The Muscular System: Structure, Function, and Physiology

The Muscular System

Muscle Tissue Types

Muscle tissue is one of the four primary tissue types in the human body, specialized for contraction. There are three main types of muscle tissue, each with distinct structure and function:

• Skeletal muscle tissue: Moves the body by pulling on bones of the skeleton; responsible for voluntary movements.

• Cardiac muscle tissue: Found only in the heart; propels blood throughout the circulatory system.

• Smooth muscle tissue: Moves fluids and solids along internal passageways (e.g., digestive tract); regulates diameter of small arteries.

Example: Skeletal muscles allow walking, cardiac muscle enables heartbeat, and smooth muscle controls peristalsis in the intestines.

Functions of Skeletal Muscle Tissue

Skeletal muscle tissue performs several essential functions for the body:

• Produce skeletal movement: Muscle contractions pull on tendons, moving bones.

• Maintain posture and position: Constant muscle activity maintains tension and supports body alignment.

• Support soft tissue: Muscles form the abdominal wall and pelvic floor, supporting organs.

• Guard entrances and exits: Muscles form sphincters around digestive and urinary tracts, controlling voluntary actions like swallowing and urination.

• Maintain body temperature: Muscle contraction generates heat as a byproduct.

• Provide nutrient reserves: Contractile proteins can be broken down to amino acids for energy during inadequate intake.

Skeletal Muscle Structure

Skeletal muscle is organized into bundles of fibers wrapped in connective tissue layers:

• Epimysium: Dense collagen layer surrounding the entire muscle.

• Perimysium: Fibrous layer dividing muscle into bundles called fascicles.

• Endomysium: Delicate connective tissue surrounding individual muscle fibers.

Tendons: Collagen fibers from these layers extend beyond the muscle, forming tendons (attach muscle to bone) or aponeuroses (attach muscle to broader areas).

Example: The Achilles tendon connects calf muscles to the heel bone.

Skeletal Muscle Fiber Terminology and Structure

Muscle fibers are specialized cells with unique terminology:

• Muscle fiber: The muscle cell.

• Sarcolemma: Cell membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum for calcium storage.

Muscle fibers contain hundreds of nuclei and are packed with myofibrils, which are composed of myofilaments:

• Thin filaments: Mostly actin.

• Thick filaments: Primarily myosin.

The arrangement of myofibrils gives muscle fibers a striated appearance.

Sarcomeres: The Functional Unit

The sarcomere is the basic contractile unit of skeletal muscle, with about 10,000 per myofibril. Its structure includes:

• Z lines: Boundaries between adjacent sarcomeres.

• M line: Center of the sarcomere, connects thick filaments.

• H band: Contains only thick filaments.

• A band: Contains both thick and thin filaments.

• I band: Contains only thin filaments.

Structure of Thin and Thick Filaments

Thin filaments are attached to Z lines and composed of:

• Actin: Twisted double strand with myosin binding sites.

• Tropomyosin: Covers myosin binding sites when muscle is relaxed.

• Troponin: Binds to actin and tropomyosin; has a calcium binding site.

Thick filaments contain about 300 myosin molecules, arranged with tails toward the M line. Myosin heads bind to actin during contraction, forming cross-bridges.

Sarcolemma, T-tubules, and Sarcoplasmic Reticulum

The sarcolemma maintains a membrane potential. Sudden changes in this potential travel along T-tubules, which are passageways encircling the sarcomere and tightly bound to the SR. The SR stores calcium ions and releases them to trigger contraction.

Skeletal Muscle Fiber Contraction: The Neuromuscular Junction

Contraction begins with a nervous signal:

1. Action potential from motor neuron reaches the neuromuscular junction (NMJ).

2. Acetylcholine (ACh) is released from vesicles in the axon terminal.

3. ACh binds to receptors on the motor end plate, increasing membrane permeability to sodium.

4. Sodium influx generates an action potential in the sarcolemma.

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

6. Triggers release of calcium from SR, initiating contraction.

Acetylcholinesterase (AChE) breaks down ACh, terminating the signal.

Sliding Filament Theory

The sliding filament theory explains how muscles contract:

• Thin filaments slide past thick filaments.

• H and I bands decrease in size; zones of overlap increase.

• Z lines move closer together; A bands remain constant.

Muscle fiber shortens as sarcomeres contract, transmitting tension to tendons and bones.

Contraction Cycle

The contraction cycle involves several steps:

1. Calcium arrives in the zone of overlap.

2. Calcium binds to troponin, exposing active sites on actin.

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

4. Myosin heads pivot (power stroke), releasing ADP and phosphate.

5. ATP binds to myosin, causing detachment from actin.

6. Myosin splits ATP, re-cocking the head; cycle repeats as long as ATP and calcium are available.

When stimulation stops, calcium is pumped back into the SR, and muscle relaxes.

Muscle Twitch and Tetanus

A muscle twitch is a single stimulus-contraction-relaxation sequence. It has three phases:

• Latent period: Time from stimulation to tension development.

• Contraction phase: Tension rises to peak.

• Relaxation phase: Tension falls to resting level.

Repeated stimuli can cause:

• Wave summation: Addition of twitches for stronger contraction.

• Incomplete tetanus: Rapid cycles with partial relaxation.

• Complete tetanus: No relaxation; continuous contraction.

Motor Units and Muscle Tone

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Smaller motor units allow more precise movements. Recruitment activates more motor units for increased tension. Muscle tone is the resting tension maintained by some active motor units.

Types of Muscle Contraction

• Isotonic contraction: Tension rises and muscle length changes (e.g., lifting a weight).

• Isometric contraction: Muscle length stays the same, but tension increases (e.g., holding a weight steady).

Example: Pushing against a wall is isometric; lifting a dumbbell is isotonic.

ATP Sources in Skeletal Muscle

Muscle fibers require ATP for contraction:

• At rest, ATP is produced by mitochondria from fatty acids and glucose.

• ATP is stored as creatine phosphate and glycogen.

• During moderate activity, ATP is produced aerobically from pyruvate.

• During peak activity, most ATP is produced anaerobically, resulting in lactic acid formation and muscle fatigue.

Equation for aerobic respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Cori cycle: Lactate is converted to pyruvate in the liver, then to glucose, which replenishes muscle glycogen.

Oxygen debt: The amount of oxygen required to restore normal conditions after exercise.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to repeated, exhaustive stimulation; more mitochondria, enzymes, glycogen, and myofibrils.

• Atrophy: Decrease in muscle size and strength due to lack of stimulation; can be temporary or permanent.

Example: Immobilization after a fracture leads to atrophy; strength training causes hypertrophy.

Rigor Mortis

After death, ATP production ceases, calcium cannot be pumped back into the SR, and cross-bridges remain attached, causing sustained contraction known as rigor mortis. Begins 2–7 hours after death and lasts 1–6 days.

Summary Table: Muscle Tissue Types

Type	Location	Function	Control
Skeletal	Attached to bones	Movement, posture	Voluntary
Cardiac	Heart	Pump blood	Involuntary
Smooth	Walls of organs	Move fluids, regulate diameter	Involuntary

Summary Table: Phases of Muscle Twitch

Phase	Duration	Events
Latent	~2 ms	Action potential, calcium release
Contraction	~13 ms	Calcium binds to troponin, cross-bridge cycling
Relaxation	~25 ms	Calcium levels fall, cross-bridges detach

Additional info: The notes have been expanded with academic context, including definitions, examples, and summary tables for clarity and completeness.