electrical activity of the heart Flashcards
what are the 2 types of cardiac muscle cells
contractile (99%)
autorythmic (1%)
t/f each type of myocardium cell has a distinctive action potential
True
cardiac has the ability to generate
action potentials
what percent of myocardial cells can generate action potentials simultaneously?
1
t/f the heart can contract without an outside signal
True
why can the heart contract without an outside signal?
it is myogenic, originating within the heart itself
define autorythmicity
The heart contracts, or beats, rhythmically due to the action potentials that it generates by itself
T/F the signal for myocardial contraction does NOT come from the nervous system but from specialized myocardial cells also called autorythmic cells
true
autorythmic cells are also called
pacemaker cells
pacemaker cell st the
rate of heartbeat
autorythmic cells do not contribute to the contractile force of the heart because
they do not have organized sarcomeres
myocardial contractile cells are also known as
working myocardium
working myocardial cells include
atrial and ventricular muscle
the conduction system is made of
specialized myocytes
myocytes can propagate
electrical current
role of the conduction system is
Initiation of the heartbeat
Coordination of the heartbeat
components of the conduction system
Sinus (sino-atrial) node
Atrioventricular (AV) node
His-Purkinje system
Bundle of His
Left bundle branch
Anterior fascicle
Posterior fascicle
Right bundle branch
Purkinje fibers
location, role and structure of the sinus node
location: right atrium at junction between cranial vena cava and atrium
role: normal pacemaker and initiates the impulse
structure: pacemaker cells
t/f In most species, the initiation of the impulse can occur anywhere between the cranial and caudal vena cava
true
what causes atrial depolarization?
Cardiac impulse propagates
-Right to left
-Top to bottom (base to apex)
Cell-to-cell propagation
No specialized conduction system within the atria
where is located and what is the role of the atrioventricular node?
location: base of the interventricular spetum
role: ONLY CONDUCTION PATHWAY
role of His left and right bundles
conducting an impulse rapidly from AV node to apex of the heart.
the His right bundle branch is isolated from
myocytes until it reaches the cardiac apex
the His left bunddle branch has connections with
myocytes un the interventricular septum
role of purkinje fibers
Propagate impulse rapidly to ventricular myocytes
in relation to the purkinje fibers, the depolarization starts at
apex and propagates to the base, ventricular contraction starts at the apex the apex and propagates to the base
t/f a pacemaker can geneate a stimulus simoultaneously
true
name the physiological pacemakers
Sinus node / AV node / Purkinje fibers
rate of depolarization can be modulated by autonomic influences
PSNS
SNS
WHICH is the faster pacemaker that controls the electrical activity?
sinus node
if the sinus node malfunctions, who takes over?
AV node
if the AV node malfunction, who takes over?
Purkinje fibers
electrical impulse transmission between myocytes is possible due to
intercalated disks that connect myocytes end to end formed by
desmosomes
gap junctions
desmosome are
mechanical connections
gap junctions are
electrical connections
The rapid propagation of impulses allows
contraction of cardiac chambers as a unit
Cells of the conduction system are wider and contain more
gap junctions = faster conduction
Gap junctions are rare between cells of
AV node = slow conduction
roles of the conduction system
initiation of electrical impulses = within the heart by pacemaker cells
propagation of electrical impulse = specialized myocytes
t/f an isolated heart can still beat at a regular rate
true
propagation of cardiac action potential
change of cell membrane voltage in pacemaker cells
ions moving across the membrane : Na, K and Ca
the inside of the cell membrane of pacemaker cells is more
negative
the outside of the cell membrane of pacemaker cells is more
positive
describe the biological curents of Na, K and Ca
more Na outside the cell, channels open for Na to enter
more K inside the cell, channels open for K to efflux
more Ca outside the cell, channels open for Ca to enter and has a extracellular and intracellular [] gradient because Ca is secuestrated in the sarcoplasmic reticulum
at rest, the ventricular myocytes are at a voltage of
-80 to -90
the duration of the cardiac action in a ventricular cell potential is
200-400
during phase 0, the ventricular cell is
depolarized
Na open
K closed
Ca closed
during phase 4, ventricular cells are
at resting potential
it is negative inside the cell
t/f during phase 0, another new action potential can be generated
false
during phase 1 od the action potential in ventricular cells, the ion channels are (Na, K , Ca)
Na+ channels inactivated
K+ channels open
Ca++ channels closed
when K flowa out of the cell, the membrane potential becomes more
negative
During phase 2 of the action potential in the ventricular cells, what happens with the ion channels (Na, K, Ca)
plateau
Na+ channels inactivated
Ca++ channels open = enters the cell
K+ channels open = leaves the cell
balance between Ca and K
During phase 3 of the action potential in the ventricular cells, what happens with the ion channels (Na, K, Ca)
repolarization
Na+ channels inactivated
Ca++ channels closed
K+ channels open = exit the cell
what enzyme is involved in phase 3 and what is it’s role?
ATPase
Na+/K+ ATPase pump restores equilibrium
Ca++ ATPase pump moves Ca++ out of cell
refractory period
Time between beginning and end of an action potential
why can’t an electrical impulse depolarize a myocyte before the end of an ongoing action potential?
due to the Na channels
They can exist in 3 states: open, inactivated and closed
They are blocked in inactivated state after phase 0
They return to a closed state when membrane potential back to -80 mV
what is the role of the refractory period?
prevents repetitive cardiac contractions
absolute refractory period = impossible to occur another action potential at this time
relative refractory period
define relative refractory period
A second action potential is unlikely but possible if high energy stimulus
stages of the action potential in myocytes ventricular cells
Depolarization
Rapid Repolarization
Plateau
Repolarization
what happens during depolarization in myocytes?
Na influx
what happens during rapid repolarization in myocytes?
K efflux
what happens during the plateau in myocytes?
Calcium Influx
what happens during repolarization in myocytes?
K efflux
what is distinct about the pacemaker cells “resting membrane potential” and what do they actually have?
do not have a stable resting membrane potential like the nerve and the skeletal muscles, they have a PACEMAKER POTENTIAL.
have an unstable membrane potential that starts at – 60mv and slowly drifts upwards towards threshold.
What causes the pacemaker potentials of these cells to be unstable?
permeability to Na and K, leading to the influx of both at the same time. This net influx slowly depolarized leading to the opening of the calcium channels.
ionic basis of the action potential in pacemaker cells
Phase 1: Pacemaker Potential
phase 2: the rising phase or depolarization
phase 3: the falling phase or repolarization
ions associated with the pacemaker potential
Opening of voltage-gated Na channels
Closure of voltage-gated K channels.
Opening of Voltage-gated Transient-type Calcium
ions associated with the pacemaker potential during the rising phase or depolarization
Opening of Long-lasting voltage-gated Calcium channels
Large influx of Calcium.
ions associated with the pacemaker potential during the falling phase or repolarization
Opening of voltage-gated Potassium channels
Closing of Long-type Ca channels.
Potassium Efflux.
nodal cells, sinus, and atrioventricular cells action potential stages
- Baseline potential = - 60 mV, Spontaneous depolarization, Net influx of Na+ (If channels)
- threshold (-40 mV), Ca++ channels open and Influx of Ca++
- When membrane potential reaches 0 mV, K+ channel open
and Efflux of K+
4.Repolarization to -60 mV
what cells are associated with these characteristics?
Spontaneously depolarize
Upstroke action potential
Depends on Ca++ channels
No Na+ channels
No plateau phase
pacemaker cells
what cells are associated with these characteristics?
No spontaneous depolarization
Upstroke action potential
Depends on Na+ channels
Na+ channels
Plateau phase (phase 2)
atrial and ventricular myocytes
what tone is associated with these characteristics
Sympathetic nerves
Neurotransmitter: Norepinephrine
Receptor: Beta receptor (beta 1 and 2)
Adrenal glands
Epinephrine, norepinephrine
Receptor: Beta receptor (beta 1 and 2)
adrenergic tone
what nerve is associated with these characteristics?
Parasympathetic nerves
Neurotransmitter:Acetylcholine (Ach)
Receptor: Muscarinic receptor
vagal nerve
what effects causes the adrenergic tone on the sinus node?
Increases heart rate
Increases firing of sinus node
Makes membrane more permeable to Na+
Makes Ca-channel more permeable to Ca++
Increases rate of depolarization
what effects causes the vagal tone (psns) on the sinus node?
slows heart rate
decreases firing of sinus node
Acetylcholine (Ach) stimulates K channels (IK(Ach))
More K+ leaves the cell
Opposes effects of If
Takes more time to reach threshold
the SNS acts on myocytes specifically on what type of receptors
Beta-receptors (mainly beta-1)
effects of the SNS on myocytes
Shortens action potential in all cells
Allows faster conduction of impulses
Increases rate of depolarization in pacemaker cells
Increases rate of sinus node discharge
Increases conduction of impulses through AV node
Increases the strength of contraction
Strength of contraction related to amount of Ca++ entering cells
Effects of parasympathetic system on myocytes
Effect on sinus node
Slows depolarization
Slows pacemaker rate discharge
Effect on atrioventricular node
Decreases conduction velocity = slows impulse propagation to the ventricles
No direct effect on ventricular myocytes (lack of innervation to the ventricles)
excitation and contraction coupling
Ca++ enters cell during phase 2 action potential
Ca++ reaches inside of the cell through T-tubules
T-tubules are invagination of the membrane
T-tubules are rich in Ca++ channels
T-tubules are close to sarcoplasmic reticulum
for the myocytes to contract, Ca has to bind to
myofilaments
myofilaments in myocytes are
thin = actin, troponin and tropomyosin
thick = myosin
actin, troponin and tropomyosin
thin filament
myosin
thick filament
contraction occurs when
myosin heads bind to actin
what prevents fixation of myosin to actin
Troponin/tropomyosin complex
what allows the binding of myosin and actin?
Ca++ binding to troponin
realxation of the myocytes occurs when Ca is
actively reabsorbed into the sarcoplasmic reticulum
Ca++ is moved out of cell
Ca++ ATPase pump
Ca++/Na+ exchanger
cardiac contraction depends on what ion?
CALCIUMMM
most of the calcium is stored in the
sarcoplasmic reticulum
3 proteins that form troponin
T I C
example of sympathomimetic drug
Dobutamine
how does Dobutamine work?
Acts on beta1-receptors
Increases intracellular calcium = positive inotrope
Increases the rate of calcium reuptake in sarcoplasmic reticulum is diastole= positive lusitrope
