Muscular System Overview and Function

Muscular System Overview

• Muscle tissue makes up 40% of male body weight and 32% of female body weight.
• Skeletal muscle is responsible for movement and is under voluntary control.
• Cardiac muscle forms the heart.
• Smooth muscle is involuntary and controls actions like digestion, blood vessel diameter, bladder function, and uterus activity.

Muscle Function

• Muscles contract to shorten the distance between bones.
• Skeletal muscles move bones via tendons.
• Synergistic muscle groups work together for actions. Examples include actions in your head turning.
• Antagonistic muscle groups work oppositely from one another to produce movements. For example, biceps contract to bend the arm and triceps muscles contract to straighten it. These groups don't contract at the same time.

Muscular System: Antagonistic Groups (Examples)

• Pectoralis major: Draws the arm forward and towards the body
• Serratus anterior: Helps raise the arm and push forward
• Biceps brachii: Bends the forearm at the elbow
• Rectus abdominis: Compresses the abdomen and chest cavity
• External oblique: Rotates the trunk & compresses the abdomen
• Adductor longus: Flexes the thigh and draws it toward the body
• Sartorius: Bends the thigh and the lower leg at the knee and rotates the thigh outward
• Quadriceps group: Flexes the thigh at the hip and extends the leg at the knee
• Tibialis anterior: Flexes the foot toward the knee
• Deltoid: Raises the arm
• Trapezius: Raises and braces the shoulder, pulls head back
• Triceps brachii: Straightens the forearm and arm at the elbow
• Latissimus dorsi: Rotates and draws the arm backward to the body
• Gluteus maximus: Extends thigh, rotates thigh laterally
• Hamstring group: Draws the thigh backward, bends the lower leg at the knee, bends foot from knee

Muscle Structure

• A muscle is composed of cells with the same insertion and origin.
• Fascicles: Bundles of cells surrounded by connective tissue.
• Myofibrils: Bundles within muscle cells.
• Myofibrils are made of sarcomeres arranged end to end.

Basic Muscle Contractile Unit (Sarcomere)

• Skeletal muscle cells are multinucleated (fused cells).
• Muscles are composed of fascicles, cells, myofibrils, and sarcomeres.

Sliding Filament Mechanism

• Thick filaments: Myosin
• Thin filaments: Actin
• Contraction: Myosin and actin filaments slide past each other, causing shortening.
• Relaxation: Filaments passively slide back into place.

Skeletal Muscle Contraction: Sarcomeres

• Actin and myosin are key proteins in contraction.
• Z-lines are attachment points for actin.
• Contraction leads to sarcomere shortening.

Skeletal Muscle Contractile Unit

• Diagrams show sarcomere contraction.

Steps of Skeletal Muscle Contraction

• Signal originates in the frontal lobe of the cerebral cortex.
• Messages travel to the muscles via the spinal cord.
• Motor neurons stimulate muscles using acetylcholine.
• Muscle receives an electrical impulse.
• Impulse stimulates calcium release from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.

Nerve Activation of Individual Muscle Cell

• Acetylcholine release causes electrical impulses in the muscle cell membrane.
• Impulses transmitted through T tubules to the interior of the cell.
• Calcium release from the sarcoplasmic reticulum triggers contraction.

Steps of Muscle Contraction

• Calcium binds to troponin, exposing actin binding sites to myosin.
• Energized myosin heads bind to actin, forming cross-bridges.
• After binding, the myosin head bends, bringing Z-lines closer together.
• ADP + P is released.
• ATP binds to myosin, causing it to release the actin filament.
• Myosin splits ATP into ADP + P, energizing the head.

Steps in Sarcomere Contraction

• Diagram shows calcium release, binding to troponin, myosin binding, and power strokes.
• ATP hydrolysis is part of the process.
• Relaxation occurs when calcium is returned to the sarcoplasmic reticulum.

Muscle Relaxation

• Nervous stimulation stops; calcium ions are no longer released.
• Calcium is pumped back into the sarcoplasmic reticulum.
• Calcium is removed from troponin.
• The actin-binding sites are covered by tropomyosin.
• Myosin can no longer bind to actin.
• The sarcomere relaxes.

Energy Required for Muscle Activity

• ATP is the primary energy source.
• ATP replenishment happens through creatine phosphate, glycogen breakdown (aerobically/anaerobically), and cellular respiration (aerobic).

Muscle Fatigue

• Muscle fatigue occurs when muscles cannot contract despite stimulation.
• Factors include low ATP supply and lactic acid buildup.
• Deep breathing/rest breaks down lactic acid and replenishes ATP.

Motor Unit

• One motor neuron can stimulate more than one muscle cell.
• A motor unit is a single motor neuron and all the muscle cells it stimulates.
• All cells in a motor unit contract together when a nerve signal arrives.
• Muscles need nerve input to contract.
• Fine control (like eye movements) involves fewer muscle cells per neuron compared to brute strength (like leg movements).

Skeletal Muscle Types

• Slow-twitch fibers: endurance activities (e.g., running marathons).
• Fast-twitch fibers: power activities (e.g., sprinting).
• Muscles contain both types, with varying proportions.

Exercise and Muscles

• Strength training increases fast-twitch fibers, leading to increased muscle mass/power.
• Endurance training increases slow-twitch fibers, leading to improved endurance.

Benefits of Exercise for Muscles

• Builds muscle mass.
• Boosts endurance.
• Slows muscle aging.
• Increases caloric burn.

Comparison of Muscle Types (Location/Function)

• Skeletal: Attached to bones, voluntary movement.
• Smooth: Found in walls of internal organs (e.g., digestive system), involuntary.
• Cardiac: Forms the heart, involuntary.

Comparison of Muscle Types (Structure)

• Skeletal: Long, cylindrical, multinucleated.
• Cardiac: Short, blunt ends, connected by gap junctions.
• Smooth: Small spindle-shaped, joined by gap junctions.

Cardiac Muscles

• Gap junctions allow contraction signals to move quickly between cells.
• Large numbers of cells contract simultaneously.

Smooth Muscles

• Actin and myosin in a net-like arrangement.
• Smooth muscles are always partially contracted.
• Smooth muscles do not fatigue (because they don't completely use their ATP).

Anabolic Steroid Use

• Mimic testosterone, increasing muscle protein synthesis.
• Result: Larger, more powerful muscles.
• Disadvantages: Liver and kidney tumors, acne, baldness, changes in sexual characteristics (men/women).

Muscular System Disorders

• Pulled muscles: Strained muscle fibers.
• Sore muscles: Damaged underused sarcomeres.
• Muscle cramps: Uncontrollable muscle contractions (often due to ATP depletion, dehydration, ion imbalances, and lactic acid buildup).

Muscular System Diseases

• Tetanus: Bacterial toxins overstimulate nerves, causing constant muscle contractions.
• Duchenne Muscular Dystrophy: Sex-linked genetic disorder, muscle wasting.