cardiac excitation (midterm 2)

Question 1

what are other functions than contraction the heart does

Answer 1

• generation of rhythmical electrical impulses to cause rhythmical contraction of heart muscle
• conduction of ^ impulses rapidly through cardiac muscle

Question 2

what are results of the rhythmical excitation of the heart

Answer 2

atria contraction before ventricular
spatial homogeneity of ventricular contraction

Question 3

what does the sinus node do

Answer 3

it’s a strip of cardiac muscle that generates the normal rhythmical impulse

Question 4

what do the internodal pathways do

Answer 4

conduct the impulse from the sinus node to the AV node

Question 5

what does the AV bundle do

Answer 5

conducts impulses from the atria into the ventricles

Question 6

what do the left and right branches of purkinje fibers do

Answer 6

conduct the cardiac impulse to all parts of the ventricles

Question 7

where is the sinus node located and how big is it

Answer 7

located in the superior posterolateral wall of the right atrium
3 mm wide, 15 mm long, 1 mm thick

Question 8

what happens because fibers connect directly with the atrial muscle fibers

Answer 8

any AP that begins in the sinus node spreads immediately into the atrial muscle wall

Question 9

how is tissue self-excitatory

Answer 9

it doesn’t contract, but hits a threshold where AP fires on its own

Question 10

how is the sinus node the pacemaker of the heart

Answer 10

the sinus node controls the heart rate of the entire heart - sinus fibers are self-excitatory

Question 11

what is the resting membrane potential of the sinus nodal fiber between discharges

Answer 11

-55 to -60 mV

Question 12

what causes the sinus nodal fiber to be less negative

Answer 12

cell membranes of the sinus fibers are naturally to Na+ and Ca++ channels,, which allows Na+ and Ca++ into the cell to eventually generate an AP via threshold value

Question 13

when are channels active/inactive in sinus nodal fiber and what is the result of this

Answer 13

• fast Na+ are inactivated
• slow Na+-Ca++ channels can open and initiate an action potential
  atrial nodal AP is slower to develop than ventricular muscle AP

Question 14

when does an AP of sinus nodal fibers occur

Answer 14

Na+ influx into cardiac muscle fiber that activates Na+-Ca++ channels

Question 15

what are internodal pathways

Answer 15

where the ends of the sinus nodal fibers connect directly with surrounding atrial muscle fibers

Question 16

what is the velocity of AP conduction in atrial muscle

Answer 16

0.3 m/sec or 1 m/sec for small bands of fibers

Question 17

how long does it take for an impulse to reach the AV node after traveling through the internodal pathways and what is the next delay

Answer 17

0.03 sec followed by a delay of 0.09 sec before impulse enters the penetrating portion of AV bundle, where it passes into ventricles

Question 18

what is the final delay of impulses in AV node

Answer 18

0.04 seconds occurring mainly in this penetrating AV bundle

Question 19

what is the total delay before the excitatory signal reaches the contracting muscle of the ventricles

Answer 19

0.16 seconds

Question 20

what is speed a function of

Answer 20

of gap junctions - less gap junctions = greater resistance to conduction of excitatory ions from one fibers to the next

Question 21

what does special purkinje fibers do

Answer 21

lead from the AV node through the AV bundle into the ventricles

Question 22

what is the velocity and size of special purkinje fibers

Answer 22

large - 6x of ventricular muscle, 150x of Av nodal fibers
velocity of 1.5-4 m/s

Question 23

why is there no retrograde transmission of AP in AV bundle

Answer 23

prevents passage of cardiac impulse between atrial and ventricular muscle

Question 24

what is the fastest rate of rhythmical dischage in sinus node

Answer 24

70-80/min = pacemaker
impulse is conducted into AV node and purkinje fibers, also discharging their excitable membranes

Question 25

what is the rate of rhythmical discharge in unstimulated AV nodal fibers

Answer 25

40-60/min

Question 26

what is the rate of rhythmical discharge in unstimulated purkinje fibers

Answer 26

15-40/min

Question 27

what is parasympathetic (vagal) stimulation

Answer 27

causes acetylcholine release, decreasing rate of rhythm of the sinus node

Question 28

what is sympathetic stimulation

Answer 28

increases rate of sinus nodal discharge and rate of conduction + force of contraction - increases overall activity of the heart