Origin and conduction of cardiac impulse (tbc)

What is autorhythmicity in the heart?

The electrical signals that control the heart are generated within the heart itself. It is capable of functioning rhythmically without any external stimuli. This is called the auto rhythmic nature of the heart.

Where does the excitation of the heart normally originate?

Pacemaker cells in the SA node

Where is the SA node located?

Upper right atrium; at the junction of the Superior vena cava and the opening of the right atrium.

What is sinus rhythm?

A heart controlled by the impulses generated by the Sino atrial (SA) node is said to be in sinus rhythm. Signup and view all the answers

Do the cells in the SA node have a stable membrane potential in a resting state?

No Signup and view all the answers

What is the pathway of generating an action potential in the cells of the SA node?

<ol> <li>Pacemaker potential- slow depolarisation of a membrane till the threshold.</li> <li>Once the threshold is reached, it triggers the rising phase of action potential through depolarisation.</li> <li>Falling phase of action potential through repolarisation.</li> </ol> Signup and view all the answers

Describe the ionic basis of the spontaneous pacemaker potential?

The pacemaker potential is the first step in the generation of an action potential in the SA node. It is the slow depolarisation of the membrane potential till the threshold. It is caused by:- <ol> <li>Funny current- depolarising cation current due to the slow influx of Na+ ions through HCN channels.</li> <li>Decreased K+ ion efflux</li> <li>Ca 2+ ion influx</li> </ol> Signup and view all the answers

Describe the ionic basis for the rising phase of the pacemaker action potential.

Once the threshold is reached via slow depolarisation, the rising phase of action potential is caused by:- <ol> <li>Opening of L type voltage gated Ca 2+ ion channels</li> <li>This results in Ca 2+ influx.</li> </ol> Signup and view all the answers

Describe the ionic basis for the falling phase of the pacemaker action potential.

Once the depolarisation reaches the peak, there is the falling phase of the action potential that is caused by:- <ol> <li>Inactivation of the L type voltage gated Ca 2+ ion channels.</li> <li>Activation of K+ ion channels resulting in K+ ion efflux. This causes the repolarisation of the pacemaker cells.</li> </ol> Signup and view all the answers

Where is the AV node located?

The AV node is located at the base of the right atrium just above the junction of the atria and ventricles. Signup and view all the answers

What is the AV node made of?

The AV node is a small bundle of specialised cardiac cells. The AV node cells are small in diameter and has a slow conduction velocity. Signup and view all the answers

What is special about the AV node in terms of connecting the atria and the ventricles?

The AV node is the only point of electrical contact between the atria and the ventricles. Signup and view all the answers

What is the pathway of the spread of excitation in the heart?

<ol> <li>From SA node to AV node- cell to cell conduction through gap junctions across the atria (there are also other internodal pathways).</li> <li>Delay of conduction in the AV node- allows the atrium to complete contraction</li> <li>AV node spreads action potential to ventricles rapidly through Bundle of His (left and right) and Purkinje fibres.</li> <li>Cell to cell conduction of impulses in the ventricular muscles.</li> </ol> Signup and view all the answers

What is the pathway of generation of action potential in the contractile myocytes?

There is a resting membrane potential at -90 mV. <ol> <li>Rising phase of action potential- depolarisation</li> <li>Plateau phase- The membrane potential is maintained near the peak of action potential for a few hundred milliseconds.</li> <li>Falling phase of action potential- repolarisation. Then it returns to the resting membrane potential.</li> </ol> Signup and view all the answers

Describe the ionic basis of the rising phase of action potential in cardiac myocytes?

The resting potential remains at -90 mV until the excitation. <ol> <li>It is the depolarisation of the membrane potential caused by the fast Na+ influx.</li> <li>This rapidly reverses the membrane potential from -90 mV to +20 mV. This is known as Phase 0 of action potential in the contractile cardiac muscle cells.</li> </ol> Signup and view all the answers

Describe the ionic basis of the plateau phase of the action potential in contractile myocytes.

The membrane potential is maintained near the peak of action potential for few hundred milliseconds causing the plateau phase of action potential. The plateau phase is due to:- <ol> <li>Closure of Na+ ion channels and transient K+ efflux. (Phase 1)</li> <li>Influx of Ca 2+ (L-type Ca 2+ channels)- Phase 2</li> </ol> The Phases 1 and 2 make the plateau phase of action potential. Signup and view all the answers

Describe the ionic basis of the falling phase of action potential in contractile myocytes.

The falling phase of the action potential through repolarisation is caused by:- <ol> <li>Inactivation of Ca 2+ channels and activation of K+ channels.</li> <li>This results in K+ efflux This is the Phase 3 of the action potential in contractile myocytes.</li> </ol> Signup and view all the answers

What is the Phase 4 in the action potential in contractile myocytes?

Resting membrane potential at -90 mV Signup and view all the answers

What is the ECG?

The ECG is a record of depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface. -The wave of depolarisation and repolarisation moves across the heart and sets up electrical currents which can be detected by surface electrodes. Signup and view all the answers

What does the P wave in the ECG correspond to?

Atrial depolarisation Signup and view all the answers

What does the QRS complex in the ECG correspond to?

Ventricular depolarisation (masks atrial repolarisation) Signup and view all the answers

What does the T wave in the ECG correspond to?

Ventricular repolarisation Signup and view all the answers

What does the PR interval in the ECG correspond to?

AV node delay Signup and view all the answers

What does the ST segment in the ECG correspond to?

Ventricular systole Signup and view all the answers

What does the TP interval in the ECG correspond to?

Diastole Signup and view all the answers

What is the influence of the autonomic nervous system on the heart rate?

Although the heart is auto rhythmic in nature and the SA node generates impulses, the autonomic nervous system modifies the heart rate. <ol> <li>Sympathetic stimulation- increases heart rate.</li> <li>Parasympathetic stimulation- decreases heart rate Their roles are reciprocal in nature.</li> </ol> Signup and view all the answers

What is the role of the vagus nerve on the heart rate?

The vagus nerve is the parasympathetic supply to the SA node of the heart. In resting conditions, the vagal tone dominates. It slows down the intrinsic heart rate from 100 bpm to 70 bpm (normal heart rate). Signup and view all the answers

What is the normal heart rate?

60-100 bpm Signup and view all the answers

What is bradycardia?

Resting heart rate lower than 60 Signup and view all the answers

What is tachycardia?

Resting heart rate above 100 bpm Signup and view all the answers

How does the vagus nerve slow down the resting heart rate?

The vagus nerve supplies to the SA and the AV node in the heart. <ol> <li>It slows down the rate of firing from the SA node</li> <li>It increases the AV node delay The neurotransmitter is acetylcholine acting through the muscarinic M2 receptors.</li> </ol> Signup and view all the answers

What is the effect of vagal stimulation on the pacemaker potential graph?

<ol> <li>The pacemaker potential takes longer to reach the threshold. Thus, the slope of pacemaker potential decreases.</li> <li>The frequency of AP decreases.</li> <li>Negative chonotropic effect</li> </ol> Signup and view all the answers

What is the effect of the cardiac sympathetic nerves on the heart rate?

The cardiac sympathetic nerves supply the SA node, AV node and the myocardium. <ol> <li>The sympathetic stimulation increases rate of firing from the SA node</li> <li>Decreases AV nodal delay</li> <li>Increases force of contraction. Noradrenaline is the neurotransmitter working through the beta 1 adrenoreceptors.</li> </ol> Signup and view all the answers

What is the effect of noradrenaline on the graph of pacemaker potential?

<ol> <li>Slope of pacemaker potential decreases</li> <li>Pacemaker potential reaches threshold quicker</li> <li>Frequency of action potential increases. This causes a positive chronotropic effect.</li> </ol> Signup and view all the answers

What causes the striation in the cardiac muscle?

It is caused by the regular arrangement of contractile proteins. Signup and view all the answers

How do cardiac myocytes transmit impulses to the next?

The cardiac myocytes are electrically coupled by gap junctions. <ol> <li>These are protein channels which form low resistance communication pathways between neighbouring myocytes.</li> <li>They ensure that each impulse reaches all cardiac myocytes through the all-or-none law of the heart.</li> </ol> Signup and view all the answers

What is the cell junction that allows transmission of mechanical tension through cardiac myocytes?

The desmosomes within the intercalated discs provide mechanical adhesion between adjacent cardiac cells. They ensure that the tension developed by one cell is transmitted on to the next. Signup and view all the answers

What is the structure of the muscle fibres?

Each muscle fibre (cell) contains many myofibrils- contractile units of muscle. The myofibrils have alternating segments of thick and thin protein filaments. (a) Actin (thin filaments): lighter appearance (b) Myosin (thick filaments): darker appearance. <ul> <li>Within each myofibril, actin and myosin are arranged into sarcomeres.</li> </ul> Signup and view all the answers

How are myofibrils arranged?

Myofibrils consist of sarcomeres- the main contractile unit of the muscle fibre. They have alternating segments of thick and thin filaments protein filaments. Actin (thin)- lighter appearance; Myosin (thick)- darker appearance Signup and view all the answers

What is the sliding filaments theory?

Muscle tension is produced by sliding of actin filaments on myosin filaments. The sliding filaments theory is the explanation how muscle shortens and produces force. Signup and view all the answers

What is the pathway of cardiac muscle fibre contraction?

<ol> <li>Muscle fibre relaxed due to troponin-tropomyosin complex covering the cross bridge binding site.</li> <li>Muscle fibre excitation caused by ventricular action potential generation (fast response).</li> <li>L type voltage gated Ca 2+ ion channels open allowing influx of Ca ions into the cytoplasm.</li> <li>This causes Calcium induced Calcium release (CICR) by the sarcoplasmic reticulum in the myocytes.</li> <li>Ca 2+ ions binds with the troponin- causing a conformational change to the myosin binding site- cross bridge occurs.</li> <li>Binding of actin/myosin cross bridge triggers power stroke pulling the actin filament (thin) close during contraction (systole).</li> <li>After contraction has occurred and the action potential has passed, Ca 2+ influx ceases due to the ion channels closing. The Ca ions need to be sequestered back to the sarcoplasmic reticulum via ATP use. This allows heart to relax- diastole.</li> </ol> Signup and view all the answers

What is the refractory period in the heart?

Period following an AP in which it is not possible to produce another AP in the heart. AP is action potential. Signup and view all the answers

Why can a new action potential not be produced in (a) the plateau phase and (b) the descending phase?

<ol> <li>During the plateau phase of ventricular action potential, the Na+ channels are in the depolarized closed state - not available for opening</li> <li>During the descending phase (repolarizing) of the AP the K+ channels are open, and the membrane cannot be depolarised.</li> </ol> Signup and view all the answers

Why do contractile myocytes have a refractory period?

The long refractory period prevents generation of tetanic contraction - protective function Signup and view all the answers

What is stroke volume?

The volume of blood ejected by each ventricle per heart beat. Contraction of the ventricular muscle ejects the stroke volume. Signup and view all the answers

What is the formula for stroke volume?

SV= End Diastolic Volume (EDV)- End Systolic Volume (ESV) Signup and view all the answers

How is stroke volume regulated?

It is regulated by extrinsic and intrinsic mechanisms. Intrinsic= within the heart muscle itself. Extrinsic= nervous and hormonal control. Signup and view all the answers

What is end diastolic volume?

The volume of each blood within each ventricle at the end of diastole. - Determined by the venous return to the heart Signup and view all the answers

What is cardiac output? How would you calculate it?

The volume of blood pumped by each ventricle per minute. CO = Stroke Volume x Heart Rate - If we regulate SV and HR, we will regulate SO Signup and view all the answers

What is the regular cardiac output in a healthy adult human?

5 litres/min Signup and view all the answers

What is the preload?

The diastolic length/stretch of myocardial fibres - Determined by EDV (end diastolic volume) Signup and view all the answers

What is afterload?

The force against which the ventricle contracts to eject blood during systole. - Determined by the MAP in the aorta, which is influenced by SVR Signup and view all the answers