Muscular System Study Notes

Introduction to the Muscular System

• Muscles are organs that generate force to cause movement.

• Examples of muscle actions include:

  • Walking

  • Breathing

  • Pumping blood

  • Moving food in the digestive tract

• The three types of muscle tissue in the body are:

  • Skeletal muscle

  • Smooth muscle

  • Cardiac muscle

• The human body contains over 600 skeletal muscles.

Skeletal Muscle Structure

Connective Tissue Coverings Over Muscles

• Layers of dense connective tissue, known as fascia, surround and separate each muscle.

• This connective tissue extends beyond the ends of the muscle and forms tendons that attach to the periosteum of bones.

• Muscles can also be connected by broad sheets of connective tissue called aponeuroses.

Detailed Layers of Connective Tissue

• Epimysium: The outer layer that surrounds each skeletal muscle.

• Perimysium: Extends inward from the epimysium and surrounds bundles of skeletal muscle fibers (fascicles).

• Endomysium: A layer that covers each muscle cell (fiber).

Myofibrils and Sarcomeres

• Myofibrils consist of many units called sarcomeres, joined end-to-end.

• A sarcomere extends from one Z line to the next.

• Striations: An alternating pattern of light (I bands) and dark (A bands) bands.

• I bands (light bands): Comprised of actin filaments anchored to Z lines.

• A bands (dark bands): Composed of overlapping thick (myosin) and thin (actin) filaments.

• The center of the A band contains the H zone, which consists solely of myosin filaments.

• M line: Found in the center of the H zone, contains proteins that hold myosin filaments in place.

Skeletal Muscle Fibers

• Each muscle fiber is a single, long, cylindrical muscle cell that responds to stimulation by exerting a pulling force.

• Sarcolemma: The cell membrane of a muscle fiber.

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

• Sarcoplasm contains parallel myofibrils, active in muscle contraction:

  • Thick filaments consist of myosin.

  • Thin filaments mainly comprise actin, along with troponin and tropomyosin.

  • The arrangement produces striations visible under a microscope.

Sarcoplasmic Reticulum and T Tubules

• Beneath the sarcolemma lies a network of membranous channels called the sarcoplasmic reticulum (SR), which is the muscle cell's endoplasmic reticulum.

• Transverse (T) tubules: Invaginations of the sarcolemma that lie between two cisternae of the sarcoplasmic reticulum. They are open to the exterior of the muscle fiber.

• The sarcoplasmic reticulum and T tubules play a vital role in activating the muscle contraction mechanism when the fiber is stimulated.

Neuromuscular Junction

• Skeletal muscle fibers contract only when stimulated by a motor neuron.

• Each muscle fiber is functionally (but not physically) connected to the axon of a motor neuron, forming a synapse (neuromuscular junction).

• The neuron communicates with the muscle fiber through neurotransmitters released at the synapse.

• The distal end of the motor neuron contains mitochondria and synaptic vesicles storing neurotransmitters.

• The muscle fiber membrane features a specialized region known as the motor end plate, where the sarcolemma is folded and contains specific receptors for the neurotransmitter acetylcholine.

• Upon reaching the motor neuron axon's end, synaptic vesicles release acetylcholine into the synaptic cleft, stimulating the muscle fiber to contract.

Skeletal Muscle Contraction (Overview)

• Muscle contraction involves a series of events that result in the shortening of sarcomeres and pull the muscle against its attachments.

• The pulling force is due to the binding of myosin to actin molecules.

• The fiber shortens through increased overlap of actin and myosin filaments as they slide past each other.

• Shortening of muscle fibers results in the shortening of the entire muscle, which then creates movement.

Myosin and Actin Role in Contraction

• Myosin molecules consist of two twisted strands with globular heads projecting outward.

• Actin molecules consist of globular proteins arranged in a double helix containing myosin binding sites.

• Troponin and tropomyosin associate with actin to form thin filaments.

Sliding Filament Model of Muscle Contraction

• During muscle contraction, a myosin head binds to an actin filament, forming a cross-bridge.

• This causes the head to bend, pulling the actin filament towards the center of the sarcomere.

• After release, the head attaches to the next binding site on actin, moving it similarly. This repeated cycling of attachment increases filament overlap, shortening the sarcomere.

• Many shortening sarcomeres result in muscle fiber shortening.

• Energy from the conversion of ATP to ADP is utilized in powering cross-bridges by the enzyme ATPase.

• Rigor mortis: After death, skeletal muscles partially contract and become rigid due to the increase in calcium permeability, cross-bridge formation, and depletion of ATP, preventing relaxation.

Stimulus for Contraction

• Acetylcholine (ACh) is the neurotransmitter crucial for skeletal muscle contraction at neuromuscular junctions.

• Synthesized in the motor neuron and stored in synaptic vesicles, ACh is released into the synaptic cleft following a nerve impulse.

• The stimulation of the muscle fiber by ACh leads to:

  1. Release of calcium ions from the sarcoplasmic reticulum into the cytosol.

  2. High concentrations of calcium bind to troponin, causing tropomyosin to move aside and expose myosin binding sites on actin filaments.

  3. Cross-bridges form, pulling actin filaments utilizing ATP energy, resulting in sarcomere shortening.

Muscle Contraction and Relaxation Process

• Muscle contraction continues as long as nerve impulses are present.

• Once the nerve impulse ceases, several key events for muscle relaxation occur:

  1. Acetylcholinesterase decomposes acetylcholine.

  2. Calcium ions are actively transported back into the sarcoplasmic reticulum, requiring ATP.

  3. ATP breaks the cross-bridge linkages between myosin and actin filaments, allowing actin to return to its resting position.

  4. The muscle remains relaxed and ready for the next stimulus.

Energy Sources for Muscle Contraction

• The energy for muscle contraction is primarily sourced from ATP, which is limited and must be regenerated.

• Creatine phosphate serves to regenerate ATP by transferring its high-energy phosphate to ADP, converting it back to ATP.

• The enzyme creatine phosphokinase facilitates ATP synthesis from creatine phosphate when ATP levels are sufficient.

• During high ATP demands, the muscle cell relies on cellular respiration to regenerate more ATP.

Overview of Cellular Respiration

• Glycolysis: The first phase, occurring in the cytoplasm, which is anaerobic and yields 2 ATP per glucose.

• Aerobic respiration: A complete breakdown of glucose in the mitochondria that yields 28 ATP per glucose.

• Oxygen transport: Hemoglobin in red blood cells carries oxygen to muscle tissue, while myoglobin in muscle enhances oxygen storage for aerobic respiration.

Oxygen Debt

• Oxygen debt arises from insufficient oxygen during strenuous exercise, leading to anaerobic respiration.

• Lactic acid forms from pyruvic acid and accumulates in muscles and the bloodstream.

• Lactic acid is then converted back into glucose in the liver, utilizing ATP.

• Oxygen debt refers to the total oxygen required for:

  1. Converting accumulated lactic acid back to glucose.

  2. Resynthesizing ATP and creatine phosphate to baseline levels.

• The repayment of oxygen debt can take hours, with physical training increasing muscle energy production capacity.

Muscle Metabolism

Heat Production and Muscle Fatigue

• Cellular respiration produces heat alongside ATP, which assists in maintaining body temperature.

• Muscle fatigue occurs when muscles lose contraction ability, potentially from electrolyte imbalances, decreased ATP levels, and acidity from lactic acid.

• Muscle cramps: Involuntary contractions resulting from changes in the muscle's extracellular fluid environment, stimulating uncontrolled muscle fiber action.

Types of Muscle Fibers

• Skeletal muscle fibers can be classified as:

Fast Fibers

• Make up the majority of muscle fibers.

• Specialized for rapid movements and exert maximum force quickly but fatigue quickly.

• Characteristics:

  • Large diameter

  • Few mitochondria

  • Store glycogen; primarily utilize anaerobic metabolism

Slow Fibers

• Have a smaller diameter and are more resistant to fatigue.

• Characteristics:

  • Higher mitochondria density

  • More capillaries

  • Primarily utilize aerobic metabolism

Exercise and Muscle Use

• Muscle response to activity level changes is significant:

Hypertrophy

• Enlargement of a muscle due to repeated exercise.

Atrophy

• Decrease in muscle size and strength due to disuse.

Muscle Contraction Mechanisms

Twitch

• The response of a single muscle fiber to one impulse includes a cycle of contraction and relaxation.

• Myogram: A graph recording the electrical stimulation of muscle contraction, demonstrating:

  • Latent period: A brief delay after stimulation before contraction begins.

  • Period of contraction followed by relaxation.

  • Each twitch generates a consistent force known as the all-or-none response.

Summation and Tetanic Contraction

• When a muscle fiber receives multiple stimuli of increasing frequency, summation occurs, leading to greater contraction than a single twitch can achieve.

• Partial tetany: When relaxation between stimuli becomes short, leading to higher contraction strength.

• Complete tetany: Sustained contraction without any relaxation, typically only achieved in laboratory settings.

Recruitment of Motor Units

• A motor unit consists of a motor neuron and all muscle fibers it activates. When activated, muscle fibers contract simultaneously.

• Motor unit recruitment: Increases the number of motor units active to boost contraction strength; maximum muscle tension occurs when all units are recruited.

Skeletal Muscle Actions

• Origin: Less movable end of the muscle.

• Insertion: More movable end.

• Muscle contraction pulls the insertion towards the origin.

• Example: Biceps brachii

  • “Biceps” indicates two origins.

  • Both heads origin from different parts of the scapula (coracoid process and tubercle above glenoid cavity).

  • Insertion at the radial tuberosity of the radius;

  • Action: Flexion of the forearm at the elbow.

Muscle Movements and Leverage

• Muscles and bones function as levers:

• Example: Bending of the arm at the elbow:

  • Rigid bar (forearm bones)

  • Fulcrum (elbow joint)

  • Resistance (weight hand must lift)

  • Force (muscles in the anterior arm).

Muscle Relationships

• Changes in angles and movements:

  • Flexion: Decrease angle between bones.

  • Extension: Increase angle between bones.

Muscle Function Groups

• Agonist (prime mover): Muscle performing the primary action.

• Synergists: Muscles assisting the agonist.

• Antagonists: Muscles opposing the motion.

• A muscle can serve as a synergist for one action and an antagonist for another.

Types of Muscle Tissue

Smooth Muscle

• Smooth muscle cells are elongated with tapered ends, lack striations, and have an undeveloped sarcoplasmic reticulum.

• Multiunit smooth muscle: Stimulated individually (e.g., blood vessels, iris).

• Visceral smooth muscle: Fibers occur in sheets (e.g., walls of hollow organs), stimulating concertedly and displaying rhythmicity for peristalsis.

Smooth Muscle Contraction

• Similarities with skeletal muscle contractions:

  • Involve actin and myosin interactions, stimulated by membrane impulses, rely on calcium ions and ATP.

• Key differences:

  • ACh and norepinephrine can stimulate/inhibit contraction, hormones may influence contractions, slower contraction and relaxation, longer lasting contractions.

Cardiac Muscle

• Cardiac muscle is exclusive to the heart, made of branching striated cells interlinked for synchronous contraction.

• Contraction mechanisms share similarities with skeletal/smooth muscle with essential variations:

  • Less developed sarcoplasmic reticulum, transverse tubules supply calcium for longer twitches.

  • Cardiac muscle is self-exciting and rhythmic due to intercalated discs for force transmission between cells.

Muscle Actions in the Body

• Different muscle groups, such as:

  • Muscles of the face and neck (e.g., temporalis, masseter for chewing)

  • Shoulder muscles (e.g., trapezius for stabilization)

  • Upper arm (e.g., biceps brachii for elbow flexion)

  • Forearm functions with flexion and extension actions among various muscles (e.g., brachioradialis, triceps brachii).

Summary of Muscle Groups

• Anterior and posterior muscle groups provide movement and stabilization across the body, from neck to limbs, all contributing to the body's kinesiology.

Figures and Visuals

• Reference diagrams and illustrations throughout the chapter for detailed anatomical structures of muscles, leverage mechanism, and distinctions among muscle tissues.