The Heartbeat

1
The Heartbeat

• A single contraction of the heart
• The entire heart contracts in series
• first the atria
• then the ventricles

2
2 Types of Cardiac Muscle Cells

• Contractile cells
• produce contractions
• Conducting system cells
• controls and coordinates heartbeat

3
The Cardiac Cycle

• Begins with action potential
• transmitted through conducting system
• produces action potentials in cardiac muscle
  cells (contractile cells)

4
What is the difference between nodal cells and
conducting cells what are the components and
functions of the conducting system of the heart?
5
The Conducting System

• A system of specialized cardiac muscle cells
• initiates and distributes electrical impulses
  that stimulate contraction
• Autorhythmicity
• cardiac muscle tissue contracts automatically

6

• Sinoatrial (SA) node
• pacemaker of the heart
• Atrioventricular (AV) node
• junction between the atria and ventricles
• Conducting cells
• connect the two nodes and distribute the
  contractile stimulus through myocardium

7
Conducting Cells

• In the atrium
• internodal pathways
• In the ventricles
• AV bundle, bundle branches and Purkinje fibers
• Electrocardiogram (ECG/EKG)

8
The Conducting System
Figure 2012
9

• SA Node in posterior wall of right atrium
• Contains pacemaker cells
• Depolarizes first, establishing heart rate
• Connected to AV node at junction of atria and
  ventricle by internodal pathways
• Delays impulse atrial contraction begins

10

• AV Bundle in interventricular septum carries
  impulse to left and right bundle branches
• Conducts impulse to Purkinje fibers
• Distribute impulse through ventricles
• Atrial contraction is completed
• Ventricular contraction begins

11
Impulse Conduction through the Heart
Figure 2013
12
Heart Rate

• Damage to the conducting system will disrupt
  normal rhythm of the heart
• Damage to the SA node or internodal pathway will
  result in the AV node acting as an ectopic
  pacemaker
• SA node generates 80100 action potentials per
  minute
• AV node generates 4060 action potentials per
  minute

13
Abnormal Pacemaker Function

• Bradycardia
• abnormally slow heart rate
• Tachycardia
• abnormally fast heart rate

14
What events take place during an action
potential in cardiac muscle?
15
Resting Potential

• Of a ventricular cell
• about 90 mV
• Of an atrial cell
• about 80 mV
• Threshold (TMP that will cause an action
  potential) is -75mV

16

• Similar to skeletal muscle in that AP leads to
  Ca release which then binds to thin filament
  causing contraction
• Differ in nature of AP, source of Ca and
  duration of contraction

17
Steps of Cardiac Contraction

• Rapid depolarization
• voltage-regulated sodium channels (fast channels)
  open

18

• Plateau
• As sodium channels close
• voltage-regulated calcium channels (slow
  channels) open
• Na ions pumped out
• holds membrane at 0 mV plateau

19

• Repolarization
• plateau continues
• slow calcium channels close
• slow potassium channels open
• rapid repolarization restores resting potential

20
The Refractory Periods

• Absolute refractory period
• long
• cardiac muscle cells cannot respond
• Relative refractory period
• short
• response depends on degree of stimulus

21
Action Potentials in Skeletal and Cardiac Muscle
Figure 2015
22
What electrical events are associated with a
normal electrocardiogram?
23
Electrocardiogram (ECG)

• Electrical events in the cardiac cycle can be
  recorded on an electrocardiogram (ECG)
• Cardiac Arrhythmias
• Abnormal patterns of cardiac electrical activity

24
Typical ECG Tracing

• P wave
• atria depolarize
• QRS complex
• ventricles depolarize
• masks atrial repolarization
• T wave
• ventricles repolarize

25
The Electrocardiogram
Figure 2014b
26
What events take place during the cardiac cycle,
including atrial and ventricular systole and
diastole?
27
The Cardiac Cycle

• The period between the start of 1 heartbeat and
  the beginning of the next
• Includes both contraction and relaxation
• systole (contraction)
• diastole (relaxation)

28

• Atrial systole
• atrial contraction begins
• right and left AV valves are open
• Atria eject blood into ventricles filling
  ventricles
• Atrial systole ends
• ventricles contain maximum volume
• end-diastolic volume (EDV)
• Atrial diastole

29

• Ventricular systole
• isovolumetric contraction
• pressure in ventricles rises all valves are
  closed
• Ventricular ejection
• semilunar valves open and blood flows into
  pulmonary and aortic trunks
• Stroke volume (SV)
• Ventricular pressure falls and semilunar valves
  close
• End-systolic volume (ESV)

30

• Ventricular diastole
• ventricular pressure is higher than atrial
  pressure
• all heart valves are closed
• ventricles relax (isovolumetric relaxation)
• AV valves open and passive ventricular filling
  occurs

31
What is cardiac output, and what factors
influence it?
32
Cardiodynamics

• The movement and force generated by cardiac
  contractions
• End-diastolic volume (EDV)
• End-systolic volume (ESV)
• Stroke volume (SV)
• SV EDV ESV

33

• Cardiac Output
• Volume pumped by each ventricle in 1 minute
• Heart rate (HR) beats/min ? Stroke volume (SV)
  ml/beat
• Cardiac Reserve
• resting CO - max CO

34
What variables influence stroke volume?
35
2 Factors Affect EDV

• Filling time
• duration of ventricular diastole
• Venous return
• rate of blood flow during ventricular diastole

36
3 Factors that Affect ESV

• Preload
• ventricular stretching during diastole
• Contractility
• force produced during contraction, at a given
  preload
• Afterload
• tension the ventricle produces to open the
  semilunar valve and eject blood

37
What variables influence heart rate?
38
Factors affecting heart rate

• nervous system activity
• hormones
• drugs
• changes in ion concentration
• body temperature