Muscular System Notes
Chapter 08: Muscular System
8.1 Introduction to the Muscular System
Definition of Muscles: Organs that generate force to facilitate various types of movement.
Examples of Muscle Actions:
Walking
Breathing
Pumping blood
Moving food through the digestive tract
Types of Muscle Tissue: There are three types of muscle tissue:
Skeletal Muscle: Responsible for voluntary movements.
Smooth Muscle: Involuntary muscle found in organs.
Cardiac Muscle: Involuntary muscle found in the heart.
8.2 Structure of a Skeletal Muscle
Number of Skeletal Muscles: The human body contains over 600 skeletal muscles.
Connective Tissue Coverings:
Fascia: Layers of dense connective tissue that surround and separate each muscle.
Tendons: Extensions of the fascia that connect muscles to bones, fused to the periosteum.
Aponeuroses: Broad sheets of connective tissue that can connect muscles to skin or other muscles.
Connective Tissue Coverings Details
Epimysium: The outer layer of connective tissue surrounding each skeletal muscle.
Perimysium:
Extends inward from the epimysium.
Surrounds bundles of skeletal muscle fibers called fascicles.
Endomysium: Connective tissue layer that covers each muscle cell (fiber).
8.3 Skeletal Muscle Fibers
Muscle Fiber Definition: A single, cylindrical muscle cell.
Response to Stimulation: Muscle fibers exert a pulling force upon stimulation.
Cell Membrane: The membrane of a muscle fiber is called the sarcolemma.
Cytoplasm: Known as sarcoplasm, contains many mitochondria and nuclei.
Myofibrils: Present in the sarcoplasm, these structures are crucial for muscle contraction.
Thick Filaments: Composed of the protein myosin.
Thin Filaments: Mainly composed of the protein actin, also contains troponin and tropomyosin.
Striations: The organization of filaments into bands produces visible striations.
Myofibrils and Sarcomeres
Sarcomeres: The functional units of myofibrils, extending from one Z line to another.
Striations: Comprised of light (I bands) and dark (A bands) bands.
I Bands: Composed of actin filaments anchored to Z lines.
A Bands: Composed of overlapping thick (myosin) and thin (actin) filaments.
H Zone: Central area of the A band with only myosin filaments.
M Line: Central line holding myosin filaments in place.
8.4 The Sarcoplasm of a Skeletal Muscle Fiber
Sarcoplasmic Reticulum (SR): A network of membranous channels beneath the sarcolemma, associated with transverse (T) tubules.
T Tubules: Extensions of the sarcolemma that are open to the exterior of the muscle fiber; lie between two cisternae of the SR.
Function: The SR and T tubules activate the muscle contraction mechanism during stimulation.
8.5 Neuromuscular Junction
Motor Neuron Connection: Skeletal muscle fibers contract only when stimulated by a motor neuron.
Synapse:
Functional connection between a neuron and a muscle fiber.
Neurotransmitters: Chemicals released to communicate from the neuron to the muscle fiber.
Neuromuscular Junction: Synapse where a motor neuron communicates with a muscle fiber.
Mechanism of Neuromuscular Junction
Synaptic Vesicles: Located in the cytoplasm of the motor neuron, containing neurotransmitters.
Motor End Plate: Specialized region of the muscle fiber’s sarcolemma, containing receptors for neurotransmitters.
Contraction Process:
Electrical impulse releases neurotransmitter into the synaptic cleft.
Neurotransmitters diffuse across the cleft, bind to the motor end plate, and stimulate contraction.
8.6 Skeletal Muscle Contraction
Process: Involves several events that lead to the shortening of sarcomeres and the muscle pulling against its attachments.
Binding Mechanism: Myosin binds to actin, increasing overlap between filaments.
Outcome: Shortening of muscle fibers leads to movement.
Role of Myosin and Actin
Myosin Characteristics: Composed of twisted strands with protruding globular heads forming thick filaments.
Actin Characteristics:
Formed of globular proteins in a double helix structure with binding sites for myosin.
Associated proteins: Troponin and Tropomyosin.
Sliding Filament Model of Muscle Contraction
Cross-bridge Formation: Myosin heads attach to actin binding sites to form cross-bridges.
Power Stroke: Myosin heads bend, pulling actin towards the center of the sarcomere.
Cycle Overview: Heads release and attach to the next binding site, facilitating contraction and shortening of the sarcomere.
ATP Requirement: The process requires ATP; ATPase enzyme provides energy for cross-bridge cycling.
Rigor Mortis: The phenomenon post-mortem where muscles partially contract due to high calcium permeability and ATP depletion.
8.7 Stimulus for Contraction
Acetylcholine: The primary neurotransmitter involved at the neuromuscular junction.
Production and Storage: Created in the motor neuron and stored in synaptic vesicles.
Release Mechanism: Triggered by an impulse, it leads to muscle fiber stimulation.
Calcium Release: Upon stimulation, the SR releases calcium into the muscle cytosol, enabling contraction.
Muscle Relaxation Process
Decomposition of Acetylcholine: By the enzyme acetylcholinesterase; halts muscle stimulation.
Calcium Reabsorption: Actively transported back into the SR, requiring ATP.
Myosin-Actin Linkage Resolution: ATP enables the breaking of cross-bridge linkages, allowing the muscle to relax.
8.8 Major Events of Muscle Contraction and Relaxation
Contraction Events:
Impulse travels down the motor neuron axon.
Release of acetylcholine.
Acetylcholine binds to receptors in muscle fiber, stimulating it.
Sarcolemma is stimulated, and impulses travel through T tubules.
Impulse triggers SR, opening calcium channels.
Calcium ions trigger interactions leading to contraction.
Cross-bridge formation occurs.
Muscles pull on attachments.
Relaxation Events:
Acetylcholinesterase decomposes acetylcholine, ceasing stimulation.
Calcium ions are returned to the SR.
ATP breaks down cross-bridges.
Muscle fiber remains relaxed until another stimulus occurs.
8.9 Energy Sources for Contraction
ATP Role: The primary source of energy limited in supply, needing regeneration.
Creatine Phosphate: Serves to regenerate ATP quickly in muscles.
Synthesized by the enzyme creatine phosphokinase when ATP is plentiful.
Cellular Respiration: Bypassed when creatine phosphate supply is low; the body must rely on aerobic respiration.
Oxygen Supply and Cellular Respiration
Glycolysis: The first phase of cellular respiration, anaerobic, yielding 2 ATP from glucose.
Aerobic Respiration: Complete glucose breakdown yielding 28 ATP, requires oxygen.
Oxygen Transport:
Hemoglobin in red blood cells carries oxygen.
Myoglobin in muscles stores oxygen, enhancing aerobic conditions.
8.10 Oxygen Debt
Definition: The amount of oxygen required for recovering from anaerobic metabolism, particularly to convert lactic acid back to glucose.
Development: Lactic acid accumulates during strenuous exercise; lactate is transported to the liver for conversion back to glucose using ATP.
Post-Exercise Recovery: The period required to repay oxygen debt may last hours; training enhances muscle energy production.
8.11 Muscle Metabolism Conditions
Low to Moderate Intensity: Sufficient oxygen supports cellular respiration via aerobic pathways.
High Intensity: Oxygen supply may fall short, relying on anaerobic processes yielding lactic acid and less ATP.
8.12 Heat Production and Muscle Fatigue
Energy Release: Less than half of energy from cellular respiration is converted to ATP; the rest converts to heat, helping maintain body temperature.
Fatigue: Results from ATP depletion; also related to electrolyte imbalances, decreased pH from lactic acid build-up, leading to muscle cramps.
8.13 Types of Muscle Fibers
Fast Fibers:
Majority in skeletal muscle.
Characteristics: Large diameter, rapid force production, fatigue quickly, fewer mitochondria (anaerobic), used for short bursts of activity.
Slow Fibers:
Smaller diameter, resistant to fatigue, used in endurance activities with more mitochondria and aerobic metabolism for sustained energy.
8.14 Exercise Effects on Muscles
Hypertrophy: Muscle enlargement due to repeated exercise.
Atrophy: Muscle size and strength decrease with disuse.
Response Variability:
Low-intensity exercise promotes endurance through increased mitochondria and capillaries.
High-intensity exercise increases muscle size and strength via increased filament content.
8.15 Muscular Responses
Single Fiber Studies: Used to analyze muscle functions through stimuli of varying strengths.
Threshold Stimulus: The minimum strength required to activate muscle fiber and contract.
Recording of Muscle Contraction
Twitch Overview: A cycle of contraction and relaxation triggered by a single impulse is referred to as a twitch.
Myogram: The graphical recording of muscle contraction.
Latent Period: A short delay before contraction begins, followed by contraction and relaxation phases.
All-or-None Response: Each twitch generates the same force regardless of strength of stimulus above threshold.
8.16 Summation and Motor Unit Recruitment
Summation: A series of stimuli leading to combined force of contractions, leading to greater strength beyond single twitch.
Tetany:
Partial Tetany: Incomplete relaxation due to high frequency stimuli.
Complete Tetanic Contraction: No relaxation between contractions, achievable only in a lab.
Motor Units: Composed of a motor neuron and the muscle fibers it controls; all fibers contract simultaneously when the motor neuron fires.
Motor Unit Recruitment: Increased recruitment leads to stronger contractions until maximum tension is achieved.
8.17 Sustained Contractions and Muscle Tone
Sustained Contractions: Allows for daily activities, produced via summation and recruitment.
Muscle Tone: A resting state of slight contraction of several motor units maintaining posture and readiness for movement.
8.18 Types of Contractions
Isotonic Contraction:
Muscle tension remains constant while shortening occurs, e.g., actual lifting of weights.
Isometric Contraction:
Muscle generates force without shortening, e.g., holding a weight in place against gravity.
8.19 Smooth Muscle
Characteristics: Elongated cells lacking striations, with undeveloped sarcoplasmic reticulum.
Types of Smooth Muscle:
Multiunit Smooth Muscle: Individually functioning fibers, e.g., in blood vessels.
Visceral Smooth Muscle: Sheets of muscle fibers functioning rhythmically as seen in hollow organs.
Smooth Muscle Contraction
Similarities to Skeletal Muscle: React in similar mechanisms involving actin and myosin, calcium influx, and ATP use.
Differences from Skeletal Muscle:
Dual stimulation (ACh and norepinephrine), slower contraction, sustained contraction with less ATP, and adaptability to length changes without tension variations.
8.20 Cardiac Muscle
Location: Exclusive to the heart.
Structure: Striated, branching cells interconnected in networks, facilitating rhythmic contractions.
Contraction Mechanisms: Utilizes calcium provided by transverse tubules; cross-border communication occurs through intercalated discs.
8.21 Types of Muscle Tissue Comparison
Skeletal Muscle: Striated, voluntary control, rapid contraction.
Smooth Muscle: Non-striated, involuntary control, slower, more sustained contractions.
Cardiac Muscle: Striated, involuntary, rhythmic contractions.
8.22 Skeletal Muscle Actions
Origin and Insertion:
Origin: Less movable end of muscle.
Insertion: More movable end of muscle.
Example: Biceps brachii, named for its two origins; involved in elbow flexion.
8.23 Muscle Relationships and Movements
Types of Movements:
Flexion: Decreasing angle at a joint (e.g., bending the arm).
Extension: Increasing angle at a joint (e.g., straightening the arm).
Muscle Functional Roles:
Agonist: Main muscle performing the action.
Synergists: Assist the agonist.
Antagonists: Oppose the action of agonists; relationships can change based on movement type.
8.24 Major Skeletal Muscles
Naming Muscles: Based on size, shape, location, action, number of attachments, or fiber direction.
Examples include pectoralis major, deltoid, and biceps brachii.
8.25 Muscles of Facial Expression
Role: Connect to bones and skin to enable various facial expressions.
Key Muscles:
Epicranius: Elevates the eyebrow.
Orbicularis oculi: Closes the eye.
Zygomaticus: Elevates the corner of the mouth.
8.26 Muscles of Mastication
Role: Facilitate chewing by moving the mandible.
Muscles:
Masseter: Elevates mandible.
Temporalis: Elevates and retracts mandible.
8.27 Muscles that Move the Head
Key Muscles:
Sternocleidomastoid: Flexes and rotates the head.
Splenius capitis, semispinalis capitis, and scalenes assist in head movement.
8.28 Muscles that Move the Pectoral Girdle
Function: Whole group involves scapula movement.
Major muscles include trapezius, rhomboid major, and serratus anterior.
8.29 Muscles that Move the Arm
Function: Connect arm's motion to the torso.
Examples:
Flexors: Pectoralis major, coracobrachialis.
Extensors: Teres major, latissimus dorsi.
8.30 Muscles that Move the Forearm
Flexors: Biceps brachii, brachialis; Extensor: Triceps brachii.
Rotators: Supinator and pronator teres/quadratus affect forearm movement.
8.31 Muscles that Move the Hand
Movements: Include flexion, extension, abduction, adduction, and include several flexor and extensor muscles.
8.32 Muscles of the Abdominal Wall
Support Role: Connect rib cage to the pelvic girdle through broad muscles, increasing abdominal pressure.
Core Muscles: External and internal oblique, transverse abdominis, rectus abdominis.
8.33 Muscles of the Pelvic Floor
Structure: Composed of two muscular sheets providing pelvic support.
Deeper pelvic diaphragm: Levator ani, coccygeus.
Superficial urogenital diaphragm: Transverse perineal and bulbospongiosus muscles.
8.34 Muscles that Move the Thigh
Classification: Muscles are grouped as anterior (hip flexors), medial (adductors), or posterior (hip extensors).
8.35 Muscles that Move the Leg
Role: Connect femur to tibia/fibula; flexors (hamstring group) and extensors (quadriceps).
8.36 Muscles that Move the Foot
Movements: Includes dorsiflexion, plantar flexion, inversion, and eversion, linked to various leg muscles.