions in the blood, and a high blood level of potassium ions. A victim will die within a few days if not treated with corticoids. Oversecretion of corticoids may result in Cushing’s disease, characterized by high blood pressure, fat deposits in the face and back, and accumulation of tissue fluids. Excessive secretion of androgens may result in early sexual development in males and excessive hair and a deep voice in females.

The Pancreas

The pancreases is an exocrine gland that produces digestive enzymes. However, it also has special cells called the islets of Langerhans that function as an endocrine gland. They secrete insulin and glucagons. Insulin is a hormone that lowers the level of glucose in the blood. It does so by stimulating the uptake of glucose by body cells and the formation of excess glucose into glycogen in the liver and muscles. Glucagon triggers the break-down of glycogen to glucose when the body needs more energy.

In the absence of insulin, glucose cannot enter body cells. As a result, the cells use their own proteins and fat for energy. The level of glucose in the blood then becomes abnormally high. This condition, called diabetes mellitus, is the third major cause of death in the US. Without proper treatment it can lead to heart disease, strike, kidney failure, severe nerve damage, or blindness. Diabetes may also result in infections so severe that limb amputation is necessary.

The two chief forms of diabetes are Type 2, or non-insulin-dependent, diabetes. In Type 1 diabetes, the islets of Langerhans produce too little or no insulin. Some researchers suspect a virus may be involved in Type 1 diabetes. Type 1 usually first appears in people under 20 years of age and can be controlled by strict diet and daily injections of insulin. Approximately 85%of all diabetics suffer from Type 2 diabetes. Type 2 generally first appears in people over 40 years of age. These diabetics may have normal or even high level of insulin, but their bodies cannot use the hormone. The causes of Type 2 diabetes are believed to be a storage of insulin receptors on body cells or a breakdown of the immune system, which causes the body to become insulin-resistant. Heredity also appears to be factor in both types of diabetes. Type 2 diabetes can generally be controlled through diet.

Excessive levels of insulin in the blood lead to hypoglycemia, a condition in which the level of glucose in the blood brain cells need a constant supply of glucose, as victim may lose consciousness due to the lack of glucose. A diet high in protein and low in carbohydrates can help control hypoglycemia.

The Gonads

Gonads, the gamete-producing organs of the reproductive system, also produce and secrete hormones. The female gonads secrete estrogens that influence the development of female secondary sex characteristics. Among these are wider hips, enlarged breasts, and rounded body contours. The male gonads produce androgens that stimulate development of the male secondary sex characteristics. These include sex hormones play roles in reproduction.

The Pituitary

The pituitary gland, located at the base of the brain, is about the size and shape of a kidney bean. It has two major sections, the anterior lobe and the posterior lobe.

The anterior lobe produces at least six hormones. Four are tropic hormones – that is, hormones that affect the secretions of other glands. Two tropic hormones, FSH (follicle stimulating hormone) and LH (luteinizing hormone), act on the gonads. The other two tropic hormones are TSH (thyroid stimulating hormone) and ACTH (adrenocorticotropic hormone). TSH stimulates the thyroid gland to secrete thyroxine, and ACTH affects the adrenal cortex. The anterior lobe also secretes somatotropin, or growth hormone (GH). Stomatotropin has many effects on metabolism. It stimulates bone and muscle growth and helps control the use of glucose and fatty acids for energy. Prolactin, another hormone of the anterior lobe, stimulates the mammary glands to produce milk after the birth of a child.

The posterior lobe of the pituitary does not produce any hormones, but it stores two hormones produced by the hypothalamus. They are antidiuretic hormone (ADH) and oxytocin. ADH, also called vasopressin, keeps the blood volume constant by controlling reabsorption of water in the kidneys. Oxytocin stimulates the contraction of uterine muscles during childbirth and the release of milk from the breasts after childbirth. Prolactin and oxytocin have no known function in males.

Most disorders associated with the pituitary gland involve somatotropin. An excess during childhood results in gigantism, or excessive growth, One victim grew to 2.7 m. An excess during adulthood results in acromegaly, in which the hands, feet and skull increase in size. Too little somatotropin during childhood results in dwarfism, characterized by a short body but otherwise normal proportions and normal mental and sexual development.

The Hypothalamus

The hypothalamus, which is a part of the brain, may be considered the master switchboard of the endocrine system. It links the endocrine system with the nervous system. The nervous system feeds information from the entire body into the hypothalamus. Based on that information, the hypothalamus then sends signals in the form of tropic hormones to stimulate o inhibit hormone secretion by the pituitary gland. At least nine such hormones have been identified. The hormones that stimulate secretion are called releasing hormones. Releasing hormones trigger secretion of TSH, GH, LH, FSH, ACTH, and Prolactin. Hormones that slow down secretion are inhibiting hormones. The hypothalamus secretes inhibitors for GH, TSH, and Prolactin. It also produces ADH and oxytocin and signals their release from the posterior pituitary.

Table 6.1

Glands: hormones and functions

Endocrine System Regulation

The endocrine system and the nervous system together control other body systems. However, the endocrine system also controls itself.

Feedback

The endocrine system controls itself through a process called negative feedback. This process is similar to the way a thermostat regulates a household furnace. When the temperature falls below the thermostat setting, the furnace switches on and begins producing heat. When the temperature reaches the thermostat setting, the furnace switches off. Similarly, the level of a hormone in the blood turns its own production off and on.

Negative feedback controls the thyroxine level in the blood. The hypothalamus plays the role of the thermostat. The hypothalamus has cells that detect the presence of thyroxine in the blood. When the thyroxine level is low, the hypothalamus secretes a releasing hormone that stimulates the pituitary to secrete TSH. TSH causes the thyroid to secrete thyroxine. When the thyroxine level returns to normal, the hypothalamus stops secreting the releasing hormone. As a result of this feedback mechanism, the pituitary stops secreting TSH, and the thyroid slows down secretion of thyroxine.

How Hormones Act

There are two types of hormones: steroids, which are fatlike organic compounds, and protein hormones. Sex hormones and corticoids are steroids. All others are protein hormones.

Steroids and protein hormones produce their effect differently. A steroid passes through the target cell membrane. It combines with a receptor molecule and moves into the cell nucleus. There it helps determine the manufacture of specific proteins. Protein hormones affect their target cells through a two-step procedure called a “two-messenger” system. The first messenger, the hormone, combines with the receptor on the target cell membrane. This combination activates an enzyme on the membrane’s inside wall. The enzyme helps change ATP into cyclic adenosine monophosphate, or cyclic AMP. Cyclic AMP triggers enzymes that bring about changes initiated by the original hormone. Thus, cyclic AMP is called the second messenger.

Date: 2014-12-22; view: 1039