Electrical activity of the heart

Question 1

what is a syncytium

Answer 1

The heart is a syncytium

A cell which is made up of different cells fused together

Question 2

How are cells in the heart connected and what does this allow

Answer 2

Physically; desmosomes
electrically: gap junctions
this forms an intercalated disc
allows the cells in the heart to communicate and contract in sync.

Question 3

How are action potentials in cardiac muscle different to skeletal muscle

Answer 3

Last longer- 250ms vs 2ms

Have a longer resting potential- they do not have tetanic potentials as the refractory period is much longer

Question 4

What are the different types of resting potentials in heart cells and where are they found

Answer 4

Resting potential

• 90m.v.
• found in non-pacemaker potential cells
• maintained by leaky potassium channels

Fluctuating resting potential :

• found in pacemaker cells
• due to decreased permeability of potassium channels, increased permeability to sodium and T-type calcium channels being opened

Question 5

Trend of an action potential in a pacemaker cell- explains AUTORHYTHMICITY

Answer 5

• fluctutating R.P.
• calcium channels open (T-type)
• calcium flows into the cell and depolarises it to threshold
• Calcium channels close and potassium channels open

Question 6

Trend of an action potential in a non-pacemaker cell

Answer 6

• resting R.P.
• cell becomes depolarised as the sodium channels open and the L-type calcium channels open
• Plateau- the sodium channels close but the calcium channels stay open, permeability to potassium decreases
• Repolarisation- potassium channels open and calcium channels close

Question 7

How can electrical activity be modulated

Answer 7

DRUGS:
-calcium channel blockers: decreases calcium entering the heart, decreases the forces of contraction
-cardiac glucosides: opposite
HYPERKALEMIA- high potassium in the plasma
- decreases the gradient so less potassium flows out of the cell
-resting potential cannot be maintained which leads to fibrillation
HYPOKALEMIA = HYPERKALEMIA
HYPERCALCEMIA:
-increases amount of calcium in the plasma
-increases amount of calcium flowing into the cell
-increases heart rate and force of contraction
HYPOCALCEMIA: does opposite
TEMPERATURE: heart rate increases by 10bpm for every 1 degree celsius- increased force of contraction

Question 8

Describe the special conducting system of the heart

Answer 8

sinoatrial node:
-found In right atrium
-fastest depolariser -0.5 ms
-spreads electrical activity through both atria
Annulus fibrosus:
-layer of non-conducting tissue
-prevents the ventricles from conducting before the atria
AV node: delays spread
Bundle of his and purkinje fibres:
-electrical activity spreads down bundle of his through to purkinje fibres at the apex of the heart
-ventricles contract from bottom up

Question 9

what is the pacemaker of the heart

Answer 9

the SA

Question 10

explain the excitation-contraction coupling theory?

Answer 10

1- A.P. enters transverse TUBULE ( folds in membrane)
2- calcium is released from LATERAL SAC and binds to sarcoplasmic reticulum
3-increased calcium ions in sarcoplasmic redticulum bind to troponin.
4-troponin is bound to tropomyosin and holds it in place. tropomyosin partially covers the myosin cross-bridge heads. when calcium binds to troponin, it causes it to change shape and detach from tropomyosin. cross-bridge moves

How does the cross-bridge work?
1- cross-bridge binds to actin
2-cross-bridge moves- ADP and Pi detach from cross-bridge head
3- ATP binds to myosin- cross-bridge breaks
4-cross-bridge head is activated by ATP hydrolysis