Muscle is one of the four tissues in the body and plays a key role in the locomotion of higher animals. Besides locomotion, muscles are also associated with digestion, respiration, circulation, excretion and reproduction. There are three main types of muscles: smooth or unstriated muscle, skeletal or striated muscle, and cardiac muscle.
This article explains the structure and characteristics of muscles, with particular focus on skeletal muscle and its role in movement.
Table of Contents
1.1 Muscle
Muscle is one of the four tissues in our body. It performs a key role in locomotion of higher animals. In addition to locomotion, it is also associated with digestion, respiration, circulation, excretion and reproduction.
There are three types of muscles-
- smooth or unstriated or unstriped muscle,
- skeletal or striated or striped muscle and
- cardiac muscle
Smooth muscles occur in the wall of internal organs such as digestive tract, arteries, veins etc. They are spindle-shaped cells having centrally located single nucleus.
The cardiac muscles are unique types of muscles found only in the heart. These are distinctive in possessing an inherent and basic rhythmic beat i.e. contraction and relaxation.
Skeletal muscles contract most rapidly and vigorously and are attached to the bones or tendons. Hence, they are mainly responsible for the movement of the skeleton and therefore of the body. Skeleton muscles are formed by long cylindrical, multinucleated cells or fibers and appear striated both in longitudinal and cross sections. Each muscle fiber is composed of fibrils or myofibrils, having characteristically alternating light and dark regions.
1.1.1 Ultrastructure of Skeletal Muscle
Skeletal muscle fiber is an elongated multinucleated cell completely invested by the cell membrane called sarcolemma. It consists of a semifluid cytoplasm called sarcoplasm containing numerous longitudinal fibrils called myofibrils. Sarcoplasm contains several proteins, enzymes, nuclei, numerous mitochondria, a small Golgi body, sarcoplasmic reticulum, (sarcosomes), lipid droplet, glycogen and many more.
The sarcoplasmic reticulum is a system of tubules (sarcotubules) that does not open to the exterior but the unit of the other system, the transverse tubules open to the exterior of the fiber. These tubules are inwardly directed extensions of the sarcolemma and their cavities are thus in continuity with the fluid which surrounds the fibrils that provide a simple means of conveying an electrical signal.
Each myofibril is the contractile unit or element of the muscle fiber. The light and dark bands of many myofibrils give its characteristically striated appearance. The striations or repeating pattern of light and dark bands are similar for each myofibril. The dense bands are called A or anisotropic while lighter bands are called I or isotropic bands and the middle of the I band is called Z- band or Z- line as a narrow dark line.
Henceforth, sarcomere is defined as that length of a myofibril bounded by two adjacent Z bands and are called the smallest functional unit of muscle. Within the center of the A band is a lighter region of low density known as the H zone which is visible in relaxed fibrils. In the center of H zone lies the 'pseudo H-zone', a region of even lower density that maintains its width, no matter how the length of the muscle changes. The light zone surrounds a dark thin line called the M line which lies in the center of the sarcomere. A thin N band may be seen within the I band, between Z and A bands.
H.E. Huxley has shown that each myofibril is composed of two types of filaments (myofilaments)-
- Primary or thick filaments and
- Secondary or thin filaments.
Each myofibril is formed of a large number of each type of filaments, arranged in such a way that each primary filament is surrounded by six secondary filaments, which are in turn shared with six surrounding primary filaments.
The repeating band pattern in the myofibrils is due to the alternate of overlapping of thick and thin filaments, the dark bands occurring both kinds of filaments overlap and the light band only one type of filament overlap. The dense Z-lines are continuous across the width of each muscle fiber and hold the myofibrils together. The two kinds of filaments are linked together by an intricate system of cross-bridges which play an important role in muscle contraction.
The cross-bridges lirk the actin (thin) and myosin (thick) filaments together which project outward from both the ends of a myosin filament at a regular interval. In the center of the myosin filament there is a bridge free region which is responsible for the pseudo H zone in the center of A bands. H.E. Huxley showed that cross bridges are the heads of the myosin molecules and all such heads (zig-zag lines) or the cross-bridges have the enzymatic and actin-binding properties. During muscle activity, each cross-bridge is attached to an active site on the actin filament at a definite rate and then is detached from it.
When an action potential i.e., changes of the potentiality of nerve fiber travels along a motor nerve to its endings on the muscle fibers, the nerve releases the neurotransmitter substance, acetylcholine which acts on the local area of the muscle fiber membrane and allows a large number of sodium ions to enter into the muscle fiber membrane establishing depolarization.
Depolarization leads to opening of voltage-gated sodium channels starting an action potential at the membrane which then travels along the muscle fiber membrane. It depolarizes the muscle membrane and the flow of the action potential ultimately causes the sarcoplasmic reticulum to release large quantities of calcium ions stored within them. Those calcium ions provide the attractive forces between the actin and myosin filaments to slide each other i.e., the contractile process. The calcium ions are pumped back into the sarcoplasmic reticulum after a fraction of a second by a calcium ion membrane pump. The ions remain stored in the reticulum until a new muscle action potential comes along.
1.1.2 Molecular Mechanism of Muscle Contraction: Sliding-filament theory
The Sliding filament or Interdigitation theory was proposed independently by A.F. Huxley and R. Niedergerke and by J. Hanson and H.S. Huxley (1958). According to this theory, the two kinds of filaments i.e., actin or thin filaments and myosin or thick filaments slide past each other so as to produce change in the length of the muscle.
In the relaxed state, the ends of the actin filaments derived from two successive Z-discs rarely overlap each other while at a same time completely overlapping the myosin filaments. On the other hand, in the contracted state these actin filaments have been pulled inward among the myosin filaments so that they now overlap each other to a major extent.
The Z-discs have been pulled by the actin filaments up to the ends of the myosin filaments. The actin filaments can be pulled together so tightly that the ends of the myosin filaments actually buckle during very intense contraction. Thus, muscle contraction occurs by a sliding filament mechanism.
The factors which cause actin filaments to slide inward among the myosin filaments are mechanical, chemical or electrostatic forces generated by the interaction of the cross-bridges from the myosin filaments with the actin filaments. Normally under resting condition, these forces are inactive, but in contraction the forces become activated.
‘Walk Along' Theory of Contraction
A hypothesis called the 'walk along' theory (or ratchet theory) of contraction is is postulated to explain the manner by which the interaction between the cross bridges and the actin causes contraction. When the head of two cross bridges attaches to active sites of an actin filament, it causes changes in the intramolecular forces between the head and arm of its cross-bridge. It again causes the head to tilt towards the arm and to drag the actin filament along with it. The tilt of head is called the power stroke.
The head breaks away from the active site immediately after tilting and returns to its extended direction. It then combines with the new active site along the actin filament and the heads tilt to cause a new power stroke. The actin filament moves another step and thus, the heads of the cross bridges walk step by step along the actin filament, pulling the ends of two successive actin filaments toward the center of myosin filament.
Energy of Contraction
The energy required for it is provided by adenosine triphosphate (ATP) which is degraded to adenosine diphosphate (ADP) to liberate the energy. The immediate source of energy for recharging of muscle are phosphagens. They transfer their high-energy phosphate and later they are reenergized by respiration. The reaction occurs as follows.
Phosphocreatine + ADP → Creatine + ATP
The ATP plays an important role in muscles as a source of energy for contraction and is responsible for:
- The interaction of actin and myosin which cause shortening
- The synthesis of phosphocreatine from creatine and phosphate.
- The synthesis of glycogen from lactic acid which is necessary because the conversion of glycogen to lactic acid is another source of energy to synthesis ATP other than the phosphocreatine reaction.
1.1.3 Chemical Basis of Muscle Contraction
Chemical composition of muscle fiber
Each muscle contains about 75% water, about 20% protein and 5% others such as glycogen, inorganic ions and a variety of organic substances like free amino acids and phosphagens (phosphocreatine in vertebrate muscle and phosphoarginine in invertebrates).
The muscle proteins are of three types:
- Stroma proteins.
- Ordinary cellular proteins.
- Special contractile proteins.
The stroma proteins constitute about 1/5th of the total muscle protein which can be identified with collagen and elastin elements that bind muscle fibers together and transmit tension to tendons.
The ordinary cellular proteins constitute 1/5th of the total muscle protein which are not specifically characteristic to muscle cells but are important as the enzymatic proteins catalyze various chemical reactions of the muscle cells.
Special contractile proteins called actin, myosin, tropomyosin are important contractile proteins characteristic of muscle cells.
Myosin is the most abundant protein and the thick filaments of the A-band are composed of it. The thin filaments of the I-band are composed of actin and very likely tropomyosin. Myosin molecules are the tad pole-shaped with high molecular weight of about 500,000. It can be broken down by digestion with trypsin into head and tail fragments.
The head is globular in shape with a molecular weight of about 3,5000 and is called Heavy meromyosin (HMM). The tail fragment is called Light meromyosin (LMM) with a molecular weight of about 150,000.
HMM has two chemically different active sites- one of which binds to actin molecule and at the other, hydrolysis of ATP occurs. LMM is a rod like structure probably consisting of two alpha helics coiled around each other like a two-stranded rope.
Actin occurs in monomeric form and polymeric form. They are called globular actin molecule, or G-actin in monomeric form and each molecule possesses a molecule of ATP firmly bound to it, but lacks the ATPase activity. The actin molecules are fiber like in polymeric form and are called fibrous actin or F-actin.
The F-actin is formed by polymerization of globular actin molecules in solutions of high ionic concentration. They are in the form of a two-stranded rope. The G-actin molecules can stick together only if the ATP bound to them becomes hydrolyzed to ADP with the release of energy.
G-actin-ATP → F-actin-ADP + Pi
Another protein, tropomyosin plays an important role in sensitizing the contractile proteins to calcium. Tropomyosin A and Tropomyosin B are two known forms of tropomyosin.
Chemical Events in Muscle Contraction
The flow of energy and chemical reactions in the muscle during contraction have been described by Loewy and Siekevitz (1966).
Firstly, myosin reacts with ATP to form a myosin-ATP complex. Then in the presence of K+ and F-actin, actomyosin-ATP complex is formed.
Myosin + ATP → Myosin - ATP complex
Myosin-ATP + F-actin K+ → Actomyosin-ATP complex
The complex in presence of Ca2+ and ATPase splits the ATP to inorganic phosphate and acto-myosin-ADP complex.
Actomyosin - ATP complex —(Ca2+, ATPase)→ Actomyosin - ADP + Pi
Again, the complex breaks down to free actin, free myosin and ADP.
Actomyosin - ADP complex → Actin + Myosin + ADP
So, muscle contraction is turned on by the release of Ca2+ and the hydrolysis of the ATP of actomyosin-ATP complex which provides required energy for the two kinds of filaments to slide past each other during contraction. The relaxation is caused by binding Ca2+ back into the sarcoplasmic reticulum sacs inhibiting the breakdown of actomyosin-ATP complex.
The Ca2+ acts as a trigger for muscle contraction and performs its role by combining with troponin which otherwise puts a restraint on interaction of actin and myosin. The muscle activity is continued by the continuous removed of ADP that accumulates in contraction or in the other words, by the conversion of ADP to ATP which is done in several ways.
Consequently, contraction of muscle occurs in following steps:
- Arrival of an impulse.
- Reduction of electric potential or depolarization of the fiber membrane.
- Release of calcium.
- ATPase activation.
- Hydrolysis of ATP at each site where cross-bridge and actin filament interact to generate a sliding force between the two kinds of filaments.
Relaxation occurs in following stages:
- Exhaustion of the nerve impulse.
- Withdrawal of calcium.
- Recreation of electric potential or repolarization of the fiber membrane.
- ATPase inhibition.
