Electrical activity of the heart Flashcards
What are electrical properties of the heart?
- Excitation contraction coupling
- Ionic basis of the non-pacemaker potential
- Modulators of electrical activity
- The special conducting system
- The electrocardiogram
What membrane surrounds muscle cells?
Sarcolemma membrane
What are intercalated discs in cardiac cells?
Junctions that connect cardiomyocytes together, some of which transmit electrical impulses between cells.
What is a sarcomere?
The basic unit of contractile muscle which contains myosin and actin, the two proteins that slide past one another to cause a muscle contraction.
What is the action of desmosomes in cardiac muscle?
They stop separation during contraction by binding filaments, joining the cells together.
What is the action of gap junctions in cardiac muscle?
allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarisation of the heart muscle.
How long is depolarisation in skeletal muscle compared to cardiac muscle?
Skeletal cells = 2ms
Cardiac cells = 250ms
Why is it important for cardiac cells to have a longer refractory period compared to skeletal muscle cells?
It is important because if the refractory periods were short, the heart could enter tetanic contraction/summation which is when muscle is stimulated so rapidly that it does not have a chance to relax at all between stimuli
- Fundamental in order to do not have a simultaneous contraction of atria and ventricles
What are cardiomyocytes?
They are the muscle cells (myocytes) that make up the cardiac muscle (heart muscle).
What do intercalated discs look like under microscopy?
Intercalated discs appear as thin lines dividing adjacent cardiac muscle cells and running perpendicular to the direction of muscle fibers.
What is tetanus in muscle cells?
The prolonged contraction of a muscle caused by rapidly repeated stimuli.
- Skeletal muscle can exhibit tetanus.
- NO tetanus in cardiac muscle.
How is contraction regulated in cardiac cells?
- Ca2+ entry from outside cell can regulate contraction.
- Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary the strength of contraction.
What cells create rhythmic impulses in the heart, setting the pace for blood pumping?
Pacemaker cells
- They directly control the heart rate. They make up the cardiac pacemaker.
Where are the pacemaker cells normally found in the heart?
The sinoatrial node
- The resultant rhythm is a sinus rhythm.
-Sometimes an ectopic pacemaker sets the pace, if the SA node is damaged
What is a cardiac arrhythmia?
Heart rhythm problems (heart arrhythmias) occur when the electrical impulses that coordinate your heartbeats don’t work properly, causing your heart to beat too fast, too slow or irregularly.
- E.g. Atrial fibrillation
What is the primary function of the sarcoplasmic reticulum?
To store Ca2+ ions.
- The sarcoplasmic reticulum of cardiac-muscle cells is not as well-developed as that of skeletal-muscle cells. Cardiac-muscle contraction is actin-regulated, meaning that the calcium ions come both from the sarcoplasmic reticulum (as in skeletal muscle) and from outside the cell (as in smooth muscle).
Features of pacemaker action potentials
- Action potential: increase in PCa2+ (L-type).
- Pacemaker potential(=pre-potential): gradual decrease in PK+, early increase in PNa+, late increase in PCa2+ (T-type)
- Pacemaker explains autorhythmicity
How does an action potential in cardiac cells occur?
Action potentials occur due to an increase in PCa2+ (L-type).
What is autorhythmicity and what causes it?
It is when the cells are able to generate the action potential at a certain rate without any external stimulus due to which the heart beats continuously and rhythmically.
- The inherent leakiness of SA node fibres to Na+ ions is what causes their self-excitation.
What are T-type calcium channels?
T-type calcium channels are low-voltage activated calcium channels that open during membrane depolarisation.
- T-type let in tiny amounts of calcium, which is why there is a late increase in PCa2+.
Features of non-pacemaker action potentials
- Resting membrane potential: high resting PK+.
- Initial depolarisation: increase in PNa+.
- Plateau: increase in PCa2+ (L-type) and decrease in PK+.
- Repolarisation: decrease in PCa2+ and increase in PK+.
Modulators of electrical activity
- Sympathetic & parasympathetic systems
- Drugs
- Temperature
- Hyperkalemia: fibrillation & heart block
- Hypokalemia: fibrillation & heart block (anomalous)
- Hypercalcemia: increased HR & force of contraction.
- Hypocalcemia: decreased HR & force of contraction.
How can drugs affect electrical activity?
- Ca2+ channel blocker: decrease force of contraction.
- Cardiac glycocides: increase force of contraction.
How can temperature affect electrical activity?
- Increase in temperature increases heart rate.
- increases: 10 beats/min/degrees celcius
