Muscle Histology & Physiology

Question 1

Main function of skeletal muscles

Answer 1

Body movement:
- All muscles must cross a joint in order to create movement
Maintain posture:
- Stabalize joints
- Contracting to keep you upright
Production of body heat:
- Breakdown and rebuilding of proteins generate heat, also produced via shivering
Communication:
- Get feedback about the body position and make changes

Question 2

Contractibility

Answer 2

• Ability to make forceful contractions

Question 3

Excitability

Answer 3

• Capacity of muscle to responde to a stimulus by producing an action potention
• Depending on the type of muscle, stimuli can be variable (ie. skeletal muscles are always contracted by neurons)

Question 4

Extensibility

Answer 4

• Muscles can be stretched beyond its resting length and still be able to contract
  Ie. Hypertension

Question 5

Elasticity

Answer 5

• Ability of muscle to recoil to original resting length after it has been stretched.

Question 6

Skeletal muscle strucutre

Answer 6

• Composed of muscle cells, connective tissue, blood vessels and nerves.
• Multinucleated
• Striated due to lighr and dark banding

Question 7

Layers of muscle

Answer 7

Skeletal muscle < epimysium < perimysium < fascicle < endomysium < muscle fiber

Question 8

Fascicle structure (muscle fibers)

Answer 8

On the outside:
- Nucleus are found on the surface of the cell
- Capillary surround to deliver nutrients and oxygen

Sarcolemma (plasma membrane) < sarcoplasmic retriculum (highly organized and stores calcium) < transverse tubules (extension of sarcolemma, creates tunnel to get to the inside of the cell, and allows action potention to move through the cell) < myofibrils < sarcomere < actin and myosin

Question 8

Myofibril structure

Answer 8

• Sarcomere is composed of contractile protein actin and myosin
• Actin is attached to z band which holds the actin myofilaments in place
• Titin is also attached to the z band which had elastic properties to being z disk back to position when stretched.
• The M line holds the myosin

Question 9

Myosin Myofilaments

Answer 9

• Thick filament
• Two protein chains twisted together form A-helix
• Comes with varying lengths of head because we want attachment to actin at all lengths.
• Myosin APTase found in the head, helps break down ATP, allows myosin head to get energy to bind and move actin
• Movement of the head creates muscle shortening/ contracting

Question 9

Actin myosfilement

Answer 9

• Thin filament
• G-actin is the monomeric form of actin. It is a small, globular protein that has a single polypeptide chain
• F-actin is the polymeric form of actin, composed of long, thin helical strands formed by the polymerization of many G-actin monomers. It is a filamentous structure.
• There are active sites where myosin can bind…however, they are not always open which helps control when muscle contraction occurs.
• Contains tropomyosin and troponin …
  TROPOMYOSIN:
• Twists around the F actin molecules to cover all active sites
  TROPONIN:
• Binds to G actin
• Binds to Ca2+, which removes the tropomysoin off the filament.

Question 10

Sarcomere Organization

Answer 10

Left to right
- Actin only, contained in the I band from Z disk to end of thick filaments, light in colour < myosin surrounded by actin, A band represents length of thick filament, darker in colour < myosin only < H zone in A band where actin and myosin do not overlap < M line, middle of H zone, delicate filamnet holding myosin in place

Question 11

Sliding filament mechanism

Answer 11

• Explains how actin and myosin shorten the muscle
• Myosin will attach to the actin and pull it towards the m-line.
• By doing so, they H zone gets smaller due to greater overlap of actin and mysin
• This also pulls the z-disks closer, creating a shortening effect of the entire muscle
• Eventually gets to a point where actin is oulled so close, they begin to overlap and the H zone fully disappears
• I band narrows/ disappears

Question 12

Cross-bridge cycle

Answer 12

1. Calcium must first expose the active site on actin myofilament
   - release of calcium via the SR, which binds to troponin
   - this changes the shape of troponin which moves the tropomyosin off the actin to expose the active site.
2. Myosin heads are able to bind, but now they have to become activated
   - Head have stored energy when right side up
   - ATPase hydrolyzes ATP to form ADP and inorganic phosphate
   - This energy gets stored in the head and are ready to go when calcium binds to troponine.
   - Once active site is available, binding occurs, where the IP is released and the ADP stays
   - This forms the cross-bridge on contact
3. Myosin cross-bridge rotates towards the center of the sarcomere via power stroke, pulling the actin towards the middle as well.
   - ADP is released at this moment
4. In order to release myosin from active site, we need ATP which will bind to the head and cause the release
5. Again, ATPase on the head will break down ATP to ADP and IP
   - Note that attachment occurs closer to Z-disk because we’ve already pull some actin to the M-line.

Question 13

Skeletal Muscle Electrical Properties

Answer 13

• Main difference is that resting membrane potential (of muscle cell) is a bit larger in muscle due to greater amount of K+ leak channels.
• High amounts of K+ inside the cell, High amounts of Na+ outside the cell
• When depolarization occurs, aka when sodium enters the cell, we get muscle action potential (MAP)

Question 14

Neuromuscular junction

Answer 14

1. An action potential arrives at the presynaptic terminal causing voltage gated calcium channels to open.
2. Once calcium enters, it initates the release of neurotransmitter ACh, from synaptic vesicles into the synaptic cleft.
3. Diffusion of ACh across the synaptic cleft and binding of ACh to ACh receptors on the postsynaptic muscle fiber membrane, causing increase in the permeability of ligand-gated sodium channels.
4. Increase of sodium results in depolarization of the postsynaptic membrane, resulting in postsynaptic action potential once threshold is reached.
5. ACh is rapidly broken down in the synaptic cleft by acetylcholinesterase to acetic acid and choline
6. Choline is reabsorbed by the presynaptic terminal and combinded with acetic acid to form more ACh. Note that Acetic acid is not taken up by the presynaptic terminal, but rather by other cell types.

Question 15

Excitation-Contraction Coupling

Answer 15

1. MAP propagated along sarcolemme and into t-tubules
2. Calcium channels in the SR open and calcium is released into the sarcoplasm
3. Calcium binds to troponin, movement of tropomyosin off the myosin binding site
4. Heads of myosin can bind active sites on actin, forming the cross-bridge.

Question 16

Motor unit

Answer 16

• contains somatic motor neuron and all its muscle fibers that it serves.
• somatic motor neuron will leave the spinal chord and go to the level of the muscle. It makes contact with the perimysium where it branches to serve individual muscle fibers.
• Once single action potential moving along somatic motor neuron contracts multiple muscle fibers at once in this manner.

Question 17

Strength and muscle contraction

Answer 17

• Subtreshold stimulus has no motor unit recruitment
• Threshold stimulus has one motor unit promoted
• You continue to increase … the more tension, the more motor units recrutied
• Reach a point of maximal stimulus where all motor units respond.
• Supramaximal stimulu where all the motor units respond but you cannot produce anymore tension because you’ve recruite every single muscle fiber.

Question 18

Supramaximal stimuli

Answer 18

• You are born with a set amount of myofibrils. The only way to change the amount of myofibrils is to change the proteins found within each muscle fiber.
• More myofibrils allow for more sarcomere and therefore more force…developed via resistance training.

Question 19

Muscle Fatigue

Answer 19

• To prevent fatigue, we alternatre between which motor unit are providing the tension or working during muscle contraction.
• Asyncronous recruitment allows muscle fibers to relax while others take over in the mean time, while they also regenerate ATP
• Recruitment of the weakest muscle fibers occur first.

Question 20

Isometric

Answer 20

• Length of the muscle doesn’t change, but tension increases.
• Force external to the muscle is equal to the force within the muscle that is being produced by the myosin head acting on actin

Question 21

Isotonic

Answer 21

• Tension produced is constand but the length of the muscle changes

Question 22

Concentric

Answer 22

• Muscles shorten when tension overcomes resistnace

Question 23

Eccentric

Answer 23

• Muscle lengthens, when resistance greater than tension

Question 24

Type I muscle fibers

Answer 24

Slow twitch, high oxidative:
- Slow ATP breakdown
- Smaller, less myofibrils inside of the fibers
- More mitochondria
- Better blood supply
- Fatigue resistant
- Slow oxidative

Question 25

Type IIA Muscle Fibers

Answer 25

Fast twitch, low oxidative:
-Fast ATP breakdown
-Larger than type I
-Less mitochondria
-Less blood supply
-Less aerobic capacity
- Fast ocidative-glycolytic (oxygen + the breakdown of carbohydrates w/o oxygen)

Question 26

Type IIX Muscle Fibers

Answer 26

• Largest
• Fewest mitochondria
• Least blood supply
• Lowest aerobic capacity
• Fatigueable
• Fast glycolytic (Non oxidative uses ATP at much faster rate)