CARDIOVASCULAR SYSTEM LECTURE 8:

Heart Has Intrinsic Rhythmicity

Heart can keep beating on its own outside of body if supplied with energy, oxygen, fluids, etc.

Cardiac Cycle (General)

Depolarisation → contraction → relaxation (refill with blood) 

Coordination required

Where does each Heart Beat Start

SA node → AV node (slow to allow atria to contract to top up ventricles before depolarisation & contraction) → purkinje networks (delivers rapid depolarisation through ventricles) 

Cardiac Muscle Action Potential: Depolarisation

Threshold → depolarisation

Rapid depolarisation due to opening of voltage-gated fast Na+ channels

Cardiac Muscle Action Potential: Plateau Phase

Plateau phase (maintained depolarisation) due to opening of slow voltage gated Ca2+ channels & closing of some K+ channels 

Long period allows Ca2+ to enter myocyte for contraction

Cardiac Muscle Action Potential: Repolarisation Phase

Due to opening of voltage-gated K+ channels (activated in depolarisation phase but opens now) & closing of Ca2+ channels

[timing of channels opening allows relaxation]

Pacemaker Cell AP

Start & regulate heartbeat:

Spontaneously depolarise to threshold → opening of fast voltage-gated Na+/Ca2+ channels → AP (Pacemaker Potential)

SA node Structure

Physically connected to CT to  generate contractions

Electrically connected via gap junctions, spreading AP throughout the heart. 

Resting Heart Rate: (Regulation of Heart Rate - Autonomic)

Resting heart rate dominated by parasympathetic nervous system

Vagus nerve carries signals from parasympathetic nervous system

Vagal activity slows down rate of SA & AV node depolarisation, decreasing HR

Cardiac Accelerator Nerves: (Regulation of Heart Rate - Autonomic)

Cardiac Accelerator Nerves of sympathetic system increases rate of SA & AV node depolarisation, increasing HR, atria & ventricle contraction & stroke volume.

Increasing HR: (Regulation of Heart Rate - Autonomic)

Decrease parasympathetic input or increase sympathetic input (both usually)

Althetes Resting Heart Rate:

Heart becomes bigger & stronger increasing pumping efficiency allowing for lower resting heart rate

Sympathetic & Parasympathetic nerves origin

Sympathetic & parasympathetic nerves originate from cardiovascular center in brain stem (all mammals)

Cardiovascular Centre

CV centre receives input from muscles, etc about exercise, oxygen level, etc
Receives constant info from heart & rest of circulatory system about blood pressure, etc.

Medulla Oblongata (CV Sensory)

Receives info from the brain, a conscious decision to exercise, increases HR.

SA Nodes (CV Sensory)

Have receptors that can detect & respond to hormones, receiving signals from other organs of the body. 

P wave: The Cardiac Cycle - Electrical & Mechanical Events (ECG)

SA node depolarisation → Atrial depolarisation → contraction

P-Q Inteval (Plateau): The Cardiac Cycle - Electrical & Mechanical Events (ECG)

AV node depolarisation delay where Ca2+ goes into myocytes

QRS: The Cardiac Cycle - Electrical & Mechanical Events (ECG)

Biggest, lots of fast AP stack up

Ventricular depolarisation

S-T Interval (Plateau): The Cardiac Cycle - Electrical & Mechanical Events (ECG)

Cardiac Cell AP plateau

T Wave: The Cardiac Cycle - Electrical & Mechanical Events (ECG)

Ventricular repolarisation

Q-T Interval: The Cardiac Cycle - Electrical & Mechanical Events (ECG)

Ventricle is fully repolarised

AV Node Clincal Implications

IF AV node is skipped, unhealthy etc

Not enough time for atria to top off ventricle → less efficient heart beat