07. Muscle Tissue

The common functions of muscle tissue include:

1. Contraction (voluntary and involuntary)
2. Movement of the skeleton
3. Facial expression
4. Speech
5. Breathing
6. Movement of internal organs
7. Pushing blood through vessels

Muscle fiber

Sarcolemma

Sarcoplasm

Sarcosomes

Sarcoplasmic reticulum

Storage and release of calcium ions necessary for contraction

• Long, cylindrical cells (fibers)
• Multiple nuclei at the periphery (below the sarcolemma)
• Cross striation of the sarcoplasm (L.M)

• Voluntary and rapid contraction
• Functions include:
  • Attach to bones and move skeleton
  • Facial expression
  • Speech
  • Breathing

At 4 weeks, mesenchymal cells differentiate into myoblasts, which begin to form aggregates and line up into tubes.

At 5 weeks, myotubes lengthen by incorporating additional myoblasts via cell fusion.

At 9 weeks, myofilaments have appeared, but the nuclei are still centralized within the developing muscle fibers.

At 20 weeks, developing muscle fibers exhibit a cross-striated appearance, and the nuclei move to a peripheral position.

Myoblasts are muscle precursor cells that fuse to form myotubes, which then differentiate into mature muscle fibers. This process is essential for muscle growth and repair.

Satellite cells are a type of stem cell located on the surface of muscle fibers. They play a crucial role in muscle repair and regeneration by differentiating into myoblasts when muscle injury occurs, aiding in the formation of new muscle fibers.

The process of myoblast fusion involves the following steps:

1. Myoblast Activation: Satellite cells are activated in response to muscle injury or stress.
2. Proliferation: Activated myoblasts proliferate and increase in number.
3. Fusion: Myoblasts align and fuse together to form myotubes.
4. Differentiation: Myotubes mature into muscle fibers with multiple nuclei, completing the muscle formation process.

The endomysium surrounds each muscle fiber, containing blood capillaries and nerves, providing support and facilitating nutrient exchange.

The perimysium surrounds a muscle fascicle, which is a bundle of 10 to 100 muscle fibers, providing structural support and organization.

The epimysium surrounds the entire muscle, composed of many muscle fascicles, and is made of dense irregular connective tissue. It is continuous with the endomysium and perimysium, and also connects to tendons.

The basic functional units of muscle contraction in skeletal muscle are sarcomeres. They contain myofilaments of actin and myosin that slide past each other to shorten the muscle fiber and produce force.

A sarcomere contains the following components:

1. Thick myofilament (myosin)
2. Thin myofilament (actin)
3. Sarcoplasmic reticulum
4. T tubule
5. Mitochondria
6. Myofibril

Actin and myosin contribute to muscle contraction by sliding past each other within the sarcomere, which leads to the shortening of the muscle fiber and the generation of force.

The sarcoplasmic reticulum plays a crucial role in regulating calcium ion concentration, which is essential for muscle contraction and relaxation processes.

The thick filament contains:

• Myosin molecules (200-500 per filament)
  • Tail part: Heavy chain, double helix
  • Head: Flexible, binds to actin filament

The thin filament consists of:

• G-actin
• Tropomyosin
• Troponin complex (including TnC, TnT, TnI)

The myosin head has two important binding sites:

• Actin-binding site: Binds to actin filament during contraction.
• ATP- and ATPase-binding site: Binds ATP, which is necessary for muscle contraction and relaxation.

In muscle fibers:

• Thick filaments are composed of myosin molecules.
• Thin filaments are primarily made of actin, with tropomyosin and troponin.
• The arrangement allows for the formation of cross-bridges during contraction, facilitating muscle movement.

The sarcomere is the basic contractile unit of muscle fiber, arranged end-to-end within myofibrils.

The arrangement of thick and thin myofilaments within the sarcomere, which are parallel and overlapping, produces alternating light (isotropic) and dark (anisotropic) bands or zones.

The arrangement of myofilaments in a sarcomere is fixed by binding to accessory proteins located in the Z disk and M line.

The main structural components of a sarcomere include thin filaments, thick filaments, M-line, H-zone, Z-discs, Titin, and the A and I bands.

The Z disc is the line that separates one sarcomere from another.

The M line is the central line of the sarcomere where myosin filaments are anchored.

The I band is the area where only actin filaments are present.

The A band includes overlapping myosin and actin filaments.

The H zone is the area of the A band where only myosin filaments are present.

The sarcomere is the basic contractile unit of muscle fiber, delineated by Z discs on either end. It contains:

• Thin filaments (actin) extending from the Z discs towards the center, overlapping with thick filaments.
• Thick filaments (myosin) located in the A band.
• The M line marks the center of the sarcomere within the H zone, where thin filaments do not overlap.
• The I band contains only thin filaments, located on either side of the A band.
• Titin protein provides structural support.

Calcium ions (Ca2+) initiate muscle contraction by binding to troponin, which causes a conformational change that exposes the actin-binding sites. This allows myosin heads to bind to actin, leading to the sliding of filaments and muscle contraction.

The Cross Bridge Cycle involves the following key steps:

1. Attachment: Myosin heads attach to actin at specific binding sites.
2. Power Stroke: Myosin heads pivot, pulling the thin filaments toward the center of the sarcomere.
3. Detachment: Myosin heads detach from actin.
4. Recovery: Myosin heads return to their original position, ready for another cycle.

• ATP is required for detachment and recovery, while ADP + Pi are involved in the power stroke.

The process of muscle contraction is often compared to a rowing cycle, where the myosin heads act like a rower pulling oars. The stages of this analogy include:

• In: Preparing for the stroke.
• Pull: Myosin pulls actin, similar to pulling oars.
• Out: Myosin heads detach.
• Push: Myosin heads recover for the next stroke.

In the relaxed state, the actin filaments are not bound to myosin. Tropomyosin covers the myosin-binding sites on actin, preventing interaction. The troponin complex is not bound to calcium ions, maintaining this inhibition of contraction.

Calcium ions bind to the troponin complex, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. This exposure allows myosin heads to bind to actin, initiating muscle contraction.

The troponin complex regulates muscle contraction by binding calcium ions, which leads to the movement of tropomyosin and the exposure of myosin-binding sites on actin filaments.

During muscle contraction, calcium ions bind to the troponin complex, causing tropomyosin to shift and expose the myosin-binding sites on actin. Myosin heads then attach to these sites, leading to the sliding of actin filaments over myosin, resulting in muscle shortening.

The main components of a relaxed sarcomere include:

• Z disc: Defines the boundaries of the sarcomere.
• Thick filament: Composed mainly of myosin.
• Thin filament: Composed mainly of actin, along with troponin and tropomyosin.
• M line: The center of the sarcomere where thick filaments are anchored.
• I band: The region containing only thin filaments.
• A band: The region containing both thick and thin filaments.
• H zone: The area within the A band where there are only thick filaments.

During muscle contraction, the sarcomere undergoes the following changes:

• Filament Sliding: The thin filaments slide past the thick filaments.
• Shortening: The overall length of the sarcomere decreases.
• Z discs: Move closer together.
• I band: Becomes narrower as the thin filaments overlap more with the thick filaments.
• H zone: Disappears as the thin filaments slide into this region.

The A band is significant because it represents the length of the thick filaments and includes overlapping regions of thick and thin filaments. It remains constant during muscle contraction, indicating that the length of the thick filaments does not change, while the thin filaments slide over them, leading to sarcomere shortening.

The key structural components of a sarcomere include:

• Z lines: Vertical green lines marking the boundaries of the sarcomere.
• Thin filaments: Extending from the Z lines, represented in red.
• Thick filaments: Located in the center, represented in blue.
• H zone: The central region of the sarcomere where only thick filaments are present.
• I bands: Light bands on either side of the Z line.
• A bands: Dark bands that contain both thick and thin filaments.

The morphological characteristics of skeletal muscle cells include:

• Many nuclei: Located at the periphery of the cells, visible as dark ovals.
• Cross striation: Alternating light and dark bands in the sarcoplasm, indicating the presence of myofibrils.
• Mitochondria: Identified as 'Mi' in electron micrographs, indicating energy production capabilities.
• Nucleus: Labeled as 'N', essential for muscle cell function and repair.

The organization of skeletal muscle is as follows:

1. Entire Skeletal Muscle

   • Composed of muscle fascicles.

2. Muscle Fascicle

   • Composed of muscle fibers (visible in light microscopy).

3. Muscle Fiber

   • Contains myofibrils (visible in electron microscopy).
   • Myofibrils are oriented longitudinally with the long axis of the muscle fiber.

4. Myofibrils

   • Composed of myofilaments:
     • Thick myosin myofilaments
     • Thin actin myofilaments

5. Molecular Level

   • Myosin myofilament: Composed of hundreds of myosin molecules (golf stick shape).
   • Actin myofilament: Composed of F-actin, tropomyosin, and troponin complex (Tn subunits: TnC, Tnl, TnT).

The key components of the neuromuscular junction include:

• Motor nerve fiber
• Axon terminal
• Synaptic vesicles containing acetylcholine (ACh)
• Sarcolemma of the muscle fiber with ACh receptors
• Synaptic cleft
• T tubules
• Sarcoplasmic reticulum
• Thick (myosin) and thin (actin) myofilaments

Acetylcholine (ACh) is released from synaptic vesicles at the axon terminal and binds to ACh receptors on the sarcolemma, leading to depolarization of the muscle fiber. This depolarization initiates muscle contraction by facilitating the bridging between actin and myosin filaments.

The structure of the neuromuscular junction facilitates muscle contraction through:

1. Synaptic vesicles releasing ACh into the synaptic cleft.
2. ACh binding to receptors on the sarcolemma, causing depolarization.
3. The depolarization traveling along the T tubules to the sarcoplasmic reticulum, triggering the release of calcium ions.
4. Calcium ions enabling the interaction between actin and myosin filaments, leading to muscle contraction.

• Short, branching cells
• 1 nucleus (up to 2 nuclei) centrally located
• Cross striation of the cytoplasm
• Intercalated discs (unique junctions)
• Diads: 1 T-tubule + 1 cisternae of SR

The primary function of cardiac muscle tissue is involuntary rapid, rhythmic contraction to propel blood into the circulatory system.

Cardiac muscle tissue occurs in the walls of the heart.

The characteristic junctions found in cardiac muscle tissue include:

1. Intercalated discs - steplike structures that connect cardiac muscle cells.
2. Adherens junctions - provide mechanical stability by linking the cytoskeletons of adjacent cells.
3. Desmosomes - provide strong adhesion between cells, preventing them from being pulled apart during contraction.
4. Gap junctions - allow for electrical communication between cells, facilitating synchronized contraction.

Key structural features of cardiac muscle tissue include:

• Elongated pink cells with a centrally located nucleus.
• Presence of intercalated disks, which are electron-dense structures that connect adjacent cells.
• A striated pattern due to the arrangement of actin and myosin filaments.
• Z lines visible in electron microscopy, indicating the boundaries of sarcomeres.
• Abundant mitochondria, reflecting the high energy demand of cardiac muscle.

Cardiac muscle fibers are arranged in a spiral pattern. This arrangement allows for effective contraction and efficient pumping of blood from the heart.

The two syncytiums in cardiac muscle tissue are the atrial syncytium and the ventricular syncytium. These syncytiums allow for coordinated contraction of the heart chambers.

Purkinje fibers are specialized cardiac muscle cells that belong to the heart conduction system, usually located in the sub-endocardium.

Purkinje fibers are pale, swollen cells resembling cotton wood balls, with few myofibrils, abundant glycogen and mitochondria, and they lack T-tubules and intercalated discs but have gap junctions and desmosomes.

Purkinje fibers are larger and paler than surrounding cardiac muscle cells, and they have a unique composition with fewer myofibrils and more glycogen and mitochondria.

1. Cardiac muscle fibers - These are the primary muscle fibers that make up the cardiac tissue.

2. Purkinje fibers - These are characterized by their larger size and paler staining compared to the surrounding cardiac muscle tissue.

• Spindle-shaped cells
• 1 oval nucleus centrally located
• No cross striation of the cytoplasm
• Cell borders are invisible
• Arranged closely to form sheets

• Involuntary slow contraction
• Propels substances along internal passageways

• Mostly in the walls of hollow organs (digestive and respiratory tracts, uterus, bladder)
• Wall of blood vessels
• Skin (arrector pili muscle for hair follicles)

The key structural components of smooth muscle cells involved in contraction include:

• Thick filaments
• Thin filaments
• Dense bodies (which are similar to Z disks and contain α-actinin)
• Nucleus

Dense bodies serve as sites for junctions between adjacent cells and myofilaments insert on these dense bodies, facilitating contraction.

Dense bodies function in smooth muscle contraction by:

• Serving as attachment points for myofilaments (thick and thin filaments).
• Acting as sites for adhesive junctions between adjacent smooth muscle cells.
• Facilitating the contraction of the whole muscle by decreasing the size of the cell during contraction.

Smooth muscle cells are distinguished from skeletal muscle cells by:

• The absence of myofibrils and sarcomeres.
• Lack of cross-striation.
• Myofilaments insert on dense bodies located in the cell membrane and deep in the cytoplasm, rather than forming organized sarcomeres as in skeletal muscle.

Dense bodies serve as attachment points for thin filaments in smooth muscle cells. When the filaments contract, they pull on the dense bodies, leading to a decrease in cell size and promoting the contraction of the entire muscle.

A relaxed smooth muscle cell appears elongated and spindle-shaped, while a contracted smooth muscle cell has a more irregular and rounded shape. The nucleus in the relaxed cell is normal, whereas in the contracted cell, the nucleus is deformed and folded into a cork-screw shape.

During contraction, the nucleus of a smooth muscle cell becomes deformed and takes on a cork-screw shape.

The main components visible in the histological images include:

1. Cross-sections of blood vessels (labeled as '1')
2. Connective tissue (labeled as '2')
3. Bundles of smooth muscle cells (labeled as '3')

The tissue is stained pink and purple, highlighting these structures.