Download Phases of Action Potential

Cardiovascular
System
• Heart – cardiac muscle physiology
–Cardiac conduction system
–Heart structures
• Vessels- arteries and veins
• Blood – WBC, RBC and Platelets, =Formed
elements plus the plasma (the fluid)
Types of Cardiac Cells
• Auto-rhythmic cells = pacemaker cells
(depolarize spontaneously)
• Contractile cells – myofibrils –connected
well together to form “functional
Syncitium” for myocardium contraction
Ease of movement between cells
(conductivity)
Mechanical and Electrical
• Critical to remember that there is
mechanical aspects to heart
function=myofibrils and electrical aspect
– Pacemaker cells and
– Resulting AP from these cells to working
muscle cells causes depolarization and
repolarization
Conduction System of Heart
Coordinates contraction of heart muscle.
20-1
Conduction System of Heart
Autorhythmic Cells
Cells fire spontaneously, act as pacemaker and form conduction
system for the heart
• SA node
– cluster of cells in wall of Rt. Atria
– begins heart activity that spreads to both atria
– excitation spreads to AV node
• AV node
– in atrial septum, transmits signal to bundle of His
• AV bundle of His
– the connection between atria and ventricles
• Bundle branches
– To right and left side of heart along ventricular septum
• Purkinje fibers,
– large diameter fibers that conduct signals quickly in ventricles
Rhythm of Conduction System
• SA node fires spontaneously 80-100 times per
minute
• AV node fires at 40-60 times per minute
• If both nodes are suppressed fibers in ventricles by
themselves fire only 20-40 times per minute
• Artificial pacemaker needed if pace is too slow
• Extra beats forming at other sites are called
ectopic pacemakers
– caffeine & nicotine increase activity
How does these cells
automatically fire??
Timing of Atrial &
Ventricular Excitation
• SA node setting pace since is the fastest
• In 50 msec excitation spreads through both atria
and down to AV node
• 100 msec delay at AV node due to smaller
diameter fibers- allows atria to fully contract filling
ventricles before ventricles contract
• In 50 msec excitation spreads through both
ventricles simultaneously
Cardiac Muscle Cells-Myofibrils
Physiology of Contraction
• Depolarization, plateau, repolarization
Important Electrolytes
• Sodium – major extracellular ion
• Potassium - major intracellular ion
• Chloride - negative ion
• Calcium - very important for muscle
contraction
Normal Resting Membrane
Protein Channels
• Special channels allow cations to go
through membrane
Will cause membrane
charge to rise towards 0mV
Action Potential Origin
•Channel proteins
•Voltage gated ion channels
•potassium
•sodium
Sodium & Potassium
Channels
• Responds to concentration gradients
• “depolarization” = movement away from
resting potential towards or above 0 mV
• “repolarization” = return to resting
potential -90 mV
• Necessity of channels
Overview of Ion Movement
Depolarization
• Action potential movement = ion
exchange
• Sodium moves into cell
• Potassium moves out of cell
•0 Membrane potential rises
-70
•this picture shows a neuron
• the principle is the same in a cardiac cell
Gap Junctions
Depolarization
Fast and Slow Channels
• Sodium and Potassium ions utilize both
fast and slow channels
– Sodium enters cells via slow channels until “threshold” is
reached
– Mass of sodium then enters cell via the fast channels
during contraction
Repolarization
• Bringing the membrane back to its
resting state
• Active pump moves ions against the
gradients
• Sodium/Potassium Pump
Sodium/Potassium Pump
• Maintains
concentration
• ~ 80% of metabolic
energy consumed
by pump
• “Leaky membrane”
– Sodium leaks in
– Potassium leaks
out
Phases of the Action
Potential
• Specific sequence of events
• Phases 0 through 4
• Important to know what happens in
each phase
– Comprehension of pathology and
pharmacotherapy
Phases of Action Potential
• Phase 0 =
Na+ channels
open slow then
fast
• Phase 1 =
K+ channels
open slow
Phases of Action Potential
• Phase 2 = Ca+
channels open
• Phase 3 = K+
channels open
Phases of Action Potential
• Phase 4 - Na/K pump
returns membrane to rest
• Na+ leaks =
• Slow phase 4 rise =
• automaticity
The Condensed Version
• Maintenance of
membranes resting
potential
• External force
(stimulus)
• Slow rise brings it to
threshold at - 70 mV
• Ion exchange begins
• Na+, then Ca+ into cell
• K+ out of cell
Depolarization & Repolarization
• Depolarization
– Cardiac cell resting membrane potential is -90mv
– excitation spreads through gap junctions
– fast Na+ channels open for rapid depolarization
• Plateau phase
– 250 msec (only 1msec in neuron)
– slow Ca+2 channels open, let Ca +2 enter from outside cell and
from storage in sarcoplasmic reticulum, while K+ channels close
– Ca +2 binds to troponin to allow for actin-myosin cross-bridge
formation & tension development
• Repolarization
– Ca+2 channels close and K+ channels open & -90mv is restored
as potassium entering the cell
• Refractory period
– very long so heart can fill with blood
Action Potential in Cardiac Muscle
Changes in cell membrane permeability.
Electrocardiogram---ECG or EKG
• EKG
– Action potentials of all active
cells can be detected and
recorded
• P wave
– atrial depolarization
• P to R interval
– conduction time from atrial to
ventricular excitation
• QRS complex
– ventricular depolarization
• T wave
– ventricular repolarization
One Cardiac Cycle
• At 75 beats/min, one cycle requires 0.8 sec.
– systole (contraction) and diastole (relaxation) of
both atria, plus the systole and diastole of both
ventricles
• Stroke volume (SV)
– the volume ejected per beat from each ventricle,
about 70ml
MEDIC Point: What would the normal amount of
blood pumped out of the Left ventricle be in 1
minute?
Phases of Cardiac Cycle
• Isovolumetric relaxation
– brief period when volume in ventricles does not
change--as ventricles relax, pressure drops and AV
valves open
• Ventricular filling
– rapid ventricular filling:as blood flows from full atria
– diastasis: as blood flows from atria in smaller
volume
– atrial systole pushes final 20-25 ml blood into
ventricle
• Ventricular systole
– ventricular systole
– isovolumetric contraction
• brief period, AV valves close before SL valves open
– ventricular ejection: as SL valves open and blood is
ejected
Ventricular Pressures
• Blood pressure in aorta is 120mm Hg
• Blood pressure in pulmonary trunk is 30mm Hg
• Differences in ventricle wall thickness allows heart
to push the same amount of blood with more force
from the left ventricle
• The volume of blood ejected from each ventricle is
70ml (stroke volume)
• Why do both stroke volumes need to be same?