Lecture 30: Excitable Tissue Flashcards

Excitable Tissue: Muscle

Introduction

• Carolyn Barrett (c.barrett@auckland.ac.nz) -- Dept of Physiology, Faculty of Medical and Health Sciences.
• Runs the "Circulatory Control Lab" focusing on blood pressure control in everyday life and cardiovascular disease, particularly preeclampsia.
• This lecture series covers skeletal, cardiac, and smooth muscle.

Learning Objectives

• Describe the structure and organization of skeletal muscle.
• Describe the structure of thick and thin filaments in skeletal muscle.
• Describe the events of the cross-bridge contraction cycle.
• Describe the role of calcium in muscle contraction.
• Describe the length-tension relationship in skeletal muscle.

Types of Muscle

1. Skeletal Muscle
   • Attached to bones, responsible for movement.
   • Striated (highly ordered contractile system).
   • Voluntary (under somatic nervous system control).
   • Long cylindrical cells with multiple peripheral nuclei.
2. Cardiac Muscle
   • Forms the bulk of the heart, ejects blood upon contraction.
   • Striated.
   • Involuntary (under autonomic nervous system control).
   • Branched cells with 1-3 central nuclei.
   • Connected via intercalated discs.
   • Located only in the heart
3. Smooth Muscle
   • Lines hollow organs and blood vessels, regulates dimensions.
   • Not striated.
   • Involuntary (under autonomic nervous system control).
   • Spindle-shaped, uninucleated cells.
   • Found in the wall of internal organs (gut, blood vessels etc).

General Organization of Muscle

• Muscle cells are interconnected either by direct cellular contacts (common in cardiac and smooth muscle) or innervation by the same neuron (skeletal and most smooth muscles).
• Interconnections coordinate activity to provide useful contractile activity.
• A motor unit is a group of muscle cells innervated by a single neuron.
• Actin and myosin are the major proteins responsible for contraction in all three muscle types.

Structure of Skeletal Muscle

• Most highly ordered muscle type.
• Cells are long (up to several cm) and relatively wide (about 0.1 mm), referred to as muscle fibers.
• Attached to bones via tendons.
• Contractile apparatus is organized into myofibrils running the length of the cell.
• Myofibrils consist of alternating bands of actin and myosin filaments.
• Sarcoplasmic reticulum (SR) surrounds the fibril and stores calcium to activate contraction.

Sarcomere

• The basic contractile element.
• Consists of thick filaments (myosin) interdigitating with thin filaments (actin), attached to z-disks at each end.
• Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
• T-tubules invaginate the sarcolemma (surface membrane) at A-I band junctions.
• Sarcomeres are surrounded by SR with terminal cisterns closely apposed to T-tubules.

Myofibril

• Contains highly organised contractile filaments.

Bands and Zones:

• I band: contains thin filaments; run the length of the I band and partway into the A band
• A band: contains thick filaments; run the entire length of an A band
• H zone: lighter mid-region where filaments do not overlap
• M line: line of protein myomesin that holds adjacent thick filaments together

Sarcoplasmic Reticulum (SR) and T-tubules

• T-tubules
  • Deep invaginations continuous with the sarcolemma (cell membrane).
  • Circle each sarcomere at A-I band junctions.
  • Carry action potentials deep within the muscle cell.
• Sarcoplasmic reticulum (SR)
  • The calcium storage site.
  • Terminal cisternae lie close to the T-tubules.

Thin Actin Filaments

• Composed of globular actin proteins.
• At either end of the sarcomere, the thin filaments are attached to the Z line.
• The protein actinin interconnects the thin filaments at the Z line.
• Consist of F-actin, nebulin, tropomyosin, and troponin.
• Accessory proteins (troponin and tropomyosin) regulate activity.

Thick Myosin Filaments

• Composed of Myosin
• High molecular weight protein formed from two subunits with a tail (double helix) and a globular head. The heads have a binding site for actin
• The myosin molecules can also form filaments with the myosin molecules polarised along the filament.
• Heads hydrolyze ATP.
• Light chains may be bound near the globular region to regulate ATP hydrolysis.
• Approximately 300 myosin molecules per thick filament.
• Titin anchors the thick filament to the Z-line

Sliding Filament Model of Contraction

• Contraction is the activation of myosin's cross bridges.
• Thin filaments are pulled over thick filaments, shortening the sarcomere.
• Z-discs are pulled toward the M-line.
• I band and H zone become narrower.
• A bands do not change in length.

Chemical Basis of Contraction

• Myosin heads are ATPases (enzymes that hydrolyze ATP to ADP and Pi).
• ATP or hydrolyzed ATP binding determines the conformation of the myosin head.
• Actin binding promotes ATP hydrolysis, leading to a new conformation.
• Relative motion occurs between actin and myosin filaments.
• If motion is prevented, force is produced between filaments.
• Rebinding 'fresh' ATP allows detachment and return to the original conformation.

Cross-Bridge Cycle

• Myosin head attaches, swings, detaches, and returns to the original conformation, consuming one ATP per cycle per head.
• Myosin forms cross bridges between actin and myosin filaments in the attached state.
• Allows myosin to move along the thin filament, producing relative motion.

Steps:

1. Energization: ATP binding and hydrolysis to form myosin-ADP-Pi complex (high affinity for actin).
2. Crossbridge formation: Myosin-ADP-Pi binds to actin in a 90° orientation.
3. Power stroke: ADP-Pi release minimizes free energy, orienting the complex at about 45°.
4. Detachment: ATP binding reduces myosin's affinity for actin, leading to detachment. No ATP results in rigor (rigor mortis).

Regulation of Contraction by Calcium

• Calcium ions initiate muscle activation.
• Calcium ions interact with troponin (actin-regulated muscles) or calmodulin (myosin-regulated muscles).
• Muscle is typically relaxed when calcium levels are < 0.0001 mM and activated at 0.001 – 0.01 mM.
• Calcium transients are produced by calcium release from outside the cell and/or from internal stores (sarcoplasmic reticulum).
• Opening calcium ion channels increases myoplasmic calcium levels.

Calcium Removal

• Active process linked to ATP hydrolysis by ion pumps in the surface membrane.
• Relaxation occurs as calcium influx channels close and pumps return calcium to stores and/or extracellular space.
• In skeletal muscle opening of calcium channels in the SR allows the movement of calcium ions into the cytosol.
• Active transport pumps (Ca^{2+} ATPase) are constantly moving Ca^{2+} from the cytoplasm back into the sarcoplasma reticulum

Importance of Calcium

• Sydney Ringer discovered the importance of calcium in muscle function in 1883.
• Calcium ions provide the “on” switch for cross-bridge cycle to begin.
• When the calcium binds with troponin the tropomyosin moves to expose the myosin binding sites on actin
• The cross-bridge cycle will continue as long as calcium levels remain above the critical threshold (0.001-0.01 mM)

Length-Tension Relationship

• The amount of overlap between thick and thin filaments determines the number of attached cross bridges when the muscle develops isometric force.
• Maximal under normal conditions at a sarcomere length of 2.0 – 2.2 µm (2.0 – 2.2 x 10-6 m).
• Joint range of motion usually limits sarcomere lengths, but injuries may increase this range.
  • At lengths < 2.0 µm, active force development is reduced because the ends of filaments collide and start to interfere with each other;
  • At lengths >2.2 µm, passive force increases as elastic connective tissue around the muscle cells is stretched (the cytoskeleton also contributes slightly to this effect). The active force declines as the extent of overlap between the filaments reduces, which reduces the number of possible cross bridge interactions along the sarcomere;
  • Maximal force is developed at between 2.0–2.2 µm, the normal working range of the muscle.

Cross-Bridge Cycle Steps

1. Cross-bridge formation
   • Myosin binds to the actin binding site to form a cross-bridge
   • Note: cross-bridges can only occur in the presence of calcium when the myosin binding site on actin is exposed.
2. Power stroke
   • ADP is released
   • The myosin head rotates to its low energy state (about 45° to the actin) pulling with it the thin filament
   • The result is shortening of the sarcomere.
3. Detachment
   • A new ATP molecule binds to the myosin
   • The actin-myosin bind is weakened and the myosin detaches
   • (Note: No ATP = no detachment)
4. Energization of the myosin head
   • Myosin head hydrolyzes the ATP to ADP + Pi
   • The myosin head moves back to its “high energy (cocked)” confirmation (about 90° to the actin)

Isometric vs isotonic

• Isometric
  • No shortening
  • Length constant
  • Tension variable
• Isotonic
  • Shortening
  • Tension constant
  • Velocity variable