Lecture 11: Initiation and Regulation of Heart Beat

Question 1

What are the 2 outputs of the ANS to the heart and what nerves does it use and where do they terminate

Answer 1

Parasympathetic pathway (vagus nerve), sympathetic pathway (cardiac accelerator nerves and vasomotor nerves). Both pathways nerves terminate in the SA node and AV node. Symp goes also to atrial myocardium goes ventricular myocardium

Question 2

What output does the parasympathetic output do

Answer 2

It decreases the heart rate by slowing the rate of spontaneous depolarisation in autorhythmic fibres. The parasympathetic pathway works at rest to make the resting heart rate slower than autorhythmic rate and when the heart rate is increased it is dampened.

Question 3

What output does the sympathetic output do

Answer 3

The nerves go from thoracic region of spinal cord and increase the rate of spontaneous depolarisation in the SA and AV node which increases heart rate. Contractility of the atria and ventricles is increased because of enhanced Ca2+ entry, therefore stroke volume is increased because increased contractility offsets decreased preload.

Question 4

How do vasomotor nerves in the sympathetic NS affect heart rate

Answer 4

It triggers vasoconstriction which increases blood pressure

Question 5

What input does the brain get that triggers its output

Answer 5

From sensory receptors: 
Baroreceptors: monitor blood pressure
Chemoreceptors: monitor blood chemistry 
Proprioceptors: monitor movements
As well as input from higher centres of cerebral cortex, limbic system and hypothalamus

Question 6

What are the 3 stages of the cardiac action potential in contractile fibres

Answer 6

1. Depolarisation: Excitation is initiated by SA node and when reaches fibre it causes voltage gated Na+ channels to open causing rapid depolarisation
2. Plateau: depolarisation is maintained by Ca2+ inflow when voltage gated Ca2+ channels open and K+ outflow
3. Repolarisation: After a delay Ca2+ channels inactivate and additional K+ channels open and outflow of K+ restores negative resting membrane potential

Question 7

What is the main difference between cardiac and normal nervous AP

Answer 7

The AP of cardiac is much longer than others because of the extended plateau phase. This ensures the AP lasts as long as the contraction of the cell. This ensures the excitation of myocardium is unidirectional because of refractory period

Question 8

What does the P wave, QRS complex, S-T and T wave relate to in the ECG of a sum of the electrical activity on the chest

Answer 8

P wave is a small bump that represents atrial depolarisation, which is initiated by the AP in the SA node and causes atrial contraction up until Q.
At QRS, there is onset of depolarisation of ventricular contractile fibres.
At S to T there is a flat section where there is ventricular contraction.
At the T wave there is repolarisation of the ventricular fibres.

Question 9

How is the cells in the SA node a ‘pacemaker’

Answer 9

they have an unstable resting membrane potential that has progressive depolarisation between successive APs.