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Cardiac Myocytes��Vincent A. Barnett, Ph.D.

Department of Integrative Biology & Physiology

Program in Human Anatomy

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The Cardiac Myocyte is a Cell

Liver Cells

Early Stage

Mammalian Embryo

Therefore, it has characteristics that are common to most mammalian cells

Cardiac Myocytes

Cellular (plasma) membrane with cytoskeletal proteins, ion pumps and receptors defines the boundaries of the cell.
Intracellular Organelles which participate in cellular functions
Nuclei which contain the genetic programing and direct cellular activity

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General Cellular Morphology

Lysosome

Ribosome

Nucleus

Golgi

Endoplasmic

Reticulum

Mitochondrion

Plasma Cell

Membrane

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Cell Membrane

d

Membrane

Receptors

Membrane

Bilayer

Ion Channels

Connective Tissue

Matrix

Integral Membrane

Protein Anchors

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The Cardiac Myocyte

Is specialized for rhythmic force generating contractions

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Cardiac Cell Morphology

Shape – tends to be irregular
Size – ~ 10 - 40μm in width and ~ 50 - 200μm or length
Organelles – similar to other cell types

Nucleus, mitochondria, golgi apparatus, ion channels, receptors, gap junctions, etc.

Cytoplasmic Space – most of space is occupied by contractile proteins in a filament lattice.

Comparison of our idealized cell to a cardiac muscle cell reveals several distinct specializations. The shapes of mammalian cardiac fibers do not conform to a simple geometry, so measurement of their size is not straightforward and a single measurement of length or width would be inadequate and misleading. They tend to have a shorter dimension (10 - 40 microns) and a longer dimension (~50 -200 microns). The individual cells sometimes have a branched appearance. Most of the internal volume of the cells is devoted to a cytoskeletal lattice of contractile proteins whose liquid crystalline order gives rise to a striated appearance under the microscope. Within the membrane bilayer there are a collection of ion channels and pumps, receptor proteins and proteins designed to connect cardiac myocytes to one another as mechanical or electrical partners.

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Microscopic anatomy of cardiac muscle

Intercalated disk

Myofibril

Sarcoplasmic

reticulum

Transverse tubules

Sarcolemma

(Cardiac cell membrane)

Mitochondrion

Mitochondria

Nucleus

Desmosomes

Gap junctions

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Cellular Landmarks

Electron micrograph of a portion of a cardiac muscle cell (2000x)

A-band

I-band

Invagination of the sarcolemma by the transverse tubule system

gap junctions

&

fascia adherens

Sarcomere

Sarcoplasmic

Reticulum (SR)

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Myofibrils, Sarcomeres and Contractile Proteins

Sarcomere

A

band

I

band

Z-line

M-line

H

band

Actin monomers

Actin (thin) filament

Myosin molecule

Myofibrils

Myofibril

Titin

Myosin (thick) filament

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The Molecular Mechanism� of Muscle Contraction

ATP

Rigor

ATP

ATP hydrolysis

ADP

P_i

ADP

P_i

Power stroke

Energy storage

Energy release

P_i

ADP

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Control of Muscle Contraction

Thin Filaments

Troponin

Tropomyosin

Troponin-C
Troponin-I
Troponin-T

Relaxed

No Ca²⁺

Thin filaments are formed with an actin backbone but also carry the regulatory proteins Tropomyosin and Troponin. Tropomyosin is a a-helical coiled-coil molecule that spans seven actin monomers. Troponin is a globular protein complex with three subunits. Tropomyosin molecules run end-to-end around the helical coil of the thin filament with one Troponin complex attached to each tropomyosin molecule. In relaxed muscle tropomyosin binds to actin in such a way as to impede the binding of myosin to actin. However calcium binding to troponin induces a conformational change that is transmitted to tropomyosin and it shifts its position on the actin thin filament to reveal the myosin binding site. Myosin can then bind to the thin filament in a manner conducive to force production. This association of tropomyosin with troponin over seven actin monomers represents a de facto regulatory unit along the thin filament.

Control of skeletal and cardiac muscle contraction is a calcium dependent process.

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Active

+ Ca²⁺

Control of Muscle Contraction

Thin Filaments

Troponin

Tropomyosin

Troponin-C
Troponin-I
Troponin-T

Thin filaments are formed with an actin backbone but also carry the regulatory proteins Tropomyosin and Troponin. Tropomyosin is a a-helical coiled-coil molecule that spans seven actin monomers. Troponin is a globular protein complex with three subunits. Tropomyosin molecules run end-to-end around the helical coil of the thin filament with one Troponin complex attached to each tropomyosin molecule. In relaxed muscle tropomyosin binds to actin in such a way as to impede the binding of myosin to actin. However calcium binding to troponin induces a conformational change that is transmitted to tropomyosin and it shifts its position on the actin thin filament to reveal the myosin binding site. Myosin can then bind to the thin filament in a manner conducive to force production. This association of tropomyosin with troponin over seven actin monomers represents a de facto regulatory unit along the thin filament.

Control of skeletal and cardiac muscle contraction is a calcium dependent process.

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Cardiac Contraction

Systole

Diastole

Adapted from review by Daniel D. Streeter Jr. (1979) “Gross morphology and fiber geometry of the heart” in Handbook of Physiology - Section 2: The Volume I: The Cardiovascular System edited by R.M. Berne and N. Sperelakis.

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Isometric Contraction and� the Length-Tension Relationship

Tension

Sarcomere Length (μm)

100%

0%

1.0

2.0

3.0

Systole

Diastole

4.0

50%

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Shortening of cardiac myofibrils leads to shortening of cardiac myocytes

Myocyte contraction leads to the

compression of cardiac chambers

Systole

Diastole

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Ion Distributions for Cardiac Cells

Ca⁺⁺

(2 mM)

Na⁺

(145 mM)

Cl^-

(120 mM)

K⁺

(4 mM)

Ca⁺⁺

(10^-7 mM)

Na+

(15 mM)

Cl-

(5 mM)

K+

(145 mM)

K⁺

Na⁺

Intracellular fluid

Extracellular fluid

-80 mV

-

+

K⁺

Leak channel

Na⁺/K⁺ ATPase

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Cardiac Myocytes:� Resting Membrane Potential

+ + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - - - - - - - -

Outside

Inside

Leak channels

Na⁺/K⁺ ATPase

3Na⁺

2K⁺

K⁺

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Cardiac Myocytes: �Ion Channels

Outside

Inside

Voltage gated

Ligand gated

Spontaneous

Leak

Mechanically gated

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Ion Gradients into Electrical Potentials: The Nernst Equation

E = (-2.3RT/zF) log₁₀ [C_i]/[C_o]

E = equilibrium potential (mV)

R = gas constant

F = Faraday constant

z = charge on the ion

2.3RT/F = 60mV @ 37°C

Na⁺

K⁺

E_Na≈ +60 mV

E_K≈ -90 mV

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Calculating the Membrane Potential

The General Case Goldman-Hodgkin-Katz equation:

for cardiac cells the equation is modified to include the cell’s calcium sensitivity:

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Ionic Conductance Changes During �the Ventricular Cardiac Action Potential

Action

Potential

Duration

Sinoatrial node 150 ms

Atrial myocytes 150 ms

Ventricles myocytes 250 ms

Purkinje fibers 300 ms

Na

Ca

K

In

Out

Relative Ion Flux

Phase	Event
0	Upstroke (Depolarization)
1	Initial repolarization
2	Plateau
3	Final repolarization
4	Resting Potential

0

2

3

4

1

-85

0

Time (msec)

mV

Cardiac action potentials occur because of transient changes in the cellular permeability to Na, Ca and K. A brief increase in sodium permeability depolarizes the cell and drives the membrane potential towards the sodium equilibrium potential. This activates voltage gated calcium and potassium channels. The opening of Calcium channels causes calcium to enter cell and sustains the depolarized state. The opening of the potassium channels allows potassium efflux from the cell and drives the membrane potential back towards the potassium equilibrium potential. The timing of these changes depends on the isoforms of the channel proteins present in each cell with sinoatrial and ventricular action potentials lasting ~ 150 ms, ventricular muscle ~250 ms and Purkinje fingers ~300ms. The primary difference between these cell types is often the duration of the plateau phase (phase 2) which is primarily a response to calcium channels.

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Cardiac Action Potentials

Slow Response Cardiac Action Potentials

Sinoatrial (SA) Nodal Cells

Fast Response Cardiac Action Potential

Atrial & Ventricular Contractile Cells

0

2

3

4

1

0

-85

mV

time

Phase	Event
0	Upstroke (Depolarization)
1	Initial repolarization
2	Plateau
3	Final repolarization
4	Resting Potential

Inward Na current (leak current) prevents the membrane potential from reaching a stable endpoint
“Slow” upstroke due to the opening of T-type Calcium channels.
No sustained plateau
Repolarization due to outward K current

Na

K

Ca

In

Out

0

-85

0

3

4

mV

time

No phase 1 or 2

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Automaticity

0

- 85

0

3

4

0

3

4

0

3

4

0

3

4

millivolts

Time (ms)

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Refractory Periods

40

0

-40

-80

-120

0

300

100

200

0

300

100

200

Time (msec)

Millivolts

Fast response

(contractile myocytes)

Slow response

(nodal cells)

0

1

2

3

4

3

0

a

b

c

d

e

ERP

RRP

Na_V

Ca_V3

Refractory periods prevent sustained contractions (tetany) and ensure rhythmic pumping. Unfortunately, if a premature beat hits during the vulnerable relative period, it can trigger dangerous arrhythmias like ventricular tachycardia.

Recording

electrode

Reference

electrode

Fast response cardiac cells, atrial and ventricular muscle and the purkinje fibers have an extremely rapid phase 0, or transition from the resting membrane potential to depolarization. As the sodium channels begin to close phase 1 is indicative of an initial repolarization. The opening of the L-type calcium channels causes in calcium influx and balances the potassium efflux via K-channels. This results in the positive plateau (phase 2) of the fast-response action potential profile. As the calcium channels close the K-channels begin to dominate and repolarization of the cells occur. From the initiation of the action potential through approximately half of the repolarization the cell is refractory, meaning that it could not respond to a new depolarization signal.

Slow response cardiac cells, like sino-atrial and atrio-ventricular nodal cells have unstable resting potentials. A gradual rise in resting potential crosses the threshold for opening of T-type calcium channels. The movement of calcium into the cells (phase 0) initiates depolarization. No initial repolarization or plateau occurs so phases 1 and 2 are said to be absent. Repolarization (phase 3) is accomplished through the opening of voltage gated K-channels. Once the cell is repolarized (phase 4), leak channels (often attributed to slow Na-channels) contribute to instability of the resting potential and a gradual rise to the threshold value of the T-type Ca-channels.

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Excitation-Contraction Coupling in Cardiac Muscle.

Systole

Ca Pump

Ca⁺⁺

Ca⁺⁺-Troponin

complex

Myofilaments

L-Type

Ca Channel

Ca⁺⁺

+

SR

Ca⁺⁺

Cardiac Action Potential

Ca⁺⁺ enters cell during the plateau

Ca⁺⁺ induced calcium release from the SR

Ca⁺⁺ binds to Troponin-C

Cross-bridge cycling

Tension

Na⁺

AP

RyR

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Excitation-Contraction Coupling in Cardiac Muscle.

Ca Pump

Ca⁺⁺

Ca⁺⁺-Troponin

complex

Myofilaments

L-Type

Ca Channel

Na⁺

SR

Ca⁺⁺

1 Ca⁺⁺

K⁺

Diastole

Ca⁺⁺

3 Na⁺

Ca Pump

Na-Ca

Exchanger

Na-K

Pump

Relaxation

Cross-bridge cycling stops

Ca⁺⁺ is released from Troponin-C

Ca⁺⁺is pumped from the cytoplasm

into the SR and interstitium

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Excitation-Contraction Coupling in Cardiac Muscle.

Systole

Ca Pump

Ca⁺⁺

Ca⁺⁺-Troponin

complex

Myofilaments

L-Type

Ca Channel

Na⁺

Ca⁺⁺

+

SR

Ca⁺⁺

1 Ca⁺⁺

K⁺

Diastole

Ca⁺⁺

3 Na⁺

Ca Pump

Na-Ca

Exchanger

Na-K

Pump

Cardiac Action Potential

Ca⁺⁺ enters cell during the plateau

Ca⁺⁺ induced calcium release from the SR

Ca⁺⁺ binds to Troponin-C

Cross-bridge cycling

Tension

Na⁺

AP

Relaxation

Cross-bridge cycling stops

Ca⁺⁺ is released from Troponin-C

Ca⁺⁺is pumped from the cytoplasm

into the SR and interstitium

RyR

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The role of Gap Junctions�in the spread of electrical activity.

Gap junctions form at cell-cell interfaces to facilitate communication.

Cardiac Cell Connectivity

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The role of Gap Junctions�in the spread of electrical activity.

Rotation of Gap Junction Subunits Opens the Pores.

Cardiac Cell Connectivity

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The Role of Gap Junctions�in the Spread of Electrical Activity.

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Progression of Electrical Signals Through the Heart

Ventricular

Muscle

SA node

Atrial

Muscle

AV node

Bundle of His

Left Bundle Branch

Purkinje Fibers

Right Bundle Branch

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Progression of Electrical Signals Through the Heart

From: http://www.vhlab.umn.edu/atlas/conduction-system-tutorial/gap-junctions.shtml

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References

Medical Physiology 2^nd edition (2003) by Rhodes & Tanner (Lippincott, Williams & Wilkins).

Chapter 10: Cardiac Muscle.

Principles of Physiology 3^rd edition (2000) by Berne & Levy (Mosby, Inc.).

Chapter 18: The Cardiac Pump

Physiology 4^th edition (2007) L. Costanzo (W.B. Saunders and Co.)

Chapter 3: Cardiovascular Physiology

Vander’s Human Physiology 10^th edition (2006) by Widmaier, Raff & Strang. (McGraw-Hill Co.)

Chapter 12: Cardiovascular Physiology

Principles of Human Physiology (2001) by Germann & Stanfield (Benjamin Cummings)

Chapter 12: The Cardiovascular System

Cardiovascular Physiology 5^th edition (2003) Mohrman & Heller (McGraw-Hill Co.)

Chapter 2: Characteristics of Cardiac Muscle Cells.