Electrical activity of the heart

Question 1

State the electrical properties of the heart

Answer 1

-

Question 2

Excitation-contraction coupling in skeletal muscle VS cardiac muscle

Answer 2

SM- can exhibit tetanus ( good when carrying heavy weight)
-AP does not trigger opening of Ca2+ channels, but ttravels down T-tubules and causes release from Ca2+ from SR onl ( no extracellular)
CM - no tetanus ( need contraction and relaxation, otherwise no beating)

Longer action potential
- due to calcium entry delaying repolarisation of membranes. Calcium can come from cytosol/calcium dependent release from sacroplasmic reticulum.

Calcium does not saturate troponin like in skeletal muscle, so amount of calcium acts as a control

• Unstable MPs in pacemaker cells
• functional syncitium

Question 3

Describe the features of the cardiac muscle ( e-c coupling)

Answer 3

• eletrically connected via gap junctions ( allows ions to pass depolarisation in neighbouring cells)
• physically connected by desmosones ( prevents cells from pulling apart during contraction)
• GJ + Des =intercelated discs

Question 4

Why can’t cardiac muscle exhibit tetanus?

Answer 4

-long refractory period so no tetanic contraction

Question 5

Describe the mechanism of Ca2+ in a muscle contraction

Answer 5

• regulation of calcium ions will regulate contraction ( can make contraction stronger/weaker depending on number). The more Ca2+, the more crossbridges will be formed.
• calcium release will not saturate all the troponin
• can regulate SV

Question 6

Non-pacemaker ( mycocytes) action potential

Answer 6

RMP
-high resting PK+ (permeability - leaky K+ channels)

Initial depolarisation
- increase in PNa+ ( sodium voltage gated channels and they close quickly)

Plateau

• increase in PCa2+(L-type which lets in more Ca2+ vs T-type) - [opens slower but stays open longer]
• decrease in PK+

Repolarisation

• decrease in PCa2+ ( L-type)
• decrease in PK+

Question 7

Pacemaker action potential

Answer 7

Action potential is slower due to:
-increase in PCa2+ ( L-type), only voltage gated Ca2+ NO Na+, hence slower.

Pacemaker potential

• there is a gradual decrease in PK+ (slow)
• BUT early increase in PNa+ ( = PF) [ voltage gated but different. Causes early AP due to Na+ flowing into cell via this funny channel]
• late increase in PCA2+ ( T-type) [ not big AP evoked due to less Ca2+ entering but threshold is still reached]

Pacemaker is autorythmicity ( self depolarising)

Question 8

Modulators of electrical activity

Answer 8

-Drugs(anti-arrythmic), temperate, hconditions (hyperkalemia/calcemia hypokalemeria/calcemia )

Drugs

• Ca2+ channel blockers decrease in force of contraction[ L-type]. They operate by making less Ca2+ available to trigger cross-bridge formation.
• cardiac glycocides ( increases force of contraction) increased Ca2+ released from int stores so more crossbridge formation]

Temperature
- increase by 1 degree celcius = increase in HR by 10beats/min

Hyperkalemia ( high plasma K+)

• fibrilation + heart block [depolarises cells due to reduced conc gradient for K+ changing eq/potential + RMP. Cardiac cells more likely to fire..unordinated firing+contraction=fib]
• Heart block: If cells depolarise, smaller ion gradient pushing ions in + things happen slower to the point heart stops.

Hypokalemia
- fibrilation + heart block (anomalous) [ hyperpolarises but then depolarises]

Hypercalcemia
-increased HR + force of contraction

Hypocalcemia
-decreased HR + force of contraction

Question 9

Name the Conduction system

Answer 9

• SA node ( pacemaker) [ self depolarises wheras cardiac cells depolarise due to neighbouring cell ]
• Annulus fibrosus ( non conducting) [ conduction stops here + NO GAP J]
• AV node ( delay box)
• Bundle of His
• Purkinje fibres ( rapid conduction system)

Question 10

Describe the mechanism of the conduction system

Answer 10

An action potential in a single myocyte evokes a small extracellular electrical potential

• lots of small extracellular eletrical potentials are evoked by many cells depolarising and repolarising at the same time and can summate to creat a llarge extracellylar eletrical waves
• This is recorded at the periphery as the electrocardiogram

Question 11

Describe the PQRST wave

Answer 11

P wave = atrial depolarisation
QRS complex = ventricular depolarisation
T wave = ventricular repolarisation

Question 12

How is the cardiac muscle classified as a functional syncitium?

Answer 12

• this is formed by fusion of many cells, so many of these cells will operate as the 1 cell
• physically and electrically connected

Question 13

Types of calcium ion channels

Answer 13

L type: allows large amount of Ca2+ ions into cell

T type: present in pacemaker cells and allows small amount of ions into cell over a long period

Question 14

Funny current

Answer 14

-pacemaker potential is stange, sodium channels are opened up by repolarisation from previous AP