Overview of the Muscular System and Its Functions

Muscle Tissue

One of four primary tissue types for contraction.

Skeletal Muscles

Composed of skeletal muscle tissue; attached to bones.

Muscular System

Includes about 700 skeletal muscles in the body.

Muscle Functions

Move skeleton, maintain posture, generate heat.

Tendons

Fibrous tissue attaching muscle to bone.

Aponeurosis

Broad sheet connecting muscles to each other.

Epimysium

Connective tissue covering the entire muscle.

Perimysium

Divides muscle into fascicles; contains blood vessels.

Endomysium

Covers each muscle fiber; ties fibers together.

Muscle Fiber

Individual skeletal muscle cell; multinucleate structure.

Skeletal Muscle Fibers

Long, striated fibers with repeating patterns.

Sarcolemma

Plasma membrane surrounding muscle cell's cytoplasm.

Transverse Tubules

Network allowing impulses to reach muscle fiber interior.

Myofibrils

Cylinder-shaped structures causing muscle fiber contraction.

Sarcoplasmic Reticulum

Smooth ER surrounding myofibrils; stores calcium ions.

Terminal Cisternae

Expanded SR portions adjacent to T tubules.

Triad

Two terminal cisternae and one T tubule combination.

Sarcomeres

Smallest functional units of skeletal muscle fibers.

Myofilaments

Thick and thin filaments within myofibrils.

Striated Appearance

Pattern from arrangement of thick and thin filaments.

Voluntary Control

Skeletal muscles require CNS stimulation for contraction.

Blood Vessels

Extensive network supplying energy to skeletal muscles.

Muscle Contraction Heat

Generated by muscle contractions to maintain body temperature.

Z lines

Boundaries of each sarcomere in muscle fibers.

M line

Center of sarcomere connecting thick filaments.

A band

Darker region with thick filaments and overlap.

I band

Lighter region containing only thin filaments.

H band

Region with only thick filaments when relaxed.

Thin filaments

Actin strands with active sites for myosin.

Thick filaments

Composed of myosin molecules with heads and tails.

Sliding Filament Theory

Explains sarcomere contraction by filament sliding.

Cross-bridges

Connections formed by myosin heads binding to actin.

Contraction Cycle

Process of muscle contraction involving cross-bridge cycling.

Neuromuscular Junction

Connection between motor neuron and muscle fiber.

Acetylcholine (ACh)

Neurotransmitter released at the neuromuscular junction.

Synaptic cleft

Gap between axon terminal and muscle sarcolemma.

Tension

Force produced by muscle fiber contraction.

Muscle Twitch

Single stimulus-contraction-relaxation cycle in muscle.

Myogram

Graph showing tension development during a twitch.

Resting length

Initial length of muscle fiber before contraction.

Frequency of stimulation

Rate affecting calcium ion concentration in fibers.

Sarcomere shortening

Causes muscle fiber contraction and tension production.

Active sites

Locations on actin for myosin head binding.

Tropomyosin

Covers active sites on actin at rest.

Troponin

Holds tropomyosin in position on actin.

Compression

Push applied to an object by muscle.

Latent period

Initial phase of muscle twitch lasting about 2 msec.

Contraction phase

Period of maximum tension within about 15 msec.

Relaxation phase

Tension decreases as calcium levels drop, lasts 25 msec.

Summation

Addition of twitches for stronger muscle contraction.

Incomplete tetanus

Rapid contractions produce near-peak tension.

Complete tetanus

Continuous stimulation eliminates relaxation, maximum tension achieved.

Motor unit

Single motor neuron and all controlled muscle fibers.

Recruitment

Activation of more motor units for increased tension.

Muscle tone

Resting tension stabilizing bones and joints.

Atrophy

Muscle wasting due to lack of stimulation.

Isotonic contraction

Muscle length changes while tension remains constant.

Isometric contraction

Muscle length remains unchanged under tension.

Energy storage (CP)

Excess ATP converted to creatine phosphate for energy.

Creatine phosphokinase

Enzyme regulating ATP recharging from creatine phosphate.

Calcium ion concentration

High levels lead to complete tetanus.

Cross-bridge interaction

Binding of myosin and actin during contraction.

Passive muscle elongation

Muscle returns to length via passive forces.

Active mechanism

No active process exists for muscle elongation.

Skeletal muscle fibers

Controlled by motor neurons for movement.

Motor neuron control

Neurons can control hundreds to thousands of fibers.

Precise movements

Involve small motor units with few fibers.

Gross movements

Involve large motor units with many fibers.

Aerobic Metabolism

Requires oxygen, produces ATP in mitochondria.

ATP Production

95% of resting muscle ATP needs met.

End Products of Aerobic Metabolism

Produces ATP, water, and carbon dioxide.

Pyruvate in Citric Acid Cycle

About 15 ATP produced per pyruvate.

Glycolysis

Breaks glucose down to pyruvate in cytoplasm.

Anaerobic Process

Glycolysis can occur without oxygen.

ATP Yield from Glycolysis

Only yields 2 ATP per glucose.

Muscle Fatigue

Muscle fails to perform due to energy depletion.

Causes of Muscle Fatigue

Depletion of reserves or pH decline from H+.

Recovery Period

Restores muscle to pre-exertion conditions.

Oxygen Debt

Increased breathing to restore oxygen levels.

Lactate Conversion

Lactate is converted back to glucose.

Muscle Performance Measurement

Measured in force and endurance.

Fast-Twitch Fibers

Reach peak tension in 0.01 sec or less.

Characteristics of Fast Fibers

Large diameter, few mitochondria, fatigue rapidly.

Slow-Twitch Fibers

Contract slowly, fatigue resistant, high oxygen supply.

Capillary Network

Oxygen supply is greater due to this.

Myoglobin Function

Stores oxygen for muscle use.

Muscle Fiber Type Variation

Percentage varies among skeletal muscles.

White Muscles

Dominated by fast fibers, appear pale.

Red Muscles

Dominated by slow fibers, appear reddish.

Muscle Conditioning

Training increases power and endurance.

Anaerobic Endurance

Supported by glycolysis and existing ATP.

Aerobic Endurance

Supported by mitochondrial activity during activity.

Cardiac Muscle Tissue

Muscle found only in the heart.

Cardiac Muscle Cells

Small cells with one central nucleus.

Intercalated Discs

Connect cardiac muscle cells for communication.

Gap Junctions

Allow rapid communication between cardiac cells.

Automaticity

Ability to contract without neural stimulation.

Pacemaker Cells

Specialized cells that regulate heart contraction timing.

Calcium Ion Role

Increases permeability for contraction in cardiac cells.

Aerobic Metabolism

Cardiac muscle cells primarily rely on oxygen.