Download Cardiac Muscle Skeletal Muscle Action Potential

BIOENGENHARIA MÉDICA
MÚSCULO CARDÍACO
Abril 31, 2008
Eduardo Infante de Oliveira
Instituto de Fisiologia, FML
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Orientation of cardiac muscle fibres:
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Unlike skeletal muscles,
cardiac muscles have to
contract in more than
one direction.
Cardiac muscle cells are
striated, meaning they
will only contract along
their long axis.
In order to get
contraction in two axis,
the fibres wrap around.
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TYPES OF MUSCLE
LOCATION
MICROSCOPIC
APPEARANCE
RELATIONSHIP
WITH THE
NERVOUS
SYSTEM
VOLUNTARY
SPEED OF
CONTRATION
SKELETAL
HEAVYILY
STRIATED
SLOW TO FAST
CONTRACTIONS
VISCERAL
NONSTRIATED INVOLUNTARY VERY SLOW
(SMOOTH)
CONTRACTIONS
CARDIAC
LIGHTLY
STRIATED
AUTORHYTHMIC SLOW
CONTRACTIONS
Characteristics of Skeletal,
Cardiac, and Smooth Muscle
Table 10–4
Cardiac Tissue
• Cardiac muscle is striated,
found only in the heart
• 7 Characteristics of Cardiocytes
• Unlike skeletal muscle, cardiac muscle cells
(cardiocytes):
– are small
– have a single nucleus
– have short, wide T tubules
– have no triads
– have SR with no terminal cisternae
– are aerobic (high in myoglobin, mitochondr
– have intercalated discs
Figure 10–22
Intercalated Discs
• Are specialized contact points between
cardiocytes
• Join cell membranes of adjacent cardiocytes
(gap junctions, desmosomes)
• Functions
– Maintain structure
– Enhance molecular and electrical
connections
– Conduct action potentials
• Because intercalated discs link heart cells
mechanically, chemically, and electrically,
the heart functions like a single, fused
mass of cells
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Structure of Cardiac Muscle Cell
15
Cardiac muscle
Cardiac muscle – section – H&E – 40x objective
striations
intercalated disc
centrally-located nucleus
One distinguishes cardiac muscle from skeletal muscle by the branching fibers,
presence of intercalated discs, and centrally-placed single nuclei/cell.
Cardiac muscle
Cardiac muscle –section – silver – 20x objective
branching
intercalated disc
nucleus
This stain clearly shows the single central nucleus, branching fibers,
intercalated discs, and striations.
T: T tubules
mit:
mitochondria
g: glycogen
contractile unit: sarcomere
Z line: the actin filaments are attached
I: band of actin filaments, titin and Z line
A: band of actin-myosin overlap
H: clear central zone containing only
myosin
AP-contraction relationship:

AP in skeletal muscle is very
short-lived


AP is basically over before an
increase in muscle tension can
be measured.
AP in cardiac muscle is very
long-lived

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AP has an extra component,
which extends the duration.
The contraction is almost over
before the action potential has
finished.
Cardiac myocyte action potential:
Refractory Period

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Absolute: Cardiac muscle cell completely
insensitive to further stimulation
Relative: Cell exhibits reduced sensitivity to
additional stimulation
Long refractory period prevents tetanic
contractions
Cardiac vs. Skeletal Muscle
Contraction
Cardiac
Muscle
Action
Potential
Extracellular
Calcium Ions
Type of
contraction
Skeletal
Muscle
Duration: 250-300
msec
Duration: 10 msec
Delay repolarization &
initiate contraction
Initiate contraction
Long refractory period Tetanus occurs
which continues while because short
relaxing = no tetanus! refractory period
Cardiac conducting system:
Pacemaker potential:
Pacemaker regulation:

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Once the pacemaker cells reach threshold, the
magnitude and duration of the AP is always the same.
In order to change the frequency, the time between
APs must vary.

The interval can only be changed in two ways.

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The rate of depolarization can be changed
The amount of depolarization required to reach threshold can be
changed.
The conduction system of the heart.
EE-515 Bioelectricity &
Biomagnetism 2002 Fall - Murat
Electrophysiology of the heart
The different waveforms for each of the specialized
cells
EE-515 Bioelectricity &
Biomagnetism 2002 Fall - Murat
• Principle of Continuity:
•
•
•
•
Conservation of mass in a closed hydraulic system
Blood is an incompressible fluid
Vascular system is a closed hydraulic loop
Vol ejected from left heart = vol received in R heart
Pressure relationships:
Curva Pressão-Volume
Ventricular
Cardiac Output and EDV
Regulation of the Heart

Intrinsic regulation: Results from normal functional
characteristics, not on neural or hormonal regulation

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Starling’s law of the heart
Extrinsic regulation: Involves neural and hormonal
control
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Parasympathetic stimulation

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Supplied by vagus nerve, decreases heart rate, acetylcholine
secreted
Sympathetic stimulation

Supplied by cardiac nerves, increases heart rate and force of
contraction, epinephrine and norepinephrine released
Heart Homeostasis

Effect of blood pressure
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Effect of pH, carbon dioxide, oxygen
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Chemoreceptors monitor
Effect of extracellular ion concentration

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Baroreceptors monitor blood pressure
Increase or decrease in extracellular K+ decreases heart
rate
Effect of body temperature

Heart rate increases when body temperature increases,
heart rate decreases when body temperature decreases