Pacemaker potentials Flashcards
Compare the **% of cells in the heart that are contractile or pace maker** cells
- 99% of cardiac cells are contractile, they are responsible for the mechanical work of pumping
- 1% of cardiac cells are pacemaker cells are specialised non contractile cells
What is the function of cardiac pacemaker cells?
- they are responsible for triggering the contraction of cardiac muscle
- exhibit automaticity - spontaneous rhythmic action potential generation
- they set the rhythm of cardiac contraction
What is the main location in the heart that contains pacemaker cells? (dominant pace maker)
the Sino Atrial node (SA node)
What is the SA node regulated by?
regulated by
* the ANS
* circulating hormones
What are the** 2 types of Ca2+ channels** involved in the pacemaker potential?
- T (transient) channel
- L (long type) channel
what is the Na+ channel needed for the pacemaker potential?
- Na+ (F)
- funny channel
How many phases are there in the pace maker potential (SA node AP)
- 4 phases
- NO PHASE 1
- 4,0,2,3
Describe what occurs during phase 4 of SA node AP?
- An **Na+ influx **occurs through the Na+ funny channels
- the F type channels are not selective between K+ and Na+, but Na+ entry is favoured (inward depolarising current)
- delayed recitifier voltage activated K+ channels (Kv) inactivate which reduces the outflow of K+
- opening of the voltage gated T-type Ca2+ channels
What are important exchanger channel that is involved in phase 4 of the SA node AP?
The 3Na+/Ca2+ exchanger
What does phase 0 involve?
- the opening of the T type Ca2+ channels leads to a membrane depolarisation
- this activates the** L type Ca2+ channels** which cause a** full depolarisation** of the membrane
What does phase 2 involve?
- The T type channels close
- the membrane begins to repolarise
What does phase 3 involve in the SA node AP?
- The PREVIOUS depolarisation has opened 2 types of K+ channels
- Ikr (rapidly activating delayed outward rectifiying current)
- Iks (slowly activating delayed outward rectifying current)
- The Kv channels activate slowly during the AP and also inactivate slowly
What are the** important anatomical parts** of the heart for the cardiac conduction system?
- SA node
- AV node (atrioventricular)
- Bundle of his
- Purkinje fibers
What is the function of the bundle of His?
- The bundle of His is an important part of the electrical conduction system in the heart
- they transmit impulses that are made in the AV node down to the ventricles
- the bundle branches out into the left and right branch
What is the function of the purkinje fibers?
- The purkinje fibers are extremely efficient at conducting action potentials from the pacemaker cells to the endocardium (the smooth membrane that lines the inside of the heart)
- from the endocardium the impulses travel from myocyte to myocyte via gap junctions
What are intercalated discs?
- they are structural formations found between cardiac muscle cells
- they play a role in connecting them together and allowing the passage of signals between them
- they contain desmosomes and gap junctions
What are the 3 types of cell junctions that make up the intercalated disc?
- **Fascia adherens **(anchor actin filaments of the sarcomere)
- desmosomes (important for cell adhesion via cadherin plaques)
- gap junctions
What is the function of the gap junctions within the intercalated discs?
- they provide electrical coupling for adjacent cardiac muscle cells (allows ions to flow between cells)
In summary, how does the SA node AP arise?
- **inflow of Na+ **through F type channels
* inflow of Ca2+ through T type channels and L type channels - decreased outflow of K+ through delayed rectifier (Kv) k+ channels
In summary, what is the AP depolarisation phase (0) caused by?
It is caused by the inflow of Ca2+ through L type Ca2+ channels
In summary, what is the repolarisation phase caused by (2) ?
The repolarisation phase is caused by
* the** K+ outflow** through delayed recitifer (Kv) K+ channels
* and the **closure of the L type Ca2+ channels **
Why are the SA node action potentials smaller and slower rising than the ventricular AP’s in cardiac myocytes?
- few fast Na+ channels
- the Ca2+ current through L type channels depolarises the membrane SLOWLY in comparision with the influence of voltage gated Na+ channels in fast response cells
