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Excitable tissue : Muscle
physiology
Dr. A.K. Dwivedi
Muscle physiology
Introduction: Nature in its supreme wisdom has therefore
given muscles about half a man’s body weight. Locomotion
is achieved by skeletal muscle so called they are attached to
bones and bring about movement at joints. Muscle cells like
neurons can excited chemically, electrically and mechanically
to produce an action potential that is transmitted along
there cell membrane, unlike neurons they have a contractile
mechanism that is activated by the action potential. The
contractile proteins actins and myosin are abundant in
muscles where they bring about contraction.
muscles may contract voluntarily or involuntarily. All
muscles movement results from contraction since
movement is possible in opposite direction when a group
of muscle contract, the opposing group of muscle must
relax.
Classification of muscles :
Basis of classification
There are three methods to classify the muscles. The basis of
this classification is as follows :
1. Depending upon the presence or absence of striations
2. Depending upon the control and
3. Depending upon the functions
Depending upon striations :depending upon the presence or
absence of the cross striations, the muscles are derived
into two groups namely
a. Striated muscle and
b. Non striated muscle
Depending upon the control
Depending upon the control, the muscles are classified
into two types namely :
a. Voluntary muscle and
b. Involuntary muscle
Depending upon the function :
The muscles are classified into three types depending
upon the function :
a. Skeletal muscle.
b. Cardiac muscle.
c. Smooth muscle.
Functions of muscle tissue
1. Body movements : total body movements like walking,
running or fine movement like writing, holding the pen
or holding light or heavy objects is integrated functions
of bones, joints and skeletal muscles.
2. Stabilizing body position : skeletal muscle contraction
stabilize joints and help to maintain body positions such
as sitting. Postural muscles contract continuously when
a person is awake, for eg : sustained contraction in neck
muscle hold the head upright.
3. Regulating organ volume :storage of food, peristalsis,
holding of urine in the urinary bladder (about 600 ml)
4. Moving substance within the body : cardiac muscle
contract and pump blood throughout the body through
the vessels contraction and relaxation of smooth muscles
in body help to regulate the rate of flow. smooth muscles
contract and also move by and other enzymes through
gastro-intestinal tract.
5. Generating heat :as the muscle tissue contract they
produce heat, this is used to maintain body temperature .
Involuntary contraction of skeletal muscle is called as
shivering can increase the late of production of heat
many lines. It is a type of metabolic activities.
Properties :
Muscle tissue have four special properties :
1.Electrical excitability : property of both muscle and nerve
cells, is the ability to respond to certain stimuli by
producing electrical signals such as action potential.
Action potential can propagate along a cells plasma
membrane due to the presence of specify voltage gated
channels for muscles cells. Two main types of stimuli
trigger action potential one is the auto systemic electrical
signals arising in the muscle tissue itself such as occurs in
the heart pacemaker. The other is chemical stimuli, such
as neurotransmitter released by neurons, hormones
distributed by the blood, or even local changes in ph.
2. Contractibility : it is ability of muscle tissue to contract
forcefully when stimulated by action potential. When muscle
contract, generates tension ( force of concentration) while
pulling on its attached points. In an isometric contraction
(iso-equal, metric-measure or length ). The muscle developed
tension but does not shorten. An e.g. : holding a book is an
out stretched hand. If the tension generated is great enough
to overcome the resistance of the object to being moved. The
muscle shortens and movement occurs. In an isotonic
contraction ( tonic – tension ), the tension developed by the
muscle remain almost constant while the muscle shortens. An
e.g. : lifting a book from a table.
3. Extensibility : it is the ability of muscle stretch
without being damaged. Extensibility allows a muscle
to contract forcefully even if it is already stretched.
Normally, smooth muscle is subject to the greatest
amount of stretching. For e.g. : each time the stomach
fills with food, the muscle in its wall is stretched.
Cardiac muscles also is stretched is each time the heart
fills with blood. During normal activities, the stretched
on skeletal muscle remains almost constant
4.Elasticity ; it is the ability of muscle tissue to return
to its original length and shape after contraction or
extension.
Description of muscle
1. Skeletal muscle
Introduction : skeletal muscle is made up of individual
muscle fibers that are the ‘building block’ of the
muscular system in the same sense that the neurons are
the building block of the nervous system. The skeletal
muscle fibers are multinucleated cylindrical structures
having a clear display of longitudinal and cross
striations. Most skeletal muscle began and end in
tendons, and the muscle fibers are arranged in parallel
between the tedious ends, so that the force of
contraction of the units is additive.
Distribution : These muscles mostly in all instances are
attached to osseous tissue, innervated with somatic nerves
through which voluntary control is performed. In the
fresh state the human skeletal muscle is pink in colour
due to the presence of muscle pigments and high
vascularity. Due to variations in colour there are red and
white muscles. In most skeletal muscles, each fiber extend
the entire length of muscle, except for about two percent
of the fibers. Each fiber is innervated by only one nerve
ending, located near the middle of the fiber.
Types of skeletal muscle
There are two types of muscle fibers – red and white with
intermediate varieties between the extremes red, white
and intermediate fibers are present within a single muscle.
However, some muscles have predominantly red fibers and
some predominantly white.
1. Red fibers : red fibers are small in diameter and rich in
mitochondria and myoglobine. Their red colour to the
higher content of myoglobin which is reddish brown in
colour. Their speed of contraction is low and they contract
less forcefully but do not get fatigued easily because
myoglobin can provide a steady supply of oxygen.
2. White muscle fibers : they are larger in diameter then
red fibers, have fewer mitochondria, less myoglobine and
a higher velocity of contraction. They contract more
forcefully but fatigue quickly. There is evidence to
suggest that the muscle fiber type is determined by the
motor nerve fibers.
Connective tissue component : connective tissue surrounds
and protect muscle tissue. Three layers of connective tissue
extend from the deep fascia to further protect and
strengthen skeletal muscle. The outer most layer including
the whole muscle is the “epimysium” , “perimysium”
surrounds groups of 10 -100 or more including muscle
fibers, separating them into bundles called fascicles many
fascicles are large enough to be seen with the naked eye,
they give a cut of meat its characteristics “grain” and if
you tear a piece of a meat , reps a part along the fascicles.
Both epimysium and perimysium are dense irregular
connective tissue. Penetrating the interior of each fascicle
and separating individual muscle fiber from one another
Is “endomysium”, a then sheath of areolar connective
tissue
Histology
The skeletal muscle fiber are cylindrical elongated cells with
multiple nuclei. The extend of muscle fiber in the bulk
may be form –
a. One end to the other
b. One end somewhere at midway or
c. Both the ands within the muscle having no attachment
with either side.
The length and breadth of the muscle fiber vary from 1.0-40
mm and 0.01 mm to 0.1 mm respectively.
The transparent cell wall of the muscle fiber is named as
“sarcolemma” under light microscope it is visible when
fresh muscle fiber are teased electron micrographs shows
that it is made up not only of the plasma lemma but
also of an extrinsic coat of amorphous layer is and by
reticular fibers. The plasma lemma is of same structure
as that of reticular fiber. The sarcoplasm contains other
constituent as that of any other cell e.g.. Numerous
mitochondria, a small gorge apparatus near each
nucleus, myoglobin, lipid, glycogen, sarcoplasmic
reticulum etc. fiber are riches in sarcoplasma.
Myofibrils :
The characteristics feature of skeletal muscle, the alternate
legit and dark shades and thick longitudinal strands can be
studied with light microscope. Electron microscope reveals
that the longitudinal striation is due to the presence of
myofibrils of different thickness where case the transverse
striation is due to presence of alternate light and dark
segments of longitudinally arranged. In cross section
myofibrils appear as fine dots either in a group poly gonad
areas and are separated from adjacent bundle by clear
sarcoplasm. The separated myofibrils by the sarcoplasmic
known as fields of conceit.
Sarcotubular system :
under electron microscope, the myofibrils are seen to have
surrounded by a canalicular network of membrane limited
tubules – “Sarcoplasmic reticulum” is identical with the
enoplasmic reticulum of other cell type but with the
difference that its membrane does not possess ribosome.
The sarcoplasmic reticulum is extended longitudinally
along the A- band with frequent anastomosis in the
region of the H- band and also in the I- band. The
sarcoplasmic reticulum is connected at its both
longitudinal and terminal ends by another set of the
transvere cisterns – terminal cisterns. The terminal cistern
have got larger calibre and are thus continsous and
confluent with the longitudinal reticulum .
H Band
Pairs of parallel terminal cisternae are separated from each
other by a slender transverse tubule which is known as Ttubule. This T- tubule is not confluent the terminal cisternae
and is a tubular invagination of the sarcolemma but not a
part of sarcoplasmic reticulum. It is continuous with the
extra cellular space. These tubules are generally called as Tsystem. The pair of terminal cisternae and the central Ttubules are collectively called as “triads”. In amphibian
muscle the triads encircle the I- band at the region of the Zline but in mammalian muscle, the same is present at the
junction of each A- band with the adjacent I- band so in
mammals are there two sets of triads in each sarcomere. The
T- system plays an important role in quick transmission of
impulse from the cell surface to each myofibril.
Muscle protein :
Myofibrils are built from three kinds of proteins :
1. Contractile proteins, which genrate force during contraction.
2. Regulatory protein which help switch the contraction process on
and off and
3. Structural proteins, which keep the thick and thin ligaments in
the proper ligament, give the myofibril elasticity and
extensibility and link the myofibrils to the sarcolemma and
extracellular matrix .
the two contractile proteins in muscle are “myosin and actin” which
are the main components of thick and thin filaments,
respectively myosin function as a motor protein in all three types
of muscle tissue.
Motor proteins push or pull their cargo to achieve
movement by converting the chemical energy in ATP to
the mechanical energy of motion of force production. In
skeletal muscle, about 300 molecules of myosin from a
single thick filament each myosin molecules of is shape
like two golf clubs handle points towards the M – line
in the center of the sarcomere. The two projections of
each myosin molecules are called myosin head or cross
bridge. The head project outward from the shaft in a
spilling fashion, each extending towards one of the six
then filament that surround each thick filament.
Thin filament extend from according point within the Z
discs. Their main component is protein actin. Individual actin
molecules four to form an actin filament that is twisted into
one each actin molecules is a myosin binding site, where
myosin head can attach smaller amount of two regulatory
protein – “Tropomyosin and Troponin” are also part of their
filament. In relaxed muscle myosin blocked from binding to
actin because strands of tropomysin could the myosin binding
sites on actin . The tropomysin strands, in turn are held in
place by Troponin molecules. Beside contractile and
regulatory proteins, muscle contains about a does not
structural proteins which contractile to the alignment,
stability, elasticity and extensibility of myofibrils.
Binding Site
Troponin
Tropomyosin
Myosin
Several key structural proteins are titan myosin and
dystrophic . Titan ( titan – gigantic ) is the third most
plentiful protein in skeletal muscle. This molecules name
reflects its huge size. with a molecular weight of about
3 million Daltons , tint is 50 times larger then an average
size protein.
Titin anchors a thick filament to both a Z disc and the
M- line . There by helping stabilize the position of the
thick filament . The part of the titin molecule that
extends from the Z disc to the beginning of the thick
filament is very elastic . Because it can stretch to at
least four times its resting length and then spring back
unharmed, titin accounts fore much of the elasticity
and extensibility of myofibrils . Titin probably help the
sarcomere return to its resting length after a muscle
has contraction or been stretched . Molecules of the
protein myosin form the M- line protein bind to titin
And connect adjust thick filament s to one another .
Dystrophin is a cytoskeletal protein that links thin
filament of the sarcomere to integral membrane proteins
of the sarcolemma . In turn the membrane proteins
attach to protein in the connective tissue matrix that
surrounds muscle fiber . Hence, Dystrophin and its
associated protein are thought reinforce the sarcolemma
and help transmit the tension generated by the
sarcomeres to the tendons .
General mechanism of contraction
Introduction : Muscle fibers cells are connected with connective
tissue . When the muscle fibers shortens, the pull is , there fore
transmitted through the connective tissue to the bone and
there in flexion / extension etc movement of the joint .
Finer details : at rest is when the muscle is in a state of
relaxation :
1. H- zone is wide
2. I- bands are wide ( because good deal of thin filament
is not overlapped by the thick filament in the
Contracted state of muscle
1. The H- zone greatly narrowed or disappear
2. The width of the band is reduced but the width of the
A- band remain we changed . Thus in each sarcomere
the I- bands becomes narrows -> the muscle fiber as a
whole short ends -> the muscle belly as a whole
shortens.
Explanation : as the muscle begins to contact , the thin
filaments starts to move -> the thin filaments move
towards the H- zone of a sarcomere move towards the
H- zone .
1. H- zone becomes obliterated ,
2. I- band is narrowed
3. The sarcomere as a whole is shortened .
Types of contraction
1. Isometric contraction : means contraction on which
there is no change in length of muscle, but there is
increase in tension .
2. Isotonic contraction : means contraction in which there
is change of length at constant tension . The tension is
equal to the weight lifted during contraction of the
muscle .
Neuromuscular junction
The skeletal muscle fibers are innervated by large,
myelinated nerve fibers that originated in the large
motoneuron of the anterior horns of the spinal cord . As
pointed out in nerve fiber, after interning the muscle
belly , normally because and stimulates from three to
several hundred skeletal muscle fiber. Each nerve ending
makes a junction called the “Neuromuscular junction” ,
with the muscle fiber near its midpoint, and the resulting
action potential in the muscle fiber travels in both
direction towards the muscle fiber ends with excention of
about 2% of the muscle fiber there is only one such
function per muscle fiber.
Neuromuscular Junction
Spread of excitation : electrical excitation spreads to the
interior of the muscle fiber by means of the transversely
oriented T- tubules which are formed by invagination of
the sarcolemma since T- tubules aig. Deep into the
muscle fiber encircle every myofibril and are repeated at
every
A-I junction . They conduct the action potential
throughout the length and breadth of the muscle fiber
within a very short time .
Role of calcium in contraction and relaxation
Calcium mediated the effect of excitation to the contractile
proteins of muscle that is why inerea in the contraction of
sarcoplasmic calcium leads to contraction. Relaxation is
due to removal of calcium ions into the cisternae by active
transport. An active calcium pump resides in the cisternal
membrane which concentrates calcium ions in the
cisternae. The pump is overpowered only movementasily
during excitation by the opening up of a large membrane
of calcium channels.
In order due to understand how calcium ions bring
about contraction it is essential to learn more about
muscle proteins.
Characteristics of muscle contraction
One have so fore discussed muscle contraction at the
level of a single sarcomere now we shall study some
characteristics of the contraction of a whole muscle
and examine how they can be explained in tern of
function at molecular and microscopic level. This
excuse will also lead to the discovery of some defects
in the cross bridge and sliding filament theories of
muscle contraction.
Z line
Z line
Mechanism of skeletal muscle
Many mechanical characteristics of skeletal muscle were
explained by means of to very similar models
proposed by Hill in 1951 and Albert in 1956.
This models are no longer very popular because of there
short comings and because of over better
understanding today of the real structure in muscle at
molecular level but it is still quiet convenient to make
these models becomes of their simplicity.
These are basically three component three components
models in which skeletal muscle is believed to have
contractile elements. Contractile elements are the
filaments which being about contraction through their
rearrangement , series elastic elements are mainly the
tendons, and parallel elastic elements are the
sarcolemmal and connective tissue element disposed
parallel to muscle fiber.
Motor unit : The unit of contraction
Skeletal muscle are supplied by motor nerves. A motor nerve
consist of several nerve fiber. Each motor nerve fiber is the
axon of a motor neuron. The cell body of leis in the
anterior column of the spinal cord. After entering the
muscle belly, a motor nerve fiber supplies several muscle
fiber. A motor neuron together with the all muscle fiber
supplied by it is called “motor unit.” the muscle fibers of a
motor unit are scattered throughout the muscle belly.
Therefore even when only the muscle fiber of one motor
unit contract, the whole muscle contracts. The larger
number of motor unit activated stronger the contraction.
Motor Unit
All the muscle cells controlled by one nerve cell
A muscle may be activated by activation of a variable
number of nerve fibers since the unit of activation is a
nerve fibers, the unit of contraction is a motor
unit.
The number of muscle fiber in a motor unit varies from
to several hundred the precision with which the
contraction of a muscle may be geaded depends on the
size of the motor unit. For e.g. if a motor unit has 5
muscle fibers, number of muscle fibers activated at a
time may 5,10,15,20 and so on the other hand.
If a motor unit has 100 muscle fibers , the number of
muscle fiber activated at a time may be
100,200,300,400 and so on. The other hand muscle of
lag have large motor units and there fore only poorly
graded contractions. The size of motor units in a muscle
seen to be related to the functional necessity for
precision in grading of the contractive strength. Further
all the motor units in a given muscle are not exactly
equal. During weak contraction of a muscle, only small
motor units are activated. Stronger contraction is
achieved by recruitment of progressively larger motor
units.
Length- tension relationship : The forcefulness of
muscle contraction depends on the length of the
sarcomere within the muscle before contraction begins
at a sarcomere length of about 2.0 – 2.4 um. The zone
of overlap in each sarcomere is optimal and the muscle
fiber can develop maximum tension.
As the sarcomeres of a muscle fiber are stretched to a
longer length the zone of overlap shortens , and few of
myosin heads can make contact with thin filaments.
So the tension, the fiber can produce decreases. when a
skeletal muscle fiber is stretched to 170% of its optimal
length, there is no overlap between the thick and thin
filaments. Because none of the myosin heads can bind to thin
filaments. The muscle fibers can not contract, and tension is
zero. As sarcomere length become increasingly shorter than
the optimum. The tension that can develop again decreases.
this is because thick filaments. Crumple as they are
compressed by the z discs, resulting in fewer myosin heads
making contact with thin filaments.
Normally resulting muscle fiber length is held very close to
the optimum by firm attachments of skeletal muscle to bones
and to other inelastic tissue, so that our stretching does not
occur.
Energetic of muscle contraction :
Work output during muscle contraction : when a muscle
contracts against a load, it perform work. This means that
energy is transferred from the muscle to the external load,
for eg, to lift an object to a greater height or to overcome
resistance to movement.
In mathematical terms work is defined by the following
equation :
W=l *d
In which w is the work output, l is the load , and d is the
distance of movement against the load , the energy required
to perform the work derived from the chemical reactions in
the muscle cells during contraction.
Skeletal muscle tone :
Even when muscles are at rest, a certain amount of
tautness usually remains. This is called muscle tone.
Because skeletal muscle fibers do not contract without an
action potential to stimulate the fibers , skeletal muscle
tone results entirely from a low rate of nerve impulse
coming from the spinal cord. This in turns are controlled
partially by impulse transmitted from the brain to the
appropriate anterior motor neurons and partly by impulses
that originated in muscle spindles located in the muscle
itself.
Motor unit recruitment :
The process in which the number of active motor units
increases is called motor unit recruitment. typically., the
different motor units in a whole muscle are not stimulated
to contract in unison. While some motor units are
contracting, others are relaxed. This pattern of motor units
actively delays muscle fatigue by allowing alternately
contracting motor units to relieve one another. In this way ,
contraction of a whole muscle can be sustained for a long
period. The weakest motor units are recruited first, with
progressively stronger motor units being added if the task
requires more force.
Recruitment is one factor responsible for producing smooth
movements rather than a series of jerks. As mentioned, the
number of muscle fibers innervated by one motor neuron
varies greatly, precise movements are brought about small
changes in muscle contraction. Therefore, the muscles that
produce precise movement are made up of small motor units
for this reason when a motor unit is recruited of turn of only
slide changes occur in muscle tension. By contract, large
motor unit are active where large tension is needed and
precision is less important.
Energy supply for muscle contraction :
We have seen that the immediate source of energy for
muscle contraction is ATP. But the ATP used must be
replenished promptly if the contraction has to
continue for any length of time because all the ATP
in a muscle can provide energy for muscle contraction
for only LS. The immediate source of replenishment of
ATP is creative phosphate.
Creatine phosphate + ADP --> creatine + ATP.
Creatine phosphate is stored in muscle set depleted
completely in just 3s. If the muscle is contracting
maximally for replenishment of creatine phosphate and
APP the next fuel, which can last much longer is glucose.
A muscle can get glucose from two source : from the blood
flowing through the muscle and from the breakdown of
glycogen stored in muscle can also last only a few minutes.
Therefore the only steady source of glucose is blood glucose
which in turn is replenished is the post absorptive state by
the breakdown of liver glycogen.
The breakdown of glucose into lower energy compounds
(glycolysis) may be divided into two stapes. One stage up to
the formation of the 3 – carbon compound pyruvic acid
yields only two molecules of ATP for each molecule of
glucose broken down. But this stapes does not need oxygen
and is therefore also known as aerobic glycolysis. Aerobic
glycolysis yields much more energy than anaerobic glycolysis
another 34 molecules of ATP. Thus a total of 36 molecules
of ATP is obtained from the oxidative breakdown of each
molecule of glucose. The body stored of glucose and glycogen
also cannot sustain heavy exercise beyond 100min.
The reason why we can continue with exercise much longer
is because the body has yet another fuel in reserve ie fat. The
fat stored in muscles and else where can be broken down to
yield free fatty acids which are a concentrated source of
energy. Theoretically, amino acids can also be broken down
to live energy they are spared for protein synthesis. It is only
in starvation conditions that protein reserves of the body are
drawn upon for providing energy.
To summarize there is a chain of fuels to replenish the ATP
which is broken down for muscle contraction there is
considerable overlap in the stapes at which different links in
the chain become operative. For example even brief bursts of
contraction for which ATP and creatine phosphate. Longer
exercise for which enough glucose is available in the body
are also partly energized by breakdown of fat. As the
duration of exercise increases, the proportion of energy
provided by fat increases and that provided by
carbohydrates decreases.
fat are a more concentrated source of energy is that they
provide more calories per gram but they are less efficient in
that they provide less energy per liter of oxygen consumed.
Also the maximum rate of energy production in case of fats
than carbohydrates. That is perhaps the reason why the
body show slight preference for using carbohydrates as
compared to fats. In general the rate of energy production by
a fuel smaller is the amount stored in the body with high
rate of energy production serve as immediate source of fuel
where as those of lower rates of energy production serve as
energy stores.
Heat production in muscle :
From a biochemical point of view, heat production by a
tissue is a by product of energy expenditure. Energy is
spent primarily for staying alive and for staying alive
for specific function of the tissue concerned. But since
the utilization of energy is not perfect part of the
energy released in the tissue is dissipated as heat.
Muscle is a tissue the energy expenditure of which
differs markedly at rest as compared to that during
activity. Therefore although an un stimulated muscle
produces heat, the heat production increases during
and immediately after contraction.
The amount of heat produced is small during any phase but
is of great theoretical and historical interest. The heat
production was first measured very accurately by A.V hill
in 1939, long before the molecular basis of contraction has
been worked out. On the basis of heat production hill had
deduced many aspects of muscle function which have been
borne out by subsequent research. Heat produced during
different stapes of rest and activity is designated by specific
terms, some of which are confusing or ambiguous.
Resting heat :
Resting heat is the heat produced in stimulated muscle
before shortening. It is likely to be a by product of energy
spent on released of calcium from the terminal cisternae
binding of calcium by the sarcoplasmic reticulum. The last
named process begins as soon as the calcium released and
accounts for about half the activation heat. There is also
a term, initial heat, which is best avoided because some
author used it as a synonym for activation heat while
others use it to imply the sum of activation heat and
shortening heat.
Shortening heat :
As the name indicates this is the heat associated with
shortening. The amount of shortening heat depends
on the degree of shortening and the velocity of
shortening. Since there is no shortening in isometric
contraction, there is no shortening heat associated
with it. shortening heat seems to be a by product of
the energy spent on ualchet mechanism involving
myosin cross bridges and the achieve sites on action
filaments.
Maintain heat :
Maintain heat is the heat produced during tetanus. It is
partly made of activation heat associated with each
stimulus and partly of the heat produced due to actin
myosin interaction.
Relaxation heat :
As the name indicates, relaxation heat is associated
with relaxation. It is due to the energy expenditure
associated with the uptake of calcium by the terminal
cisternae.
Recovery heat or delayed heat :
This is due to the over and above the resting heat –
after contraction and relaxation are over. This is due
to the restoration of the resting state. Replenishment
of the energy stored of the muscle and correction of
the slight imbalance of in sodium and potassium
concentration brought about by the action potential
need energy expenditure. Consequently this process
also lead to dissipation of heat. Although these
process soon side by side with contraction, they also
continue for some time afterwards, especially in case
of problem contraction.
Neuromuscular Junction. :
The functional region between the motor nerve fiber
and the corresponding skeletal nerve fiber is called
neuromuscular junction.
A typical neuromuscular is seen only in the skeletal
muscle, smooth muscle or cardiac muscle do not have
such typical structure.
Load and Tension When a muscle contract ( Isotonic or
Isometrically) it generated a force. This force is called
tension. This tension is applied to an object which is moved
(lifted). The resistance offered by the object is called the
load and is expressed in engineering units. E.g. kg wt is
mass in kg x gravity )
To take a concrete example assume that
the biceps ( flexor of elbow joint ) is contracting. When it
contracts, the weight of the forearm ( i.e. the mass of
forearm x gravity)
opposes the shortening of the biceps, therefore the wt of the
forearm is the load against which the tension generated by
the contraction of the biceps is working.
Muscle Twitch :- A single action potential causes a brief
contraction followed by relaxation. This response is called a
muscle twitch. The action potential and the twitch are
plotted on the same time scale. The twitch starts about 2 m
after the start of depolarization
of the membrane before repolarization is complete.
the duration of the twitch varies with the type of muscle
being tasted “fast” muscle fibers, primarily those concerned
with fine, rapid. precise movement have twitch duration as
short
as muscle “slow” muscle fibers principally. Those involved in
strong, gross, sustained Movements, have twitch duration
up to 100 ms.
The twitch contraction is the brief contraction of all the
muscle fibers in a motor unit in response a single. Action
potential in its motor neuron of its muscle fibers.
The record of muscle contraction called a myogram is shown
in twitches of skeletal muscle fibers last anywhere from 20
to 200 on sec. this duration is very long compared to the
brief 1to 2 m sec duration of an action potential.
Note that a brief occurs b/w application of the
stimulation (time zero on the graph) and the beginning of
contraction. The delay, which lasts, about two milliseconds,
is termed the latent period. During the latent period,
calcium ions are being related from the sarcoplasmic
retinaculum, the filament start to exert tension.
Others, such as those that move the legs are slow-twitch
fibers, with contraction and relaxation period of about m
sec each.
If two stimuli are applied one immediately after
the other, the muscle well respond to the first stimulus but
not to the second, when a muscle fibers receives enough
stimulation to contract. It temporarily loses its excitability,
called the refractory period, is a of all muscle and nerve
cells. The duration of the refractory period varies with the
muscle involved.
skeletal muscle has a short refractory period of about five
milliseconds, whereas cardiac muscle has a long refractory
period of about 300 milliseconds.
Types of contraction :- Isotonic contraction are used for body
movements and for moving objects. The two types of
isotonic contraction are concentric and eccentric. In a
concentric isotonic contraction a muscle shortens and pulls
on another structure such as a tendon, to produce movement
and to reduce the angle at a joint.
picking a book up off a table involves concentric isotonic
contraction of the biceps brachia muscle in the arm. By
contract, as you lowers the book to place it back. On the
table the previously shortness biceps lengthens in a
controlled manner while it continues to contract. when the
length of a muscle arises during a contraction the
contraction is an eccentric isotonic contraction. During an
eccentric contraction, the tension exerted by a myosin cross
bridges resists movements of a load (the book in this case)
and slows the lengthening process.
for isotonic contraction produce more muscle damage and
more delayed – onset muscle soreness than to concentric
isotonic contraction.
In isometric contractions, the myosin cross bridges generate
considerable tension but the muscle doesn’t shorten because
the force of the load equals the muscle tension. An example
would be holding a book steady using an outstretched arm.
These contraction are important for maintaining posture
and for supporting objects as a fixed position.
although isometric contractions. Do not result in body
movement, energy is still expanded. The book pulls the arm
downward stretching the shoulders and arm muscles. The
isometric contraction of the shoulder and arm muscles
counteracts the stretch. Isometric contraction are important
because they stabilize. Some joints as others are moved.
Most activities include both isotonic and isometric
contraction.
Benefits of stretching :
The overall goal of stretching is to achieve normal range
of motion of joints and mobility of soft tissues
surrounding the joints. For most individuals, the best
stretching routine involves static stretching, that is,
slow sustained the muscle should be stretched to the
point of slight discomfort ( not pain ) and held for
about 15 to 30 seconds. Stretching should be done
after warming up as this will increase range of
motion, among the benefits of stretching are the
following.
1.
Improved physical performance : a flexible joint has
the ability to move through a greater range of motion,
which improves physical performance.
2. Decreased risk of injury :stretching decreases
resistance in various soft tissues and thus there is less
likelihood of exceeding maximum tissues extensibility
during all activity. This decreases the risk of injury.
3. Reduced muscle sources : stretching can reduce some of the
muscle soreness that results from delayed on set muscles
soreness (DOMS) after exercise.
4. improved posture : poor posture results from improper
posture and gravity over a number of years, stretching can
help realign soft tissues to improve and maintain good
posture.
Applied physiology :
Rigor mortis : After death cellular membrane start to
become leaky. Ca++ ion leak out of the sarcoplasmic
reticulum into the cytosol and allow myosin heads to
bind to actin. ATP synthesis has ceased . However ,
so the cross bridge can not detach from actin. The
resulting condition, in which muscles are in a state of
rigidity (can not contract or stretch) is called rigor
mortis (rigidity of death).
Rigor mortis begins 3 – 4 hours after death and lasts about
24 hours, then it disappears as proteolytic enzymes from
lysosomes digest the cross bridge.
Aerobic training versus strength training : regular the
supply of oxygen reach blood available to skeletal muscle
for aerobic cellular respiration by contract, activities such
as weight lifting rely more on anaerobic production of ATP
through glycolysis. Such anaerobic activities stimulate
synthesis of muscle proteins and result, over time, in
increased muscle size.
As a result aerobic training builds endurance for prolong
ate activities, where as aerobic training builds muscle
strength for short term feat. Internal training is a work out
regimen that incorporates both types of training for
example alerting sprints with jogging.
Anabolic steroids : the illegal use of anabolic steroids by
athletes has received wide spread attention. This steroid
hormones, similar to testosterone, are taken to increase
muscle size and thus strength during athletic contests.
The large doses needed to produce an effect. Hormones have
damaging , some times even demonstrating side effects,
including lever cancer , kidney damage increased rest of
heart disease, stunted growth and increased irritability,
females who take anabolic steroid may experience atrophy
of the breast and uterus, menstrual regulates, sterility,
facial hair growth, and deepening of the voice. Males may
experience diminished testosterone secretion atrophy of the
testis, and baldness.
Tenosynovitis : Tenosynovitis commonly known as
tendonitis, is a pain full inflammation of the tendon.
Tendon sheaths and synovical membranes of joint the
tendon most often affected are at the wrist shoulders,
elbows resulting finger joints, ankles, and feet. The
affected sheaths some times become visibly swollen due to
fluid accumulation . The joint is tender and movement of
the body part often causes pain. Trauma, strain, or
excessive exercise may cause Tenosynovitis. For instance,
tying shoelaces too tightly may cause Tenosynovitis of the
dorsum of the foot. Also gymnasts are prone to developing
maximum hypertension of the wrists.
Smooth muscle :
Introduction : smooth muscle is distinguished
anatomically from skeletal and cardiac muscle
because cross striation. Actin and myosin-2 present
and they slide on each other to produce contraction.
Smooth muscle also contain tropomyosin, but it is
poorly developed.
Types :
Smooth muscle like cardiac tissue, smooth muscle is usually activated
involuntarily of the two types at smooth muscle tissue. The more
common type is visceral smooth muscle tissue. It is found in up
round sheets that from part of the walls of small arteries and veins
and hollow organs such as the stomach, intestine, uterus and
urinary bladder like cardiac muscle, visceral smooth muscle is
autothymic because the fiber connect to one another by gap
junctions, muscle action potential spread throughout the network
when a neurotransmitter, hormones, or autothymic signals
stimulates one fiber.
Distribution :
This muscle are present in all hollow viscera eg.
Gastro intestinal tract, ducts of the glands, blood
vessels, respiratory, and lymphatic systems at the
body. This are also present in the dermis, ciliary
body and iris of the eye. The automatic
contraction at the smooth muscle fibers facilitates
the movement at the contents that are passing
though the above mentioned viscera.
Origin and Development
The smooth muscles are mesenchymal in origin. The
mesenchymal cells first start to stretch out. The nucleus
becomes elongated and myofilament appear in the wall of
the tube at regular intervals and developed in the same
fashion as already mentioned and ultimately forming the
continuous circular and longitudinal muscular layers of
blood vessels. There is also evidence that is smooth muscle
cells themselves can divide by mitosis.
Physiology of smooth muscle
Although the principles of contraction are similar in all
three types of muscle tissue, smooth muscle tissue
exhibits some important physiological differences
compared with contraction in skeletal muscle fiber.
Contraction in smooth muscle fiber starts more slowly
and lots much longer, moreover , smooth muscle can
both shorten and stretch to a greater extent than
other muscle types.
An increase in the concentration and of calcium in smooth
muscle cytosol initiates contraction, just as in striated
muscle. Calcium ions flow into smooth muscle cytosol from
both the interstitial fluid and sarcoplasmic reticulum.
Several mechanisms regulate contraction and relaxation of
smooth muscle cells. In one a regulatory protein called
calmodulin binds to calcium in the cytosol . Myosin light
chain kinase work rather slowly, also contribution to the
slowness of smooth muscle contraction. The prolonged
presence of calcium in the cytosol provides for smooth muscle
tone, a state of continuous partial contraction.
smooth muscle tissue can thus sustained long term tone,
which is important in the gastrointestinal tract, where the
walls maintain a steady pressure on the contents of the
tract, and in the walls of blood vessels called arterioles,
which maintain a steady pressure on blood. Most of smooth
muscle fibers contract or relax in response to action
potential from the autonomic nervous system. In addition,
many smooth muscle fibers contract or relax in response to
stretching, hormones, or local factors such as changes in ph,
oxygen and carbon dioxide levels, temperature and ion
concentration.
Histology :
The smooth muscle fibers are elongated, fusiform with a
wider central portion where the single nucleus is situated.
Both the ends of the fibers are tapering towards the
periphery. The average length is about 0.2mm which varies
much and the width is about 6u at the central widest
portion. The cells of the smooth muscles are so arranged that
the thick middle portion of one is juxtaposed by the thin end
portion of the other.
So in the transverse section there have round or irregularly
polygonal profile of various size only the largest profile will
demonstrate the centrally placed nucleus. Delicate uniform
chromatin network is present in the nucleoplasm. There are
two or more nucleoli in ordinary preparation, the sarcoplasm
is quite homogeneous, but in special preparation. “fine
longitudinal striations” may be demonstrate running full
length of the fiber .
This are the myofibrils and are interpreted as the myofibrils
and are of the smooth muscles. A small golgi apparatus is
also present near the nucleus . Sarcoplasm contents
considerable amount of glycogen. The surface of the smooth
muscles has a thick basal lamina.
Fine structure :
In electron micrograph, the nucleus appears as
elongated, smooth surfaced and rounded at the ends.
The mitochondria are present at the poles of the
nucleus. Myofilaments are intepressed by the
mitochondria which are also arranged along the long
axis.
Cell to cell relation :
the adjacent smooth muscle cells are separated by the thick
basilar lamina . These are the suggested site for cell to cell
cohesion. At some areas the basal lamina are absent where
the unit membranes are coming in close contact to each other
having a similar fusion pattern as that at the tight junctions
at the cardiac intercalated disc. The fascia occludents or
nexuses. The nexuses is probably a low resistance area
through which rapid propagation of excitation impulse is
possible from one cell to other in case at the smooth muscle.
Mechanism of contraction, excitation,
contraction, coupling :
Smooth muscle contain actin and myosin. At the time of
contraction, myosin heads bind with the actin
filaments as in cardiac or skeletal muscle leading to
sliding of the actin filaments shortening.
However, mechanism of excitation contraction coupling
is different smooth muscle, is as follows :
When the smooth muscle is activated, say by an action
potential or a chemical like ACH, Ca ions from ECF,
via Ca++ channels of cell membrane enter the inside
of the cell.
Here, the ca ions combine with a protein calmodulin to a ca
calmodulin complex .
Calcium calmodulin complex now binds with an enzyme
MLCK
myosin light chain kinase
now MLCK is activated
myosin is now phosphorylated by the active MLCK enzyme.
ATP acting as phosphate donor.
This phosphorylated myosin can attach with actin so that
the sliding of actin becomes possible.
General properties of smooth muscle
The properties can be described under the following
headings:
1. Autorythmicity
2. Factors influencing the performance of the smooth
muscle
3. Tone
1. Autorythmicity : in the organs where the single unit
are present (e.g. : intestine, uterus, ureter.), there are
“pace makers”. The pace maker generate impulses which
are propagated and stimulate the whole bundle of the
smooth muscle.
Sympathetic and parasympathetic stimulation can alter the
activity of the pace maker, and hence of the Autorythmicity,
but enervation cannot abolish the spontaneous contraction
totally.
Factors influencing the performance of smooth muscle
a. Stretch : stretch causes excitation of smooth muscle,
when spontaneous rhythmic contractions are already
present. Stretch causes sustained contractions.
b. Effects of catecholamine : effect of catecholamine
depends upon the organ in which the smooth muscle is
present, the nature of receptors as well as on the species.
c. Effect of acetyl choline : acetyl choline stimulates the
smooth muscle and increases the force of contraction.
d. Effects of autonomic stimulations
Like the catecholamines , stimulations of the autonomic
nerves produces effects which depend on the organ,
the receptors and the species.
e. Gastro intestinal hormones :
Gastrointestinal hormones have profound effects on the
contractions of the smooth muscles of the
gastrointestinal tract.
f. Effects of the classical hormones:
On uterus, estrogen causes excitation where as,
progesterone causes depression of the excitability,
oxytocin causes contraction of both, uterine muscles
and myoepithelial cells of the mammary gland and
thus helps in parturition and milk ejection.
Tone :
Tone of skeletal muscles is completely last when its
motor nerves are destroyed. On the other hand the
isolated intestine, in the tissue organ both, maintains
a tone.
Nerve supply :
Smooth muscles are supply by sympathetic and
parasympathetic nerves. This control activities of
smooth muscle but are not responsible for initiation
actively.
Neuromuscular junctions of smooth muscle :
The physiologic anatomy of smooth muscle
neuromuscular junction : neuromuscular junctions of
the highly structured type found on skeletal muscle
fibers do not occur in smooth muscle. Instead, the
autonomic nerves fibers that innervate smooth muscle
generally branch diffusely on top of a sheet of muscle
fibers.
In most instances , this fibers, do not make direct contact
with the smooth muscle fibers but instead from so called
“diffuse junction”. That secrete their transmitter substance
into the matrix coating of the smooth muscle a few
nanometers to a few micrometer away from the muscle cells,
the transmitter substance than diffuses to the cells, the
nerve fibers often innervate only the outer layers , and the
muscle excitation than travels from this outer layer to the
inner layer by action.
Potential conduction in the muscle mass by additional
diffusion of the transmitter substance.
Excitatory and inhibitory transmitter substances secreted at
the smooth muscle neuromuscular junction. The most
important transmitter substances secreted by the autonomic
nerves innervating smooth muscle are acetylcholine and nor
epinephrine but, they are never secreted by the same nerve
fibers.
Acetylcholine is a excitatory transmitter substance for
smooth muscle fibers in some organs but an inhibitory
transmitter for smooth muscle fiber, nor epinephrine
ordinarily inhibits it. Conversely, when acetylcholine
inhibits a fiber nor epinephrine usually excites it some of the
receptors proteins are excitatory receptors, whereas others
are inhibitory receptors.
Thus, the types of receptors determine whether the smooth
muscle is inhibited or excited and also determines which of
the two transmitters, acetylcholine or nor epinephrine is
effective in causing the excitation or inhibition.
Effects of hormones on smooth muscle contraction :
Most of the circulatory hormones in the body affect
smooth muscle contraction to some degree in some
have profound effects. Among the more important
blood born hormones that affect contraction are nor
epinephrine, epinephrine, acetylcholine angiotensin
vasopressin, oxytocin, serotonin and histamine.
A hormone causes contraction of a smooth muscle when the
muscle cell membrane contains hormones- gated excitatory
receptors for the respective hormones.
Type :
Smooth muscle at not two sites is the same. But
conventionally it is divided into two broad categories,
single unit and multi unit.
Single unit smooth muscle :
This type of smooth muscle behaves as it the entire muscle
mass is a single unit. The gap junction between muscle
fiber are so generous that the impulse of activation can
spread rapidly from one cell to another. Such an
arrangement is also called as functional syncytium.
Although single unit smooth muscle has a nerve supply,
the nerve supply only reverse to modulate its activity,
which can still be modulated to a considerable extent by
hormones intrinsic factors.
Multiunit smooth muscle :
Multiunit smooth muscle resembles skeletal muscle in
that each muscle fiber has independent nerve supply,
and activation of a muscle fiber does not lead to
activation of its neighboring fibers. Autonomy and
automicity of multiunit smooth muscle is much less
than that of single unit smooth muscle. Classical
examples of this type of smooth muscle are found in
the ciliary muscles of the eye, iris and vas deferens.
Cardiac muscle :
The cardiac muscle (involuntary striated ) contracts
rhythmically and automatically, which is particularly
maintained the life process of the living system by
assisting the supply of nutrients oxygen and removal
of metabolic waste products.
Distribution :
The cardiac muscle are actually forming the muscular
body of the heart. This muscles are also present in
small amounts in the great vessels ending in or
opening from the heart.
Histology :
The cardiac muscle fibers are separated from each other
by the connective tissue endomysium along with
blood vessel and lymphatic. The cardiac muscle fiber
are not made at one straight simple cylinder but they
have gat short cylindrical branches in all directions.
This branches are coming in contact with that of the
adjacent fibers, ultimately forming a three
dimensional network.
Under like microscope these network appears is syncytium
which was also supported as the property of the cardiac
muscle that if contracts it will contact as a whole. But
electron micrograph reveals these branches are not acting as
cytoplasmic bridge but they are separated from each other
by special surface specialization, the intercalated disc. The
sarcolemma of the cardiac muscle is more or less similar to
skeletal muscle. The mitochondria are more numbers and
the cytoplasm is more abundant. The mitochondria are
arranged longitudinally and are present in between the
myofibrils in rows.
A small gorge apparatus is present at one pole with a few lipid
drop lets. As age increase the lipofuchsin pigments are
deposited near the nucleus and may be much extensive to give
the brownish appearance of the heart the brown atrophy of the
heart.
Fine structure under electron microscope the myofibrils made
up of Myofilaments, are more are less similar to that of the
skeletal muscle. This myofibrils are delineated by the
sarcoplasmic reticulum and sarcoplasm. Sarcoplasm contends a
large amount of mitochondria along the long axis.
Mitochondria often remain completely surrounded by
Myofilaments. so the Myofilaments of the cardiac muscle
from a continuum which looks like a large cylindrical mass
made up of a parallel myofilaments subdivided by fusiform
clefts of the sarcoplasm occupied by mitochondria.
Sarcotubular system :
1. T –system : this T-system is the tubular imagination of
the sarcolemma of the cardiac muscle and is larger in
diameter than that of the skeletal muscle. The T- tubules
are present at the z line in the cardiac muscle fiber, but
the same are at the A-I junction in case of the mammalian
skeletal muscle. The functional significance of the
locational difference is not yet understood. These tubules
also increase the surface for metabolic exchange in
between the interior of the cardiac muscle fiber and the
intercellular space, over and above they function for quick
propagation in pulse from the cell surface to the interior.
2. Sarcoplasmic reticulum : this reticulum of the cardiac
muscle fiber is ill developed. They are consisted of
longitudinal interconnected tubules which are expanded into
small terminal sacs at the Z- line. There is not well
developed transverse cisternae in the cardiac muscle.
Physiology of cardiac muscle :
The heart is composed of three major types of cardiac
muscle. Atrial muscle, ventricular muscle, and
specialized excitatory and conductive muscle fiber.
The atrial and ventricular types of muscle contract in
much the same way skeletal muscle. The except that
the duration of contraction is much longer conversely.
The specialized excitatory and conductive fibers
contract only feebly because they contain few
contractile fibrils instead, they exhibit rhythmicity
and varying rates of conduction, providing an
excitatory system that controls the rhythmical beating
of the heart.
Bibliography :
1.
2.
3.
4.
5.
6.
7.
C.C. CHATTERJEE
R.L. BIJLANI
TORTORA
WILLIAM F. GANONG
GUYTON AND HALL
S.K. CHAUDHARY
K. SEMBULINGAM