Chapter 6: The Muscular System

Overview

Muscles are responsible for all body movements. Three basic muscle types include:

• Skeletal Muscle

• Cardiac Muscle

• Smooth Muscle

Muscle Types

Muscle cells are elongated and known as muscle fibers. Muscle contraction is due to the movement of microfilaments, which consist of actin and myosin proteins that interact during the contraction process.

Muscle Terminology

• Prefixes "myo-" and "mys-" refer to "muscle."

• Prefix "sarco-" refers to "flesh."

Skeletal Muscle Characteristics

• Skeletal muscles are attached to bones by tendons, which are strong cord-like structures made of collagen fibers.

• They are large, cigar-shaped cells that are multinucleate (having multiple nuclei).

• Skeletal muscles have a striated appearance due to the arrangement of filaments and are known as voluntary muscles, meaning they are under conscious control of the nervous system.

Connective Tissue Wrappings

• Endomysium: A thin connective tissue wrapping that encloses a single muscle fiber, providing support and insulation.

• Perimysium: Connective tissue that wraps around a fascicle (bundle) of muscle fibers, containing blood vessels and nerves that supply the fibers.

• Epimysium: The outer layer that covers the entire skeletal muscle, helping to protect and shape it.

• Fascia: A connective tissue layer that surrounds the muscle outside of the epimysium, which may connect with surrounding tissues or muscles.

Muscle Connective Tissue Attachments

• Tendons: These cordlike structures cross joints due to their toughness and size, anchoring muscles to bones.

• Aponeuroses: Sheet-like structures that can attach muscles indirectly to bones, cartilage, or other connective tissues, providing a broader area of attachment than tendons.

Smooth Muscle Characteristics

• Smooth muscle lacks striations and is controlled involuntarily by the autonomic nervous system.

• It is located in the walls of hollow visceral organs such as the stomach, bladder, and blood vessels.

• Smooth muscle fibers are spindle-shaped and uninucleate (having one nucleus), allowing for slow, sustained contractions that help regulate body functions such as digestion and blood flow.

Cardiac Muscle Characteristics

• Cardiac muscle is striated and also involuntary, found only in the walls of the heart.

• The cells are branching and interconnected by intercalated discs, which include gap junctions that facilitate rapid electrical signal conduction.

• Cardiac muscle contracts at a steady rate set by pacemaker cells, ensuring that the heart pumps blood efficiently.

Muscle Functions

Skeletal Muscle Functions:

• Produces movement by contracting and pulling on bones.

• Maintains posture and stabilizes body position by counteracting the pull of gravity.

• Stabilizes joints through maintaining muscle tone and support around joint areas.

• Generates heat as a byproduct of muscle activity, which is essential for maintaining body temperature.

Microscopic Anatomy of Skeletal Muscle

• Sarcolemma: The specialized plasma membrane of a muscle fiber that conducts electrical impulses.

• Myofibrils: Long, rod-like organelles that fill the muscle cells; they consist of repeating units called sarcomeres that create the striated appearance through their arrangement of light (I) and dark (A) bands.

Sarcomere Structure

• Sarcomere: The smallest contractile unit of a muscle fiber; produces distinctive banding patterns.

• Thick Filaments: Composed of myosin; these filaments contain ATPase enzymes that are crucial for muscle contraction.

• Thin Filaments: Made of actin; these filaments are anchored to the Z disc, providing structure to the sarcomere and aiding in contraction.

Muscle Contraction

Sliding Filament Theory:

• Calcium ions bind to proteins on thin filaments, exposing myosin-binding sites.

• Myosin heads attach to actin, pivoting to pull thin filaments towards the center of the sarcomere.

• Contraction continues as long as calcium ions are present, and ATP supplies the necessary energy.

Stimulation and Action Potential

• Motor Unit: Consists of one motor neuron and all the skeletal muscle cells it stimulates, enabling coordinated muscle contractions.

• Neuromuscular Junction: The specialized connection site between the motor neuron and the muscle sarcolemma, facilitating signal transmission.

• Acetylcholine (ACh): A neurotransmitter released at the neuromuscular junction that initiates muscle contraction by stimulating the muscle fiber.

Muscle Energetics

• ATP: The only energy source directly used for muscle contraction, produced through various metabolic pathways.

• ATP is stored in small amounts within muscle fibers and is depleted quickly during activity.

Three Pathways to Regenerate ATP:

1. Direct Phosphorylation of ADP by Creatine Phosphate: The fastest method, providing immediate energy.

2. Aerobic Respiration: A slower process that requires oxygen, generating more ATP for extended activity, primarily occurring in mitochondria.

3. Anaerobic Glycolysis: Breaks down glucose without oxygen, producing ATP quickly but resulting in lactic acid and lower energy yield.

Muscle Fatigue

Muscle fatigue occurs with prolonged or intense activity, characterized by:

• Ion imbalances that disrupt normal cellular function.

• Oxygen deficits and lactic acid accumulation, leading to discomfort and reduced muscle performance.

• A decrease in energy supply (ATP) that impairs muscle contraction.

Types of Muscle Contractions

• Isotonic Contractions: Occur when the muscle shortens and movement happens (e.g., bending the knee).

• Isometric Contractions: Involves muscle tension increase without shortening (e.g., pushing palms together).

Muscle Tone

Refers to continuous partial contractions that keep muscles firm, healthy, and ready for action, providing stability and posture support.

Effect of Exercise on Muscles

Regular exercise increases muscle size, strength, and endurance:

• Aerobic (Endurance) Exercise: Enhances muscle flexibility, cardiovascular fitness, and metabolic efficiency.

• Resistance (Isometric) Exercise: Increases muscle size and strength through focused exertion against resistance, stimulating muscle hypertrophy.