Untitled Flashcards Set

Here’s an improved one-sheet summary with clearer explanations while keeping it easy to memorize:

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# Muscle Physiology Cheat Sheet

## 1. Types of Muscle

| Feature | Skeletal Muscle | Cardiac Muscle | Smooth Muscle |

|----------|---------------|---------------|--------------|

| Striated? | Yes | Yes | No |

| Nuclei per Cell | Many | One | One |

| Sarcomeres? | Yes (organized structure) | Yes | No (randomly arranged) |

| Control | Voluntary (Somatic NS) | Involuntary (Autonomic NS) | Involuntary (Autonomic NS) |

| Speed of Contraction | Fastest | Intermediate | Slowest |

| Location | Attached to bones | Heart | Internal organs, blood vessels |

| Function | Movement, posture | Pump blood | Move substances (food, blood, etc.) |

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## 2. General Function & Structure

- Fundamental Muscle Function: Convert neural signals into mechanical force (movement).

- Why is skeletal muscle considered an organ?

- Made of muscle fibers (cells), connective tissue, blood vessels, and nerves, working together to contract and generate force.

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## 3. Muscle Fiber & Sarcomere Structure

- Myofiber = A single muscle cell containing multiple myofibrils.

- Myofibrils = Chains of sarcomeres, the contractile units of the muscle.

- Sarcomere Composition:

- Thick filaments (myosin) → Attach to actin to create movement.

- Thin filaments (actin) → Pulled toward the M-line for contraction.

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## 4. Sliding Filament Theory (How Muscles Shorten)

- Sarcomere contracts when myosin pulls actin filaments inward.

- What shortens?

- I-band & H-zone shrink.

- A-band stays the same (thick filaments don’t change length).

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## 5. Neuromuscular Junction (NMJ) & Excitation-Contraction Coupling

### Events at the NMJ (Muscle Activation)

1. Action potential arrives at the axon terminal.

2. Acetylcholine (ACh) is released into the synapse.

3. ACh binds to nicotinic receptors → Na+ influx → Depolarization.

4. Action potential spreads down the muscle fiber via T-tubules.

5. Ca²⁺ release from the sarcoplasmic reticulum (SR) → Contraction begins.

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## 6. Role of Calcium & ATP in Muscle Contraction

- Ca²⁺ binds to troponin → Moves tropomyosin, exposing myosin-binding sites on actin.

- Myosin binds to actin → Cross-bridge cycle begins.

- Relaxation:

- Ca²⁺ is pumped back into the SR (via SERCA pump) → Muscle relaxes.

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## 7. The Cross-Bridge Cycle & ATP's Role

1. Myosin binds to actin (Cross-bridge formation).

2. Power stroke: Myosin head pulls actin, releasing ADP + Pi.

3. ATP binds to myosin → Myosin detaches from actin.

4. ATP is hydrolyzed → Myosin head resets for the next cycle.

- ATP is essential: Without it, myosin remains attached (causing rigor mortis after death).

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## 8. Types of Muscle Contractions

| Type | Description | Example |

|----------|---------------|------------|

| Isometric | Muscle contracts but does not shorten | Holding a plank |

| Isotonic | Muscle contracts and shortens | Lifting a weight |

| Eccentric | Muscle lengthens under tension | Lowering a dumbbell |

| Concentric | Muscle shortens under tension | Raising a dumbbell |

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## 9. Muscle Force & Summation

| Concept | Description |

|------------|---------------|

| Length-Tension Relationship | Muscle force is strongest at mid-length where myosin and actin overlap optimally. |

| Force-Velocity Relationship | Higher force = Lower velocity (max force at zero velocity = isometric contraction). |

| Summation | More action potentials before relaxation → Increased force. |

| Tetanus | Unfused tetanus (partial relaxation) vs. Fused tetanus (continuous contraction). |

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## 10. Motor Units & Recruitment

- Motor Unit = 1 motor neuron + all the muscle fibers it controls.

- Smaller motor units (slow-twitch fibers) are recruited first, followed by larger ones as force demand increases (**size principle**).

| Motor Unit | Function |

|---------------|------------|

| Small | Precise control, fatigue-resistant (e.g., eye muscles) |

| Intermediate | Moderate force & fatigue resistance |

| Large | High force, fast fatigue (e.g., leg muscles for sprinting) |

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## 11. Muscle Fiber Types & Function

| Fiber Type | Speed & Function | Energy Source |

|---------------|----------------------|-------------------|

| Slow Oxidative (Type I) | Slow, high endurance | Aerobic respiration (many mitochondria, myoglobin) |

| Fast Oxidative (Type IIa) | Intermediate speed & endurance | Aerobic & anaerobic metabolism |

| Fast Glycolytic (Type IIx) | Fast, powerful, fatigues quickly | Glycolysis (low endurance, few mitochondria) |

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## 12. Smooth Muscle Differences

- No sarcomeres → Uses dense bodies for contraction.

- Innervated by varicosities (instead of NMJs).

- Single-Unit vs. Multi-Unit Smooth Muscle

- Single-unit: Connected by gap junctions → Cells contract together (e.g., intestines).

- Multi-unit: Cells contract independently (e.g., iris, piloerector muscles).

### Smooth Muscle Contraction vs. Skeletal Muscle

| Feature | Smooth Muscle | Skeletal Muscle |

|---------|--------------|----------------|

| Regulatory Proteins | Calmodulin & MLCK | Troponin & Tropomyosin |

| Calcium Source | Extracellular & SR | Mainly SR |

| Control | Autonomic (involuntary) | Somatic (voluntary) |

| Contraction Speed | Slow, sustained | Fast, short duration |

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This version keeps everything clear and easy to understand while including enough detail for strong memorization. Let me know if you need further simplifications! 🚀