Download ECG and the Heart*s Internal Conduction System

1-2 March 2016
The heart continues to beat after being removed from the
body!
Unlike skeletal muscle, the heart does NOT need to be
stimulated by the nervous system to contract. Instead, the
heart has its own pacemakers and intrinsic conduction
system to regulate contraction.

What events lead to skeletal
muscle contraction?
What part of this process
involves an electric current?
◦ Nerve releases neurotransmitter
into synaptic cleft
The action potential!
◦ Neurotransmitter causes action
potential along sarcolemma
During the action potential, the
membrane first depolarizes
then repolarizes.
◦ Action potential causes
sarcoplasmic reticulum to release Contraction occurs shortly after
the depolarization of the
calcium
membrane.
◦ Calcium exposes the myosin
binding sites on actin, leading to
muscle contraction

SA node is the heart’s natural pacemaker.
◦ It depolarizes ~75 times / min, to start each heartbeat.

Impulse travels
throughout the atria and
to the AV node
o

Travels cell-to-cell through
gap junctions in
intercalated discs – another
unique feature of cardiac
muscle
AV node delays impulse
transmission to allow
atria to finish contracting

The AV bundle (aka bundle of His) transmits the
impulse to the bundle branches and Purkinje fibers.
o
These bundles and fibers
speed the transmission of
the impulse throughout the
ventricles, to coordinate
ventricular contraction
Interestingly, all the cells of the
conduction system are
autorhythmic. That is, all will
depolarize at a certain rate. The
SA node has the fastest rate of
depolarization, though, so it sets
the pace for the entire heart.

What is the function of:
◦ AV node –
delays impulse so atria can
finish contracting, and conducts
to AV bundle
◦ Gap junctions –
Allow impulse to spread cell-tocell
◦ SA node
heart’s pacemaker – depolarizes
to start contraction
◦ Purkinje fibers
carry impulse to cells of
ventricles
Name 3 differences between cardiac and skeletal muscle
contraction & explain why each is important to cardiac function.
1.
2.
3.
Cardiac muscles do not need to be stimulated by nerves. This
allows heart to beat even if nervous system doesn’t function
well.
Cardiac muscles have autorhythmic / natural pace-makers to
keep the heart beating an an appropriate rate.
Cardiac muscle has special features to speed impulse
conduction from one cell to another, including special cells
(the AV bundle, branch bundles and Purkinje fibers) and gap
junctions between cells.


An electrocardiogram (ECG or
EKG) records the electrical
current through the heart.
A normal ECG has three
distinct waves:
◦ P wave – atrial depolarization
◦ QRS complex – ventricular
depolarization AND atrial
repolarization
◦ T wave – ventricular repolarization
Watch me!
A normal ECG has three distinct
waves:
◦ P wave – atrial depolarization
◦ QRS complex – ventricular
depolarization AND atrial
repolarization
◦ T wave – ventricular repolarization
What is responsible for these delays?
• Delay between P wave and QRS complex is caused by
AV node
• Delay between T and next P wave is determined by
heart rate / speed at which SA node depolarizes
“LUB” - closing
of AV valves as
ventricle begins
to contract
systole
diastole
systole
diastole
“DUP” - closing
of SL valves as
ventricle begins
to relax
Examine this normal ECG.
 When would the ventricles be in systole?
 When would they be in diastole?
 When would the heart sounds occur, and
what do the heart sounds correspond to?

Identify how the abnormal ECGs differ from
the normal one, and match them to their
disease.
a)
All waves present and
normal shape.
Heart beat is about
twice as fast.
TACHYCARDIA

Identify how the abnormal ECGs differ from
the normal one, and match them to their
disease.
b)
No relationship
between p waves and
QRS complex
AV block

Identify how the abnormal ECGs differ from
the normal one, and match them to their
disease.
c)
Waves are more-orless correct shape.
Heart rate is about 1.5
times slower than
normal.
Bradycardia

Identify how the abnormal ECGs differ from
the normal one, and match them to their
disease.
d)
No obvious p waves,
wandering baseline
(not so obvious)
Atrial fibrillation
Earlier, we said that the SA node depolarizes at a rate
of ~75 beats per minute, and that this acts as a pace
maker for heart contraction.
Does this mean our hearts always beat at ~75 bpm?
Of course not!
Our hearts do not require stimulation from the
nervous system to beat, however, they do respond to
stimulation from the nervous system by either
speeding up or slowing down.
Earlier, we said that the SA node depolarizes at a rate
of ~75 beats per minute, and that this acts as a pace
maker for heart contraction.
Does this mean our hearts always beat at ~75 bpm?
Of course not!
Our hearts do not require stimulation from the
nervous system to beat, however, they do respond to
stimulation from the nervous system by either
speeding up or slowing down.
What are the two branches of the autonomic nervous
system, and how do they affect heart rate?


Sympathetic (fight-or-flight / stress) – increases HR
Parasympathetic (rest & digest) – decreases HR
Other factors that influence heart rate
 Age (fastest in fetus, young children)
 Gender (faster in females)
 Temperature (faster in heat)
 Exercise (faster with exercise)
 Ions imbalances & medicines (faster or slower)
 Weak / damaged heart (can be either faster / slower)



Think back to the list of questions we made last class.
Were any answered today?
Have you developed any new questions?
Remember HW assignment: research one question and
be ready to answer.
1)
2)
3)
4)
5)
What serves as the heart’s natural pacemaker?
What event does a T wave correspond with?
Name one the stimulation of cardiac muscle differs
from skeletal muscle.
What does tachycardia refer to?
Which system slows down the heart?
E.C. What does ECG stand for?