The Rhythmical excitation of the Heart and the Normal ECG

Question 1

What is the resting membrane potential of the SA node and of the ventricle?

Answer 1

SA node: -55mV to -60mV
Ventricle: -85mV to -90mV

Question 2

How are electrical impulses generated automatically in the SA node?

Answer 2

The SA node has leaky sodium channels, which causes influx of Na+, thus raising the membrane potential from -55mV to -60mV, to -40mV.
Then the -40mV membrane potential causes L gated calcium channels to open

Question 3

What stops the SA node from remaining depolarized?

Answer 3

-L-type Calcium channels close.
-Potassium ion channels open.
-Membrane potential repolarize.

Question 4

Explain how the electric impulses move through the heart (conducting system)

Answer 4

SA node depolarize.
Internodal pathways to the AV node.
AP spreads across the atrial muscle fiber and slowly through the AV node.
Ventricular conducting system to the apex of the heart.
AP moves upward from apex of the heart to base.

Question 5

Explain what an ectopic pacemaker is, and its effect on heart contractions.

Answer 5

Ectopic Pacemaker is electrical impulses generated at other places in the heart besides the SA node.
Rhythmical discharge rate that is faster than SA node (overrides the SA node).
Ectopic pacemaker causes an arrhythmic contraction of the heart.

Question 6

Where is the sympathetic innervation in the heart, and where is the parasympathetic innervation?

Answer 6

Sympathetic innervation: The whole heart muscle
Parasympathetic innervation: the SA and AV node

Question 7

Define bradycardia and Tachycardia

Answer 7

Bradycardia refers to a resting heart rate of less than 60 BPM(beats/min).
Tachycardia is a resting heart rate of greater than 100 BPM.

Question 8

What neurotransmitter do parasympathetic nerves release

Answer 8

Acetylcholine

Question 9

What are the effects of acetylcholine in the heart?

Answer 9

-Acetylcholine increases the permeability potassium ions.
-Efflux of potassium ions down concentration gradient to the ECF.
-Cell membrane hyperpolarization.

Question 10

What neurotransmitter/hormone do sympathetic nerves release?

Answer 10

Norepinephrine

Question 11

What is the effects of NE in the heart?

Answer 11

Norepinephrine increase permeability of the sodium and calcium ions.
Increase the rate of electrical impulse discharged by the SA node.
Increases the rate of electrical impulse conduction and lower threshold.
Increases strength of cardiac muscle contraction.