Cellular And Molecular Events

Question 0

Describe changes in pacemaker cell membrane potential

Answer 0

There is the pacemaker potential (funny current) - influx of sodium by opening of HCN channels.
Opening of voltage gated Ca causes depolarisation once threshold is reached
Opening of voltage gated potassium channels causing repolarisation

Question 1

Describe and explain changes in membrane potential of ventricular cells

Answer 1

• Opening of sodium channels causes an influx - depolarisation
• Transient outflow of potassium causing a slight, sharp hyperpolarisation
• Opening of voltage-gated Ca channels causes calcium influx while potassium efflux is still occurring - slows down hyperpolarisation
• Ca channels are gradually inactivated and voltage gated potassium channels open, causing further hyperpolarisation

Question 2

What activates the funny current?

Answer 2

When membrane potential drops to less than -50mV

Question 3

What are HCN channels activated by?

Answer 3

Hyperpolarisation and cAMP

Question 4

What are cardiac cells joined electrically and mechanically by?

Answer 4

Electrically - gap junctions

Mechanically - desmosomes

Question 5

What are gap junctions?

Answer 5

Connection proteins which have come together to form a large, unselective ion channel.

Question 6

How does depolarisation lead to cardiac contraction?

Answer 6

Calcium enters via VOCCs/L-type Ca channels which are activated by membrane depolarisation from calcium in T tubules
During depolarisation, NCX channel is reversed, pumping sodium out and calcium in.
Also enters by ligand gated ion channel
Calcium-induced calcium release
A Gq protein on SR can be activated by the binding of an agonist. Phospholipase C causes PIP2 to be converted to IP3. IP3 binds to an IP3 receptor leading to calcium outflux because IP3 receptor contains an ion channel.
Calcium binds to troponin C, causing a conformational change. Shifts the tropomyosin to to reveal myosin binding site on actin filament

Question 7

How are cardiac myocytes relaxed?

Answer 7

Calcium needs to be returned back to resting levels
Most pumped back into SR by SERCA
Some exits across the cell membrane by the Na-Ca exchange and the sarcolemmal Ca-ATPase

Question 8

What stimulates SERCA?

Answer 8

Raised calcium concentration

Question 9

What ion channels are open at rest?

Answer 9

Leak potassium channels

Question 10

When intracellular calcium concentration increases, where does the calcium come from?

Answer 10

25% extracellulary

75% SR

Question 11

How are vascular smooth muscle cells stimulated to contract?

Answer 11

Calcium enters through L type calcium channels
4xCa binds to a calmodulin
This activates myosin light chain kinase (MLCK)
MLCK phosphorylates a light chain on the myosin head which enables myosin head to bind to actin

Question 12

How does smooth muscle relax?

Answer 12

Dephosphorylation by myosin light chain phosphatase

Question 13

What happens if MLCK is phosphorylated to protein kinase A

Answer 13

Inhibits muscular contraction

Question 14

Where are smooth muscle cells found in blood vessels and in which vessels?

Answer 14

The tunica media

Arteries, arterioles and veins

Question 15

Structure of cardiac myocytes?

Answer 15

Single central nucleus
Cells joined at intercalated discs
Have gap junctions which permit movement of ions and electrically couple the cells
Desmosomes rivet cells together

Question 16

What is calcium-induced calcium release?

Answer 16

Calcium binds to ryanodine on the sarcoplasmic reticulum

This causes a conformational change opens the channel in the ryanodine so calcium can move from the SR into the cell.