Characteristics of Cardiac Muscle Cells Flashcards
Cardiac muscle APs differ from skeletal APs and promote rhythmic excitation of the heart that has 3 characteristics.
Self-generating
Conducted directly from cell to cell
Long duration (long refractory period)
Potassium has what equilibrium potential
-90mv
Activation Gates: Responds to
membrane depolarization QUICKLY by opening.
Inactivation Gates: Responds to
membrane depolarization SLOWLY by closing. Limits the time a channel can remain open, despite continued stimulation. Not in all channels.
What are the main differences in AP between the contractile and the pacemaker cells in the phases?
Phase 0: Mediated by Na+ in contractile cells, Ca2+ in pacemaker cells
Phase 1 & 2: Absent in pacemaker cells
Phase 3: No difference
Phase 4: Resting vs Pacemaker Potential
Intercalated Disks:
specialized adherens junction structure that connects ends of two adjacent cells.
Three types of adhering junctions make up intercalated discs:
Fascia Adherens
Macula Adherens (aka Desmosomes)
Gap Junctions
Fascia Adherens:
anchoring sites for actin, connect to the closest sarcomere
Macula Adherens (aka Desmosomes):
join cells together by binding intermediate filaments
Gap Junctions:
channels formed of proteins called connexin permit passage of ions between cells, thus allowing the spread of action potentials
Fascia adherens and macula adherens (desmosomes)
ENSURE THE TRANSMITTION OF
FORCE FROM ONE MUSCLE CELL TO THE NEXT
The speed at which AP travels between cells is largely determined by the
# of gap junctions in the intercalated discs Fewer gap junctions = slower
AV node has_________ gap junctions than the SA node and atrial myocardium
AV node has fewer gap junctions than the SA node and atrial myocardium
P wave is the
atrial depolarization
PR interval
is the conduction time through atria & AV node
R wave
is the ventricular depolarization
is larger due to the larger amount of tissue depolarizing
QT interval
is the total duration of ventricular systole
ST segment
plateau phase of ventricular APs
T wave
Ventricular repolarization
Cardiac Parasympathetic Fibers:
work via vagus nerves
Release acetylcholine
Increases permeability of resting membrane to K+
Decreases diastolic funny current through HCN channels
Cardiac Sympathetic Fibers:
Release norepinephrine
Increase diastolic inward currents (if) through HCN channels
Reversal Potential for Ca
123
Reversal Potential for Na
67
Reversal Potential for K
-92
Fascia adherens and macula adherens (desmosomes): ensure what happens?
ENSURE THE TRANSMITION OF FORCE FROM ONE MUSCLE CELL TO THE NEXT
Cardiac Parasympathetic Fibers via vagus nerves (LEFT)
Release acetylcholine – slows the rate of firing of AP in the SA node –> slows heart rate
Increases permeability of resting membrane to K+
Decreases diastolic if current through HCN channels
Cardiac Sympathetic Fibers: (RIGHT)
Release norepinephrine – speeds up heart rate, SHORTENS time to depolarization
Increase diastolic inward currents (if) through HCN channels (increases permeability)
Preload:
passive/resting tension placed on cardiac muscle cells before contraction
A function of the volume and pressure at the end of diastole
Increased in hypervolemia, regurgitation of cardiac valves, heart failure
Afterload:
the active tension placed on cardiac muscle cells during contraction
A function of the resistance the left ventricle must overcome to circulate blood
Increased in hypertension, vasoconstriction
Isometric Contraction:
“fixed length”
Activation of muscle whose ends are held rigidly means it can develop tension, but cannot shorten
Active tension developed during isometric contraction depends on the muscle length at which contraction
There is an optimal muscle fiber length in which to achieve maximum contraction
Isotonic Contraction:
“fixed tension”
occurs (which is linked to the resting tension)
Activation of unrestrained muscle causes it to shorten without force development because it has nothing to develop force against
Total load is the summation of preload and afterload
Inotrope: what is it
Can be positive or negative
Influences the amount of tension the muscle can develop
Influences the amount of shortening the muscle can achieve
Norepinephrine and cardiac function
= positive inotrope
Increases afterloaded shortening
Increases isometric tension – muscle fiber is able to shorten beyond what it normally could have
The Law of Laplace
The total ventricular wall tension (T) depends on both intraventricular pressure (P) & the internal ventricular radius (r)
T = P × r
easier for muscle cells to produce adequate internal pressure at end of ejection (small radius) than beginning of ejection (large radius)
Clinical relevance for cardiac dilation and hypertrophy
Bachmann’s bundle
Carries depolarization from SA node (in the right atrium) to the left atrium.
Bundles off of the Bundle of His
Left and right bundles
Left splits into Left Posterior Fascicle and Left Anterior Fascicle
Right bundle does not split into sub bundles
All 3 bundles end up spreading out and that is called the purkinje fibers
Negative Chronotropic Effect means what
prolongs time to depolarization
ex: parasympathetic effect on heart
Positive Chronotropic Effect means what
shortens time to depolarization
ex. sympathetic effect on the heart
Activation of muscle whose ends are held rigidly means it can develop
tension, but cannot shorten
Active tension developed during isometric contraction depends on the
muscle length at which contraction occurs (which is linked to the resting tension)
Inotrope:
Influences the amount of tension the muscle can develop
Influences the amount of shortening the muscle can achieve
Norepinephrine:
Positive Inotrope
Increases afterloaded shortening
Increases isometric tension
