Electron Microscopic Appearance of a Skeletal Muscle | Medical Science Optional Notes for UPSC PDF Download

Overview

• Muscle fibers are composed of myofibrils, which are further divided into individual filaments.

• Myofilaments within the muscle contain multiple proteins, collectively forming the contractile machinery of the skeletal muscle.

• The contractile mechanism in skeletal muscle is heavily reliant on proteins such as myosin-II, actin, tropomyosin, and troponin.

• Troponin is comprised of three subunits: troponin I, troponin T, and troponin C.

• Each myofibril consists of interdigitating thick and thin filaments arranged longitudinally within sarcomeres.

• Sarcomeres, the repeating units within myofibrils, contribute to the distinctive banding pattern observed in striated muscle.

• A sarcomere extends from the Z line to another Z line, defining the structural and functional unit of muscle contraction.

  

Thick Filaments

• Found in the A band at the center of the sarcomere and are composed of myosin.

• Myosin consists of six polypeptide chains, comprising one pair of heavy chains and two pairs of light chains.

• Each myosin molecule possesses two "heads" attached to a single "tail."

• The myosin heads play a crucial role in the process of muscle contraction, as they bind ATP and actin, participating in the formation of cross-bridges.

  
  Thick Filament

Thin Filaments

• Anchored at the Z lines and located in the I bands of the sarcomere.

• Interdigitate with the thick filaments within a portion of the A band.

• Composed of actin, tropomyosin, and troponin.

• Troponin acts as the regulatory protein, allowing cross-bridge formation when it binds to Ca²⁺.

• Troponin is a complex of three globular proteins:

  1. Troponin T ("T" for tropomyosin): Attaches the troponin complex to tropomyosin.
  2. Troponin I ("I" for inhibition): Inhibits the interaction of actin and myosin.
  3. Troponin C ("C" for Ca²⁺): The Ca²⁺-binding protein that, when bound to Ca²⁺, facilitates the interaction of actin and myosin.

T Tubules

• Form an extensive tubular network open to the extracellular space, facilitating the transmission of depolarization from the sarcolemmal membrane to the cell interior.

• Positioned at the junctions of A bands and I bands.

• Contain a voltage-sensitive protein known as the dihydropyridine receptor (DHPR).

• Depolarization induces a conformational change in the dihydropyridine receptor.

Sarcoplasmic Reticulum

• Internal tubular structure crucial for Ca²⁺ storage and release in excitation-contraction coupling.

• Features terminal cisternae in intimate contact with T tubules, forming a triad arrangement.

• Membrane includes the Ca²⁺-ATPase (Ca²⁺ pump) responsible for transporting Ca²⁺ from the intracellular fluid into the SR interior, maintaining low intracellular [Ca²⁺].

• Contains a Ca²⁺ release channel called the ryanodine receptor.

  

Latch Phenomenon in Smooth Muscle

• Dephosphorylation of myosin light chain kinase: Does not always result in smooth muscle relaxation.

• Latch Bridge Mechanism: Myosin cross-bridges stay attached to actin even after the cytoplasmic Ca²⁺ concentration decreases.

• Outcome: Sustained contraction with minimal energy expenditure, particularly crucial in vascular smooth muscle.

• Relaxation: Presumably happens when the Ca²⁺-calmodulin complex eventually dissociates.

  

There is no troponin; instead, Ca²⁺ regulates myosin on the thick filaments.

Nerve muscle physiology-Repeats

Muscle physiology
Q1: Differentiate between isometric and isotonic contraction in skeletal muscle. Give one example of each type. (2015)
Q2: What are modes of contraction in a skeletal muscle? Give suitable examples. Explain length-tension relationship in skeletal muscle. (2018)
Q3: Name the contractile proteins in skeletal muscle. What is the electron microscopic appearance of muscle? Write sequence of events in muscular contraction. (2011)
Q4: Compare with the help of a diagram the length-tension relationship for skeletal, cardiac and smooth muscles. What is the latch phenomenon in smooth muscle (2001)?