Skeletal Muscle Tissue Notes

Overview of Muscle Tissue

• Definition: Muscle tissue is uniquely composed of cells capable of shortening or contracting to generate movement of body parts.

• Characteristics: It is highly cellular and possesses a rich blood supply.

• Types of Muscle Tissue:

  • Striated Muscle: Characterized by regularly arranged contractile units.

    • Skeletal Muscle:

      • Contraction: Quick, vigorous, and under voluntary control.

      • Cells: Long, cylindrical, multinucleated cells with peripherally placed nuclei.

      • Functions: Used for locomotion, mastication (chewing), and phonation (sound production).

    • Cardiac Muscle:

      • Contraction: Involuntary, vigorous, and rhythmic.

      • Cells: Elongated, branched cells with a single, centrally placed nucleus and distinguishing intercalated discs at their ends.

      • Functions: Responsible for the heart's contraction and relaxation.

  • Non-striated Muscle: Possesses contractile machinery, but it is irregularly arranged.

    • Smooth Muscle (Visceral Muscle):

      • Cells: Fusiform (spindle-shaped) with a central nucleus.

      • Contraction: Involuntary, slow, and long-lasting.

      • Functions: Examples include moving food along the digestive tract.

Comparison of Muscle Tissue Types

Skeletal Muscle Tissue: General Characteristics

• Definition: Muscles primarily attached to the bones of the body (the skeleton).

• Movement: Their contraction permits a wide range of movements including extremities, eyeballs, tongue, and the chest cavity.

• Control: Contraction is voluntary, meaning it is consciously controlled.

• Force Exertion: Skeletal muscles can exert tremendous forces.

  • Example 1: Uncontrolled contractions (spasming) during seizure activity can be strong enough to snap the diaphysis of long bones.

  • Example 2: The human calf muscle can, on its own, withstand forces exceeding a ton.

Terminology

(Derived from the Greek root "sarkos" meaning "flesh")

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm within a muscle fiber.

• Sarcoplasmic Reticulum (SR): The endoplasmic reticulum of a muscle fiber.

• Muscle Fiber: A muscle cell.

Location

• Attachment: Usually attached to bone.

  • Note: The periosteum (connective tissue covering bones) is continuous with the tendon, which in turn is continuous with the fascia (dense connective tissue surrounding muscles).

Cell Structure

• Arrangement: Long, cylindrical, parallel fibers arranged as "bundles within bundles." The hierarchical construction is as follows:

  • Actin & myosin $\rightarrow$ myofilaments

  • Bundles of myofilaments $\rightarrow$ myofibrils

  • Bundles of myofibrils $\rightarrow$ muscle fibers (muscle cells)

  • Bundles of muscle fiber bundles $\rightarrow$ muscle

Myofibril Arrangement

• Density: Densely fill the cell from end to end.

• Order: Exhibit an orderly arrangement.

• Appearance: Possess visible dark striations.

Location of Nucleus/Nuclei

• Multinucleated: Skeletal muscle fibers contain multiple nuclei.

• Peripheral Location: These nuclei are typically located at the periphery of the cell, just beneath the sarcolemma.

• Striations: A strong presence of striations is notable microscopically.

Contraction and Control Factor

• Contraction: Vigorous, of relatively short duration, with individual fibers contracting independently.

• Control: Voluntary contraction.

• Alternative Names: Voluntary, striated muscle.

Vascular Supply and Relative Rank

• Blood Supply: Possesses a good blood supply, which is variable depending on the activity level.

• Rank: Ranks second among muscle tissues in terms of vascular supply.

Skeletal Muscle Tissue: Structure

• Description: Often described as "bundles within bundles within bundles."

• Muscle Fiber Formation: Each individual skeletal muscle fiber is a single large cell formed by the fusion of $10$ to $100$ (hundreds) embryonic muscle cells (myoblasts).

  • Consequently, a skeletal muscle fiber has $10$ to $100$ nuclei.

  • A fiber can range from $10$ to $100 \, \mu m$ in width and $1$ to $300 \, mm$ in length (average length is $100 \, mm$).

Connective Tissue Layers

• Endomysium:

  • Description: A connective tissue casing that surrounds each individual bundle of myofibrils (i.e., each muscle fiber).

  • Composition:

    • Inner layer: Mostly laminin.

    • Middle layer: Collagen IV.

    • Outer layer: Type I and Type III collagen.

  • Significance: Since skeletal muscle fibers are rarely as long as the entire muscle, this collagen is critical for the muscle's ability to generate useful force.

  • Location: Found outside the sarcolemma, surrounding the muscle fiber.

• Perimysium:

  • Description: A thin layer of areolar connective tissue that surrounds a bundle of muscle fibers, forming a structure called a fascicle.

• Epimysium:

  • Description: A layer of dense connective tissue that encloses groups of fascicles, thereby forming individual muscles. Some skeletal muscles can be divided into entire skeletal muscles and heads of skeletal muscles, both surrounded by epimysium.

• Fascia:

  • Description: A tough layer of fibrous connective tissue that wraps each individual muscle.

  • Location: Located outside the epimysium, surrounding and separating muscles.

  • Continuity: Continuous with tendons and the periosteum of bones.

Short Account of Structure

• Sarcolemma: Cell membrane of the muscle fiber.

• Endomysium: Surrounds individual muscle fibers (cells).

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Epimysium: Surrounds entire muscles and muscle heads.

• Fascia: Envelops and separates muscles, located outside the epimysium.

Skeletal Muscle Tissue: Muscle Fibers

• Basic Unit: Muscle fibers are the fundamental units of the muscle itself.

• Composition: Muscle fibers are composed of smaller subunits called myofibrils.

  • Myofibrils: These are $1$ to $3 \, \mu m$ in diameter and can be as long as the muscle fiber.

  • Development: During early development, embryonic myoblasts (single, spherical/elongated, uninucleated precursors) fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei allow for multiple copies of genes, enabling the production of large quantities of proteins and enzymes essential for muscle contraction.

• Functional Unit: The functional unit of a skeletal muscle fiber is the sarcomere.

Sarcoplasmic Reticulum (SR)

• Structure: A network of tubules and saccules located between the myofibrils in a skeletal muscle fiber. Myofibrils are separated from each other by the SR.

• Specialization: It is a specialized smooth endoplasmic reticulum (SER).

• Distinguishing Features: Characterized by the presence of a calcium-activated ATPase and gated calcium channels.

  • The calcium-activated ATPase concentrates $Ca^{++}$ within the saccules of the SR.

  • The calcium channels release $Ca^{++}$ to initiate muscle contraction.

• Function: Stores, releases, and retrieves calcium ions ($Ca^{++}$).

Sarcomere

• Definition: A highly organized arrangement of contractile myofilaments (actin and myosin) along with other support proteins.

• Size: Each sarcomere is approximately $2 \, \mu m$ in length.

• Arrangement: Exhibits a three-dimensional, cylinder-like arrangement.

• Borders: Bordered by structures known as Z-discs.

Myofilaments

• Thin Filaments:

  • Components: Composed of actin and its associated troponin-tropomyosin complex.

  • Projection: Project from the Z-discs toward the center of the sarcomere.

  • Thickness: Thinner than myosin.

  • Light Property: They are isotropic (transparent to polarized light), forming the lighter I-bands.

  • Dimensions: Actin filament is $0.007 \, \mu m$ across and $1 \, \mu m$ long.

• Thick Filaments:

  • Components: Composed of myosin strands and their multiple heads.

  • Projection: Project from the center of the sarcomere, toward but not all the way to the Z-discs.

  • Mass/Thickness: Have more mass and are thicker.

  • Light Property: They are anisotropic (opaque to polarized light), forming the darker A-bands.

  • Dimensions: Myosin filament is $0.015 \, \mu m$ thick and $1.5 \, \mu m$ long.

Sarcomere Band and Zone Descriptions

• Z-disc (Z-line):

  • Composition: A plate of $\alpha\text{-actinin}$ that anchors the thin filaments.

  • Microscopic View: At extremely high magnification ($\text{X } 150,000$), the Z-line appears as a puckered disk made of a fine network of $\alpha\text{-actinin}$ fibers.

  • Filament Anchorage: Thin filaments are anchored by one end to each side of a Z-disc, with the other end being free.

  • Titin: Each thick filament is attached to Z-discs by a filament of titin, whose elasticity is responsible for the passive rebound of a stretched muscle.

  • Arrangement: Two groups of thin filaments run parallel, one on each side of a Z-disc. Thick filaments lie between the thin filaments, spanning the gaps between their free ends.

  • Connections to Neighboring Myofibrils: The Z-lines of one myofibril are connected to the Z-lines of neighboring myofibrils by $11 \, nm$ intermediate filaments of desmin, a protein specific to muscle cells.

  • Anchorage to Cell Membrane (Costameres):

    • The Z-lines closest to the cell membrane are anchored to it by protein complexes called costameres.

    • The $\alpha\text{-actinin}$ plate is connected to a membrane protein called integrin by a protein called vinculin.

    • Defective vinculin: Lethal in the first trimester.

    • Dystrophin: The filamentous protein dystrophin connects points on the thin filaments (adjacent to the Z-disk and closest to the cell membrane) to dystroglycan in the cell membrane.

    • Dystroglycan: Bonds to laminin in the basal lamina.

    • Integrin: Seems to bond directly to collagen IV in the basal lamina.

• Muscular Dystrophy:

  • Cause: Defective dystrophin or dystroglycan.

  • Effect: Causes a slow deterioration of muscle, often leading to death before age $30$.

  • Prevalence: A rare condition.

• I bands:

  • Composition: Sections of the myofibril that consist only of thin filaments.

  • Appearance: Appear pale under a light microscope.

  • Structure: The Z-disc splits the I-band into halves.

  • Size: The resting length of an I-band is $0.8 \, \mu m$; each half is $0.4 \, \mu m$ long.

  • Contraction: Decrease in length during contraction.

• A bands:

  • Composition: Sections of the myofibril that contain thick filaments (and overlapping thin filaments).

  • Appearance: Appear dark under a light microscope.

  • Size: $1.5 \, \mu m$ wide.

  • Contraction: Do not shorten during contraction.

• H band:

  • Composition: The central portion of the A band that contains only thick filaments.

  • Size: $0.3 \, \mu m$ wide.

  • Appearance: Can sometimes be seen at the highest magnification of a light microscope as a slightly paler line in the middle of the A band.

  • Contraction: Shorten during contraction.

• M (middle) band:

  • Composition: A thin dark line in the middle of the H (or A) band.

  • Function: Contains fine transverse filaments that link the thick filaments together to maintain their alignment and anchor myosin during contraction.

Sarcomere Summary

• A sarcomere is the part of a myofibril between two Z-lines.

• It includes two half I bands and one A band.

• A myofibril consists of many sarcomeres arranged end-to-end.

Morphology under the Microscope

• Overlapping Structure: The thin and thick filaments have an overlapping structure, held together by chemical cross-bridges.

• Banded Appearance: This overlapping structure gives skeletal muscle a banded or striped look under the microscope.

• Striations: These alternating light (I) and dark (A) bands are called striations, lending skeletal muscle its more accurate name: voluntary, striated muscle.

• Nuclei: Multiple peripheral nuclei are also visible.

T-tubules & Triad

• T-tubules (Transverse tubules):

  • Structure: Invaginations of the sarcolemma that run transversely across the cell, between myofibrils, adjacent to the boundary between the A and I bands (A–I junction).

  • Membrane: The membrane of a T-tubule is an extension of the sarcolemma.

  • Contents: The contents of the T-tubule are tissue fluid, which contains both $Na^{+}$ and $Ca^{++}$.

• Triad:

  • Structure: An enlarged cistern (terminal cisternae) of the Sarcoplasmic Reticulum lies close to each side of the T-tubule.

  • Components: The T-tubule and the two cisternae of the SR are collectively called a triad.

Skeletal Muscle Tissue: Muscle Contraction

• Mechanism: Involves the sliding of thin filaments over and between thick filaments, moving towards the center of the sarcomere.

• Effect: Shortens the distance from Z-disc to Z-disc.

• Requirement: Depends on the interaction of actin and myosin.

Filaments for Contraction

• Myosin (Thick Filament Protein):

  • Shape: A protein commonly described as shaped like two golf clubs with their shafts twisted together to form a dimer.

  • Thick Filament Formation: Hundreds of these dimers combine side-to-side to create a thick filament.

  • Heads: The heads protrude from the thick filament.

  • ATP Binding: Each myosin head contains an ATP-binding site.

  • Enzymatic Activity: The myosin head functions as a myosin ATPase.

• Actin (Thin Filament Protein):

  • Monomers: G-actin (globular actin), a globular protein, is the product of a single gene.

  • Polymer: F-actin (filamentous actin) is a chain of G-actin units. The basis of a thin filament is a double helix of two F-actin filaments twisted around each other.

  • Myosin Binding Site: Each G-actin unit has a myosin binding site for the head of a myosin molecule.

Sliding of Filaments

• Cross Bridges: The sliding is produced by the actions of cross-bridges, which are myosin proteins that extend out (forming arms that terminate in heads) toward actin.

• Binding and Contraction: When the myosin head binds to the G-actin unit, this binding flexes the head of the myosin molecule, producing a power stroke and thus contraction.

Powerstroke Cycle

• Mechanism: Myosin motor protein exhibits ATPase activity and functions cyclically.

• Coupling: This cycle couples ATP binding and hydrolysis to a conformational change in the protein, known as the 'powerstroke cycle'.

Regulation of Contraction

Cross Bridge Attachment Regulation

• Proteins: Regulated by two key proteins: tropomyosin and troponin.

  • Tropomyosin: Lies within the groove between the double row of G-actin units.

  • Troponin: Attached to tropomyosin and serves as a crucial switch for muscle contraction and relaxation.

• Relaxed Muscle State: In a relaxed muscle, tropomyosin blocks the myosin binding sites on the actin filaments, preventing cross-bridge attachment.

Excitation-Contraction Coupling

1. End-Plate Potential (EPP):

   • $Na^{+}$ diffusion into the muscle fiber produces an end-plate potential, leading to depolarization.

   • Definition of EPP: The voltages that cause depolarization of skeletal muscle fibers, induced by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction.

2. Action Potential (AP) Generation:

   • If depolarization is sufficient, a threshold is reached, leading to the generation of Action Potentials (APs).

3. AP Propagation:

   • APs travel down the sarcolemma and into the T-tubules.

4. T-Tubule Depolarization and DHPR Activation:

   • The APs depolarize the T-tubule membrane, which opens Dihydropyridine Receptors (DHPRs). These are voltage-gated cation channels located on the T-tubule membrane.

5. Ion Influx and Further DHPR Activation:

   • $Na^{+}$ and $Ca^{2+}$ flow into the cell through the open DHPRs, which in turn opens more DHPRs.

6. RyR Activation and Calcium Release:

   • DHPRs are directly linked to Ryanodine Receptors (RyRs), which are calcium release channels located in the membrane of the SR terminal cisternae.

   • Activation of DHPRs opens RyRs, leading to the release of $Ca^{2+}$ from the SR into the sarcoplasm.

7. Troponin-Tropomyosin Complex Change:

   • $Ca^{2+}$ attaches to troponin.

   • This binding causes a conformational change in the tropomyosin-troponin complex.

8. Cross-Bridge Attachment:

   • The change in the troponin-tropomyosin complex moves tropomyosin away from the myosin binding sites on actin, allowing cross-bridges (myosin heads) to attach to actin.

Muscle Relaxation

1. Acetylcholine (ACh) Degradation: Acetylcholinesterase (ACh-esterase) degrades ACh in the synaptic cleft.

2. RyR Closure: $Ca^{2+}$ release channels (RyRs) close.

3. Calcium Reuptake: $Ca^{2+}$ is actively pumped back into the SR through $Ca^{2+}\text{-ATPase}$ pumps.

4. Choline Recycling: Choline is recycled to synthesize more ACh.

5. Troponin-Tropomyosin Reset: As $Ca^{2+}$ concentration decreases, troponin returns to its original conformation, causing tropomyosin to block the myosin-binding sites on actin again, leading to muscle relaxation.

Sources of Energy

• ATP from Mitochondria: ATP is primarily supplied by mitochondria, which are abundant around the myofibrils and beneath the sarcolemma.

• Phosphocreatine Reserve: A small reserve of phosphocreatine rapidly rephosphorylates ADP to ATP for several seconds (e.g., long enough to fuel a $100 \, m$ dash).

• Glycolysis: Some ATP is supplied by glycolysis in the cytosol, but lactic acid buildup limits this source to about a minute or so.

Short Account of Contraction

Acetylcholine (ACh) $\rightarrow$ (+) Acetylcholine-gated sodium channels $\rightarrow$ (+) $Na^{+}/K^{+}$ ATPase $\rightarrow$ depolarize cell by $Na^{+}$ (from $-70 \, mV$ to $+30 \, mV$) $\rightarrow$ Action Potentials (APs) $\rightarrow$ APs travel down sarcolemma and T-tubules $\rightarrow$ depolarize T-tubule membrane $\rightarrow$ open Dihydropyridine Receptors (DHPRs) (voltage-gated cation channels on T-tubule membrane) $\rightarrow$ $Na^{+}$ & $Ca^{2+}$ flow in $\rightarrow$ open more DHPRs $\rightarrow$ open Ryanodine Receptors (RyRs) (directly linked to DHPRs) in the SR terminal cisternae membrane $\rightarrow$ $Ca^{2+}$-release into cytosol $\rightarrow$ $Ca^{2+}$ attaches to troponin $\rightarrow$ troponin changes conformation $\rightarrow$ moves tropomyosin to unmask myosin binding sites on actin $\rightarrow$ Muscle Contracts!

Short Account of Relaxation

Cholinesterase $\rightarrow$ destroys acetylcholine $\rightarrow$ (-) Acetylcholine-gated sodium channels $\rightarrow$ (-) (Closes) $Na^{+}/K^{+}$ ATPase, pumps out $Na^{+}$ ($\downarrow$) $\rightarrow$ Cell repolarizes $\rightarrow$ (-) (closes) dihydropyridine receptors $\rightarrow$ $Ca^{2+}$ pump pumps $Ca^{++}$ into sarcoplasmic reticulum $\rightarrow$ troponin changes conformation $\rightarrow$ troponin positions tropomyosin over myosin-binding sites on actin $\rightarrow$ Muscle Relaxes!

Summary

• Skeletal muscle fibers are very long, multinucleated cells.

• Perimysium, containing arterioles, surrounds fascicles.

• Sarcoplasmic reticulum surrounds myofibrils inside each fiber.

• Myofibrils are long arrays of sarcomeres.

• Acetylcholine from the motor end plate allows $Na^{+}$ into the muscle fiber.

• $Na^{+}$ influx opens dihydropyridine receptors and depolarizes the fiber.

• Dihydropyridine receptors open ryanodine receptors.

• Ryanodine receptors release $Ca^{++}$ into the cytosol to interact with troponin and initiate contraction.

• Cytosolic $Ca^{++}$ concentrations critically control muscle contraction and relaxation.

• $Ca^{++}$ concentrations in the cytosol are regulated by the Sarcoplasmic Reticulum (SR).

• ATP enables myosin heads to move along F-actin filaments, facilitating contraction.