Human Phys Chapter 13: Muscle

13.1 Skeletal Muscle

What is the function of skeletal muscles, and how do they attach to bones?

Skeletal muscles move and position the skeleton. They usually attach to bones through collagen-rich tendons.

Example: Biceps pulls on its tendon to move your forearm.

Memory: Tendon = Rope connecting muscle to bone.

What is the difference between a muscle's origin and insertion?

• Origin = More stationary attachment, usually closer to the trunk.

• Insertion = More movable attachment, usually farther from the trunk.

Memory: Origin = Stationary, Insertion = In motion.

What is the difference between flexion and extension?

• Flexion = Brings bones closer together (bending).

• Extension = Moves bones farther apart (straightening).

Example: Biceps curls = Flexion; triceps straightens elbow = Extension.

What are antagonistic muscle groups, and why are they necessary?

Muscle pairs that produce opposite movements because muscles can pull, not push.

Example: Biceps flexes the elbow, triceps extends it.

Memory: Opposite muscles = Opposite movements.

Skeletal Muscles Are Composed of Muscle Fibers

What is the organization of skeletal muscle from largest to smallest?

Whole muscle → Fascicle → Muscle fiber → Myofibril → Sarcomere → Myofilaments.

Memory: Big muscle → Bundles → Cells → Contractile structures.

What is a skeletal muscle fiber, and what are satellite cells?

A muscle fiber is one long, multinucleated muscle cell.

Satellite cells are stem cells that help with muscle growth and repair.

Memory: Muscle fiber = One giant cell. Satellite cells = Repair crew.

What are the muscle-specific names for common cell structures?

- Cell membrane = Sarcolemma.

- Cytoplasm = Sarcoplasm.

- Modified ER = Sarcoplasmic reticulum.

Memory: Sarco = Muscle.

What are myofibrils and sarcomeres?

Myofibrils are long contractile structures containing repeating sarcomeres.

Sarcomeres are the basic contractile units of striated muscle, made from thick and thin filaments.

Example: Sarcomeres shorten during contraction.

How does Ca2+ control muscle contraction?

- Ca2+ OUT of SR → Muscle contracts 💪

- Ca2+ IN to SR → Muscle relaxes 😴

Memory trick: Calcium comes OUT to WORK, goes BACK IN to REST.

What are T-tubules, and why are they important?

Extensions of the sarcolemma that carry action potentials deep into muscle fibers.

Example: They allow the inner parts of a muscle fiber to be activated rapidly.

Memory: T-tubules = Electrical hallways.

What is a triad?

One T-tubule between two terminal cisternae of the SR.

Function: Allows electrical signals to rapidly trigger calcium release.

Memory: Triad = 3 parts (SR + T-tubule + SR).

Myofibrils Are Muscle Fiber Contractile Structures

What is the difference between actin and myosin?

- Actin = Thin filament.

- Myosin = Thick filament with heads that grab and pull actin.

Together, they produce muscle contraction.

🧠 Analogy:

Actin = Rope, Myosin = Hands pulling the rope.

Memory trick: Myosin MOVES actin.

What are myosin crossbridges, and what does ATP do?

Crossbridges form when myosin heads attach to actin.

- Myosin grabs actin.

- Uses energy from ATP to power repeated pulling cycles.

- Actin moves toward the center of the sarcomere.

🧠 Analogy:

Like rowing a boat. You grab, pull, release, and repeat.

Memory trick: Myosin = Motor, ATP = Fuel.

What's the difference between G-actin and F-actin?

- G-actin = One round actin protein.

- F-actin = A long chain of G-actin proteins.

Two F-actin chains twist together to help form a thin filament.

🧠 Analogy:

G-actin = One bead. F-actin = A necklace of beads.

What are the five major structures of a sarcomere?

- Z disk = Ends of sarcomere.

- I band = Actin only.

- A band = Entire length of myosin.

- H zone = Myosin only.

- M line = Middle, holds myosin in place.

🧠 Memory tricks:

Z = Zone boundary.

I = thIn filament only.

A = All of myosin.

H = Heavy filament only.

M = Middle.

What changes during sarcomere contraction?

- I band → SHORTER

- H zone → SHORTER

- A band → SAME

- Z disks → CLOSER together

Actin and myosin stay the same length; they slide past each other.

🧠 Analogy:

Two ropes sliding toward each other. Neither rope actually shrinks.

Memory trick: I and H SHRINK, A ALWAYS stays.

What are titin and nebulin?

- Titin = Stretchy protein that stabilizes myosin and helps muscles return to resting length.

- Nebulin = Stiff protein that keeps actin lined up.

🧠 Analogy:

Titin = Rubber band. Nebulin = Ruler.

Memory trick: Titin = Spring, Nebulin = Alignment.

Why are skeletal muscles called striated?

Because they look striped under a microscope due to the repeating arrangement of actin and myosin.

Memory trick: Striated = Striped.

Muscle Contraction Creates Force

What are muscle tension, load, contraction, and relaxation?

- Tension = Force produced by muscle.

- Load = Weight or force resisting the muscle.

- Contraction = Muscle uses ATP to create tension.

- Relaxation = Muscle releases tension.

🧠 Analogy: Tug-of-war!

Your pulling force = Tension.

Opposing team's force = Load.

Memory trick:

Contraction = WORK,

Relaxation = REST.

What are the major steps of muscle contraction?

1. NMJ: Neuron releases ACh → Muscle action potential.

2. E-C coupling: Muscle action potential → Ca2+ released from SR.

3. Calcium signal: Ca2+ allows myosin to bind actin.

4. Contraction: Myosin pulls actin → Muscle produces tension.

5. Relaxation: Ca2+ returns to SR → Muscle relaxes.

🧠 Analogy:

Doorbell rings → Workers come out → Work gets done → Workers go home.

Memory trick:

ACh → Electricity → Ca2+ OUT → PULL → Ca2+ IN → REST.

What are excitation-contraction coupling and a muscle twitch?

- E-C coupling = Electrical signal triggers Ca2+ release, starting contraction.

- Muscle twitch = One contraction followed by relaxation.

🧠 Analogy:

E-C coupling = Flipping the power switch.

Muscle twitch = One quick pull and release.

Memory trick: Electricity → Calcium → Contraction.

Actin and Myosin Slide Past Each Other during Contraction

What is sliding filament theory?

During muscle contraction, myosin pulls actin toward the middle of the sarcomere.

- Actin slides toward the M line.

- Z disks move closer.

- Sarcomere gets shorter.

- Actin and myosin do NOT shrink.

🧠 Analogy:

Pulling two ropes toward the middle. The ropes stay the same length but move closer together.

Memory trick: SLIDE, DON'T SHRINK.

What determines how much tension a muscle fiber produces?

The number of high-force crossbridges between actin and myosin.

- More high-force crossbridges → MORE tension 💪

- Fewer high-force crossbridges → LESS tension.

🧠 Analogy:

Tug-of-war! More people pulling = More force.

Memory trick: MORE PULLERS = MORE POWER.

Myosin Crossbridges Move Actin Filaments

What is the power stroke?

The power stroke is when a myosin head swivels and pulls actin toward the center of the sarcomere.

- Myosin attaches to actin.

- Myosin head swivels → Actin moves inward.

- The process repeats during contraction.

🧠 Analogy:

Pulling a rope toward yourself, hand over hand.

Memory trick: POWER STROKE = THE PULL.

What does myosin ATPase do?

Myosin ATPase breaks down ATP into ADP + Pi, releasing energy that prepares the myosin head to pull.

- ATP breaks down → Energy released.

- Myosin stores energy → Head becomes cocked.

- Power stroke → Stored energy moves actin.

🧠 Analogy:

Pulling back a slingshot and then releasing it.

Memory trick: ATP = CHARGE, POWER STROKE = FIRE.

Why don't all myosin heads release actin simultaneously?

Because muscle needs to maintain tension while contraction occurs.

Myosin heads take turns grabbing, pulling, and releasing actin.

🧠 Analogy:

People climbing a rope don't let go with both hands at once, or they'd fall.

Memory trick: TAKE TURNS = KEEP PULLING.

Calcium Signals Initiate Contraction

What do troponin and tropomyosin do?

• Tropomyosin = BLOCKS myosin-binding sites on actin.

• Troponin = MOVES tropomyosin when Ca2+ binds.

• Ca2+ = Turns contraction ON by binding troponin C.

🧠 Analogy:

Tropomyosin = Locked door, troponin = Door controller, calcium = Key.

Memory trick: Tropomyosin BLOCKS, Troponin MOVES, Calcium UNLOCKS.

How does Ca2+ START muscle contraction?

1. Ca2+ comes OUT of the SR.

2. Ca2+ binds to troponin C.

3. Tropomyosin moves away from actin's binding sites.

4. Myosin grabs and pulls actin.

5. Muscle contracts!

🧠 Analogy:

Calcium unlocks the door so myosin can grab the rope and pull.

Memory trick: Ca2+ OUT → Troponin → Blocker MOVES → Myosin PULLS. 💪

How does Ca2+ cause muscle RELAXATION?

1. Ca2+ goes BACK INTO the SR.

2. Ca2+ detaches from troponin.

3. Tropomyosin blocks actin again.

4. Myosin stops producing strong pulling forces.

5. Muscle relaxes!

🧠 Analogy:

Calcium goes home, the door closes, and the workers stop working.

Memory trick: Ca2+ IN → Door CLOSED → Muscle RESTS. 😴

Myosin Heads Step along Actin Filaments

What happens when ATP binds to myosin?

ATP binds → Myosin DETACHES from actin.

ATP binding makes myosin release its grip.

🧠 Analogy:

ATP is the signal telling your hand to let go of a rope.

Memory trick: ATP ATTACHES → MYOSIN DETACHES.

What triggers the power stroke, and what happens?

Ca2+ binds troponin C, allowing tropomyosin to move and myosin to bind strongly.

• Pi leaves myosin → Power stroke begins.

• Myosin head swivels from about 90° to 45°.

• Actin gets pulled toward the M line.

🧠 Analogy:

Releasing a loaded slingshot.

Memory trick:

Pi OUT = POWER STROKE.

What is the rigor state, and why does rigor mortis happen?

Rigor = Myosin tightly stuck to actin with no nucleotide bound.

After death, ATP runs out → Myosin can't detach → Muscles become stiff.

🧠 Analogy:

Your hand is gripping a rope but can't open.

Memory trick:

NO ATP = NO RELEASE.

Acetylcholine Initiates Excitation-Contraction Coupling

What are the steps of excitation-contraction coupling?

1. ACh released → Na+ enters muscle.

2. Muscle action potential travels through T-tubules.

3. DHP activates RyR.

4. Ca2+ comes OUT of SR.

5. Ca2+ binds troponin → Myosin can pull actin.

🧠 Analogy:

Fire alarm goes off → Garage opens → Firefighters come out → Work begins.

Memory trick:

ACh → Electricity → DHP → RyR → Ca2+ OUT → WORK. 💪

What do DHP and RyR do?

• DHP = Senses the electrical signal in the T-tubule.

• RyR = Opens the calcium door on the SR.

• DHP activates RyR → Ca2+ comes OUT → Contraction can begin.

🧠 Analogy:

DHP = Light switch. RyR = Door. Calcium = Workers coming out.

Memory trick:

DHP DETECTS, RyR RELEASES.

What is an end-plate potential (EPP)?

A small, local depolarization at the motor end plate caused by ACh opening receptor-channels.

• ACh binds receptors → Net Na+ enters.

• Muscle becomes more positive.

• EPP reaches threshold → Muscle action potential starts.

🧠 Analogy:

EPP is like turning your car key. It starts the electrical signal.

Memory trick:

EPP = First electrical push.

What does SERCA do during muscle relaxation?

SERCA is a calcium pump that uses ATP to move Ca2+ back into the SR.

• Ca2+ OUT of SR → CONTRACTION 💪

• SERCA pumps Ca2+ IN → RELAXATION 😴
  
  

🧠 Analogy:

SERCA = School bus bringing the workers home.

Memory trick:

SERCA = Sends Calcium Away (into SR).

What are the three phases of a muscle twitch?

1. Latent = PREPARING. Electrical signal triggers calcium release, but tension hasn't risen yet.

2. Contraction = PULLING. Muscle tension increases.

3. Relaxation = RESTING. Calcium returns to SR, and tension decreases.

🧠 Analogy:

Getting ready to lift a backpack → Lifting → Relaxing afterward.

Memory trick:

PREPARE → PULL → REST.

Skeletal Muscle Contraction Requires a Steady Supply of ATP

What are the three main jobs ATP performs in muscle?

1. Myosin ATPase = Powers contraction and crossbridge release.

2. SERCA = Pumps Ca2+ back into SR for relaxation.

3. Na+/K+-ATPase = Restores ion gradients after action potentials.

🧠 Analogy:

ATP pays the workers, takes them home, and cleans up afterward.

Memory trick:

ATP = WORK → REST → RESET.

What is phosphocreatine, and what does it do?

Phosphocreatine is the muscle's quick backup energy supply.

• Muscle RESTING → ATP helps make phosphocreatine (stores energy).

• Muscle WORKING → Phosphocreatine gives phosphate to ADP → Makes ATP.

🧠 Analogy:

Phosphocreatine is a portable phone charger. You charge it while resting and use it when your battery gets low.

Memory trick:

REST = STORE ENERGY.

WORK = MAKE ATP FAST.

What does creatine kinase (CK) do?

CK is the enzyme that transfers phosphate groups between creatine/phosphocreatine and ATP/ADP.

It helps store energy at rest and regenerate ATP during activity.

High CK levels in blood can also indicate muscle damage.

🧠 Analogy:

CK is the worker who charges and uses your backup battery.

Memory trick:

CK = Creatine's energy-transfer helper.

ow do aerobic and anaerobic glucose metabolism differ?

Aerobic = Uses oxygen-dependent pathways.

• About 30 ATP per glucose.

• Greater ATP yield.

Anaerobic = Relies on glycolysis without oxygen.

• 2 ATP per glucose.

• Faster ATP supply, but much less ATP per glucose.

• Produces lactate.

🧠 Analogy:

Aerobic = Efficient power plant.

Anaerobic = Quick emergency generator.

Memory trick:

OXYGEN = MORE ATP.

NO OXYGEN = ONLY 2 ATP per glucose.

Why doesn't muscle fatigue simply mean running out of ATP?

Muscles usually still have ATP when fatigue occurs.

The textbook notes that intense exercise typically uses only about 30% of the muscle fiber's ATP pool.

Fatigue occurs because other changes reduce the muscle's ability to generate force.

🧠 Analogy:

Your phone can become slow or overheated even when its battery isn't empty.

Memory trick:

FATIGUE ≠ ZERO ATP.

Muscle Fatigue Has Multiple Causes

What is muscle fatigue, and what are its two types?

Muscle fatigue = Temporary decrease in a muscle's ability to produce force.

• Central fatigue = Starts in the CNS (brain/spinal cord).

• Peripheral fatigue = Problems at the NMJ or inside the muscle fiber.

🧠 Analogy:

Central = Coach tells the team to slow down.

Peripheral = Players are struggling even though the coach is still giving instructions.

Memory trick:

CENTRAL = COMMAND PROBLEM.

PERIPHERAL = MUSCLE PROBLEM.

How can problems with Ca2+ cause muscle fatigue?

Less Ca2+ released from SR → Less Ca2+ binds troponin → Fewer strong crossbridges → Less muscle force.

🧠 Analogy:

Calcium is like factory workers. If fewer workers come out, less work gets done.

Memory trick:

LESS Ca2+ OUT = LESS POWER. 💪

What are the main possible causes of peripheral muscle fatigue?

• Increased Pi → May interfere with power strokes and Ca2+.

• Less glycogen → May disrupt muscle function during long exercise.

• K+ buildup outside muscle → Changes membrane potential.

• Impaired Ca2+ release → Weakens contraction.

🧠 Analogy:

A factory can slow down because of low supplies, faulty electricity, or workers not showing up.

Memory trick:

Pi = PULLING PROBLEM.

K+ = ELECTRICAL PROBLEM.

Ca2+ = CONTRACTION SIGNAL PROBLEM.

Does muscle fatigue happen because ATP runs out or lactate builds up?

Usually, no.

Muscles generally still have ATP during fatigue, and lactate accumulation is no longer considered a likely primary cause.

The strongest evidence points toward problems with E-C coupling and muscle force production.

🧠 Analogy:

A phone can slow down even when its battery isn't dead. The problem could be overheating or something else.

Memory trick:

FATIGUE ≠ NO ATP.

FATIGUE ≠ TOO MUCH LACTATE.

Skeletal Muscle Is Classified by Speed and Fatigue Resistance

What are the three types of skeletal muscle fibers?

Type I (Slow oxidative): Slow contraction, excellent endurance, resists fatigue.

Type IIA (Fast oxidative-glycolytic): Fast contraction, good endurance, uses both aerobic and glycolytic metabolism.

Type IIX (Fast glycolytic): Very fast contraction, fatigues quickly, relies heavily on glycolysis.

🧠 Analogy:

Type I = Marathon runner 🏃

Type IIA = Soccer player ⚽

Type IIX = Sprinter ⚡

Memory trick:

I = LASTS LONG.

IIA = FAST + LASTS.

IIX = FAST BUT TIRES.

Why do fast-twitch fibers contract and relax faster?

They have faster myosin ATPase and higher SR Ca2+-ATPase activity.

• Fast myosin ATPase → Faster crossbridge cycling.

• Fast Ca2+ pumps (SERCA) → Faster relaxation.

🧠 Analogy:

Fast-twitch muscles have fast workers AND a fast cleanup crew.

Memory trick:

FAST ATPase = FAST PULL.

FAST SERCA = FAST REST.

Why are slow-twitch fibers red and fatigue-resistant?

They have:

• Lots of myoglobin → Holds and helps supply oxygen.

• Lots of mitochondria → Makes ATP using oxygen.

• Lots of capillaries → Delivers oxygen.

• Small diameter → Oxygen travels a shorter distance.

🧠 Analogy:

A factory with lots of oxygen deliveries, storage tanks, and power plants can keep working all day.

Memory trick:

MORE OXYGEN + MORE MITOCHONDRIA = MORE ENDURANCE.

Why do fast glycolytic fibers fatigue quickly?

Type IIX fibers are large, pale fibers with fewer mitochondria, capillaries, and less myoglobin.

They rely mainly on glycolysis for rapid ATP production but cannot sustain intense activity as long as oxidative fibers.

🧠 Analogy:

A race car is built to go extremely fast, but it isn't designed to keep sprinting forever.

Memory trick:

WHITE = FAST BUT FATIGUES.

RED = SLOWER BUT LASTS.

What is muscle plasticity?

Muscle plasticity means muscle fibers can adapt their characteristics to different activity and training demands.

For example, endurance training can make Type IIA fibers more oxidative and fatigue-resistant.

🧠 Analogy:

A soccer player who practices long-distance running can improve their endurance.

Memory trick:

MUSCLES ADAPT TO THEIR JOB.

Resting Fiber Length Affects Tension

What is the length-tension relationship?

The starting length of a sarcomere determines how much tension it can produce.

• Too SHORT → Too much overlap → Less force.

• Too LONG → Not enough overlap → Less force.

• JUST RIGHT → Best overlap → Maximum force.

🧠 Analogy:

Think of tug-of-war.

Too close together = No room to pull.

Too far apart = Can't reach the rope.

Perfect distance = Strongest pull.

Memory trick:

TOO SHORT = CROWDED.

TOO LONG = CAN'T REACH.

JUST RIGHT = MAX POWER. 💪

Why does optimal sarcomere length produce maximum tension?

Because actin and myosin have the best amount of overlap to form many effective crossbridges.

Optimal resting length = Approximately 2.0–2.3 µm.

🧠 Analogy:

Imagine a group of people grabbing a rope. When everyone's positioned correctly, the most people can pull effectively.

Memory trick:

BEST OVERLAP = MOST EFFECTIVE PULLERS = MAXIMUM TENSION.

Force of Contraction Increases with Summation

What is muscle summation, and why does it happen?

Summation happens when another action potential arrives before the muscle fully relaxes.

• Signals FAR APART → Separate twitches.

• Signals CLOSER → Less relaxation → More tension.

• Ca2+ remains elevated → More force can be produced.

🧠 Analogy:

Pushing someone on a swing. If you push again before the swing slows down, it goes higher!

Memory trick:

SIGNALS CLOSER = CONTRACTIONS ADD UP.

What is the difference between unfused and complete tetanus?

Unfused tetanus:

Muscle stays contracted but relaxes a little between signals.

Graph = WAVY at the top.

Complete tetanus:

Muscle doesn't relax between signals and maintains maximum tension.

Graph = SMOOTH, FLAT top.

🧠 Analogy:

Unfused = Working with tiny breaks.

Complete = Working nonstop.

Memory trick:

UNFUSED = LITTLE DIPS.

FUSED = NO DIPS.

How does increasing stimulation frequency affect muscle tension?

• Low frequency → Separate twitches.

• Higher frequency → Summation.

• Even higher → Unfused tetanus.

• Very high → Complete tetanus.

• Prolonged activity → Fatigue may eventually decrease tension.

🧠 Analogy:

Think of turning up the speed on a machine. It works harder and harder until it reaches maximum output. If it keeps working too long, performance may drop.

Memory trick:

FASTER SIGNALS → MORE FORCE → MAXIMUM → POSSIBLE FATIGUE.

A Motor Unit Is One Motor Neuron and Its Muscle Fibers

What is a motor unit?

A motor unit is ONE somatic motor neuron and ALL the muscle fibers it controls.

- One neuron can control many muscle fibers.

- When the neuron fires → All its muscle fibers contract.

- Each muscle fiber is controlled by only one motor neuron.

🧠 Analogy:

A light switch connected to several light bulbs. Flip the switch, and all the bulbs turn on.

Memory trick:

ONE NEURON = ALL ITS FIBERS CONTRACT.

What is the difference between small and large motor units?

Small motor units:

• Few muscle fibers per neuron.

• Precise, fine movements.

• Example: Eye or hand muscles.

Large motor units:

• Hundreds or thousands of fibers per neuron.

• Larger force changes, less precision.

• Example: Calf muscles.

🧠 Analogy:

Small motor unit = Tiny paintbrush for details.

Large motor unit = Paint roller for big jobs.

Memory trick:

SMALL = PRECISION.

LARGE = POWER.

What is motor unit recruitment, and how is it different from summation?

Recruitment = Activating MORE motor units to increase force.

Summation = Stimulating the SAME muscle fibers more frequently to increase force.

• Recruitment → More muscle fibers working.

• Summation → Same fibers producing more tension.

🧠 Analogy:

Recruitment = Hiring more workers.

Summation = Making your existing workers work harder.

Memory trick:

RECRUITMENT = MORE WORKERS.

SUMMATION = FASTER SIGNALS.

Contraction Force Depends on the Types and Numbers of Motor Units

What are graded contractions, and how are they possible if motor units are all-or-none?

Graded contractions mean a whole muscle can produce different amounts of force.

• One motor unit = All its fibers activate together (all-or-none).

• Whole muscle = Can activate different numbers and types of motor units to adjust force.

🧠 Analogy:

Imagine students clapping. Each student either claps or doesn't, but more students clapping makes the room louder.

Memory trick:

ONE UNIT = ALL OR NONE.

WHOLE MUSCLE = ADJUSTABLE FORCE.

In what order are motor units recruited as more force is needed?

1. Type I (Slow oxidative) → First, low force, resists fatigue.

2. Type IIA (Fast oxidative-glycolytic) → Next, more force.

3. Type IIX (Fast glycolytic) → Last, maximum force, fatigues quickly.

Motor units are recruited from lowest to highest activation threshold.

🧠 Analogy:

Moving furniture: One person moves a chair, more helpers move a couch, and the strongest helpers come in for the refrigerator.

Memory trick:

SLOW FIRST → FAST SECOND → POWER LAST.

What is asynchronous recruitment, and why is it important?

Asynchronous recruitment = Different motor units take turns contracting to maintain force.

• Some motor units work while others rest.

• Helps prevent fatigue during sustained, submaximal contractions.

• Less effective at maximum force because nearly all units are needed.

🧠 Analogy:

Workers taking shifts at a hospital. Some work while others rest, keeping the hospital running without exhausting everyone.

Memory trick:

TAKE TURNS = LAST LONGER.

MAX FORCE = EVERYONE WORKS.

13.2 Mechanics of Body Movement

What is the difference between isotonic and isometric contractions?

Isotonic = Muscle produces force AND moves the load.

• Muscle shortens while lifting a weight.

• Movement occurs.

Isometric = Muscle produces force WITHOUT moving the load.

• Overall muscle length stays approximately the same.

• No movement occurs.

🧠 Analogy:

Pushing a shopping cart:

Cart MOVES = Isotonic.

Cart stuck against a wall = Isometric.

Memory trick:

ISOTONIC = MOVEMENT.

ISOMETRIC = SAME LENGTH.

What is an eccentric contraction?

An eccentric contraction happens when a muscle produces tension while LENGTHENING.

Example:

• Lifting dumbbell → Biceps shortens.

• Slowly lowering dumbbell → Biceps lengthens while still producing force.

Eccentric contractions are associated with exercise-induced muscle damage and delayed muscle soreness.

🧠 Analogy:

Slowly lowering a heavy box instead of dropping it. Your muscles are still working to control the weight.

Memory trick:

ECCENTRIC = ACTIVE WHILE LENGTHENING.

How can an isometric contraction create tension without changing overall muscle length?

During an isometric contraction:

1. Sarcomeres shorten slightly.

2. Series elastic elements stretch.

3. Their length changes balance out.

4. Overall muscle length stays approximately constant.

🧠 Analogy:

Pulling on a stretchy rubber band attached to something too heavy to move. The band stretches, but the object stays still.

Memory trick:

SARCOMERES SHORTEN + ELASTIC ELEMENTS STRETCH = NO MOVEMENT.

Bones and Muscles around Joints Form Levers and Fulcrums

What are levers, fulcrums, and loads in the body?

• Lever = Bone that moves.

• Fulcrum = Joint that acts as a pivot.

• Applied force = Muscle pulling on the bone.

• Load = Weight or resistance being moved.

Example: Biceps curl

Forearm = Lever, elbow = fulcrum, biceps = force, dumbbell = load.

🧠 Analogy:

Your arm works like a fishing rod.

Your elbow is the pivot, your biceps pulls, and the weight is the fish.

Memory trick:

BONES = LEVERS.

JOINTS = FULCRUMS.

MUSCLES = FORCE.

How do you calculate the force needed to hold a load stationary?

Muscle force × Muscle distance = Load × Load distance.

Example:

• Biceps attaches 5 cm from elbow.

• Load = 2 kg, located 15 cm from elbow.

Biceps force × 5 = 2 × 15

Biceps force = 6 kg-force.

The biceps needs more force because it attaches closer to the fulcrum.

🧠 Analogy:

Pushing a door near its hinges is harder than pushing near its handle.

Memory trick:

FORCE × DISTANCE = FORCE × DISTANCE.

CLOSER TO PIVOT = MORE FORCE NEEDED.

Why does the arm's lever system increase movement speed?

The biceps attaches close to the elbow, while the hand is farther away.

• Small biceps movement → Large hand movement.

• Example: Biceps shortens 1 cm → Hand moves 5 cm.

• Advantage = Greater speed and movement distance.

• Disadvantage = Muscle must produce more force.

🧠 Analogy:

Swinging a long stick. Your hand moves a little near the handle, but the tip moves much farther and faster.

Memory trick:

SMALL MUSCLE MOVEMENT = BIG HAND MOVEMENT.

How does increasing the load affect muscle shortening speed?

• Light load → Muscle shortens FAST.

• Heavy load → Muscle shortens SLOWLY.

• Load too heavy to move → Speed = 0 (isometric).

On your professor's graph:

A = Maximum velocity.

E = Zero velocity, isometric contraction.

🧠 Analogy:

Lifting a pencil is quick. Lifting a heavy dumbbell is slower. Trying to lift something too heavy = No movement.

Memory trick:

MORE WEIGHT = LESS SPEED.

NO MOVEMENT = ISOMETRIC.

Muscle Disorders Have Multiple Causes

What causes muscle cramps, and why can stretching help?

Muscle cramps are painful, sustained contractions often caused by hyperexcitable somatic motor neurons.

Motor neurons fire repeatedly → Muscle stays contracted.

Stretching may send sensory signals to the CNS that inhibit the motor neuron and stop the cramp.

🧠 Analogy:

Someone keeps pressing a doorbell, so it won't stop ringing.

Stretching helps tell the person to stop pressing it.

Memory trick:

CRAMP = TOO MANY SIGNALS.

STRETCHING = HELPS STOP THE SIGNALS.

What are muscle atrophy and botulinum toxin's effects?

Muscle atrophy: Muscle fibers shrink and weaken from prolonged inactivity.

Botulinum toxin: Decreases ACh release at the NMJ, causing muscle weakness or temporary paralysis.

🧠 Analogy:

Atrophy = A machine gets weaker from sitting unused.

Botulinum toxin = The machine isn't receiving its start signal.

Memory trick:

ATROPHY = DON'T USE IT → LOSE MUSCLE.

BOTOX = LESS ACh → LESS CONTRACTION.

What causes Duchenne muscular dystrophy?

Duchenne muscular dystrophy is an inherited disorder where the muscle lacks dystrophin.

Dystrophin normally helps connect actin to the muscle cell membrane.

Without it:

• Muscle membrane becomes vulnerable.

• Excess Ca2+ may enter.

• Enzymes damage muscle components.

• Progressive muscle weakness develops.

🧠 Analogy:

A building missing support beams becomes damaged more easily.

Memory trick:

NO DYSTROPHIN = NO SUPPORT → MUSCLE DAMAGE.

What is McArdle's disease?

McArdle's disease is caused by a missing enzyme called myophosphorylase.

Without this enzyme, muscles cannot properly break down stored glycogen for energy.

Result = Reduced exercise tolerance.

🧠 Analogy:

Having a refrigerator full of food but no key to open it.

Memory trick:

McARDLE = GLYCOGEN LOCKED AWAY.

13.3 Smooth Muscle

What is the difference between phasic and tonic smooth muscle?

Phasic = Alternates between contraction and relaxation.

Examples: Esophagus and intestines.

Tonic = Maintains some contraction most of the time.

Examples: Sphincters and blood vessel walls.

🧠 Analogy:

Phasic = A door that opens and closes repeatedly.

Tonic = A door that usually stays closed but can open when needed.

Memory trick:

PHASIC = ON, OFF, ON, OFF.

TONIC = STAYS ON, CAN ADJUST.

What is the difference between single-unit and multiunit smooth muscle?

Single-unit:

• Cells connected by gap junctions.

• Electrical signals spread between cells.

• Cells contract together.

• Example: Intestines.

Multiunit:

• Cells are NOT electrically connected.

• Each cell must be stimulated independently.

• Allows precise control.

• Example: Iris of the eye.

🧠 Analogy:

Single-unit = One light switch controls the entire room.

Multiunit = Every light has its own switch.

Memory trick:

SINGLE = TOGETHER.

MULTI = INDEPENDENT.

How do single-unit and multiunit smooth muscles increase contraction force?

Single-unit: Changes Ca2+ signaling to adjust contraction strength because cells are activated together.

Multiunit: Recruits additional muscle cells to increase total force.

🧠 Analogy:

Single-unit = Make the entire team work harder.

Multiunit = Hire more workers.

Memory trick:

SINGLE = MORE Ca2+ → MORE FORCE.

MULTI = MORE CELLS → MORE FORCE.

Why does uterine smooth muscle become more coordinated before childbirth?

Near labor, uterine cells develop more gap junctions.

These connections allow electrical signals to spread between cells so they contract together more effectively.

🧠 Analogy:

Imagine a group of people trying to push a car.

If everyone pushes at different times, movement is inefficient.

If everyone pushes together, the car moves more easily.

Memory trick:

GAP JUNCTIONS = TEAMWORK.

MORE GAP JUNCTIONS = COORDINATED CONTRACTIONS.

Smooth Muscle Is More Variable Than Skeletal Muscle

Which muscle type contracts and relaxes fastest?

Skeletal muscle: Fastest.

Cardiac muscle: Intermediate.

Smooth muscle: Slowest.

Smooth muscle contractions can last several seconds or longer, allowing sustained tension.

🧠 Analogy:

Skeletal = Sprinter.

Cardiac = Steady drummer.

Smooth = Slow machine that keeps working.

Memory trick:

SKELETAL → CARDIAC → SMOOTH.

FASTEST → SLOWEST.

Why can smooth muscle maintain tension without using much ATP?

Smooth muscle can slow myosin ATPase activity and crossbridge cycling while maintaining force.

• Uses less ATP.

• Can maintain contractions for a long time.

• Resists fatigue.

🧠 Analogy:

Instead of repeatedly pushing a heavy door, you hold it in place.

Memory trick:

SLOW CROSSBRIDGES = SAVE ATP = LAST LONGER.

How does smooth muscle anatomy differ from skeletal muscle?

Smooth muscle:

• Small, spindle-shaped cells.

• One nucleus.

• No sarcomeres or striations.

• Works over a wide range of lengths.

Skeletal muscle:

• Long, cylindrical fibers.

• Many nuclei.

• Has sarcomeres and striations.

• Operates over a narrower range of lengths.

🧠 Analogy:

Smooth muscle = Stretchy bag.

Skeletal muscle = Organized machine.

Memory trick:

SMOOTH = ONE NUCLEUS, NO STRIPES.

SKELETAL = MANY NUCLEI, STRIPES.

How does calcium trigger contraction differently in skeletal and smooth muscle?

Skeletal muscle:

Ca2+ from SR → Binds troponin → Tropomyosin moves → Contraction.

Smooth muscle:

Ca2+ from SR and ECF → Activates pathway that phosphorylates myosin light chains → Contraction.

Smooth muscle has NO troponin.

🧠 Analogy:

Two machines have different ON switches.

Skeletal turns on through troponin.

Smooth turns on through myosin activation.

Memory trick:

SKELETAL = TRO PONIN.

SMOOTH = MYOSIN PHOSPHORYLATION.

How do smooth muscles receive signals and change organ shape?

Smooth muscle is controlled by the autonomic nervous system and responds to electrical and/or chemical signals.

Neurotransmitters are released from varicosities rather than specialized motor end plates.

Different muscle layers can change an organ's shape:

• Circular layer contracts → Tube becomes narrower.

• Longitudinal layer contracts → Tube becomes shorter.

🧠 Analogy:

Circular = Squeezing a straw.

Longitudinal = Pushing both ends of a straw together.

Memory trick:

CIRCULAR = NARROWER.

LONGITUDINAL = SHORTER.

Smooth Muscle Lacks Sarcomeres

What are dense bodies and intermediate filaments in smooth muscle?

Dense bodies are anchor points where actin filaments attach.

They perform a role similar to Z disks in skeletal muscle.

Intermediate filaments form supporting connections between dense bodies, helping hold the cell together and transmit force.

🧠 Analogy:

Imagine a spiderweb.

Dense bodies = Anchors.

Intermediate filaments = Support cables.

Myosin pulls actin, and the pulling force spreads through the cell.

Memory trick:

DENSE BODIES = Z DISKS OF SMOOTH MUSCLE.

INTERMEDIATE FILAMENTS = SUPPORT CABLES.

How do actin and myosin differ in smooth and skeletal muscle?

Smooth muscle:

• More actin relative to myosin (10–15:1).

• Longer myosin filaments.

• Myosin heads along the entire filament.

• Has tropomyosin but NO troponin.

Skeletal muscle:

• Lower actin-to-myosin ratio (2–4:1).

• Myosin heads clustered toward the ends of thick filaments.

• Has troponin and tropomyosin.

Smooth muscle's arrangement helps it contract over a wider range of lengths.

🧠 Analogy:

Smooth muscle is like a stretchy rope system with pulling hooks spread along its length, so it can keep working even when stretched.

Memory trick:

SMOOTH = MORE ACTIN + LONGER MYOSIN + NO TROPONIN.

What is the difference between the SR and T-tubules in smooth versus skeletal muscle?

Skeletal muscle:

• Has organized SR that stores Ca2+.

• Has T-tubules that carry action potentials deep into the cell.

Smooth muscle:

• Has less organized SR that stores Ca2+.

• Does NOT have T-tubules.

• Has caveolae, small membrane indentations involved in signaling.

🧠 Analogy:

Skeletal muscle = Long tunnels (T-tubules).

Smooth muscle = Little pockets (caveolae).

Both have a calcium storage room (SR).

Memory trick:

SKELETAL = T-TUBULES.

SMOOTH = CAVEOLAE.

BOTH STORE Ca2+ IN SR.

Myosin Phosphorylation Controls Contraction

What are the steps of smooth muscle contraction?

1. Ca2+ increases in the cytosol (from SR and ECF).

2. Ca2+ binds to calmodulin (CaM).

3. Ca2+-CaM activates MLCK.

4. MLCK adds phosphate to myosin light chains.

5. Myosin ATPase activity increases → Myosin pulls actin → Contraction.

🧠 Analogy:

Ca2+ = Manager.

Calmodulin = Assistant.

MLCK = Worker who turns on the machine.

Myosin = Machine that starts working.

Memory trick:

Ca2+ → CaM → MLCK → PHOSPHATE ON → CONTRACT.

How does smooth muscle relax?

1. Ca2+ leaves the cytosol (back into SR or outside the cell).

2. Ca2+ unbinds from calmodulin.

3. MLCK activity decreases.

4. MLCP removes phosphate from myosin.

5. Myosin ATPase activity decreases → Muscle tension usually decreases.

Ca2+-ATPase and the Na+-Ca2+ exchanger help remove calcium from the cytosol.

🧠 Analogy:

The manager (Ca2+) leaves the factory, the ON switch stops working, and the OFF worker (MLCP) removes the activation tag.

Memory trick:

Ca2+ DOWN → MLCK OFF → MLCP REMOVES PHOSPHATE → RELAX.

What is the difference between MLCK and MLCP?

MLCK (myosin light chain kinase):

Adds phosphate to myosin light chains → Increases myosin ATPase activity → Promotes contraction.

MLCP (myosin light chain phosphatase):

Removes phosphate from myosin light chains → Decreases myosin ATPase activity → Promotes relaxation.

The balance between MLCK and MLCP determines smooth muscle's contraction state.

🧠 Analogy:

MLCK = Turns the machine ON.

MLCP = Helps turn the machine OFF.

Memory trick:

KINASE = PHOSPHATE ON.

PHOSPHATASE = PHOSPHATE OFF.

What is the latch state in smooth muscle?

Latch state = Smooth muscle maintains tension even after myosin has been dephosphorylated.

• Some myosin crossbridges remain attached to actin.

• Muscle maintains force.

• Very little ATP is used.

This helps smooth muscle sustain contractions for long periods without fatiguing.

🧠 Analogy:

A door latch keeps the door shut without someone constantly pushing against it.

Memory trick:

LATCH = STAY ATTACHED + SAVE ATP.

MLCP Controls Ca2+ Sensitivity + Calcium Initiates Smooth Muscle Contraction

How does MLCP change smooth muscle calcium sensitivity?

LOW MLCP activity:

Less phosphate removed from myosin → More phosphorylation → Stronger contraction at the same Ca2+ level.

HIGH MLCP activity:

More phosphate removed → Less phosphorylation → Weaker contraction at the same Ca2+ level.

🧠 Analogy:

MLCK is a faucet adding water.

MLCP is a drain removing water.

Smaller drain = More water stays in the tub.

Memory trick:

LESS MLCP = MORE FORCE.

MORE MLCP = LESS FORCE.

LEFT SHIFT = MORE Ca2+ SENSITIVE.

What is the difference between electromechanical and pharmacomechanical coupling?

Electromechanical coupling:

Electrical changes in membrane potential trigger calcium signaling and contraction.

Pharmacomechanical coupling:

Chemical signals change contraction or relaxation through signaling pathways, with little or no change in membrane potential.

🧠 Analogy:

Electromechanical = Pressing a power button.

Pharmacomechanical = Using a remote control.

Memory trick:

ELECTRO = ELECTRICAL.

PHARMACO = CHEMICAL.

What is the difference between slow-wave and pacemaker potentials?

Slow waves:

Membrane potential rises and falls rhythmically but only triggers action potentials if threshold is reached.

Pacemaker potentials:

Spontaneously depolarize to threshold and repeatedly trigger action potentials.

🧠 Analogy:

Slow wave = Someone jumping toward a hoop, but sometimes missing.

Pacemaker = Someone reaching the hoop every time.

Memory trick:

SLOW WAVES = MAY REACH THRESHOLD.

PACEMAKER = ALWAYS REACHES THRESHOLD.

How does smooth muscle release Ca2+ from the SR?

Two major channels:

1. RyR (ryanodine receptor): Ca2+ enters the cell → Triggers more Ca2+ release from SR. Called CICR.

2. IP3 receptor: Chemical signal → GPCR → PLC → IP3 → Opens SR Ca2+ channel.

Both increase cytosolic Ca2+ to help initiate contraction.

🧠 Analogy:

RyR = One person claps, and everyone joins.

IP3 = A key opening the calcium storage room.

Memory trick:

RyR = Ca2+ RELEASES MORE Ca2+.

IP3 = OPENS THE Ca2+ DOOR.

What opens Ca2+ channels in smooth muscle?

• Voltage-gated: Membrane depolarization.

• Ligand-gated: Chemical binding.

• Stretch-activated: Physical stretching.

• Store-operated: Low SR Ca2+ activates STIM1, which opens Orai1 channels to help refill calcium stores.

Stretch can cause a myogenic contraction without direct stimulation from a neuron.

🧠 Analogy:

Four ways to open a door:

Voltage = Electrical switch.

Ligand = Chemical key.

Stretch = Pulling the door open.

Store-operated = Low-storage alarm opens the refill door.

Memory trick:

VOLTAGE = ELECTRICAL.

LIGAND = CHEMICAL.

STRETCH = PRESSURE.

STIM1 + ORAI1 = REFILL Ca2+.

Some Smooth Muscles Have Unstable Membrane Potentials

How do depolarization and hyperpolarization affect smooth muscle contraction?

Depolarization = Membrane becomes LESS negative → Generally increases the chance of contraction.

Hyperpolarization = Membrane becomes MORE negative → Decreases the chance of contraction.

Smooth muscle can contract after an action potential, a graded potential, or chemical signaling without an electrical change.

🧠 Analogy:

Depolarization = Turning the activity level UP.

Hyperpolarization = Turning the activity level DOWN.

Memory trick:

DEPOLARIZE = MORE LIKELY TO CONTRACT.

HYPERPOLARIZE = LESS LIKELY TO CONTRACT.

Why can some smooth muscle action potentials continue even when Na+ channels are blocked by TTX?

Some smooth muscle action potentials depend on Ca2+ entering the cell rather than Na+ entry.

TTX blocks certain voltage-gated Na+ channels.

If the action potential continues, Ca2+ entry can be responsible for depolarization.

🧠 Analogy:

A building has two doors. TTX locks the Na+ door, but Ca2+ can still enter through the other door.

Memory trick:

TTX BLOCKS Na+.

AP STILL WORKS → Ca2+ CAN CAUSE DEPOLARIZATION.

Chemical Signals Influence Smooth Muscle Activity

How do chemical signals control smooth muscle contraction?

Chemical signals (neurotransmitters, hormones, and paracrines) can cause contraction OR relaxation by:

• Changing cytosolic Ca2+ concentrations.

• Altering MLCK or MLCP activity.

• Changing membrane potential and Ca2+ entry.

The response depends on the receptor and signaling pathway activated.

🧠 Analogy:

Chemical signals can press the gas pedal or brake in a car.

Memory trick:

CHEMICAL SIGNAL → Ca2+ OR MLCK/MLCP → CONTRACT OR RELAX.

How does epinephrine affect smooth muscle through α and β2 receptors?

α-adrenergic receptors:

Generally promote smooth muscle CONTRACTION.

Examples: Many blood vessels and bladder sphincter.

β2-adrenergic receptors:

Generally promote smooth muscle RELAXATION.

Examples: Airways and some blood vessels.

The same chemical can have opposite effects depending on its receptor.

🧠 Analogy:

The same remote sends signals to two different devices, but they respond differently.

Memory trick:

ALPHA = CONTRACT.

BETA-2 = RELAX.

How do IP3, DAG, and cAMP influence smooth muscle?

IP3:

Opens SR Ca2+ channels → Increases cytosolic Ca2+ → Promotes contraction.

DAG:

Can indirectly inhibit MLCP → More myosin stays phosphorylated → Promotes contraction.

cAMP:

Generally promotes smooth muscle relaxation.

🧠 Analogy:

IP3 opens the calcium storage door.

DAG slows the phosphate-removing worker.

cAMP sends a relaxation signal.

Memory trick:

IP3 + DAG = CONTRACT.

cAMP = RELAX.