Cardiac, smooth and skeletal muscle

Question 1

Where are the 3 different muscle types found?

Answer 1

Question 2

Describe the structure of cardiac muscle cells.

Answer 2

• Small with a single, central nucleus
• Striated
• Adjacent cells are linked to each other through mechanical (adherens junctions and desmosomes) and electrical (intercalated disks) gap junctions.

Question 3

Describe the structure of smooth muscle cells.

Answer 3

• Not striated, even though they have actin and myosin filaments
• Long, with a single, central nucleus
• Adjacent cells are coupled to each mechanically (by paired Membrane Dense Areas - MDAs) and electrically (by gap junctions)
• Lack T-tubules and troponin
• Minimal SR

Question 4

Describe how smooth muscle contraction occurs.

Answer 4

1. Ca2+ ions bind to calmodulin.
2. Calmodulin binds to a kinase enzyme that phosphorylates the myosin light chains.
3. The myosin heads can bind to actin to start the contraction.
4. Contraction stops when a phosphatase enzyme dephosphorylates the myosin light chains.

Question 5

In skeletal muscle contraction, how does the AP in the T-tubule cause intracellular [Ca2+] to increase?

Answer 5

1. Confirmation change in voltage-sensor (L-type) Ca2+ channel in T-tubule membrane, which is in physical contact with RyR channel.
2. Opening of Ca2+ release channels of SR (RyR - Ryanodine receptor) allows Ca2+ to exit the SR.

Question 6

Where does the Ca2+ in cardiac muscle contraction come from?

Answer 6

• Some is released from SR via T-tubule AP
• Some enter the cell via voltage-gated Ca2+ channels
• Most is released from the SR by the rise in Ca2+ concentration inside the cell - ‘Calcium-induced Calcium Release’
• Sarcolemmal Ca2+/Na+ exchanger helps regulate strength of contraction by controlling the intracellular concentration of Ca2+

Question 7

Describe how [Ca2+]i is increased for contraction in smooth muscle cells.

Answer 7

Question 8

Describe how [Ca2+]i is decreased for relaxation in smooth muscle cells.

Answer 8

Question 9

How are tetanic contractions prevented in cardiac muscle?

Answer 9

• Both AP and contraction in cardiac muscle lasts 300ms
• So the twitch has finished by the time a new AP can be generated.
• This contrasts with skeletal muscle where the twitch contractions lasts 200ms but the AP duration is only 2ms.

Question 10

What are the two ways in which cardiac muscle regulates force?

Answer 10

1. Intracellular Ca2+ concentration
2. Muscle length

Question 11

Question 12

What are the similarities between cardiac muscle, smooth muscle and skeletal muscle?

Answer 12