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How do sarcomeres coordinate contraction?


As can be seen from the figure

  1. if myosins from both sides apply equal force then how does muscle contract? And also how do actins resist tearing?

  2. Is there any kind of coordination between different sarcomeres? How does they achieve it? I know that acetylcholine is released in response to action potential so does the release of acetylcholine controlled in each individual sarcomere?

  3. Or simply there is an increase in the number of myosins as the distance of each sarcomere increases from the neuromuscular junction?


I'm answering question 1, I think it'd be better to post these questions separately.

Consider action and reaction: myosins (thick) filament is experiencing force F towards Z line.

If the muscle is acting on something that is movable (e.g. you're pulling on a drawer which isn't locked), the force F towards Z line is greater than the force that maintains your thick filaments in place, and they'll slide towards Z line. The speed at which they slide is limited by the rate at which myosin heads can detach once they have performed power-stroke.

If the drawer is locked, then force F cannot overcome the external force. This force however is normally lower than the force required to tear actin or z-line, it is limited by two molecular factors: myosin will stall and not effectuate power stroke under too strong an opposing force, and myosin heads will detach after some time and relax the tension they have built up. This is how you get the maximum force under clamped conditions.


MCAT Biology : Sarcomeres

Which two proteins are the major components of myofibrils, allowing for muscle fiber contraction?

Myosin and actin are the two major proteins in muscle cells that allow for contraction. Actin is the thin filament myosin is the thick filament. During muscle contractions the overlap between these two proteins results in a shorter muscle fiber, and a shorter muscle, that pulls on the tendon. The result is movement. The other answers contain other structural elements of muscles but are not the direct cause of muscle contraction.

Example Question #2 : Musculoskeletal System And Muscle Tissue

Which of the following proteins does not play a functional role in creating the force-tension curve of muscle contraction?

All of these are involved in creating the force-tension curve

All of these are involved in creating the force-tension curve

The force-tension curve is used to measure the optimal muscle length for maximum muscle contraction. This length corresponds to the optimal overlap of actin and myosin filaments to generate force. The length of actin and myosin filaments determines the minimum and maximum possible overlap. Titin is the protein responsible for the elasticity of the sarcomere after it is stretched past maximum actin-myosin overlap. Titin allows force production to exist at a maximum tension slightly beyond only actin and myosin, thus affecting the force-tension curve.

Example Question #3 : Musculoskeletal System And Muscle Tissue

What type of enzyme is myosin?

In addition to the subunits of myosin that link it to actin, myosin is also an ATP hydrolase, or ATPase. Myosin must hydrolyze ATP to ADP to allow for the power stroke that propels myosin forward on the actin polymers.

Example Question #4 : Musculoskeletal System And Muscle Tissue

Which of the following sections of a sarcomere does not shorten during contraction?

Upon contraction, actin filaments will be pulled by myosin heads resulting in the shortening of the sarcomeres. The I band is composed of only actin filaments, and will begin to overlap with the myosin filaments, shortening the band. The A band, however, is the section composed of myosin filaments. Since this section is not altered by contraction, it stays the same length. Unlike the I band, the A band can contain regions of overlap without changing length.

The H zone, in contrast, refers to the region of myosin that is not overlapped by action. As the region of overlap increases, the H zone decreases. The distance between Z discs represents the total length of the sarcomere and must shorten in order for the muscle to contract.

Example Question #5 : Musculoskeletal System And Muscle Tissue

What structure marks the separation between two sarcomeres?

Z-discs are the dividing points between sarcomeres. Actin filaments extend from this region and are joined together by several complex protein structures.

The M-line is the middle of the sarcomere, marking the central point of the myosin filaments. The I-band consists of actin filaments that are not overlapped by myosin this region contains the Z-disc. The A-band marks the length of an entire thick filament (myosin), including the overlap region with actin.

Example Question #1 : Sarcomeres

What protein, present in sarcomeres, is responsible for the passive elasticity of muscle?

Titin is a massive protein that spans the length of half of a sarcomere (from the Z-disc to the M-line) and allows for the passive elasticity of muscle. It is not directly involved in the process of contraction that function is performed by actin and myosin.

Collagen proteins play an important role in providing tensile strength and building connective tissue throughout the body, but play only a minor role in the properties of muscle tissue in the extracellular matrix. Collagen is not found in the sarcomere.

Example Question #1 : Musculoskeletal System And Muscle Tissue

Which of the following changes length during sarcomere contraction?

Recall that during sarcomere contraction, the myosin filaments attach to actin filaments and slide along the actin filaments. By this mechanism, the region of overlap between the fibers is increased and the total sarcomere length shortens. Neither actin, nor myosin actually change length they simply move in relation to one another.

The H zone refers to the region of myosin at the center of the sarcomere that is not overlapped by actin. When the sarcomere shortens, the region of overlap increases and the H zone decreases.

Example Question #8 : Musculoskeletal System And Muscle Tissue

Which of the following is true about sarcomeres?

Actin filaments are only found in the I band

The sarcomeres contribute to the striated appearance of smooth muscle cells

The A band contains both actin and myosin filaments

Sarcomeres are functional units of skeletal and smooth muscle cells

The A band contains both actin and myosin filaments

Recall that sarcomeres are functional units of muscles that facilitate muscle contraction. Myosin heads bind to actin filaments and cause the filaments to overlap, shortening the sarcomere and, subsequently, the muscle.

Inside a sarcomere there are several regions. One such region is the I band, which consists of the actin filaments in the region where they are not superimposed by the myosin filaments. This means that the I band consists only of actin filaments however, actin filaments aren’t exclusive to the I band. They are also found in other regions, such as A band. The A band is the region of the sarcomere that contains the myosin (thick) filaments, regardless of overlap. This means that myosin is exclusive to the A band, but that this region contains both actin and myosin due to overlap.

Sarcomeres are functional units of muscles, but they are only found in skeletal and cardiac muscle cells smooth muscle cells do not contain sarcomeres. Actin and myosin filaments still cause the contraction seen in smooth muscle, but are not organized into alignment. This means that smooth muscle cells do not contract linearly and can essentially shrink in size during contraction, which can allow for things like constriction around organs and vessels.

Example Question #9 : Musculoskeletal System And Muscle Tissue

A researcher observes a sarcomere through a microscope. He notices that a single myosin filament is forty micrometers long and that a single actin filament is fifty micrometers long. What can the researcher conclude from this information?

To answer this question you need to understand the structural regions of the sarcomere. The I band, A band, and H zone are regions in a sarcomere that constitute of actin (thin) and myosin (thick) filaments. I band is the region of actin filaments that are not superimposed by myosin filaments. The H zone is the region of myosin filaments that are not superimposed by actin filaments. To calculate the length of the I band, you need the length of the myosin filament, the actin filament, and the H zone. Since we don’t have the length of H zone, we can’t solve for the length of I band. Essentially, without knowing the degree of overlap, we cannot determine the length of un-overlapped actin.

The A band is the region of the sarcomere that consists of the entire length of the myosin filament. The question states that the length of the myosin filaments is micrometers therefore, the length of the A band is micrometers.

Example Question #10 : Musculoskeletal System And Muscle Tissue

Which of the following is true about the organization of actin filaments and myosin in sarcomeres?

Myosin filaments appear thinner than actin filaments

Prior to contraction, there is no overlap between actin and myosin

The degree of overlap of actin and myosin affects the overall contractile strength

The degree of overlap of actin and myosin affects the overall contractile strength

The only choice that is actually true is that the degree of overlap of myosin and actin plays a role in contractile strength. If there is little to no overlap, contractile strength is low however, if there is too much overlap then contractile strength is also low. This trend can be represented in a force-tension curve, which demonstrates that maximum force generation occurs when the sarcomere begins at equilibrium.

In a normal sarcomere there is always a small area of overlap of myosin and actin prior to contraction. Myosin appears thicker than actin, and is considered the "thick filament."

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Microscopic muscles: the source of all movement

A muscle is made of microscopic contractile units arranged in series and bundles: the sarcomeres, tiny packages of proteins (especially myosin II, a famous molecule). Muscles contract because sarcomeres contract. These molecular machines are the best example of how life is chemistry. Although proteins have many impressive properties and do many dazzling things, none is more defining of living things than this ability to generate movement.

Most molecular biology is amazing if you can understand it, but hard to connect to anything as familiar as wiggling your toes. Sarcomeres are an unusual explanatory bridge between weird science and ordinary experiences because they actually resemble the muscles they power. There’s something simple and beautiful about how they are so much like miniature versions of the muscles they power.

You know how kids are so good at asking a chain of “why” and “how” questions? Sarcomeres are the deepest possible answer to the chain of kid-questions that starts with, “How do we move?” (Well, almost the deepest answer.2)

Sarcomeres are how chemistry lifts barbells. Without sarcomeres, your heart could not beat, your guts could not digest, your jaw could not flap. You would never blink, breathe, or burp. Sarcomeres are the ultimate source of all movement, and they are powered by the weird properties of mind-bogglingly complicated molecules.

And sarcomeres can probably screw up.


Functions

The main function of the sarcomere is to allow a muscle cell to contract. For this, the sarcomere must be shortened in response to a nervous impulse.

The thick and thin filaments do not shorten, but slide around each other, which causes the sarcomere to shorten while the filaments retain the same length. This process is known as the sliding filament model of muscle contraction.

The sliding of the filament generates muscular tension, which is undoubtedly the main contribution of the sarcomere. This action gives the muscles their physical strength.

A quick analogy to this is the way a long ladder can be extended or folded depending on our needs, without physically shortening its metal parts.

Myosin involvement

Fortunately, recent research offers a good idea of ​​how this slippage works. The theory of the sliding filament has been modified to include how myosin is able to pull actin to shorten the length of the sarcomere.

In this theory, the globular head of myosin is located near actin in an area called the S1 region. This region is rich in segments with hinges that can bend and thus facilitate contraction.

The flexion of S1 may be the key to understanding how myosin is able to"walk"along the actin filaments. This is achieved by binding cycles of the S1 myosin fragment, its contraction and its final release.

Union of myosin and actiba

When myosin and actin come together, they form extensions called"crossed bridges". These crossed bridges can be formed and break with the presence (or absence) of ATP, which is the energy molecule that makes contraction possible.

When ATP binds to the actin filament, it moves it to a position that exposes its myosin binding site. This allows the globular head of the myosin to attach to this site to form the cross bridge.

This union causes the phosphate group of ATP to dissociate, and thus myosin initiates its function. Then, the myosin enters a state of lower energy where the sarcomere can be shortened.

To break the cross bridge and allow again the binding of myosin to the actin in the next cycle, the binding of another ATP molecule to the myosin is necessary. That is, the ATP molecule is necessary for both contraction and relaxation.


MicroRNAs in Cardiac Development and Function

MiRNA Regulation of Cardiomyocyte Sarcomere Organization

Sarcomeres are the basic contractile units of cardiac muscle. They are composed of thick and thin filaments essential for generation and propagation of mechanical force. Myosin, the major component of the thick filament, is comprised of MHC subunits and myosin light-chain (MLC) subunits. α-MHC (Myh6) and β-MHC (Myh7) are both expressed in the heart during development and in the adult. In rodents, expression of β-MHC is downregulated after birth so that in the adult, α-MHC is the dominant MHC isoform in the heart ( Lyons et al., 1990 England and Loughna, 2013 ). MHC isoform switches in response to cardiac stress or hypothyroidism. Pathologic hypertrophy is associated with upregulation of β-MHC and downregulation of α-MHC ( Krenz and Robbins, 2004 Gupta, 2007 ).

Expression of α- and β-MHC isoforms is controlled by miRNA-208a, miRNA-208b, and miRNA-499 ( van Rooij et al., 2007, 2009 Callis et al., 2009 ). miRNA-208a and miRNA-208b are encoded in an intron of the α-MHC and β-MHC gene, respectively. Mice null for miRNA-208a are viable but show abnormalities in sarcomere structure and declined cardiac function at 6 months of age ( van Rooij et al., 2007 ). However, miRNA-208a null mice are resistant to cardiac hypertrophy in response to stress induced by transverse aortic banding or calcineurin ( van Rooij et al., 2007 Callis et al., 2009 ). This is concomitant with decreased expression of slow skeletal muscle contractile protein, β-MHC, in the miRNA-208a null heart. The function of miRNA-208a is mediated, in part, by repression of thyroid hormone receptor associated protein 1 (Thrap1), which negatively regulates the expression of β-MHC.

miRNA-208a controls not only the expression of β-MHC in the heart but also that of the closely related slow myosin isoform, Myh7b ( van Rooij et al., 2009 ). Both β-MHC and Myh7b genes encode intronic miRNAs, miRNA-208b, and miRNA-499, respectively ( Berezikov et al., 2006 Landgraf et al., 2007 ). Mice lacking the miRNA-208b or miRNA-499 gene have no obvious developmental defects ( van Rooij et al., 2009 ). However, miRNA-208b/-499 double null mutant mice display reduced expression of slow myofiber β-MHC and increased expression of fast type myosin isoforms. By contrast, overexpression of miRNA-499 leads to increased expression of β-MHC and drives muscle toward a slow myofiber phenotype. Forced cardiac miRNA-499 expression promotes hypertrophy in mice ( Shieh et al., 2011 Matkovich et al., 2012 ). Together, these miRNAs are important in the specification of the identity of muscle fibers by stimulating slow myofiber gene programs at the expense of those that control fast myofiber gene expression ( Hodgkinson et al., 2015 ).

It has been demonstrated that miRNA-1 and miRNA-133 can act as specific activators or suppressors of sarcomere formation and muscle gene expression. Deletion of both miRNA-1-2 and miRNA-1-1 in mice (miRNA-1 null) leads to sarcomere disruption in cardiomyocytes and impaired cardiac function. All the miRNA-1 null mice died before weaning age ( Heidersbach et al., 2013 Wei et al., 2014 ). miRNA-1 functions to negatively regulate myocardin, the major regulator of smooth muscle gene expression, and telokin, the smooth muscle-specific inhibitor of MLC-2 phosphorylation ( Heidersbach et al., 2013 Wystub et al., 2013 ). The upregulation of myocardin and telokin in miRNA-1 null hearts may, in part, contribute to the defect in sarcomere organization. Furthermore, studies from Wei et al. indicated that miRNA-1 directly represses nuclear receptor estrogen-related receptor β (Errβ). The increased level of Errβ in miRNA-1 null heart activates the expression of fetal sarcomere-associated genes ( Wei et al., 2014 ).

miRNA-133a represses smooth muscle gene expression in the heart by directly targeting myocardin and SRF for repression ( Liu et al., 2008 Wystub et al., 2013 ). Deletion of both miRNA-133a-1 and mIRNA-133a-2 (miRNA-133a null) causes late embryonic and neonatal lethality due to ventricular septal defect (VSD) and chamber dilation ( Liu et al., 2008 ). miRNA-133a null mice display sarcomere disorganization and ectopic activation of the smooth muscle gene program ( Liu et al., 2008 ). In addition, mice lacking both miRNA-1 and miRNA-133a displayed severe cardiac dysfunction and died before embryonic day 11.5 (E11.5). Mice with a null mutation in miRNA-1/133a showed increased expression of myocardin and smooth muscle genes in the heart. These studies indicate that miRNA-1 and miRNA-133a clusters are important in cardiomyocyte differentiation and sarcomere formation during embryonic and postnatal life. They act cooperatively to govern the gene transition program from an immature state characterized by expression of smooth muscle genes to a mature phenotype ( Wystub et al., 2013 ).


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In: Anatomical Record , Vol. 297, No. 9, 09.2014, p. 1663-1669.

Research output : Contribution to journal › Article › peer-review

T1 - Cell biology of sarcomeric protein engineering

T2 - Disease modeling and therapeutic potential

N2 - The cardiac sarcomere is the functional unit for myocyte contraction. Ordered arrays of sarcomeric proteins, held in stoichiometric balance with each other, respond to calcium to coordinate contraction and relaxation of the heart. Altered sarcomeric structure-function underlies the primary basis of disease in multiple acquired and inherited heart disease states. Hypertrophic and restrictive cardiomyopathies are caused by inherited mutations in sarcomeric genes and result in altered contractility. Ischemia-mediated acidosis directly alters sarcomere function resulting in decreased contractility. In this review, we highlight the use of acute genetic engineering of adult cardiac myocytes through stoichiometric replacement of sarcomeric proteins in these disease states with particular focus on cardiac troponin I. Stoichiometric replacement of disease causing mutations has been instrumental in defining the molecular mechanisms of hypertrophic and restrictive cardiomyopathy in a cellular context. In addition, taking advantage of stoichiometric replacement through gene therapy is discussed, highlighting the ischemia-resistant histidine-button, A164H cTnI. Stoichiometric replacement of sarcomeric proteins offers a potential gene therapy avenue to replace mutant proteins, alter sarcomeric responses to pathophysiologic insults, or neutralize altered sarcomeric function in disease. Anat Rec, 297:1663-1669, 2014.

AB - The cardiac sarcomere is the functional unit for myocyte contraction. Ordered arrays of sarcomeric proteins, held in stoichiometric balance with each other, respond to calcium to coordinate contraction and relaxation of the heart. Altered sarcomeric structure-function underlies the primary basis of disease in multiple acquired and inherited heart disease states. Hypertrophic and restrictive cardiomyopathies are caused by inherited mutations in sarcomeric genes and result in altered contractility. Ischemia-mediated acidosis directly alters sarcomere function resulting in decreased contractility. In this review, we highlight the use of acute genetic engineering of adult cardiac myocytes through stoichiometric replacement of sarcomeric proteins in these disease states with particular focus on cardiac troponin I. Stoichiometric replacement of disease causing mutations has been instrumental in defining the molecular mechanisms of hypertrophic and restrictive cardiomyopathy in a cellular context. In addition, taking advantage of stoichiometric replacement through gene therapy is discussed, highlighting the ischemia-resistant histidine-button, A164H cTnI. Stoichiometric replacement of sarcomeric proteins offers a potential gene therapy avenue to replace mutant proteins, alter sarcomeric responses to pathophysiologic insults, or neutralize altered sarcomeric function in disease. Anat Rec, 297:1663-1669, 2014.


The biology of desmin filaments: how do mutations affect their structure, assembly, and organisation?

Desmin, the major intermediate filament (IF) protein of muscle, is evolutionarily highly conserved from shark to man. Recently, an increasing number of mutations of the desmin gene has been described to be associated with human diseases such as certain skeletal and cardiac myopathies. These diseases are histologically characterised by intracellular aggregates containing desmin and various associated proteins. Although there is progress regarding our knowledge on the cellular function of desmin within the cytoskeleton, the impact of each distinct mutation is currently not understood at all. In order to get insight into how such mutations affect filament assembly and their integration into the cytoskeleton we need to establish IF structure at atomic detail. Recent progress in determining the dimer structure of the desmin-related IF-protein vimentin allows us to assess how such mutations may affect desmin filament architecture.


Muscle Fiber Action

The skeletal muscles of the body can act by contraction of the points of contact with the bones to produce forces in essentially any direction under voluntary control. This control uses the somatic nervous system. The typical muscle cell is 10 to 100 μm in diameter and over 100 mm in length and contains hundreds of nuclei. It is commonly referred to as a muscle fiber. A muscle is made up of hundreds of thin cylindrical strands called myofibrils. Each myofibril consists of a linear array of contractile units called sarcomeres. The sarcomeres contain an array of partially overlapping thin actin filaments and thicker myosin-containing filaments. The basic structure of a sarcomere is shown below, following the form from Karp's development and using his terminology to describe them.

The current understanding of muscle fiber contraction is called the sliding-filament model, which was developed by two groups of British investigators, Andrew Huxley and Rolf Niedergerke, and Hugh Huxley and Jean Hanson. Upon initiation of muscle contraction, the actin filaments are drawn into the framework of the Myosin-containing filament, shortening the fiber. With hundreds of sarcomeres in series, a substantial contraction of the muscle is obtained.

Upon initiation of muscle contraction, the myosin-II molecules in the thicker fiber are seen as the agents for moving the actin filaments toward the center of the sarcomere. The heads of the myosin molecules reach out and bind to the thinner actin filaments and move them by lever action by about 10nm. This is powered by the universal fuel molecule ATP, and within a collection of the filaments of a muscle fiber there are available mitochondria to produce the ATP.


How do our muscles work?

Scientists led by Kristina Djinović-Carugo at the Max F. Perutz Laboratories (MFPL) of the University of Vienna and the Medical University of Vienna have elucidated the molecular structure and regulation of the essential muscle protein &alpha-actinin. The new findings allow unprecedented insights into the protein's mode of action and its role in muscle disorders. The findings, made in collaboration with King's College London (KCL), may lead to improved treatments, and are published in the top-class journal Cell.

Most animals rely on muscles to move, irrespective whether it is feed, fight or flee. The smallest building block of a muscle is the sarcomere, hundreds of which are successively arranged to form muscle fibers. "Sarcomeres are mainly made up of actin and myosin protein strands called filaments. Muscle shortening or contraction depends on these filaments sliding against each other, and requires that the actin filaments are anchored in planes, called Z-discs. The major Z-disc protein is &alpha-actinin, which is also responsible for anchoring another protein called titin. Titin ensures that the actin and myosin filaments of a sarcomere are positioned correctly and re-adjusts the sarcomere to its original length after muscle activity," explains structural biologist Kristina Djinović-Carugo.

Important Role in Heart and Muscle Disorders

&alpha-Actinin is clearly an essential protein -- embryos of organisms that cannot produce it die. If, however, &alpha-actinin is produced but does not function properly, the organism suffers from diseases such as muscular dystrophies and cardiomyopathies. Knowing the detailed structure of &alpha-actinin and muscle would not only better our understanding of the role of mutant proteins in disease, but would also aid accurate diagnosis and may ultimately assist designing new therapies.

Muscle protein structure is a research topic of Kristina Djinović-Carugo at the Max F. Perutz Laboratories (MFPL) of the University of Vienna and the Department for Structural and Computational Biology. The head of the "Laura Bassi Centre of Optimized Structural Studies" explains: "We determined the structure of &alpha-actinin using a technique called X-ray crystallography. It took us several years to produce sufficient high-quality protein necessary to obtain quality crystals for our analysis." With patience and innovative ideas the team eventually teased out the detailed structure of &alpha-actinin, and in collaboration with Mathias Gautel at the British Heart Foundation Centre of Research Excellence at KCL could also show how the protein is regulated.

The Structure is Similar to Fusilli Pasta

The structure showed that &alpha-actinin forms a symmetrical complex made of two molecules. Each molecule has a head, short neck region and a rod-shaped body that looks like four fusilli pasta aligned in a zigzag. The head of &alpha-actinin binds actin whilst two small L-shaped parts sit at the end of the rod and interact with the neck of the other molecule. However, the structure revealed to be more than the sum of its parts: placing two diametrically opposing &alpha-actinin molecules does not only allow simultaneous binding of actin and titin filaments and so anchoring them in the Z-disc, but also confers regulation.

The Fatty Acid PIP2 Regulates &alpha-actinin Function

"It was a long standing hypothesis, that the interaction of &alpha-actinin with titin is regulated via a fatty acid molecule termed PIP2 which switches the molecule on and off. Our structural data now for the first time reveals how a fatty acid opens and closes the muscle protein &alpha-actinin, modifying its ability to bind actin and titin," explains Kristina Djinović-Carugo. If there is no PIP2, one of the L-shaped parts of an &alpha-actinin molecule binds a titin-lookalike region in the neck of the opposing molecule. If PIP2 is present, the L-shaped part detaches from the neck and binds titin. The team complemented the structural picture of the interaction of PIP2 with &alpha-actinin by analyzing the dynamic life of &alpha-actinin in collaboration with Katharina Pirker (University of Natural Resources and Life Sciences Vienna), and Bojan Žagrović (MFPL). Mathias Gautel's team also discovered that, when they destroyed &alpha-actinin's binding site for PIP2 or locked it in a position which can permanently bind titin, ordered sarcomeres were lost the muscle was broken.

Long-term Funding and International Collaborations were Essential

"Our findings give new insights into how muscle is built and flexes at the molecular level. This will help to better understand both inherited and acquired muscular diseases, and aid therapy development," sums up Kristina Djinović-Carugo. She adds: "Without long-term funding through the FWF, the University of Vienna, the FFG, the EU-FP7 Marie Curie Action 'Networks for Initial Training' program, the British Heart Foundation, as well as the long standing collaborations with Mathias Gautel and Katharina Pirker, this research would have been impossible. This paper is the fruit of eight years of hard of labor!"


Muscle Anatomy 101: How Does A Muscle Contract? Part Two

Time for our Workout Of The Month. Each month we take a look at something fitness related and break it down in a way that you can understand it so that you can apply it to your life. Some months we take a look at a specific workout and break it down – other times we’ll take a look at the latest fitness research and separate fact from fiction. This month we’ll be talking about one of everyone’s favorite things: muscle. Last month we broke down what a muscle looks like. This month, we’re taking a look at how a muscle does the things that it does.

What’s In A Muscle Review

This is the view of a muscle cell under the microscope. The black lines represent the Z-Line: the space between one muscle cell and another.

Last month we took a look at some simple muscle anatomy and broke down what’s in a muscle. The muscle is made up of three different layers: an outer layer, a middle layer and an inner layer. All layers come together at the end of the muscle to form thick connective tissue that will either connect to other muscles or bones. The inner layer of a muscle fiber is where most of the magic happens. In the inner layer, energy is made and stored and where movement happens. The inner layer is where nerve endings and capillaries that supply oxygen to the muscles are. Part two in muscle anatomy takes place where part one left off: how does a muscle contract?

How Does A Muscle Contract?

The majority of muscle contraction takes place on the inner layer of the muscle fibers. It’s in this layer that a muscle receives its signal from the brain to move. It’s also in this layer that the muscle cells have a rich blood supply and abundant mitochondria. Mitochondria are cells which are responsible for producing energy. They’re like the power plant of the cell.

Components Of Muscle Contraction

  • Sarcomere: The functional unit of muscle found in the inner layer which produces contraction.
  • Myofilaments: The portion of muscle cell contains even smaller muscle cells called filaments. There are two different kinds of filaments, thin and thick. Thin filaments contain actin and thick filaments contain myosin. Actin and myosin are two different proteins responsible for “pulling” muscle cells closer together to make a contraction.
  • Z-Line: A small space in the cell where one sarcomere ends another begins.

The Sliding Filament Theory

The two important things here are the red and the blue. They both represent a myofilament. The myofilaments come together to make the muscle contract.

The sliding filament theory is the current accepted proposal of how a muscle contracts. Let’s say that you wanted to lift a bag of groceries or a forty-five pound dumb-bell. This is the order of events that would take place:

  1. You think of lifting the weight in your hands and your brain sends a signal to your muscle.
  2. In the inner layer of the muscle, the sarcomere gets the message from the neuron.
  3. The myofilaments communicate with each other through different cell transmitters.
  4. The different components of myfilaments pull closer together shortening the z-lines between all sarcomeres.
  5. All the sarcomeres pull all laywers of the muscle together resulting in a muscle contraction.

BOOM! That’s muscle contraction in a nutshell.

Muscle Contraction Summary

Muscles are incredible organs. They do incredible things and at times are responsible for super human strength. It takes an intricate timing of neurons, electrolytes, and specialized cells to come together to make a muscle contract. Our muscles can do three types of contractions: isometric, eccentric, and concentric. Each type of contraction can be used for specific reasons. Each type of contraction can also be used to help you get the most of your workouts.

Read About Different Types Of Muscle Contractions To Get The Best Of Your Workout

1. Clark, M., Corn, R., Lucett, S. “Muscle Contractions.” NASM Essentials Of Personal Training. National Academy Of Sports Medicine. Baltimore: Lippincott Williams & Wilkins. 2008. Print. 31-34.

2. Sherwood, Lauralee. Fundamentals of Physiology. Third Edition. Thomson Brooks/Cole. 2006. Print. 204-210.


Watch the video: The Mechanism of Muscle Contraction: Sarcomeres, Action Potential, and the Neuromuscular Junction (November 2021).