Lecture 11 : The Initiation and Regulation of the Heart Beat

Question 1

What does the Depolarisation period of the Cardiac Action Potential Cycle consist of?

Answer 1

Fast initial upstroke,due tot he rapid depolarisation as Na2+ inflow when voltage gated fast Na2+ channels open - causes contraction

Question 2

What does the Plateau period of the Cardiac Action Potential cycle consist of?

Answer 2

Plateau phase of depolarisation (maintained depolarisation) due to the inward sustained Ca2+ current of movement, causing AP to last almost as long as the contraction of the cell
Because the muscle if refractory, both during and shortly after the passage of action potential- the long plateau phase ensures the unidirectional excitation of the myocardium. (means that a second contraction cannot be triggered and gives chance for heart to relax) Caused by the Ca2+ inflow when voltage-gated slow Ca2+ channels open and K+ outflow when some K+ channels are open
Ca2+ Inflow = K+ outflow
-much longer than nerves
-no ca2+ channel for nerve AP
-less likely to tetanus as long

Question 3

How are ventricular myocytes arranged?

Answer 3

In sheets
Lots of CT between those sheets
Coronary vessels

Question 4

What is the moment of the heart?

Answer 4

“Twisting and ringing motion”
Contracting and Turning
with wavering of pressure through coronary vessels

Question 5

What is the inherent rhythmicity of the heart?

Answer 5

inherent rhythmicity based on SA node 90-100 beats per min

no nervous input, but will still receive hormonal input

Question 6

What is the SA Node?

Answer 6

A specialised knot/bundle of cells in the heart which has an auto rhythmic function, generating a pattern of activation
the cells go through a cycle of generating AP with a known delay refractory period, and generate a known electrical activity which spread through the heart, causing coordinated contractile function (myocytes active in a pattern)

Question 7

What doe the phrase “ wavefront of electrical activity mean”?

Answer 7

Not all parts of the heart are activated at once

The Myocytes of the heart are activated in a sequence, which spread through the heart and down the Purkinje fibres

Question 8

What is the whole basis of the heart’s electrical activity?

Answer 8

To co-ordinate the mechanical activation and generate pressure

Question 9

What happens when the electrical activity does occur perfectly?

Answer 9

There electrical activity may not go through the tissue (dead tissue)
May be a defect in Purkinje Fibres so it doesn’t transfer
Region is now not acting in a co-ordinate fashion
Generations of force or pressure is less/not as efficient
seen through EF or Echo

Question 10

What does the Repolarisation Period of the Cardiac Action Potential consist of?

Answer 10

Occurs when Voltage gated Ca2+ channels Inactivate, causing repolarisation of myocardial cells
Closure of Ca2+ channels and K+ outflow when additional K+ channels open.
Resets back to Negative membrane potential.

Question 11

What happens if you change the ionic composition in blood?

Answer 11

Sensitive to changes in circulating K+, Na+ and Ca2+ levels in the plasma, and it could affect the potential to Generate AP,
therefore you need to Maintain Osmolarity(# or quantity in the plasma) and Ionic Gradient
Relies on Stability of sodium in the plasma
Affects ECG
Can alter saltiness/osmolarity of the plasma if you drink a lot of water, or become very dehydrated, worst case could have large effects on euro physiology and cardiac action potentials

Question 12

What does ECG stand for?

Answer 12

Electrocardiogram

Question 13

What is the role of an ECG?

Answer 13

Measures the Electrical activity in the heart
ECG is the summation of tall cardiac Action Potential from different parts of the body, as they spread through the chest wall, at different positions and different times of the cardiac cycle
Electrical activity can be measured throughout the body
The biggest electrical acuity in the body that we can measure is about 1-1.5 mV spread throughout tissue, and this can be picked up in different regions of the body
Connected by different (12) Lead Placement of different parts of the body, about 4 electrodes, getting different “view” of the heart (relative position is different)
you can get different Patterns relative to where the chest pattern view has been taken

Question 14

What is the feature of having water in our body?

Answer 14

Our body is mostly water
Water is a good conductor of electrical activity
this allows us to measure the spread of the measure of action potentials out over the heart and throughout the body
Comes out to the surface of the skin - where we can make the measurement of ECG

Question 15

What is an installed pacemaker?

Answer 15

Leaves have been put down one of the veins, and cathode gets pretty close to the RA, and are able to pulse and Resynchronise the electrical activity
Lots are sensing and analysing the ECG, but when ECG is disturbed, the pacemaker will act (give the person an electric shock) and get the heart back into rhythm.

Question 16

What happens if the AV node overtakes some of the roles of the Pacemaker?

Answer 16

The Inherent pacemaker like activity of the AV node isn’t as fast 50-40 bp
can replicate via having

Question 17

What is the RMP of Cardiac Action potential?

Answer 17

• 90 mV

Question 18

Which ions are involved in Cardiac Action Potential?

Answer 18

Sodium
Calcium
Potassium

Question 19

What is the duration of a cardiac action potential?

Answer 19

0.25s - 0.3 s minimum duration

Much longer than nervous AP

Question 20

What is the difference between Cardiac Action Potential and ECG?

Answer 20

ECG is measured on the Chest wall, measuring the Summation of all of the AP, getting to different parts of muscle fibres at the same time, spreading through not only the heart and water of our body (enabling us to measure it on the chest wall)
Cardiac Action Potential is the individual component which makes up the ECG

Question 21

What are one of the main treatments for improving contractile function/contractility?

Answer 21

Give Calcium Channel Blockers

Restoring and improving particular Contractility

Question 22

What are the differences between Cardiac AP and Nervous System AP?

Answer 22

Much longer duration that nerves (300ms vs 1-2ms)
No Ca2+ channels for Nerves AP
Cardiac AP is less likely to Tetanus as long

Question 23

What is the need for the plateau phase?

Answer 23

Has a long refractory period
Cannot induce another AP during this period
Means that the heart cannot contract any faster (max 200ms)

Question 24

What are the features of a ECG recording?

Answer 24

1. AP in the SA node
2. P wave = the electrical activity does to Atria first and causes Atrial Depolarisation (contract)
3. P-Q interval = represents the conduction time of the AP, from the beginning of atrial excitation(through atria, AV node and fibres)(Atrial systole/contraction), to the beginning of ventricular excitation
4. Q wave= Beginning of Ventricular Depolarisation
5. QRS Complex = Rapid Ventricular Depolarisation and Contraction (prior to)
6. S-T segment = represents the time when ventricular contraction fibres are depolarising the plateau of AP (ventricular systole/contraction)
7. T wave = repolarisation of ventricles. slower than depolarisation - wave wider
   roughly 300ms later
   After is ventricular diastole/relaxation
8. Q-T interval = time from ventricular depolarisation –> to repolarisation

Question 25

What does an enlarged P wave indicate?

Answer 25

Enlarged (Amplitude) of P wave indicated a enlarged atrium of atrial hypertrophy - preload on the heart

Question 26

What happens if there is an extended P-Q interval?

Answer 26

If there is a long P-Q interval, could be due to coronary artery disease
If so SA–> atria –> AV –> P Fibres
- the Path is LONGER due to/via the Scar tissue

Question 27

What does an enlarged Q wave indicate?

Answer 27

Enlarged Q wave may indicate presence of Myocardial infarction

Question 28

What does an enlarged R wave indicate?

Answer 28

Enlarged R wave indicates Enlarged Ventricles

Question 29

What does having a flat T wave indicate?

Answer 29

Having a flat T wave indicates Ischaemia

- receiving insufficient O2 –> coronary artery disease

Question 30

What does having an elevated T wave indicate?

Answer 30

Hyperkalemia –> high blood Potassium (K+) level

Question 31

What does having an extended Q-T interval indicate?

Answer 31

Myocardial damage/ischemia (low blood flow), conduction problem

Question 32

What is Ventricular Arrhythmia?

Answer 32

Heart attack
When the electrical activity generated become uncoordinated
in order to get contractile function working well to generate pressure, and eject blood LV–> Aorta you need to have COORDINATED Muscular EXCITATION via the ECG
If the ECG becomes uncoordinated it , the ventricles still contract, but are not co-ordinated/not contracting in a pattern
(shivering), and hence the heart isn’t pumping blood
- some pacemakers sense this arrhythmia and cause a defibrillation to occur, releasing an electric shock by discharging quickly, to try and resynchronise the heart

Question 33

How do you measure heart rate from ECG?

Answer 33

Intervals between R waves-

RR interval

Question 34

What is the Autonomic Nervous System?

Answer 34

Method in extrinsic regulation
Normal Nervous AP
Automatic heart rate set/adjusted on Demand
Bunch of cell groups based primarily (but no exclusively) on the brain stem
(not a control centre, but is a network)
–> Adjustments of heart rate are important in the short term control of CO (cardiac output) and BP (blood pressure)
Heart rate is lower than that of intrinsic pace of SA node (100 beats per min-1) due to the input received by the CNS (central nervous system)

Question 35

What is the Cardiovascular Centre responsible for?

Answer 35

Cardiovascular Centres in the medulla is responsible for the regulation of HR (para + sym) and SV (sym)

Question 36

When does ventricular contraction occur?

Answer 36

A little after the R wave - Ventricular systole

Question 37

What does the Parasympathetic Nervous System Consist of?

Answer 37

“brake”
Vagus Nerve (10th cranial nerve, down by the carotid), Goes to Heart (specifically SA node), bladder and Gut is involved in SLOWING down the Heart Rate (normal nervous AP firing)
Releases ACh which SLOWS HR by slowing down the Rate of Spontaneous Depolarisation of the SA node
40-50 bpm or lower
Vagal Activity
This nervous system is also centrally involved in blood pressure control
Tonic Activity resting level of activity/baseline
- continuous ongoing activity of ANS to keep heart down
Parasympathetic Nervous system ONLY affects HR –> not contractility as only few vocal fibres innervate

Question 38

What does the Sympathetic Nervous System Consist of?

Answer 38

Increases HR
Sympathetic Nerves run from the Medulla –> Spinal Cord
-cardiac accellerator nerves extend out to SA node, AV node and Myocardium
Increases Heart Rate
Slow to affect Heart Rate –> Parasympathetic is faster than Sympathetic
Regulates BOTH HR and Contractility, therefore (it is the only part of the NS which changes SV)
- Regulates Inotrophy (forcefulness of contraction (SV))
-Regulates Chronotrophy
(Heart Rate)
Comes down to different sections of the spinal cord, mostly in the thoracic regions
Receives input from “Higher Centres”

Question 39

What is the pathway of the Sympathetic Nervous system?

Answer 39

Uses different cell groups in the brain stem to generate its activity (Cardiac Accelerator Nerves)
Sympathetic nerves goes to the SA Node, and releases Neurotransmitters
Noradrenalin or Norepinephrine increases stimulation of innervating nerves which binds to Beta-1 Receptors on cardiac Muscle Fibres of Ventricles (can give compounds targeting these receptors HBP –> Beta blocker slows HR = lower SV = lower CO)
Noradrenalin speeds up the rate of spontaneous depolarisation, thus HR increases SA node and AV node fibres
Noradrenalin in contractile fibres promotes Ca2+ entry through voltage gated channels, therefore increasing contractility and more blood is ejected in contractile fibres

Question 40

What are the inputs to the cardiovascular centres?

Answer 40

We have a lot of input to the cardiovascular centres, coming from multiple pathways around the body, some located within the brain and some in the periphery, and is processed and then results in an adjustment via the sympathetic and parasympathetic nervous system
Feedback Pathway
From higher brain centres: Cerebral cortex, limbic system, and hypothalamus
From Sensory Receptors:
Proprioceptors - monitor movements
Chemoreceptors - monitor blood chemistry
Baroreceptors (arterial) - monitor blood pressure

Question 41

What are the outputs from the Cardiovascular centre to the heart?

Answer 41

1. Cardiac Accelerator Nerves (sympathetic): A. Increase rate of spontaneous depolarisation in SA node (and AV node) increases heart rate. B. Increased contractility of atria and ventricles increases stroke volume
2. Vagal Nerves (cranial Nerve X (10), Parasympathetic) : Decreased rate of spontaneous depolarisation in SA Node (and AV node) decreases Heart Rate

Question 42

What is the Cardiovascular centre?

Answer 42

Autonomic Nervous system
Is NOT a control centre
It is just a groups of cells/network which are located in the back of the brain in a range of cells in the brain stem

Question 43

How do you activate the higher centres in the brain?

Answer 43

Mental stress situation, will lead to an activation of the sympathetic nervous system, which will increase HR and SV(contractility)