Electrical activity of the heart, electromechanical coupling Flashcards
Types of myocytes/excitable tissues in the myocardium
- Pacemaker
- Conductive system
- Working fibers
Additional elements:
- Anulus fibrosus
- AV node
What is membrane potential?
electrical differences measured between the outer and inner side of the membrane (sarcolemma)
What is Resting membrane potential (RMP)?
electrical differences measured between the outer and inner side of the membrane in resting state
- Ca -90 mV
- In pacemaker cells RMP does not exist.
- In diastole, spontaneous deplarization occurs, followed by an AP.
What is Action Potential (AP)?
Following certain stimuli, ion channels of the membrane open and the ion exchange between the two sides lead to electrical changes called Action Potential
Pacemaker cells:
- Has no permanent resting membrane potential, but turns into constant depolarization (lifetime pacemaker)
- They are specialized heart muscle cells located in the sinoatrial and atrioventricular node.
- Their main role is the continuous generation of the excitation
- during repolarization the transmembrane potential reaches -55mV an automatic depolarization folloes.
Cardiac cells that depolarize spontaneously toward treshold are called?
Pacemaker cells. Becuase they initiate heartbeats and therefore determine the rate, or pace, og the heart.
Types of pacemaker cells:
- Round pacemaker cells: sites of the generation of the excitation
- Elongated or slender cells: Conduct and synchronize the excitation generated in the round pacemaker cells
explain MDP
- Pacemaker fiber
Maximal Diastolic Potential (virtual resting membrane potential):
- slow Na+ channels open spontaneously - slow depolarization begins
explain SDD
- Pacemaker fiber
Spontaneous Diastolic Depolarization:
- there is no RMP, from MDP till treshold potential
explain overshoot
- Pacemaker fiber
Much lower positive values (+5/15mV) than in working fibers (+25 mV)
Repolarization:
- pacemaker fiber
K+ efflux till MDP
What type of calcium channels to the depolarization open in the pacemaker?
Type-T, rianodin sensitive and Type-L, DHP sensitive.
This opening causes calcium to flow from the EC into the cell creating a Transient Ca-influx.
The deppolarization of the SA-node is due to?
- pacemaker cells
the long-lasting Ca channels (strong Ca influx)
What happens by opening of the K-channels?
- pacemaker cells
It causes the efflux of K ions from the cell, repolarization begins.
Natural factors influencing the heart rate:
by altering the activity of round cells in SA:
- Sympathetic effect
- Parasympathetic effect
And the Vagus escape
What is the sympathetic effect stimulated by?
Beta-1-rec, which stimulates G-protein mediated IC cAMP increase, consequently non specific Na+ and K+ channels open.
What happens when the sympathetic effect is stimulated?
- Non-specific Na? and K+ channel open.
- MDP shifted upwards,
- steepness of SDD increases
- treshold is reduced
= the heart rate (Fr) is increased!
The effect of the sympathetic effect can also be triggered by other signals, like what?
- Norepinephrine (neural signal)
- Epinephrine (endocrinological)
What is the Parasympathetic effect stimulated by?
Acetylcholine (from vagal nerve endings) stimulate muscarinic acetylcholine receptors on round cells.
What happens when the Parasympathetic effect is stimulated?
- cAMP decreases
- MDP is shifted down
- SDD slope decreases
- Treshold potential elevates
- Hyperpolarization occurs.
= heart rate is decreased!
What is the function of the n.vagus
It continous AP discharge slows down the original activity of the heart.
What does chronotrop mean?
frequency, time of contraction
What does Dromotrop mean?
speed of contraction
What does Bathmotrop mean?
Treshold
What does Inotrop mean?
Force of contraction
Conduction system in small animals:
Subendocardial conduction = conductive fibers do not go deeply into working muscle
Conduction system in Large animals:
Subepicardial conduction = deeply into ventricle
what happens in the conduction system if the signal comes from the SA-node?
it results in nomotop excitation
what happens in the conduction system if the signal comes from the AV-node?
that results in heterotop excitation
What is the role of the Anulus Fibrosus in the conduction system?
it represents electric resistance - it synchronizes te atrio-ventric cooperation
What are responsible for fast conduction?
His bundle -> Tawara bundles -> Purkinje fibers
What is the role of Working fibers?
Generate unique, elongated AP (plateau), which prevents the heart from early secondary contraction.
Ion flow during Action Potential in Working Fibers:
Phase 0: Depolarization: voltage-gated Na+ opens. These channels are inactivated at -25mV (overshoot)
Phase 1: Rapid repolarization: Short Cl- influx and K´efflux (early K+ channels)
Phase 2: Plateau: Slow Ca++ influx, and slow K+ efflux. Plateau phase is shorter closer to the epicardium.
Phase 3: Rapid repolarization: strong K+ (late K+ channels), Na + Ca ++ channels are active
Phase 4: Late hyperpolarization: At the end of the AP, membrane potential sinks below the RMP.
what are the electrical properties of the heart?
- RMP
- Ion flow during Action Potential
What are the voltage-gated channels?
The potential (voltage)- dependent Na channels open, inactivate and close in function of the actual potential difference. These channels are responsible for the generation of the AP
What is the reason for that the Potassium conductance increases until the end of the AP?
In phase 1 of the AP, the early K channels, in phase 2 the slow K channels and in phae 3 the late K channels opens. This is the reason why the K concuctance increases until the end of the AP
What is the role of the Plateau phase?
To prevent a premature generation of a new AP
refractory phases of Working heart muscle fibers:
- Absolute refractory phase
- Relative refractory phase
- Supernormal phase
Explain Abosolute refractory phase:
- No stimulus (independetly of the stregth) can elicit a new AP before the end of phase 2(plateau) in the working fiber of the heart
- rational: in this phase all necessary ion channels that participate in the depolarization are either in use or inactive.
Explain Relative refractory phase:
- A stimulus given after the end of the plateau phase but before reaching the treshold potential elicits an answer in function of the strength of the stimulus.
- Strong stimuli can elicit the formation of a new AP. This phase is therefore named Relative Refractory Phase..
- Rational: some ion channels are active again (after inactive state), but not all. Therefore only stronger stimuli can cause enough strong depolarization effect.
Explain Supernormal phase:
- from the practical point of view the most important period is the one between the treshold and the RMP
- A slight stimulus can elicit a new AP, and produce a premature new contraction.
- A non-working (sick) fiber generates a small battery and stimulates the normal working fibers, when they go into their SNP. this lead to renewed premature contractions and can be fatal when it happens in the ventricle (fibrillation)
Explain Atrial Fibrillation:
- i.e electrical stimulation of the atrium, repeated premature contrctions, the normal atrial stimulus stops.
- The ventricle maintains the normal circulatory pressure
- It is not lethal!
Explain Ventricular Fibrillation:
- The normal blood pressure can nt be maintained, it may drop to 0, normal systole and diastole disappear
- this is lethal!
Explain defibrillation
With strong electric current, desynchronization is stopped, SA node is synchronized again, and we have normal rythm generation, so nomotop excitation is back
Difference between AP and mechanogram of cardiac and skeletal muscle:
- There is no plateau phase of the skeletal AP
- the skeletal AP lasts for millisec in contrast to the 200msec AP of the heart.
- The cardiac mechanogram is almost parallel with the AP
- the mechanogram of the skeletal develops only after the AP has vanished.
Definition of Electromechanical coupling
Connection between electric stimulus (excitation) and mechanical signal (contraction)
what is the structural unit of the cardiac muscle in electromechanical coupling?
DIAD, (in skeletal muscle: Triad)
Steps in electromechanical coupling:
- AP spreds onto the cell
- AP reaches the T-tubules and activates the tubular L-type Ca++ channel (voltage-gated, DHP sensitive channels)
- Conformation changes of L-type channels opens the T-type channels on the SR (rianodin-sensitive channels)
A large amount of Ca gets into the sarcoplasma from SR. - Elevating sarcoplasmic level of Ca++ opens the Ca++ dependent Ca++ channels on the SR.
- And elevating sarcoplasmic level of Ca++ also opens the Ca++-dependent Ca++ channels on the cell membrane (sarcolemma).
- RESULT = huge amount of Intra Cytoplasmic Ca++ is around the sarcomers –> CONTRACTION
Elimination of Calcium signal in electrochemical coupling
Happens after the contraction (but starts during the contraction).
- Na+/Ca2+ antiporter into extracellular space.
- ATP-dependent Ca2+ transporter into SR.
- IC Ca2+ concentration decreases, leading to RELAXATION
