Download Muscle tissue

Physiology
Lect(4)
Asst.Lect/Shaimaa Hussein
Muscle tissue
If you weight 120 pounds about 50 pounds of your weight
comes from your muscle. Under the microscope these muscles
appear an bundles of fine threads with many crosswise stripes.
Each fine thread is a muscle cell or, as it is usually called (a
muscle fiber). This type of muscle tissue has three names
striated muscle because of its cross stripe, skeletal muscles
because it attaches to bone and voluntary muscle because its
contraction can be controlled voluntarily.
Besides skeletal muscle the body also contain two other kinds of
muscle tissue:1- Cardiac muscle 2-and nonstriated,Smooth or
involuntary muscle.
Cardic muscle as its names suggests composes the bulk of the
heart. Cardic muscle branch frequently. Nonsrtriated or smooth
muscle lacks the cross stripes which seen in skeletal muscle. It
called involuntary because we normally do not have control over
its contraction.
This muscle forms an important part of blood vessel walls and
of many internal organ(viscera) such as the gut.
Muscle cells specialize in the function of contraction or
shortening. Every movement we make is produced by
contraction of skeletal muscle cells. Contraction of cardiac
muscle cells keep the blood circulating through do many things
for instance, move food into and through the stomach and
intestine and make a major contraction to the maintenance of
normal blood pressure.
Skeletal muscle
A skeletal muscle is an organ composed mainly of striated
muscle cells and connective tissue. Most of skeletal muscle
attach to two bones that have a movable joint between them.
Most muscle extend from one bone a cross a joint to another
bone. Also one of the two bones moves less easily than the
other. The muscle s attachment to this more stationary bone is
called its origin. Its attachment to the more movable bone is
called the muscle s insertion. The rest of the muscle( all of it
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except its two ends) is called the body of the muscle. Tendons
anchor muscles firmly to bone. Made of dense fibrous
connective tissue.
Small fluid filled sacs called bursae lie between some tendons
and the bones. These small sac are made of connective tissue
and lined with synovial membrane. The synovial membrane
secretes a slippery. Lubricating fluid that fills the bursa.
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Microscopie Structure
Muscle tissue consist of specialized contractal cells or muscle
fibers that are grouped together and arranged in a highly
organized way. Each skeletal muscle fiber is itself e filled with
two kinds of very fine and thread like structure called thick and
thin myofilaments. The thick are formed from a protein called
myosin and thin are composed of the protein actin.
Find the label Sarcomere.
The sarcomere as the basic functional or contractile unit of
skeletal muscle. The submicroscopie structure of a sarcomere
consist of numerous actin and myosin myofilaments arranged so
that when viewed under microscop, dark and high stripes or
cross-striae are seen. The repetitise units or sarcomere are
separated from each other by dark band called Z lines.
Look at the structure of sarcomere in figure that the thick and
thin myofilaments overlap each other to form a dark area are
called A band. A light area between the Z line and A band is
called I band. Each A band is composed of both thick and thin
myofilaments and each I band is composed only of thin. Another
specialized structure found in skeletal musccle and cardiac
muscle is the system of transverse tubule (T-tube).
During the contraction process energy obtained from ATP
molecules eneble the two type of myofilament to slide toward
each other and shorten the sarcomer.
The explantation of muscle contraction resting from the
movement of thick and thin sliding-filament theory.
Function :
The three primary function of the muscular system are:
1-movement
2-posture or muscle tone
3-heat production
1-Movement:
Muscle move bones by pulling on them. Because the length of
skeletal muscle becomes shorter as the fibers contract the bone
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to which muscle attached move coloser together. The shortening
of the muscle pulls the insertion bone toward the origin bone.
2-Posture:
We are able to maintain our body position because of a
specialized type of skeletal muscle contraction called tonic
contraction. Skeletal muscle tone maintains posture by
contraction the pull of gravity.
3-Heat production:
Healthy survival depends on our ability to maintain a constant
body temprature. A fever or elevation in body temperature of
only a degree or two above 37c will result in illness.
Any decrease below normal, a condition called hypothermia.
The contraction of muscle fiber produces most of the heat
required to maintain our body temperature. Energy required to
produce a muscle contraction obtained from ATP. Most of
energy released during the break down of ATP during muscle
contraction is used to shorter the muscle fibers however some of
energy is lost as heat during the reaction. It is this heat that help
us to maintain our body temperature at a constant level.
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Exaltation of muscle cells:
Skeletal muscle is stimulated to contract by nerve impulses
traveling at a rate of about 100 meter per second. When a
stimules reaches the membrane of muscle cells. The
permeability of the membrane to sodium and potassium ions is
altered.
The result is a change in the balance of electrical charges,
creating an electrical potential or action potential. The action
potential moves along the muscle cell membrane spreading
down in to T. tubes. When the action potential part regin of the
sarcoplasmic reticulum to release calcium ions (Ca++) in the
sarcoplasm. It take roughly amillisecond (1/1000 second) Ca ion
act as mediaters between excitation, an electrical event and
contraction, a mechanical event.
The contraction process:
The sliding filament theory of contraction proposed in the 1950
by H.E. Huxely and based on electron microscop studies.
Acording to this theory muscle shortening occur when the thin
filament of a sarcomer slide over thick filament so that greater
overlap exist between them.
Actin and myosin are protein molecule that bring about the
generation of force in the contraction process. Actin is a
globular protein, actin molecule are arranged like two string of
bread wound in a spiral to form an actin filament.
Myosin is much large molecule with a globular head and along
tall picee. The head of myosin molecule project laterally from a
thick filament toward the surrounding actin filament. These
heads are called cross-bridges. ‫الجسور العرضية‬
The head of each myosin molecule contain an enzyme capable
of releasing energy from ATP.
Magnesium ion are required for this enzyme to function.
The protein troponin and tropomyosin which are closely
associated with actin also are important in regulation the
attachment of actin to the cross- bridges. Calcium ions can a
change in its molecular configuration. This in turn alteres the
configuration of tropomyosin to which tropomyosin binds.This
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shift exposes previously covered area called active site on the
surface of actin molecule. Myosin cross-bridges are then able to
bind to there active site.
The step in the contraction process show in figure
(a) ATP is bound to head of a myosin molecule.Thought
energy is available for contraction no contraction occure
because the troponin-tropomyosin complex on the thin filament
blocks the myosin binding sites on the actin. Following
excitation Ca ions released into sarcoplasm cause the troponintropomyosin complex to withdraw into agroove between the
chains of actin molecule.
(b) An enzyme in the myosin head splits ATP to ADP and
phosphate energizing the cross-bridge and cross-bridges bind to
actin.
(c) Each time across-bridge completes a swiveling movement , a
new molecule of ATP replaced the ADP. The formation of a
new ATP- myosin bound breaks the existing cross-bridge
attachment and allow a new one to form.
If Ca++ remain available in sufficient quantity c,d,e occur over
and over again rate of 50-100 times/second. These repetitive
steps constitute the contraction.
*It is important to note that in the contraction process,no
shortening of either the actin or the myosin filaments occurs.
The sarcomere shortens because of the sliding of the actin
filaments produced by cross-bridge movements.
H-zones and I-bands, but the width of the A-band remains
constant.
*The most signifigantevent in this process is the cyclic changes
in the position of the myosin heads and the resultant movement
of the filaments in relation to each other. This movement is a
little like the movement of the oars ‫ مجداا‬of a boat. ‫حركدة الججداا‬
‫على الزورق‬
Rowing ‫ التجداف‬with oars puches a boat across the water. In
some what the same way that myosin and actin filaments slide
along one another.
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The relaxation process:
Like contraction, relaxation also requires ATP. When a muscle
is stimulated, the SR releases Ca++ for only a few milliseconds
(repetitive stimuli are required for Ca++ to continue to be
released).
As soon as stimulation ceases ‫فتوق‬, Ca++ beigns to be returned to
the SR by active transport (a process that requires ATP).
However, contraction continues to occur for several handred
milliseconds, or until enough Ca++ is removed from the
troponin- tropomyosin complex to allow these molecules to
block reactive sites on myosin when myosin cannot attach to
actin the muscle relaxes. However, the actin filaments slide back
to their relaxed position, and the sarcomeres lengthen only when
an external force (load) is exerted on the muscle.
The Motor unit:
A motor unit consists of a single motor neuron(nerve cell) and
the muscle fibers it innervates.
Motor units controlling precise ‫ دقيد‬movements such as those
in the fingers have 3-6 muscle fibers / motor neroun.
Motor units controlling gross movements such as those in the
back have several handred fibers/ motor neroun.
Motor units in some leg muscles such as the gastrocnemius
muscle have as many as 1900 muscle fibers/motor neroun.
The branch of an axon that serves a particular muscle fibers
has many small terminals branches that lie in grooves in the
muscle cell membrane. The portion of the muscle plasma
membrane that lies beneath ‫ تحد‬these nerve ending is called
the motor end plate. The terminal ends of the axon and the
motor end plate together constitutes the myoneural junction.
As a nerve impulse reaches the terminal end of the axon,
small sacs called synaptic vesicles fuse with the axon
membrane and release a chemical transmitter (acetyl-cholin).
A.ch diffuses across the synaptic cleft (the space between the
axon membrane and the motor end plate). In this area the
muscle cell membrane is folded into subneural clefts and has
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A.ch. receptor sites on its surface. When a sufficient number
of these sites are stimulated by A.ch., the motor end plate
becomes depolarized. ‫زوال أستقطاب‬
To understand depolarization, it is important to realize that a
resting muscle cell membrane is polarized that is it has a small
positive charge at the outer surface and a small negative
charge at the inner surface.
This polarized state is maintained by the transport of Na+ and
K+ the membrane. A.ch.acts to depolarize the muscle cell
membrane by causing it to become suddenly more permeable
to Na+. The positive Na+ entering the cell reduce the electrical
charge across the membrane (the membrane is then said to be
depolarized). When depolarization of the motor end plate
reaches a certain level, it creates an action potential. This
action potential is propagated along the T-tubules and causes
the SR to release Ca++.
Along with receptor sites for A.ch. the motor end plate also
contain an enzyme (cholin esterase) that breaks down A.ch.
About 5 milliseconds after the release of A.ch., cholinesterase
has broken A.ch. down into it components parts (acetate and
choline). The effect of cholinesterase is to prevent continous
stimulation of a muscle fiber after the release of A.ch.
Aportion of the choline diffuses back to the axon and is reused
to synthesize more A.ch. for transmission of subsequent
impulses.
Muscle Tension:
The amount of tension (force) the force produced by a whole
muscle when it contraction.
The amount of tension that can be developed by a skeletal
muscle depends on several factors:
1- Frequency of stimulation of muscle fibers by motor
neurons.
2- The number of motor fibers contraction (number of active
motor units) at a particular time.
3- The components of the muscle fibers themselves:
a- contractile elements: are those components that are actively
involved in muscle contraction (thin and thick myofilaments).
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The tension generation by contractile elements is called active
tension.
b- Elastic elements: are structures that are capable of being
stretched. They include the C.T. surrounding each motor
fibers and the tension that attaches muscle to bone . The
tension generation by elastic elements is called passive
tension.
4- The length of the muscle fibers at the time of contraction.
Muscle Tone :
A sustained partial contraction of portionsof a skeletal muscle
in response to activation of stretch receptors‫ مسدتقالت الددا‬result
in muscle tone and occurs even in relaxed muscle. Tone is
essential for maintaining posture.
For example: when the muscle in the back of the neck are in
tonic contraction, they keep the head in the anatomical
position and prevent it from slumping forwared onto the chest,
but they do not apply enough force to pull the head back into
hyperextension. The degree of tone in a skeletal muscle is
monitored by receptors in the muscle called muscle spindles.
They provide feedback information on tone to the brain and
spinal cord so that adjustments can be made.
The load : is the force exerted on a muscle by a weight.
Eg. When you pick up a book, the book is the load and the
force produced by the muscle in your arm is the tension. Thus,
load and tension are opposing forces.
Types of muscle contraction:
1- Isotonic contraction (same tention):
When the tension in a muscle in a muscle is sufficient to lift a
load, the contraction is said to be isotonic. One the other hand,
during isotonic contraction, the muscle is shorten.
2-Isometric contraction (same length):
When the tension in a muscle is used to support a load (a book
you are carrying,for example) or to push against an
immovable object, the contraction is said to be isometric. In
isometric contraction the muscle maintains the same length
but develops tension during the period of the contraction.
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Simple muscle twitch:
There are three phases of twitch can be distinguish:
1- The latent period: is the time between the stimulation and
the beginning of the contraction
During this period: a-the nerve action potential is conducted to
the axon terminals.
b-A.ch. is released and binds to the motor end plate.
c- The action potential passes over the membrane.
d- Ca++ inhibits the action of troponin and tropomyosin or
Ca++ exposes the myosin active site.
2-The contraction period: is the time during which tension is
rising or shortening is occurring.
3- The relaxation period: is the time during which tension is
declines or muscle back to its original length.
Energy for Contraction and Relaxation:
1- Phosphagen system
a-ATP is the immediate source of energy for contraction. When
a muscle action potential stimulates a motor fiber, ATP, in the
presence of enzyme ATPase, breaks down into ADP+P, and
energy is released. Like other cells of the body, motor fibers
synthesize ATP as follows
ADP+P+ energy
ATP
*The amount of ATP present in skeletal muscle fibers is
sufficient to maintain muscle contraction during vigorous
exercise for only 5-6 seconds.
b- Creatine phosphate: keletal muscle fibers contain a high
energy molecule called phosphocreatine that is used to generate
ATP rapidly. The amount of phosphocreatine is about 2-3 times
greater than ATP.
Phosphocreatine
creatine+P+energy
ADP+P+energy
ATP
*The transfer of energy from phosphocreatine to ATP takes
place in a fraction of a second.
*Phosphagen system provide only enough energy for muscle to
contract maximally for about 15 seconds.
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2-Glycogen-Lactic acid system
When muscle activity is continued so that even the supply of
phosphocreatine is depleted, then the source of energy is
glucose, which is derived from the break down of glycogen and
also picked up from the blood. Glycogen is always present in
skeletal muscle and the liver.
In some cases there may no be sufficient oxygen for the
complete catabolism of pyruvic acid. Then most of the pyruvic
acid is converted to lactic acid and energy that can be used to
produce ATP.
*The glycogen-lactic acid system can provide sufficient energy
for about 30-40 seconds of maximal muscle activity.
3-Aerobic system
If sufficient oxygen is present, pyruvic acid can be completely
catabolized to form CO2,H2O and energy(ATP) in the
mitochondria. This aerobic system along with glycogen, is used
for prolonged muscular activity and will continue as long as
nutrients and adequate oxygen last. These nutrients include fatty
acid (from fat) and amino acids (from proteins) as well as
glucose derived from glucose derived from glycogen breakdown
or delivered to the muscle by way of blood.
In summary
1-The phosphogen system will provide enough ATP to sustain
maximal muscular activity for about 15 seconds.
2-The glycogen-lactic acid system will provide sufficient ATP
to support about 30-40 seconds of maximum muscular activity.
3-The aerobic system will provide sufficient ATP for a
prolonged activity as sufficient oxygen and nutrients are
available.
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