Skeletal Muscles

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Introduction

Skeletal muscles are the most abundant tissue forum in the human body. More than 40% or our body is made up of skeletal muscles. These are the voluntary muscles that are under the control of the somatic nervous system. Most of the skeletal muscles are attached to bones via specialized connective tissues.

In this article, we will study the organization of skeletal muscles, the general structure of a skeletal muscle fiber, and the excitation and contraction of skeletal muscles. 

Organization

Each skeletal muscle is a tissue made up of multinucleated cells called skeletal muscle fibers. These fibers extend throughout the length of the muscle. The skeletal muscle fibers are arranged in the form of groups called muscle fasciculi. Many muscle fasciculi join together to form a skeletal muscle. 

Skeletal Muscle Fiber 

A skeletal muscle fiber is a multinucleated cell and acts as a single contractile unit of skeletal muscles. It can undergo contraction and relaxation independent of the rest of fibers. The following are the various components of a skeletal muscle.

Sarcolemma

Sarcolemma is the plasma membrane that encloses the entire skeletal muscle fiber. It is also sometimes called myolemma. It is known to be made up of two layers;

  • An inner layer called true plasma membrane
  • An outer layer of polysaccharides called glycocalyx

The true plasma membrane is a lipid bilayer and serves the same function in skeletal muscle fibers as in the other cells of the body. It separates the intracellular and extracellular fluids acting as a selectively permeable barrier between the two media. It contains several ion channels that help in the contraction of the skeletal muscles. The innervation of the skeletal muscle fiber also takes place via sarcolemma. 

The glycocalyx above the plasma membrane also contains several collagen fibers. It serves to hold the fibers together. At each end of the skeletal muscle fibers, the glycocalyx layers of several fibers merge and fuse with the tendon, attaching the muscle fibers to the bones. 

T-tubules

At some places along the length of the fiber, the sarcolemma invaginates into the cytoplasm to form transverse tubules called T-tubules. Each T-tubule is connected with smooth endoplasmic reticulum on either side to form a triad. These triads serve a special role in the contraction of skeletal muscles. 

Myofibrils

Each skeletal muscle fiber is composed of several hundred to several thousand of myofibrils. These myofibrils extend throughout the length of the skeletal muscle fiber. 

Each myofibril is further composed of two types of filaments, thin filaments and thick filaments.  There are about 3000 thin filaments and 1500 thick filaments in each myofibril. The two filaments are arranged in a myofibril in such a way that they interdigitate and give a striated appearance to the myofibril. The overall striated appearance of skeletal muscles is due to these interdigitating filaments. 

Thin filaments

The thin filaments are made up of two types of proteins, actin and tropomyosin. The backbone of thin filaments or actin filaments is made up of F-actin filaments that are the polymerized G-actin molecules. Each G-actin molecule in these filaments have a molecule of ADP that acts as an active site for the formation of cross-bridges. 

The tropomyosin is another filamentous protein that is wrapped around the F-actin to form thin filaments. Tropomyosin is wrapped in such a way that it covers the active sites and prevents the formation of actin-myosin cross-bridges in resting state.

The part of myofibril containing only thin filaments appear as light bands called the I bands. They are called so because they are isotropic to polarized light.

Thick filaments

These are made up of myosin protein. Myosin molecules polymerize to form thick filaments in myofibrils. These thick filaments have small projections at both ends called the myosin cross-bridges. An interaction occurs between the thin filaments and cross-bridges of thick filaments that results in the contraction of skeletal muscle fiber. 

Each myosin molecule in thick filaments is made up of six polypeptide chains, two heavy chains, and four light chains. The two heavy chains are coiled around each other to form a double helix that forms the tail of myosin thick filaments. Each end of the heavy chain is folded onto itself to form a globular structure along with two light chains. The globular structure is called myosin head. There are two myosin heads at the end of each tail. The cross-bridges of thick filaments are made up of myosin heads along with the arms by which they are attached to the myosin filament.

The areas of myofibril containing thick filaments along with the interdigitating ends of thin filaments have a dark appearance and are thus called dark bands. They are also called A bands because they are anisotropic to polarized light. 

Z-Disk

The ends of thin filaments are attached to a filamentous structure called Z-disk. The actin filaments extend from Z-disk and interdigitate with myosin filaments. 

The Z-disk is made up of a filamentous protein, having a different structure than actin or myosin. Each Z-disk extends from one myofibril to next, connecting several myofibrils in a skeletal muscle fiber to one another. The area between two successive Z disk has alternate light, dark and light bands. The same pattern is seen in all the myofibrils giving a striated appearance to the skeletal muscle fibers that together make the entire skeletal muscle. 

Sarcomere

The part of myofibril between the two successive Z-disks is called sarcomere. Sarcomeres are the contractile units of myofibrils. Each sarcomere is composed of an I band, an A band and again an I band, respectively. 

Each A band in sarcomere has a slightly light area called the H zone as it has only thick filaments. The rest of the A band on both sides of the H zone has interdigitating thick and thin filaments and appears darker. A line bisects the H zone, called the M line. This line divides the sarcomere into two halves. 

The length of the sarcomeres decreases whenever the skeletal muscle contracts. In a fully contracted state, each sarcomere in skeletal muscle fibers has a length of 2 micrometers with the actin filaments completely overlapping the myosin filaments. At this length, the H zone disappears, and the tips of actin filaments just begin to overlap. A skeletal muscle generates maximum contractile force at this length of sarcomeres.

Titin filaments

The actin and myosin filaments are kept in proper position by filaments of another protein called titin. The titin filaments are very springy and elastic in nature. 

One end of each titin molecule is attached to the Z-disk while the other end is attached to the myosin filaments present in the center of the sarcomere. The Z-disk end is very elastic and acts as a spring that can charge length during cycles of contraction and relaxation. 

Sarcoplasm

It is the cytoplasm of skeletal muscle fibers. Sarcoplasm is the fluid that lies between the successive myofibrils in a skeletal muscle fiber. They lie suspended in this fluid. 

Sarcoplasm is rich in potassium, magnesium and phosphate ions. A large amount of enzymes is also present in it.

Mitochondria also lie among the myofibrils in sarcoplasm. They are arranged parallel to the myofibrils and serve to supply energy in the form of ATP so that contraction can occur. 

Sarcoplasmic Reticulum

It is the specialized endoplasmic reticulum present in the skeletal muscle fibers. it appears as a network of tubules that is present among the myofibrils in the sarcoplasm. They have dilated terminal sacs called the terminal cisternae. In between the two terminal cisternae lies the invagination of the sarcolemma, the t-tubule. The two-terminal cisternae along with at-tubule in between them form a structure called triad. 

The function of the sarcoplasmic reticulum is to store calcium ions. These ions are released for the contraction of muscle fiber once the nerve impulse reaches the sarcoplasmic reticulum. 

Innervation

The contraction of skeletal muscle takes place upon excitation by a nerve fiber. A bundle of axons innervating several muscle fibers together form a neuromuscular junction.  Each skeletal muscle is innervated by an axon that lies outside its sarcolemma forming a structure called the motor end plate.

Motor End Plate

The junction between a single axon terminal innervating a single skeletal muscle fiber is called a motor end plate. This junction is surrounded by a Schwann cell that provides insulation from the surrounding fluid. 

The sarcolemma of muscle fiber undergoes an invagination at the site of innervation forming a gutter called synaptic gutter. The space between the terminal axon and the sarcolemma is called the synaptic cleft. The sarcolemma at the synaptic gutter also has several infoldings called the subneural folds. These fold greatly increase the surface area for the action of neurotransmitters. 

Process of Innervation

The process of innervation begins once the nerve impulse reaches the terminal axon. It can be divided into two phases;

  1. Release of neurotransmitters
  2. Action of neurotransmitters
  3. Release of Neurotransmitters

The terminal axon has several voltage-gated calcium channels. As the axon terminal is depolarized by the nerve impulse, the voltage-gated calcium channels open. The inflowing calcium ions form a complex with calmodulin forming a calcium-calmodulin complex. This complex activates the Ca-calmodulin dependent protein kinase that phosphorylates the synapsin protein. 

Synapsin is an anchoring protein that holds the synaptic vesicles containing acetylcholine within the axon. Upon phosphorylation, it releases the synaptic vesicle. These vesicles fuse with the neural membrane at the endplate releasing the acetylcholine molecules into the synaptic cleft. 

  1. Action of Neurotransmitters

The molecules of acetylcholine act as neurotransmitters at the motor end plate. Once they are released into the synaptic cleft, they diffuse to the subneural folds of the postsynaptic membrane.

Several acetylcholine receptors are located on the mouth of subneural folds and are, in fact, the ion channels. The binding of acetylcholine to these receptors causes the opening of ion channels, and the sodium ions start diffusing into the muscle fiber. This influx of sodium ions causes depolarization of skeletal muscle fiber. 

If the action potential thus generated is more than the threshold potential, it causes the opening of voltage-gated sodium channels at the bottom of the subneural cleft. In this way, an action potential is generated in the muscle fiber. 

Excitation-Contraction Coupling

An important phenomenon about the contraction of skeletal muscle is excitation-contraction coupling. It means that the process of skeletal muscle contraction is coupled with its excitation, the two processes being occurring simultaneously. 

This coupling occurs via the triads formed by T-tubules and the terminal cisternae. The T-tubules begin at the sarcolemma of the muscle fiber and penetrate throughout the fiber. Remember that these T-tubules are the part of the sarcolemma and are in contact with the extracellular fluid. 

Once a nerve impulse travels through the sarcolemma of muscle fiber, it is also carried to the T-tubules. The action potential of T-tubules at the triad causes a current to flow the adjacent sarcoplasmic reticulum via terminal cisterna. This current flow is detected by specialized DHP receptors that are linked to calcium channels. 

The activation of DHP receptors by the action potential causes the opening of calcium channels and the calcium ions start diffusing outside the sarcoplasmic reticulum. These calcium ions, in turn, initiate the process of skeletal muscle contraction as described in the subsequent heading. 

Contraction and Relaxation

The contraction and relaxation of skeletal muscles occur cyclically, one after the other. It involves the formation and breaking of the actin-myosin cross-bridges. Before studying the molecules events that happen during contraction and relaxation of a skeletal muscle fiber, it is important to know the following points. 

ATPase activity of Myosin Head

The myosin heads that are made up of the end of a heavy chain and two light chains have an intrinsic ATPase activity, meaning that they can cleave ATP to ADP. This plays an important role in the contraction of skeletal muscles. 

Role of Troponin in Contraction

Troponin is another protein that is attached periodically along the tropomyosin molecules in actin filaments. It serves to attach tropomyosin to actin. Each troponin molecule has three subunits;

  • Troponin I, having high affinity for actin
  • Troponin T, having affinity for tropomyosin
  • Troponin C, having high affinity for calcium ion

Troponin serves to initiate the process of skeletal muscle contraction. The calcium ions that are released from the sarcoplasmic reticulum bind to troponin C and cause a conformational change in its structure in such a way that it pulls the tropomyosin. As a result, the myosin-binding sites on actin are exposed and the contraction begins. 

Phases of Contraction

The contraction of skeletal muscles is described by walk-along theory. It states that the contraction occurs in a serious of steps in which the myosin heads continuously walk along the actin filaments, pulling them towards the center of the sarcomere. The following steps are involved in this process.

  1. Charging of Myosin Heads

Myosin heads become charged as a result of ATP hydrolysis via intrinsic ATPase activity. The ADP and inorganic phosphate remain attached to the myosin head. The head stands erect and is said to be in high energy state. 

  1. Cross-bridge formation

The charged myosin heads bind to the exposed binding sites on the actin filaments, resulting in the formation of actin and myosin cross-bridges. 

  1. Power stroke of Myosin Heads

In the next step, the myosin heads bend toward the center of sarcomere utilizing the energy stored during the process of hydrolysis. As a result, ADP and inorganic phosphates are released from the myosin head and the actin filament is pulled towards the center of the sarcomere. After this step, the myosin head attains a low energy state while the cross-bridge remains attached. 

  1. Breaking of Cross-Bridge

After the power stroke is over, a molecule of ATP binds to the myosin head. It results in the release of the myosin head from the actin filament and the cross-bridge is broken. A molecule of ATP becomes attached to the myosin head.

  1. Recharging and Continuation Cycle

The myosin head again hydrolyzes ATP, becomes charged, binds to another binding site down the actin filament and the cycle repeats. A series of these cycles results in pulling the actin filaments towards the center so that the H zone disappears, and the maximum contractile state is reached. 

Relaxation 

Once the nerve singles stop, the calcium ions are no longer released into the sarcoplasm. The process of contraction continues as long as the calcium ions remain in high concentration within the sarcoplasm. However, a calcium pump located in the sarcoplasmic reticulum continuously pumps the ions into the reticulum. Once the calcium ions concentration falls below a certain level, no more contraction cycles can occur. The myosin-binding sites are hidden, and the skeletal muscle relaxes.

Energy Sources

The contraction of skeletal muscle requires energy in the form of ATP. This ATP is used by muscle for three major purposes;

  1. Walk-along cycles of actin and myosin cross-bridges
  2. Pumping calcium ions back into the sarcoplasmic reticulum
  3. Sodium-Potassium pump to maintain proper ionic concentrations within the muscle fibers

Skeletal muscles obtained ATP from three major sources. All these sources are used to resynthesize ATP from ADP and inorganic phosphate. 

Phosphocreatine

The most important source of ATP in the skeletal muscles is phosphocreatine. This organic compound has a high-energy phosphate bond much similar to the one found in ATP. The phosphate bond of phosphocreatine has slightly greater energy as compared to that in ATP. When this bond is broken, the released energy is used to phosphorylate ADP to ATP. 

Glycolysis

It is the second major source of energy production in skeletal muscles. Glycolysis involves the breakdown of glucose previously stored in the skeletal muscles as glycogen. The energy obtained from glucose is then used to make additional molecules of ATP. 

The process of glycolysis has two major benefits;

  • It can occur even in the absence of oxygen
  • The rate of ATP synthesis is rapid

These two energy sources are used for short term contractions of skeletal muscles. 

Oxidative Metabolism

This is the final source of ATP synthesis. More than 95% of ATP for sustained skeletal muscle contractions is obtained via oxidative metabolism. This process involves the breakdown of glucose and other food molecules like fatty acids in the presence of oxygen. A large amount of energy is released during the metabolic process which is used to make ATP. 

Fats are the major source of energy for long term sustained contraction of skeletal muscles. However, stored carbohydrates provide energy for muscle contractions that last for 2 to 4 hours. 

Summary

Skeletal muscles are the most abundant muscular tissue making more than 40% of our bodies. 

They are arranged in the form of muscle fasciculi that are further composed of skeletal muscle fibers, the cells. 

Each skeletal muscle fiber has the following structure;

  • Surrounded by a membrane called the sarcolemma
  • Have thousands of myofibrils that are made up of thick and thin filaments
  • Myofibrils are divided into sarcomeres, the contractile units of muscle fiber
  • Have cytoplasm called sarcoplasm
  • Have a specialized network of tubules that store calcium called sarcoplasmic reticulum

Each skeletal muscle fiber is innervated by an axon, forming a motor end plate that is surrounded by a Schwann cell for insulation. Each motor end plate has;

  • An invagination of postsynaptic membrane called synaptic gutter
  • Folds on synaptic gutter called subneural folds
  • A space between the axon terminal and postsynaptic membrane called the synaptic cleft

The innervation involves two steps;

  • Release of acetylcholine into the synaptic cleft
  • Opening of ion channels by acetylcholine to cause depolarization of skeletal muscle fiber

The action potential thus generated causes the release of calcium ions from the sarcoplasmic reticulum. 

The calcium ions bind to troponin, uncovering the myosin-binding sites, and initiate the walk-along  cycle that involves the following steps;

  1. Charging of myosin heads
  2. Binding of myosin heads to actin filaments forming cross-bridges
  3. Power stroke that pulls the actin filaments towards the center 
  4. Release of cross-bridges after the ATP binds to myosin head

The process repeats and the muscle continues to contract. 

When the innervation stops, no more calcium is released. The available calcium is pumped back into the sarcoplasmic reticulum and the muscle relaxes. 

The ATP required in this process is obtained from three sources;

  • Phosphocreatine
  • Glycolysis
  • Oxidative Metabolism

References

  1. Guyton and Hall, Textbook of Medical Physiology, 13th Edition.
  2. Zammit, PS; Partridge, TA; Yablonka-Reuveni, Z (November 2006). “The skeletal muscle satellite cell: the stem cell that came in from the cold”. Journal of Histochemistry and Cytochemistry. 54 (11): 1177–91. doi:10.1369/jhc.6r6995.2006PMID 16899758.
  3. Saladin, Kenneth S. (2010). Anatomy and Physiology (3rd ed.). New York: Watnick. pp. 405–406. ISBN 9780072943689.
  4. Martini, Frederic H.; Timmons, Michael J.; Tallitsch, Robert B. (2008). Human Anatomy (6 ed.). Benjamin Cummings. pp. 251–252. ISBN 978-0-321-50042-7.
  5. Lieber, Richard L. (2002) Skeletal muscle structure, function, and plasticity. Wolters Kluwer Health.
  6. https://en.wikipedia.org/wiki/File:Cardiac_calcium_cycling_and_excitation-contraction_coupling.png
  7. https://en.wikipedia.org/wiki/Muscle_contraction#/media/File:1009_Motor_End_Plate_and_Innervation.jpg
  8. https://en.wikipedia.org/wiki/Sarcolemma