CVS 4 - Electrical Properties of the Heart Flashcards
Briefly describe the potassium hypothesis.
2 chambers separated by a semi permeable membrane (K+). KCl added.
Chemical gradient opposes electrical gradient (but chemical gradient stronger).
K+ moves down its conc gr, carrying its +ve charge. Makes positive area more positive.
Eventually, chemical gradient and electrical gradient are equal - EQUILIBRIUM.
NO net movement of ions
How is resting membrane potential established?
Through movement of potassium through channels.
NOT SODIUM POTASSIUM PUMP
What is the Nernst Eqn.
Eqn to predict resting membrane potential.
What does the sodium-potassium pump maintain?
K+ conc
Why is the Goldman-Hodgkin-Katz eqn better to account for the relative permeabilities of the membrane?
It takes into account relative permeabilities of membrane to different ions.
Nernst eqn only takes into account 1 ion (K+).
How long do APs in the heart last?
200-400ms
Longer than in nerves which are 2-3ms.
Heart is required to be long and slow to produce an effective pump - action potential duration controls duration of contraction of the heart
What are the 5 phases of Cardiac AP?
- Phase 0 = upstroke
- Phase 1 = early repolarisation
- Phase 2 = plateau
- Phase 3 = repolarisation
- Phase 4 = Resting membrane potential (diastole)
Explain the process of a cardiac action potential.
- Phase 0 - upstroke. Occurs due to large increase in membrane permeability to Na.
(2. Phase 1 - early Repolarisation. Caused by sodium channels inactivating and brief increase in potassium membrane permeability. K efflux causes some repolarisation - TRANSIENT OUTWARD K current.) - Phase 2 - plateau. During the early part of plateau, Ca influx occurs to trigger Ca release from intracellular stores. Ca influx balances K efflux so membrane depolarised at plateau value.
- Phase 3 - repolarisation. K efflux gradually overcome Ca influx. Once partial repolarisation has occurred, large K currents becomes activated.
- Ik1 fully repolarises the cell.
Describe IK1 potassium channels. What do they do?
Large K current and flows during diastole. Activated by gradual repolarisation.
IT STABILISES RESTING MEMBRANE POTENTIAL AND REDUCES THE RISK OF ARRHYTHMIAS (by requiring large stimulus to excite the cells)
Name 3 drugs that can inhibit Ca permeability and what phase of Cardiac AP do they act on?
- Nifedipine
- Nitrendipine
- Nisoldipine
They block Ca entry by binding to L type Ca channels.
They act on phase 2 of the AP (Plateau)
What is the Absolute Refractory Period? (ARP)
The period which no AP can be generated regardless of stimulus intensity.
Occurs as Na channels are inactivated and cannot be activated for a long period.
Na channels reactivated when membrane is repolarised
What is the benefit of a long ARP?
It means the muscle cannot be restimulated quickly so summation of contraction cannot occur, meaning the cardiac muscle will not tetanise.
(in cardiac muscle you can’t restimulate until the process of contraction is well underway).
What is relative refractory period? (RRP)
Period after ARP whereby AP can be generated only with stimulus strength larger than normal.
What is full recovery time?
Time at which normal AP can be generated with normal stimulus.
Why can the heart continue beating independent of its nerve supply?
Due to the myogenic nature of the nerve impulses, as the heart has its own electrical impulse generation and propagation system.
It is modulated by sympathetic and parasympathetic nervous system. (AUTONOMIC)
Why do different parts of the heart have different AP shapes?
Different ionic currents are flowing, which is due to different expression of ion channels.
Describe differences between SAN and Ventricular cells
- SAN cells have no IK1.
- SAN therefore do not have a very stable membrane potential.
- SAN cells do not have many Na channels, so little Na influx (hence the slower upstroke).
- SAN Upstroke produced by Ca influx - T-type Ca channels.
- T type Ca channels activate at more negative potentials than L type Ca channels - SAN has a pacemaker current present.
(6. SAN has a very small transient outward)
GRADUAL UPWARD SLOPE BEFORE THRESHOLD POTENTIAL IS PACEMAKER POTENTIAL
Explain what effect sympathetic stimulation and parasympathetic stimulation has.
- Sympathetic stimulation - makes pacemaker potential steeper = threshold potential reached quicker.
- Parasympathetic stimulation means decrease in pacemaker potential gradient = takes longer to reach threshold, decreasing heart rate.
Which 2 nerves are involved in autonomic modulation of HR?
- Vagus nerve - parasympathetic
2. (Cardiac) Sympathetic nerves - sympathetic
Where is the SAN located?
- Lies just below epicardial surface between RA and SVC.
SAN cells mark the start of the conduction pathway
What are the 4 basic components of the hearts conduction system?
- SAN
- Internodal Fibre Bundles
- Atrioventricular Node
- Ventricular Bundles (bundle branch and Purkinje fibres)
How are impulses propagated?
- Gap junctions mean that there is low resistance between cells. Allows depolarisation of one cell to be carried to the next.
- The effect of depolarisation gradually decays meaning it has less effect on cells further away.
- Gap junctions exist in CLUSTERS at INTERCALATED DISCS.
4 basic principles of ECG (upward/downward deflections)
UPWARD:
- Depolarisation towards positive electrode
- Repolarisation away from positive electrode
DOWNWARD:
- Depolarisation away from positive electrode
- Repolarisation towards positive electrode
Describe the excitation sequence of the heart.
- SAN fires, AP propagates along atria. (upward deflection as towards electrode).
- AVN, which has a 0.1s delay to allow atrial systole. AP downwards (BIG UPSTROKE). Once AP reaches apex, AP goes upwards through Purkinje fibres, downstroke.
- Depolarisation, goes upwards from epicardium to endocardium. Causes upstroke (T wave)
LEARN ECG VALUES
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