Muscle Physiology and Function

Prime Movers (Muscle Actions)

Prime movers, also known as agonists, are muscles primarily responsible for generating a specific movement at a joint. Understanding which muscles act as prime movers for various body parts is essential in anatomy and physiology.

• Definition: A prime mover is the main muscle responsible for a particular movement.

• Examples:

  • Head/Neck: Sternocleidomastoid (flexes neck)

  • Vertebral Column: Erector spinae (extends spine)

  • Scapula: Trapezius (elevates, retracts scapula)

  • Glenohumeral (Shoulder): Deltoid (abducts arm)

  • Elbow: Biceps brachii (flexes forearm)

  • Radiocarpal (Wrist): Flexor carpi radialis (flexes wrist)

  • Hip: Iliopsoas (flexes hip)

  • Knee: Quadriceps femoris (extends knee)

  • Ankle: Gastrocnemius (plantar flexes foot)

Muscle Fiber Excitation-Contraction Coupling (Steps 1–13)

Excitation-contraction coupling is the physiological process linking muscle fiber stimulation to contraction. The following steps outline this process:

1. Action potential arrives at the neuromuscular junction.

2. Acetylcholine (ACh) is released from the motor neuron.

3. ACh binds to receptors on the sarcolemma (muscle cell membrane).

4. Sodium ions enter the muscle fiber, generating an action potential.

5. Action potential travels along the sarcolemma and down T-tubules.

6. Sarcoplasmic reticulum releases calcium ions (Ca2+).

7. Calcium binds to troponin, causing a conformational change.

8. Tropomyosin shifts, exposing myosin-binding sites on actin.

9. Myosin heads bind to actin, forming cross-bridges.

10. Myosin heads pivot, pulling actin filaments (power stroke).

11. ATP binds to myosin, causing it to detach from actin.

12. ATP is hydrolyzed, re-cocking the myosin head.

13. Cycle repeats as long as Ca2+ and ATP are present.

Example: During a biceps curl, these steps occur rapidly to produce muscle contraction.

Muscle Terminology: Myo- & Sarco- Terms

Understanding muscle-related terminology is crucial for mastering anatomy and physiology. Many terms use the prefixes myo- (muscle) and sarco- (flesh).

• Myoglobin: Oxygen-binding protein in muscle cells.

• Myofibril: Rod-like unit within muscle fibers, composed of sarcomeres.

• Myofilament: Protein filaments (actin and myosin) within myofibrils.

• Myosin: Thick filament protein involved in contraction.

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases Ca2+.

• Sarcomere: Functional contractile unit of muscle, defined by Z-lines.

Facial Muscle Functions

Facial muscles are responsible for a wide range of expressions and movements. Knowing their functions is important for understanding human communication and anatomy.

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes the lips.

• Zygomaticus major: Elevates the corners of the mouth (smiling).

• Buccinator: Compresses the cheek.

• Frontalis: Raises eyebrows and wrinkles forehead.

• Platysma: Tenses skin of the neck.

Anaerobic vs. Aerobic Respiration

Muscle cells generate ATP through two main processes: aerobic (with oxygen) and anaerobic (without oxygen) respiration.

Example: Sprinting uses anaerobic respiration; marathon running relies on aerobic respiration.

Fast Glycolytic vs. Slow Oxidative Muscle Fibers

Muscle fibers are classified based on their contraction speed and metabolic pathways.

Causes of Muscle Fatigue

Muscle fatigue is the decline in ability of a muscle to generate force. Several factors contribute to fatigue:

• Depletion of ATP and glycogen stores

• Accumulation of lactic acid

• Impaired calcium release or uptake

• Electrolyte imbalances

• Reduced oxygen supply

Factors Determining Muscle Tension

The amount of tension a muscle can produce depends on several physiological factors:

• Number of motor units recruited

• Frequency of stimulation (rate of nerve impulses)

• Muscle fiber length at the start of contraction

• Muscle fiber type

• Degree of fatigue

Trapezius Muscle Functions

The trapezius is a large, superficial muscle of the upper back and neck. It performs several important functions:

• Elevates, retracts, and rotates the scapula

• Extends the neck

• Stabilizes the shoulder girdle

• Assists in head movement

• Supports arm abduction

Muscles That Move More Than One Joint/Body Part

Some muscles cross multiple joints, allowing them to produce movement at more than one location.

• Biceps brachii: Flexes both the shoulder and elbow

• Rectus femoris: Flexes hip and extends knee

• Gastrocnemius: Flexes knee and plantar flexes ankle

• Sartorius: Flexes, abducts, and laterally rotates hip; flexes knee

Example: The biceps brachii is involved in both elbow flexion and shoulder flexion.

Additional info: Some content and examples were inferred based on standard Anatomy & Physiology curriculum to ensure completeness and clarity.