Download Physiology 5

Physiology 5:
Today we are going to talk about:
-autonomic innervations of the heart
-ECG
Innervations of the heart:
It comes from the sympathetic and parasympathetic system.
Sympathetic innervations come from the cardiac plexus (T1-T4)
to supply all parts of the heart.
Parasympathetic innervations comes from the vagus nerve to
both atria SA node and AV node.(i.e. it does not supply the
ventricles or very minimal ).
The sympathetic system usually stimulates the heart (i.e. cardio
acceleratory center that is found in the medulla oblongata) and
works at the postsynaptic junction through NE and E
(epinephrine and nor epinephrine) to increase the cardiac
membrane permeability to sodium and calcium which in turn
increases the depolarization rate (much less time is needed to
reach the threshold) thus increasing the heart rate. This effect is
called positive chronotropic effect (increase of the heart rate).as
it increases permeability to sodium and calcium it increase the
force of contraction of the heart. This is called positive iontropic
effect. It also affects the conduction of AP by the positive
dromotropic effect.
The parasympathetic system usually inhibits the heart
decreasing its rate (HR).this system has a negative chromtropic
effect and negative dromotropic effect BUT with no effect on
the force of contraction (i.e no iontropic effect).
This system releases ACTL that decreases the membrane
permeability to sodium and calcium AND increase the
permeability to potassium.
This shifts the resting membrane potential from -60 mV to -70
mV, so more time is needed to reach the threshold thus the HR
is decreased.
Important note:
Changes of membrane ion permeability do not change the peak
of the AP (all or none), only manipulate with the rate of AP
(either faster to slower).
Example:
Stimulation of the vagus nerve -----decrease of HR-----HR
rate keeps on decreasing till the heart stops by the
overstimulation of the parasympathetic system…after 30s the
heart starts to contract again .Why?
Since the ventricles (and the purkinje fibers) are not affected by
the parasympathetic system the purakinje fibers will override the
effect of the parasympathetic system allowing it to contract
again (i.e. ventricular escape).
Stock Adam s syndrome:
Is a syndrome which stimulates the parasympathetic system
through its pressure on the vagus nerve----decrease HR--Coma for 30s------Ventricular escape
the S.A node will return to be the heart pacemaker after the
stimulus or pressure on the vagus nerve is removed.
--------------------------------------------------------------------------Now to the second part of our lecture.
ECG:
What is ECG?
The recording of the electrical changes that occur in the heart
during a cardiac cycle that keeps repeating itself each time. It
does not record any of the mechanical changes , muscle
contraction or relaxation that occurs in the heart.
During this cycle, there is atrial depolarization followed by
ventricular depolarization then atrial and ventricular
repolarization.
We say that the AP is monophasic,what does that
mean?
It means that we have one wave for depolarization and
repolarization that occur at the same side above the isoelectric
line (the positive side)
The ECG is bipolar, meaning that depolarization and
repolarization waves should occur on opposite sides and are two
different waves.
Back to ECG.
In the ECG recording we have the P wave,QRS complex and the
T wave.
P represents the atrial depolarization. The P wave immediately
precedes atrial contraction.
QRS complex represents the depolarization of the ventricles.
The QRS complex immediately precedes ventricular
contraction.
T represents the repolarization of ventricles.
Notice in the ECG:
--there is no repolaztion of atria because repolarzation occurs at
the same time as the depolarization of ventricles which
overrides it.
--The T wave is on the positve side of the isoelectric line
although it is a repolarizing wave ( Repolarization wave should
occur opposite to depolarizing wave as mentioned )..the reason
of this will be explained later on (star)
Depolarization starts from the endocardium to the epicardium,
from the base to the apex.
Repolarization starts from the epicardium to the
endocardium,from apex to base.
So both of them will have a positive deflection in the graph.
(repolarization will be deflected to the positive side rather than
the negative one).because they started from opposite
origins(star)
Why do the depolarization and repolarization waves start from
different origins?
There are two theories that explain this:
1- There is an intrinsic property of the muscles of the epicerium
so that they have a short action potential while endocardium
muscle have a long action potential to the extent that
repolarization of the epicardium occur before the endocarium
although they were depolarized after it.
2- After depolarization and when the heart contracts,it develops
pressure inside it that will affect the endocardium more than the
epicardium,this pressure will change the permeability of the ions
to the membrane to the extent that it will prolong the
repolarization wave of the endocardium more than the
epicardium.
Important note:this pressure will not affect the atria considerably
because its propotionaly low in atria compared to ventricles ,so
depolarization and repolarization waves start from the same
origin… Depolarization starts from the endocardium to the
epicardium and repolarization starts from the endocardium to
the epicardium.so IF we record an atrial repolarization it will be
a downward deflection (negative deflection).Atrial
repolarization can only be recorded when the heart rate is
low,for example in AV block.