Physiology 1 Flashcards
What is the heart?
An electrically controlled muscular pump
Where are the electrical signals that control the heart generated?
Within the heart itself
What is autorhythmicity?
Heart capable of beating rhythmically in absence of external stimuli
Where in the heart does excitation normally originate?
Pacemaker cells in sino-atrial node
What in the heart initiates the heart beat?
Pacemaker cells in sino-atrial node
Where is SA node located?
Upper right atrium
What is the function of the SA node?
Sets pace for entire heart
What is a heart controlled by SA node said to be?
In sinus rhythm
How does cardiac excitation normally originate?
* Cells in SA node do not have stable resting membrane potential
* Exhibit spontaneous pacemaker potential
* Spontaneous pacemaker potential take membrane potential above threshold to generate an action potential
What is the pacemaker potential?
Slow depolarisation of membrane potential to threshold
What is the ionic basis for spontaneous pacemaker potential? (3)
* Decrease in K+ efflux
* Na+ and K+ influx (funny current)
* Transient Ca++ influx
What channel type causes transient Ca++ influx?
T-type Ca++ channels
What is the ionic basis for pacemaker action potential?
Once threshold is reached:
* Depolarisation caused by activation of long lasting L-type Ca++ channels
* Results in Ca++ influx
What is repolarisation caused by? (2)
* Inactivation of L-type Ca++ channels
* Activation of K+ channels causing K efflux
Understand diagram
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How does cardiac excitation normally spread across the heart?
What allows the cell-to-cell current flow in the heart?
Gap junctions
What exists between cardiac myocytes?
Intercalated disc
What is the atrioventricular node?
Small bundle of specialised cardiac cells
Where is AV node located?
Base of right atrium
What is the only point of electrical contact between atria and ventricles?
AV node
Why is conduction delayed in AV node?
To allow atrial systole to precede ventricular systole
What is the function of the Bundle of His and Purkinje fibres?
Allow rapid spread of action potential to ventricles
How does AP spread through ventricular muscle?
Cell-to-cell conduction via gap junctions
Is the action potential in cardiac muscle cells the same as the action potential is pacemaker cells?
No, different
Do cardiac myocytes have an unstable resting potential like pacemaker cells?
No, resting membrane potential remains at -90 mV until cell is excited
What is phase 0 of action potential in atrial and ventricular myocytes?
Fast Na+ influx reverses membrane potential to +20 mV (depolarisation) (NOT calcium influx like in pacemaker cells)
What is phase 1, 2, 3 and 4 of action potential in atrial and ventricular myocytes?
* Phase 1 - closure of Na+ channels and transient K+ efflux
* Phase 2 - mainly Ca++ influx
* Phase 3 - repolarisation, closure of Ca++ channels and K+ efflux
* Phase 4 - resting membrane potential
What is plateau phase of action potential?
Unique to cardiac muscle cells - membrane potential is maintained near the peak of action potential for a few hundred milliseconds
What causes plateau phase of action potential?
Influx of Ca++ through L-type Ca++ channels (phase 2)
What causes repolarisation in cardiac muscle action potential?
Inactivation of Ca++ channels and activation of K+ channels resulting in K+ efflux
What is heart rate mainly influenced by?
Autonomic NS
What is the effect of sympathetic stimulation of the heart?
Increases HR
What is the effect of parasympathetic stimulation of the heart?
Decreases HR
Explain autonomic NS influence on normal resting heart rate
* Vagus nerve (parasympathetic) exerts continuous influence on SA node under resting conditions
* Vagal tone dominates under resting conditions
* Vagal tone slows intrinsic HR from 100 ppm to a normal resting HR of 70 ppm
Which group of people tend to have increased vagal tone?
Athletes
What is normal resting heart rate?
60 - 100 bpm
What is a resting heart rate less than 60? More than 100?
\<60 = bradycardia \>100 = tachycardia
What does vagus nerve supply in the heart?
SA and AV node
What is the effect of vagal stimulation of the heart? (2)
* Slows heart rate
* Increases AV nodal delay
What neurotransmitter is responsible for vagal parasympathetic stimulation of the heart?
Acetylcholine acting on muscarinic M2 receptors
What drug is given to patients with extreme bradycardia to speed up the heart?
Atropine - a competitive inhibitor of acetylcholine
What is the effect of vagal stimulation on pacemaker potentials? (4)
* Cell hyperpolarises
* Takes longer to reach threshold
* Slope of pacemaker potential decreases
* Frequency of AP decreases
What is the effect of vagal stimulation on pacemaker potentials, or anything that slows down heart rate known as?
Negative chronotropic effect
What do cardiac sympathetic nerves supply?
SA, AV node and myocardium
What is the effect of sympathetic stimulation on the heart?
* Increases HR
* Decreases AV nodal delay
* Increases force of contraction
What neurotransmitter is responsible for sympathetic stimulation of the heart?
Noradrenaline acting through B1 adrenoceptors
What is the effect of noradrenaline on pacemaker cells? (3)
* Slope of pacemaker potential increases
* Pacemaker potential reaches threshold quicker
* Frequency of action potential increases
What is the effect of noradrenaline on pacemaker potentials, or anything that speeds up heart rate known as?
Positive chronotropic effect
What is an ECG?
Depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface
What are standard limb leads of ECG? (3)
* Lead I: right arm - left arm
* Lead II: right arm - left leg
* Lead III: left arm - left leg
What are the ECG waves?
* P - atrial depolarisation
* QRS complex - ventricular depolarisation/arial repolarisation * T - ventricular repolarisation
What are the ECG intervals?
* PR interval: AV node delay
* ST segment: ventricular systole
* TP interval: diastole
