Chapter 6: The Muscular System – Structure, Function, and Physiology

Muscular System Overview

Functions of the Muscular System

• Movement: Muscles contract to produce body movements.

• Posture Maintenance: Muscles help maintain posture and body position.

• Heat Production: Muscle activity generates heat, contributing to body temperature regulation.

• Facial Expression: Muscles enable facial expressions for communication.

Types of Muscle Tissue

Comparison of the Three Muscle Types

• Skeletal Muscle:

  • Cylindrical, striated, multinucleate cells.

  • Voluntary control.

  • Attached to bones, responsible for body movement.

• Cardiac Muscle:

  • Branched, striated, usually uninucleate cells.

  • Intercalated discs connect cells for synchronized contraction.

  • Involuntary control.

  • Found only in the heart, pumps blood throughout the body.

• Smooth Muscle:

  • Spindle-shaped, non-striated, uninucleate cells.

  • Involuntary control.

  • Found in walls of hollow organs (e.g., stomach, intestines, blood vessels).

Histology of Skeletal Muscle

Connective Tissue Coverings

• Endomysium: Surrounds each individual muscle fiber (cell).

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Epimysium: Surrounds the entire muscle.

• Fascia: Connective tissue outside the epimysium, separates muscles from each other.

Muscle Cell Structure

Muscle Fiber Anatomy

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell, containing many nuclei and mitochondria.

• Myofibrils: Rod-like structures within muscle fibers, composed of protein filaments (actin and myosin) responsible for contraction.

• Sarcomere: The functional contractile unit of a muscle fiber, defined as the segment between two Z-discs.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions (Ca2+).

• Mitochondria: Abundant in muscle cells, provide ATP for contraction.

Physiology of Skeletal Muscle Contraction

Neuromuscular Junction and Stimulation

• Neuromuscular Junction (NMJ): The synapse between a motor neuron and a muscle fiber.

• Acetylcholine (ACh): Neurotransmitter released from the neuron, binds to receptors on the sarcolemma, initiating an action potential.

• Action Potential: Electrical signal that travels along the sarcolemma and triggers muscle contraction.

Steps of Muscle Contraction

1. Stimulation: Motor neuron releases ACh at the NMJ.

2. Action Potential: ACh binds to receptors, causing depolarization of the sarcolemma.

3. Calcium Release: Action potential travels down T-tubules, causing the SR to release Ca2+ into the sarcoplasm.

4. Cross-Bridge Formation: Ca2+ binds to troponin, exposing binding sites on actin. Myosin heads attach to actin, forming cross-bridges.

5. Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere (muscle shortens).

6. Detachment: ATP binds to myosin, causing it to detach from actin.

7. Reactivation: ATP is hydrolyzed, re-cocking the myosin head for another cycle.

8. Relaxation: When stimulation ends, Ca2+ is pumped back into the SR, and the muscle fiber relaxes.

Muscle Action Potential and Repolarization

• After depolarization, the inside of the sarcolemma becomes negatively charged again as K+ ions exit the cell, restoring the resting potential (repolarization).

• Muscle contraction ceases, and the sarcolemma returns to its resting state.

Energy Sources for Muscle Contraction

ATP and Muscle Metabolism

• ATP: The immediate source of energy for muscle contraction.

• Creatine Phosphate: In the first few seconds of contraction, creatine phosphate donates a phosphate group to ADP to rapidly regenerate ATP.

• Anaerobic Glycolysis: Glucose is broken down to pyruvate, producing ATP without oxygen. If oxygen is limited, pyruvate is converted to lactic acid.

• Aerobic Respiration: In the presence of oxygen, pyruvate enters the mitochondria and is fully oxidized to CO2 and H2O, producing much more ATP.

Summary Table: Energy Pathways in Muscle

Pathway	Oxygen Required?	ATP Yield	Byproducts
Creatine Phosphate	No	1 ATP per CP	Creatine
Anaerobic Glycolysis	No	2 ATP per glucose	Lactic acid
Aerobic Respiration	Yes	~36 ATP per glucose	CO2, H2O

Muscle Attachments and Actions

Definitions

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Action: The movement produced when a muscle contracts (e.g., flexion, extension).

Golden Rules of Muscle Action

• Muscles cross at least one joint.

• The bulk of the muscle lies proximal to the joint crossed.

• Muscles have at least two attachments: origin and insertion.

• Muscles can only pull; they never push.

• During contraction, the insertion moves toward the origin.

Major Skeletal Muscles

Muscle Identification (Lab Focus)

• Know the major muscles, their origins, insertions, and actions.

• Examples: biceps brachii, triceps brachii, quadriceps, hamstrings, gastrocnemius, gluteus maximus, deltoid, pectoralis major, etc.

Neuromuscular Diseases

Examples

• Muscular Dystrophy: Genetic disorder causing progressive muscle weakness and degeneration.

• Myasthenia Gravis: Autoimmune disease where antibodies block ACh receptors, causing muscle weakness and fatigue. Symptoms may include drooping eyelids and difficulty swallowing.

Treatment and Prognosis

• Some neuromuscular diseases can be managed with medication, physical therapy, or surgery.

• Early diagnosis and intervention can improve quality of life.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.