Skeletal Muscle Tissue Notes

Skeletal Muscle Tissue

Overview of Muscle Tissue

General Characteristics of Skeletal Muscle Tissue

Formation of Muscle Fibers:
- Each individual skeletal muscle fiber is a large, single cell formed by the fusion of $10$ to $100$ (hundreds) embryonic muscle cells called myoblasts.
- This fusion results in a multinucleated fiber, typically possessing $10$ to $100$ nuclei.
- Multiple nuclei are crucial as they provide multiple copies of genes, enabling the production of large quantities of proteins and enzymes necessary for muscle contraction.
Dimensions:
- A skeletal muscle fiber can range from $10$ to $100 ext{ µm}$ in width.
- Its length can vary from $1$ to $300 ext{ mm}$ , with an average length of $100 ext{ mm}$ .

Structure: "Bundles Within Bundles Within Bundles"

Endomysium:
- Encases each individual skeletal muscle fiber (or bundle of myofibrils).
- It is a connective tissue composed of multiple layers:
  - Inner layer: primarily laminin.
  - Middle layer: collagen IV.
  - Outer layer: type I and type III collagen.
- Crucial Role of Collagen: 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.
Perimysium:
- Surrounds a bundle of muscle fibers, which forms a fascicle.
- It is a thin layer of areolar connective tissue.
Epimysium:
- Encloses groups of fascicles, forming individual muscles.
- It is a layer of dense connective tissue.
- Some skeletal muscles can be divided into entire skeletal muscles and heads of skeletal muscles, both of which are surrounded by epimysium.
Fascia:
- A tough layer of fibrous connective tissue that wraps each individual muscle.
- It lies outside the epimysium, surrounding and separating the muscles.
- It is continuous with tendons and the periosteum of bones.

Muscle Fibers

Basic Units and Sub-units

Muscle fibers are the basic unit of the muscle itself.
They are composed of smaller sub-units called myofibrils.
- Myofibrils are $1$ to $3 ext{ µm}$ in diameter and can be as long as the muscle fiber.
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 within a skeletal muscle fiber.
- Myofibrils are separated from each other by the SR.
- It is a specialized smooth endoplasmic reticulum (SER).
Distinctive Features:
- Presence of a calcium-activated ATPase.
- Presence of gated calcium channels.
Function:
- Calcium-activated ATPase: Concentrates calcium ions ( $ext{Ca}^{2+}$ ) within the saccules of the SR.
- Calcium channels: Release $ext{Ca}^{2+}$ to initiate muscle contraction.
- The SR stores, releases, and retrieves $ext{Ca}^{2+}$ ions.

Sarcomere

Organization:
- A highly organized arrangement of contractile myofilaments:
  - Actin (thin filament).
  - Myosin (thick filament).
- It also contains other support proteins.
Dimensions and Borders:
- Each sarcomere is approximately $2 ext{ µm}$ in length.
- It has a three-dimensional, cylinder-like arrangement.
- Bordered by structures called Z-discs.
Myofilaments:
- Thin Filaments (Actin):
  - Composed of actin and its troponin-tropomyosin complex.
  - Project from the Z-discs toward the center of the sarcomere.
  - Thinner than myosin filaments.
  - Are isotropic (transparent to polarized light).
  - Actin filament dimensions: $0.007 ext{ µm}$ across and $1 ext{ µm}$ long.
  - G-actin: A globular protein, the product of a single gene.
  - F-actin: 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.
  - Each G-actin unit has a myosin binding site for the head of a myosin molecule.
- Thick Filaments (Myosin):
  - Composed of myosin strands and their multiple heads.
  - Project from the center of the sarcomere toward (but not all the way to) the Z-discs.
  - Have more mass and are thicker than thin filaments.
  - Are anisotropic (opaque to polarized light).
  - Myosin filament dimensions: $0.015 ext{ µm}$ thick and $1.5 ext{ µm}$ long.
  - Myosin protein is shaped like two golf clubs with their shafts twisted together to form a dimer.
  - Hundreds of these dimers combine side-to-side to form a thick filament, with the heads sticking out.
Sarcomere Bands and Lines:
- Z-disc (Z-line):
  - A plate of $ext{alpha-actinin}$ that anchors the thin filaments.
  - At extremely high magnification ( $ext{X} ext{ } 150,000$ ), it appears as a puckered disk composed of a fine network of $ext{alpha-actinin}$ fibers.
  - Thin filaments are anchored by one end to each side of a Z-disc, with the other end being free.
  - Each thick filament is attached to Z-discs by a filament of titin.
    - The elasticity of titin is responsible for the passive rebound of a stretched muscle.
  - The Z-lines of one myofibril are connected to the Z-lines of neighboring myofibrils by $11 ext{ nm}$ intermediate filaments of desmin, a protein peculiar to muscle cells.
  - Z-lines nearest to the cell membrane are anchored to it by protein complexes called costameres.
    - The $ext{alpha-actinin}$ plate is connected to a membrane protein called integrin by a protein called vinculin. Defective vinculin is lethal in the first trimester.
- I bands:
  - Sections of the myofibril that consist only of thin filaments.
  - Appear pale under a light microscope.
  - The Z-disc splits the I-band into halves.
  - The resting length of an I-band is $0.8 ext{ µm}$ , with each half being $0.4 ext{ µm}$ long.
  - Decrease in length during contraction.
- A bands:
  - Sections of the myofibril that contain thick filaments.
  - Appear dark ( $1.5 ext{ µm}$ wide) under a light microscope.
  - Do not shorten during contraction.
- H band:
  - The portion of the A band that contains only thick filaments.
  - Usually $0.3 ext{ µm}$ wide.
  - Can sometimes be seen at the highest magnification of a light microscope as a slightly paler line in the middle of the A-band.
  - Shortens during contraction.
- M, or middle, band:
  - A thin dark line in the middle of the H (or A) band.
  - Contains fine transverse filaments that link the thick filaments together to maintain their alignment (anchor myosin during contraction).
Definition of a Sarcomere:
- The part of a myofibril between two Z lines.
- Includes two half I bands and one A band.
- A myofibril consists of many sarcomeres arranged end-to-end.

Muscular Dystrophy

Costamere: Sub-membranous, Z-line associated structures in striated muscle.
Dystrophin: A filamentous protein that 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: Appears to bond directly to collagen IV in the basal lamina.
Clinical Relevance: Defective dystrophin or dystroglycan causes a slow deterioration of muscle, leading to death before age $30$ . Muscular dystrophy caused by these defects is rare.

Morphology under the Microscope

The overlapping structure of thin and thick filaments, held together by chemical cross bridges, gives skeletal muscle a banded or striped appearance under the microscope.
These alternating light (I) and dark (A) bands are called striations, which give skeletal muscle its alternate and more accurate name: voluntary, striated muscle.
Multiple peripheral nuclei can also be observed in muscle fibers.

T-tubules & Triad

T-tubules (Transverse Tubules):
- Invaginations of the sarcolemma (muscle cell membrane).
- Run transversely across the cell between myofibrils, specifically next to the boundary between the A and I bands (A–I junction).
- The membrane of a T-tubule is an extension of the sarcolemma.
- The contents of the T-tubule are tissue fluid, which contains both $ext{Na}^+$ and $ext{Ca}^{2+}$ .
Triad:
- An enlarged cistern (terminal cisterna) of the SR lies close to each side of the T-tubule.
- The T-tubule along with the two adjacent cisternae of the SR constitute a triad.

Muscle Contraction

Mechanism of Contraction

Sliding Filament Model: Muscle contraction involves the sliding of thin (actin) filaments over and between thick (myosin) filaments towards the center of the sarcomere.
This action shortens the distance from one Z-disc to the next Z-disc.
Muscle contraction is dependent on the interaction of actin and myosin.
Cross Bridges: Myosin proteins extend out from the thick filament, forming arms that terminate in heads. These heads act as cross bridges, interacting with actin.
- Each myosin head contains an ATP-binding site and functions as a myosin ATPase.
- When a myosin head binds to the myosin binding site on a G-actin unit, it flexes, producing a contraction (the "powerstroke cycle").
- The powerstroke cycle involves the myosin motor protein's ATPase activity, coupling ATP binding and hydrolysis to a conformational change in the protein.

Regulation of Contraction

Regulation of Cross Bridge Attachment:
- Regulated by tropomyosin and troponin.
- Tropomyosin: Lies within the groove between the double row of G-actin units.
- Troponin: Attached to tropomyosin.
- Together, they act as a switch for muscle contraction and relaxation.
- In a relaxed muscle, tropomyosin blocks the myosin binding sites on actin, preventing cross bridge attachment.
Excitation-Contraction Coupling: The process linking electrical excitation to muscle contraction.
1. End-plate potential (EPP): Na+ diffusion into the muscle fiber at the neuromuscular junction produces depolarization, forming EPPs.
  - EPPs are voltages that cause depolarization of skeletal muscle fibers, triggered by neurotransmitters binding to the postsynaptic membrane.
2. Action Potentials (APs): If depolarization is sufficient to reach threshold, action potentials are generated.
3. AP Propagation: APs travel down the sarcolemma and into the T-tubules.
4. T-tubule Depolarization: Depolarization of the T-tubule membrane opens Dihydropyridine Receptors (DHPRs).
  - DHPRs are voltage-gated cation channels located on the T-tubule membrane.
  - Opening of DHPRs allows $ext{Na}^+$ and $ext{Ca}^{2+}$ to flow into the cell.
  - This flow opens more DHPRs.
5. Calcium Release from SR: DHPRs are directly linked to Ryanodine Receptors (RyRs) located in the membrane of the SR terminal cisternae.
  - Opening of DHPRs triggers the opening of RyRs, leading to a massive release of $ext{Ca}^{2+}$ from the SR into the cytosol.
  - Summary of initial events: Acetylcholine (ACh) release $\rightarrow$ activates ACh-gated sodium channels $\rightarrow$ activates skeletal muscle voltage-gated sodium channels $\rightarrow$ depolarizes cell (from $-70 ext{ mV}$ to $+30 ext{ mV}$ ) $\rightarrow$ APs $\rightarrow$ travel down sarcolemma and T-tubules $\rightarrow$ depolarize T-tubule membrane $\rightarrow$ open DHPRs (voltage-gated cation channels) letting $ext{Na}^+$ and $ext{Ca}^{2+}$ in $\rightarrow$ open RyRs (directly linked to DHPRs) in SR $\rightarrow$ $ext{Ca}^{2+}$ release into cytosol.
6. Actin-Myosin Interaction:
  - $ext{Ca}^{2+}$ attaches to troponin.
  - This attachment causes a configuration change in the tropomyosin-troponin complex.
  - This change moves tropomyosin away from the myosin binding sites on actin, unmasking them.
  - Cross bridges (myosin heads) can then attach to actin, initiating contraction.

Muscle Relaxation

ACh Degradation: Acetylcholinesterase degrades ACh in the synaptic cleft, ending the neural stimulus.
Closure of Ca2+ Channels: $ext{Ca}^{2+}$ release channels (RyRs) in the SR close.
Ca2+ Reuptake: $ext{Ca}^{2+}$ is actively pumped back into the SR through $ext{Ca}^{2+}$ -ATPase pumps.
- This decreases cytosolic $ext{Ca}^{2+}$ concentrations.
Troponin/Tropomyosin Reset: As cytosolic $ext{Ca}^{2+}$ levels drop, $ext{Ca}^{2+}$ detaches from troponin.
- Troponin changes conformation, allowing tropomyosin to cover the myosin-binding sites on actin once more.
Choline Recycling: Choline is recycled to synthesize more ACh.
Overall: The absence of $ext{Ca}^{2+}$ allows tropomyosin to block myosin binding sites, preventing cross bridge formation and leading to muscle relaxation.

Sources of Energy for Contraction

ATP: Supplied by mitochondria located around the myofibrils and under the sarcolemma.
Phosphocreatine: A small reserve of phosphocreatine rapidly rephosphorylates ADP to ATP for several seconds (e.g., sufficient for a $100 ext{ m}$ dash).
Glycolysis: Some ATP is supplied by glycolysis in the cytosol, but lactic acid buildup limits this energy source to about a minute.

Short Account of Contraction and Relaxation

Contraction

Acetylcholine (ACh) $\rightarrow$ Activates Acetylcholine-gated sodium channels $\rightarrow$ Activates $ext{Na}^+/ ext{K}^+$ ATPase $\rightarrow$ Depolarizes cell by $ext{Na}^+$ influx $\rightarrow$ Activates dihydropyridine receptors (DHPRs) in T-tubule membrane $\rightarrow$ Opens ryanodine receptors (RyRs) $\rightarrow$ Releases $ext{Ca}^{2+}$ into cytosol $\rightarrow$ $ext{Ca}^{2+}$ binds to troponin $\rightarrow$ Troponin undergoes conformational change $\rightarrow$ Leads to movement of tropomyosin $\rightarrow$ Unmasks myosin binding sites on actin $\rightarrow$ Muscle contracts!

Relaxation

Cholinesterase $\rightarrow$ Destroys acetylcholine $\rightarrow$ Closes Acetylcholine-gated sodium channels $\rightarrow$ Closes $ext{Na}^+/ ext{K}^+$ ATPase, pumps out $ext{Na}^+$ $\rightarrow$ Cell repolarizes $\rightarrow$ Closes dihydropyridine receptors $\rightarrow$ $ext{Ca}^{2+}$ pump ( $ext{Ca}^{2+}$ -ATPase) pumps $ext{Ca}^{2+}$ into sarcoplasmic reticulum $\rightarrow$ $ext{Ca}^{2+}$ detaches from troponin $\rightarrow$ Troponin changes conformation $\rightarrow$ Repositions tropomyosin over myosin-binding sites on actin $\rightarrow$ Muscle relaxes!

Summary of Key Points

Skeletal muscle fibers are very long, multinucleated cells.
Perimysium with arterioles surrounds fascicles.
Sarcoplasmic reticulum surrounds myofibrils inside the fiber.
Myofibrils are long arrays of sarcomeres.
Acetylcholine from the motor end plate allows $ext{Na}^+$ into the cell.
$ext{Na}^+$ influx opens dihydropyridine receptors (DHPRs) and depolarizes the fiber.
DHPRs open ryanodine receptors (RyRs).
RyRs release $ext{Ca}^{2+}$ into the cytosol, which binds to troponin and initiates contraction.
Cytosolic $ext{Ca}^{2+}$ concentrations control muscle contraction and relaxation.
Cytosolic $ext{Ca}^{2+}$ concentrations are primarily controlled by the SR.
ATP is essential, allowing myosin heads to move along F-actin, driving the contraction cycle.