Regulatory Systems

 

Regulatory Systems

To stay alive ,  organisms must respond to external and internal changes (stimulus).Organisms require systems to maintain internal balance (homeostasis ) and the normal function of the bodies.There are two main regulatory systems in the body:

             I.      Nervous system

          II.      Endocrine system

I. The Nervous System

     The environment of an organism is always changing. Some of these changes take place in the external environment—that is, outside the body. Examples of such external changes are a change in temperature; the appearance of food; the appear­ance of a natural enemy. Changes also occur within the or­ganism. For example, the concentration of a waste product may increase; a disease-causing organism may enter the body; the supply of a necessary substance may decrease.

         To stay alive, an organism must respond to these external and internal changes. The organism must maintain homeostasis: it is ability of an organism to keep conditions inside its body the same even though conditions in its external environment change. It must keep all the factors of its internal environment within certain limits.

       In one-celled and some simple multicellular organisms, the regulation and coordination of responses is a function of each cell as a whole, including the special activities of its organelles. This capacity of a cell to respond is often called irritabil­ity. In more complex multicellular animals, the regulation and coordination of responses are controlled by regulatory systems; they are the nervous system and the endocrine system- chemicals.

      The endocrine system is found in both in animals and plants. The nervous system however, is found only in animals. Hormonal coordination in organisms developed before neural coordination. 

Central Nerves

         The nervous system controls all of the activi­ties of the body. It is made up of the brain, spinal cord, and nerves that are found throughout the body.

The nervous system allows you to react to stimuli (singular: stimulus). A stimulus is a change in the environment. At their simplest, these reac­tions are involuntary, or automatic. If an insect or other object zooms toward your eye, you blink with­out thinking. Your body reacts quickly and automat­ically to avoid damage to the eye. Such a response, or action caused by the stimulus, is controlled by your nervous system.

     Although some responses to stimuli are involun­tary, such as blinking your eye, many responses of the nervous system are far more complex.

     For ex­ample, leaving a football game because it begins to rain is a voluntary reaction. It is a conscious choice that involves the feelings of the moment, the mem­ory of what happened the last time you stayed out in the rain, and the ability to reason.

Because the human nervous system controls re­actions that involve emotion, reason, and habit, it often has been compared to a computer. The cir­cuits of this human computer are located through­out the body. This computer controls your emo­tions, your thoughts, and every movement you make. Without it, you could not feel pain, move, or think. You also would not be able to enjoy the taste of food.

Attempts at understanding the human nervous system usually begin by dividing it into two parts. One part of the human nervous system is the central nervous system. It is made up of the brain and the spinal cord. The central nervous system is the control center of the body. All information about what is happening in the outside world or within the body itself is brought here.

The other part of the human nervous system branches out from the central nervous system. It is a network of nerves and sense organs, which makes up the peripheral nervous system. Peripheral means "outer." Included in the peripheral nervous system are all the nerves that connect the central nervous system to other parts of the body. A division of the peripheral nervous sys­tem controls all involuntary body processes, such as heartbeat and peristalsis. This divides in two, somatic nervous system the autonomic nervous system.

Mechanisms of Nervous Regulation

      The functioning of a true nervous system involves three basic types of structures—receptors, nerve cells or neurons, and effectors.

      Receptors, or sense organs, are specialized structures that are sensitive to certain changes, physical forces, or chemicals in the internal and external environments. Stimulation of a re­ceptor causes "messages," or impulses, to be transmitted over a pathway of nerve cells. These impulses eventually reach an effector, which is either a gland or a muscle. If the effector is a gland, it will respond to the impulse by either decreasing or increasing its activity, depending on the nerve pathways in­volved. However, if the effector is a muscle, a nerve impulse can only cause it to contract.

       Any factor that causes a receptor to trigger, or initiate, im­pulses in a nerve pathway is called a stimulus. The stimulus causes electrical and chemical changes in the receptor, and these, in turn, initiate the nerve impulses. Thus, the basic sequence of events in regulation by the nervous system involves

(1)a stimulus that activates a receptor,

(2)the starting of impulses in associated nerve pathways,

(3)a response by an effector.

     Multicellular animals possess several different types of re­ceptors, each sensitive to a different type of stimulus. Among the sense organs found in animals are those sensitive to heat, cold, light, sound, pressure, and chemicals.

Structure of Neurons

   In the nervous systems of all multicellular animals, the basic unit of structure and function is the nerve cell, or neuron. Neurons are specialized for the rapid conduction of impulses, which are both electrical and chemical (electrochemical) in nature. The capacity to conduct impulses is a property of the nerve-cell membrane. The changes associated with the impulses do not enter or pass through the cytoplasm of the cell; they are transmitted only along the cell membrane.

    A nerve cell usually consists of three basic parts—a cell body, dendrites, and an axon. The cell body contains the nucleus and the cell organelles. The metabolic activities common to all cells are carried out in the cell body, which also controls the growth of the nerve cell. Materials necessary for the maintenance of the nerve cell are generally synthesized in the cell body and then moved to other parts of the cell where they are needed.

         The dendrites are short, highly branched fibers that are specialized for receiving impulses. Dendrites gener­ally conduct impulses toward the cell body. In some neurons, the branching of the dendrites around the cell body gives the cell a bushy appearance.

The axon is usually a long, thin fiber that extends from the cell body. Axons carry impulses away from the cell body and transmit them either to other neurons or to effectors. Axons range in length from a fraction of a centimeter to more than a meter. Nerve fibers may be made up of either the axons or dendrites of neurons.

    All axons are surrounded by cells called Schwann cells. On some axons, the Schwann cells produce layers of a white fatty substance called myelin. The myelin terms a sheath around the axon, and axons having such a sheath are said to be myelinated. The myelin insulates the axon.

The nerve cells of mature animals cannot divide, so there is no periodic replacement of neurons as there is with other cells in the body. However, if the cell body is unhurt, axons and dendrites outside the brain and spinal cord can regenerate, or grow back, if they are damaged.

Types of Neurons and Nerves    

Neurons are generally grouped according to their function.

Sensory brain. Motor neurons carry impulses neurons carry impulses from receptors toward the spinal cord and from the brain and spinal cord toward effectors, usu­ally muscles. Interneuron relay impulses from one neuron to another in the brain and spinal cord.

Nerves are bundles of axons or dendrites that are bound together by connective tissue. Nerves are called Sensory nerves if they conduct impulses from receptors toward the spinal cord and brain; motor nerves if they conduct impulses from the brain and spinal cord toward effectors; and mixed nerves if they are composed of both sensory and motor fibers.

Nerve Impulse

    An impulse travels along a nerve by a complex combination of chemicals and electric­ity. A nerve impulse is not a flow of electricity. Nerve impulses move much more slowly than electricity. They move at about 100 meters per second. Electricity moves at about 300,000 kilometers  a sec­ond. Also, an impulse uses oxygen and gives off carbon dioxide as it travels along a nerve. This indicates that some chemical reaction is involved in its movement. This is why it is said that a nerve impulse is an electrochemical charge moving along a neuron.

     Neuron at Rest: The transmission of a nerve impulse is made possible by a difference in electrical charge between the outer and inner surfaces of the nerve-cell membrane. When the neuron is resting (not transmitting an impulse), The outside of the cell membrane has an excess of sodium ions (Na⁺) and the membrane becomes charged positively. The inside of the cell membrane con­tains a high concentration of potassium ions (K⁺), chloride ions (Cl^-) and negative organic ions. This results in a neg­ative charge within the nerve fiber. This resting neuron is said to be polarized.

       All this changes when an impulse moves along the neuron. The polarity reverses. The sodium ions (Na⁺) rush inside the fiber until they are in excess, while the potassium ions (K⁺) move outside the membrane. This change in polarity sweeps along the neuron like a wave, carrying the impulse. After the impulse passes a given spot, the orig­inal polarity returns. The balance of sodium and potassium ions along the cell membrane affects this polarity change.

The Synapse

The axon of a neuron usually has no branches along its length, but it may have many branches at its end. Each of these terminal branches makes contact with another cell. The junction between the terminal branch of a neuron and the membrane of another cell is called a synapse. The synapse includes a microscopic gap between the end of the terminal branch and the adjoining cell. Impulses are transmit­ted from the axon to the adjoining cell across this gap.

Transmission at the Synapse

       The axon ends in a synaptic knob. The cell membrane at the knob is called the presynaptic membrane. The cell membrane of the adjacent cell is called the postsynaptic membrane. Between the pre- and postsynaptic membranes is a very narrow space called the synoptic cleft. When an im­pulse arrives at the synaptic knob, it must be transmitted from the presynaptic membrane, across the synaptic cleft, to the postsynaptic membrane of the adjoining cell.

The transmission of the impulse across the synaptic cleft is a chemical process. Within the synaptic knob are many small sacs called synoptic vesicles. The vesicles contain substances called neurotransmitters. Among the most common of these chemical transmit­ters are acetylcholine and norepinephrine. When an impulse reaches the synaptic knob, some of the synaptic vesicles fuse with the membrane of the synaptic knob and release their contents into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and initiates impulses in the adjacent nerve cell by changing the permeability of its membrane. There are spe­cial receptor proteins embedded in the membrane of the dendrites, and it is at these receptors that the neurotransmitters produce their effects.

        Because neurotransmitters are released only by the ends of axons and because they exert their effects only at specialized receptor sites, impulses can travel in only one direction across synapses—from axons to dendrites or cell bodies. Thus, synapses control the direction of flow of information over nerve pathways.

       Different types of neurons release different neurotransmit­ters. Some neurons release excitatory neurotransmitters. These chemicals initiate impulses in adjacent neurons. Acetylcholine, norepinephrine, and the amino acids histamine and glutamic acid are excitatory neurotransmitters. Still other neurons release neurotransmitters that do not ini­tiate impulses in adjacent neurons. Instead, they have the op­posite effect—they inhibit the firing of impulses. Inhibitory neurotransmitters include serotonin, epinephrine, and the amino acid glycine. Thus, while some synapses transmit im­pulses from one neuron to the next, other synapses can block the transmission of impulses.

Neuromuscular Junctions

The passage of impulses from motor neurons to muscles occurs at special points of contact called neuromuscular junctions. The axons of motor neurons end in structures called motor end plates. Like synaptic knobs, motor end plates contain synaptic vesicles. When impulses reach the motor end plates, they cause the release of the chemical transmitter acetylcholine. The acetylcholine diffuses across the gap between the end of the axon and the muscle cell and combines with receptor molecules on the muscle cell membrane. The effect of the acetylcholine is to increase the permeability of the muscle cell membrane to sodium, causing impulses to travel along the muscle cell membrane. These impulses cause the muscle cell to contract. As in the synapses between neurons, the acetyl­choline at the neuromuscular junction is quickly destroyed by enzyme action.

Drugs and the Synapse

Many poisons and drugs affect the activity of chemical transmitters at synapses. Nerve gas, curare, botulin toxin (a bacterial poison), and some insecti­cides are poisons that interfere with the functioning of acetyl­choline at neuromuscular junctions and cause muscle paralysis. If the muscles of the respiratory system become paralyzed, death follows.

Drugs that affect the mind and the emotions or that alter the activity of body systems also act on synapses.

Stimulants are drugs that produce a feeling of well-being, alertness, and excitement. Among the stimulants, amphetamines ("uppers") produce their effects by binding to certain receptors, thereby mimicking norepinephrine. Caffeine, which is found in coffee, tea, and cola drinks, aids synaptic transmission.

Depressants are drugs that slow down body activities. Bar­biturates ("downers") produce a depres­sant effect by blocking the formation of norepinephrine.

Some of the mind-altering or hallucinatory drugs, such as LSD ("acid") and mescaline, interfere with the effect of the inhibitory transmitter serotonin

THE CENTRAL NERVOUS SYSTEM

The human nervous system, like that of other vertebrates, can be divided into two main subdivisions. One of these is the central nervous system, which consists of the brain and the spinal cord. The other is the peripheral nervous system, which is a vast network of nerves that conduct impulses between the central nervous system and the receptors and effectors of the body. This system consists of sensory neurons, including their cell bodies, and the axons of motor neurons.

       Most of the activities of the body are controlled by the cen­tral nervous system—the brain and the spinal cord. Impulses from sense receptors throughout the body bring a constant flow of information about the internal state of tissues and or­gans and about the external environment. In the brain and the spinal cord the information is interpreted, and impulses are sent out to muscles and glands, causing appropriate responses.

The Skull and Spinal Column

The brain and the spinal cord are protected by bone. The brain is enclosed by the skull, while the spinal cord is surrounded by the vertebrae of the spinal col­umn, or backbone. The brain and the spinal cord are also covered and protected by three tough membranes known as the meninges. A liquid, the cerebrospinal fluid, protect the delicate nervous tissues against shock. Within the brain are four spaces, or ven­tricles, that are filled with cerebrospinal fluid. These spaces connect with a space between the meninges and with the central canal of the spinal cord, which are also filled with fluid.

The brain

     The brain is one of the most active organs in the body. It receives 20 percent of the blood pumped from the heart, it replaces most of its protein every three weeks, and it is the major user of glucose in the body. Unlike the cells of other tissues the cells of the brain generally metabolize only glucose for the release of energy. The major parts of the brain are the cerebrum, cerebellum, and medulla. Other parts of the brain are the thalamus, hypothalamus, and pons. The thalamus serves as a relay center between vari­ous parts of the brain and the spinal cord; it also receives and modifies all sensory impulses except those involved in smell before they travel to the cerebral cortex; and it may be in­volved in pain perception and maintenance of consciousness. The hypothalamus is involved in control of body temperature, blood pressure, sleep, and emotions; it is also involved in the functioning of the endocrine system. The pons  serves as a relay system linking the spinal cord, medulla, cerebellum, and cerebrum

The cerebrum.

 The cerebrum is the largest part of the human brain, making up about two-thirds of the entire organ. The cere­brum is divided in half from front to back by a deep groove, or fissure, which separates it into the right and left cerebral hemispheres. Nerve fibers from each hemis­phere pass to the other hemisphere and to other parts of the nervous system.

     The outermost layer of the cerebrum is the cerebral cortex, or gray matter, which is made up of the cell bodies of motor neurons and a huge number of interneurons, interconnected by unmyelinated fibers. The outer surface of the cortex is highly folded. These greatly increase the surface area of the gray matter.

     The cerebral cortex performs three major types of functions—sensory, motor, and associative functions. Each part of the cortex is specialized to carry out a particular func­tion.

     The sensory areas of the cortex receive and interpret im­pulses from the sense receptors, including the eyes, ears, taste buds, and nose, as well as the touch, pain, pressure, heat, and cold receptors in the skin and other organs.

     The motor areas of the cortex initiate impulses that are responsible for all volun­tary movement and for the position of the movable parts of the body. Impulses from the motor cortex may be modified by other parts of the brain.

     The associative areas of the brain are responsible for memory, learning, and thought.

          Recent research indicates that the two cerebral hemi­spheres do not perform identical functions. Instead, some functions are performed by the left hemisphere and others by the right hemisphere.

     Beneath the gray matter of the cerebrum is an inner area called the white matter. This area consists of myelinated nerve fibers. One of the bundles, or tracts, of fibers in the white matter connects the right and left hemispheres, so that there is an exchange of information between the two

halves of the cerebrum. Other tracts from the white matter connect the cortex with other parts of the nervous system.

Nerve fibers leaving the cerebral hemispheres pass down through the brain and spinal cord. At some point along their pathway, these fibers cross over to the opposite side of the brain or spinal cord and then continue to various parts of the body. Thus the left cerebral hemisphere controls the right side of the body, and the right hemisphere controls the left side of the body. Therefore, an injury to one side of the cere­brum will affect the opposite side of the body.

The cerebellum.

   The cerebellum is located below the rear part of the cerebrum. The cerebellum, like the cerebral cortex, is divided into two hemispheres. The highly folded outer layer of the cerebellum consists of gray matter, while the inner portion is white matter.

   The cerebellum coordinates and controls all voluntary movements and some involuntary movements. The cere­bral cortex correct and coordinate the movement of the muscles. The cerebral cortex and the cerebellum work together to produce smooth and orderly voluntary movement. With certain involuntary movements, the cere­bellum functions in cooperation with other parts of the brain. The cerebellum, using information from receptors in the inner ear, maintains balance, or equi­librium. It is also involved in the maintenance of muscle tone (keeping the muscles slightly tensed). Damage to the cerebel­lum results in jerky movements, tremor, or loss of equilibrium. Staggering and other signs of coordination loss seen with alcohol intoxication reflect a temporary loss of cerebellar function.

The medulla.

     Beneath the cerebellum and continuous with the spinal cord is the medulla. In this lowest part of the brain, the white matter makes up the outer layer, while the gray matter is the inner layer. The medulla consists mainly of nerve fillers connecting the spinal cord to the various other parts of the brain. Nerve cen­ters in the medulla control many involuntary activities, in­cluding breathing, heartbeat, blood pressure, and coughing.

The Spinal Cord

The spinal cord, which is about 45 centimeters long, ex­tends from the base of the brain down through the vertebrae of the spinal column. A cross section of the spinal cord shows an inner H-shaped region of gray matter surrounded by an outer layer of white matter. The gray matter con­tains many interneurons, as well as the cell bodies of motor neurons. The white matter contains myelinated fibers that carry impulses between all parts of the body and the spinal cord and brain. In the center of the cord is the spinal canal, which is filled with cerebrospinal fluid.

The spinal cord performs two main functions. First, it con­nects the nerves of the peripheral nervous system with the brain. Impulses reaching the spinal cord from sensory neurons travel up the cord through interneurons to the brain. Impulses from the brain are transmitted down the spinal cord by inter­neurons to motor neurons. These impulses travel through peripheral nerves to muscles and glands. Second, the spinal cord controls certain reflexes, which are automatic responses not involving the brain.

Reflexes

A reflex is an involuntary, automatic response to a given stimulus. It involves a relatively simple pathway between a receptor, the spinal cord or brain- and an effector. Many normal body functions are controlled by reflexes. These include blink­ing, sneezing, coughing, breathing movements, heartbeat, and peristalsis. The knee-jerk reflex and the reflex constriction and dilation of the pupil of the eye in response to light are used by doctors to cheek the condition of the nervous system. The absence of a reflex response, or excessive slowness in a reflex response, may indicate a nervous system disorder.

Reflex arcs:

The pathway over which the nerve impulses travel in a reflex is called a reflex arc. The simplest reflex arcs involve only two neurons—one sensory and one motor. The pathway of the knee-jerk reflex is of this type. Most reflexes, however, involve three or more neurons. Withdrawal reflexes, for example, involve a three-neuron reflex arc.

  When your hand touches a hot stove, it is pulled back before you feel the sensation of heat or pain. This removal of your hand is accomplished by a withdrawal reflex. The parts of this reflex arc are as follows;

1.  A receptor in the skin is stimulated by the heat.

2.  The receptor initiates impulses in a sensory neuron, which carries impulses to the spinal cord.

3.  Within the spinal cord, the sensory neuron synapses with an interneuron, which synapses with a motor neuron. Impulses are also carried to the brain, but this is not part of the reflex arc.

4.  The motor neuron transmits impulses to the effector. In this example impulses are carried to certain muscles of the arm.

5.  The muscles receiving impulses from the motor neuron contract, moving the hand and arm.

The withdrawal reflex is accomplished without the in­volvement of the brain. However, shortly after the hand is withdrawn from the hot object, there may be sensations of heat and pain. These result from impulses passing up the spinal cord to the brain.

The Sense Organs

 You know what is going on inside your body and around you because of special sense receptors. Many of these receptors are found in sense organs. Sense organs are structures that carry messages about your surroundings to the central nervous system. Sense organs respond to light, sound, heat, pressure, and chemicals and detect changes in the position of your body. The eyes, ears, nose, mouth, and skin are examples of sense organs.

Most sense organs respond to stimuli from your body's external environment. Other kinds of sense organs keep track of the environment inside your body. Without your being aware of it, these sense organs send messages to the central nervous system about body temperature, carbon dioxide and oxygen levels in your blood, and the amount of light enter­ing your eyes.

 

1). Eyes and Seeing

      Seeing is not possible with your eyes alone. Peo­ple whose vision center in the brain is damaged can­not see. Your eyes are designed to focus light rays to produce images of objects. But your eyes are use­less without a brain to interpret these images.

Your eyes are made of three layers of tissue. The  outer protective layer is called the sclera. This is the "white" of your eyes. At the center front of the eyeball, the sclera is transparent. The trans­parent tissue forms a protective shield called the cornea. Beneath the sclera is the choroid, in middle layer of the eye. It contains nourishing blood vessels. The choroid layer also includes the circular, colored portion of the eye called the iris. When people say someone's eyes are blue, brown, or hazel, they are actually describing the color of the Iris.

At the center of the iris is a circular opening called the pupil. The size of this opening is controlled by muscles in the iris. They relax or contract to make the pupil larger or smaller.

Watch your pupils change size by looking at them in a mirror as you vary the amount of light in the room. Your pupils open in dim light and close as the light gets brighter. Pupils narrow in bright light to prevent light damage to the inside of the eye. They widen in dim light to let more light in.

       As the light passes through the pupil, it travels  through the aqueous humor. The aqueous humor is a watery fluid that is found be­tween the cornea and lens. The lens focuses the light rays coming into the eye. A human eye lens is different from a camera lens in that the human eye lens adjusts its focus by actually changing shape. The focus of a camera lens is adjusted by moving the lens forward or backward.                 

       A human lens focuses light on the back surface of the eyeball, an area known as the retina. The retina is the eye's third layer of tissue. It contains light-sensitive cells called rods and cones. There are three different types of cones in the retina—one type is sensitive to red light, one to green light, and one to blue light. The retina contains about 125 million rods and 6.5 million cones. Rods react to dim light, while cones react to colors and to bright light. Both pro­duce nerve impulses that travel along the optic nerve. The point where the optic nerve leaves the eye contains no rods or cones and is called the blind spot. The optic nerve carries these impulses to the vision center of the brain. Between the lens and ret­ina is a large compartment that contains a fluid called the vitreous humor. This fluid gives the eyeball its roundish shape.             

      Vision, of course, does not end at the retina. The nerve impulses passing through the optic nerve from the retina have to be interpreted by the brain. Because of the way the lens bends light rays, the brain receives images from the retina upside down and must automatically turn them right side up. The brain must also combine the two slightly differ­ent images provided by each eye into one three-dimensional image.

    A severe deficiency of vitamin A leads to a condition called night blindness, which is an inability to see in dim light. In this condition the amount of retinal in both the rods and cones is decreased, and both become less sensitive to light. Retinal is synthesized from vitamin A Retinal combines with proteins within the rods and cones. Thus, vision in dim light is greatly affected. However, there is enough pigment left for vision in bright light. Color blindness, which is an inability to see certain colors, is a hereditary con­dition in which the proteins of one or more of the three types of cones do not function properly.

EYE DEFECTS

1. Myopia

     While at rest, instead of focusing on the retina, the light rays focus in front of it. This type of eye defect is termed myopia, or short sight. The major cause of this defect is the difference in diameter between the anterior and posterior portion of the eye. In such cases, the posterior portion is wider than the anterior. This condition can be corrected by wearing glasses or contact lenses with concave lenses.  This defect also can be corrected by the latest techniques in laser surgery.

2. Hypermetropia

       At rest, the light rays focus behind, instead of on the retina. This type of eye defect is termed hypermetropia, or long sight. The major prominent cause of this defect is again the difference in the diameter between the anterior and posterior portion of the eye, the anterior por­tion being wider than that of the posterior. The condition can be cor­rected by wearing glasses or contact lenses with convex lenses.

3. Astigmatism

    This describes the condition where the image is constantly unclear due to non-uniformity of the cornea. This defect can be corrected by wearing cylindrical edged glasses.

4. Prestism

This describes the condition of the lens losing its elasticity due to age. After middle age, the ability of the eye to focus clearly on near objects is reduced. A young man for example, can clearly see an object 10-15 cm in front of him. An old man can only clearly see an object which is more than 80 cm away from his eyes. This problem can be cor­rected by wearing concave glasses-

2). The Ear

     The human ear has two sensory functions. One of course, is hearing. The other is maintaining balance, or equilibrium.

Structure of the ear

The three parts of the ear are the outer ear, the middle ear, and the inner ear. The outer ear is the visible part of the ear. It consists of the Pinna, a flap of skin supported by                        cartilage, and a short ear canal. Stretched across the inner end of the ear canal is the delicate membrane, eardrum.

Oval window

Eustachian tube

   The middle ear is an air-filled chamber that begins at the eardrum. It contains three tiny bones—the hammer, anvil, and stirrup. These bones form a chain across the middle ear link­ing the eardrum to another membrane, the oval window. The hammer is attached to the eardrum, the anvil connects the hammer to the stirrup, and the stirrup is connected to the oval window. Extending between the middle ear and the throat is the Eustachian tube. Its function is to equalize the pressure in the middle ear with that of the atmosphere outside.

    The inner ear consists of the cochlea and the semicircular canals. The cochlea is the organ of hearing. It consists of coiled, liquid-filled tubes that are separated from one another by membranes. Lining one of the membranes are specialized hair cells that are sensitive to vibration.

     The semicircular canals enable the body to maintain bal­ance. They consist of three interconnected loop-shaped tubes at right angles to one another. These canals contain fluid and hail-like projections that detect changes in body position.

Hearing.

     Sound waves are vibrations in air or some other medium, such as water. Hearing takes place when these vibra­tions are transmitted to the inner ear, where they initiate im­pulses that are carried to the brain by the auditory nerve.

     Sound waves collected by the outer ear pass down the ear canal to the eardrum. They cause the eardrum to vi­brate, and the vibrations are transmitted across the middle ear by the hammer, anvil, and stirrup. Vibrations of the stirrup cause vibrations in the oval window, which in turn cause the fluid within the cochlea to vibrate. The movement of the fluid causes vibrations in specialized hair cells lining one of the membranes within the cochlea. This initiates impulses in nerve endings around the hair cells. These impulses are car­ried to the cerebral cortex, where their meaning is interpreted.

Balance.

      Balance, or equilibrium, is a function both of the inner ear and the cerebellum. In the inner ear, the fluid-filled semicircular canals lie at right angles to one another in the three different planes of the body. As the head changes posi­tion, the fluid in the canals also changes position, which causes movement of hair-like projections. This in turn stimulates nerve endings, which initiate impulses that travel through a branch of the auditory nerve to the cerebellum. The cerebel­lum interprets the direction of movement, and sends impulses to the cerebrum. Impulses initiated by the cerebrum correct the position of the body.    

If you spin around for a time, the fluid in the semicircular canals also moves. When you stop suddenly, you feel as though you are still moving and are dizzy because the fluid in the canals continues to move and stimulate the nerve endings. In some people, the rhythmic motions of a ship, plane, or car may over stimulate the semicircular canals, resulting in motion sickness.

3)Tongue and Tasting

Everyone has a favorite food. Maybe yours is ice cream or spaghetti or a peanut-butter sandwich. Taste buds on your tongue enable you to taste foods like these. Each taste bud contains several taste receptors. When chewed food mixes with saliva, the liquid produced enters a taste bud. The receptors send impulses along sensory neurons to the cere­brum. The cerebrum interprets the impulses, and you taste the food.

There are four types of taste receptors; each senses a dif­ferent taste. Look at Figure. Taste receptors for sweet, sour, salty, and bitter are located on different parts of your tongue. Most foods are combinations of the four basic tastes.

       

4). Nose and Smelling

          The receptors for smell, the olfactory cells, are located in the mucous membrane lining the upper nasal cavity. Odor is detected when molecules of a gaseous sub­stance enter the nose, dissolve in the mucus, and stimulate the olfactory receptors. The olfactory cells are specialized nerve cells. When they are stimulated, impulses are carried by the olfactory nerves to the brain, where they are interpreted.

      Unlike taste buds, which respond to only four basic tastes, olfactory cells appear to respond to more than fifty different basic odors. Like the taste buds, each olfactory cell appears to be more sensitive to one basic odor than to all the others. Continuous exposure to a specific odor quickly leads to an inability to detect that odor, but does not interfere with the detection of other odors. This is called adaptation, and is thought to be partly a response of the central nervous system.

Both taste and smell result from the chemical stimulation of receptors. However, olfactory receptors are much more sensi­tive than the cells of the taste buds. They are stimulated by much lower concentrations of chemicals and they are sensi­tive to a much greater variety of chemicals.

       

5)Skin and Sensing

Remember the last time you cut your finger or skinned your knee? The pain you felt was caused by pain receptors in your skin. In addition to pain receptors, your skin contains receptors for heat, cold, touch, and pressure. Figure shows the difference between the five kinds of receptors. Whenever a receptor is stimulated, it sends impulses along a sensory neuron to the cerebrum. The cerebrum interprets the impulses, and you feel one of the five sensations.

Skin receptors are spread unevenly throughout your body. For example, your fingertips and lips contain many touch receptors, while your forehead contains many pain receptors.

Disorders of the Sense Organs

You have probably heard about certain disorders that affect the senses. Table 17-2 lists some well-known disorders. What is the difference between near- and farsightedness?

Table:  Disorders of the Sense Otyans''^-.^;'^-4.'1^-'^''-•• '^^T^-^ ' ^ff^' '^f '"^
Disorder	Description
Nearsightedness	Inability to focus on distant objects due to an long eyeball abnormally
Farsightedness	Inability to focus on closeup objects due to an short eyeball abnormally
Astigmatism	Blurred vision caused by an irregular cornea or lens
Earache	Pain, ringing, discharge, or temporary hearing infection of the outer or middle ear loss caused by
Hearing Impairment	Loss or lack of hearing often caused by disease the ear 3 or injury to
Motion Sickness	Nausea and dizziness due to movement of the semicircular canals liquid in the

Chemical Regulation

Glands and Hormones

        The systems of the body are never at rest. They are continually making adjustments to changing conditions both outside and inside the body in order to maintain homeostasis. We have already seen how the nervous system takes part in this process. The body has another system, called the endocrine system, that also helps to regulate and coordinate its functions.

        The nervous system operates by means of electrical im­pulses in nerve fibers and neurotransmitters that cross the tiny gaps that separate adjacent neurons. This system acts quickly and directs its messages to specific parts of the body. The endocrine system, on the other hand, operates by means of chemicals released into the bloodstream, which then carries them to all tissues of the body. It takes time for these substances to reach their target organs and produce an effect. The endo­crine system is therefore slower in its action than the nervous system. Its effects also tend to last longer. Generally speaking, the nervous system enables the body to make rapid responses of short duration. The endocrine system produces effects that last for hours, days, or even years. However, the endocrine and nervous systems work together. When you run from danger, for example, nerves control your muscle activity, while the endo­crine system controls the blood sugar level and respiration rate.

Glands       

Glands are organs made up of epithelial cells specialized for secretion of substances needed by the organism. Some glands, such as the digestive glands, discharge their secretions into ducts, which carry the secretions to where they are used. Such glands are called exocrine glands. Other glands release their secretions directly into the bloodstream. These glands are called endocrine glands, and they make up the endocrine system. Endocrine glands are also called ductless glands, or glands of internal secretion. The secretions of the endocrine glands are called hormones. These endocrine glands are the hypothalamus, pituitary, thyroid, parathyroid, pancreas, adrenal, and reproductive glands, testes and ovary.

Hormones

      Hormones are released into the bloodstream by cells in one part of the body, but they exert their effect somewhere else in the body. Because of this, hormones are sometimes called "chemical messengers."

Hormones are usually present in the bloodstream in very low concentrations. Each type of hormone is recognized only by specific tissues. The tissues regulated by a given hormone are called the target tissues of that hormone. The hormone may stimulate the target tissue and increase its activities, or it may inhibit the target tissue and decrease its activities.

      Hormones affect the functioning of target tissues by chang­ing the rates of certain biochemical reactions in those tissues. A hormone may cause a reaction to start, to speed up, to slow down, or to stop. However, hormones do not produce their effects by acting directly on the reacting substances, as en­zymes do. They appear to act always through some inter­mediate cellular process. The processes in the body that are chiefly regulated by hormones include;

(1)   overall metabolism

(2)   maintenance of homeostasis

(3)   growth

(4)   reproduction.

       In terms of their chemical makeup, most hormones fall into two classes. Protein-type hormones consist of chains of amino acids or related compounds. Insulin, oxytocin, and ACTH are examples of this type of hormone. Steroid hor­mones are lipid-like, carbon-ring compounds that are chemically similar to cholesterol and bile. Cortisone, testosterone, and estrogen are examples of steroid hormones.

II. The Human Endocrine System

Functioning of Endocrine Glands

        The human endocrine system consists of a number of en­docrine glands that regulate a wide range of activities. In addi­tion, there are a few tissues that are not organized as separate glands, but which do secrete hormones. For example, certain cells in the lining of the stomach and small intestine function in this way. The improper functioning of an endocrine gland may result in a disease or disorder of the body. An excess, or hypersecretion, of a hormone may cause one type of disorder, while a deficiency, or hyposecretion, of a hormone may cause another disorder. In the following sections we will describe the struc­ture and function of the human endocrine glands.

Hypothalamus

The hypothalamus, a small part of the brain located close to the pituitary gland, is known to secrete at least nine hormones from its nerve endings to the pituitary. Among these hormones is one (TRH, thyrotropin-releasing hormone) that causes the pituitary to release a hor­mone (TSH, thyrotropin-stimulating hormone) that stim­ulates the production of thyroxin by the thyroid gland. Another hormone controls the release of gonad-stimulating hormones by the pituitary. A third hormone inhibits the secretion of prolactin from the pituitary. Another hormone, oxytocin, is produced in the hypothalamus and stored in the posterior pituitary until its release to act on the uterus during childbirth. As a part of the brain, the hypothalamus serves as a major link between the ner­vous system and the endocrine system.

     Hypothalamus hormones

-          Growth hormone releasing hormone GRH - (GH-RH)

-          Adrenocorticotropic hormone releasing hormone CRH - (ACTH-RH)

-          Thyroid releasing hormone          TRH - (TSH-RH)

-          Gonadotrophic releasing hormone    GnRH. (LH-FSH-LTH-RH)

-          Oxytocin

The activities of the hypothalamus are closely relat­ed to the activities of the pituitary gland and of the other endocrine glands that are, in turn, stimulated by it. For example, if the concentration of thyroxin rises in the blood, the hypothalamus reduces the supply of its TRH hormone to the pituitary; this inhibits the produc­tion of thyroid-stimulating hormone (TSH) by the pitu­itary; the result is a decrease in the secretion of thyrox­in by the thyroid gland. This type of regulation is referred to as a negative feedback mechanism. It helps maintain homeostasis throughout the body. In the case of the thyroxin feedback system, the rate of metabolism is kept at a constant level. A similar nega­tive feedback mechanism affects the production rate of the other hormones,

Pituitary Gland

The pituitary is a small gland about 1 centimeter in diameter. It consists of an anterior lobe, or front, and a posterior lobe, or back. Between these two lobes there is a very small intermediate zone that is not functional in humans, but which is larger and functional in other animals. The pituitary is often called the "master gland" of the body because it controls the activity of a number of other endocrine glands.

Anterior pituitary. The anterior lobe of the pituitary secretes several different hormones, many of them very important in controlling metabolic functions. The release of hormones from the anterior pituitary is controlled by hormones produced by the hypothalamus. The hormones from the hypothalamus are called releasing hormones, or releasing factors.

It is thought that each different hormone of the anterior pituitary is produced by a different type of cell. The major hormones of the anterior pituitary and their functions are as follows;

1.         Thyroid-stimulating hormone, or TSH, stimulates the production and release of thyroid hormone by the thyroid gland.

2.         Adrenocorticotropic hormone, or ACTH, stimulates the production and release of hormones from the cortex layer of the adrenal glands. ACTH is used in the treatment of arthritis, asthma, and allergies.

3.         Growth hormone, or GH, controls growth of the body. It affects the growth of bone and cartilage. This is accomplished indirectly by its control of the production of another factor that acts directly on these tissues. Growth hormone directly affects protein, carbohydrate, and fat metabolism at a cellular level. In rare cases, too much or too little of it may be produced in children. If there is an excess of it, a child continues to grow to extraordinary height (Gigantism); if there is too little, the child remains small (Dwarfism). If an adult begins to produce too much growth hor­mone, the extremities of the body (hands, feet, face) enlarge to produce a condition known as acromegaly; this may be treated with X rays.

4.          Follicle-stimulating hormone, or FSH, stimulates the de­velopment of egg cells in the ovaries in females. In males, it controls the production of sperm cells in the testes.

5.          Luteinizing hormone, or LH, causes the release of egg cells from the ovaries in females, and it controls the production of sex hormones in both males and females.

6.         Prolactin stimulates the secretion of milk by the mammary glands of the female after she gives birth. Otherwise, it is secreted only in very small amounts. It is thought that the production of prolactin is normally inhibited by a factor secreted by the hypothalamus. Following childbirth, the secretion of this inhibitory factor is blocked, and prolactin is produced.

Posterior pituitary. The posterior lobe of the pituitary is di­rectly connected to the hypothalamus. Two tracts of nerve fibers originating in the hypothalamus have their endings in the posterior pituitary. Two hormones, oxytocin and vasopressin, are produced by these nerve cells in the hypothalamus. The hormones then pass down the axons to the posterior lobe of the pituitary for storage and eventual release.

1.          Oxytocin stimulates contraction of the smooth muscles of the uterus during childbirth.

2.          Vasopressin, which is also known as Antidiuretic hormone or ADH, controls the reabsorption of water by the nephrons of the kid­neys. ADH increases the permeability of the tubules to water, so that water is reabsorbed by osmosis.

Pancreas—Islets of Langerhans

    The pancreas is both an exocrine gland and an endocrine gland. The exocrine portion secretes digestive juices into the pancreatic duct. The endocrine portion consists of small clusters, or islands, of hormone-secreting cells—the islets of Langerhans. These are dispersed throughout the pancreas. There are two types of cells in the islets—alpha (a) cells, which secrete the hormone glucagon, and beta (b) cells, which secrete the hormone insulin. Both of these hormones function in the control of carbohydrate metab­olism.

          

Insulin.

     Insulin affects glucose metabolism in several ways. It increases the rate of transport of glucose through cell membranes in most of the tissues of the body. When the level of glucose in the blood is high, the beta cells of the pancreas are stimulated to secrete insulin. The insulin promotes the passage of the glucose into the body cells, thereby lowering the blood glucose level. Within the cells of the liver and skeletal muscle, insulin promotes the conversion of glucose to glycogen, and in fatty tissues, it promotes the conversion of glucose to fat. It also increases the rate of oxida­tion of glucose within cells.

Glucagon.

     The effects of glucagon on glu­cose metabolism are generally opposite, or antagonistic, to those of insulin. While insulin lowers the blood glucose level, glucagon raises it. When the glucose concentration in the blood falls below a certain level, the alpha cells of the pancreas are stimulated to secrete glucagon. Glucagon promotes the conver­sion of glycogen to glucose in the liver. This glucose quickly diffuses out of the liver into the bloodstream.

       When the supply of liver glycogen is exhausted, glucagon pauses the conversion of amino acids and fatty acids to glu­cose. Thus, when adequate carbohydrates are not available, body fat and proteins are broken down to provide glucose to meet energy requirements.,

Diabetes]

        When the islets of Langerhans fail to produce enough insulin, the amount of glucose that can enter the body cells is greatly decreased. Instead, the concentration of glu­cose in the blood increases, and the excess sugar is excreted in the urine. This condition is called diabetes. Symptoms of diabetes include loss of weight despite in­creased appetite, thirst, and general weakness. If untreated, diabetes causes death. Proper diet and daily injections of insu­lin can control the disease.

Reproductive Organs: Gonads

      The gonads, or sex glands, are the ovaries of the female and the testes of the male. The ovaries produce egg cells and the testes produce sperm cells. The gonads also secrete sex hor­mones, which control all aspects of sexual development and reproduction.

      The ovaries. The ovaries produce two hor­mones, estrogen and progesterone. During development, estrogen stimulates the development of the female reproductive system. Estrogen also promotes the development of the female secondary sex characteristics, such as broadening of the hips and develop­ment of breasts. Estrogen acts with progesterone to regulate the menstrual cycle.

      The testes. The testes secrete male sex hormones called androgens. The most important androgen is testosterone. During fetal development, testosterone stimulates development of the male reproductive system. This hormone also promotes development of male secondary sex characteristics, such as a deep voice, beard, body hair, and the male body form.

Stomach and Small Intestine

Special cells in the lining of the stomach secrete the hormone gastrin, which stimulates the flow of gastric juice. In the lining of the small intestine there are cells that secrete the hormone secretin, which stimulates the flow of pancreatic juice. Secretin was the first hormone to be discovered.

Thymus

      The thymus is a gland located in the upper chest cavity near the heart. It is large in infants and children, but shrinks after the start of adolescence. Early in life, the thymus is involved in the processing of lymphocytes, which are part of the body's defense against infection. Current research indicates that through childhood the thymus produces a hormone called thymosin. Thymosin is thought to stimulate development of T lymphocytes, which are impor­tant in immunity. The thymus appears to serve no function in adults.