Muscle Physiology

Question 1

sarcolemma

Answer 1

“sarco-“ = pertaining to muscle

sarcolemma = muscle plasma membrane
sarcoplasma = muscle cytoplasm

etc

Question 2

which type of muscle does NOT acquire calcium for contraction from an extracellular source?

Answer 2

skeletal muscle: intracellular calcium source (stored in sarcoplasmic reticulum)

cardiac and smooth: extra and intracellular calcium sources

Question 3

muscle strength vs power

Answer 3

strength = max force/torque that is generated during a max voluntary contraction

power = work performed per unit time

Question 4

what are the components of actin and myosin, respectively, that allow them to participate in muscle contraction?

Answer 4

actin: filamentous double-helix F actin
- troponin I: anchors to actin
- troponin C: Ca2+ binding
- troponin T: shields tropomyosin
- tropomyosin: myosin binding domain

myosin: 2 heavy chains (tail) + 4 light chains (head)
- ATPase

Question 5

describe the mechanism calcium release from the sarcoplasmic reticulum of cardiac and skeletal muscle cells to initiate contraction

Answer 5

1. action potentials conducted along T-tubules
2. DHPR (dihydropyridine receptor) on SR are activated (voltage-gated)
3. activated DHPR activates RyR (ryanodine receptor) on SR
4. activated RyR are Ca2+ channels in SR
5. Ca2+ floods out of the SR due to concentration gradient

Question 6

what is the role of calsequesterin?

Answer 6

calsequesterin: located in the sarcoplasmic reticulum, binds Ca2+ and maintains it in low energy state while housed in the SR

Question 7

describe the structure of a sarcomere

Answer 7

A band = overlapping thick (myosin) and thin (actin) filaments
I band = thin (actin) filaments
H band = thick (myosin) filaments, centrally located
Z band = end boundaries of each sarcomere, actin anchored to Z discs

Question 8

after calcium is released from the SR, how is skeletal and cardiac muscle contraction initiated?

Answer 8

1. Ca2+ floods out of SR and binds troponin C on actin, causing conformational change
2. tropomyosin (myosin binding site) is released
3. high energy myosin head (which during rest hydrolyzed ATP with intrinsic ATPase in light chains) binds actin
4. conformational change is induced which causes myosin head to pull actin closer (power-stroke, shortening of sarcomere via overlap)
5. ADP+Pi is released by power stroke, and when ATP binds myosin head again it causes myosin to dissociate from actin (and cycle begins again)

Question 9

what is the determining factor of whether muscle contraction will continue cycling or cease?

Answer 9

concentration of sarcoplasmic Ca2+

when sarcoplasmic (read: cytoplasm of myocytes) calcium drops below threshold, muscle contraction ceases

Question 10

what is the function of SERCA and NCX?

Answer 10

both function in facilitating muscle relaxation by lowering concentration of sarcoplasmic Ca2+

SERCA (sarco/endoplasmic reticulum Ca2+ ATPase): in membrane of SR, pumps Ca2+ against its concentration back into SR

NCX (Na+/Ca2+ exchanger): in plasma membrane of CARDIOmyocytes, secondary active transporter that exchanges extracellular Na+ for intracellular Ca2+, very important to help cardiac muscle relax in between contractions

Question 11

preload vs afterload

Answer 11

preload = force potential a muscle can generate

afterload = anything resisting muscle contraction

preload must overcome afterload to cause movement (displacement of load - isotonic)

Question 12

what is different about the amount of force that can be generated by a skeletal muscle vs a cardiac muscle fiber?

Answer 12

skeletal muscle has a set/given amount of force it can generate (if more force is needed, more motor units are recruited)

cardiac muscle contractive force is dependent on the concentration of intracellular calcium

Question 13

what is the preload and afterload of heart contraction?

Answer 13

preload = force potential a muscle can generate = myocardial contractile force

afterload = anything resisting muscle contraction = blood pressure (the “weight”)

Question 14

fill in the following equations:

BP = ____ x ____

MAP = ____ x ____

Answer 14

BP = CO x TPR
where TPR = total peripheral resistance (via vascular smooth muscle)

MAP = CO x TPR
where MAP = mean arterial pressure

these are the same equations, the MAP is just a more accurate way of describing this relationship

Question 15

what are beta blockers used for?

Answer 15

beta blockers = beta1 antagonists

antagonize beta1 adrenal receptors expressed in myocardium of SA node (activated by nor/epinephrine to increase HR) —> slow down HR and CO, used to lower blood pressure

[recall BP = CO x TPR]