Transcript Electrical Properties of the Heart Chapters 9 and 10

Electrical Properties of the Heart
Chapters 9 and 10
Review of Heart Muscle
•
•
•
Cardiocytes, myocardium
Branched cells
Intercalated discs- (desmosomes)
and electrical junctions (gap
junctions).
• Has actin and myosin
filaments
• Has low resistance (1/400 the
resistance of cell membrane)
•
•
•
Atrial syncytium
Ventricular syncytium
Fibrous insulator exists between
atrium and ventricle (what would
this do to any electrical activity
trying to go through?)
Figure 9-1; Guyton & Hall
• If the electrical signals from the atria
were conducted directly into the
ventricles across the AV septum, the
ventricles would start to contract at the
top (base). Then the blood would be
squeezed downward and trapped at
the bottom of the ventricle.
• The apex to base contraction
squeezes blood toward the arterial
opening at the base of the heart.
• The AV node also delays the
transmission of action potentials
slightly, allowing the atria to complete
their contraction before the ventricles
begin their contraction. This AV nodal
delay is accomplished by the naturally
slow conduction through the AV node
cells. (Why are they slow conductors?
Small diameter cells, fewer channels.
Refer to text)
Fibers within the heart
• Specialized Fibers
– are the fibers that can spontaneously
initiate an AP all by themselves!
– The AP will spread to all other fibers via
gap junctions
– AKA “leading cells”
– But they are also muscle, so they do
contract, albeit feebly!
– They are not nerves!!!!
• Contractile Fibers
– These maintain their RMP forever, unless
brought to threshold by some other cell
– They cannot generate an AP by
themselves
– AKA “following cells”
– But they do have gap junctions, so once
they’re triggered, they will help spread the
AP to neighbors.
Pathway of Heartbeat
• Begins in the sinoatrial (S-A)
node
• Internodal pathway to
atrioventricular (A-V) node
• Impulse delayed in A-V node
(allows atria to contract
before ventricles)
• A-V bundle takes impulse
into ventricles
• Left and right bundles of
Purkinje fibers take
impulses to all parts of
ventricles
KEY
Red = specialized cells;
all else = contractile cells
• Specialized cardiac muscle
connected to atrial muscle.
• Acts as pacemaker
because membrane leaks
Na+ and membrane
potential is -55 to -60mV
• When membrane potential
reaches -40 mV, slow Ca++
channels open causing
action potential.
• After 100-150 msec Ca++
channels close and
K+channels open more thus
returning membrane
potential toward -55mV.
Sinus Node
Internodal Pathways
• Transmits cardiac impulse throughout atria
• Anterior, middle, and posterior internodal
pathways
• Anterior interatrial band carries impulses to left atrium.
A-V Node
• Delays cardiac impulse
• Most delay is in A-V node
• Delay AV node---0.09 sec.
• Delay AV bundle--0.04 sec.
A-V Bundles
• Only conducting path
between atria and
ventricles
• Divides into left and
right bundles
• Time delay of 0.04sec
Purkinje
System
• Fast conduction;
many gap junctions at
intercalated disks
Time of Arrival of Cardiac Impulse
SA Node
H
(0.19)
(0.0)
T
(0.03)
(0.12)
Left Bundle
Branch
(0.19)
AV Node
(0.22)
AV Bundle
(0.21)
Main Arrival Times
S-A Node
0.00 sec
A-V Node
0.03 sec
A-V Bundle 0.12 sec
Ventricular Septum 0.16 sec
Base
0.22 sec
Copyright © 2006 by Elsevier, Inc.
(0.18)
Right Bundle
Branch
(0.17)
(0.18)
How can these Specialized fibers
spontaneously “fire?”
• Can’t hold stable resting
membrane potential
• Potentials drift (gradual
depolarization) –
”prepotential” or
“pacemaker potential”
• During this time, they have
a gradually increasing perm
to Na+ and less leaky to K+
(more “+” inside causes cell
to depolarize, remember?)
Na+
Specialized fibers
Notice slow rise from rest to
threshold.
This is called the “prepotential” or
“pacemaker potential”
Only specialized fibers of the heart
can do this.
This is what gives the heart it’s
rhythm.
Rhythmical Discharge of Sinus Nodal Fiber
Slow Ca++
Channels Open
Membrane Potential (mV)
Sinus Nodal
Fiber
K+ Channels
Open more
+20
Ventricular
Muscle fiber
Threshold
0
-20
-40
-60
-80
-100
Na+ Leak
And less leaky to potassium
0
1
2
Seconds
3
4
Specialized fibers of conductive
system
•
•
•
•
Each region generates its own rhythm.
If cells didn’t touch, then….
Faster at SA vs AV node, etc.
SA is faster than AV- “pacemaker”
– SA 60-80 depol/min
– AV 40-60 depol/min
– Purkinje 15-30 depol/min
• Draw it!
Because SA node has the highest
intrinsic rhythm, it is called the
cardiac pacemaker. What if
damaged…?
SA
AV
Pur
Time (min)
Specialized fibers
of conductive
system
• These rhythms can
ALSO be modified by
the ANS
• NTS can change slope of
prepotentials…faster or
slower rise to threshold
(bringing them closer or
further from threshold.)
by altering ion
permeability.
• ACh (psymp
postganglionic); NE
(symp postganglionic)
K+ efflux
Sympathetic and Parasympathetic
• Sympathetic – speeds heart rate by  Ca++ & Na+
channel influx and  K+ permeability/efflux
(positive chronotropy)
• Parasympathetic – slows rate by  K+ efflux & 
Ca++ influx (negative chronotropy)
Figure 14-17: Modulation of heart rate by the nervous system
Other effect of ANS
• Symp (NE) also affects
inotropy in ALL fibers,
specialized and contractile
• Inotropy is the “force of
contraction” or the tension
development in the muscle
fiber (strength of the
contraction.)
• Sympathetic firing causes
positive inotropy! (the
pounding heart)
• Parasympathetics have little
effect on inotropy
Terminology: Chrono, Inotropy
Symp (+,+) Parasym ( -, )
http://www.phschool.com/science/biology_place/biocoach/cardio1/electrical.html
Regulators of the Heart: Reflex
Controls of Rate
• Your HR at any moment is the
balance between symp and
parasym discharge rates.
(“tone”/ reserve)
• Tonic discharge
• How to speed up? Two ways
(faucet analogy)
• How to slow down? Two ways
• Range: about 50 – near 200
• Typical resting HR: near 70 -SA would normally beat at 6080 bpm- but vagal tone slows it
down. Parasympathetic slowsdown (20bpm or even stop)• Sympathetic speeds-speed up
(230bpm)
K+
Contractile Fibers of Heart
• Bulk of heart mass
• Review APs
– Still need calcium to
initiate contraction
• Differences from
Skeletal Muscle and
neurons
– Nature of AP
– Source of calcium (EC
vs. SR)
– Duration of contraction
– Resting potential
EC Coupling –
how it works (skeletal muscle)
Sequence of Events:
1.
2.
3.
4.
5.
6.
AP moves along T-tubule
The voltage change is sensed
by VONa+ Channels
Is communicated to the
(VOCC) (voltage operated
calcium channel; VOCR) how
much calcium released depends
on voltage
Contraction occurs.
Calcium is pumped back into
SR. Calcium binds to
calsequestrin to facilitate
storage.
Contraction is terminated.
AP
Ca2+ pump
calsequestrin
EC Coupling – Cardiac Muscle
Sequence of Events:
1. AP moves along T-tubule.
2. Activation – voltage sensors
that release a small amount of
Ca into the fiber.
3. Ca then binds to a receptor
which opens, releasing a large
amount of Ca. (Calcium
Activated Calcium Release)
How much calcium released
depends on how much calcium
gets through cell membrane
4. Calcium is pumped (a) back
into SR, and (b) back into T
tubule.
5. Contraction is terminated.
AP
Ca2+/Na+
exchanger
(Ca2+ out / Na+ in)
Ca2+ pump
requires
ATP
calsequestrin
Ventricular Muscle Action Potential-RMP -85mV;
Fast Na+ close
Slow Ca++ Channels open and decreased
K+ permeability
K+ Channels
Open
1
Membrane Potential
(mV)
+20
2
0
-20
3
-40
-60
-80
-100
Na+
0
4
Fast Na+
Channels Open
0
1
phase 0- Fast
channels open
phase 1- Fast Na+ channels close
phase 2- slow Ca++ open and decreased K+ permeability
phase 3- K+ channels open
phase 4- Resting membrane potential
Copyright © 2006 by Elsevier, Inc.
2
3
Seconds
4
Important things to
consider
• Cardiac muscle cells have a
long absolute refractory
period
• Twitches can not summate
• Tetanus not possible (this is
good!)
• If average heart beats 72bpm;
what does the heart do for the
rest of the time?
• Answer : It “rests” and fills
Spread of Depolarization
Direction of Depol
Resting Cell
++++++++++++++++++++++++++++++++
------------------------------------------------------------------------------------++++++++++++++++++++++++++++++++
Direction of Depol
++++++++++++++++++++++++++++++++
------------------------------------------------------------------------------------++++++++++++++++++++++++++++++++
Stim
microelectrode
Direction of Depol
++++++++++++++++++++++++++++++++
------------------------------------------------------------------------------------++++++++++++++++++++++++++++++++
+
+++
Depolarizing Current!
Stim
microelectrode
Direction of Depol
++++++++++++++++++++++++++++++++
------------------------------------------+ ------------------------------------------++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+
Depolarizing Current!
Stim
microelectrode
Direction of Depol
++++++++++++++++++++++++++++++++
------------------------------------------++
+ ------------------------------------------++++++++++++++++++++++++++++++++
Depolarizing Current!
Stim
microelectrode
Direction of Depol
---+++++++++++++++++++++++++++++
++---------------------------------------++-------------------------------------------++++++++++++++++++++++++++++++
Stim
microelectrode
Direction of Depol
--------+++++++++++++++++++++++++
+++++-----------------------------------+++++-------------------------------------------++++++++++++++++++++++++++
Stim
microelectrode
Direction of Depol
----------------++++++++++++++++++++
++++++++++----------------------------++++++++++-------------------------------------------- ++++++++++++++++++++
Stim
microelectrode
Direction of Depol
---------------------++++++++++++++++
++++++++++++++----------------------++++++++++++++-------------------------------------------++++++++++++++++++
Stim
microelectrode
Direction of Depol
--------------------- ----- ++++++++++++
+++++++++++++++++++ ---------------+++++++++++++++++++------------------------------------ ------+++++++++++++
Stim
microelectrode
Direction of Depol
--------------------- ----- --------++++++
+++++++++++++++++++ +++++--------++++++++++++++++++++++++----------------------------- --------------+++++++
Stim
microelectrode
Direction of Depol
--------------------- ----- ---------------++
+++++++++++++++++++ +++++ ++++--+++++++++++++++++++++++++++++---------------------- --------------------- ++
Stim
microelectrode
Direction of Depol
--------------------- ----- -----------------+++++++++++++++++++ +++++ +++++++
+++++++++++++++++++++++++++++++
-------------------- -------------------------
Stim
microelectrode
Direction of Depol
--------------------- ----- -----------------+++++++++++++++++++ +++++ +++++++
+++++++++++++++++++++++++++++++
-------------------- -------------------------
Stim
microelectrode
Direction of Depol
--------------------- ----- -----------------+++++++++++++++++++ +++++ +++++++
+++++++++++++++++++++++++++++++
-------------------- ------------------------Depol = spread of surface NEG charge
Stim
microelectrode
Direction of Repolarization
Begin
Repolarization
--------------------- ----- -----------------+++++++++++++++++++ +++++ +++++++
+++++++++++++++++++++++++++++++
-------------------- ---------------------------
Stim
microelectrode
Direction of Repolarization
--------------------- ----- -----------------+++++++++++++++++++ +++++ +++++++
--+++++++++++++++++++++++++++++++
-------------------- -------------------------
Stim
microelectrode
Direction of Repolarization
++++++------------- ----- -----------------------++++++++++++++ +++++ +++++++
------+++++++++++++++++++++++++++
+++++ ------------- ------------------------
Stim
microelectrode
Direction of Repolarization
++++++++++++------- ----- --------------------- ------++++++++++ +++++ +++++++
----------- ++++++++++++++++++++++
+++++ +++++-------- ----------------------
Stim
microelectrode
Direction of Repolarization
++++++++++++++++++ ----- -------------------- --------------++++ +++++ +++++++
----------- ----------+++++++++++++++
+++++ ++++++++++++ --------------------
Stim
microelectrode
Direction of Repolarization
++++++++++++++++++ ++++++++-------------- ------------------------- - -++++++
------- ------------------------- - -++++++
++++++++++++++++++ ++++++++--------
Repolarization= spread of positive surface charge
Stim
microelectrode
Direction of Repolarization
++++++++++++++++++ +++++++++++++
------- ------------------------- - ---------------- ------------------------- - --------+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ +
Stim
microelectrode
Direction of Repolarization
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ +
------- ------------------------- - ---------------- ------------------------- - --------+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ +
Repolarization= spread of POS surface charge
Stim
microelectrode
Depolarization/Repolarization
Cycle in the Atria
Depolarization Begins
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
Depolarization Complete
=Resting cell
=Depol cell
Repolarization Begins
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
Repolarization Complete
=Resting cell
=Depol cell
Depolarization/Repolarization
Cycle in the Ventricles
Depolarization Begins
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
Depolarization Complete
=Resting cell
=Depol cell
Repolarization Begins
=Resting cell
=Depol cell
=Resting cell
=Depol cell
=Resting cell
=Depol cell
Repolarization Complete
=Resting cell
=Depol cell