Chapter 8 (PART ONE)
Chapter 08 - Muscular System
8.1: Introduction to the Muscular System
Muscles are considered organs and are comprised of:
Skeletal muscle tissue
Nervous tissue
Blood vessels
Connective tissues
Functions of muscles include generating force for movement.
Examples of muscle actions:
Walking
Breathing
Pumping blood
Moving food through the digestive tract
There are three types of muscle tissue in the body:
Skeletal muscle - Voluntary and striated muscles responsible for locomotion.
Smooth muscle - Involuntary muscles found in walls of hollow organs.
Cardiac muscle - Involuntary and striated muscles found in the heart.
8.2: Structure of a Skeletal Muscle
The human body contains over 600 skeletal muscles.
Connective tissue coverings over muscles:
Fascia: layers of dense connective tissue that separate and hold each muscle in position.
Fascia extends beyond the muscle ends to form tendons that attach to the periosteum of bones.
Aponeuroses: Broad sheets of connective tissue that may connect muscles to one another, such as in the palms (palmar aponeurosis).
Connective Tissue Layers
Epimysium: Connective tissue surrounding the entire skeletal muscle.
Perimysium: Connective tissue surrounding groups of muscle fibers, forming bundles called fascicles.
Endomysium: Connective tissue layer covering each muscle fiber (cell).
Major Components of a Skeletal Muscle Fiber
Muscle fiber (cell) features:
Long, cylindrical shape
Covered by a cell membrane known as the sarcolemma.
Contains cytoplasm called sarcoplasm with inclusions such as:
Mitochondria for energy processing
Multiple nuclei for regulation of protein synthesis.
The sarcoplasm contains myofibrils, which are involved in muscle contraction.
Myofibrils and Sarcomeres
Myofibrils: Subunits that make up the skeletal muscle fibers; composed of repeated units called sarcomeres.
A sarcomere extends from one Z line to the next.
Striations in muscle fibers arise from the alternating pattern of light (I bands) and dark (A bands) bands:
I bands: Composed mainly of actin filaments anchored to the Z lines.
A bands: Contain both thick (myosin) and thin (actin) filaments.
H zone: Central area of the A band with only myosin filaments.
M line: Proteins in the center of the H zone that hold myosin filaments together.
8.3: Skeletal Muscle Contraction
Muscle contraction results in the shortening of sarcomeres and pulling of muscles against attachments.
The process involves:
Binding of myosin to actin to exert forces.
Overlapping of actin and myosin filaments.
Shortening of muscle fibers causing entire muscle contraction.
Myosin and Actin in Contraction
Myosin: Thick filament proteins with globular heads that project outward.
Actin: Thin filament proteins that form a double helix structure with myosin binding sites.
Tropomyosin and Troponin: Proteins associated with actin that regulate contraction by blocking interaction sites when not stimulated.
Sliding Filament Model
During contraction, a myosin head binds to an actin site forming a cross-bridge:
The myosin head bends, pulling the actin filament toward the center of the sarcomere.
ATP is converted to ADP to provide energy for the power stroke.
After death, increased calcium permeability results in partial muscle contraction known as rigor mortis.
Neuromuscular Junction
Neuromuscular Junction: Synapse where a motor neuron connects with a skeletal muscle fiber.
Muscle contraction requires stimulation by a motor neuron.
Motor neurons communicate with muscle fibers through neurotransmitters (primarily acetylcholine).
At the junction, the muscle fiber has a motor end plate with specialized receptors for neurotransmitters.
When an impulse reaches the motor neuron, acetylcholine is released into the synaptic cleft, activating muscle contraction.
Stimulus for Contraction
Muscle fibers contract when stimulated by acetylcholine:
Binding of the neurotransmitter prompts sarcoplasmic reticulum to release calcium.
Calcium interacts with troponin and tropomyosin, exposing myosin binding sites on actin filaments.
The contraction continues as long as stimulation from the motor neuron persists.
Events Leading to Relaxation
After stimulation, muscle relaxation occurs through:
Breakdown of acetylcholine by acetylcholinesterase.
Active transport of calcium back to sarcoplasmic reticulum using ATP.
ATP binding to myosin heads breaking the cross-bridge.
Restoration of original positions of actin and myosin filaments leading to relaxation.
Major Events of Muscle Contraction and Relaxation
Muscle Fiber Contraction:
Impulse travels down motor neuron axon.
Motor neuron releases acetylcholine.
ACh binds to receptors on muscle fiber membrane, stimulating it.
Impulse travels along sarcolemma and T-tubules.
Impulse reaches sarcoplasmic reticulum, opening calcium channels.
Calcium ions enter muscle cytosol, binding to troponin.
Tropomyosin moves to expose actin binding sites.
Cross-bridges form linking actin and myosin.
Filaments pull towards the center of sarcomere.
Contraction occurs, exerting force on attachments.
Muscle Fiber Relaxation:
ACh decomposed; muscle fiber membrane is unstimulated.
Calcium ions reabsorbed into sarcoplasmic reticulum.
ATP breaks the link between actin and myosin.
Myosin heads are cocked for another contraction cycle.
Muscle fiber remains in a relaxed state ready for stimulation again.
Energy Sources for Contraction
Energy for muscle contraction mainly comes from ATP, which requires regeneration due to its limited supply.
Creatine Phosphate: Plays a key role in regenerating ATP from ADP and phosphate when ATP levels are sufficient.
Initial sources of ATP are critical, and as demand increases, muscles switch to cellular respiration to maintain ATP levels.
Oxygen Supply and Cellular Respiration
Glycolysis: Anaerobic process; occurs in cytoplasm yielding 2 ATP per glucose.
Aerobic Respiration: Complete glucose breakdown; occurs in mitochondria yielding 28 ATP per glucose.
Hemoglobin: Carries oxygen to muscle tissue; Myoglobin: Stores oxygen in muscle tissue, facilitating aerobic respiration.
Oxygen Debt
Oxygen Debt occurs when oxygen supply is insufficient, particularly during strenuous exercise.
Anaerobic respiration occurs, producing lactate, which must be converted back to glucose post-exercise.
Replenishment of oxygen debt may take several hours and is crucial for recovery post-exercise.
Muscle Metabolism
Low to moderate intensity exercise: Sufficient oxygen for aerobic respiration.
High intensity exercise: Insufficient oxygen, leading to lactate formation.
ATP Production: Varies based on exercise intensity:
Aerobic exercise: 30 ATP/glucose.
Anaerobic exercise: 2 ATP/glucose.
Waste products differ: CO2 vs. lactic acid.
Heat Production and Muscle Fatigue
Less than half of the energy from cellular respiration forms ATP; excess energy becomes heat.
Heat helps maintain body temperature while muscle fatigue results from electrolyte imbalances and ATP depletion.
Muscle cramps may occur due to extracellular fluid changes affecting fiber stimulation.
Types of Muscle Fibers
Fast Fibers (White Muscles):
Majority of muscle fibers.
Rapid contraction, but fatigue quickly.
Less mitochondria, reliant on anaerobic metabolism.
Slow Fibers (Red Muscles):
Smaller in diameter; more resistant to fatigue.
Contain more mitochondria and capillaries; suited for endurance activities.
Exercise and Muscle Use
Hypertrophy: Muscle enlargement from repeated exercise.
Atrophy: Muscle size reduction due to disuse.
Effects of exercise depend on intensity:
Low intensity increases mitochondrial density but maintains size/strength.
High intensity increases actin/myosin filament numbers allowing stronger contractions.
Recruitment of Motor Units
A motor unit is a motor neuron and all muscle fibers it controls.
Motor Unit Recruitment increases strength of contraction by activating more fibers.
Maximum tension occurs when all motor units are recruited.
Types of Contractions
Isotonic Contraction: Muscle shortens during contraction.
Associated with movement (e.g., lifting weight); tension remains constant.
Isometric Contraction: Muscle generates force without changing in length; stabilizes positions.