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Physiology, anatomy, structure, and traits of muscle cells. Description of the sliding filament theory (how muscles contract), neurological recruitment of muscle cells/fibers, connective tissue, and muscle fiber arrangement. 

Muscle Cell Structure and Function

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This course describes the amazing structure and function of the skeletal muscle cell. The human body is comprised of 4 cell types; epithelial cells (e.g. skin), nerve cells (e.g. sciatic nerve), connective tissue cells (e.g. ligaments, tendons, and fascia), and muscle cells (e.g. biceps brachii). There are even 3 distinct types of muscle tissue:

Smooth muscle tissue: involuntary and non-striated muscle tissue

Cardiac muscle tissue: involuntary and striated muscle tissue

Skeletal muscle tissue: voluntary and striated muscle tissue

Each cell type has components and attributes common to all cells, and unique modifications to components that give that cell type its unique characteristics. Muscle cells are specialized to maximize force production via "contraction", and have a unique combination of responsiveness, excitability, conductivity, contractility, extensibility, and elasticity.

This course details cell classifications, muscle cell organelles, muscle cell function, action potentials, sliding filament theory (actin and myosin), excitation-contraction coupling, motor unit recruitment, striation, fascial layers, and muscle structure and organization. By the time you complete this course, you will understand why the interaction of actin and myosin, and the sliding filament theory help us understand why muscles are weaker in a lengthened position; or, how excitation-contraction coupling and the organization of motor units explain why you cannot target or shape one part of a muscle. Movement professionals (personal trainers, fitness instructors, physical therapists, athletic trainers, massage therapists, chiropractors, occupational therapists, etc.) should consider this course to be foundational physiology content, which is essential for understanding future courses including acute variables, corrective exercise, strength training program design, physical rehabilitation, etc.

Muscle Fiber Dysfunction and Trigger Points

Motor unit by Electron Microscope: Note how you can see the neuromuscular junction and motor end plate, and if you look closely the striations in the muscle cells.

Understanding this process should bring feelings of sheer

 amazement at the intricacy of the human body

. Consider that all of the steps detailed above (and summarized below) happen every time you need to generate force. In fact, consider that this sequence happens many times per second, at every muscle cell, of every motor unit, of every muscle you recruit, at every joint, and every time you move or resist movement.

Stimulus (intent, reflex, motor program, etc)

ACh released into the neuromuscular junction

ACh binds to receptors on the motor endplate of the muscle cell

Sodium-ion channels open on the motor endplate

An action potential is propagated across the sarcolemma

Calcium is released and binds to troponin

The change in the shape of the troponin-tropomyosin complex exposes binding sites

Hydrolyzed ATP becomes ADP and extends the myosin head

The myosin head binds to an available binding site on troponin

Release of ADP results in flexing of the myosin head

Flexing the myosin head pulls actin resulting in a power stroke

ATP binds to myosin releasing the head from troponin

The process starts over at a binding site further down actin.

This step is multiplied by the number of binding sites on each protein, the number of proteins in each sarcomere, the number of proteins in parallel in each myofibril, and the number of myofibrils in each muscle cell.

Force output is then multiplied again, and calibrated for function, by the number of cells in the motor unit recruited, and the number of motor units recruited.

Note: Even this step-by-step account is a simplified summary of this process. Understanding each step represents years of work from incredible minds. It is not necessary that you memorize this entire sequence today, (although, you may be able to write down most of these steps in your own words after studying this course). But, stop studying for a moment, and take the time to appreciate how amazing the muscle cell really is. This knowledge is the first step in understanding the human movement system.

Course Description: Muscle Cell Structure and Function

Muscle Cell Structure and Function Course Study Guide

Study Guide: Muscle Cell Structure and Function 

Structure of a Muscle Cell

The human body is comprised of 4 cell types (epithelial cells, muscle cells, nerve cells, and connective tissue cells). Each cell type has components that are found in all cells and unique modifications to components that give the cell type its unique attributes. These components may have special names specific to the cell type, for example, the "cytoplasm" in a muscle cell is called "sarcoplasm." Muscle cells are specialized to maximize force production via "contraction." In the table below, you can find a list of the muscle cell organelles with their specialized names, the function of those organelles, and the equivalent of those organelles in a "standard" cell.

 (n.) Late 14c., from Middle French muscle "muscle, sinew" (14c.) and directly from Latin musculus "a muscle," literally "little mouse," diminutive of mus "mouse". So-called because the shape and movement of some muscles (notably biceps) were thought to resemble mice ( 

 - before vowels sarc-, word-forming element meaning "flesh, fleshy, of the flesh," from Latinized form of Greek sark-, combining form of sarx "flesh" ( 

Cell structure with cell components labeled

By Kelvinsong - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=22952603

A detailed image of skeletal muscle fibers

Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Muscle Tissue Traits

All cells have "traits". Some of the characteristics that make a muscle cell unique are responsiveness, conductivity, contractility, extensibility, and elasticity. These traits are explained in further detail below:

: The ability of a cell to respond or react to stimuli. For example, a muscle cell may respond to chemicals/hormones in the bloodstream by increasing the intracellular production of certain proteins.

 The ability of a cell to change in membrane conductance as a response to stimulation. Although all cells are "excitable", muscle cells are specialized to react to a change in membrane potential with contraction.

: Stimulation results in more than a local effect. For example, the stimulation of a motor endplate results in a change in cell membrane polarity that rapidly propagates along the entire length of the muscle cell (origin to insertion).

: Muscle cells are unique in the amount they can shorten when stimulated. This is due to the highly organized arrangement of myofilaments.

: This is the ability of a cell to be stretched or lengthened without damage. Most cells would rupture with even a modest stretch, but muscles can stretch up to 3 times their contracted length.

: Although this term is often confused with extensibility; elasticity refers to the ability of a cell to return to its resting length after being stretched.

Although all cells possess these traits to various degrees, the evolution of the cellular components of the muscle cell has made them "specialists" in these traits. Take a moment to compare these traits to the description of the muscle cell components above.

Excitation from a Motor (Efferent) Nerve

The next section will focus on changes that occur at the cellular level in a muscle due to excitation from a motor (efferent) nerve. That is, how an action potential goes from motor neuron to stimulating the 

, where a cascade is stimulated that results in a change in the shape of the 

words were discussed in the sections above).

The excitation of a muscle cell starts at a motor neuron (found in the brain stem or spinal cord). An impulse is generated when the motor neuron is sufficiently stimulated to result in an "action potential." The action potential is conducted along the length of an axon to the neuromuscular junction. In some cases that signal is conducted along an axon for nearly a meter, for example, from the spinal cord to the plantar interossei of the foot. Once the action potential reaches the synapse of the motor nerve it signals the release of the neurotransmitter, acetylcholine (ACh), which then leaks into the neuromuscular junction. This ACh binds to receptors on the muscle cell's "motor endplate" which open sodium ion channels that result in a change in polarity, an action potential, and the propagation of that action potential along the sarcolemma.

Note: Relaxation occurs due to the presence of the basal lamina in the synaptic cleft which contains an enzyme called acetylcholinesterase. This enzyme is responsible for the breakdown of ACh. The breakdown of ACh returns the cell to a relaxed state.

Diagram of the neuromuscular junction with acetylcholine release


Muscles will contract or relax when they receive signals from the nervous system. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows: (1) The action potential reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine (ACh) is released into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium ions to flow across the membrane into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential that travels to the myofibril and results in muscle contraction. Labels: A: Motor Neuron Axon B: Axon Terminal C. Synaptic Cleft D. Muscle Cell E. Part of a Myofibril

The wave of the action potential is conducted across the sarcolemma, down the T tubules into the sarcoplasm. This change in polarity opens voltage-regulated gates linked to calcium channels (in the terminal cisternae of the sarcoplasmic reticulum). The released calcium binds to troponin (of the troponin-tropomyosin complex on actin filaments) and causes a shape change in the protein that makes binding sites available for myosin.

Note: Troponin is a protein that is bound to tropomyosin which wraps around actin (the thin filament). In this way, you could view troponin, tropomyosin, and actin as a single braided complex, that functionally acts as a single strand of proteins.

Image of the actin, troponin, tropomyosin, and myosin relationship

Actin (Thin Filament), troponin, tropomyosin and myosin - Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436.

For a "power stroke" to occur (the source of the force produced), myosin must be bound to an adenosine triphosphate (ATP) molecule. The ATP is then "hydrolyzed" (broken down) to adenosine diphosphate (ADP) which "cocks" the myosin head into an extended position. If there is an active site available on

, the myosin head will bind to it, this is known as "cross-bridging." The release of the ADP results in the "flexing" of the myosin head which pulls actin with it. This forceful pull on actin by myosin is known as the "power stroke." The myosin head stays bound to troponin until more ATP is available to bind to the myosin head. The binding of "new" ATP "releases" the myosin head from troponin so it can start the process over on a binding site further down the actin filament.

The specific number of myosin heads that attach to actin-binding sites (cross-bridging) at any one time dictates the amount of force that can be produced by the muscle cell (2).

This process occurs in a cyclical fashion, attaching, flexing, and detaching, to produce (concentric contraction), reduce (eccentric contraction), or maintain (isokinetic) force during movement.

Illustration of the molecular changes leading to a muscular contraction.

This illustration looks a bit complex, but if you look at each slide slowly you will note that it is just a visual representation of what we just reviewed. Sliding Filament Theory - Muskel-molekular.png: Hank van Helvete
derivative work: GravityGilly

Contraction (Sliding Filament Theory)

So far, we have covered the process from impulse (intent resulting in an action potential) to conformational (shape) changes in contractile proteins, resulting in a "power stroke." Sliding filament theory (and the diagram below) explains the organization and structure of these contractile proteins. This aids in understanding how the power stroke process is multiplied 1,000,000's of times per muscle to generate force and facilitate function. It also explains why these cells are striated (striped) under a microscope.

In the diagram below, note the thick myosin filaments (horizontal and red) and the thin actin filaments (horizontal in blue). The bumps on the myosin proteins represent the myosin heads. This is the portion of myosin that binds to troponin (of the troponin-tropomyosin complex) embedded on the actin filaments. Note, there are multiple myosin heads ready to bind to each actin protein. The multiple heads on the myosin protein are due to the myosin chains in muscles being comprised of many myosin proteins, with their "tails" braided together, and their "heads" sticking out ready for a power stroke. The best analogy I have heard describing this arrangement is rowers in a rowboat. The legs of the rowers are in the boat like the tails of the myosin proteins, while their oars are sticking out ready to stroke the water, like the myosin heads. At either end of the myosin proteins, note the "z-disks" (vertical pink line). This represents a protein structure that borders and separates two adjacent sarcomeres. When the contraction is initiated, the synchronized "power strokes" (like synchronized rowers in a race) "pull" on actin, sliding the filaments across one another, and pulling the z-disks closer to one another. Now consider a 3-dimensional representation of this diagram (or at least a sagittal cross-section), each myosin protein is surrounded by approximately 6 actin, creating a hexagonal honeycomb-like arrangement. This honeycomb-like arrangement creates a grid of dozens or hundreds of contractile proteins, running parallel to one another in each myofibril (tube of contractile proteins), with many opportunities for cross-bridging on each myofilament. Then consider how many microscopic sarcomeres must be connected in series to span the full length of a muscle fiber, and how many parallel tubes of the sarcomere (myofibril) are then clustered into muscle cells. Muscle cells are then clustered into fascicles (fibers), finally fascicles into muscle bellies. It is not hard to imagine how a small, seemingly insignificant force generated by a single protein, is multiplied millions of times, to generate significant force every time you contract a muscle. The reason muscle cells are so unique in their ability to contract and produce force is this repeated pattern of parallel proteins throughout the entirety of a muscle cell (and muscle as a whole) ensures that the force produced is along one axis. Like rowers in a boat (power stroke), all stroking the water in the same direction (parallel filaments), with a million other boats stroking in the same direction (e.g. sarcomere, myofibril, muscle cells, fascicles), all tied to a single rope (e.g. tendon), all pulling the same object (e.g bone), with the same goal (e.g. move or stabilize a joint).

Each myosin filament has multiple heads capable of producing a power stroke.

Approximately 6 actin, with multiple binding sites, surround each strand of myosin filament.

There are many parallel myosin and actin per myofibril (tubes of contractile proteins in a honeycomb-like arrangement).

There are many sarcomeres in series per myofibril along the length of a muscle cell.

Muscle cells (myocytes) are comprised of many myofibrils
There are many muscle cells per fascicle.

Muscles are generally comprised of many fascicles.

Diagram of a sarcomere, used to describe the sliding filament theory

Diagram of a sarcomere (Sliding Filament Theory) By David Richfield (User:Slashme)When using this image in external works, it may be cited as follows:Richfield, David (2014). "Medical gallery of David Richfield". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2002-4436. - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2264027

Striations

The electron microscope image below (and others like it) is what led researchers to our current understanding of the organization of contractile proteins in a muscle cell. Take a second to consider how what we have discussed up to this point is represented below; consider how the length of a muscle may influence the number of cross-bridges available, how the structure of a muscle cell may limit the capacity to shorten, how stretching may be limited by protein length, and then consider how those traits affect motion, exercise selection, rehab, and performance.

 - A segment of a myofibril (muscle cell) from one Z disk to the next. This is the smallest functional unit of a muscle cell. A muscle shortens because all of the individual sarcomeres shorten, pulling Z disks closer to one another.

 - Refers to a dark line between adjacent I-bands under a microscope. This area is the border, or walls of the "sarcomere", and is comprised of connectin protein that acts as an anchor for the actin filaments.

 (not labeled below but visible down the middle of the sarcomere)- Refers to a dark line through the middle of a sarcomere, bisecting the two halves between Z disks when viewed under a microscope. The M-band is composed of proteins myomesin, C-protein, the M-line portion, as well as other cross-links between the thick filament system (myosin).

 - Refers to the central zone of a sarcomere that looks lighter under a microscope. This area of the sarcomere is a lighter portion of the A-band where there is no overlap between the thinner filaments (actin) and the thicker filaments (myosin).

 - Refers to the part of the muscle cell that looks lighter under a microscope. This area of the cell is lighter due to the parallel arrangement of thinner filaments (actin), and no overlap with the thicker filaments (myosin).

 - Refers to the part of the muscle cell that looks darker under a microscope. This area of the cell is darker due to the parallel arrangement of thicker filaments (myosin) and overlap with the thinner filaments (actin).

Examples of how the Sliding Filament Theory Influence our Understanding of Motion:

The number of cross-bridges, which is correlated with the amount of overlap of actin and myosin (A-band), is correlated with the amount of force a muscle can produce. Changes in sarcomere length (Z disk to Z-disk), both shorter and longer, result in a decrease in the number of cross-bridges. This results in a relationship between length and tension discussed below under "Length/Tension Relationship."

There are limits to the capacity of a muscle cell to shorten and lengthen and produce force at the extreme ends of these ranges. This is sometimes referred to as active insufficiency (too short) and passive insufficiency (too long).

A loss of the total number of sarcomeres (as is found in long-term casting and contractures) will reduce strength, extensibility, and function.

Sarcomere under electron microscope matched to diagram of zones. Author User: Sameerb in English WP

Sarcomere under electron microscope matched to a diagram of zones. Author User: Sameerb in English WP

Striations (Why are muscle cells striped under a microscope?)

Motor Unit Recruitment (Multiplication by Clustering):

The information above is presented as one motor neuron stimulating a single muscle cell, but there is an additional layer of organization that increases the efficiency of the neuromuscular system and allows for the control of force output. When an action potential reaches the end of an axon it spreads out over many terminal branches, and each branch stimulates a different muscle cell. This results in all muscle cells at the end of these terminal branches contracting simultaneously from the impulse of a single motor neuron. In essence, a motor neuron and all of the muscle cells it innervates function as a unit. A motor neuron, axon (nerve fiber), terminal branches, and all of the muscle cells (muscle fibers) it innervates are known as a 

. The muscle cells of a single motor unit are generally dispersed throughout a muscle, resulting in a weak diffuse contraction of the muscle, rather than a strong local twitch. This simultaneous excitation of a unit of muscle cells improves the efficiency of the neuromuscular system.

Basics of Human Movement Systems (Connective Tissue, 3 Rules of Muscles, Intro to Motor Unit Recruitment)

When a muscle cell is stimulated the cell is stimulated completely, and contraction is carried out to the best of the cell's ability. Regardless of the strength of the impulse or the intensity of the external stimulus, an action potential is either initiated or not, and that action potential results in contraction of the stimulated muscle cell, or it does not. It is analogous to a simple light switch and lamp. You cannot move the switch to the halfway position to turn on half the bulb or turn the switch on completely, but adjust the bulb to "half brightness." Motor neurons and muscle cells are either "on" or "off", there is no "dimmer switch." This is known as the 

- a nerve or muscle cell's response to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential (threshold of an action potential) the nerve and/or muscle fiber will respond completely; if it does not exceed threshold potential there is no response at all. This begs the question of how we control the force output of any muscle. How do we pick up a pencil and not impart the same force we would impart on a 30lb dumbbell? The answer is 

. Although each motor unit contracts completely when sufficiently stimulated, motor units may include as few as 3 - 6 muscle fibers as found in the eye or 1000 or more as found in the larger muscles of the body like that gastrocnemius. Smaller motor units are well-suited for fine motor skills, whereas larger motor units are better for larger crude outputs of force. Larger motor units are attached to larger motor nerves that are harder to stimulate. The size and number of motor units recruited determine the force output of any muscle. To continue the lamp and light switch analogy, although we do not have a dimmer switch, we have multiple lamps of varied brightness and a switch for each. If we need a little light, we can easily flip small switches to tiny lamps with more exacting control of brightness. If we need more light, we have to flip large switches for large diffuse amounts of light.

Motor units are not only an efficient means of recruiting muscle cells, this type of organization also allows motor units to work "in-shifts". When tasks that require multiple repetitions, or a force must be held for a long duration, fatiguing motor units may be replaced by previously unrecruited motor units, who can then be replaced by more unrecruited motor units, who may then be replaced by previously recruited motor units who have now had sufficient time to rest (replenish ATP), etc., in a coordinated and sequential fashion.

Force Output (Strength) is Determined By:

The number and coordination of motor units working "in-shifts"

Note: In later lessons, we will discuss how muscle cells adapt to exercise to enhance strength, endurance, and power.

Neuromuscular junction, motor endplates, striated cells.

Motor Unit by Electron Microscope: Note how you can see the neuromuscular junction and motor end plate and if you look closely the striations in the muscle cells.

First, it is important to be aware of muscle cell physiology to 

during interventions for rehab, fitness, or performance. For example, the knowledge that available ATP is necessary for continued contraction may influence our understanding of research on energy systems and acute variables (reps, load, tempo). The physiology of contraction may also aid in understanding how creatine phosphate supplementation plays a role in augmenting training. Last, it may aid in understanding pathological processes like myasthenia gravis, muscular dystrophy, or understanding how Botox injections work by disrupting the neuromuscular junction.

Second, knowledge of physiology becomes a

. Too many myths, misguided recommendations, and physiologically impossible promises are peddled to the professional and consumer alike. Having a strong foundation of physiology may aid in deciding which recommendations have the potential to work and which do not. For example, there is no place in the steps above that suggests this process can be "confused" to enhance adaptation. This process does not depend on which exercise or technique is chosen; it is the only way a muscle will contract. Another example, consider how an action potential is propagated along the entire sarcolemma of a cell. This results in a complete contraction of that cell from origin to insertion - it is not possible to contract, be tight, or target the distal or proximal portions of a muscle.

Motor Unit Recruitment (Multiplication by Clustering)

Connective Tissue and Muscle Fiber Arrangement

Skeletal muscle is surrounded by layers of connective tissue, traditionally subdivided into 3 "levels." These connective tissue layers add to the strength and integrity of a muscle, as well as play a key role in transmitting the force generated by muscular contraction to the bones they are attached to. For example, by forming a tendon that attaches muscle to bone (a connective tissues structure).

Epimysium: Layer of dense connective tissue encasing the entire muscle. (The epimysium is often what is implied by the word "fascia" as these are sheaths of fascia covering a muscle.)

Perimysium: Layer of dense connective tissue surrounding each fascicle (bundle of muscle fibers).

Endomysium: A thin layer of areolar (loose) connective tissue around each individual muscle cell (myocyte).

The epimysium either fuses with the periosteum of bone directly (e.g. the origin of the 

 ) or continues to fuse and form into tendons that fuse with the periosteum (e.g. the insertion of the 

The layers of fascia in a muscle fiber, including the epimysium, perimysium, and ectomysium

Layers of fascia. (Note the column projecting from the bottom picture is the myofibril, each cell contains these columns of myofilaments, there are not individually wrapped in connective tissue)

The amount of overlap between actin and myosin filaments (A-band) is correlated with the number of cross-bridges that can form and the amount of force that can be generated. As a muscle cell is elongated the amount of overlap decreases, decreasing the amount of force that can be produced. Further, the available distance myosin can glide over actin before abutting z-disks also has an impact on the amount of force that can be produced. That is, a muscle cell will produce less force when approaching a maximally shortened length. This results in an "upside-down U" shaped relationship between length and tension, with most muscles producing the most force close to their resting length.

The length/tension relationship: Note as length of a muscle gets longer or shorter the tension decreases

Length/Tension Relationship: Note, the "active" portion of this graph is explained by the interaction between actin and myosin described above, the passive relationship is as if the muscle was being stretched and resistance to extensibility is being imparted by proteins like titin and the connective tissue layers. By Mokele, via Wikimedia Commons

Middle length muscle = optimal overlap between actin/myosin and optimal force production

Long muscle = little overlap between actin/myosin and less force production

Short muscle = less ability to contract further and less force production

Note: In later lessons, we will discuss how the central nervous system aids in the control of resting muscle length via gamma motoneurons, muscle spindles, and tone. The intent of this system may be to maintain muscles close to optimal length to ensure optimal force production. In the video below, the relationship between length/tension, posture, performance, and injury is highlighted.

Schoenfield, B. (2016). Science & Development of Muscle Hypertrophy. Human Kinetics; Champaign, Il.

Haff, G.G., Triplett, N.T. (2016). Essentials of Strength and Conditioning Training. (4th Ed.). Human Kinetics; Champaign, Il.

Tiidus, P. (2008). Skeletal Muscle Damage and Repair. Human Kinetics; Champaign, Il.

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