MUSCLE

MUSCLE

7.   MUSCLE

Muscle cells are specialized cells to generate the forces and movement that are used by the multicellular organism in the regulation of the internal environment and to produce the movements of the entire organism in its external environment.
Muscle cells act as chemical apparatus for these activities. Indeed, it is only by controlling the activity of muscles that the human mind ultimately express itself.
*    One the basis of structure, contractile properties and control mechanism, three types of muscle can be identified 
(1)     Skeletal muscle         
(2)     Smooth muscle and     
(3)     Cardiac muscle

Most skeletal muscles are attached to bone and its contraction is responsible for supporting and moving the skeleton. Sheets of smooth muscle surround various hollow organ and tubes, including the stomach, intestinal tract, urinary bladder, uterus, blood vessels and the airways in the lungs. Single smooth muscle cells are also found distributed throughout organs and in small bundles of cells attached to the hair in the skin and the iris of the eye.
Contraction of the smooth-muscle surrounding hollow organ or it may propel the luminal contents through the organ, or it may regulate internal flow by changing the tube diameter. 
Smooth-muscle contraction is controlled by the autonomic nervous system, hormones, paracrine and other local chemical signals, some smooth muscles contract spontaneously, however, even in the absence of such signals.
Cardiac muscle is the muscle of the heart. Its contraction propels blood through the circulatory system like the smooth muscle. It is regulated by the autonomic nervous system and hormones and a certain portion of it undergoes spontaneous contraction.
Structure of skeletal muscle and muscle fibers
A single skeletal muscle is known as a muscle fiber. each muscle fiber is formed during development by the fusion of a number of undifferentiated, mononucleated cells, known as myoblasts into a single, cylindrical, multi nucleated muscle fiber. 
This stage of muscle differentiation is completed around the time of birth, after which new fibers are not normally formed. The existing fiber continues to increase in the size with the growth of the child.
If skeletal muscle fibres are destroyed after birth due to injury, they can not be replaced by the division of other existing muscle fibre, However, new fibre can be formed from undifferentiated cells known as satellite cells located in the extracellular compartment adjacent to the muscle cell.

Muscle fibre : Each muscle fiber has a diameter between 10 and 100 mm and a length that may extend upto 20 cm. The term muscle refers to a number of muscle fibre bond together by connective tissue and usually linked to bones by the bundle of collagen fibres known as tendons located at each end of the muscles.
Tendons: The relation between single muscle fibre and a muscle is analogous to that between a single neuron and a nerve, which is composed of the axons of many neurons. In some muscle, the individual fibres extend the entire length of the muscle, but in most the fibres are shorter, often oriented at an angle to the longitudinal axis of the muscle. These short fibers are anchored to the connective tissue network surrounding the muscle fibres. The transmission of force from muscle to bone is like a number of people puling on a rope each person corresponding to a single muscle fibre and the rope corresponding to the connective tissue and tendons.
Microscopic structure of skeletal muscle fibre : 
The most striking feature seen when observing a muscle fibre with a light microscope is a series of light and dark bands oriented perpendicular to the long axis of the fibre. Both skeletal and cardiac muscle fiber have this characteristic banding and are known as striated muscles.
This striated pattern is due to the presence of cytoplasmic fibrils of approximately cylindrical elements (1 to 2 mm in diameter) known as myofibril. Most of the cytoplasm is filled with myofibril, each of which extends from one end of a fiber to the other.
Each myofibril is composed of thick and thin filaments arranged in a repeating pattern along the length of the myofibril, one unit of this repeating pattern is known as a sarcomere.
The thick filament is composed almost entirely of the contractile protein myosin and the thin filament (about half the diameter of the thick filament) contain the contractile protein actin along with as two other proteins troponin and tropomyosin. That play important role in regulating contraction.
The thick filament is located in the middle of each sarcomere, where their arrangement produces the wide, dark band known as the A band.
In contrast, each sarcomere contains two sets of thin filament, one at each end. One end of each thin filament is anchored to a network of the interconnecting protein known as a Z line. Two successive Z lines define the limit of one sarcomere. Thus, thin filament from two adjacent sarcomeres is anchored to each Z line.
The I band lies between the ends of the A band of two adjacent sarcomeres and contain those portions of the thin filament that do not overlap the thick filament. It is bisected by the Z lines.
Two additional bands are present in the A-band region of each sarcomere. The H zone is a relatively light region in the center of the A band. It corresponds to the space between the ends of the two sets of thin filament in each sarcomere. Hence, only the central part of thick filament is found in the H zone. Finally, the narrow, dark line in the center of the H zone is known as the M line.
A cross-section through the A band of six adjacent myofibrils shows the regular, almost crystalline arrangement of the thick and thin filament. Each thick filament is surrounded by a hexagonal array of six thin filaments and each thin filament is surrounded by a triangular arrangement of thick filaments.
The space between adjacent thick and thin filament is bridged by projection known as cross bridges which portions of myosin molecules that extend from the surface of the thick filament toward the thin filament.
During muscle contraction, these cross bridges make contact with the thin filament and apply force on them. The cross bridges are the force generating sites in muscle cells.
When the muscle fibre is contracted, the length of sarcomere is about 2 mm. At this length, the actin filament completely overlaps the myosin filament and the tips of the actin filaments are just beginning to overlap one another. At this length, the muscle is capable of generating its greatest force of contraction.
The sarcolemma is a thin membrane enclosing a skeletal muscle fibre. The sarcolemma consist of a true cell membrane called the plasma membrane and an outer coat made up of a thin layer of polysaccharide material that contain numerous thin collagen and fibrils at each end of the muscle fibre, the surface layer of the sarcolemma fuses with a tendon fibre. The tendon fibres, in turn, collect into bundles to form the muscle tendons that then insert into the bones.

Titian filamentous molecules keep the myosin and actin filaments in place. The relationship between action and myosin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin. The molecular weight of a titin molecule is about 3 million, which makes it largest protein molecule present in the body. It is very springy.
These springy titin molecule act as a framework that holds the myosin and actin filament in place so that the contractile machinery of the sarcomere will work. One end of the titin molecule is elastic and attached to the Z-disk acting as a spring and changing length as the sarcomere contract and relaxes. The other part of the titin molecule tethers to the myosin thick filament.
Sarcoplasm is the intractacellular fluid between myofibrils containing large quantities of potassium, mg+2 and phosphate and multiple protein enzyme. The tremendous number of mitochondria are also present. These supply energy in the form of adenosine triphosphate (ATP) for contracting myofibril. The sarcoplasmic reticulum is a specialized endoplasmic reticulum of skeletal muscle present in sarcoplasm surrounding the myofibril of each muscle fiber. This reticulum has a special organization that is extremely important in controlling muscle contraction.
The general mechanism of muscle contraction :
*    An action potential travels along a motor nerve to it's ending on muscle fiber.
*    At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine.
*    The acetylcholine act on a local area of the muscle fibre membrane to open a multiple "acetylcholine gated" cation channels through protein molecule floating in the membrane.
*    This allows large quantities of sodium ions to diffuse into the interior of the muscle fibre membrane. This causes a local depolarization that in turn leads to the opening of the voltage-gated sodium channel. This initiates an action potential at the membrane.
*    This action potential travels along the muscle fibre membrane in the same way that action potential travel along nerve fibre membrane.
*    The action potential depolarizes the muscle membrane and much of the action potential electricity flows through the centre of the muscle fibre. Here, it causes the sarcoplasmic reticulum to release large quantities of Ca++ ions that have been stored within the reticulum.
*    The Ca++ ions initiate attractive forces between actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.
After a second action potential or after a second stimulus, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along. This removal of Ca++ ions from the myofibrils causes muscle contraction to cease.

Molecular mechanism of muscle contraction :
When a skeletal muscle fiber is activated, the cross bridges bind to the thin filament and exert a force on them. In other words, for shortening of the muscle, the forces exerted on the thin filament must be greater than the force opposing shortening. The term contraction, as used in muscle physiology does not necessarily mean shortening rather it shows only to the turning on of the force generating sites. After contraction, the mechanism that initiates force generating site is turned off and tension generation declines, producing relaxation of the muscle fibre. In a relaxed state, the ends of the actin filament extending from two successive Z discs begin to overlap one another conversely. In the contracted state, these actin filaments have been inward along the myosin filaments. So their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filament upto the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism.
But what causes the actin filaments to slide inward between the myosin filament? This is caused by forces generated by the interaction of the cross-bridges with actin filament. 
Under the resting condition, these forces are inactive But when an action potential travels along the muscle fibre, the forces between actin and myosin are activated. This occurs through Ca++ ions release by sarcoplasmic reticulum.
During shortening, each cross bridges attached to the thin filament moves in a swivelling motion just like oars of a boat. This swivelling motion forces the thin filament to either end of the bond toward the M line thus shortening the sarcomere occur.
One stroke of a cross bridge produces only a small movement of a thin filament relative to a thick filament while the force generating mechanism remains active, the cross bridges will repeat their swivelling motion many times, producing a large movement that is made up of a series of very small steps.
But energy is needed in this whole process. This energy comes from high energy bonds in the ATP molecule, which is degraded to ADP to liberate energy.
The sequence of events that occurs between the time when a cross bridge binds to a thin filament the time which it again binds to a thin filament to repeat the process is known as cross bridge cycle. This interaction between activated actin filament and the myosin cross bridge is known as "The walk-along theory of contraction" or ratchet theory of contraction. Each cycle consist of four steps-
(1)    Attachment of a cross bridge to a thin filament.
(2)    Movement of the cross bridge producing movement of the thin filament.
(3)    The detachment of the cross bridge from the thin filament.
(4)    And the movement of the cross bridge into a position where it can again reattach to a thin filament and repeat the cycle.
Note: Each cross bridge undergoes its own cycle of movement independently to the other cross bridge, so that at any instant during contraction, only about 50 per cent of the cross-bridge is attached to the thin filaments and are producing movement.
A muscle fibres ability to generate force and movement depends on the interaction of the two contractile proteins myosin in the thick filament and actin in the thin filament and energy provided by ATP. An enzymatic site that is ATPase present on myosin that catalyzes the hydrolysis of ATP to ADP and Pi releasing the chemical energy for the process. The chemical and physical events occuring during the four steps of a cross bridge cycle are as follow.
Step-1: Initially the ATP bound to myosin was split, releasing the chemical energy. This energy was transferred in myosin (M), producing an energized form of myosin, which is a product of ATP hydrolysis. ADP and inorganic phosphate are still bound.

Step-2: When a muscle fiber is stimulated to contact the energized myosin cross bridges binds to an action (A) molecule in a thin filament.
Step-3: The binding of this energized myosin to action triggers the discharge of the energy stored in myosin, with the resulting movement of the bound cross bridge. The ADP and Pi are released from myosin during the cross bridge movement.

Step-4: During the cross-bridge movement, myosin bind very firmly to actin and this linkage must be broken in order to allow the cross-bridge to reattach to a new actin molecule and repeat the cycle. The binding of a molecule of ATP to myosin is responsible for breaking the link between actin and myosin.
The dissociation of actin and myosin by ATP is an example of allosteric regulation of protein activity. The binding of ATP at one site on myosin decreases the affinity of myosin for actin found at another site on myosin.
Following the dissociation of actin and myosin, the ATP bound to myosin is split (Step-4) thereby reforming the re-energized state of myosin. Which can now reattach to the actin filament and repeat the cycle as long as the fibre remain activated?
ATP plays two distinct roles in the cross-bridge cycle
(1)    The energy released from ATP hydrolysis ultimately provides energy for cross-bridge movement.
(2)    The binding (not hydrolysis) of ATP to myosin breaks the link formed between actin and myosin during the cycle allowing the cycle to the repeated.
The importance of ATP in dissociating actin and myosin during step-3 of a cross-bridge cycle is illustrated by rigour mortis (the stiffening of skeletal muscle) which begins several hours after death and is complete after about 12h. Because the ATP production in cells including muscle cells decline after death because the nutrients are no longer supplied. In the absence of ATP, cross bridges can bind to actin but the subsequent movement of the cross bridge and breakage link between actin and myosin does not occur because they require the binding of ATP. The thick and thin filament become bound to each other by immobilized cross bridges, producing the rigid condition of a dead muscle. Thus, the absence of ATP caused muscle stiffness.

Molecular characteristics of the contractile fillaments -

As we discussed above, actin and myosin filaments are two important contractile filaments which have a very important role or we can say the principal role in muscle contraction and cross-bridge movement.
Myosin Filaments are composed of multiple myosin molecules. Each of the myosin molecules has an M.W. about 480,000. The organization of many molecules forms myosin filament. The interaction of this filament on one side with the ends of two actin filament. The myosin molecule is composed of six polypeptide chains two heavy chains and four light chains. Two heavy chains wrap spirally around each other to form a double helix, which is called the tail of myosin molecule. One end of each of these chains folded bilaterally into a globular polypeptide structure called a myosin head. Thus, there are two free heads at one end of the double-helical myosin molecule.
The four light chains are also part of the myosin head two to each head. These light chains help control the function of the head during muscle contraction.
Cross-bridges : 

The myosin filaments made up of 200 or more individual myosin molecule. The central portion of one of these filaments display the tails of the myosin molecule bundled together. Form the body of the filament, heads of the molecule protrude outside the body and also some part of the body is hanging outward along with the head and form arm.
The protruding arm and head together called cross-bridge; which are principle force generating site of muscle fibre. 
The globular head contains a binding site for actin and an enzymatic site an ATPase that catalyzes the hydrolysis of ATP to ADP+ Pi releasing the chemical energy stored in ATP.
*    Actin filaments are composed of actin, tropomyosin and troponin.
The backbone of the actin filament is a double-stranded F-actin protein molecule. These two strands are wound in a helix in the same manner as the myosin molecule. Each strand of the double F-actin helix is composed of polymerized G-Actin molecules. Each has a M.W around 42,000 one molecule of ADP attached with each G-actin molecule.
It is believed that these ADP molecules are the active sites on the actin filament with which the cross-bridges of the myosin filament interact to cause muscle contraction. The active site on the two F-actin strands of the double-helix is staggered, giving one active site on the overall actin filament about every 2.7 nanometers. Each actin filament is about 1 mm long.
Tropomyosin molecules: The actin filament also contains another protein tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. The molecules are wrapped spirally around the sides of the F-actin helix.
In resting state, tropomyosin lies on the active sites of actin filament so that attraction cannot occur between the actin and myosin filament to cause contraction.
Troponin and its role in muscle contraction or role of Ca++ ion in muscle contraction - 
Troponin molecules are attached along the side of the tropomyosin molecule. These are actually complexes of the three solely bound protein molecule or subunits, each of which plays a specific role in muscle contraction. One of the subunits (troponin I) has a strong affinity for actin.
Troponin I →    affinity for actin
Troponin T →    affinity for tropomyosin
Troponin C →    affinity for Ca++ ions
Role of Ca++ ions in muscle contraction - Since every muscle fiber contains all the ingredents necessary for cross bridge activity - actin, myosin and ATP - the question arises why are muscles not in a continous state of contractile activity? The answer is [in a continuous state of resting muscle fibre, the cross-bridge is unable to bind to actin. This inhibition is due to troponin and tropomysin, which are located on the actin filament. Tropomyosin is a rod-shaped molecule composed of two inter-twined polypeptides with a length approximately equal to that of seven actin molecule. Two chains of tropomyosin molecule are arranged end to end along the two strand of actin double helix, where they partially preventing the cross bridge from making contact. With this tropomyosin held in this blocking position by troponin, i.e. a smaller globular protein is bound to both tropomyosin and actin. One molecule of troponine bound to each molecule of tropomysin.
What allows the cross bridge activity to occur?
In order for the cross bridge to bind with actin, the tropomysin molecule must be moved away from the actin blocking position. This occurs when Ca++ binds to a specific binding site on troponin i.e. 'troponin C'. The binding of Ca++ produces a change in the shape of troponin such that it pulls the tropomyosin out of it's blocking position, uncovering the cross-bridge binding site on actin. Conversly, removal of actin from troponin reverses the process and tropomyosin moves back into its blocking position so that cross-bridge activity is prevented.
Thus cytosolic Ca++ ion concentration determines the number of troponin sites occupied by Ca++ which in turn determines the number of cross-bridges that can bind to actin and exert the force on the thin filament. These changes in cytosolic Ca+2 ion concen is controlled by electrical occuring in the muscle plasma membrane.
Effect of muscle length on the force of contraction in the whole intact muscle :

Relation of muscle length to tension in a muscle fibre in given figure (Length-tension diagram) for a single fully contracted sarcomere. Showing maximum strength of contraction when the sarcomere is 2.0 to 2.2 micrometres in length.
The relative position of actin and myosin filament at different sarcomere length from point A to point D is also shown in the above diagram.
The amount of tension developed by a muscle fiber and thus its strength can be altered not only by changing the frequency of stimulation but also by changing the length of the fibre.
The length at which the fibre develops the greatest tension is termed the optimal length (lo)
-    At longer length, the tension is increased up to a maximum at lo and further lengthening leads to a drop in muscle.
When muscle fibre length is 60% of the fibre,  no tension develops when stimulated.

Relation of muscle length to tension in the muscle both before and during contraction:-
Above figure depict tension of the intact whole muscle rather than of a single muscle fiber. The whole muscle has a large amount of connective tissue in it. Also, the sarcomeres in the different part of muscle do not always contract equally. Therefore, the curve has somewhat different dimensions from those shown for the individual muscle fibre.
When the muscle is at its normal resting length, with a sarcomere length of about 2 micrometres, It contract upon activation with the approx maximum force of contraction.
However, the Increase in tension that occurs during contraction called active tension, decreases as the muscle is streached beyond its normal length-i.e. sarcomere length greater than about 2.2 mm. 
Note: In tension-length relation at length less than Lo tension declines because the overlapping of the two actin filament interfere with cross-bridge binding and less Ca++ release from sarcoplasmic reticulum at short fibre length, the reason behind this is unknown.

Loud-velocity Relation: A skeletal muscle contracts rapidly when it contracts against no load. When loads are applied, the velocity of contraction becomes progressively less as the load increases. Contraction velocity decrease while undergoing stimulation decreases with increasing load.
The Shortening velocity is determined by the rate at which the Individual cross bridge undergo their cyclic activity.
For complex reasons, increasing load on a cross bridge decreases the rate at which the bridge proceeds through step Z, i.e. the force generating step in the cross-bridge cycle. The slower the rate of cross-bridge cycling, the slower the shortening velocity.
Energetics of muscle contraction :
Work output during muscle contraction: When a muscle contract against a load, it performs work. Energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement.
W   =  L  X  D
W = Work output    L = Load        
D = Distance of movement against the load.
Source of energy for muscle contraction: ATP provide energy for muscle contraction. There are three major functions of ATP in skeletal muscle contraction-
1.    Hydrolysis of ATP by myosin, energizes the cross-bridges providing the energy for force generation.
2.    Binding of ATP to myosin dissociate cross bridge from actin, allowing the bridges to repeat their cycle of activity.
3.    Hydrolysis of ATP by the ca+2 ATPase in sarcoplasmic reticulum provides the energy for the active transport of calcium ions (Ca++) into the lateral sacs of the reticulum, lowering cytosolic calcium, ending the contraction and allowing the muscle fibre to relax.
In no other cell type, the rate of ATP break-down increase so much from one moment to the next as in a skeletal muscle fiber when it consumes preformed ATP that exists at the start of the contractile activity. If this were the only source of ATP, cross-bridge movement will be blocked and a state of rigour mortise achieved.
There are three ways a muscle fibre can form ATP during contractile activity : 
(1)     Phosphorylation of ADP by creatine phosphate 
(2)    Oxidative phosphorylation of ADP in mitochondria
(3)    Substrate phosphorylation of ADP primerily by the glycolytic pathway in the cytosol.
*    Metabolic pathway producing the ATP-

Types of muscle contraction: The force exerted on an object by a contracting muscle is known as muscle tension and the force exerted on the muscle by an object is known as a load.
Isometric versus isotonic contraction: When a muscle develops tension but does not shorten or lengthen, the contraction is said to be Isometric (constant length). Such contraction occurs when the muscle supports a load in a constant position or attempts to move a load that is greater than the tension developed by the muscle.
-    A contraction under the condition in which muscle length shortening occur. It is said to be Isotonic contraction. A muscle undergoes an isotonic contraction when moving a load. In short, Isometric muscle does not shorten during contraction. When it does shorten but the tension on muscle remains constant throughout contraction.
The characteristics of Isotonic muscle depends on load against which, muscle is contracted whereas isometric muscle's characters do not depend on the object.
Therefore, the isometric system is most often used when comparing the functional characteristics of different muscles types.
The third type of contraction is a lengthening contraction. This occurs when the load on a muscle is greater than the tension being generated by the cross bridges. In this action, the load pulls the muscle to a longer length. For e.g. such action occurs when you sit down from a standing position or walk down a flight of stairs. It is not an active process produced by contractile proteins but a consequence of the external forces being applied to the muscle.
Note: In isometric contraction, there is no change in the position of actin and myosin. But force is generated whereas in Isotonic contraction actin slide over myosin and sarcomeres come close. So shortening of muscle occurs.
Ocular Muscles respond rapidly and having a short contraction time. Wheras muscles like soleus respond very slowly.
Duration of Isometric contraction -
Ocular muscle        →    1/50 second
Gastro cnemius muscle    →    1/15 second
Soleus muscle        →    1/5 second
Twitch contractions
The mechanical response of a muscle fiber to a single action potential is known as a twitch.
Isometric and Isotonic twitches: A major difference between isometric and isotonic twitches is that in isometric twitches, there is a short latent period (few milliseconds) whereas, in isotonic twitches, latent period is longer as compare to Isometric.
Latent period:-
The patent period is a time required by muscle to show the response after stimulation.
The latent period show lag phase of muscle contraction due to-
*    Opening of voltage gated Na+
*    Releasing the Ca++ ion from the sarcoplasmic reticulum.
*    Binding of Ca++ ion with Troponin.
*    ATPase activity during depolarization (Contraction)

Skeletal muscle receives stimulus from neurons. When neurons are in a relative retracted period, it can be stimulated by a strong stimulus. When neurons are in R.R.P. muscle are in repolarization stage. So muscle can generate the nerve action potential.
Isometric twitch of skeletal muscle fiber -

An Isotonic twitch of a skeletal muscle fiber

In isometric twitch, following the action potential, there is an interval of a few milliseconds known as the latent period, before initiation of the tension in the muscle fibre. During this latent period, the process associated with excitation-contraction coupling is occurring.
The time from the end of the latent period to the peak tension is known as contraction time. Comparing an isometric twitch with an isotonic twitch in same muscle fibre we can see that the latent period is longer in an isotonic twitch but the duration of shortening is longer i.e. the contraction time is longer than the isometric twitch.
The Characteristics of an isotonic twitch depend upon the magnitude of the load being lifted.
Isotonic twitches with different loads :

At heavier load, the latent period is longer. the velocity of shortening (distance shortened), the duration of twitch decreased.
Muscle fatigue
Muscle fatigue is a feature of slow twitch fibre. 
When a skeletal muscle fiber is continuously stimulated at a frequency that produces maximal contraction, the tension development by the fibers eventually declines. This failure of muscle fibre to maintain tension as a result of the previous contractile activity is known as muscle fatigue. If twitches are repeated frequently muscle fibres undergo fatigue. In short, prolonged and strong contraction of muscle leads to muscle fatigue. Therefore, fatigue results from Inability of contractile and metabolic processes of muscle fiber. When a muscle is excited during the relaxation stage of muscle twitch, the muscle show response and undergo contraction without completing Ist relaxation. But the 2nd stimulation should be stronger that 1st one. Successive strong stimulation cause "muscle fatigue" means in this condition, ATP ends up. So muscle becomes the permanently contract.


Frequency summation and tetenization : 
If individual twitch contractions occurring one after another at the low frequency of stimulation, then as the frequency increases, there comes a point where each new contraction occur before the proceeding one is over. As a result, the second contraction is added partially to the first, so the total strength of contraction rises progressively with increasing frequency. When the frequency reaches a critical level, the successive contraction eventually becomes so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous. This is called tetanization any additional increase in frequency beyond this point has no further effect in increasing contractile force. This occurs because enough Ca++ ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials.
Types of skeletal muscle fibre 
Different types of fiber can be identified on the basis of 
(1)     Their maximal velocities of shortening: fast and slow fibers
(2)    The major pathways used to form ATP: Oxidative and glycolytic fibers.
Fast versus slow muscle fibres : 
Every muscle of the body is composed of a mixture of fast and slow muscle fibre.
Muscles that react rapidly, including the anterior tibialis are composed of mainly 'fast fibres' conversly, muscles such as soleus that respond slowing but with prolonged contraction are composed mainly of 'slow' fibres. The differences between these two types of fibres are as follow-
Slow fibres (red muscles) →
(1)    Smaller fibres (fatigue resistant fibre) 
(2)    Innervated by smaller nerve fibres.
(3)    More extensive blood vessel system and capillaries to supply extra amount of oxygen.
(4)    Greatly increased number of mitochondria, to support oxidative metabolism.
(5)    A large amount of myoglobin combines with oxygen and store it until needed. The myoglobin gives the slow muscle a reddish appearence named red muscle.
-    Slow twitch fibers are good for endurance activities like long distance running or cycling. They can work for a long time without getting tired.
Example: Soleus muscle
Fast fibers (white muscle) → 
(1)     Large fibers for great strength of contraction.
(2)    Extensive sarcoplasmic reticulum for rapid release of Ca++ to initiate contraction.
(3)    A large amount of glycolytic enzyme for the rapid release of energy by the glycolytic process.
(4)    Less extensive blood supply.
(5)    Fewer mitochondria.
Fast twitch fibres are used for a rapid response like running, jumping. They are faster-fatigue fibre.
Fast and slow twitch fiber : 
Relaxation time short in fast fiber and long in slow twitch fiber.
The second means of classifying skeletal-muscle fibres is according to the type of enzymatic machinary available for synthesizing ATP. Some fibers contain numerous mitochondria thus use oxidative phosphorylation. They are oxidative fibers. Often present in Red muscles. In contrast, glycolytic fibres-have few mitochondria but posses a high concentration of glycolytic enzyme and a large store of glycogen. They often present in white muscle.
Slow oxidative fibre - Combine low myosin-ATPase activity with high oxidative capacity.
Fast oxidative fibre - Combine high myosin - ATPase activity with high oxidative capacity. Both are present in red muscle and have a large number of mitochondria, myoglobin, blood vessels and capillaries.
Fast-glycolytic fibres: Combine high myosin-ATPase activity with high glycolytic capacity.
Smooth muscle :
Smooth muscles are present in hollow organs like stomach etc. Two characters are more common in all smooth muscles.
They lack the cross-striated bonding pattern found in skeletal and cardiac. They are directly under involuntary control and nerves of autonomic nerves system direct innervate them.
Smooth muscle, like skeletal muscle, uses cross bridge movement between actin and myosin filament to generate force and Ca++ to control cross-bridge activity.
-    Organization of contractile proteins or filament and the mechanism of contractile-excitation coupling is also quite different.
Types of smooth muscle: Based on the excitability characters of the muscle membrane and on the conduction of electrical activity from fibre to fibre within the muscle tissue 
(1)    Single - Unit smooth muscle    
(2)    Multiple-unit smooth muscle 


(1)    Single-unit smooth muscle: Membrane is capable of propagating action potential from cell to cell and may manifest spontaneous action potential.
Ex.- Muscles of the intestinal tract, the uterus and small diameters blood vessels.
All the fibres in single-unit smooth muscle undergo synchronous activity, both electrical and mechanical.
-    That means the whole muscle responds to stimulation as a single unit.
-    Each muscle fibre link to adjacent muscle fibre by gap junction through which small ions can move from cell to cell, carrying electric current. So, action potential occurring in one cell is propagated by local current through gap junction into adjacent cells.
Multiunit smooth muscle: Exhibit little propagation, of electrical activity from fibre to fibre and whose contractile activity is closely coupled to the neural activity.
Ex.- Smooth muscle in the large airways to the lungs in the large arteries and attached to the hairs in the skin.
-    Because this type of muscle have fed gap junction so each muscle fibre responds independently.
-    These muscles are richly innervated by branches of the autonomic nervous system.
-    The contractile response of the whole muscle depends on the number of muscle fibre that is activated and on the frequency of nerve stimulation.


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