Excitation Contraction Coupling in Cardiac and Skeletal Muscle

Question 1

What is excitation contraction coupling?

Answer 1

It is the linkage between excitation of the muscle fibre membrane and the onset of contraction

Question 2

What is the latent period?

Answer 2

It is the period of time between peak membrane potential and when contraction begins

Question 3

Describe the role of the transverse tubule (T-tubule) in the contraction of muscle

Answer 3

Action potential propagates from the motor end plate along the sacolemma down the t-tubule. The change in voltage is sensed by dihydrapyridine receptors on the T-tubule which then undergo a conformational change which allows it to interact with the ryanodine receptor via mechanical coupling. The ryanodine receptor will then open releasing calcium.

Question 4

What are the junctional foot proteins?

Answer 4

• Dihyrapyridine receptor protein (L-type voltage gated calcium channel) found in the T-tubule membrane
• Ryanodine receptor protein (RYR1) which is found in the SR membrane

Question 5

What occurs when the Ryanodine receptors open?

Answer 5

It allows for calcium to flow only its concentration gradient into the cytoplasm. The cytoplasmic Ca concentration increases from 10(-7)M to 10(-5)M

Question 6

Name a voltage gated calcium channel blocking drug and what is it used for

Answer 6

Nifedipine. It is used to treat hypertensions, migranes and atherosclerosis (disease where plaque builds up in arteries)

Question 7

Name a drug that acts as a skeletal muscle relaxant and what it is used for

Answer 7

Dantrolene and it is used to treat malignant hypothermia

Question 8

What is SERCA and how is it activated?

Answer 8

SERCA - Sarcoplasmic Endoplasmic Reticulum Calcium ATPase (Ca-ATPase). It is activated by an increase in intracellular calcium concentration

Question 9

What is the role of the SERCA/Ca-ATPase

Answer 9

It facilitates the active transport of calcium from the cytoplasm into the sarcoplasmic reticulum. (Two Ca ions per ATP calcium molecule) Which causes the intracelluar concentration to decrease to 10(-7)M causing relaxation.

Question 10

What is Calsequestrin and what is its role?

Answer 10

It is a protein found in the sarcoplasmic reticulum that stores calcium at high concentrations so it establishes a concentration gradient

Question 11

How do action potentials in cardiac muscle differ from skeletal muscle

Answer 11

Cardiac - Pacemaker cells which undergo automatic rhythmical depolarisation. The action potentials are much longer in cardiac but contraction can occur during this action potential

Question 12

Describe how the parasympathetic NS innervates cardiac muscle

Answer 12

It releases acetyl-choline which slows the heart rate and is localised to pacemaker cells

Question 13

Describe how the sympathetic NS innervates cardiac muscle

Answer 13

It releases nor-adrenaline which increase heart rate and strength, It just diffuses

Question 14

What is key in BOTH cardiac and skeletal muscle to cause contraction

Answer 14

Increasing the intracellular calcium concentration

Question 15

How does the time between action potentials and contractions differ between cardiac and skeletal muscle?

Answer 15

In cardiac muscle, contraction occurs during an action potential however in skeletal muscle, the contraction occurs after an action potential propagates. So cardiac muscle doesn’t have a latent period

Question 16

In cardiac muscle what causes the release of calcium from the SR and how does this differ from skeletal

Answer 16

An action potential is propagated down the T-tubule which causes the dihydrapyridine to open. This allows an influx of calcium into the cell which binds to the ryanodine receptor (calcium ligand gated channel) causing it to open and allow calcium to flow out of the SR. Therefore it is different from skeletal as it does not involve mechanical coupling of the receptors

Question 17

Explain the structural differences in T-tubule and terminal cisternae in cardiac and skeletal muscle

Answer 17

Cadriac - Diad so one t-tubule to one terminal cisternae

Skeletal - Triad so one t-tubule sandwiched between two terminal cisternae

Question 18

How does relaxation occur in cardiac muscle?

Answer 18

By a decrease in intracellular calcium concentraion from 10(-5)M to 10(-7)M. This is achieved by the used of Ca-ATPase pumping calcium back into the SR and the sodium-calcium exchanger located in the sarcolemma