Origin and Conduction of Cardiac Impulse

Question 1

Excitation of the heart normally originates in the ______ cells in the ________ node

Answer 1

pacemaker

sino-atrial node

Question 2

The cluster of the specialised pacemaker cells in the _____ initiate the heartbeat

Answer 2

SA node

Question 3

Where is the Sino-atrial node located?

Answer 3

The upper RIGHT ATRIUM close to where the Superior Vena Cava enters the right atrium

Question 4

A heart controlled by the Sino-Atrial node is said to be in ______

Answer 4

sinus rhythm

Question 5

The cells in the SA node _____ have a stable resting membrane potential

Answer 5

do not

Question 6

The cells in the SA NODE exhibit ________ _______ ______

Answer 6

spontaneous pacemaker potential

Question 7

The spontaneous pacemaker potential takes the membrane to a threshold to generate an _____ ______ in the SA nodal cells.

Answer 7

Action potential

Question 8

In the pacemaker cells the permeability to ____ does not remain constant between action potentials

Answer 8

K+ ions

Question 9

The pacemaker potential (i.e. the slow depolarisation of membrane potential to a threshold is due to: (3)

Answer 9

Decrease in K+ efflux Na+ and K+ influx (the funny current) Transient Ca++ influx (T-type Ca++ channels)

Question 10

during the rising phase of pacemaker action potential (depolarisation) what happens on a chemical level?

Answer 10

activation of long lasting (L-TYPE Ca++ CHANNELS) Resulting in Ca++ influx

Question 11

Repolarisation (falling phase on pacemaker action potential) is caused by…

Answer 11

Inactivation of L type Ca++ channels Activation of K+ CHANNELS Resulting in K+ EFFLUX

Question 12

How does cardiac excitation normally spread across the heart?

Answer 12

From the SA node to AV node via intranodal tracts to LA and AV node AV node - Bundle of His - Purkinje fibers

Question 13

Desmosomes - what is their function?

Answer 13

Desmosomes form links between cells, and provide a connection between intermediate filaments of the cell cytoskeletons of adjacent cells. This structure gives strength to tissues.

Question 14

Gap junctions allow _____

Answer 14

Cell-to-cell current flow via gap junctions

Question 15

Where is the AV node located

Answer 15

Located at the base of the right atrium; just above the junction of the atria and ventricles

Question 16

What makes the AV node have slow conduction velocity?

Answer 16

The AV node cells are small in diameter and has slow conduction velocity

Question 17

What is specific to the AV node?

Answer 17

Is the only point of electrical contact between atria and ventricles The AV node cells are small in diameter and has slow conduction velocity

Question 18

Why is conduction delayed in the AV Node?

Answer 18

To allow atrial systole (contraction) to precede ventricular systole

Question 19

Action potential on atrial and ventricular myocytes graph

Answer 19

Phase 0: Fast Na+ influx Phase 1: Closure of Na+ channels and Transient K+ efflux Phase 2: Mainly Ca++ influx Phase 3: Closure of Ca++ channels and K+ efflux Phase 4: Resting membrane potential

Question 20

The plateau phase of action potential is phase ___ and it is a unique characteristic of contractile muscle cells, the plateau phase is mainly due to ______

Answer 20

2 Influx of Ca++ through L-type Ca++ channels

Question 21

The falling phase of action potential (repolarisation) is phase ___ , the falling phase is caused by ______

Answer 21

3 inactivation of Ca++ channels and activation of K+ channels Resulting in K+ EFFLUX

Question 22

What changes Heart rate?

Answer 22

Autonomic Nervous system Sympathetic stimulation increases heart rate Parasympathetic stimulation decreases heart rate - The VAGUS NERVE (PARASYMPATHETIC supply to the heart) exerts a CONTINUOS influence on the SA node under resting conditions - VAGAL TONE DONIMATES under normal resting conditions - Vagal tone SLOWS the INTRINSIC HEART RATE from ~100bpm to produce a normal resting heart rate of ~70 bpm

Question 23

Define Bradychardia

Answer 23

Heart rate of <60

Question 24

Define Tachycardia

Answer 24

Heart rate of >100

Question 25

Parasympathetic supply of the heart: What nerve supplies the SA and AV node

Answer 25

Vagus

Question 26

What effect does vagal stimulation have on the heart?

Answer 26

Vagal stimulation slows heart rate and increases AV nodal delay (increasing the length of each cardiac cycle)

Question 27

Describe what happens in each phase

Answer 27

Phase 0: Fast Na+ influx

Phase 1: Closure of Na+ channels and Transient K+ efflux

Phase 2: Mainly Ca++ influx

Phase 3: Closure of Ca++ channels and K+ efflux

Phase 4: Resting membrane potential