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## Muscle Contraction and Physiology

### Three Phases of Twitch (These are also the steps in the Sliding Filament Theory!)

1. Latent Period Before Contraction:

- The action potential moves through the sarcolemma and down the T-tubules.

- Causing Ca²⁺ release from the sarcoplasmic reticulum.

- Ca²⁺ binds to troponin.

- Troponin changes shape and pulls the tropomyosin off binding sites on actin.

- Myosin tails flex, causing myosin to bind to actin, forming cross-bridges.

2. Contraction Phase:

- Myosin heads flex (power stroke) using ATP, causing sliding of actin across myosin (and shortening of the muscle).

- This is sequential (like playing tug of war – you don’t let go until the next hand grabs).

- Tension builds to peak of contraction.

3. Relaxation Phase:

- Ca²⁺ levels fall as ATP is used to pump Ca²⁺ back into the sarcoplasmic reticulum.

- ATP is used to disconnect myosin from actin, returning myosin to the “high-energy” conformation (shape).

- Active sites are covered and the muscle is relaxed.

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## Muscle Energy Systems – How Your Muscles Generate ATP

### Cellular Respiration: Anaerobic vs. Aerobic, Creatine Phosphate

- We exhaust our supply of ATP in about 5 seconds.

- We can rely on our supply of Creatine Phosphate for approximately 10-15 seconds.

- After that, it depends on whether or not we have oxygen available.

Key Terms:

- Adenosine Triphosphate (ATP): Active energy molecule.

- Creatine Phosphate (CP): Storage molecule for excess ATP energy in resting muscle.

- Energy recharges ADP to ATP using the enzyme creatine phosphokinase (CPK or CK).

- When CP is used up, other mechanisms generate ATP.

### Cellular Respiration Process

- During cellular respiration, MOST energy flows in this sequence:

- Glucose → NADH → ETC → ATP.

- Each glucose yields up to 32 ATP during cellular respiration.

- Estimated efficiency in eukaryotic cells is ~ 40%; energy lost in the process is released as heat.

### Metabolism Types:

- Aerobic Metabolism: Primary energy source of resting muscles.

### Lactic Acid Fermentation

- Process: NADH oxidized to NAD⁺ & pyruvate reduced to lactate.

- Occurs when muscle contractile activity reaches 70% of maximum:

- Bulging muscles compress blood vessels, impairing oxygen delivery.

- Pyruvic acid is converted into lactate.

### Muscle Fatigue

- Results of Muscle Fatigue:

- Depletion of metabolic reserves.

- Damage to sarcolemma and sarcoplasmic reticulum.

- Low pH (lactic acid).

- Muscle exhaustion and pain.

- Muscle fatigue occurs due to:

- ATP production too slow.

- Deficit of ATP.

- Lactic acid buildup.

- Ionic imbalances.

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## Muscle Fiber Types

| Characteristic | Fast-Twitch | Slow-Twitch |

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

| Color of Muscle | | |

| Contains Myoglobin | | |

| More Blood Flow | | |

| High Oxygen Supply | | |

| More Mitochondria | | |

| Produces More ATP | | |

| Speed of Contraction | Quick and powerful | Slow but resistant to fatigue |

| Fatigue Rate | Fatigues quickly | Slow to fatigue |

### Physical Conditioning:

- Anaerobic Activities (e.g., 50-meter dash, weightlifting):

- Use fast fibers, fatigue quickly.

- Improved by frequent, brief, intense workouts.

- Aerobic Activities (prolonged activity):

- Supported by mitochondria, require oxygen and nutrients.

- Improved by repetitive training (neural responses) and cardiovascular training.

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## Muscle Growth and Atrophy

Muscle Hypertrophy:

- Muscle growth from heavy training.

- Increases diameter of muscle fibers.

- Increases number of myofibrils.

- Increases mitochondria and glycogen reserves.

Muscle Atrophy:

- Lack of muscle activity leads to reduced muscle size, tone, and power.

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## Muscle Naming and Terminology

- Origin: Muscle attachment that remains fixed.

- Insertion: Muscle attachment that moves.

- Action: What joint movement a muscle produces.

### Primary Action Categories

- Prime Mover (Agonist): Main muscle in an action.

- Synergist: Helper muscle in an action.

- Fixator: Immobilizes a bone or muscle’s origin.

- Antagonist: Opposed muscle to an action.

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## Divisions of the Muscular System

- Axial Muscles: Position head and spinal column, move rib cage (60% of skeletal muscles).

- Appendicular Muscles: Support pectoral and pelvic girdles, support limbs (40% of skeletal muscles).

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## Effects of Aging on the Muscular System

- Skeletal muscle fibers become smaller in diameter.

- Skeletal muscles become less elastic.

- Develop increasing amounts of fibrous tissue (fibrosis).

- Decreased tolerance for exercise and recovery from muscular injuries.

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## Skeletal System Overview

### Functions of the Skeletal System

- Supports body structure.

- Protects vital organs.

- Facilitates movement with muscles.

- Produces blood cells (hematopoiesis).

- Stores minerals (calcium, phosphorus).

### Gross Anatomy of a Typical Long Bone

- Diaphysis: Shaft of the bone.

- Epiphysis: End part of a long bone.

- Medullary Cavity: Cavity inside the diaphysis containing marrow.

- Periosteum: Outer membrane covering the bone.

- Articular Cartilage: Covers the ends of bones at joints.

- Compact Bone: Dense mineralized matrix of bone tissue.

- Spongy Bone: Porous bone tissue consisting of trabeculae.

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## Bone Structure

### Compact and Spongy Bone

- Haversian System (Osteon): Structural unit of compact bone.

- Lamella: Column-like matrix tubes composed mainly of collagen.

- Haversian (Central) Canal: Central channel containing blood vessels and nerves.

- Volkmann’s Canals: Channels connecting blood and nerve supply of the periosteum to that of the Haversian canal.

- Osteocytes: Mature bone cells.

- Lacunae: Cavities in bone that contain osteocytes.

- Canaliculi: Hairlike canals that connect lacunae.

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## Hematopoietic Tissue (Red Marrow)

- Where blood cell production occurs after birth.

- Supplies nutrients to osteocytes.

- Found in all bony spaces in children; in adults, found only in spongy bone (pelvis, ribs, sternum, vertebrae, skull, and ends of arm and thigh bones).

### Yellow Bone Marrow

- Stores triglycerides in adipose cells; also contains a few blood cells.

- In newborns, all bone marrow is red; in adults, yellow marrow is found in the medullary cavities of long bones.

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## Bone Cells

- Osteocytes: Maintain bone matrix; live in lacunae.

- Osteoclasts: Dissolve bone matrix and release stored minerals.

- Osteoblasts: Secrete matrix compounds (osteogenesis); become osteocytes when surrounded by bone.

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## Calcium Homeostasis

- Parathyroid Hormone (PTH): Increases calcium ion levels by stimulating osteoclasts, increasing intestinal absorption, and decreasing calcium excretion at kidneys.

- Calcitonin: Decreases calcium ion levels by inhibiting osteoclast activity and increasing calcium excretion at kidneys.

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## Ossification and Bone Development

- Ossification: Process of replacing other tissues with bone.

- Intramembranous Ossification: Bone develops from a fibrous membrane (produces flat bones).

- Endochondral Ossification: Bone forms by replacing hyaline cartilage.

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## Bone Remodeling

- Maintains and replaces mineral reserves.

- Involves osteocytes, osteoblasts, and osteoclasts.

- Turnover rate varies; deposition greater than removal strengthens bones.

### Nutritional and Hormonal Factors for Bone Growth

- Calcium and phosphate salts, Vitamin D3 (calcitriol), Vitamin C (collagen synthesis), and hormones like Growth Hormone and Thyroid Hormone stimulate growth.

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## Joint Anatomy and Classification

- Joints categorized by range of motion and anatomical organization.

- Joint strength decreases as mobility increases; stability determined by articular surfaces, ligaments, and muscle tone.

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