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Muscle Cell Structure and Function Audio

Muscle cells, also known as myocytes, are specialized cells designed for contraction and force production. This course covers structure, excitation-contraction coupling, sliding filament theory, motor unit recruitment, connective tissue layers, and more.

Muscle Cell Structure and Function

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Muscle cells, also known as myocytes, are specialized cells designed for contraction and force production. 

: These cells are long, cylindrical, and have a striated (or striped) appearance due to their highly organized structure into functional units known as sarcomere. Additionally, these cells are multi-nucleated to increase the rate of protein synthesis and growth. Further, these cells have the highest number of mitochondria per cell in the human body to aid in energy production. However, the most important difference between skeletal muscle cells and other muscle cell types is that they can be voluntarily recruited. That is, skeletal muscle is the only type of muscle that can be consciously activated via the intent to move.

 Note that skeletal muscle cells are also known as "muscle fibers" because of their long fiber-like shape (0.001 - 0.01 cm thick but 2 - 12 cm long). However, "myofibrils" are an organelle within a muscle cell (muscle fiber). Myofibrils are strands of repeating sarcomeres that run parallel to one another and are bundled into muscle fibers by the sarcolemma and endomysium - a thin layer of areolar (loose) connective tissue.

This course covers the structure and function of skeletal muscle cells. The human body consists of four main types of cells: epithelial cells (e.g., skin), nerve cells (e.g., sciatic nerve), connective tissue cells (e.g., tendons), and muscle cells (e.g., biceps brachii). Muscle tissue is classified into three distinct types: smooth muscle tissue, cardiac muscle tissue, and skeletal muscle tissue. Skeletal muscle cells are unique due to their highly organized structure, their ability to generate force through contraction, and their ability to be voluntarily controlled. 

Muscle contraction results from an interaction between two proteins, actin and myosin, which are collectively known as "contractile filaments." These contractile filaments are organized parallel to one another in the smallest functional unit of a muscle cell, known as a sarcomere. When a motor nerve is stimulated, it signals to all of the sarcomeres in all of the innervated muscle cells (a motor unit) to contract. This is known as excitation-contraction coupling. Contraction occurs when myosin (a "motor protein") changes in shape and pulls actin toward the center of the sarcomere, sliding the filaments passed one another. This process of contraction is known as the sliding filament theory. 

This course covers all of these amazing processes and more in greater detail, with practical applications throughout. See an example of "practical applications" below.

Practical Application of Understanding Excitation-Contraction Coupling

Because an action potential is all-or-none, and the stimulus from a nerve propagates across the entire sarcolemma, a muscle fiber has to contract completely or not at all (all-or-none principle). You cannot contract half a muscle fiber (or muscle). This should have dispelled the myths about targeting your "distal bicep brachii," "medial 

 Several injuries and pathologies may disrupt normal excitation-contraction coupling. For example, radiculopathies (e.g., sciatica) can cause intense muscle pain, as these nerve irritations often result in irregular but intense bursts of action potentials that result in muscle spasms. Additionally, some pathologies directly affect the excitation-contraction coupling. Myasthenia gravis is an autoimmune disorder in which antibodies attack the acetylcholine receptors at the neuromuscular junction, reducing the ability of muscles to respond to neural signals.

 Discussions about ions, calcium, and ATP might remind you of sports supplements. Although ingesting calcium alone may not significantly impact performance (unless correcting a deficiency), and ATP cannot be directly ingested to increase muscle stores, two common supplements can enhance performance by affecting excitation-contraction coupling. While calcium itself may not provide a performance boost, consuming sodium and potassium, along with smaller amounts of the other electrolytes, can improve performance during exercise lasting longer than an hour. These electrolytes, found in sports drinks like Gatorade, play a key role in maintaining muscle and nerve function during the rapid depletion of electrolytes during intense physical activity. Additionally, creatine phosphate (CP) supplements increase CP stores in muscles, which helps regenerate ATP from ADP. Even though ATP cannot be ingested directly, increasing CP stores through supplementation supports energy production during intense physical activity.

Muscle fiber arrangement (Pennation Angle). 

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Muscle Fiber Dysfunction and Trigger Points

Contractile filament arrangement within a sarcomere.

Course Description: Muscle Cell Structure and Function

Course Summary Webinar: Muscle Cell Structure and Function

Brookbush Institute Course Study Guide "Muscle Cell Structure and Function"

Course Study Guide: Muscle Cell Structure and Function 

The human body consists of four cell types: epithelial cells, nerve cells, connective tissue cells, and muscle cells. Muscle cells, also known as myocytes, are specialized cells designed for contraction and force production. There are three types of muscle cells: skeletal, cardiac, and smooth muscle cells.

 These cells have a striated (or striped) appearance due to the highly organized structure of the cell into sarcomeres. They are long, cylindrical, and multi-nucleated, allowing efficient force generation during movement. Additionally, these cells have the highest number of mitochondria per cell when compared to all other cells in the body. However, 

the most important difference between skeletal muscle cells and other muscle cell types is that they can be voluntarily recruited

. That is, skeletal muscle is the only type of muscle that can be consciously activated via motion.

All cells have unique traits, and muscle cells possess several characteristics that distinguish them. These traits include contractility, extensibility, elasticity, responsiveness, excitability, and conductivity.

 Muscle cells have a remarkable ability to shorten significantly when stimulated. This is due to the highly organized arrangement of contractile proteins known as myofilaments (actin and myosin) within the muscle fiber.

 Muscle cells can be stretched or lengthened without damage. Unlike most cells, which would rupture with moderate stretching, muscle fibers can stretch up to three times their contracted length.

 Although often confused with extensibility, elasticity refers to the ability of muscle cells to return to their resting length after being stretched. This trait allows muscles to maintain their shape and function following lengthening.

 Muscle cells are highly responsive to stimuli, such as chemicals or hormones. For instance, in response to certain signals in the bloodstream, muscle cells may increase the intracellular production of specific proteins.

 Muscle cells have a specialized ability to change their membrane potential in response to stimulation. While many cells are excitable, muscle cells are particularly adept at converting changes in membrane potential into contraction.

 In muscle cells, stimulation doesn’t remain localized. For example, stimulation at a motor endplate results in a change in cell membrane polarity that propagates rapidly along the entire length of the muscle fiber, from origin to insertion.

 These protein filaments (myofilaments) are the contractile proteins responsible for the contraction of a muscle cell (5).
Myosin is the thicker of the two structures, with globular heads that attach to binding sites on actin. Actin is the thinner protein filament that myosin "pulls on."

 A sarcomere is the basic contractile unit of striated muscle tissue found in both skeletal and cardiac muscles. It is the repeating unit between two Z-lines (or Z-discs), contains highly organized contractile proteins, and is responsible for the muscle's ability to contract. This highly organized repeated structure is a unique feature of muscle cells. This also gives muscle cells their striated appearance under a microscope.

 Mitochondria are responsible for producing adenosine triphosphate (ATP), the cell's "energy currency." Compared to other cells in the human body, muscle cells have the highest number of mitochondria per cell.

 The sarcoplasmic reticulum is a specialized type of smooth endoplasmic reticulum found in muscle cells. Its primary function is to regulate the storage, release, and reabsorption of calcium ions (Ca²⁺), which are crucial for muscle contraction.

 The nuclei contain all genetic material (DNA), allowing for replication, growth, and repair of cell structures. Muscle cells have multiple nuclei, improving their ability to repair and grow.

Components of a Muscle Cell

E-C coupling refers to the steps in the process of muscular contraction from action potential (excitation) to the power stroke (contraction).

 Stimulus (intent, reflex, motor program, etc.) opens sodium ion channels on a motor neuron's membrane, propagating the action potential across the motor neuron. The action potential is propagated by the motor neuron axon to the synapse. Acetylcholine (ACh) is released into the neuromuscular junction.

 The ACh released into the neuromuscular junction binds to receptors on the motor endplate of the muscle cell, opening sodium ion channels. The action potential is propagated across the sarcolemma. It reaches the sarcoplasmic reticulum via t-tubules in the cell membrane, initiating the release of calcium.

 Calcium enters the sarcoplasm and binds to troponin, changing the shape of tropomyosin and exposing binding sites on actin. Adenosine triphosphate (ATP) binds to the myosin head. ATP is hydrolyzed, causing the myosin head to extend.

 Myosin attaches to the binding sites on actin (cross-bridge formation). ADP is released, resulting in the flexing of the myosin head (power stroke). ATP binds again, releasing the myosin head from actin.

 Step 4 continues to repeat as long as the stimulus is continued, releasing calcium to ensure there are available binding sites on actin, and ATP is available for the power stroke.

Myosin heads attaching to actin in preparation for the power stroke.

The sliding filament theory explains the organization and structure of the sarcomere (the smallest function unit of a muscle cell) and the contractile proteins. It helps us understand muscle cell contraction via a power stroke. The power stroke (discussed in excitation-contraction coupling ) is the process of the myosin head attaching to actin and pulling the actin filament toward the center of the cell (sliding the "actin" filament across the "myosin" filament).

 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 actin. Note that 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 being comprised of many myosin proteins "braided" together, and the myosin protein "heads" staggered and sticking out, ready for a power stroke. The power stroke (discussed in excitation-contraction coupling) is the process of the myosin head attaching to actin and pulling the actin filament toward the center of the cell (sliding the "actin" filament across the "myosin" filament).

 The sliding filament theory explains the bell-shaped curve of the relationship between muscle length and force production, known as the 

. This relationship is determined by the number of cross-bridges that can form between actin and myosin filaments, which depends on their overlap (

 ). As a muscle elongates, the overlap between actin and myosin decreases, reducing the force that can be produced. Similarly, as myosin and actin exceed optimal overlap and start to approach the maximally shortened position, force production also decreases. This results in the relationship between length and force production exhibiting an "upside-down U-shaped curve" when graphed.

Sarcomere Structure (Myosin and Actin Filaments)

: A motor unit includes a motor nerve (nerve cell axon) and all of the muscle cells (fibers) it innervates. More specifically, the motor unit consists of 1 anterior horn cell, its axon, and all the muscle fibers innervated by that motor neuron.

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 (action potential) of a single motor neuron. In essence, a motor neuron and all of the muscle cells it innervates function as a unit. 

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 or muscle cell responds completely; otherwise, there is no response.

 Force is modified by adjusting the size, number, and type of motor units recruited. Smaller motor units are well-suited for fine motor skills. Larger motor units are well suited for generating massive amounts of force that do not require the same fine motor control provided by smaller motor units. And, motor units are generally comprised of 1 muscle fiber type. Fiber types are uniquely adapted for endurance (type I), strength (type IIa), and power (type IIb).

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

Motor Unit (Electron Microscope)

Three layers of connective tissue aid in the organization of skeletal muscle fibers. These layers play a crucial role in maintaining the structure, strength, and function of muscle cells. They are also essential for transmitting the force generated by muscle contractions to the tendons and bones they are attached to.

 A thin layer of areolar (loose) connective tissue that surrounds each muscle cell (myocyte).

 This layer of dense connective tissue surrounds each fascicle or bundle of muscle fibers.

 This is a dense layer of connective tissue that encases the entire muscle. The epimysium either fuses directly with the periosteum of bones (as seen in the origin of the subscapularis) or merges into tendons that attach to bones (as in the insertion of the soleus via the Achilles tendon).

Layers of Connective Tissue

Muscle fibers can be arranged in 4 orientations, which influence their function and strength. These alignments include:

 Muscle fibers are organized in long strings of parallel myofibrils coursing from end to end of a muscle. Parallel muscles are common in long muscles of the body. There are two types of parallel muscles.

Fusiform: These muscles are wider in the middle and taper at both ends, like a spindle. An example is the biceps brachii.

Non-fusiform: These muscles have a uniform with along their length, creating a rectangular shape. An example is the rectus abdominis.

 Muscle fibers originate from a broad area and converge to form a single tendon, creating a fan shape. An example is the pectoralis major.

 Muscle fibers are aligned at an angle to the tendon, which increases the number of muscle fibers per cross-sectional area (fiber density) of the tendon, allowing for greater force production than parallel muscles. There are 3 types of pennate muscle fiber arrangements.

Unipennate: Muscle fibers are arranged at an oblique angle to the tendon on one side. An example of this is the lateral and medial heads of the gastrocnemius.

Bipennate: Muscle fibers are arranged at an oblique angle on both sides of the tendon in a feather-like pattern. An example is the rectus femoris.

Multipennate: Muscle fibers are arranged in multiple directions. The deltoid muscles are an example.

 The muscle fibers of the circular muscles are arranged concentrically around an opening or recess. As the muscle contracts, the opening gets smaller. These muscles are often found at the entrances and exits of external and internal passageways. For example, the orbicularis oris is a circular muscle that is responsible for contracting the lips, as seen when sucking through a straw.

Summary: Muscle Cell Structure and Function

The human body consists of four cell types: epithelial cells, nerve cells, connective tissue cells, and muscle cells. 

, also known as myocytes, are specialized cells designed for contraction and force production. There are three types of muscle cells: skeletal, cardiac, and smooth muscle cells.

All cells have unique traits, and muscle cells possess several characteristics that distinguish them. These traits include contractility, extensibility, elasticity, responsiveness, excitability, and conductivity. These traits are explained in further detail below:

 Muscle cells have a remarkable ability to shorten significantly when stimulated. This is due to the highly organized arrangement of myofilaments (actin and myosin) within the muscle fiber.

 Muscle cells can be stretched or lengthened without damage. Unlike most cells, which would rupture with moderate stretching, muscle cells can stretch up to 3 times their contracted length.

Although often confused with extensibility, elasticity refers to the ability of muscle cells to return to their resting length after being stretched. This trait allows muscles to maintain their shape and function following lengthening.

Although all cells possess these traits to varying degrees, muscle cells have evolved to become "specialists" in these areas. These specialized traits are essential for their primary role in movement and force generation.

BFR with Assisted Squats

Understanding the unique traits of muscle cells significantly influences how we address these tissues. Many of the practical applications of this knowledge are so common that we often forget the underlying reasons behind them.

 Since muscle cells are specialized for contraction, we stimulate these tissues by causing them to contract under external resistance. This is the basis of resistance training. In contrast, we would never use this method to stimulate tissues like the epithelial cells of the liver or the nerve cells of the spinal cord, as they are not designed for contraction.

Muscle cells' ability to stretch and return to their original shape allows us to improve mobility using various stretching techniques. The relatively extreme changes in length that muscle tissue can withstand would never be applied to connective, nerve, or epithelial tissues, which lack this same degree of flexibility.

 Muscle tissue’s responsiveness to changes in hormones, inflammatory markers, and enzymes has led to treatments like blood flow restriction (BFR) therapy. This technique enhances muscle growth (hypertrophy) with significantly less load by increasing the concentration of these substances, leveraging the muscle cell’s natural responsiveness to such stimuli.

Because muscle cells are excitable and capable of conducting electrical signals, therapies like electrical stimulation (e-stim) are used to help improve muscle strength and function. This is particularly useful when injury has made it difficult for the body to stimulate muscle contractions naturally.

Muscle Cell Definition and Traits

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.

Structure of a Muscle Cell

Labels: A: Motor Neuron Axon B: Axon Terminal C. Synaptic Cleft D. Muscle Cell E. Part of a Myofibrill

This section builds upon previous sections and focuses on the "neural activation" of a muscle cell. In this section, we cover 

Excitation-Contraction (E-C) Coupling. E-C coupling 

refers to the steps in the process of muscular contraction from action potential (excitation) to the power stroke (contraction).

The excitation of a muscle cell begins at a motor neuron, which is a nerve cell located in the brainstem or spinal cord that innervates a muscle cell. When the motor neuron is sufficiently stimulated, either by conscious intent or through a reflex, a nerve impulse called an action potential is generated. 

An "action potential" is a rapid temporary change in the electrical charge of a cell membrane. This occurs when a stimulus reaches a threshold that results in sodium channels in the cell membrane to open. A rapid influx of Na⁺ ions rushes into the cell and changes the polarity of the cell membrane from positive to negative (depolarization). Note that this "all or none" process has many implications on the function of motor nerves and muscle cells, including that a nerve cell or muscle cannot be partially stimulated to create a smaller response. That is, there is no "dimmer switch" on a cell. 

The action potential then initiates a cascade of membrane depolarization that travels (is propagated) down the length of the axon. An axon is the slender projection of a neuron (nerve cell) that conducts electrical impulses away from the neuron's cell body. In the case of a motor nerve, the axon extends from the nerve cell to the neuromuscular junction (the connection between the synapse of the nerve cell and the motor end plate on the muscle cell sarcolemma), the site where the neuron communicates with the muscle fiber. In some cases, this signal is transmitted along an axon for nearly a meter, such as when traveling from the spinal cord to the muscles of the foot (like the plantar interossei). The speed of these impulses is so rapid that it’s nearly impossible to perceive any delay between your intent to flex your toes and the actual movement.

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 at the "motor endplate." the motor end plate is the junction between the nerve and muscle cells, which opens the sodium ion channels. The opening of these channels results in a rush of ions that change the polarity of the cell membrane (an action potential). And that action potential is propagated along the sarcolemma (muscle cell membrane).

Stimulus (intent, reflex, motor program, etc.) opens sodium ion channels on the membrane of a motor neuron.

An action potential is propagated across the motor neuron.

The action potential is propagated by the motor neuron axon to the synapse.

ACh is released into the neuromuscular junction.

ACh binds to receptors on the motor endplate of the muscle cell, opening sodium ion channels.

An action potential is propagated across the sarcolemma.

 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 reduces ACh to pre-stimulation concentrations, and this returns the cell to a relaxed state.

Sarcolemma and T-tubules

Muscle contraction occurs following the last step of the previous section, in which a motor nerve stimulates a muscle cell at the neuromuscular junction, initiating an action potential across the muscle cell membrane (sarcolemma). 

The action potential spreads across the entire sarcolemma and into the muscle fiber through the T-tubules (transverse tubules). T-tubules are deep crevices in the muscle cell membrane that assist in quickly propagating the action potential to the sarcoplasmic reticulum. In skeletal muscle, there is a "triad structure," which consists of one T-tubule flanked by two terminal cisternae

 The terminal cisternae are enlarged areas of the sarcoplasmic reticulum where calcium ions are stored.

 Voltage-regulated calcium ion channels (stimulated by an action potential) on the terminal cisternae of the sarcoplasmic reticulum release stored calcium ions (Ca²⁺) into the muscle’s sarcoplasm (fluid inside the cell). The released calcium binds to troponin, a component of the troponin-tropomyosin complex. This troponin-tropomyosin complex is comprised of two proteins that are bound to one another, wrap around the actin filament, and cover the myosin-binding sites on actin (preventing contraction). However, when calcium binds to troponin, there is a conformational shift (change in shape) in tropomyosin that exposes the myosin-binding sites on actin.

With the binding sites on actin exposed, 

attach to actin, forming cross-bridges. Myosin then undergoes its conformational change (change in shape), referred to as a "power stroke. " This pulls the actin filaments toward the center of the sarcomere, shortening the muscle fiber and generating a contraction.

The motor nerve stimulates a muscle cell, initiating an action potential across the muscle cell membrane (sarcolemma).

The action potential reaches the sarcoplasmic reticulum via t-tubules in the cell membrane, which initiates the release of calcium.

Calcium enters the sarcoplasm and binds to troponin, changing the shape of tropomyosin and exposing binding sites on actin.

Troponin and tropomyosin and the binding sites on actin.

Muscle contraction relies on a process known as the power stroke, which is the source of the force produced during contraction. This process involves the interaction between myosin and actin filaments, regulated by the availability of adenosine triphosphate (ATP) and binding sites on the actin filaments.

For a power stroke to begin, a molecule of ATP must bind to the myosin head. ATP is the cell's primary energy molecule and is crucial for energizing the myosin head and in preparation for the next step.

ATP is then hydrolyzed (broken down) into adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis causes the myosin head to "cock" into an extended position, ready for interaction with actin.

 If an active binding site is available on actin, the myosin head attaches to it, forming a cross-bridge. These binding sites are exposed when calcium binds to troponin, shifting tropomyosin and uncovering the sites (see above).

The release of ADP and Pi triggers the flexing of the myosin head, pulling the actin filament along with it. This pulling action is known as the power stroke, generating the force needed for muscle contraction.

After the power stroke, a new molecule of ATP binds to the myosin head, causing it to release from the actin-binding site. This detachment allows the myosin head to reset for the next cycle.

The process repeats as the myosin head moves to a new binding site further down the actin filament, resulting in continued contraction as long as ATP and calcium are present.

ATP is hydrolyzed, causing the myosin head to extend.

The myosin head binds to actin (cross-bridge formation).

ADP is released, resulting in the flexing of the myosin head (power stroke).

ATP binds again, releasing the myosin head from actin.

The cycle repeats as myosin binds to a new site on actin.

, an enzyme in the synaptic cleft, breaks down ACh, reducing the concentration of ACh and terminating the signal. Calcium is then pumped back into the terminal cisternae of the sarcoplasmic reticulum, and the troponin-tropomyosin complex returns to its original state, covering the binding sites on actin. 

Muscle Contraction (Power Stroke)

 Several injuries and pathologies may disrupt normal excitation-contraction coupling. For example, radiculopathies (e.g., sciatica) can cause intense muscle pain, as these nerve irritations often result in irregular but intense bursts of action potentials that result in muscle spasms.  Additionally, some pathologies directly affect the excitation-contraction coupling. Myasthenia gravis is an autoimmune disorder in which antibodies attack the acetylcholine receptors at the neuromuscular junction, reducing the ability of muscles to respond to neural signals.

Excitation-Contraction Coupling

So far, we have covered the process from impulse (intent resulting in an action potential) to contraction, including the conformational (shape) changes in contractile proteins that result in a "power stroke." 

 explains the organization and structure of the sarcomere (the smallest function unit of a muscle cell) and the contractile proteins

 It helps us understand how the power stroke process is multiplied millions 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 actin. Note that 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 being comprised of many myosin proteins "braided" together, and the myosin protein "heads" staggered and sticking out, ready for a power stroke. The power stroke (discussed in excitation-contraction coupling) is the process of the myosin head attaching to actin and pulling the actin filament toward the center of the cell (sliding the "actin" filament across the "myosin" filament).

 Note the "z-disks" (vertical pink line) at either end of the myosin proteins. The z-disk is a protein structure that creates the borders of each adjacent sarcomere. When the contraction is initiated, the synchronized "power strokes" (like synchronized rowers in a race) "pull" on actin, sliding the filaments across one another toward the center of the sarcomere and pulling the z-disks closer to one another. 

 The best analogy for describing the sliding filament theory is likely the "rowers of a boat." The rowers' legs in the boat are like the majority of the myosin protein, with the oars hanging out of the boat like the myosin heads. In this analogy, we equate the action of the multiple myosin heads repeatedly attaching to actin, performing a power stroke, detaching, and reattaching further along the actin to repeat the process, as similar to the oars (myosin) stroking the water (actin), coming out of the water, resetting their position, and performing another stroke.  As the water (actin) is pulled behind the oars (myosin heads) toward the center of the sarcomere, the Z-disks are pulled along with them.

Muscle Cell Structure and Function

Related Courses:

Course Description: Muscle Cell Structure and Function

Course Summary Webinar: Muscle Cell Structure and Function

Course Study Guide: Muscle Cell Structure and Function

Summary: Muscle Cell Structure and Function

Muscle Cell Definition and Traits

Structure of a Muscle Cell

Excitation-Contraction Coupling

Sliding Filament Theory

Motor Unit Recruitment

Connective Tissue and Muscle Fiber Arrangement

Bibliography

Related Courses:

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