9. Smooth and Heart Muscle

Question 1

Describe multiunit muscle

Answer 1

Gaps between cells
Each has own nerve supply: very dependent on nerve
Gap between neuromuscular junction is small
No action potentials

Question 2

Describe visceral muscle

Answer 2

Cells are close together
Share nerves: metabolites are more important
Poorly developed neuromuscular junction
Pacemaker cells (spontaneous activity)
Linked by gap junctions of nexi
Action Potentials and slow waves

Question 3

Describe excitation-contraction coupling in smooth muscle

Answer 3

1. Ca++ binds to calmodulin
2. Activates myosin light chain kinase
3. Activates myosin, allowing cross-bridge formation
4. Ca++/calmodulin binds caldesmon which allows cross-bridge formation

Question 4

Describe how smooth muscle relaxes

Answer 4

Ca++ is pumped out of the cell

Allows myosin phosphate to inactivate myosin

Question 5

What is plasticity?

Answer 5

Stretching causes contraction, but prolonged stretching causes relaxation

Question 6

Where does calcium come from in smooth muscle contraction?

Answer 6

ECF

Question 7

Where does calcium come from in heart muscle contraction?

Answer 7

ECF and SR

Question 8

Describe excitation-contraction coupling in heart muscle

Answer 8

1. Dihydropyridine receptor allows Ca++ from the T system into the cell
2. Activates ryanodine receptor to release Ca++ from T system into the cell
3. Cross-bridge cycling is similar to skeletal muscle
4. Ca++ exits cell via Na/Ca exchanger

Question 9

What is hypocalcaemic tetany?

Answer 9

Spasm of skeletal muscle due to increased motoneuron excitability

Question 10

What effect do Ca++ channel blocking drugs have on the cardiovascular system?

Answer 10

Reduce contractibility of the heart

Dilate blood vessels

Question 11

How might heart damage be indicated in the blood?

Answer 11

Part of the troponin complex is released into the blood after an MI

Question 12

Which parts of muscle mechanics does heart muscle display?

Answer 12

Length-tension relationship

Staircase phenomenon

Question 13

Which parts of muscle mechanics does heart muscle not display?

Answer 13

Summation

Tetanus

Question 14

What is the equivalent to preload in heart muscle?

Answer 14

Filling of the ventricle

Question 15

What is the equivalent to afterload in heart muscle?

Answer 15

Aortic pressure

Question 16

Why can heart muscle not be tetanised?

Answer 16

Action potential is so long that the muscle has begun to relax before the end of the refractory period