Download Cardiac Muscle Excit.

PHYSIOLOGY 1
LECTURE 23
CARDIAC MUSCLE
EXCIT. - CONT. - COUPL.
ACTION POTENTIALS
Cardiac Muscle

Cardiac muscle differs from both
smooth muscle and skeletal muscle,
being somewhere in between the two.
Therefore, cardiac muscle is considered
an intermediate form between the more
primitive smooth muscle and advanced
skeletal muscle. Thus, it has properties
of both smooth and skeletal muscle as
well as it’s own unique properties.
Cardiac Muscle
Structure
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SIZE Cardiac muscle
tends to be smaller
than skeletal muscle
but larger than
smooth muscle.
About 10 to 15 mm
in diameter and
around 50 mm in
length.
Cardiac Muscle
Structure
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Contractile ProteinsIn cardiac muscle
the contractile
proteins are isomers
of the skeletal
muscle sarcomeres
and operate in a
similar manner.
Cardiac Muscle
Structure
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T- Tubules In cardiac muscle
only one T -tubule
occurs per
sarcomere and Triad
formation is
somewhat more
rudimentary due to
the small size of the
sarcoplasmic
reticulium.
Cardiac Muscle
Structure
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Organelles Cardiac muscle is
normally
mononucleated but
a few may be
multinucleated.
Otherwise
organelles are
similar to most other
cells with the
exception of the SR
Cardiac Muscle
Structure

The SR in cardiac muscle
cells is similar to smooth
muscle and takes up less
than 5% of cellular
volume. Therefore, it
lacks space for large
amounts of Ca++ and
the cell is dependent on
external Ca++ for
contraction.
Cardiac Muscle
Structure
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Intercellular Connections
Cardiac muscle cells are
connected by a intercalated
disk. In reality these are
interdigitated bundles of
tightly bound collagen and
elastin fibers containing
many gap junctions (An
electrical syncytium)
Cardiac Muscle
Structure

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Sarcomere Size Due to the very
heavy quantity of
connective tissue
surrounding cardiac
myocytes the resting
sarcomere length is
somewhat
compressed at 1.6
mM.
Cardiac Muscle
Structure

There are four types of cardiac muscle
histologically “P” cells or pacemaker cells,
transition cells, Purkinje cells, and normal
cardiomyocytes. All of these are classified as
cardiac muscle cells. However the “P”,
transition, and Purkinje cells are all part of
what is known as the cardiac conduction
system and do not participate in muscle
contraction but generate and transmit the
cardiac action potential.
Cardiac Muscle
Structure
Cardiac Muscle
Excitation - Contraction - Coupling
In cardiac muscle external Ca++ is once
again required due to the small volume
of the SR.
 1. Calcium induced calcium release
 2. Mechanism of calcium influx
 3. Role of calcium in tension
development
 4. Relaxation and calcium efflux

Cardiac Muscle
Excitation - Contraction - Coupling

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Calcium Induced Calcium
Release Passage of the cardiac
action potential opens
the slow calcium/sodium
voltage gated channels.
Ca++ influx opens Ca++
sensitive calcium
channels in the SR.
Cardiac Muscle
Excitation - Contraction - Coupling

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Mechanism of Calcium
Influx Calcium will influx through the
slow voltage gated channels.
But catacholamines also activate
cAMP which in turn activates a
cAMP dependent Protein Kinase
which phosphoralates Ca++
channels causing them to open
increasing [Ca++].
Cardiac Muscle
Excitation - Contraction - Coupling

Role of Ca++ in
Tension development
-

Increased cytosolic Ca++
concentrations increase
crossbridge cycling rates
thereby, increasing both
the force and speed of
myocyte contraction.
Cardiac Muscle
Excitation - Contraction - Coupling
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Relaxation and Calcium
efflux Calcium is removed from the
cardiac muscle cytosol by our
standard Ca++ATPase into
the SR and into the
interstitial space, but it is
joined by a Na+Ca++
exchanger 1 Ca++ out for 3
Na+ in.
Cardiac Muscle
Fast Cardiac Action Potential

There are two cardiac action potentials
produced by coronary myocytes, the
fast AP and the slow AP. The fast
cardiac action potential is characterized
by containing a plateau phase. We will
take up the slow cardiac AP when we
discuss the conduction system of the
heart.
Cardiac Muscle
Fast Cardiac Action Potential
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The Fast Cardiac Action
Potential Four phases
Phase 4 or resting phase
Phase 0 Depolarization
Phase 1
Phase 2
Phase 3 Repolarization
Cardiac Muscle
Fast Cardiac Action Potential
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Refractoriness Due to the long plateau
phase ( phase 2) of the
fast cardiac action
potential the cardiac
cells have time to
contract before the
action potential is
finished. Can not
generate tetany.
Cardiac Muscle
Slow Cardiac Action Potential
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension

In cardiac muscle the
resting sarcomere length
is 1.6 mm due to the
large amount of
connective tissue. This
places the resting
sarcomere length on the
left hand ascending limb
of the active tension
curve.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension

The passive tension
curve in cardiac muscle
begins with the active
tension curve and
increases rapidly
eliminating the right
hand descending limb of
the active tension curve.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension
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The cardiac muscle total
tension curve then is
nearly a straight line
steadily increasing.
Frank-Starling law of the
heart. – Increasing
sarcomere length
increase force and speed
of contraction
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

The isotonic
condition -
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

B. Affect of changing afterload (Aortic
Pressure or Diastolic Pressure) constant preload
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
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C. Affect of changing preload constant afterload
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
CARDIAC MUSCLE MECHANICS
Cardiac Contractility

D. Concept of contractility or inotrophy

Contractility or inotrophy is an increase in cytosolic
calcium concentration. This phenomenon is under the
control of the sympathetic nervous system. As NE is
released it activates both the b1 and a1 receptors
which activate both cAMP and IP3 second messengers
which cause phosphorylation of calcium channels and
thus, increased Ca++ influx. Therefore, more
Troponin is activated - > force & Vel.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
CARDIAC MUSCLE MECHANICS
Cardiac Pressure-Volume Loops

Pressure volume loops are an excellent
way of illustrating cardiac performance
through out the cardiac cycle. They can
be used to illustrate contractility,
afterload, and preload effects on
cardiac performance. The drawback is
that they can only be utilized on one
side of the heart therefore, the
following discussion involves only the
left ventricle but the right is similar.
SUMMARY
1. How does cardiac muscle differ from
skeletal or smooth muscle?
 2. What is important about cardiac intercalated
disks and connective tissue?
 3. What is important about resting cardiac
sarcomere length?
 4. What is Frank-Starling’s law of the heart?
 5. What is contractility (inotropy)?
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SUMMARY
6. What is preload, afterload to cardiac tissue?
 7. How does preload or afterload change
cardiac performance?
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