1. Physiology of a Muscle
Unit 4
Part 2 Notes

2. 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

3. 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

4. What role does the nervous system play in muscle movement?
Motor Unit – one neuron and all the skeletal muscle cells it stimulates

5. Within a motor unit… Muscle Fibers Axons
Axon Terminals (neuromuscular junctions)

7. 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

8. 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:
2nd Muscle Contraction
Once the muscle has the message from the brain to move, the muscle will move its fibers and contract.
Video #2

9. 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

10. 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!)

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

12. 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

13. 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???

14. 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

16. The Sliding Filament Theory Note: 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!

17. 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!

19. 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!!!

20. ADP Pi
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transport
into the SR after the action
potential ends. SR
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+ 1 2 3 4 5 6

21. 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!!

22. 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

23. 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 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…..

24. 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……..

25. 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!

26. 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!

27. 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

28. 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!

29. Muscle Contraction Isotonic contractions
Myofilaments slide, shortening the muscle
Movement occurs – bending knee, rotating arms, smiling

30. 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

31. 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!)

32. Effect of Exercise on Muscles
Aerobic exercises include…
Running
Jogging
Biking
Elliptical

33. 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!

34. 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!