Muscle Ultrastructure and Bioelectricity

Myosin Heavy Chain Structure

• The rod-shaped tail domain of myosin heavy chains is formed by two α-helices that intertwine to form a coiled-coil structure. This coiled-coil structure is essential for the assembly of myosin filaments, which are responsible for muscle contraction.

Sliding Filament Theory Evidence

• The sliding filament theory of muscle contraction is supported by several observations:
  • The shortening of the sarcomere during muscle contraction is due to the sliding of the thin filaments (actin) relative to the thick filaments (myosin).
  • The I-band, which contains only thin filaments, decreases in length during contraction, while the A-band, which contains both thick and thin filaments, remains the same length.
  • The H-zone, which contains only thick filaments, also shortens during contraction.

Energy Storage in Myosin Head

• The myosin head stores energy in the form of a high-energy phosphate bond between ADP and Pi. The hydrolysis of ATP to ADP and Pi releases energy, which is used to power the conformational change in the myosin head and drive the sliding of thin filaments.

Tropomyosin Conformational Change

• Calcium ions (Ca2+) trigger the conformational change in tropomyosin during muscle contraction. When calcium binds to troponin, it causes a conformational change in the troponin-tropomyosin complex, which exposes the myosin binding sites on actin.

Initiation of Power Stroke

• The power stroke is initiated by the release of inorganic phosphate (Pi) from the myosin head. This release causes a conformational change in the myosin head that pulls the actin filament towards the M-line.

Actin-Myosin Cross-Bridge Interaction

• The binding of a new ATP molecule to the myosin head causes the detachment of the actin-myosin cross-bridge. This detachment allows the myosin head to re-cock and bind to a new actin binding site further down the filament.

Role of Calcium in Muscle Contraction

• Calcium plays a crucial role in muscle contraction by exposing the myosin binding sites on actin:
  • When a nerve impulse stimulates a muscle fiber, calcium is released from the sarcoplasmic reticulum.
  • The calcium ions bind to troponin, causing a conformational change in the troponin-tropomyosin complex.
  • This conformational change moves tropomyosin away from the myosin binding sites on actin, allowing myosin to bind to actin and initiate the power stroke.

Calcium Reabsorption Consequences

• The reabsorption of calcium back into the sarcoplasmic reticulum leads to muscle relaxation:
  • When calcium is pumped back into the sarcoplasmic reticulum, the concentration of calcium in the sarcoplasm decreases.
  • Troponin returns to its original conformation, and tropomyosin blocks the myosin binding sites on actin.
  • Myosin can no longer bind to actin, and muscle contraction ceases.

I-Band During Contraction

• The I-band, which is composed of only actin filaments, shortens during muscle contraction as the actin filaments slide towards the M-line.

A-Band During Contraction

• The A-band, which contains both actin and myosin filaments, remains the same length during contraction. This is because the A-band represents the entire length of the thick filaments.

ATP Role in Contraction

• ATP plays a crucial role in the ratchet mechanism of contraction by providing energy for the myosin head to detach from actin and re-cock for another power stroke.

Myosin-Actin Binding Initiation

• The binding of myosin to actin is initiated by the release of inorganic phosphate (Pi) from the myosin head. This release causes a conformational change in the myosin head that allows it to bind to actin.

Actin Filament Movement

• During the power stroke, the actin filament is dragged approximately 5-10 nanometers towards the M-line by the myosin head.

Events During Power Stroke

• The following events occur during the power stroke:
  • Myosin head binds to actin
  • Myosin head undergoes a conformational change
  • Actin filament is pulled towards the M-line
  • ADP is released from the myosin head

Myosin Binding Site in Resting Muscle

• In resting muscle, the myosin binding site on actin is blocked by tropomyosin.

ATP Molecules Per Power Stroke

• One ATP molecule is used per myosin head for each power stroke.

ATP-Myosin Complex

• When ATP replaces ADP in the myosin head, a complex called ATP-myosin is formed. This complex is essential for the detachment of the myosin head from actin and the re-cocking of the myosin head.

Acetylcholine Role at Neuromuscular Junction

• Acetylcholine is a neurotransmitter that is released from the motor neuron at the neuromuscular junction. Acetylcholine binds to receptors on the muscle fiber membrane, leading to depolarization of the muscle fiber and initiating the release of calcium from the sarcoplasmic reticulum.

Troponin Response to Calcium

• When calcium is released into the sarcoplasm, it binds to troponin. This binding causes a conformational change in the troponin-tropomyosin complex, which exposes the myosin binding sites on actin.

Depolarizing Muscle Relaxant Function

• Depolarizing muscle relaxants, such as succinylcholine, act as acetylcholine agonists. They bind to and activate acetylcholine receptors, leading to prolonged depolarization of the muscle fiber membrane and ultimately paralysis.

Calcium Reabsorption Effect

• Pumping Ca2+ back into the sarcoplasmic reticulum causes relaxation of the muscle. This is because the reduced Ca2+ concentration in the sarcoplasm leads to the removal of Ca2+ from troponin, allowing tropomyosin to block the myosin binding sites on actin.

T-Tubule Role

• T-tubules are invaginations of the sarcolemma that carry the nerve impulse deep into the muscle fiber. They facilitate the rapid transmission of the nerve impulse to the sarcoplasmic reticulum, triggering calcium release.

Myosin After ATP Hydrolysis

• After ATP is hydrolyzed to ADP and Pi, myosin undergoes a conformational change that causes its head to bind to actin, initiating the power stroke.

Non-Depolarizing Muscle Relaxant

• A non-depolarizing muscle relaxant, such as pancuronium, competes with acetylcholine for binding to its receptors on the muscle fiber membrane. This blocks the action of acetylcholine, preventing depolarization of the muscle fiber and leading to paralysis.

Sarcomere: Basic Repeating Unit

• The basic repeating unit of muscle is the sarcomere. It is the functional unit of muscle contraction and is responsible for generating force during muscle contraction.

Calcium Collection Structure

• The sarcoplasmic reticulum, a network of interconnected tubules that surround the myofibrils, serves as the primary calcium store and release site in muscle fibers.

Sarcomere Protein Filaments

• The two main protein filaments in a sarcomere are:
  • Thin filaments, primarily composed of actin, troponin, and tropomyosin
  • Thick filaments, primarily composed of myosin

Myofibril Band Nomenclature

• The light and dark bands in a myofibril are called:
  • I-band: Light band, contains only actin filaments
  • A-band: Dark band, contains both actin and myosin filaments

Transverse Tubules Function

• Transverse tubules (T-tubules) are invaginations of the sarcolemma that carry the nerve impulse deep into the muscle fiber. They facilitate the rapid transmission of the nerve impulse to the sarcoplasmic reticulum, triggering calcium release.

Sarcoplasm

• The cytoplasm of a muscle fiber is referred to as sarcoplasm.

I-Band Distinguishing Feature

• The I-band is characterized by the presence of only thin filaments (actin) and the absence of thick filaments (myosin).

Active Structures in Muscle Contraction

• The active structures in muscle contraction are the thick filaments (myosin) and the thin filaments (actin). These filaments interact to generate force and shorten the sarcomere.

Skeletal Muscle Striated Appearance

• The striated appearance of skeletal muscle under the light microscope is due to the alternating arrangement of the I-bands and A-bands in the myofibrils.

Thin Filament Proteins

• The thin filaments are composed of the following proteins:
  • Actin: Provides the binding sites for myosin heads
  • Troponin: Binds calcium and regulates the interaction between actin and myosin
  • Tropomyosin: Blocks the myosin binding sites on actin in the absence of calcium

Tropomyosin in Muscle Contraction

• Tropomyosin regulates the interaction between actin and myosin by blocking the myosin binding sites on actin in the absence of calcium. When calcium is released, it binds to troponin, causing a conformational change that moves tropomyosin away from the myosin binding site, allowing myosin to bind to actin.

Tropomyosin Movement Initiation

• The movement of tropomyosin from the myosin binding site on actin is initiated by the release of calcium from the sarcoplasmic reticulum and the subsequent binding of calcium to troponin.

Thick Filament Composition

• Thick filaments in muscle tissue are primarily composed of myosin. Each thick filament is made up of hundreds of myosin molecules.

Myosin Molecules in Thick Filament

• Typically, around 200-300 myosin polypeptides make up a single thick filament.

Myosin Head Components

• The head region of a myosin molecule consists of:
  • Two globular heads that bind to actin
  • A neck region that connects the heads to the tail

A-Band Association

• The A-band in muscle fibers is associated with the entire length of the thick filaments (myosin filaments).

Troponin Composition

• Troponin is composed of three subunits:
  • Troponin T: Binds to tropomyosin
  • Troponin I: Inhibits the interaction between actin and myosin
  • Troponin C: Binds calcium

Myosin Tail Structure

• The tails of myosin filaments intertwine to form a thick filament, thus forming the central structure of the A-band.

This quiz covers the intricate details of muscle ultrastructure, specifically focusing on the arrangement of myofibrils and filaments, the molecular structures of myosin and actin, and the sliding filament theory. You'll also explore the biochemical processes involved in muscle action. Perfect for students studying anatomy and physiology.