Physiology of a Muscle

Unit 4Part 2 Notes

What functional properties allow a muscle to perform its duties?

• Irritability–Ability to receive and respond to a

stimulus• Contractility

–Ability to shorten when adequate stimulus is received

What functional properties allow a muscle to perform its duties?

• Conductivity–Ability for impulse to travel along

plasma membrane of muscle cell• Elasticity

–Ability to recoil and resume original length

What role does the nervous system play in muscle movement?

• Motor Unit – one neuron and all the skeletal muscle cells it stimulates

Within a motor unit…

• Muscle Fibers

• Axons

• Axon Terminals (neuromuscular junctions)

Nerve endings and muscle fibers don’t physically touch…

• Neuromuscular juction – where axon terminals match up with muscle fibers

• Snyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle

Two Major Steps Of Muscle Contractions(Not in your packet-o-notes)

1st Stimulation of Muscle by Nerve• The brain sending a

message through a neuron to the muscles to simply tell that muscle to MOVE!

Video #1: http://

tinyurl.com/NerveImpulse

2nd Muscle Contraction

• Once the muscle has the message from the brain to move, the muscle will move its fibers and contract.

Video #2http://tinyurl.com/aqspwh

What steps occur to stimulate muscle movement?

• 1. Nerve impulse reaches axon terminals

• 2. Chemical Neurotransmitter (ACh – acetylcholine) released

• 3. ACh diffuses across synaptic cleft and attaches to receptors

What steps occur to stimulate muscle movement?

• 4. ACh causes the sarcolemma to become temporarily permeable to Na+

• 5. Na+ rush into the muscle cell

• 6. Excess of positive ions creates electric current (action potential)

• 7. Muscle contracts (another whole set of steps!)

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

More animations which may help you understand how a muscle contracts!

• So, we know how muscle contraction is stimulated… but now we need to know the steps that help muscle contraction to happen!

• Called the Sliding Filament Theory

The Sliding Filament Theory

• Muscle fibers activated by nervous system due to action potential

• Calcium ions (Ca+2) happen to be released– Do you remember which structure releases those

calcium ions???

The Sliding Filament Theory

• Muscle fibers activated by nervous system due to action potential

• Calcium ions (Ca+2) happen to be released– Do you remember which structure releases those

calcium ions???– THE SARCOPLASMIC RETICULUM

The Sliding Filament TheoryNote: this part is worded a little differently than your packet-o-notes

• Release of Ca+2 binds to troponin which then moved tropomyosin out of the way and stops it from blocking binding sites on the thin filament (actin).

• Once Actin is unblocked, the cross-bridges on the thick filaments(Myosin) attach to the binding sites on the Actin

• Let the sliding begin!

The Sliding Filament Theory

• Energized by energy from ATP, cross-bridges attach and detach from thin filaments

• Works like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!)

Check this video to see how it works! http://tinyurl.com/aqspwh

The Sliding Filament Theory

• As this process is happening in every sarcomere throughout the muscle, the muscle itself is contracting!

• The whole series of events (beginning with the nervous system signal) takes just a few thousandths of a second!!!

ADP

Pi

Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.

T tubuleSarcolemma

SR tubules (cut)

SynapticcleftSynaptic

vesicle

Axon terminal

ACh ACh ACh

Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.

Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+

release from terminalcisternae of SRinto cytosol.

Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.

Contraction; myosin heads alternately attach toactin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.

Removal of Ca2+ by active transportinto the SR after the actionpotential ends.

SR

Tropomyosin blockage restored,blocking myosin binding sites onactin; contraction ends andmuscle fiber relaxes.

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

1

2

3

4

5

6

The Sliding Filament Theory• Notice in the contracted

muscle, the H zone has disappeared

• The I band has shortened significantly (all that’s left is the Z disc)

• The A band (the dark striations!) have stayed the same thickness

• Check out this animation to see what happens to the sarcomere!! http://tinyurl.com/5u8q25b

Where’s all this energy coming from?

• As a reminder, energy comes from ATP because of breaking a phosphate bond

• Breaking a bond releases energy• When this energy is used by your body,

it releases heat• Because ATP is the only energy source

that can be used to move the cross-bridges back and forth (which contract the muscle), ATP must be regenerated continuously

ATP Regeneration – 3 Sources

• Direct phosphorylation of ADP by creatine phosphate– When ATP used, changes to ADP– Creatine phosphate adds that missing phosphorous

back on!– PROBLEM: only makes 1 ATP at a time… so not very

much. And, only supplies energy for 15-20 seconds of activity!

– Your body will always do this, but it’s not very effective. Therefore, we have to have other ways of supplying energy…..

ATP Regeneration – 3 Sources• Aerobic respiration

– Occurs in the mitochondria– Glucose broken down to pyruvic acid (releasing 2

ATP), and then into carbon dioxide and water (releasing 34 ATP)

– 36 ATP made for 1 glucose! A lot of energy! And, can supply energy for hours at a time!

– PROBLEM: NEEDS OXYGEN

– But what if you’re out of oxygen??? Then your muscles will begin……..

ATP Regeneration – 3 Sources

• Anaerobic glycolysis and lactic acid formation– Glucose broken down to pyruvic acid, releasing 2

ATP– If oxygen present, process continues to the rest of

aerobic respiration…– BUT… if oxygen is inadequate, or muscle activity is

intense, pyruvic acid is instead changed to lactic acid

– PROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!

ATP Regeneration

• 95% of ATP produced through aerobic respiration

• C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATP

• If you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respiration

• If they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!

Muscle Fatigue• A muscle is fatigued when it is unable to contract

even though it is being stimulated – means you don’t have ATP to move the cross-bridges!

• Lack of oxygen can cause…– Lactic acid buildup (anaerobic glycolysis)– ATP supply low (production can’t keep up with

usage)– Muscle will contract less and less effectively,

eventually stopping contraction completely

Muscle Fatigue

• FYI: When you breathe heavy after physical activity, your muscles are trying to get enough oxygen for aerobic respiration to replace all of the ATP you used!

Muscle Contraction

• Isotonic contractions–Myofilaments slide, shortening the

muscle–Movement occurs – bending knee,

rotating arms, smiling

Muscle Contraction

• Isometric contractions–Myofilaments trying to slide, but

can’t – just building up tension (crossbridges are “rowing”, but actin is not moving together)

–Movement doesn’t occur – object too heavy to lift, push against wall

Muscle Tone

• Muscle tone – sustained partial contraction of a muscle; muscle stays firm, healthy, and ready for action

• Muscle inactivity can lead to muscle weakness and wasting (this is why Range of Motion exercises on bedridden people is important!)

Effect of Exercise on Muscles

• Aerobic exercises include…–Running–Jogging–Biking–Elliptical

Effect of Exercise on Muscles

• Increase endurance of muscles because muscle cells will form more mitochondria and store more oxygen (meaning more energy for the muscles)

• Also – improves body metabolism, improve digestion, enhance coordination, strengthens skeleton, heart & lungs more efficient

• Muscles do NOT increase in size!

Effect of Exercise on Muscles• Resistance exercises include…

– Pushing against wall– Contracting muscles (likes gluteus maximus)– Lifting weights

• Does increase muscle size!– Due to enlargement of individual muscle cells (makes

more myofilaments)– You don’t add more muscle cells – you just bulk up

the ones you already have!!!–

• NEED BOTH TYPES OF EXERCISES IN ANY TRAINING PROGRAM!