Physiology Flashcards
Excitation of the heart normally originates in?
Pacemaker cells in the SA node
Where is the SA node located?
The upper right atrium close to where the Superior Vena Cava enters the right atrium.
A heart controlled by the sino-atrial node is said to be….
in sinus rhythm
Explain why potential is generated in the SA node
The cells in the SA node have no stable resting membrane potential.
The cells in the SA node generate regular spontaneous pacemaker potentials.
The spontaneous pacemaker potential takes the membrane potential to a threshold
Every time the threshold is reached an action potential is generated
This results in the generation of regular spontaneous action potentials in the SA nodal cells
How is the potential generated in the SA node spontaneous
In the pacemaker cells the permeability to K+ does not remain constant between action potentials
The pacemaker potential (i.e. the slow depolarisation of membrane potential to a threshold) is due to:
Decrease in K+ efflux
Na+ and K+ influx (the funny current)
Transient Ca++ influx (T-type Ca++ channels)
Once the threshold is reached the rising phase is caused by:
Activation of long lasting (L-TYPE Ca++ CHANNELS) resulting in Ca++ influx
The falling phase is caused by:
Inactivation of L-type Ca++ channels and
Activation of K+ channels resulting in K+ efflux
Explain how excitation spreads in the heart
Originates in SA node, spreads by cell to cell conduction to atrioventricular node.
From AV node impulse passes by special pathway through bundle of His.
This has left and right ventricles, passes through purkinje fibres.
The Bundle of His and its branches and the network of Purkinje fibers allow rapid spread of action potential to the ventricles
Excitation of the ventricles spreads by cell to cell conduction again.
How does cell to cell conduction work?
Flows by gap junctions
Purpose of the AV node
The AV node is the only point of electrical contact between atria and ventricles
The AV node cells are small in diameter and has slow conduction velocity
This allows atria to complete contraction before the ventricles contract.
Where is the AV node located?
The base of the right atrium; just above the junction of atria and ventricles
Explain the plateau phase of action potential
The membrane potential is maintained near the peak of action potential for few hundred milliseconds
This is called the plateau phase of action potential
It is a unique characteristic of contractile cardiac muscle cells
The plateau phase is mainly due to influx of Ca++ through L-type Ca++ channels
The heart rate is mainly influenced by the _________
Sympathetic stimulation _____the heart rate
Parasympathetic stimulation _____ the heart rate
ANS
increases
decreases
Under resting conditions what does the vagus nerve do?
Exerts a continuous influence on the SA node. Vagus nerve supplies both the SA and the AV node. Vagal tone dominates under resting conditions. Vagal tone slows the intrinsic heart rate from ~100 bpm to produce a normal resting heart rate of ~70 bpm
Normal resting heart rate is usually
60-100bpm
Define bradycardia and tachycardia
Bradycardia= resting heart rate under 60bpm Tachycardia= resting heart rate more than 100bpm
Vagal stimulation ____1____ heart rate and ______2____ AV nodal delay (_____3___ the slope of pacemaker potential)
Neurotransmitter is ____4___ acting through _______5____ Receptors
Vagus exerts a _______6_____ effect on heart (decreases heart rate)
1 slows 2 increases 3 decreases 4 acetylcholine 5 muscarinic M2 receptors 6 negative chronotropic effect
Cardiac sympathetic nerves supplies ______1_____
Sympathetic stimulation ____2____ heart rate and ____3____ AV nodal delay.
Neurotransmitter is _____4____
1 SA node and AV node and myocardium
2 increases
3 decreases
4 noradrenaline acting through B1 adrenoceptors
Positive inotropes?
Negative inotropes?
Positive inotropes increase contractility
Negative inotropes decrease contractility
Normally heart valves produce a sound when they __1___, but normally not when they ___2___
1 shut
2 open
5 stages of the cardiac cycle?
1 Passive filling 2 Atrial contraction 3 Isovolumetric ventricular contraction 4 Ventricular ejection 5 Isovolumetric ventricular relaxation
Describe passive filling
- Pressure in atria and ventricles close to zero
- AV valves open so venous return flows into the ventricles
- Aortic pressure ~ 80 mmHg, and aortic valve is closed
- Similar events happen in the right side of the heart, but the pressures (right ventricular and pulmonary artery) are much lower
- Ventricles become ~ 80% full by passive filling
Describe atrial contraction
Atria contract to fill the rest of the ventricles
Describe Isovolumetric ventricular Contraction
Ventricles contract
Ventricular pressure rises
When the ventricular pressure exceeds atrial pressure the AV valves shut
This produces the first heart sound (LUB)
The aortic valve is still shut, so no blood can enter or leave the ventricle
Describe ventricular ejection
When the ventricular pressure exceeds aorta/pulmonary artery pressure the Aortic/pulmonary valves open. The SV is ejected by each ventricle leaving behind the ESV. Aortic Pressure rises. There will be ventricular depolarisation. The ventricles relax and the ventricular pressure start to fall. When the ventricular pressure falls below aortic/pulmonary pressure: aortic/pulmonary valves shut. This produces the second heart sound (DUB)
Describe isovolumetric ventricular relaxation
Closure of aortic/and pulmonary valves signals the start of the isovolumetric ventricular relaxation
As the ventricles are shut the tension falls around a closed volume. When the ventricular pressure falls below atrial pressure, AV valves open and the heart starts a new cycle.