Excitation-Contraction Coupling

Question 1

Which bands do not shorten in the sarcomere?

Answer 1

A band doesn’t change length

Question 2

Which bands in the sarcomere do shorten?

Answer 2

H zone (band) and I band

Question 3

What filament does titin anchor in the sarcomere?

Answer 3

titin anchors myosin

Question 4

What are terminal cisternae

Answer 4

SR which is part of the internal membrane system which function to store calcium

Question 5

What are the thin filaments of the muscle?

Answer 5

actin, tropomyosin, troponin which is composed of troponin-T, troponin - I, and troponin - C

Question 6

What does troponin-T bind to?

Answer 6

binds to tropomyosin

Question 7

What does troponin-C bind to?

Answer 7

binds to Ca2+

Question 8

What does troponin-I bind to?

Answer 8

binds. to actin and inhibits contraction

Question 9

Difference between isotonic and isometric contraction.

Answer 9

muscle shortens in isotonic contraction;

muscle doesn’t shorten in isometric contraction (force produced not sufficient to move the load)

Question 10

Does calcium flux through the dihydropyridine DHP in sk. muscle?

Answer 10

no although is is called a V-gated Ca2+ channel it is more of a voltage sensor

Question 11

How is Ca2+ taken back into the SR?

Answer 11

Ca2+ is pumped back into the SR by a Ca2+ ATPase on the SR membrane called sarcoplasmic reticulum calcium ATPase (SERCA)

Question 12

What are the 2 ATPases involved in contraction?

Answer 12

myosin ATPase
SERCA

Question 13

What is summation?

Answer 13

because membrane has depolarized well before force development multiple APs can be generated prior to force development

This summation can continue until the muscle tetanizes

Question 14

What is recruitment?

Answer 14

A single alpha motor neuron innervates multiple muscle fibers

The alpha motor neuron and all fibers it innervates is a motor unit

recruitment means activating more motor units; which in turn means more muscle fibers, causing greater force production

Question 15

What are the differences in cardiac muscle and striated muscle on creating contraction?

Answer 15

• Extracellular Ca2+ is involved in cardiac contractions, but not skeletal muscle. This extracellular Ca2+ causes calcium-induced calcium release in cardiac cells
• Magnitude of SR Ca2+ release can be altered in cardiac mechanics but not sk. muscle.
• Cardiac cells are electrically coupled to gap junctions
• Cardiac myocytes remove cytosolic Ca2+ in 2 ways:
  SERCA and
  Na+/Ca2+ exchanger

Question 16

Dysfunction in titin protein has been associated with what conditions of the heart?

Answer 16

dilated and restrictive cardiomyopathies

Question 17

Describe how smooth muscle contracts.

Answer 17

1. Increasing IP3 evokes Ca2+ efflux from SR. IP3 is increased by an agonist binding to Gq receptors (e.g alpha 1 receptor)
2. cytosolic Ca2+ binds to calmodulin (CAM) which activates MLCK which in turn phosphorylates MLC
3. phosphorylation of MLC causes binding of actin and myosin, in turn eliciting a contraction of smooth muscle
4. ATP dissociates actin and myosin. I MLC remains phosphorylated, then actin and myosin rebind (sim. to cross-bridge cycling
5. MLC phosphatase dephosphorylates myosin, reducing affinity of myosin for actin > relaxation
6. When cytosolic Ca2+ is high, MLCK dominates. When cytosolic Ca2+ is low, MLC phosphatase dominates
7. smooth muscle reduces cytosolic Ca2+ via same mechanism described above for cardiac cells