Initiation of the Heartbeat

Question 1

What is the resting membrane potential of a neuronal cell?

Answer 1

-70mV

Question 2

What is the resting membrane potential of a cardiac cell?

Answer 2

-90mV

Question 3

How long is the action potential of a neuronal cell?

Answer 3

SHORT = <1ms

Question 4

How long is the action potential of a cardiac cell?

Answer 4

MUCH LONGER = 250ms

Question 5

What is the refractory period of an action potential?

Answer 5

The time during an action potential during which a new potential can not be elicited?

Question 6

Which has a longer refractory period and why, cardiac or neuronal action potentials?

Answer 6

Cardiac = much longer to prevent wave summation and tetany.

Question 7

What is the relative refractory period?

Answer 7

The time during an action potential where the cell is hyperpolarised, therefore a 2nd action potential may be elicited but will only be triggered by a larger than normal stimulus.

Question 8

What are the X3 currents (in order) of a cardiac action potential?

Answer 8

1) INa (influx)
2) ICa (influx)
3) IK (efflux)

Question 9

Do a small or large number of ions cross the membrane in each cardiac cycle?

Which pump do they use to get back to their original sides of the membrane to maintain the ion gradients?

Answer 9

Only a small number

They use Na/K ATPase pumps

Calcium leaves via a Na/Ca exchanger pump

Question 10

Which ecg interval corresponds to the duration of one full cardiac action potential?

Answer 10

The Q-T interval

Question 11

As HR increases, what happens to the Q-T interval and therefore the action potential duration?

Answer 11

They decrease in time to allow more beats (and therefore potentials) to occur per minute

Question 12

What is a normal Q-T interval time?

Answer 12

350-380ms

Question 13

What is QTc and what should the value ideally be?

Answer 13

QTc is a graph of QT interval times corrected for HR, for which the QT interval time should be <400ms

Question 14

What is a re-entrant arrhythmia and what aspect of the cardiac action potential protects against this?

Answer 14

It occurs where the impulse fails to die and re-excites the heart after the refractory period ends. The refractory period aims to protect against this.

Question 15

Which areas of the heart show diastolic depolarisation (will depolarise on their own) and which do not?

Answer 15

The SA and AV nodes, and the rest of the conducting system of the heart so (at decreasing rates). All other myocardiocytes do not.

Question 16

What are the X2 theories surrounding diastolic depolarisation?

Answer 16

The membrane clock theory

The calcium clock theory

Question 17

Describe the membrane clock theory.

Answer 17

Cyclical changes in ionic currents drive the membrane to threshold (as the membrane gets more negative it stimulates the funny current)

Question 18

Describe the calcium clock theory.

Answer 18

Cyclical release of intracellular calcium stores drive the membrane potential to threshold.

Question 19

What ion makes up the funny current?

Answer 19

Sodium and potassium

Question 20

What stimulates (SNS), inhibits (PNS) and blocks (DRUG) the funny current?

Answer 20

Stimulates = adrenaline (SNS neurotransmitter - therefore speeds up HR)

Inhibits = acetylcholine (PNS neurotransmitter - therefore slows down HR)

Blocks = ivabradine

Question 21

What is the name of the gap junctions that link ventricular myocytes called?

Answer 21

Intercalated discs

Question 22

What is the difference between tight junctions and gap junctions?

Answer 22

Tight junctions = between epithelial cells and form a seal between the cells

Gap junctions = between cells in a tissue and allows passage of ions/small molecules

Question 23

Explain the structure of a gap junction, from the smallest component to the whole junction.

Answer 23

Smallest structure = connexINS = sit IN the membrane

X6 connexINS form a hemi-channel called a connexON, as it joins ONTO another hemi-channel in a neighbouring myocyte to form the gap junction

Question 24

What is anisotropic conductance?

Answer 24

Where impulses travel easier ALONG fibres instead of across them due to the connexons forming gap junctions.

Question 25

What are the X2 methods by which calcium is removed from the myocardiocytes to cause relaxation?

Answer 25

SERCA pump in the sarcoplasmic reticulum

Na/Ca exchanger in the cell membrane

Question 26

Which accessory protein regulates the activity of the SERCA pump?

Answer 26

Phospholamban

Question 27

Which adrenergic receptor is responsible for increasing inotropy and chronotropy, and by which pathway?

How does it work on different parts of the cell?

Answer 27

Beta-1 adrenergic receptors, via the cAMP/PKA pathway

PKA phosphorylates:

• “L-type” calcium channels, increasing their opening (chronotropy and inotropy)
• Phospholamban, increasing calcium uptake (lusitropy)
• Ryanadine receptors (RYR2), increasing calcium release (inotropy)
• Troponin 1 and myosin binding protein c, increasing rates of cross bridge cycling (inotropy)

Question 28

The resting membrane potential is principally determines by which ion?

Answer 28

Potassium (K)

Question 29

What is the resting membrane potential of SA node cells?

Answer 29

More positive than regular myocardiocytes and closer to that of neuronal cells (around -70mV).

Question 30

Which connexin isoform is most abundant?

Answer 30

CX43

Question 31

What name is given to the process where by impulses travel easier ALONG fibres instead of across them due to the connexons forming gap junctions.

Answer 31

Anisotropic conductance.