# Human Physiology/The Muscular System

The **muscular system** is the biological system of humans that produces
movement. The muscular system, in vertebrates, is controlled through the
nervous system, although some muscles, like cardiac muscle, can be
completely autonomous. **Muscle** is contractile tissue and is derived
from the mesodermal layer of embryonic germ cells. Its function is to
produce force and cause motion, either locomotion or movement within
internal organs. Much of muscle contraction occurs without conscious
thought and is necessary for survival, like the contraction of the heart
or peristalsis, which pushes food through the digestive system.
Voluntary muscle contraction is used to move the body and can be finely
controlled, such as movements of the finger or gross movements that of
the biceps and triceps.!Muscle
structure{width="300"}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. These bundles are then grouped together to form
muscle, and is lined by epimysium. Muscle spindles are distributed
throughout the muscles, and provide sensory feedback information to the
central nervous system. Skeletal muscle, which involves muscles from the
skeletal tissue, is arranged in discrete groups. An example is 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, respectively).!A top-down view of skeletal
muscle{width="300"}There
are approximately 640 skeletal muscles in the human body (see list of
muscles of the human body). Contrary to popular belief, the number of
muscle fibers cannot be increased through exercise; instead the muscle
cells simply get bigger. It is however believed that myofibrils have a
limited capacity for growth through hypertrophy and will split if
subject to increased demand. There are three basic types of muscles in
the body (smooth, cardiac, and skeletal). While they differ in many
regards, they all use actin sliding against myosin to create muscle
contraction and relaxation. In skeletal muscle, contraction is
stimulated at each cell by nervous impulses that releases acetylcholine
at the neuromuscular junction, creating action potentials along the cell
membrane. All skeletal muscle and many smooth muscle contractions are
stimulated by the binding of the neurotransmitter acetylcholine.
Muscular activity accounts 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. Glycogen can be rapidly converted to
glucose when more energy is necessary.

## Types

There are three types of muscles:

![](Illu_muscle_tissues.jpg‎ "Illu_muscle_tissues.jpg‎")

-   **Smooth muscle** or \"involuntary muscle\" consists of spindle
    shaped muscle cells found within the walls of organs and structures
    such as the esophagus, stomach, intestines, bronchi, uterus,
    ureters, bladder, and blood vessels. Smooth muscle cells contain
    only one nucleus and no striations.

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-   **Cardiac muscle** is also an \"involuntary muscle\" but it is
    striated in structure and appearance. Like smooth muscle, cardiac
    muscle cells contain only one nucleus. Cardiac muscle is found only
    within the heart.

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-   **Skeletal muscle** or \"voluntary muscle\" is anchored by tendons
    to the bone and is used to effect skeletal movement such as
    locomotion. Skeletal muscle cells are multinucleated with the nuclei
    peripherally located. Skeletal muscle is called \'striated\' because
    of the longitudinally striped appearance under light microscopy.
    Functions of the skeletal muscle include:
    -   Support of the body
    -   Aids in bone movement
    -   Helps maintain a constant temperature throughout the body
    -   Assists with the movement of cardiovascular and lymphatic
        vessels through contractions
    -   Protection of internal organs and contributing to joint
        stability

Cardiac and skeletal muscle are striated in that they contain sarcomere
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 several 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, *fast twitch*, muscle has three major kinds that are, in
    order of increasing contractile speed:
    -   a\) Type IIa, which, like slow muscle, is aerobic, rich in
        mitochondria and capillaries and appears red.
    -   b\) Type IIx (also known as type IId), which is less dense in
        mitochondria and myoglobin. This is the fastest muscle type in
        humans. It can contract more quickly and with a greater amount
        of force than oxidative muscle, but can sustain only short,
        anaerobic bursts of activity before muscle contraction becomes
        painful (often attributed to a build-up of lactic acid). N.B. in
        some books and articles this muscle in humans was, confusingly,
        called type IIB
    -   c\) Type IIb, which is anaerobic, glycolytic, \"white\" muscle
        that is even less dense in mitochondria and myoglobin. In small
        animals like rodents or rabbits this is the major fast muscle
        type, explaining the pale color of their meat.

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.

## Muscular System Working With Other Body Systems

-   1\. Homeostasis
-   2\. Protection
-   3\. Calcium Metabolism
-   4\. Maintaining Body Temperature

## Skeletal Muscle Contractions

Steps of a skeletal muscle contraction:

-   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 skeletal muscle fiber is excited by large myelinated nerve
    fibers which attach to the neuromuscular junction. There is one
    neuromuscular junction for each 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 ions(Ca++).
-   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 cross-bridge; once calcium binds to the troponin, the
    troponin forces the tropomyosin to move out of the way, unblocking
    the binding sites.
-   The cross-bridge (which is already in a ready-state) binds to the
    newly uncovered binding sites. It then delivers a power stroke.
-   ATP binds the cross-bridge, forcing it to conform in such a way as
    to break the actin-myosin bond. Another ATP is split to energize the
    cross bridge again.
-   Steps 7 and 8 repeat as long as calcium is present on thin filament.
-   Throughout this process, the calcium is actively pumped back into
    the sarcoplasmic reticulum. When no longer present on the thin
    filament, the tropomyosin changes back to its previous state, so as
    to block the binding sites again. The cross-bridge then ceases
    binding to the thin filament, and the contractions cease as well.
-   Muscle contraction remains as long as Ca++ is abundant in
    sarcoplasm.

Types of Contractions:

-   Isometric contraction\--muscle does not shorten during contraction
    and does not require the sliding of myofibrils but muscles are
    stiff.
-   Isotonic contraction\--inertia is used to move or work. More energy
    is used by the muscle and contraction lasts longer than isometric
    contraction. Isotonic muscle contraction is divided into two
    categories: concentric, where the muscle fibers shorten as the
    muscle contracts (ie. biceps brachialis on the up phase of a biceps
    curl); and eccentric, where the muscle fibers lengthen as they
    contract (ie. biceps brachialis on the down phase of a biceps curl).
-   Twitch\--exciting the nerve to a muscle or by passing electrical
    stimulus through muscle itself. Some fibers contract quickly while
    others contract slowly.
-   Tonic -maintaining postural tone against the force of gravity.

The Efficiency of Muscle Contraction:

-   Only about 20% of input energy converts into muscular work. The rest
    of the energy is heat.
-   50% of energy from food is used in ATP formation.
-   If a muscle contraction is slow or without movement, energy is lost
    as maintenance heat.
-   If muscle contraction is rapid, energy is used to overcome friction.

Summation of Muscle Contraction: It is the adding together of individual
muscle twitches to make strong muscle movements.

-   Multiple motor unit summation\--increasing number of motor units
    contracting simultaneously.
-   Wave summation\--increasing rapidity of contraction of individual
    motor units.
-   Tetanization\--higher frequency successive contractions fuse
    together and cannot be distinguished from one another.

## Sliding Filament theory

When a muscle contracts, the actin is pulled along myosin toward the
center of the sarcomere until the actin and myosin filaments are
completely overlapped. The H zone becomes smaller and smaller due to the
increasing overlap of actin and myosin filaments, and the muscle
shortens. Thus when the muscle is fully contracted, the H zone is no
longer visible (as in the bottom diagram, left). Note that the actin and
myosin filaments themselves do not change length, but instead slide past
each other.

### Cellular Action of Skeletal Muscles

During cellular respiration the mitochondria, within skeletal muscle
cells, convert glucose from the blood to carbon dioxide and water in the
process of producing ATP (see  cell
physiology). ATP is needed
for all muscular movement. When the need of ATP in the muscle is higher
than the cells can produce with aerobic respiration, the cells will
produce extra ATP in a process called anaerobic respiration. The first
step of aerobic respiration(glycolysis) produces two ATP per glucose
molecule. When the rest of the aerobic respiration pathway is occupied
the pyruvate molecule can be converted to lactic acid. This method
produces much less ATP than the aerobic method, but it does it faster
and allows the muscles to do a bit more than if they relied solely on
ATP production from aerobic respiration. The drawback to this method is
that lactic acid accumulates and causes the muscles to fatigue. They
will eventually stop contracting until the breakdown of lactic acid is
sufficient to allow for movement once again. People experience this most
noticeably when they repeatedly lift heavy things such as weights or
sprint for a long distance. Muscle soreness sometimes occurs after
vigorous activity, and is often misunderstood by the general public to
be the result of lactic acid buildup. This is a misconception because
the muscle does fatigue from lactic acid buildup, but it does not stay
in the muscle tissue long enough to cause tissue breakdown or soreness.
During heavy breathing, following exercise, the cells are converting the
lactic acid either back into glucose or converting it to pyruvate and
sending it through the additional steps of aerobic respiration. By the
time a person is breathing normally again the lactic acid has been
removed. The soreness is actually from small tears in the fibers
themselves. After the fibers heal they will increase in size. The number
of mitochondria will also increase if there is continued demand for
additional ATP. Hence, through exercise the muscles can increase in both
strength and endurance.

Another misconception is that as the muscle increases in size it also
gains more fibers. This is not true. The fibers themselves increase in
size rather than in quantity. The same holds true for adipose
tissue\--fat cells do not increase in number, but rather the amount of
lipids (oil) in the cells increase.

Muscle fibers are also genetically programmed to reach a certain size
and stop growing from there, so after a while even the hardest working
weightlifter will only reach a certain level of strength and endurance.
Some people will get around this by taking steroids. The artificial
steroids cause all sorts of trouble for the person. They can cause the
adrenal glands to stop producing corticosteroids and glucosteroids. This
leads to the atrophy of the gland\'s medulla and causes permanent loss
of the production of these hormones. The testicles may also atrophy in
response to steroids. Eventually the testes will stop making
testosterone and sperm, rendering the male infertile.

One of the more serious problems associated with abnormal gain of muscle
mass is heart failure. While for most people gaining muscle and losing
fat is desirable, a body builder is at risk of producing more muscle
mass than the heart can handle. One pound of fat contains about 3.5
miles of blood vessels, but one pound of muscle has about 6.5 miles.
Hence, additional muscle causes the heart to pump more blood. Some
people that have too much muscle will be very strong but will not have a
healthy aerobic endurance, in part because of the difficulty of
providing oxygenated blood to so much tissue.

Sliding filament theory:This link shows the animation of the sliding filament theory.\
explanation and image of sliding filament theory:this link gives a better demonstration of the theory with the explanation.

## Involuntary Muscle Movement

Spasms

When Smooth and skeletal muscles go through multiple spasms it is
referred either as seizure or convulsion.

Cramps

Strenuous activities can cause painful spasms that are long, this is
referred to as cramps.

## Injury

Sprain

An injury to a joint that involves a stretched or torn ligament.

Muscle Strain

A strain occurs when a muscle or the tendon that attaches it to the bone
is overstretched or torn. Muscle strains are also called pulled muscles.
Who gets it?

Anyone can strain a muscle. However, people involved in sports or other
forms of strenuous exercise are more likely to strain a muscle. *What
causes it?*

Muscles are bunches of fibers that can contract. Muscle strains usually
occur during activities that require the muscle to tighten forcefully.
The muscle is strained either because it is not properly stretched, or
warmed up, before the activity; it is too weak; or because the muscle is
already injured and not allowed time to recover. So, many muscle strains
occur during exercise or sports activities. They can also occur when
lifting heavy objects. What are the symptoms?

When a muscle is strained, it hurts and is difficult to move. You may
also feel a burning sensation in the area of the injured muscle, or feel
as though something has \"popped.\" Sometimes the area of the strained
muscle looks bruised or swells. A strained muscle might spasm, which
means it contracts suddenly and involuntarily, causing severe pain. How
is it diagnosed?

To diagnose a muscle strain, your doctor will examine the painful area,
and ask how and when the injury happened. He or she may order other
diagnostic tests, such as x-rays, to rule out any injury to the bone.

*What is the treatment?*

Muscle strains are treated with rest, ice, compression, and elevation,
or RICE. You will be told to rest the injured area to reduce pain and
swelling. If the strain is in the leg or foot area, you may need to use
crutches. Ice packs are recommended at regular intervals (as recommended
by your doctor) over the first few days after the injury. Ice causes the
blood vessels to constrict, which reduces inflammation and pain.
Anti-inflammatory medications might also be used to relieve pain.
Compression and elevation help to reduce swelling. Your doctor may also
recommend physical therapy to speed your recovery. You should avoid the
type of activity that caused the injury until the muscle is completely
healed. Self-care tips

You can prevent muscle strains by warming up for at least 10 minutes
before participating in any strenuous exercise or heavy lifting. When
you warm up, you increase the blood circulation to the muscle and
prepare it for exercise. When starting any new exercise program or
sport, it\'s important to begin gradually so your muscles are
conditioned for the activity.

### Steroids

Anabolic steroids, which are synthetic versions of the primary male sex
hormone testosterone, can be injected, taken orally, or used
transdermally. These drugs are Controlled Substances that can be
prescribed to treat conditions such as body wasting in patients with
AIDS, and other diseases that occur when the body produces abnormally
low amounts of testosterone. However, the doses prescribed to treat
these medical conditions are 10 to 100 times lower than the doses that
are used for performance enhancement.

Let me be clear:- while anabolic steroids can enhance certain types of
performance or appearance,they are dangerous drugs, and when used
inappropriately, they can cause a host of severe, long-lasting, and
often irreversible negative health consequences. These drugs can stunt
the height of growing adolescents, masculinize women, and alter sex
characteristics of men. Anabolic steroids can lead to premature heart
attacks, strokes, liver tumors, kidney failure and serious psychiatric
problems. In addition, because steroids are often injected, users risk
contracting or transmitting HIV or hepatitis.

Abuse of anabolic steroids differs from the abuse of other illicit
substances because the initial use of anabolic steroids is not driven by
the immediate euphoria that accompanies most drugs of abuse, such as
cocaine, heroin, and marijuana, but by the desire of the user to change
their appearance and performance, characteristics of great importance to
adolescents. These effects of steroids can boost confidence and strength
leading the user to overlook the potential serious long-term damage that
these substances can cause.

Government agencies such as NIDA support research that increases our
understanding of the impact of steroid use and improves our ability to
prevent abuse of these drugs. For example, NIDA funding led to the
development of two highly effective programs that not only prevent
anabolic steroid abuse among male and female high school athletes, but
also promote other healthy behaviors and attitudes. The ATLAS (targeting
male athletes) and ATHENA (targeting female athletes) programs have been
adopted by schools in 29 states and Puerto Rico. Both Congress and the
Substance Abuse and Mental Health Services Administration have endorsed
ATLAS and ATHENA as model prevention programs, which could and should be
implemented in more communities throughout the country.

In addition to these prevention programs and other research efforts,
also has invested in public education efforts to increase awareness
about the dangers of steroid abuse. We have material on our website
about steroid abuse at www.steroidabuse.gov and in April 2005 we again
will distribute a \"Game Plan\" public service announcement designed to
bring attention to abuse of anabolic steroids.

Research has shown that the inappropriate use of anabolic steroids can
have catastrophic medical, psychiatric and behavioral consequences.

I hope that students, parents, teachers, coaches and others will take
advantage of the information on our website about anabolic steroids
abuse and join us in our prevention and education efforts. Participating
in sports offers many benefits, but young people and adults shouldn\'t
take unnecessary health risks in an effort to win.(Nora D. Volkow, M.D.)

-Human-made substances related to male sex hormones. Some athletes abuse
anabolic steroids to enhance performance. Abuse of anabolic steroids can
lead to serious health problems, some of which are irreversible.

Major side effects can include liver tumors and cancer, jaundice, high
blood pressure, kidney tumors, severe acne, and trembling. In males,
side effects may include shrinking of the testicles and breast
development. In females, side effects may include growth of facial hair,
menstrual changes, and deepened voice. In teenagers, growth may be
halted prematurely and permanently.

The therapeutic use of steroids can be realized by patients and their
doctors by using them in a manner that is beneficial to the person.

### MyoD and other muscular factors

MyoD is a protein and a transcription factor that activates muscle cell
differentiation by turning on transcription of specific regulatory
genes. It turns stem cells into myoblasts, a cell that can turn into
many muscle cell, also called \"muscle stem cell\". MyoD belongs to a
family of proteins knowns as myogenic regulatory factor(MRFs). MyoD can
also turn on transcription of its own regulatory genes (MyoD protein
coding genes), and this means that it can produce more of itself. The
positive feedback turns on transcription of other muscle proteins, cell
cycle blockers, and microRNA-206. One of the main actions of MyoD is to
remove cells from the cell cycle by enhancing the transcription of
p21The function of MyoD is to commit mesoderm cells to a skeletal
lineage. MyoD can also regulate muscle repair. One of the main actions
of MyoD is to remove cells from the cell cycle by enhancing the
transcription of p21. Bidirectional Signalling- muscle cells and nerves
cells send signals back and forth to each other. Amyotrophic Lateral
Sclerosis(ALS) is a loss of motor neuron and this blocks the formation
of neuromuscular junctions. Therefore, no muscle growth which means a
potential of leading to paralysis. Stephen Hawking suffers from this
disease.

### Muscle Homeostasis

MicroRNA-206 indirectly forms neuromuscular junctions with motor
neurons. Neuromuscular junction sends synaptic signals to MyoD and this
blocks MyoD and stops or limits muscle development. Myostatin is a
protein that also blocks MyoD. Without myostatin, muscle development
increases.

Myostatin Mutations In Sheep: they can have a mutant myostatin that
causes microRNA-206 to block myostatin translation

Myostatin Mutations In Humans: humans with mutant myostatin will develop
lots of muscle (like a body builder) is possible to create a drug that
blocks myostatin production.

## 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 using
    ATP, causing them to interact with actin filaments.
-   Powerstroke.
-   Calcium is actively pumped out of the cell by receptor regulated
    channels. A second messanger, IP3, causes the release.
-   As calcium is removed the calcium-calmodulin complex breaks away
    from the myosin light-chain kinase, stopping phosphorylation.
-   Myosin phophatase dephosphorylates the myosin. If the myosin was
    bound to an actin molecule, the release is slow, this is called a
    latch state. In this manner, smooth muscle is able to stay
    contracted for some time without the use of much ATP. If the myosin
    was not bound to an actin chain it loses its affinity for actin.
-   

It should be noted that ATP is still needed for crossbridge cycling, and
that there is no reserve, such as creatine phosphate, available. Most
ATP is created from aerobic metabolism, however anaerobic production may
take place in times of low oxygen concentrations.

## Cardiac Muscle

Cardiac muscle is found in the hearts of humans. They are shaped like
skeletal muscles but work like smooth muscles. The cardiac muscle is
responsible for the contractility of the heart and, therefore, the
pumping action.

## ATP in the Human Body

Muscles cells, like all cells, use ATP as an energy source. The total
quantity of ATP in the human body at any one time is about 0.1 Mole. The
energy used by human cells requires the hydrolysis of 200 to 300 moles
of ATP daily. This means that each ATP molecule is recycled 2000 to 3000
times during a single day. ATP cannot be stored, hence its consumption
must closely follow its synthesis. On a per-hour basis, 1 kilogram of
ATP is created, processed and then recycled in the body. Looking at it
another way, a single cell uses about 10 million ATP molecules per
second to meet its metabolic needs, and recycles all of its ATP
molecules about every 20-30 seconds.

## Lactic Acid

Catabolized carbohydrates is known as glycolysis. The end product of
glycolysis, pyruvate can go into different directions depending on
aerobic or anaerobic conditions. In aerobic it goes through the Krebs
cycle and in anaerobic it goes through the Cori cycle. In the Cori cycle
pyruvate is converted to lactate, this forms lactic acid, lactic acid
causes muscle fatigue. In the aerobic conditions pyruvate goes through
the Krebs cycle. For more about Krebs cycle refer to chapter 2 Cell
Physiology.

## Muscle Disorders

### Dermatomyositis and Polymyositis

Dermatomyositis and polymyositis cause inflammation of the muscles. They
are rare disorders, affecting only about one in 100,000 people per year.
More women than men are affected. Although the peak age of onset is in
the 50s, the disorders can occur at any age.

Signs and symptoms --- Patients complain of muscle weakness that usually
worsens over several months, though in some cases symptoms come on
suddenly. The affected muscles are close to the trunk (as opposed to in
the wrists or feet), involving for example the hip, shoulder, or neck
muscles. Muscles on both sides of the body are equally affected. In some
cases, muscles are sore or tender. Some patients have involvement of the
muscles of the pharynx (throat) or the esophagus (the tube leading from
the throat to the stomach), causing problems with swallowing. In some
cases, this leads to food being misdirected from the esophagus to the
lungs, causing severe pneumonia.

In dermatomyositis, there is a rash, though sometimes the rash resolves
before muscle problems occur. A number of different types of rash can
occur, including rashes on the fingers, the chest and shoulders, or on
the upper eyelids (show picture 1-3). In rare cases, the rash of
dermatomyositis appears but myopathy never develops.

Other problems sometimes associated with these diseases include fever,
weight loss, arthritis, cold-induced color changes in the fingers or
toes (Raynaud phenomenon), and heart or lung problems.

### Muscle Atrophy

Alternative names : Atrophy of the muscles, Muscle wasting, Wasting

The majority of muscle atrophy in the general population results from
disuse. People with sedentary jobs and senior citizens with decreased
activity can lose muscle tone and develop significant atrophy. This type
of atrophy is reversible with vigorous exercise. Bed-ridden people can
undergo significant muscle wasting. Astronauts, free of the
gravitational pull of Earth, can develop decreased muscle tone and loss
of calcium from their bones following just a few days of weightlessness.

Muscle atrophy resulting from disease rather than disuse is generally
one of two types, that resulting from damage to the nerves that supply
the muscles, and disease of the muscle itself. Examples of diseases
affecting the nerves that control muscles would be poliomyelitis,
amyotrophic lateral sclerosis (ALS or Lou Gehrig\'s disease), and
Guillain-Barre syndrome. Examples of diseases affecting primarily the
muscles would include muscular dystrophy, myotonia congenita, and
myotonic dystrophy as well as other congenital, inflammatory or
metabolic myopathies.

Even minor muscle atrophy usually results in some loss of mobility or
power.

Common Causes

-   some atrophy that occurs normally with ageing
-   cerebrovascular accident (stroke)
-   spinal cord injury
-   peripheral nerve injury (peripheral neuropathy)
-   other injury
-   prolonged immobilization
-   osteoarthritis
-   rheumatoid arthritis
-   prolonged corticosteroid therapy
-   diabetes (diabetic neuropathy)
-   burns
-   poliomyelitis
-   amyotrophic lateral sclerosis (ALS or Lou Gehrig\'s disease)
-   Guillain-Barre syndrome
-   muscular dystrophy
-   myotonia congenita
-   myotonic dystrophy
-   myopathy

### Muscular Dystrophy

Muscular dystrophy (MD) is a group of rare inherited muscle diseases in
which muscle fibers are unusually susceptible to damage. Muscles,
primarily voluntary muscles, become progressively weaker. In the late
stages of muscular dystrophy, muscle fibers are often replaced by fat
and connective tissue. In some types of muscular dystrophy, heart
muscles, other involuntary muscles and other organs are affected.

The most common types of muscular dystrophy appear to be due to a
genetic deficiency of the muscle protein dystrophin. There\'s no cure
for muscular dystrophy, but medications and therapy can slow the course
of the disease.

## Medical Mysteries

#### Sleep Twitches

The twitching phenomenon that happens in the early stage of sleep is
called a hypnagogic massive jerk, or simply a hypnic jerk. It has also
been referred to as a sleep start. There has been little research on
this topic, but there have been some theories put forth. When the body
drifts off into sleep, it undergoes physiological changes related to
body temperature, breathing rate and muscular tone. Hypnic jerks may be
the result of muscle changes. Another theory suggests that the
transition from the waking to the sleeping state signals the body to
relax. But the brain may interpret the relaxation as a sign of falling
and then signal the arms and legs to wake up. Electroencephalogram
studies have shown sleep starts affect almost 10 percent of the
population regularly, 80 percent occasionally, and another 10 percent
rarely.

Muscle movement or twitching also may take place during the Rapid Eye
Movement, or REM, phase of sleep. This also is the time when dreams
occur. During the REM phase, all voluntary muscular activity stops with
a drop in muscle tone, but some individuals may experience slight eyelid
or ear twitching or slight jerks. Some people with REM behavioral
disorder, or RBD, may experience more violent muscular twitching and
full-fledged activity during sleep. This is because they do not achieve
muscle paralysis, and as a result, act out their dreams. Researchers
think that people with RBD lack neurological barriers that define the
different stages of sleep. New research done by the Mayo Clinic and
published in the July 2003 issue of Sleep Medicine shows that melatonin
can help lessen RBD symptoms.

Resources:

:   Sleep twitches, or myoclonic jerks, as they are sometimes called,
    are explained in easily understood language on this website.
:   Learn more about REM Behavior Disorder, or RBD, and treatment for
    sufferers.
:   View information about various sleep disorders such as insomnia,
    apnea, and narcolepsy.

## Microbiology

*Clostridium tetani*
:   Tetanus
:   Normally a nerve impulse initiates contraction of a muscle. At the
    same time, an opposing muscle receives the signal to relax so as not
    to oppose the contraction. Tetanus toxin blocks the relaxation, so
    both sets of muscle contract. The usual cause of tetany is lack of
    calcium, but excess of phosphate (high phosphate-to-calcium ratio)
    can also trigger the spasms.

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*Clostridium botulinum*
:   Infant botulism (floppy baby syndrome) the most common form of
    botulism in the U.S. of the four forms of botulism.
:   If ingested, the toxin is absorbed in the intestine, goes to the
    blood, and on to the nervous system. It acts on the peripheral
    nervous system by blocking the impulse that is normally passes along
    to the nervous system. By blocking the impulse that is normally
    passed along to motor end plates so the muscle contraction can be
    released, resulting in paralysis.

## Glossary

Actin:A protein that forms a long polymer rods called microfilaments; Interacts with myosin to cause movement in muscles.

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ATP:\"Adenosine Triphosphate\" is a nucleotide that comes from adenosine that takes place in muscle tissue: This provides a large source of energy for cellular reactions.

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Cardiac muscle: is also an \"involuntary muscle\" but it\'s a specialized kind of muscle found only within the heart.

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Clostridium botulinum: A pathogen that causes botulism, gram stain positive, morphology is rod shaped, grows in anaerobic conditions, and produces spores.

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Clostridium tetani: A pathogen that causes lock jaw, gram stain positive, morphology is tennis racket shaped rod, grows in anaerobic conditions, and produces spores.

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Cori cycle: In anaerobic conditions produces lactic acid.

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Cramp: A localized muscle spasm that happens after strenuous activity.

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Glycogen: Glucose that has been converted for energy storage. Muscles store energy for their own use in this form.

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Lactic acid: Causes muscle fatigue.

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Muscle:Contractile tissue that is derived from the mesodermal layer of embryonic germ cells.

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Muscular Dystrophy:A hereditary disease characterized by progressive atrophy of muscle fibers

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Myosin: The fibrous motor protein that uses ATP to drive movements along actin filaments.

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Sarcoplasmic Reticulum: Smooth-surfaced tubules forming a plexus around each myofibril that function as a storage and release area for calcium ions (CA+2).

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Skeletal muscle: this \"voluntary muscle\" is anchored by tendons to the bone and is used to affect skeletal movement such as locomotion.

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Smooth muscle: this \"involuntary muscle\" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, ureters, bladder, and blood vessels.

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Sprain: Injuries that involves a stretched or torn ligament.

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Strain: A injury to the muscle or tendon attachment

charitin; a form of drug use to ensure muscle growth.

## References

-   Van De Graaff (2002) *Human Anatomy 6th ed.* McGraw-Hill Higher
    Education
-   Windmaier, P.W. Raff, H. Strang, T.S. (2004) *Vander, Sherman, &
    Luciano\'s Human Physiology, the Mechanisms of Body Function 9th
    ed.* Mcgraw-Hill

Neil A. Campbell , Jane B. Reece \"Biology 8th edition\"
