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What is the respiratory system? Yourrespiratory system is made up of the organs
in your body that help you to breathe. Remember,that Respiration = Breathing. The goal of breathing isto deliver oxygen to the body and to take awaycarbon dioxide.
Parts of the respiratory system
LungsThe lungs are the main organs of the respiratory
system. In the lungs oxygen is taken into the bodyand carbon dioxide is breathed out. The red bloodcells are responsible for picking up the oxygen in the
lungs and carrying the oxygen to all the body cellsthat need it. The red blood cells drop off the oxygen tothe body cells, then pick up the carbon dioxide whichis a waste gas product produced by our cells. The redblood cells transport the carbon dioxide back to thelungs and we breathe it out when we exhale.Contents
Trachea
The trachea (TRAY-kee-uh} is sometimes called the windpipe. The trachea filters the air webreathe and branches into the bronchi.Contents
BronchiThe bronchi (BRAHN-ky) are two air tubes that branch off of the trachea and carry air directly
into the lungs.Contents
DiaphragmBreathing starts with a dome-shaped muscle at the bottom of the lungs called the diaphragm
(DY-uh-fram). When you breathe in, the diaphragm contracts. When it contracts it flattens out
and pulls downward. This movement enlarges the space that the lungs are in. This larger spacepulls air into the lungs. When you breathe out, the diaphragm expands reducing the amount ofspace for the lungs and forcing air out. The diaphragm is the main muscle used in breathing.Contents
Why Do I Yawn?
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When you are sleepy or drowsy the lungs do not take enough oxygen from the air. Thiscauses a shortage of oxygen in our bodies. The brain senses this shortage of oxygen and sendsa message that causes you to take a deep long breath---a YAWN.Contents
Why Do I Sneeze? Sneezing is like a cough in the upper breathing passages. It is the body's way of removing anirritant from the sensitive mucous membranes of the nose. Many things can irritate the mucousmembranes. Dust, pollen, pepper or even a cold blast of air are just some of the many thingsthat may cause you to sneeze.
What Causes Hiccups? Hiccups are the sudden movements of the diaphragm. It is involuntary --- you have no controlover hiccups, as you well know. There are many causes of hiccups. The diaphragm may getirritated, you may have eaten to fast, or maybe some substance in the blood could even havebrought on the hiccups.
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Lymphatic and Immune Systems
A number of mechanisms operate within the bodies of birds and mammals that either preventinfection or fight infection by foreign particles and cells. Nonspecific immunityrefers tomechanisms that are generally effective against a variety of infections.
Specific immunityrefers to mechanisms that are specific for one type of infection. Specificimmunity is generally acquired after exposure to the infecting particles or cells.
Barriers to Entry
The skin is the main barrier preventing the entry of foreign organisms and particles.
Skin oils weaken or kill bacteria.
Cilia lining the respiratory tract sweep mucus and trapped particles to the throat where they areswallowed.
The low pH of the stomach kills microorganisms.
Tears wash the eyes.
Saliva helps clean teeth, preventing dental caries.
Urine flow prevents colonization of the urinary tract.
Vaginal secretions move microorganisms out of the reproductive tract.
The normal bacterial colonists of the skin, gut, and vagina prevent harmful microorganisms from
colonizing the areas.
Inflammatory Reaction
The inflammatory reaction is a local response to injury.
Damaged tissue releases bradykinin, which causes pain and stimulates mast cells to releasehistamine.
Bradykinin and histamine produce vasodilation, ( increased blood vessel diameter) to increaseblood flow to the area.
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Bradykinin and histamine also cause increased permeability (allows fluid to leak out). Thisbrings more defensive cells and chemicals to the area.
Neutrophils and monocytes are amoeboid white blood cells (leukocytes) that squeeze out ofthe capillaries and enter the damaged tissue.
Neutrophils phagocytize foreign material.
Monocytes are transformed into macrophages, which can phagocytize a large number ofviruses and bacteria.
Macrophages release white blood cell growth factor. This hormone stimulates the bone marrowto produce leukocytes (white blood cells).
Pus is a large # of dead leukocytes that fought infection.
Antibody-Mediated Immunity
Antigens and Antibodies
Antibodies are proteins that protect against foreign invaders, either foreign molecules, viruses,or cells. They are capable of recognizing specific particles due to their shape. Their ability torecognize foreign shapes makes them useful in defending against foreign invaders.
Antigensare molecules that antibodies are capable of being recognized. They are usually aprotein or carbohydrate chain. The body can recognize bacteria and viruses as being foreignbecause they have antigens on their surface which are different than the bodies "self" antigens.
Antibodies are Y-shaped molecules with a constant region and two binding sites that vary from
one antibody to the next.
Antibodies fit together with and bind with antigens like a lock and key.
The body does not produce antibodies that bind to its own (self) antigens. Therefore all particlesthat are bound to antibodies are foreign.
Cells, particles, or molecules that are marked with antibodies:
1. may be phagocytized (engulfed) by neutrophils or macrophages.
2. may agglutinate (clump together) because each antibody is capable of binding to twoantigens. If the antigens are chemicals that are dissolved in the body fluids, the clumps ofantibody-bound particles will precipitate. Antigens attached to cells will cause the cells to clumptogether. The clumps are then phagocytized.
3. may activate the complement system (discussed below). The complement system is asystem of blood proteins that enhances the elimination of foreign cells or particles.
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During our life, we will encounter over 1 million different antigens, so we need at least 1 milliondifferent antibodies, one for each kind of antigen.
There are 5 different classes of antibodies (IgA, IgD, IgG, IgH, IgM). One class containspentamers, another contains dimers.
Antibodies are produced by B lymphocytes.
B Lymphocytes
B lymphocytes (B cells) mature in the bone marrow.
B lymphocytes have receptors (antibodies) attached to their surface which function to detectantigens.
There is only one specific kind of receptor on the surface of a lymphocyte. A single Blymphocyte can therefore detect only one kind of antigen. Our bodies have millions of different
kinds of B lymphocytes.
Clonal selection
B cells that encounter the correct antigen with their antibody receptors become activated andbegin to divide many times producing plasma cells, which, in turn produce antibodies.
B lymphocyte + antigen more B-cells (called memory B-cells) andplasma cellsantibodies
B lymphocyte + incorrect antigen no reaction
Plasma cells produce antibodies that are identical to the receptors on the surface of the B cellthat was initially stimulated by antigen. The antibodies therefore can adhere to the type ofinvader that initially activated the B cell.
Memory B cells are B cells that are produced as a result of stimulation by the antigen. Becausethere are now many of these to fight off future infection, they are called memory B cells.
Large numbers of B-cells are found in the lymph nodes and in the spleen.
The Complement System
The complement system consists of a number of different proteins that help defend the bodywhen they are activated.
Each activated complement protein activates many others so that a large number of activeproteins are produced.
The following may initially activate the complement system:
Antigen-Antibody interaction
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Substances on the capsules or cell walls of microorganisms; substances produced bymicroorganisms
Functions of the Complement System:
1. The activated proteins stimulate mast cells causing inflammation andattract phagocytes (neutrophils, macrophages) to the area.2. Complement proteins bind to microorganisms and other particlesenhancing their recognition by phagocytes.3. Other complement proteins produce holes in bacterial cell walls allowingsalts and fluids to enter, rupturing the cell.
It is called complement because it enhances (complements) other immune responses such asthe inflammatory reaction and the antibody-mediated response (the proteins bind to microbesthat already have antibodies attached, improving recognition by phagocytes).
Some Important Molecules
Interferons
Interferons are proteins produced by virus-infected animal cells that stimulate other cells toproduce substances that interfere with viral replication.
Lysozyme
Lysozyme is an enzyme capable of breaking down the cell walls of gram-positive bacteria. It isfound in perspiration, tears, saliva, nasal secretions, and tissue fluids.
Cell-Mediated Immunity
T lymphocytes (T-cells) are lymphocytes that mature in the thymus.
This type of immunity is used tofight cells such as cancer cells, virus-infected cells, single-celled fungi, parasites, and cells of an organ transplant.
T Lymphocytes
Activating T Cells
T cells cannot recognize antigens unless an antigen-presenting cell (usually a macrophage)presents the antigens to them.
The macrophage first engulfs the antigen (or bacterium, virus, etc.) and brings fragments of theforeign antigens to its surface linked to its own (self) antigens.
The "self" antigen is referred to as an "MHC" protein. (MHC = major histocompatibility complex)
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If receptors on a virgin T cell match both the self and foreign antigens, the T cell becomesactivated and undergoes clonal expansion (cell reproduction) producing the 4 kinds of T cellsdescribed below.
Cytotoxic T Cells
Cytotoxic T cells (also called killer T cells) attack antigen-MHC bearing cells. Because MHC is a"self" marker and antigens are part of foreign particles, Cytotoxic T cells attack the bodys owncells that are infected viruses and microorganisms. They also attack cancer cells because theyhave mutated (therefore foreign) antigens.
The cytotoxic T cell releases proteins that penetrate the target cell membrane. Salts and fluidenter through the holes and the cell ruptures.
Helper T Cells
When exposed to an antigen-MHC complex, Helper T cells secrete lymphokines, which
enhance the response of other immune cells. For example, they stimulate T cells to clone,macrophages to phagocytize, and B cells to become plasma cells and produce antibodies.
HIV (the virus that causes AIDS) attacks helper T cells as well as others in the immune system.HIV therefore prevents the immune system from becoming activated.
Suppressor T Cells
Suppressor T cells regulate the immune response by suppressing the activity and developmentof B cells and helper T cells. They do this by secreting inhibitory chemicals in response todeclining antigen levels.
Memory T Cells
Memory T cells are T cells that persist after infection. They will secrete lymphokines if the sameantigen reenters the body.
Active and Passive Immunity
Active immunityis produced in individuals by administering foreign antigens. These antigensmay come from weakened or dead microorganisms. This process is called vaccination.
Genetically engineered bacteria are currently being used to produce some antigens. Examples:malaria, hepatitis B.
After exposure to antigens in a vaccine, the level of antibodies in the blood begins to increaseafter several days, levels off, then declines. After a secondary exposure (called a booster), thelevel increases rapidly.
Memory B cells and memory T cells allow the individual to be actively immune. If they areexposed to the disease, a rapid immune response will occur because they already have largenumbers of the correct B and T cells.
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Passive immunityoccurs when an individual receives antibodies instead of making their own.Passive immunity is short-lived because the persons B and T cells have not been stimulated toproduce antibodies. The immunity lasts only as long as the antibodies they received remain intheir bloodstream.
Examples of Passive Immunity
Newborn babies have antibodies they received from their mother.
Breast-fed babies receive antibodies from their mothers milk.
Allergies
Allergies are due to an overactive immune system.
Mast cells contain antibody receptors to allergens (antigens) and when stimulated, they secretehistamine.
Histamine causes mucus secretion, airway constriction, and inflammation due to blood vesselsleaking. Leaky blood vessels cause the tissues to swell.
Allergy shots stimulate the body to produce high levels of antibodies. The antibodies react withthe allergens before they have a chance to interact with the mast cells.
Components of the Immune System
Leukocytes
Leukocytes are white blood cells. The following kinds of leukocytes were discussed in this
chapter:
neutrophilsmonocytes (become macrophages)macrophageslymphocytesB cells - mature in bone marrowT cells - mature in thymus, small intestine, skin
Lymphatic System
Functions of the Lymphatic System
1. take up excess tissue fluid and return it to the bloodstream
2. absorb fats at the intestinal villi and transport to the circulatory system
3. defend against disease
Lymphatic Vessels
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Lymphatic vessels are similar to veins, including the presence of valves. They depend on themovement of skeletal muscles to move the fluid inside.
The fluid they contain is called lymph.
They empty into the circulatory system via the thoracic duct and the right lymphatic duct. Thethoracic duct is much larger than the right lymphatic duct.
Lymph Nodes
Lymph nodes are small (1-25 mm), spherical or ovoid structures that are connected to lymphaticvessels. They contain open spaces (sinuses), each with many lymphocytes and macrophages.
As lymph passes through, macrophages purify it of infectious organisms and particles.
The structures listed below are groups of nodules that also function to purify lymph:
tonsils - back of mouthadenoids - back of mouth above the soft palatePeyers patches - intestinal wall
Spleen
The spleen stores blood.
It helps purify blood that passes through it by removing bacteria and worn-out or damaged redblood cells.
Thymus Gland
T lymphocytes mature in the thymus.
Bone Marrow
Macrophages and lymphocytes (B cells and T cells) are produced in the bone marrow. T cellsmature in the thymus gland, small intestine, and in the skin.
Autoimmune Diseases
Autoimmune diseases result when the body is attacked by its immune system.
They often appear in individuals that have recovered from other infections. Somehow the bodyseems to have learned to recognize itself (its own antigens).
Examples
Myasthenia gravis - neuromuscular junctions are weakened
Multiple sclerosis - the myelin sheath of nerve fibers is attacked
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Lupus erythematosus - Lupus is a chronic inflammatory disease. The skin, joints, kidneys andblood are most often affected but other organs may be affected as well.
Rheumatoid arthritis - the membranes that surround the joints are attacked
Summary of Leukocytes
Nonspecific response
Mast cells - secrete histamine
Neutrophils - participate in inflammatory response; phagocytize
Monocytes - become macrophages which phagocytize; produce white blood cell growth factor
Specific Immune Response
B-lymphocytes
give rise to plasma cells that produce antibodies
give rise to more B lymphocytes (also called memory B lymphocytes)
T-lymphocytes
Cytotoxic T cells - attack cells that bear antigens
Helper T cells - secrete lymphokines which enhances the response of other immune cells
Suppresser T cells - suppress helper T cells and B cells
Memory T cells - remain after the infection and produce the 4 kinds of T cells if activated.
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/Immune%20System/lymphati.htm
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Introductory Anatomy: Digestive System
Dr D.R.Johnson, Centre for Human Biology
Purpose
The digestive system prepares food for use by hundreds of millions of body cells. Food wheneaten cannot reach cells (because it cannot pass through the intestinal walls to the bloodstreamand, if it could would not be in a useful chemical state. The gut modifies food physically andchemically and disposes of unusable waste. Physical and chemical modification (digestion)depends on exocrine and endocrine secretions and controlled movement of food through thedigestive tract.
Mouth
Mouth Food enters the digestive system via the mouth or oral cavity, mucous membrane lined.The lips (labia) protect its outer opening, cheeks form lateral walls, hard palate and soft palate
form anterior/posterior roof. Communication with nasal cavity behind soft palate. Floor ismuscular tongue. Tongue has bony attachments (styloid process, hyoid bone) attached to floorof mouth by frenulum.Posterior exit from mouth guarded by a ring of palatine/lingual tonsils. Enlargement = sorethroat, tonsillitis.Food is first processed (bitten off) by teeth, especially the anterior incisors. Suitably sizedportions then retained in closed mouth and chewed or masticated (especially by cheek teeth,premolars, molars) aided by saliva Ducted salivary glands open at various points into mouth.This process involves teeth (muscles of mastication move jaws) and tongue (extrinsic andintrinsic muscles). Mechanical breakdown, plus some chemical (ptyalin, enzyme in saliva).Taste buds allow appreciation, also sample potential hazards (chemicals, toxins)
Swallowing
In leaving the mouth a bolus of food must cross the respiratory tract (trachea is anterior tooesophagus) by a complicated mechanism known as swallowing or deglutination which emptiesthe mouth and ensures that food does not enter the windpipe.Swallowing involves co-ordinated activity of tongue, soft palate pharynx and oesophagus. Thefirst (buccal) phase is voluntary, food being forced into the pharynx by the tongue. After this theprocess is reflex. The tongue blocks the mouth, soft palate closes off the nose and the larynxrises so that the epiglottis closes off the trachea. Food thus moves into the pharynx andonwards by peristalsis aided by gravity. If we try to talk whilst swallowing food may enter therespiratory passages and a cough reflex expels the bolus.
Oesophagus
The oesophagus (about 10") is the first part of the digestive tract proper and shares itsdistinctive structure. Basic tissue layers of the gut are1. mucosa. Innermost, moist lining membrane. Epithelium (friction resistant stratified squamousin oesophagus, simple beyond) plus a little connective tissue and smooth muscle.2. submucosa. Soft connective tissue layer, blood vessels, nerves, lymphatics3. muscularis externa. Typically circular inner layer, longitudinal outer layer of smooth muscle4. serosal fluid producing single layer.
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Stomach
C shaped, left side abdominal cavity (because liver is on right). Cardioesophageal sphincterguarding entrance from oesophagus is of doubtful anatomical integrity (though functionally thediaphragmatic pinch cock serves). Pyloric sphincter guarding the outlet is much better defined.Fundus, body and pylorus recognised as distinct regions. Stomach secretes both acid and
mucus (for self protection). Surface area increased by rugae. Serves as a temporary store forfood which is also churned by muscular layers (three here) to form chyme, creamy substancevoided via pyloric sphincter to duodenum
Duodenum
First part of small intestine. C shaped 10" long and curves around head of pancreas and entry ofcommon bile duct (accessory organs of digestion, pancreas, liver see below). Chemicaldegradation of small controlled amounts of food controlled by pyloric sphincter begins here,enzymes secreted by pancreas and duodenum itself aided by emulsifying bile (which alsolowers pH). Duodenal ulcers caused by squirting of acid stomach contents into duodenal wallopposite sphincter.
Small Intestine
Jejunum (8 feet) and ileum (12 feet) continue degenerative process. Surface area increased byplica circulares (circular folds) carrying villi: cells of villi carry microvilli. Each villus has acapillary and a lacteal (lymphatic capillary) Absorption of digested foodstuffs is via these to therich venous and capillary drainage of the gut. Towards the end of the small intestineaccumulations of lymphoid tissue (Peyer's patches) more common. Undigested residue of foodis rich in bacteria.
Large Intestine
Jejunum terminates at caecum. Caecum is small saclike evagination, important in some animalsas a repository for bacteria/other organisms able to digest cellulose. A blind ending appendixmay give trouble (appendicitis) if infected. The large intestine has three longitudinal musclebands (taenia coli) with bulges in the wall (haustra) between them. These may evaginate in theelderly to become diverticuli and infected in diverticulitis.The large intestine resorbs water then eliminates drier residues as faeces. Regions recognisedare the ascending colon, from appendix in right groin up to a flexure at the liver, transversecolon, liver to spleen, descending colon, spleen to left groin, then sigmoid (S-shaped) colonback to midline and anus. Anus has voluntary and involuntary sphincter and ability to distinguishwhether contents are gas or solid. No villi in large intestine, but many goblet cells secretinglubricative mucus.
Accessory digestive organs
Salivary glands
Three pairs, parotid, submandibular, sublingual. Mumps begins as infective parotitis in theparotid glands in the cheek. The others open into the floor of the mouth. Saliva is a mixture ofmucus and serous fluids, each produced to various extents in various glands. Also contains
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salivary amylase, (starts to break down starch) lysozyme (antibacterial) and IgA antibodies. Insome mammals (and snakes!) saliva may be poisonous, quietening down living prey.
Pancreas
Endocrine and exocrine gland. Exocrine part produces many enzymes which enter theduodenum via the pancreatic duct. Endocrine part produces insulin, blood sugar regulator.
Liver and gallbladder
Bile, a watery greenish fluid is produced by the liver and secreted via the hepatic duct and cysticduct to the gall bladder for storage, and thence on demand via the common bile duct to anopening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments (mainlybilerubin, essentially the non-iron part of haemoglobin) cholesterol and phospholipids. Bile saltsand phospholipds emulsify fats, the rest are just being excreted. Gallstones are usuallycholesterol based, may block the hepatic or common bile ducts causing pain, jaundice.
Liver
Multifunctional: important in this context since the capillaries of the small intestine drain fat andother nutrient rich lymph into it via the hepatic portal system.
http://www.leeds.ac.uk/chb/lectures/anatomy8.html
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What is the Skeletal System?
Your Skeletal system is all of the bones in the body and the tissues such as tendons, ligamentsand cartilage that connect them.Your teeth are also considered part of your skeletal system but they are not counted as bones.Your teeth are made of enamel and dentin. Enamel is the strongest substance in your body.
How does the Skeletal System help us?
SupportThe main job of the skeleton is to provide support for our body. Without your skeleton your bodywould collapse into a heap. Your skeleton is strong but light. Without bones you'd be just apuddle of skin and guts on the floor.
ProtectionYour skeleton also helps protectyour internal organs and fragile
body tissues. The brain, eyes,heart, lungs and spinal cord areall protected by your skeleton.Yourcranium (skull) protects yourbrain and eyes, the ribs protectyour heart and lungs and yourvertebrae (spine, backbones)protect your spinal cord.
MovementBones provide the structure formuscles to attach so that our
bodies are able to move.Tendons are tough inelasticbands that hold attach muscle tobone. Contents
Who has more bones a baby or an adult?
Babies have more than adults! At birth, you have about 300 bones. As you grow older, smallbones join together to make big ones. Adults end up with about 206 bones.
Are bones alive?
Absolutely. Old bones are dead, dry and brittle. But in the body, bones are very much alive.They have their own nerves and blood vessels, and they do various jobs, such as storing bodyminerals like calcium. Bones are made of a mix of hard stuff that gives them strength and tonsof living cells which help them grow and repair themselves.
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What is a bone made of?
A typical bone has an outer layer of hard orcompact bone, which is very strong, denseand tough. Inside this is a layer of spongybone, which is like honeycomb, lighter and
slightly flexible. In the middle of some bonesis jelly-like bone marrow, where new cellsare constantly being produced for the blood.Calcium is an important mineral that bonecells need to stay strong so keep drinkingthat low-fat milk!
Contents
How do bones break and heal?
Bones are tough and usually don't break even when we have some pretty bad falls. I'm sure youhave broken a big stick at one time. When you first try to break the stick it bends a bit but withenough force the stick finally snaps. It is the same with your bones. Bones will bend a little, but ifyou fall the wrong way from some playground equipment or maybe your bike or skateboard youcan break a bone. Doctors call a broken bone a fracture. There are many different types offractures.
Luckily, bones are made of living cells. When a bone is broken your bone will produce lots ofnew cells to rebuild the bone. These cells cover both ends of the broken part of the bone andclose up the break.
How do I keep my bones healthy?
Bones need regular exercise to stay as strong as possible. Walking, jogging, running and other
physical activities are important in keeping your bones strong and healthy. Riding your bike,basketball, soccer, gymnastics, baseball, dancing, skateboarding and other activities are allgood for your bones. Make sure you wear or use the proper equipment like a helmet, kneepads,shin guards, mats, knee pads, etc... to keep those bones safe.
Strengthen your skeleton by drinking milk and eating other dairy products (like low-fat cheese,frozen yogurt, and ice cream). They all contain calcium, which helps bones harden and becomestrong.
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The human body contains more than 650 individual muscles which are attached to the skeleton,which provides the pulling power for us to move around. The main job of the muscular system isto provide movement for the body. The muscular system consist of three different types ofmuscle tissues : skeletal, cardiac, smooth. Each of these different tissues has the ability tocontract, which then allows body movements and functions. There are two types of muscles inthe system and they are the involuntary muscles, and the voluntary muscles. The muscle in
which we are allow to control by ourselves are called the voluntary muscles and the ones wecan? control are the involuntary muscles. The heart, or the cardiac muscle, is an example ofinvoluntary muscle.
CARDIAC MUSCLE:
The cardiac muscles is the muscle of the heart itself. The cardiac muscle is the tissue thatmakes up the wall of the heart called the mydocardium. Also like the skeletal muscles, thecardiac muscle is striated and contracts through the sliding filament method. However it isdifferent from other types of muscles because it forms branching fibers. Unlike the skeletalmuscles, the cardiac muscle is attached together instead of been attach to a bone.
SKELETAL MUSCLE:
The skeletal muscle makes up about 40 % of an adults body weight. It has stripe-like markings,or striations. The skeletal muscles is composed of long muscle fibers. Each of these musclesfiber is a cell which contains several nuclei. The nervous system controls the contraction of themuscle. Many of the skeletal muscle contractions are automatic. However we still can controlthe action of the skeletal muscle. And it is because of this reason that the skeletal muscle is alsocalled voluntary muscle.
SMOOTH MUSCLE:
Much of our internal organs is made up of smooth muscles. They are found in the urinarybladder, gallbladder, arteries, and veins. Also the digestive tract is made up of smooth muscleas well. The smooth muscles are controlled by the nervous system and hormones. We cannotconsciously control the smooth muscle that is why they are often called involuntary muscles.
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Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmitnerve messages. Glial cells are in direct contact with neurons and often surround them.
Nerve Cells andAstrocyte (SEM x2,250). This image is copyright Dennis Kunkel atwww.DennisKunkel.com, used with permission.
The neuronis the functional unit of the nervous system. Humans have about 100 billion neuronsin their brain alone! While variable in size and shape, all neurons have three parts.Dendritesreceive information from another cell and transmit the message to the cell body. The cell bodycontains the nucleus, mitochondria and other organelles typical of eukaryotic cells. Theaxonconducts messages away from the cell body.
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Structure of a typical neuron. The above image is fromhttp://eleceng.ukc.ac.uk/~sd5/pics/research/big/neuron.gif.
Three types of neurons occur. Sensory neuronstypically have a long dendrite and short axon,and carry messages from sensory receptors to the central nervous system.Motor neuronshavea long axon and short dendrites and transmit messages from the central nervous system to themuscles (or to glands). Interneurons are found only in the central nervous system where theyconnect neuron to neuron.
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Structure of a neuron and the direction of nerve message transmission. Image from Purves etal., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WHFreeman (www.whfreeman.com), used with permission.
Some axons are wrapped in a myelin sheath formed from the plasma membranes of specializedglial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and servicefacilities for neurons. The gap between Schwann cells is known as the node of Ranvier, andserves as points along the neuron for generating a signal. Signals jumping from node to nodetravel hundreds of times faster than signals traveling along the surface of the axon. This allowsyour brain to communicate with your toes in a few thousandths of a second.
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Cross section ofmyelin sheaths that surround axons (TEM x191,175). This image is copyrightDennis Kunkel at www.DennisKunkel.com, used with permission.
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Structure of a nerve bundle. Image from Purves et al., Life: The Science of Biology, 4th Edition,by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used withpermission.
The Nerve Message | Back to Top
The plasma membrane of neurons, like all other cells, has an unequal distribution of ions andelectrical charges between the two sides of the membrane. The outside of the membrane has apositive charge, inside has a negative charge. This charge difference is a resting potential andis measured in millivolts. Passage of ions across the cell membrane passes the electricalcharge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (restingpotential). Resting potential results from differences between sodium and potassium positivelycharged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentratedoutside the membrane, while potassium ions are more concentrated inside the membrane. Thisimbalance is maintained by the active transport of ions to reset the membrane known as thesodium potassium pump. The sodium-potassium pumpmaintains this unequal concentration byactively transporting ions against their concentration gradients.
Transmission of an action potential. The above image is from
http://eleceng.ukc.ac.uk/~sd5/pics/research/big/actpot.gif.
Changed polarity of the membrane, the action potential, results in propagation of the nerveimpulse along the membrane. An action potential is a temporary reversal of the electricalpotential along the membrane for a few milliseconds. Sodium gates and potassium gates openin the membrane to allow their respective ions to cross. Sodium and potassium ions reversepositions by passing through membrane protein channel gates that can be opened or closed tocontrol ion passage. Sodium crosses first. At the height of the membrane potential reversal,potassium channels open to allow potassium ions to pass to the outside of the membrane.
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Potassium crosses second, resulting in changed ionic distributions, which must be reset by thecontinuously running sodium-potassium pump. Eventually enough potassium ions pass to theoutside to restore the membrane charges to those of the original resting potential.The cellbegins then to pump the ions back to their original sides of the membrane.
The action potential begins at one spot on the membrane, but spreads to adjacent areas of the
membrane, propagating the message along the length of the cell membrane. After passage ofthe action potential, there is a brief period, the refractory period, during which the membranecannot be stimulated. This prevents the message from being transmitted backward along themembrane.
Steps in an Action Potential
1. At rest the outside of the membrane is more positive than the inside.2. Sodium moves inside the cell causing an action potential, the influx of positive sodium
ions makes the inside of the membrane more positive than the outside.3. Potassium ions flow out of the cell, restoring the resting potential net charges.4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell,
restoring the original distribution of ions.
Synapses
The junction between a nerve cell and another cell is called a synapse. Messages travel withinthe neuron as an electrical action potential. The space between two cells is known as thesynaptic cleft. To cross the synaptic cleft requires the actions ofneurotransmitters.Neurotransmitters are stored in small synaptic vessicles clustered at the tip of the axon.
A synapse. Image from Purves et al., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
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Excitatory Synapse from the Central Nervous System (TEM x27,360). This image is copyrightDennis Kunkel at www.DennisKunkel.com, used with permission.
Arrival of the action potential causes some of the vesicles to move to the end of the axon anddischarge their contents into the synaptic cleft. Released neurotransmitters diffuse across thecleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell toopen. Some neurotransmitters cause an action potential, others are inhibitory.
Neurotransmitters tend to be small molecules, some are even hormones. The time forneurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are eitherdestroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by
the cell. More than 30 organic molecules are thought to act as neurotransmitters. Theneurotransmitters cross the cleft, binding to receptor molecules on the next cell, promptingtransmission of the message along that cell's membrane.Acetylcholine is an example of aneurotransmitter, as is norepinephrine, although each acts in different responses. Once in thecleft, neurotransmitters are active for only a short time. Enzymes in the cleft inactivate theneurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.
Diseases that affect the function of signal transmission can have serious consequences.Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive death of
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brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is achemical related to dopamine that eases some of the symptoms (by acting as a substituteneurotransmitter) but cannot reverse the progression of the disease.
The bacterium Clostridium tetaniproduces a toxin that prevents the release of GABA. GABA isimportant in control of skeletal muscles. Without this control chemical, regulation of muscle
contraction is lost; it can be fatal when it effects the muscles used in breathing.
Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes theprogressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in thesynapses: cocaine, LSD, caffeine, and insecticides.
Nervous Systems |Back to Top
Multicellular animals must monitor and maintain a constant internal environment as well asmonitor and respond to an external environment. In many animals, these two functions arecoordinated by two integrated and coordinated organ systems: the nervous system and theendocrine system. Click herefor a diagram of the Nervous System.
Three basic functions are prformed by nervous systems:
1. Receive sensory input from internal and external environments2. Integrate the input3. Respond to stimuli
Sensory Input
Receptors are parts of the nervous system that sense changes in the internal or externalenvironments. Sensory input can be in many forms, including pressure, taste, sound, light,
blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.
Integration and Output
In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and aresponse is generated. The response, a motor output, is a signal transmitted to organs than canconvert the signal into some form of action, such as movement, changes in heart rate, releaseof hormones, etc.
Endocrine Systems
Some animals have a second control system, the endocrine system. The nervous system
coordinates rapid responses to external stimuli. The endocrine system controls slower, longerlasting responses to internal stimuli. Activity of both systems is integrated.
Divisions of the Nervous System
The nervous system monitors and controls almost every organ system through a series ofpositive and negative feedback loops.The Central Nervous System (CNS) includes the brain
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and spinal cord. The Peripheral Nervous System (PNS) connects the CNS to other parts of thebody, and is composed of nerves (bundles of neurons).
Not all animals have highly specialized nervous systems. Those with simple systems tend to beeither small and very mobile or large and immobile. Large, mobile animals have highlydeveloped nervous systems: the evolution of nervous systems must have been an important
adaptation in the evolution of body size and mobility.
Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net.These creatures haveradial symmetry and lack a head. Although lacking a brain or eithernervous system (CNS or PNS) nerve nets are capable of some complex behavior.
Nervous systems in radially symmetrical animals. Image from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman(www.whfreeman.com), used with permission.
Bilaterally symmetrical animals have a body plan that includes a defined head and a tail region.Development of bilateral symmetry is associated withcephalization, the development of a headwith the accumulation of sensory organs at the front end of the organism. Flatworms have
neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrateshave a spinal cord in addition to a more developed brain.
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Some nervous systems in bilaterally symmetrical animals. Image from Purves et al., Life: TheScience of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman(www.whfreeman.com), used with permission.
Chordates have a dorsal rather than ventral nervous system. Several evolutionary trends occurin chordates: spinal cord, continuation of cephalization in the form of larger and more complexbrains, and development of a more elaborate nervous system. The vertebrate nervous system isdivided into a number of parts. The central nervous system includes the brain and spinal cord.The peripheral nervous system consists of all body nerves. Motor neuron pathways are of twotypes: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands). Theautonomic system is subdivided into the sympathetic and parasympathetic systems.
Peripheral Nervous System | Back to Top
The Peripheral Nervous System (PNS)contains only nerves and connects the brain and spinalcord (CNS) to the rest of the body. The axons and dendrites are surrounded by a white myelin
sheath. Cell bodies are in the central nervous system (CNS) organglia. Ganglia are collectionsof nerve cell bodies. Cranial nerves in the PNS take impulses to and from the brain(CNS).Spinal nerves take impulses to and away from the spinal cord. There are two major subdivisionsof the PNS motor pathways: the somatic and the autonomic.
Two main components of the PNS:
1. sensory (afferent) pathways that provide input from the body into the CNS.2. motor (efferent) pathways that carry signals to muscles and glands (effectors).
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Most sensory input carried in the PNS remains below the level of conscious awareness. Inputthat does reach the conscious level contributes to perception of our external environment.
Somatic Nervous System | Back to Top
The Somatic Nervous System (SNS) includes all nerves controlling themuscular system andexternal sensory receptors. External sense organs (including skin) are receptors. Muscle fibersand gland cells areeffectors. The reflex arc is an automatic, involuntary reaction to a stimulus.When the doctor taps your knee with the rubber hammer, she/he is testing your reflex (or knee-
jerk). The reaction to the stimulus is involuntary, with the CNS being informed but notconsciously controlling the response. Examples of reflex arcs include balance, the blinkingreflex, and the stretch reflex.
Sensory input from the PNS is processed by the CNS and responses are sent by the PNS fromthe CNS to the organs of the body.
Motor neurons of the somatic system are distinct from those of the autonomic system. Inhibitorysignals, cannot be sent through the motor neurons of the somatic system.
Autonomic Nervous System | Back to Top
TheAutonomic Nervous System is that part of PNS consisting of motor neurons that controlinternal organs. It has two subsystems. The autonomic system controls muscles in the heart,the smooth muscle in internal organs such as the intestine, bladder, and uterus. TheSympathetic Nervous Systemis involved in the fight or flight response. The ParasympatheticNervous System is involved in relaxation. Each of these subsystems operates in the reverse ofthe other (antagonism). Both systems innervate the same organs and act in opposition tomaintain homeostasis. For example: when you are scared the sympathetic system causes yourheart to beat faster; the parasympathetic system reverses this effect.
Motor neurons in this system do not reach their targets directly (as do those in the somaticsystem) but rather connect to a secondary motor neuron which in turn innervates the targetorgan.
Click here for a diagram of the Autonomic Nervous System.
Central Nervous System | Back to Top
The Central Nervous System(CNS) is composed of the brain and spinal cord. The CNS issurrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain and spinalcord.
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Areas of the brain. The above image is fromhttp://www.prs.k12.nj.us/schools/PHS/Science_Dept/APBio/pic/brain.gif.
The brain is composed of three parts: thecerebrum (seat of consciousness), the cerebellum,and the medulla oblongata (these latter two are "part of the unconscious brain").
The medulla oblongata is closest to the spinal cord, and is involved with the regulation ofheartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting,coughing, sneezing, swallowing, and hiccuping. The hypothalamus regulates homeostasis. Ithas regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure,and links the Nervous System to the Endocrine System. Themidbrain and pons are also part ofthe unconscious brain. The thalamus serves as a central relay point for incoming nervousmessages.
The cerebellum is the second largest part of the brain, after the cerebrum. It functions formuscle coordination and maintains normal muscle tone and posture. The cerebellumcoordinates balance.
The conscious brain includes the cerebral hemispheres, which are are separated by thecorpuscallosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory data and motorfunctions. The cerebrum governs intelligence and reasoning, learning and memory. While thecause of memory is not yet definitely known, studies on slugs indicate learning is accompaniedby a synapse decrease. Within the cell, learning involves change in gene regulation andincreased ability to secrete transmitters.
The Brain | Back to Top
During embryonic development, the brain first forms as a tube, the anterior end of whichenlarges into three hollow swellings that form the brain, and the posterior of which develops intothe spinal cord. Some parts of the brain have changed little during vertebrate evolutionaryhistory. Clickhere to view an diagram of the brain, andhere for a clickable map of the brain.
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