Physiology: Circulation, Respiration, and Renal

Course Introduction and Contact Information

• Instructor: Dr. Robert de Bruijn

• Email: $Robert.deBruijn@tufts.edu$

• Office Hours: Via Zoom, by appointment through email.

• Students are encouraged to contact the instructor anytime for assistance.

Overview of Today's Topics

• Action Potentials

• Central Nervous System (CNS) / Peripheral Nervous System (PNS)

• Muscle Physiology

Muscle Structure

• Muscles are attached to bones via tendons.

• A muscle is organized into fascicles (muscle bundles).

• Each fascicle contains multiple muscle fibers (muscle cells).

• Within each muscle fiber are numerous myofibrils.

• Myofibrils are composed of repeating functional units called sarcomeres.

• Sarcomeres are the basic contractile units of muscle.

• Sarcomeres are delimited by Z lines (or Z-discs).

• Within a sarcomere, actin (thin filaments) and myosin (thick filaments) interact.

• Key regions of the sarcomere include:

  • A-band: Contains the entire length of the thick filaments (myosin), overlapping with thin filaments.

  • I-band: Contains only thin filaments (actin) and extends across two adjacent sarcomeres, bisected by the Z-line.

  • H-zone: Central part of the A-band, containing only thick filaments (myosin) in a relaxed muscle, without overlapping thin filaments.

Thin Filament Proteins: Troponin and Tropomyosin

Tropomyosin

• Structure: A rod-shaped molecule formed by two intertwined polypeptides.

• Length: Approximately equal to that of $7$ actin monomers.

• Arrangement: Chains of tropomyosin molecules are arranged end-to-end along the actin thin filament.

• Function: In a relaxed muscle, tropomyosin partially covers the myosin-binding site on each actin monomer, thereby preventing the cross-bridges (from myosin) from making contact with actin.

Troponin

• Structure: A smaller globular protein, composed of three subunits:

  • Troponin I (inhibitory): Binds to actin, preventing myosin binding.

  • Troponin T (tropomyosin-binding): Binds to tropomyosin.

  • Troponin C (Ca$\text{2+}$-binding): Binds calcium ions.

• Arrangement: One molecule of troponin binds to each molecule of tropomyosin.

• Function: Holds tropomyosin in its blocking position over the myosin-binding sites on the $7$ actin monomers in contact with that tropomyosin.

Role of Cytosolic Calcium (Ca$\text{2+}$)

• Low Cytosolic Ca$\text{2+}$ (Relaxed Muscle):

  • Troponin and tropomyosin cooperatively block the interaction of myosin cross-bridges with the thin filament.

  • Myosin-binding sites on actin are covered.

• High Cytosolic Ca$\text{2+}$ (Activated Muscle):

  • Ca$\text{2+}$ binds to Troponin C.

  • This binding induces a conformational change in troponin.

  • This change relaxes troponin's inhibitory grip, causing it to pull tropomyosin away from the myosin-binding sites on each actin molecule.

  • Myosin-binding sites on actin are now exposed, allowing for cross-bridge formation and muscle contraction.

• Removal of Ca$\text{2+}$: Reverses the process, leading to the restoration of tropomyosin's blocking action and muscle relaxation.

Mechanism of Cytosolic Calcium (Ca$\text{2+}$) Increase

• Sarcolemma: The muscle cell plasma membrane.

• T-tubules (Transverse tubules):

  • Invaginations of the sarcolemma that extend deep into the muscle fiber.

  • They run perpendicular to the myofibrils.

  • Their primary role is to rapidly propagate the action potential from the sarcolemma into the interior of the muscle fiber.

• Sarcoplasmic Reticulum (SR):

  • A specialized endoplasmic reticulum within muscle cells that stores and releases Ca$\text{2+}$.

  • Terminal Cisternae: Enlarged regions of the SR that are in close proximity to the T-tubules.

• Junction between T-tubules and SR: Involves two integral membrane proteins:

  • Dihydropyridine (DHP) receptor: Located in the T-tubule membrane.

    • It is a modified voltage-sensitive Ca$\text{2+}$ channel, but primarily acts as a voltage sensor in skeletal muscle.

  • Ryanodine receptor (RyR): Embedded in the sarcoplasmic reticulum membrane.

    • It forms a Ca$\text{2+}$ channel.

• Process of Ca$\text{2+}$ Release:

  • During a T-tubule action potential, charged amino acid residues within the DHP receptor protein undergo a conformational change due to voltage changes.

  • This conformational change in the DHP receptor mechanically pulls open the adjacent ryanodine receptor channel.

  • Ca$\text{2+}$ is then released from the terminal cisternae of the sarcoplasmic reticulum directly into the cytosol of the muscle fiber.

Sliding-Filament Mechanism of Muscle Contraction

• Basic Principle: When a skeletal muscle fiber shortens, the overlapping thick (myosin) and thin (actin) filaments in each sarcomere move past each other.

  • This movement is propelled by the cyclic movements of the myosin cross-bridges.

• Key Insight: There is no change in the lengths of either the thick or thin filaments during shortening.

  • Muscle shortening is due to an increase in the amount of overlap between the thick and thin filaments.

Contracted State / Power Stroke

• Myosin head pulls actin: The myosin head binds to actin and pivots, pulling the actin thin filament toward the center of the sarcomere (M-line).

• Filaments slide: The thin filaments slide past the thick filaments.

• Shortening: As sarcomeres shorten, so do the myofibrils and the entire muscle fiber.

After Power Stroke

• Detachment: Myosin detaches from the active site on actin.

• Re-cocking: Myosin head rotates back to its original, energized position, preparing for the next stroke.

• Re-attachment: Myosin attaches to another active site farther down the actin filament.

Termination of Contraction

• The sliding process continues until:

  • The Z-disk reaches the myosin filaments (i.e., maximum shortening).

  • The action potential (AP) stops, and Ca$\text{2+}$ is actively pumped back into the sarcoplasmic reticulum (SR).

Visualizing States (Based on diagrams)

• Relaxed Muscle:

  • Wider I-band (region with only thin filaments).

  • Visible H-zone (region with only thick filaments in the center of the A-band).

• Contracted Muscle: (Implied, but not fully detailed in transcript's diagram caption)

  • The I-band shortens.

  • The H-zone shortens or disappears as actin filaments move into the center of the A-band.

  • The A-band length remains unchanged.

Excitation-Contraction Coupling: Detailed Event Sequence

$TABLE 9.2$: Sequence of Events Between a Motor Neuron Action Potential and Skeletal Muscle Fiber Contraction

1. An action potential is initiated in a motor neuron and propagates to its axon terminals.

2. Ca$\text{2+}$ enters the axon terminals through voltage-gated Ca$\text{2+}$ channels.

3. Ca$\text{2+}$ entry triggers the release of acetylcholine (ACh) from the axon terminals into the synaptic cleft.

4. ACh diffuses from the axon terminals to the motor end plate in the muscle fiber.

5. ACh binds to nicotinic receptors on the motor end plate, increasing their permeability to Na$\text{+}$ and K$\text{+}$.

6. More Na$\text{+}$ moves into the muscle fiber at the motor end plate than K$\text{+}$ moves out. This net influx of positive charge depolarizes the membrane and produces an end-plate potential (EPP).

7. Local currents from the EPP depolarize the adjacent muscle cell plasma membrane to its threshold potential, generating an action potential that propagates over the muscle fiber surface and into the fiber along the T-tubules.

8. The action potential in the T-tubules induces DHP receptors to pull open ryanodine receptor channels, allowing the release of Ca$\text{2+}$ from the terminal cisternae of the sarcoplasmic reticulum into the cytosol.

9. Ca$\text{2+}$ binds to troponin on the thin filaments, causing tropomyosin to move away from its blocking position, thereby uncovering cross-bridge binding sites on actin.

10. Energized myosin cross-bridges on the thick filaments bind to actin. The chemical state is represented as:
    $A + M \cdot ADP \cdot Pi \rightarrow A \cdot M \cdot ADP \cdot Pi$

11. Cross-bridge binding triggers the release of ATP hydrolysis products (ADP and Pi) from myosin, producing an angular movement of each cross-bridge (the power stroke).
    $A \cdot M \cdot ADP \cdot Pi \rightarrow A \cdot M + ADP + Pi$

12. ATP binds to myosin, breaking the linkage between actin and myosin and thereby allowing cross-bridges to dissociate from actin.
    $A \cdot M + ATP \rightarrow A + M \cdot ATP$

13. ATP bound to myosin is split (hydrolyzed) by myosin ATPase, energizing the myosin cross-bridge and re-cocking it for the next cycle.
    $A + M \cdot ATP \rightarrow A + M \cdot ADP \cdot Pi$

14. Cross-bridges repeat steps $10$ to $13$, producing continuous movement (sliding) of thin filaments past thick filaments. Cycles of cross-bridge movement continue as long as Ca$\text{2+}$ remains bound to troponin.

15. Cytosolic Ca$\text{2+}$ concentration decreases as Ca$\text{2+}$-ATPase (calcium pump) actively transports Ca$\text{2+}$ back into the sarcoplasmic reticulum.

16. Removal of Ca$\text{2+}$ from troponin restores the blocking action of tropomyosin, the cross-bridge cycle ceases, and the muscle fiber relaxes.

Myosin Filament Structure (from diagram)

• A G-actin molecule has:

  • Myosin binding sites.

  • ATP binding site.