Muscles 3

Question 1

Appearance of cardiac muscle

Answer 1

Striated

Question 2

Where are myocytes joined in cardiac muscle?

Answer 2

Intercalated disks

Question 3

Where does electric coupling happen?

Answer 3

At intercalated disk (between adjacent myocytes by means of gap junctions)

Question 4

What initiates an action potential?

Answer 4

A pacemaker cell in the sinoatrial node

Question 5

What causes a change in heart rate?

Answer 5

Changing sinoatrial node ap firing

Question 6

What are multiunit and unitary classes of?

Answer 6

Smooth muscle

Question 7

Apperance of smooth muscle

Answer 7

Non striated

Question 8

Describe a multiunit smooth muscle.

Answer 8

Electrical isolation of cells (fine control of ACh muscle fibre)
Uses varicosities (synaptic contacts) to connect to autonomic neurons in each smc

Question 9

Describe a unitary smooth muscle

Answer 9

Gap junctions for coordinated contraction
Autonomic neurons connect to only a few smc via varicosities (not every cell is innovated)

Question 10

What is a dense body?

Answer 10

Multiple actin fibres joining a point

Question 11

Do all smc generate action potentials?

Answer 11

No, some change depending on membrane potential

Question 12

Describe the mechanism for increasing intracellular calcium (skeletal)

Answer 12

1. Membrane depolarisation opens L type Ca2+ channel
2. Coupling between L type and Ca2+ release channel triggers Ca2+ release channel to open
3. Ca2+ exits sr (into cytoplasm) and activates troponin C, causing a muscle contraction
   Ca2+ release can also activate the Ca2+ release channels.

Question 13

What process occurs in T tubules?

Answer 13

Increasing intracellular calcium

Question 14

What is a Ca2+ release channel also known as?

Answer 14

Ryanodine receptor

Question 15

Describe the mechanism for increasing intracellular calcium (cardiac)

Answer 15

T tubule is only close to one branch of sr (dyad).
The influx Ca2+ through the T-tubule channel activates ryanodine receptors, triggering the release of Ca2+ from sr.
Calcium induced calcium release

Question 16

Describe the mechanism from which calcium is removed from the cytoplasm

Answer 16

1. Na-Ca exchanger and Ca2+ ATPase in the plasma membrane extrude Ca2+ from cell.
2. Ca2+ pump sequesters Ca2+ within sr.
3. Ca2+ is bound the sr by calreticulin and calsequestrin

Question 17

Describe the mechanism for increasing intracellular calcium in smooth muscle.

Answer 17

1. Change in Vm or action potentials activate L type Ca2+.
2. This leads to CIRC via activation of ryanodine receptors in sr membrane.
3. Activation of Gq coupled receptors leads to IP3 production and IP3 receptor stimulation in sr membrane.

Question 18

Describe sarcomere contraction

Answer 18

Tropomyosin wraps around myosin and interacts with actin
Troponin complex sits over actin sites
Regulated by changes in Ca2+

Question 19

Describe the events that follow calcium binding to troponin C in cross bridge formation.

Answer 19

A conformational change occurs, causing TnT to pull tropomyosin and TnI (troponin) out of the way.
Myosin can now interact with actin.

Question 20

Describe the mechanism of contraction in smooth muscle

Answer 20

Ca2+ activates calmodulin to form calcium calmodulin complex.
Activation of myosin light chain kinase causes phosphorylation of mlc, removing c and c inhibitory effects.
This facilitates cross bridge formation and therefore contraction.

Question 21

How is smooth muscle contraction inhibited?

Answer 21

Dephosphorylation of mlc
- using myosin light chain phosphatase