:: wikimiki.org ::

Muscle

Muscle

Muscle is a contractile form of tissue. It is one of the four major tissue types, the other three being epithelium, connective tissue and nervous tissue. Muscle contraction is used to move parts of the body, as well as to move substances within the body.

Types

There are three general types of muscle:
- Cardiac muscle is a specalized kind of muscle found only within the heart.
- Skeletal muscle or "voluntary muscle" is anchored by tendons to bone and is used to effect skeletal movement such as locomotion.
- Smooth muscle or "involuntary muscle" is found within structures such as the intestines, throat and blood vessels. Cardiac and skeletal muscle are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. Striated muscle is often used in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions. Skeletal muscle is further divided into two subtypes:
- Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
- Type II, glycolytic, fast twitch, or "white" muscle is less dense in mitochondria and myoglobin. It can contract more quickly and with a greater amount of force than Type I muscle, but can sustain only short, anaerobic bursts of activity before a build-up of lactic acid in tissue begins to interfere with muscular contraction and causes pain.

Anatomy

Muscle is composed of muscle cells (sometimes known as "muscle fibers"). Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle, which is lined by epimysium. Muscle spindles are distributed throughout the muscles and provide feedback sensory information to the central nervous system. Skeletal muscle is arranged in discrete groups, examples of which include the biceps brachii. It is connected by tendons to processes of the skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin (in which it controls erection of body hair) to the blood vessels and digestive tract (in which it controls the caliber of a lumen and peristalsis).

Physiology

The three types of muscle have significant differences, but all use the movement of actin against myosin to produce contraction and relaxation. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine. Muscles and muscular activity account for most of the body's energy consumption. Muscles store energy for their own use in the form of glycogen, which represents about 1% of their mass. This can be rapidly converted to glucose when more energy is necessary.

Nervous control

Efferent leg

Vertebrates move muscles in response to voluntary and autonomic signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the brain. In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain, but most muscle activity is the result of complex interactions between various areas of the brain. Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor cortex of the brain's cerebral cortex. Commands are routed though the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback loops such as that of the extrapyramidal system contribute signals to influence muscle tone and response. Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia.

Afferent leg

Sometimes known as muscle memory, the sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses. Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and nucleus ruber in particular continuously sample position against movement and make minor corrections to assure a smooth projection.

Role in health and disease

Exercise

Exercise is often recommended as a means of improving motor skills, fitness and muscle strength. Exercise has several effects upon muscles, connective tissue and bone, and the nerves that stimulate the muscles.

Disease

Symptoms of muscle disease may include weakness or spasticity/rigidity, myoclonus (twitching) and myalgia (muscle pain). Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). Neuromuscular diseases are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A large proportion of neurological disorders leads to problems with movement, ranging from cerebrovascular accident (stroke) and Parkinson's disease to Creutzfeldt-Jakob disease. Diseases of the motor end plate include myasthenia gravis, a form of muscle weakness due to antibodies to the acetylcholine receptor, and its related condition Lambert-Eaton myasthenic syndrome (LEMS). Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively. The myopathies are all diseases affecting the muscle itself, rather than its nervous control. Muscular dystrophy is a large group of diseases, many of them hereditary, where the muscle integrity is disrupted. It leads to progressive loss of strength, high dependence and decreased life span. Inflammatory muscle disorders:
- Polymyalgia rheumatica (or "muscle rheumatism") is an inflammatory condition that mainly occurs in the elderly; it is associated with giant-cell arteritis. It often responds dramatically to glucocorticoids (e.g. prednisolone).
- Polymyositis, dermatomyositis and inclusion body myositis are autoimmune conditions in which the muscle is affected. Rhabdomyolysis is the breakdown of muscular tissue due to any cause. While it may not lead to any muscular symptoms at all, the myoglobin thus released may cause acute renal failure. Tumors of muscle include:
- Smooth muscle: leiomyoma (benign, very common in the uterus), leiomyosarcoma (malignant, very rare)
- Striated muscle: rhabdomyoma (benign) and rhabdomyosarcoma (malignant) - both very rare
- Metastasis from elsewhere (e.g. lung cancer) Smooth muscle has been implicated to play a role in a large number of diseases affecting blood vessels, the respiratory tract (e.g. asthma), the digestive system (e.g. irritable bowel syndrome) and the urinary tract (e.g. urinary incontinence). These disease processes are not usually confined to the muscular tissue.

The strongest human muscle

Depending on what definition of "strongest" is used, many different muscles in the human body can be characterized as being the "strongest." In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 975 lbf (4337 N) for two seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles. If "strength" refers to the force exerted by the muscle itself, e.g. on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area at their belly. This is because the tension exerted by an individual skeletal (striated) muscle fiber does not vary much, either from muscle to muscle, or with length. Each fiber can exert a force on the order of 0.3 micronewtons. By this definition, the strongest muscle of the body is usually said to be the Quadriceps femoris or the Gluteus maximus. Again taking strength to mean only "force" (in the physicist's sense, and as contrasted with "energy" or "power"), then a shorter muscle will be stronger "pound for pound" (i.e. by weight) than a longer muscle. The uterus may be the strongest muscle by weight in the human body. At the time when an infant is delivered, the human uterus weighs about 40 oz (1.1 kg). During childbirth, the uterus exerts 25 to 100 lbf (100 to 400 N) of downward force with each contraction. The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." Eye movements, however, are and probably "need" to be exceptionally fast. The unexplained statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that technically the tongue consists of sixteen muscles, not one. The tongue may possibly be the strongest muscle at birth. The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime. Estimates of the power output of the human heart range from 1 to 5 watts. This is much less than the maximum power output of other muscles; for example, the quadriceps can produce over 100 watts, but only for a few minutes. The heart does its work continuously over an entire lifetime without pause, and thus can "outwork" other muscles. An output of one watt continuously for seventy years yields a total work output of 2 to 3 ×10⁹ joules.

Efficiency

The efficiency of human muscle has been measured (in the context of rowing and cycling) at 14% to 27%. The efficiency is defined as the ratio of mechanical work done to the total energy output (heat plus work).

Muscle evolution

According to a recent study published in 1999 [http://www.umbi.umd.edu/~collins/myoinformatics/muscle-evolution.pdf], specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line. This indicates that these types of muscle developed in a common ancestor sometime before 700 million years ago (mya). Vertebrate smooth muscle (smooth muscle found in humans) was found to have evolved independently from the skeletal and cardiac muscles.

References

- Costill, Jack H. and Wilmore, David L. (2004). Physiology of Sport and Exercise. Champaign, Illinois: Human Kinetics. ISBN 0736044892.
- Phylogenetic Relationship of Muscle Tissues Deduced from Superimposition of Gene Trees, Satoshi OOta and Naruya Saitou, Mol. Biol. Evol. 16(6) 856-7, 1999

External links

- [http://www.straightdope.com/mailbag/mmuscle.html The Straight Dope] (Masseter "strongest;" 975 pounds for 2 seconds; gluteus maximus and quadriceps strongest if leverage not included)
- [http://hypertextbook.com/facts/2003/IradaMuslumova.shtml Physics factbook] (Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×10⁹ joules)
- [http://www.akoa.org/story.dbm?sid=167 Alaska optometric association] (External eye muscles "100 times as strong as they need to be")
- [http://www.courses.vcu.edu/DANC291-003/unit_7.htm course notes for a Virginia Commonwealth dance course] (Quadriceps "strongest")
- [http://www.ergo-fit.de/english/scripts/produkteneu/produktgruppe.php?gruppe=17&hgruppe=7&familie=1 a body-building equipment website] (Quadriceps "strongest")
- [http://www.dundee.ac.uk/medther/StrokeSSM/ClinExamNeuro.htm University of Dundee] article on performing neurological examinations (Quadriceps "strongest")
- [http://www.hartnell.cc.ca.us/faculty/asteinhardt/development.htm course notes from a Hartnell College course] (Uterus "strongest pound for pound")
- [http://pregnancytoday.com/resource/definitions/uterus.htm the Pregnancy Today website] (Uterus "strongest")
- [http://www.coachesinfo.com/category/rowing/77/ Muscle efficiency in rowing]
- [http://www.gssiweb.com/reflib/attachment.cfm?id=11 "Gatorade Sports Science Institute" on muscle efficiency in cyclists (PDF)]

Contraction

Contraction can mean:
- Contraction (childbirth), a contraction during childbirth;
- Contraction (linguistics), a new word formed from two or more individual words;
- Contraction (science), one that can occur to solid matter as it cools;
- Contraction mapping, in mathematics, a type of function on a metric space;
- Muscle contraction, one that occurs when a muscle fiber shortens;
- Tensor contraction in tensor theory (in mathematics); Contraction can also refer to:
- A structural rule in proof theory, see idempotency of entailment

Tissue (biology)

Biological tissue is a substance made up of cells that perform a similar function. The study of tissues is known as histology, or, in connection with disease, histopathology. The classical tools for studying the tissues are the wax block, the tissue stain, and the optical microscope, though developments in electron microscopy, immunofluorescence, and frozen sections have all added to the sum of knowledge in the last couple of decades. With these tools, the classical appearances of the tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.

Animal Tissues

There are four basic types of tissue in the body of all animals, including the human body and lowar multicellular organisms such as insects. These compose all the organs, structures and other contents.
- Epithelium - Tissues composed of layers of cells that cover organ surfaces such as surface of the skin and inner lining of digestive tract. The tissues serve for protection, secretion, and absorption.
- Connective tissue - As the name suggests, connective tissue holds everything together. Blood is considered a connective tissue.
- Muscle tissue - Muscle cells contain contractile filaments that move past each other and change the size of the cell.
- Nervous tissue - Cells forming the brain, spinal cord and peripheral nervous system.

Plant Tissues

Examples of tissue in other multicellular organisms are vascular tissue in plants, such as xylem and phloem. Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
- Epidermis - Cells forming the outer surface of the leaves and of the young plant body.
- Vascular tissue - The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally.
- Ground tissue - Ground tissue is less differentiated than other tissues. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients.

References

- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (1986). Biology of Plants (4th ed.). New York: Worth Publishers. ISBN 0-87901-315-X. Category:Anatomy Category:Tissues ms:Tisu biologi ja:組織 (生物学) simple:Tissue (biological)

Connective tissue

Connective tissue is any type of biological tissue with an extensive extracellular matrix and often serves to support, bind together, and protect organs. There are four basic types:
- Bone contains specialized cells called osteocytes embedded in a mineralized extracellular matrix, and functions for general support.
- Blood functions in transport. Its extracellular matrix is the blood plasma, which transports dissolved nutrients, hormones, and carbon dioxide in the form of bicarbonate. The main cellular component is red blood cells.
- Cartilage makes up virtually the entire skeleton in the osteichthyes. In most other vertebrates, it is found primarily in joints, where it provides cushioning. The extracellular matrix of cartilage is composed primarily of collagen.
- Connective tissue proper
- Dense connective tissue or Fibrous connective tissue forms ligaments and tendons. Its densely packed collagen fibers have great tensile strength.
- Loose connective tissue or Areolar connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibers, including collagen and elastin. It is also important in inflammation.
- Reticular connective tissue is a network of reticular fibers (fine collagen) that form a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, and spleen.)
- Adipose tissue contains adipocytes, used for cushioning, insulation, lubrication (primarily in the pericardium) and energy storage.
-

Disorders of connective tissue

Various connective tissue conditions have been described, these can be both inherited and environmental.
- Marfan syndrome - a genetic disease causing abnormal fibrillin.
- Scurvy - caused by a dietary deficiency in vitamin C, leading to abnormal collagen.
- Ehlers-Danlos syndrome - a genetic disease causing progressive deterioration of collagens, with different EDS types affecting different sites in the body, such as joints, heart valves, organ walls, arterial walls, etc.
- Osteogenesis imperfecta (brittle bone disease) - caused by insufficient production of good quality collagen to produce healthy, strong bones. See aalso: zootomy Category:Tissues

Muscle contraction

A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) shortens. Locomotion is possible only through the repeated contraction of many muscles at the correct times. For most muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to originate from the brain. Reflexes are fast, unconscious muscular reactions that occur due to unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the brain. There are three general types of muscle contractions, skeletal muscle contractions, heart muscle contractions, and smooth muscle contractions.

Skeletal muscle contractions

Skeletal muscles contract according to the sliding-filament model: #An action potential reaches the axon of the motor neuron. #The action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in. #The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber. #The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, and potassium rushes out. However, because sodium is more permeable, the muscle fiber membrane becomes more positively charged, triggering an action potential. #The action potential on the muscle fiber causes the sarcoplasmic reticulum to release calcium. #The calcium binds to the troponin present on the thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically obstructs binding sites for myosin on the thin filament; once calcium binds to the troponin, the troponin forces the tropomyosin move out of the way, unblocking the binding sites. #Myosin (which is bound to ADP and is in a ready state) binds to the newly uncovered binding sites on the thin filament. It then releases ADP and delivers a power stroke. #ATP binds myosin, forcing it to change conformation in such a way as to break the actin-myosin bond, causing myosin to assume its ready state. Myosin then hydrolyzes ATP to ADP and inorganic phosphate. #Steps 7 and 8 repeat as long as calcium is present on thin filament. #All the while, the calcium is actively pumped back into the sarcoplasmic reticulum. When calcium is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the contractions cease. The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sacoplasm. As the calcium ions are being actively pumped by the calcium pumps present in the membrane of the sarcoplasmic reticulum creating a deficiency in the fluid around the myofibrils.This causes the removal of calcium ions from the troponin.Thus the tropomyosin-troponin complex again covers the binding sites on the actin fiaments and contraction ceases.

Smooth muscle contraction

#Contractions are initiated by an influx of calcium which binds to calmodulin. #The calcium-calmodulin complex binds to and activates myosin light-chain kinase. #Myosin light-chain kinase phosphorylates myosin light-chains, causing them to interact with actin filaments. This causes contraction. Category:Physiology Category:Muscular system

Cardiac muscle

Cardiac muscle is a type of striated muscle found within the heart. Its function is to "pump" blood through the circulatory system by contracting. Unlike skeletal muscle, which contracts in response to nerve stimulation, and like smooth muscle, cardiac muscle is myogenic, meaning that it stimulates its own contraction without a requisite electrical impulse. A single cardiac muscle cell, if left without input, will contract rhythmically at a steady rate; if two cardiac muscle cells are in contact, whichever one contracts first will stimulate the other to contract, and so on. This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although the cells are connected by intercalated discs, which conduct electrical potentials directly, rather than the chemical synapses used by neurons. Specialized pacemaker cells normally determine the overall rate of contractions. The nervous system does contact the heart, but only sends signals to speed up or slow down the heart rate, rather than controlling each beat. Since cardiac muscle is myogenic, the pacemaker serves only to modulate the cells; the cardiac muscles would still fire in the absence of a pacemaker, albeit randomly, and the heart would go into fibrillation. Cardiac muscle exhibits cross striations like those seen in skeletal muscle. A unique aspect of cardia muscle is the number of nuclei found inside the cell. Skeletal muscle cells are multinucleated from the fusion of muscle cells and smooth muscle cells are strictly mononucleated, while cardiac muscle cells are mononucleated, binucleated and multinucleated. In the fetus and post parturition infant most cardiac muscle cells are mononucleated. Shortly after birth (within a few months)most cardiac muscles undergo a change of nucleation from mononucleated to primarly binucleated, and some go on to become multinucleated. Generally among species the cardiac muscle is 90% binucleated cells and 5% both mono and multinucleated cells, but exact numbers depend upon the species in question.

Heart

The heart (Latin cor) is a hollow, muscular organ that pumps blood through the blood vessels by repeated, rhythmic contractions. The term cardiac means "related to the heart", from the Greek kardia (καρδια) for "heart".

The human heart

Structure

In the human body, the heart is normally situated slightly to the left of the middle of the thorax, underneath the sternum (breastbone). It is enclosed by a sac known as the pericardium and is surrounded by the lungs. In normal adults, it weighs 250-350 g, but extremely diseased hearts can weigh up to 1000 g. It consists of four chambers, the two upper atria (singular: atrium) and the two lower ventricles. A septum divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. Valves between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles. The function of the right side of the heart (see right heart) is to collect deoxygenated blood from the body and pump it into the lungs so that carbon dioxide can be dropped off and oxygen picked up. this happens through a process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs and pumps it out to the body. On both sides, the lower ventricles are thicker than the upper atria. lung Oxygen-depleted or deoxygenated blood from the body enters the right atrium through two great veins, the superior vena cava which drains the upper part of the body and the inferior vena cava that drains the lower part. The blood then passes through the tricuspid valve to the right ventricle. The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. In the lungs gaseous exchange takes places and the blood releases carbon dioxide into the lung cavity and picks up oxygen. The oxygenated blood then flows through pulmonary veins to the left atrium. From the left atrium this newly oxygenated blood passes through the mitral valve to enter the left ventricle. The left ventricle then pumps the blood through the aorta to the entire body. Even the lungs take some of the blood supply from the aorta via bronchial arteries. The left ventricle is much more muscular (1.3 - 1.5 cm thick) than the right (0.3 - 0.5 cm thick) as it has to pump blood around the entire body, which involves exerting a considerable force to overcome the vascular pressure. As the right ventricle needs to pump blood only to the lungs, it requires less muscle. Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top side. The contractile nature of the heart is due to the presence of cardiac muscle in its wall which can work continuously without fatigue. The heart wall is made of three distinct layers. The first is the outer epicardium which is composed of a layer of flattened epithelial cells and connective tissue. Beneath this is a much thicker myocardium made up of cardiac muscle. The endocardium is a further layer of flattened epithelial cells and connective tissue which lines the chambers of the heart. The blood supply to the heart itself is supplied by the left and right coronary arteries, which branch off from the aorta.

The cardiac cycle

See main page cardiac cycle cardiac cycle cardiac cycle The function of the heart is to pump blood around the body. Every single beat of the heart involves a sequence of events known as the cardiac cycle, which consists of three major stages: atrial systole, ventricular systole and complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood into the ventricles. Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the closing of the valves that produces the familiar beating sounds of the heart, commonly referred to as the "lub-dub" sound. The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.

Regulation of the cardiac cycle

Cardiac muscle is myogenic, which means that it is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger. The rhythmic sequence of contractions is coordinated by the sinoatrial and atrioventricular nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See action potential) that initiates atria contraction. Once the wave reaches the atrioventricular node, situated in the lower right atrium, it is conducted through the bundles of His and causes contraction of the ventricles. The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart. In the event of severe pathology, the Purkinje fibers can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the other pacemakers and hence is overridden.

Other physiological functions

The heart also secretes ANF (atrial natriuretic factor), a powerful peptide hormone, that affects the blood vessels, the adrenal glands, the kidneys and the regulatory regions of the brain to regulate blood pressure and volume.

Diseases and treatments

The study of diseases of the heart is known as cardiology. Important diseases of the heart include:
- Coronary heart disease is the lack of oxygen supply to the heart muscle; it can cause severe pain and discomfort known as Angina.
- A heart attack occurs when heart muscle cells die because blood circulation to a part of the heart is interrupted.
- Congestive heart failure is the gradual loss of pumping power of the heart.
- Endocarditis and myocarditis are inflammations of the heart.
- Cardiac arrhythmia is an irregularity in the heartbeat. It is sometimes treated by implanting an artificial pacemaker
- Congenital heart defects. If a coronary artery is blocked or narrowed, the problem spot can be bypassed with coronary artery bypass surgery or it can be widened with angioplasty. Beta blockers are drugs that lower the heart rate and blood pressure and reduce the heart's oxygen requirements. Nitroglycerin and other compounds that give off nitric oxide are used to treat heart disease as they cause the dilation of coronary vessels. At Groote Schuur Hospital in Cape Town, South Africa, 53-year-old Louis Washkansky on December 3, 1967 became the first human to receive a heart transplant (however he died 18 days later from double pneumonia). The transplant team was headed by Christiaan Barnard. See also: Cardiology diagnostic tests and procedures

First aid

See cardiac arrest for emergencies involving the heart If a person is encountered in cardiac arrest (no heartbeat), cardiopulmonary resuscitation (CPR) should be started, and help called. If an automated external defibrillator is available, this device may automatically administer defibrillation if this is indicated.

The hearts of other animals

Heartbeat

Smaller animals have faster heartbeats. This is evident within a species as well, as the young beat their hearts faster than the adults. The Gray Whale's heart beats 9 times per minute, Harbour Seal 10 when diving, 140 when on land, elephant 25, human 70, sparrow 500, shrew 600, and hummingbird 1,200 when hovering. The earthworm has a series of multiple primitive hearts.

Food use

The hearts of cattle, sheep, pigs and certain fowl are consumed as food in many countries. They are counted among offal, but being a muscle, the taste of heart is much more like regular meat than that of other offal. It resembles venison in structure and taste. Different species have different heart chambers. It can vary from one to four chambers (2 atria and 2 ventricle)

As an icon

The heart may also be illustrated as an icon (♥), symbolizing love. See Heart (symbol).

External links

- [http://www.zygote.com/DF/Heart-Anatomy-Pictures.htm Free 3D Heart Images]
- [http://library.thinkquest.org/C003758/home.htm Very Comprehensive Heart Site]
- [http://www.invisionguide.com/heart The InVision Guide to a Healthy Heart] An interactive website Category:Cardiovascular system Category:Thorax ko:심장 ms:Jantung ja:心臓 simple:Heart

Skeletal muscle

Skeletal muscle is a type of striated muscle, attached to the skeleton. Skeletal muscles are used to facilitate movement, by applying force to bones and joints; via contraction. They generally contract voluntarily (via nerve stimulation), although they can contract involuntarily. Muscles have an elongated, cylindrical shape, and are multinucleated. The nuclei of these muscles are located just under the plasma membrane, which vacates the central part of the muscle fiber for myofibrils. This unique arrangement of the nuclei allows for higher efficiency. These muscles usually have one end (the "origin") attached to a relatively stationary bone, (such as the scapula) and the other end (the "insertion") is attached across a joint, to another bone (such as the humerus). There are two types of fibers for skeletal muscles: Type I and Type II. Type I fibers appear reddish. They are good for endurance and are slow to tire because they use oxidated metabolism. Type II fibers are whitish; they are used for short bursts of speed and power, use anaerobic metabolism, and are therefore quicker to tire.

How skeletal muscle works

The strength of skeletal muscle is directly proportional to its cross-sectional area. The strength of a body, however, is determined by a number of biomechanical principles (the distance between muscle insertions and joints, muscle size, and so on). Muscles are normally arranged in opposition so that as one group of muscles contract, another group relaxes or expands. Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. Once the cells are "excited", their sarcoplasmic reticulums will release ionic calcium (Ca²⁺), this interacts with the myofibrils and, thus, induces muscular contraction (via the sliding filament mechanism). Besides calcium, this process requires adenosine triphosphate (ATP). The ATP is produced by metabolizing creatine phosphate and glycogen, which are stored within the muscle cells; as well by metabolizing glucose and fatty acids, obtained from blood. Each motor neuron "controls" a group of muscle cells, known as "motor units". When more strength is required than can be obtained from a single motor unit, more units will be stimulated; this is known as "motor unit recruitment". If more strength is required than can be obtained from the current degree of unit contraction, the motor neurons will send additional stimuli; this causes a process of contractile summation, which increases the degree of contraction. If a muscle is maximally contracted, it is said to be in a state of tetanic contraction.

Red and white fibers

Skeletal muscles contain two types of fibers, which differ in the mechanism they use to produce ATP; the amount of each type of fibre varies from muscle to muscle and from person to person.
- Red ("slow-twitch") fibers have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, and are associated with endurance; these produce ATP more slowly. Marathoners tend to have more red fibers.
- White ("fast-twitch") fibers have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, and are more likely to accumulate lactic acid. Weightlifters and Sprinters tend to have more white fibers.

Characteristics of muscle types

Fibre Type	Type I fibres	Type II A fibres	Type II B fibres
Contraction time	Slow	Fast	Very Fast
Size of motor neuron	Small	Large	Very Large
Resistance to fatigue	High	Intermediate	Low
Activity Used for	Aerobic	Long term anaerobic	Short term anaerobic
Force production	Low	High	Very High
Mitochondrial density	High	High	Low
Capillary density	High	Intermediate	Low
Oxidative capacity	High	High	Low
Glycolytic capacity	Low	High	High
Major storage fuel	Triglycerides	CP, Glycogen	CP, Glycogen

Genes that define skeletal muscle phenotype

Skeletal muscle fiber-type phenotype is regulated by several independent signaling pathways. These include pathways involved with the Ras/mitogen-activated protein kinase (MAPK), calcineurin, calcium/calmodulin-dependent protein kinase IV, and the peroxisome proliferator γ coactivator 1 (PGC-1). The Ras/MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve-dependent induction of the slow program in regenerating muscle. Calcineurin, a Ca2+/calmodulin-activated phosphatase implicated in nerve activity-dependent fiber-type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow-type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins. Calcium-dependent Ca2+/calmodulin kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow-type calcineurin-generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis. Contraction-induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium/calmodulin-dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle. Sprinter PGC1-α, a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective ST muscle genes and also serves as a target for calcineurin signaling. A peroxisome proliferator-activated receptor δ (PPARδ)-mediated transcriptional pathway is involved in the regulation of the skeletal musclefiber phenotype. Mice that harbor an activated form of PPARd display an “endurance” phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers. Thus—through functional genomics—calcineurin, calmodulin-dependent kinase, PGC-1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against insulin resistance and obesity. The transition from aerobic to anaerobic metabolism during intense work requires that several systems are rapidly activated to ensure a constant supply of ATP for the working muscles. These include a switch from fat-based to carbohydrate-based fuels, a redistribution of blood flow from nonworking to exercising muscles, and the removal of several of the byproducts of anaerobic metabolism, such as carbon dioxide and lactic acid. Some of these responses are governed by transcriptional control of the FT glycolytic phenotype. For example, skeletal muscle reprogramming from a ST glycolytic phenotype to a FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family. Moreover, the Hypoxia Inducible Factor-1α (HIF-1α) has been identified as a master regulator for the expression of genes involved in essential hypoxic responses that maintain ATP levels in cells. Ablation of HIF-1α in skeletal muscle was associated with an increase in the activity of rate-limiting enzymes of the mitochondria, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway in these animals. However, hypoxia-mediated HIF-1α responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria. Category:Muscular system Category:Somatic motor system ms:Otot rangka ja:骨格筋

Tendons

:Tendon is also the name of a commune in the Vosges département in France. :Tendon is also the abbreviated word of Tenpura-Donburi. :Sinew is also a part of a structure or system that provides support and holds it together. A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone. They are similar to ligaments except that ligaments join one bone to another.

Composition

ligament]Tendons are composed mainly of water, type-I collagen and cells called tenocytes. Minor fibrillar collagens, fibril-associated collagens and proteoglycans are present in small quantities and are critical for tendon structure. Most of the strength of tendon is due to the parallel, hierarchical arrangement of densely-packed collagen fibrils. Tenocytes are specialised fibroblasts responsible for the maintenance of collagen structure.

Anatomy

The origin of a tendon is where it joins to a muscle. Collagen fibers from within the muscle organ are continuous with those of the tendon. A tendon inserts into bone at an enthesis where the collagen fibres are mineralised and integrated into bone tissue. Tenocytes produce collagen molecules which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organised by tenocytes to form fibres. Collagen fibres coalesce into macroaggregates. Groups of macroaggregates are bounded by connective tissue endotendon and are termed fascicles. Groups of fascicles are bounded by the epitendon and peritendon to form the tendon organ. Blood vessels may be visualised within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses. The internal tendon bulk is thought to contain no nerve fibres, however the epi- and peritendon contain nerve endings, while Golgi tendon organs are present at the junction between tendon and muscle. Tendonitis refers to swelling of a tendon. Achilles tendon is a particularly large tendon connecting the heel to the muscles of the calf. It is so named because the mythic hero Achilles was said to have been killed due to an injury at this spot. Sinew was also widely used in the medieval times as a form of ancient elastic.

References

Tendons & Ligaments in Journal of Musculoskeletal & Neuronal Interactions; 2005, 5(1): Eds. Lyritis GP & Jee WSS Category:Musculoskeletal system

Bone

, a typically recognized bone.]] Bone, also called osseous tissue, (Latin: "os") is a type of hard endoskeletal connective tissue found in many vertebrate animals. Bones support body structures, protect internal organs, and (in conjunction with muscles) facilitate movement; are also involved with cell formation, calcium metabolism, and mineral storage. The bones of an animal are, collectively, known as the skeleton. Bone has a different composition than cartilage, and both are derived from mesoderm. In common parlance, cartilage can also be called "bone", certainly when referring to animals that only have cartilage as hard connective tissue, such as cartilaginous fish (Chondrichthyes) like sharks. True bone is present in bony fish (Osteichthyes) and all tetrapods. There are several evolutionary alternatives to bone. These evolutionary solutions are not completely functionally analogous to bone.
- Exoskeletal protection is offered by shells, carapaces (consisting of calcium compounds or silica) and chitinous exoskelotons.
- A true endoskeleton (that is, protective tissue derived from mesoderm) is also present in Echinoderms. Porifera (sponges) possess simple endoskeletons that consist of calcareous or siliceous spicules and a spongin fiber network. Bones and skeletons are studied in osteology. Bones can be prepared for study by several methods, such as maceration. Maceration is done by boiling fleshed bone with dish detergent and a little bleach until all large particles are off. The bones are then cleaned by hand, usually with a toothbrush and a degreaser.

Functions

Long bones can be connected to muscles via tendons. Bones connect at joints by ligaments. The interaction between bone and muscle is studied in biomechanics.

Post-mortem functions

Cut and polished bone from a variety of animals is sometimes used as material for jewelry and other crafts. Ground cattle bone is sometimes used as fertilizer. In the Stone Age bone was used to manufacture art, weapons, needles, etc.

Structure

art art Bone is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite. It has relatively high compressive strength but poor tensile strength. While bone is essentially brittle, it does have a degree of significant elasticity contributed by its organic components (chiefly collagen). Bone has an internal mesh-like structure, the density of which may vary at different points. Bone can be either compact or cancellous (spongy). Cortical (outer layer) bone is compact; the two terms are often used interchangeably. Cortical bone makes up a large portion of skeletal mass; but, because of its density, it has a low surface area. Cancellous bone is trabecular (honeycomb structure), it has a relatively high surface area, but forms a smaller portion of the skeleton. Bone can also be either woven or lamellar. Woven bone is put down rapidly during growth or repair. It is so called because its fibres are aligned at random, and as a result has low strength. In contrast lamellar bone has parallel fibres and is much stronger. Woven bone is often replaced by lamellar bone as growth continues. Long bones are tubular in structure (e.g. the tibia). The central shaft of a long bone is called the diaphysis, and has a hollow middle—the medullar cavity filled with bone marrow. Surrounding the medullar cavity is a thin layer of cancellous bone that also contains marrow. The extremities of the bone are called the epiphyses and are mostly cancellous bone covered by a relatively thin cortical of compact bone. In children, the bones are filled with red marrow, which is gradually replaced with yellow marrow as the child ages. Short bones (e.g. finger bones) have a similar structure to long bones, except that they have no medullar cavity. Flat bones (e.g. the skull and ribs) consist of two layers of compact bone with a zone of cancellous bone sandwiched between them. Irregular bones are bones which do not conform to any of the previous forms (e.g. vertebrae). All bones consist of living cells embedded in a mineralised organic matrix that makes up the main bone material.

Cells

Bone Heads include osteoblasts, so called Bone Lining Cells, osteocytes and osteoclasts. Osteoblasts are typically viewed as bone forming cells. They are located near to the surface of bone and their functions are to make osteoid and manufacture hormones such as prostaglandin which act on bone itself. Osteoblasts are mononucleate. Active osteoblasts are situated on the surface of osteoid seams and communicate with each other via gap-junctions. They contain alkaline phosphatase—a chemical which has a role in the mineralisation of bone. Bone Lining Cells (BLCs) share a common lineage with osteogenesis (bone forming) cells. They function as a barrier for certain ions, induced osteogenetic cells. They are flattened, mononucleate cells which line bone. However, osteocytes do originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The space which they occupy is known as a lacuna. Osteocytes have many processes which reach out to meet osteoblasts probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. They possibly act as mechano-sensory receptors—regulating the bones' response to stress. If osteoblasts can be described as bone forming cells, the osteoclasts can be described as bone destroying cells. Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae. These lacunae, or resorption pits, are left behind after the breakdown of bone and often present as scalloped surfaces. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with engulfment strategies similar to circulating macrophages. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival active enzymes, such as acid phosphatase, are secreted against the mineral substrate. This process, called bone resorption, allows stored calcium to be released into systemic circulation and is an important process in regulating calcium balance. As bone formation actively fixes circulating calcium in its mineral form, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations and are known as bone turnover, or remodeling. Osteoblasts and osteoclasts, coupled together via paracrine cell signalling, are referred to as bone remodeling units. The iteration of remodeling events at the cellular level is influential on shaping and sculpting the skeleton both during growth as well as after.

Matrix

The matrix comprises the other major constituent of bone. It has inorganic and organic parts. The inorganic is mainly crystalline mineral salts and calcium, which is present in the form of hydroxyapatite. The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting vesicles containing alkaline phosphatase. This cleaves phosphate groups and acts as the foci for calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on. The organic part of matrix is mainly Type I collagen. This is made intracellularly as tropocollagen and then exported. It then associates into fibrils. Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include GAGs, osteocalcin, osteonectin, bone sialo protein and Cell Attachment Factor.

Formation

bone sialo protein The formation of bone occurs by two methods: intramembranous and endochondral ossification. Intramembranous ossification mainly occurs during formation of the flat bones of the skull; the bone is formed from mesenchyme tissue. Endochondral ossification occurs in long bones, such as limbs; the bone is formed from cartilage. Endochondral ossification begins with points in the cartilage called "primary ossification centers." They mostly appear during fetal development, though a few short bones begin their primary ossification after birth. They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones. The diaphyses and the epiphyses of long bones remain separated by a growing zone of cartilage (the metaphysis) until the child reaches skeletal maturity (18 to 25 years of age), whereupon the cartilage ossifies, fusing the two together (epiphyseal closure). Marrow can be found in most any bone that holds cancellous tissue. In newborns, all such bones are filled exclusively with red marrow (or hemopoietic marrow), but as the child ages it is mostly replaced by yellow marrow (or fatty marrow). In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones. Remodeling is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. Its purpose is the release of calcium and the repair of micro-damaged bones (from everyday stress). Repeated stress results in the bone thickening at the points of maximum stress. It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.

Bone pathologies

One of the most common bone illnesses is a bone fracture. Bones heal by natural processes, but untended and unsupported can lead to misgrown bone. Other illnesses are for example osteoporosis and bone cancer (osteosarcoma). The joints can be affected by arthritis.

Terminology

: There are also names for specific parts of long bones. :

External links

- [http://silver.neep.wisc.edu/~lakes/BoneElectr.html Review (including references) of piezoelectricity and bone remodelling] Category:Anatomy Category:Skeletal system Category:Bone products ko:뼈 ja:骨 simple:Bone

Locomotion

In a general sense, locomotion simply means active movement or travel, applying not just to biological individuals.
- In biology, locomotion is the self-powered, patterned motion of limbs or other anatomical parts by which an individual customarily moves itself from place to place. Forms of locomotion are walking, running, crawling, climbing, swimming and flying. See animal locomotion, plantigrade, digitigrade, unguligrade, and fish locomotion.
- In engineering, Locomotion No. 1 is the name of an early steam railway locomotive, even older than Stephenson's Rocket.
- Chris Sawyer's Locomotion is a computer game by Chris Sawyer. It was released on September 7 2004.
- Locomotion is a 1981 arcade game by Konami.
- "The Loco-Motion" is a 1960s hit song and accompanying dance, which was covered in the 1980s by Kylie Minogue.
- "Locomotion" is a museum in the town of Shildon, County Durham, England
- "Locomotion" was a cable TV channel in Latin America. ja:ロコモーション

Intestine

right The intestine is the portion of the alimentary canal extending from the stomach to the anus and, in humans and mammals, consists of two segments, the small intestine and the large intestine. In humans, the small intestine is further subivided into the duodenum, jejunum and ileum while the large intestine is subdivided into the cecum, colon and rectum. The intestine is the part of the body responsible for extracting nutrition from food. While the stomach's role mainly consists in "breaking" food molecules into nutrients, the intestine allows these nutrients to enter the blood via its dedicated membrane. The small intestine has a particular folded texture in order to increase the surface area available for diffusion of nutrients through the intestinal wall so they can be absorbed. These microscopic folds are called microvilli. In an adult human, the small intestine is, on average, about seven meters long. The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to destroy itself. This is an example of symbiosis. These bacteria also account for the production of gases inside our intestine (which is released as flatulence when removed through the anus).

Diseases of the intestine

Gastroenteritis is inflammation of the intestines and is the most common disease of the intestines. It can arise as the result of food poisoning. Ileus is a blockage of the intestines. Appendicitis is inflammation of the vermiform appendix located at the cecum. This is a potentially fatal disease if left untreated; most cases of appendicitis will require surgical intervention. Crohn's disease and ulcerative colitis are examples of autoimmune diseases affecting the intestines. Category:Digestive system Category:Gastroenterology

Blood vessel

The blood vessels are part of the circulatory system and function to transport blood throughout the body. The most important types, arteries and veins, are so termed because they carry blood away from or towards the heart, respectively.

Types

Blood vessels exist in varying calibers:
- Arteries
- Aorta (the largest artery, carries blood out of the heart)
- Branches of the aorta, such as the carotid artery, the subclavian artery, the celiac trunk, the mesenteric arteries, the renal artery and the ileac artery.
- Arterioles
- Capillaries (the smallest blood vessels)
- Venules
- Veins
- Large collecting vessels, such as the subclavian vein, the jugular vein, the renal vein and the iliac vein.
- Venae cavae (the 2 largest veins, carry blood into the heart) They are roughly grouped as arterial and venous, determined by whether the blood in it is flowing toward or away from the heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary artery carries "venous blood" and blood flowing in the pulmonary vein is rich in oxygen.

Anatomy

All blood vessels follow the same histological makeup. The inner lining is the endothelium, followed by subendothelial connective tissue. Then follows a muscular layer of vascular smooth muscle, which is highly developed in arteries. Finally, there is a further layer of connective tissue termed the adventitia, which contains nerves that supply the muscular layer, as well as nutrient capillaries in the larger blood vessel. Capillaries consist of little more than a layer of endothelium and occasional connective tissue. In anatomy, the term for when a blood vessel joins another to form a region of diffuse vascular supply is known as anastamosis. This is important in several areas around the body, as blockages in one area can mean that anastamoses (plural of anastamosis) makes an alternative route for blood flow.

Physiology

Blood vessels do not actively engage in the transport of the blood (they have no appreciable peristalsis), but arteries - and veins to a degree - can regulate their caliber by contraction of the muscular layer. This determines the blood flow to particular downstream organs, and is determined by the autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as a method of thermoregulation in homeotherms. Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95-100%) with oxygen. In all veins apart from the pulmonary vein, the hemoglobin is desaturated at about 70%. (The values are reversed in the pulmonary circulation.) The blood pressure in blood vessels is traditionally expressed in millimetres of mercury (1 mmHg = 133 Pa). In the arterial system, this is usually around 120 mmHg systolic (high pressure wave due to contraction of the heart) and 80 mmHg diastolic (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg. Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth muscle in the vessel walls. It is regulated by vasoconstrictors (agents that cause vasoconstriction). These include paracrine factors (e.g. prostaglandins), a number of hormones (e.g. vasopressin and angiotensin) and neurotransmitters (e.g. adrenalin) from the nervous system. Vasodilation is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason). Permeability of the endothelium is pivotal in the release of nutrients to the tissue. It is also increased in inflammation in response to histamine, prostaglandins and interleukins, which leads to most of the symptoms of inflammation (swelling, redness and warmth).

Role in disease

Blood vessels play a role in virtually every medical condition. Cancer, for example, cannot progress if the tumor does not cause angiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand. Atherosclerosis, the formation of lipid lumps (atheromas) in the blood vessel wall, is the prime cause of cardiovascular disease, the main cause of death in the Western world. Blood vessel permeability is increased in inflammation. Damage, due to trauma or spontaneously, may lead to hemorrhage. In contrast, occlusion of the blood vessel (e.g. by a ruptured atherosclerotic plaque, by an embolised blood clot or a foreign body) leads to downstream ischemia (insufficient blood supply) and necrosis (tissue breakdown). Vasculitis is inflammation of the vessel wall, due to autoimmune disease or infection.

Sarcomere

A sarcomere is the basic unit of a cross striated muscle's myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems. The thick filament system is composed of myosin protein, the thin filaments are assembled by actin monomers and the elastic filament system is composed of the giant protein titin (also called connectin). A muscle cell, from a bicep, may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres. The sarcomeres are what give skeletal and cardiac muscles their striated appearance. A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs). In electron micrographs of cross striated muscle the Z-line appears as a series of dark lines. Surrounding the Z-disc is the region of the I-band. Following the I-band is the A-band. Within the A-band is a paler region called the H-band. Finally, inside the H-band is a thin M-line (or M-band). A-bands and I-bands were named after anisotropic and isotropic, respectively; their properties under a polarizing microscope. Actin filaments are the major component of the I-band and extend into the A-band. Myosin filaments extend throughout the A-band and are thought to overlap in the M-band. The giant protein titin (connectin) extends from the Z-disc of the sarcomere, where it binds to the thin filament system, to the M-band, where it is thought to interact with the thick filaments. The Titin protein (and its splice isoforms) is the biggest single protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as sarcomeric ruler and as blueprint for the assembly of the sarcomere. Several proteins important for the stability of the sarcomeric structure are found in the Z-disc as well as in the M-band of the sarcomere. Actin filaments and Titin molecules are cross-linked in the Z-disc via the Z-disc protein alpha-Actinin. The M-band proteins Myomesin as well as M-protein crosslink the thick filament system (Myosins) and the M-band part of Titin (the elastic filaments). The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the muscle contraction (sliding filament model). Upon muscle contraction, the A-bands maintain their length (1.6 micrometer in mammalian skeletal muscle) whereas the I-bands shorten. The A-band, I-band and Z-line are the only components visible at the light-microscope level. The protein tropomyosin covers the myosin binding sites of the actin molecules in the muscle cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcuim ions bind with troponin molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross bridge binding site on the actin. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum. Muscle contraction ends when calcium ions are pumped back out of the sarcomere. Skeletal muscle only contracts when an impulse is received from a motor neuron. During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine which travels across the neuromuscular junction (the synapse between the terminal button of the neuron and the muscle cell). The action potential then travels along T (transverse) tubules until it reaches the sarcoplasmic reticulum; the action potential from the motor neuron changes the permeability of the sarcoplasmic reticulum, allowing the flow of calcium ions into the sarcomere. The outflow of calcium allows the myosin heads access to the actin cross bridge binding sites, permitting muscle contraction. At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into ADP and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and inorganic phosphate ion, changing its configuration back to one of low energy. As the filament of actin moves away from the myosin head and back toward the center of the sarcomere, the myosin head is unable to preserve its bond with the actin. After cross bridge dissociation, ATP binds with the myosin head and the head is ready for another cycle of muscle contraction. Most muscle cells only store enough ATP for a small number of muscle contractions. While muscle cells also store glycogen, most of the energy required for contraction is derived from phosphagens. One such phosphagen is creatine phosphate, which is used to provide ADP with a phosphate group for ATP synthesis in vertebrates. Category:Muscular system

Capillary

Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm. They connect arteries and veins, and most closely interact with tissues. Capillaries have walls composed of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillary permeability can be increased by the release of certain cytokines. The endothelium also actively transports nutrients, messengers and other substances. Large molecules may be too big to diffuse across endothelial cells. In some cases, vesicles contained in the capillary membrane use endocytosis and exocytosis to transport material between blood and the tissues. In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus "catching" immune cells as they pass by the site of infection and aiding extravasation of these cells into the tissue. The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients. The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through autoregulation (e.g. active muscle cells) by constricting smooth muscle.

Types

Capillaries come in 3 types:
- Continuous - Continuous capillaries have a sealed epithelium and only allow small molecules, water and ions to diffuse.
- Fenestrated - Fenestrated capillaries (as thier name implies "fenster") have opening that allow larger molecules to diffuse.
- Sinusoidal - Sinusoidal capillaries are special forms of fenestrated capillaries that have larger opening allowing RBCs and serum proteins to enter.

Details

The total length of capillaries in an average adult human is approximately 40,000 km (25,000 mi). Category:Cardiovascular system ja:毛細血管

Myoglobin

Myoglobin is a single-chain protein of 153 amino acids, containing a heme (iron-containing porphyrin) group in the center. With a molecular weight of 16,700 Daltons, it is the primary oxygen-carrying pigment of muscle tissues. Unlike the blood-borne hemoglobin, to which it is structurally related, this protein does not exhibit cooperative binding of oxygen. Instead, the binding of oxygen by myoglobin is unaffected by the oxygen pressure in the surrounding tissue. In 1957, John Kendrew and associates successfully determined the structure of myoglobin by high-resolution X-ray crystallography. X-ray crystallography For this discovery, John Kendrew shared the 1962 Nobel Prize in chemistry with Max Perutz.

Role in disease

Myoglobin is the putative protein that causes acute renal failure in rapid breakdown of muscle (e.g. rhabdomyolysis, severe crush trauma, malignant hyperthermia, status epilepticus and neuroleptic malignant syndrome), due to its toxicity to renal tubular epithelium. Myoglobin is a sensitive marker for muscle injury, making it a potential marker for myocardial infarction in patients with chest pain. Its specificity and the cost of the analysis has prevented its widespread use.

External links

- oxy-myoglobin at 0.1 nm resolution: PDB [http://www.rcsb.org/pdb/cgi/explore.cgi?pid=105291034356398&page=0&pdbId=1A6M 1A6M]
- Sperm whale myoglobin at 0.17 nm resolution: PDB [http://www.rcsb.org/pdb/cgi/explore.cgi?pid=255731034609024&page=80&pdbId=1VXH 1VXH]
- [http://www.rcsb.org/pdb/molecules/mb1.html Protein Database featured molecule]
- human genetics Category:Biochemistry Category:Hemoproteins ja:ミオグロビン

Aerobic metabolism

Cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Oxidation of organic material—in a bonfire, for example—is an exothermic reaction that releases a large amount of energy rather quickly. The equation for the oxidation of glucose is: :::C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy released In a fire there is a massive uncontrolled release of energy as light and heat. Cellular respiration is the same process but it occurs in gradual steps that result in the conversion of the energy stored in glucose to usable chemical energy in the form of ATP.

Aerobic respiration

exothermic reaction Aerobic respiration requires oxygen in order to generate energy. It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion to be fully oxidised by the Krebs cycle. The product of this process is energy in the form of ATP, by substrate-level phosphorylation, NADH and FADH2. The reducing potential of NADH and FADH2 is converted to more ATP via an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by cellular respiration is by oxidative phosphorylation, ATP molecules are made due to the chemiosmotic potential driving ATP synthase. Respiration is the process by which cells obtain energy when oxygen is present in the cell. Theoretically, 36 ATP molecules can be made per glucose during cellular respiration, however, such conditions are generally not realized due to such losses as the cost of moving pyruvate into mitochondria. Aerobic metabolism is rather more efficient than anaerobic metabolism. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxydative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and at the cell membrane in prokaryotic cells.

Glycolysis

:Main article: Glycolysis Glycolysis is a metabolic pathway that is found in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced but two are consumed for the preparatory phase. The initial phosphorylation of glucose is required to destablise the molecule for cleavage into two triose sugars. During the pay-off phase of glycolysis four phosphate groups are transfered to ADP by substrate-level phosphorylation to make four ATP and two NADH are produced when the triose sugars are oxidised. Glycolysis takes place in the cytoplasm of the cell. The overall reaction can be expressed this way: :Glucose + 2 ATP + 2 NAD⁺ + 2 P_i + 4 ADP → 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H₂O + 4 H⁺

Oxidative decarboxylation

:Main Article: Oxidative decarboxylation Produces acetyl-CoA from pyruvate. This oxidation reaction also releases carbon dioxide as a product.

Krebs cycle/Citric Acid cycle

:Main article: Citric acid cycle When oxygen is present, acetyl-CoA enters the citric acid cycle, and gets oxidised to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron tranport chain to create further ATP as part of oxidative phosphorylation.

Oxidative phosphorylation

In eukaryotes, oxidative phosphorylation ocurrs in the mitochondria. It comprises of the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidising the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP.

Theoretical yields

The yields in the table below are for one glucose and molecule being fully oxidised to carbon dioxide. It is assumed that all the reduced coenzymes are oxidised by the electron transport chain and used for oxidative phosphorylation.

Anaerobic respiration

In the absence of oxygen pyruvate is not metabolized by cellular respiration, but undergoes fermentation.

External links

- [http://www.people.virginia.edu/~rjh9u/glycol.html A detailed diagram of glycolysis]
- [http://departments.oxy.edu/biology/bio130/lectures_2000/metabolic_products.htm Chart of Important Metabolic Products]
- [http://www.ufp.pt/~pedros/bq/respi.htm A detailed description of respiration vs. fermentation]
- [http://www.ufp.pt/~pedros/anim/2frame-iien.htm Interactive Molecular models of electron-transfer complexes] Category:Cellular respiration Category:Metabolism ja:呼吸

Anaerobic metabolism

Glycolysis is a series of biochemical reactions by which a molecule of glucose (Glc) is oxidized to two molecules of pyruvic acid (Pyr)<