Initiation of the Heartbeat Flashcards
What is the resting membrane potential of a neuronal cell?
-70mV
What is the resting membrane potential of a cardiac cell?
-90mV
How long is the action potential of a neuronal cell?
SHORT = <1ms
How long is the action potential of a cardiac cell?
MUCH LONGER = 250ms
What is the refractory period of an action potential?
The time during an action potential during which a new potential can not be elicited?
Which has a longer refractory period and why, cardiac or neuronal action potentials?
Cardiac = much longer to prevent wave summation and tetany.
What is the relative refractory period?
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.
What are the X3 currents (in order) of a cardiac action potential?
1) INa (influx)
2) ICa (influx)
3) IK (efflux)
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?
Only a small number
They use Na/K ATPase pumps
Calcium leaves via a Na/Ca exchanger pump
Which ecg interval corresponds to the duration of one full cardiac action potential?
The Q-T interval
As HR increases, what happens to the Q-T interval and therefore the action potential duration?
They decrease in time to allow more beats (and therefore potentials) to occur per minute
What is a normal Q-T interval time?
350-380ms
What is QTc and what should the value ideally be?
QTc is a graph of QT interval times corrected for HR, for which the QT interval time should be <400ms
What is a re-entrant arrhythmia and what aspect of the cardiac action potential protects against this?
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.
Which areas of the heart show diastolic depolarisation (will depolarise on their own) and which do not?
The SA and AV nodes, and the rest of the conducting system of the heart so (at decreasing rates). All other myocardiocytes do not.
What are the X2 theories surrounding diastolic depolarisation?
The membrane clock theory
The calcium clock theory
Describe the membrane clock theory.
Cyclical changes in ionic currents drive the membrane to threshold (as the membrane gets more negative it stimulates the funny current)
Describe the calcium clock theory.
Cyclical release of intracellular calcium stores drive the membrane potential to threshold.
What ion makes up the funny current?
Sodium and potassium
What stimulates (SNS), inhibits (PNS) and blocks (DRUG) the funny current?
Stimulates = adrenaline (SNS neurotransmitter - therefore speeds up HR)
Inhibits = acetylcholine (PNS neurotransmitter - therefore slows down HR)
Blocks = ivabradine
What is the name of the gap junctions that link ventricular myocytes called?
Intercalated discs
What is the difference between tight junctions and gap junctions?
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
Explain the structure of a gap junction, from the smallest component to the whole junction.
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
What is anisotropic conductance?
Where impulses travel easier ALONG fibres instead of across them due to the connexons forming gap junctions.
What are the X2 methods by which calcium is removed from the myocardiocytes to cause relaxation?
SERCA pump in the sarcoplasmic reticulum
Na/Ca exchanger in the cell membrane
Which accessory protein regulates the activity of the SERCA pump?
Phospholamban
Which adrenergic receptor is responsible for increasing inotropy and chronotropy, and by which pathway?
How does it work on different parts of the cell?
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)
The resting membrane potential is principally determines by which ion?
Potassium (K)
What is the resting membrane potential of SA node cells?
More positive than regular myocardiocytes and closer to that of neuronal cells (around -70mV).
Which connexin isoform is most abundant?
CX43
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.
Anisotropic conductance.
