Heart

1
Mariebs Human Anatomy and Physiology Marieb w
Hoehn

• Chapter 18
• Heart
• Lecture 2Part 3a Cardiac Muscle Fibers,
  Cardiac conduction, the EKG

2
Cardiac Muscle Fibers

• Cardiac muscle fibers form functional syncytium -
  group of cells that function as a unit
• atrial syncytium
• ventricular syncytium

Separate units that are 1) separated by the
cardiac skeleton 2) connected by fibers of the
cardiac conduction system
3
Muscle Fibers in Ventricular Walls
Figure from Holes Human AP, 12th edition, 2010
4
Cardiac Conduction System
Figure from Holes Human AP, 12th edition, 2010
Specialized myocardial cells. Instead of
contracting, they initiate and distribute
impulses throughout the heart.
S-A node Pacemaker
Pacemaker firing rates SA Node 80-100 / min AV
Node 40-60 / min Purkinje 30-40 / min
5
Cardiac Conduction Pathway
Figure from Holes Human AP, 12th edition, 2010
Know the sequence of this pathway
(of His)
6
Electrocardiogram

• recording of electrical changes that occur in
  the myocardium during the cardiac cycle
• used to assess hearts ability to conduct
  impulses, heart enlargement, and myocardial
  damage

Important points to remember - Depolarization
precedes contraction - Repolarization precedes
relaxation
P wave atrial depolarization QRS wave
ventricular depolarization T wave ventricular
repolarization
Three waves per heartbeat
7
Electrocardiogram
PR Interval 0.12 0.20 sec QT Interval 0.20
0.40 sec QRS Interval lt 0.10 sec
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
8
Review

• Cardiac conduction system
• Specialized myocardial cells
• Conduct rather than contract
• Autorhythmic (self-exciting)
• S-A node is pacemaker of heart (80 - 100 bpm)
• A-V node (40 - 60 bpm)
• Delays impulse to allow ventricular filling
• Located in interventricular septum
• A-V Bundle (Bundle of His)
• Only electrical connection between atria and
  ventricles
• Give rise to right and left bundle branches
• Purkinje fibers
• Large diameter, rapid conduction for ventricles
• Causes apex to contract first, then toward base

9
Review

• There are three major events (waves) in the ECG
• P wave atrial depolarization
• QRS complex ventricular depolarization
• T wave ventricular repolarization
• The different leads of an ECG can be used to
  localize heart muscle abnormalities
• Abnormalities in ECG presentation can be
  indicative of heart damage
• Several common cardiac abnormalities
• Arrhythmia
• Tachycardia (and bradycardia)
• Atrial flutter