Chapter 12 - Muscles

Question 1

Skeletal Muscle (intro info)

Answer 1

-responsible for interaction with external
environment
-striated (skeletal) muscle approx. 40-50% of
adult humans by mass
-at rest, accounts for 25% of oxygen (every)
consumed, can triple during exercise
-skeletal muscle group composed of fibers
-multi-nucleated due to fusion of myoblast
during development

Question 2

Micro anatomy of muscle (6)

Answer 2

1) Skeletal Muscle
2) Epimysium: connective tissue of muscle
3) Perimysium: connective tissue
surrounding fascicles
4)Fascicle: bundle of muscle fibers
5)Muscle fiber (cell): bundle of myofibrils
6)Myofibril: organizational protein that forms
striation, contains think and thin filaments,
composed of sarcomeres

Question 3

Myofibril anatomy

Answer 3

1) Sarcomere: functional unit of striated
muscle, composed of think and thin fil.
2) Sarcolemma: plasma membrane
3) Sarcoplasmic reticulum: (ER) has terminal
cisterna
4) Terminal cisterna: “lateral sacs” adjacent to
t-tubules, provide storage

Question 4

Ultra Structure of Muscle composed of two types of proteins

Answer 4

1) Contractile proteins

2) Regulatory proteins

Question 5

Contractile Proteins

Answer 5

1) Myosin
-part of the thick filament
-approx. 250 individual myosin molecules
join to form the thick filament
-each single myosin molecule has 4
subunits: 2 heavy chains forming a tail
and 2 light chains forming a head
-each head contains an actin binding site
and an ATPase binding site
2) Actin
-part of the thin filament
-double helix of G-actin (globular)
molecules
-each G-actin has binding site for myosin
-stand of G-actin called F-actin (fibers)

Question 6

Regulatory Proteins

Answer 6

1) Tropomyosin: regulatory protein that covers the binding sites on F-actin
2) Troponin: regulatory protein (composed of 3 subunits) bound to tropomyosin and has a binding site for Ca+

Question 7

Sliding Filament Theory (def)

Answer 7

force is generated as fixed-length filaments slide past one abother

Question 8

Actin-Myosin Cross Bridge Cycle

Answer 8

1) increased intracellular Ca+ levels, Ca+ binds to troponin
2) Troponin changes shape and pulls tropomyosin off binding sites
3) Actin and myosin attach
4)Pi released
5) Powerstroke occurs, shortens sarcomere
6) ADP released
7) Fresh ATP, if available, binds to myosin head
-No ATP, actin and myosin remain bound,
rigor complex
8) Myosin detaches from actin
9) ATP hydrolyzed, energy re-cocks myosin head
10a) If Ca+is present, myosin attaches to actin, cycle repeats
10b) If Ca+is removed, tropomyosin covers binding sites preventing actin-myosin cross-bridge

Question 9

How does muscle contraction begin?

Answer 9

p. 34-39

Question 10

Excitation-Contraction Coupling

Answer 10

• APs travel across the sarcolemma down the T-tubules
• Voltage-gated receptors on sarcolemma called dihydropyridine (DHP) receptors to AP
• DHP receptor acts on “foot-protein” to open ryanodine (Ryr) receptor on SR
• Ryanodine receptor acts as Ca+ channel allowing Ca+ to flood into cytoplasm

Question 11

How does muscle contraction stop?

Answer 11

1) AchE breaks down Ach at the neuromuscular junction
2) Nicotinic receptors close and AP along the sarcolemma stops
3) Ryanodine channels close and Ca+ is pumped back into SR by Ca+ ATPase

Question 12

EPPs

Answer 12

are always excitatory and are initiated by nicotinic receptor Na influx

Question 13

All muscle contractions are

Answer 13

all or nothing. Strength of muscle contraction is controlled by the number of muscle fibers

Question 14

Twitch (def and phases)

Answer 14

• A twitch is a single contraction-relaxation cycle
• Latent period: AP to muscle contraction beginning; takes Ca time to diffuse throughout the cell after being released from SR
• Contraction phase: cross-bridge cycle, takes place as long as Ca present
• Relaxation phase: Ca returning to SR and muscle lengthens

Question 15

Single twitches

Answer 15

muscle relaxes completely between stimuli, p. 43

Question 16

Summation of twitches

Answer 16

stimuli closer together do not allow muscle to relax fully, p. 44

Question 17

Summation leading to unfused tetanus

Answer 17

Stimuli are far enough apart to allow muscle to relax slightly between stimuli; Treppe

• tetanus: maximum tension achieved through multiple stimulus
• maximum tension achieved if sarcomeres are completely shortened, Z lines run into each other p. 45

Question 18

Summation leading to complete tetanus

Answer 18

Muscle reaches steady tension

• complete tetanus: smooth sustained maximum tension of smoother muscle fiber
• fatigue causes muscles to lose tension despite continuing stimuli p. 46

Question 19

Subtypes of Skeletal Muscle

Answer 19

• Rate of muscle contraction depends on rate of ATPase activity, also affects force
• Type I: slow oxidative
  
  • slow rate of ATPase cycling
  • glycolytic ad oxidative metabolism produces increased ATP
  • high mitochondrial content
  • small fiber diameter
  • dense capillary supply
  • high myoglobin content
• Type II: fast glycolytic
  
  • high rate ATPase cycling
  • anaerobic glycolytic metabolism predominates
  • low mitochondrial content
  • large fiber diameter
  • low capillary density
  • low myoglobin content

Question 20

Remember: muscle contraction is

Answer 20

energetically expensive and uses a lot of AT

Question 21

Motor Unit

Answer 21

A single motor neuron and all of its target muscle cells

• one motor neuron can have many target cells
• but each muscle cell receives input from one (and only one) motor neuron

Question 22

Recruitment of Motor Units

Answer 22

To perform a light contraction (p. 49)
-could recruit any one motor unit and get the force generated by 3 muscle fibers
A stronger contraction could be had
-by recruiting any tow of the motor units resulting in force generated by 6 muscle cells
The strongest contraction of this muscle would occur
-by recruiting all three of the motor units resulting in the force generated by all 9 muscle cells

Question 23

Muscle fatigue

Answer 23

Occurs after excitation-contraction coupling, Ca is released from SR and after follows a decreased amount of Ca-troponin interaction and contraction-relaxation follows
-Two theories
1) Depletion theory: loss of ATP and glycogen and muscle runs out
of ATP as it contracts
2) Accumulation theory: acid (H+) changes pH and influences protein
function, lactic acid changes pH, inorganic phosphate (ATP breakdown
by product)

Question 24

Effects of Traning

Answer 24

• Oxidative Fibers
  
  • increased: mitochondria number, myoglobin content, and capillary density
• Glycolytic fibers
  
  • increased: number of myofibrils, glycogen storage, and fiber diameter