The Muscular System

Myology

Study of muscles

Skeletal Muscles

Voluntary, striated muscles attached to bones, enabling movement.

Cardiac Muscles

Involuntary, striated muscles found in the heart, responsible for pumping blood automatically.

Smooth Muscles

Involuntary, non-striated muscles located in organs, controlling functions like digestion.

Functions of Muscles

• Movement

• Posture Maintenance

• Control of Organ Size

• Internal Movement

• Heat Production

Movement

Muscles contract to facilitate movement of the body and its parts.

Posture Maintenance

Muscles help maintain body posture and stability.

Control of Organ Size

Muscles regulate the size of organs such as blood vessels.

Internal Movement

Muscles move within the body (food thru the digestive tract).

Heat Production

Muscles contractions generate heat, helping to maintain body temperature.

Muscle Fibers

These are long, cylindrical cells that make up muscle tissue, responsible for generating movement in the body and internal organs.

Epimysium

Layer of connective tissue that covers the whole muscle.

Perimysium

Layer of connective tissue that wraps small bundles called fascicles.

Endomysium

Layer of connective tissue that surrounds each fiber.

Tendons

It connects muscles to the bones.

Fascicles

Bundle of muscle fibers grouped together in parallel within the perimysium.

Myofibril

Long, cylindrical structure within muscle cells; responsible for muscle contraction.

Myofilaments

Protein filaments composed of myosin (thick) and actin (thin); contractile units.

Sarcolemma

It acts as a plasma membrane wrapped in each muscle fiber.

T-tubules

These are tunnels that transmit electrical signals.

Sarcoplasm

Cell fluid

Nuclei & Mitochondria

Creates energy

Myoglobin

It stores oxygen, similar to golgi complex.

Sarcoplasmic Reticulum

It stores calcium.

Sarcomeres

These are repeated units that creates striations in the muscle fibers; also helps in contraction.

Z-discs

It marks the edges of each sarcomere.

A band

It is the dark area with thick filaments.

H zone

It is located in the center where all are thick filaments.

I band

It is the light area with only thin filaments.

Motor Neuron

This is where an electrical signal originates, needed for a muscle to contract.

Motor Unit

Consists of one motor neuron and all the muscle fibers it controls.

Neuromuscular Junction (NMJ)

This is where the nerve and muscle meet. It includes the nerve endings and the part of the muscle membrane they connect with.

Synaptic End Bulb

It is the terminal end of a motor neuron that contains synaptic vessicles filled with ACh.

Motor End Plate

It is where a motor neuron’s axon terminal communicates with a muscle fiber, initiating muscle contraction.

Contraction & Relaxation of Skeletal Muscle (NMJ)

1. Nerve releases acetylcholine (ACh)

2. ACh crossed synapse and attach itself to the muscle, starting a muscle action potential

3. Sodium (Na+) enters the muscle, triggering movement

4. Acetylcholinesterase breaks down ACh to stop the signal.

What happens when muscles contract?

Filaments slide past each other, making the muscle fiber shorter.

Calcium (Ca2+)

It turns the contraction on (increases) and off (drops).

ATP

It powers the movement.

Contraction Cycle

1. Split ATP (Myosin uses energy from ATP)

2. Attach (Myosin connects to actin)

3. Power Stroke - Myosin pulls actin inward, shortening the muscle

4. Detach & Reset - Myosin lets go, uses new ATP, and reset to repeat the cycle

Ways Muscles Get Energy (ATP)

• Creatine Phosphate

• Anaerobic Glycolysis

• Aerobic Respiration

Creatine Phosphate

It is a molecule stored in muscle cells that acts as a readily available source of energy, rapidly regenerating ATP during short muscle contractions, lasting for about 15 seconds.

Anaerobic Glycolysis

It breaks down glucose without oxygen, thereby, producing ATP that lasts for about 2 minutes.

Aerobic Respiration

Most efficient way to produce ATP, utilizing oxygen to break down glucose, fatty acids, or amino acids. It can sustain muscle activity for extended periods.

Muscle Fatigue

It happens when muscles can't contract well.

Recovery Oxygen Uptake

It occurs when the body uses more oxygen to recover especially after exercise.

Twitch

It is an quick muscle contraction from one nerve signal.

Myogram

It is a graph showing the twitch.

Latent Period

A phase shown in the myogram wherein the muscle is stimulated; no visible muscle shortening or tension.

Contraction Phase

A phase in the myogram where the muscle begins to shorten as cross-bridges form between the actin and myosin filaments, leading to an increase in muscle tension.

Relaxation Phase

Phase wherein the muscle returns to its resting length as calcium ions are pumped back to the sarcoplasmic reticulum and cross-bridges detach. Muscle tension decreases.

Wave Summation

It happens when another signal comes before the muscle relaxes, causing a stronger contraction.

Incomplete Tetanus

Occurs when repeated signals causing small relaxations between muscle twitch.

Complete Tetanus

Occurs when repeated signals disrupts muscle relaxation, leading to a smooth, strong contractions.

Motor Unit Recruitment

It means activating more muscle units to increase strength.

SO Fibers (slow oxidative)

It is a type of muscle fiber that uses aerobic metabolism (oxygen) to produce low power contractions over long periods, slow to fatigue.

FOG Fibers (fast oxidative-glycolytic)

It is a type of muscle fiber that uses aerobic metabolism to generate ATP but produce higher tension contractions for mixed use.

FG Fibers (fast glycolytic)

It is a type of muscle fiber that uses anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.

Muscle Fibers in Order

Muscle use these fiber in order, depending on how much force is needed.

SO —> FOG —> FG

More FG (fast glycolytic) Fibers

Better at sprinting or lifting

More SO (slow oxidative) Fibers

Better at endurance activities like running

Strength Training

It increases the size and strength of FG Fibers by building more muscle filaments.

Intercalated Discs

These are specialized structures found in cardiac muscle tissue that connects cardiomyocytes. They are crucial for the coordinated contraction of the heart, allowing for rapid and mechanical communication between cells.

Cardiomyocytes

These are muscle cells that make up the heart, responsible for it's rhythmic contractions and pumping blood throughout the body.

Autorhythmic Fibers

These are specialized cardiac muscle cells that can generate their own electrical impulses (mainly from aerobic respiration), setting the pace of heart contractions.

Visceral (single-unit)

These fibers work together as one unit; they can be found in organs like the stomach.

Multiunit

These fibers work independently; they are found in big vessels such as the lungs and eyes.

Muscle Tone

It refers to the continuous and passive partial contraction of the smooth muscles.

Sarcopenia

It is characterized by the degenerative loss of skeletal muscle mass, quality, and strength that occurs with aging and immobility.

How muscles move bones?

Muscles pull on tendons, and tendons pull on bones to create movements.

Origin

It refers to the region where the muscle attaches to a stationary bone.

Insertion

It refers to the region where the muscle attaches to a movable bone.

Agonist/ Prime Mover

It is the main muscle doing the action.

Antagonist

It is the muscle that does the opposite action of the agonist.

Synergist

It is the muscle that helps the prime mover in doing the action.

Fixator

It is the muscle that holds the origin steady for better movement.