Lecture 2 and 3 Unit 1

Question 1

what does cardiac muscle consist of?

Answer 1

branched, striated fibers with one or two centrally located nuclei

Question 2

what are actin and myosin packaged in and how are they arranged?

Answer 2

they are packaged in myofibrils and arranged in sarcomeres (just like skeletal muscle)

Question 3

what does the branching pattern in cardiac muscle form?

Answer 3

a network that can facilitate the transmission of electrical impulses in all directions

Question 4

what are gap junctions?

Answer 4

small channels which allow electrical impulses to pass quickly from one cell to the next

Question 5

where are gap junctions located?

Answer 5

in intercalated disks which lie between adjacent muscle fibers

Question 6

what are the two structures that intercalated disks have?

Answer 6

desmosomes and gap junctions

Question 7

what is the function of desmosomes?

Answer 7

they hold adjacent cells together

Question 8

what is the pacemaker in the heart?

Answer 8

the SA node

Question 9

what is the SA node?

Answer 9

a mass of cells in the right atrial wall

Question 10

what do pacemaker cells do?

Answer 10

spontaneously discharge action potentials at a rate of 100-120 per minute or more

Question 11

what do autonomic nerves do in the intact animal?

Answer 11

modify the rate of discharge so that the resting heart rate is ~70 beats per minute

Question 12

how must the atria and ventricles contract?

Answer 12

in a coordinated fashion

Question 13

what is the first event in cardiac muscle excitation?

Answer 13

depolarization of the SA node

Question 14

what is the second event in cardiac muscle excitation

Answer 14

the impulses travel down and across both atria causing atrial muscle fiber contraction

Question 15

what is located at the AV border?

Answer 15

poorly conducting tissue (small fibers, few gap junctions)

Question 16

what happens in the presence of poorly conducting tissue?

Answer 16

the impulse is slowed by about 0.1 seconds

Question 17

why is it beneficial for the impulse to be slowed by the poorly conducting tissue?

Answer 17

it gives the atria time to fully empty before the ventricles begin to contract

Question 18

what are right and left AV bundles connected by?

Answer 18

the short Bundle of His

Question 19

where does the bundle of his transmit action potential?

Answer 19

bundle branches, purkinje fibers, and the muscle of both ventricles

Question 20

What happens in the sequence of excitation? (what part of the heart contracts first)

Answer 20

the lower portions of the ventricles contract first, pushing the blood upwards

Question 21

what are the steps of action potentials of contractile fibers?

Answer 21

1. rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open
2. plateau due to Ca2+ inflow when voltage gated slow Ca2+ channels open and K+ outflow when some K+ channels open
3. depolarization due to closure of Ca2+ channels and K+ outflow when additional voltage gated K+ channels open

Question 22

what does refractory mean

Answer 22

unresponsive or stubborn

Question 23

what does refractory mean in physiology?

Answer 23

refers to a period of time when the muscle or nerve cell is unresponsive to stimulation

Question 24

what is the absolute refractory period?

Answer 24

it refers to the time when the cell will not respond regardless of the strength of the stimulus

Question 25

what is the relative refractory period?

Answer 25

refers to the time when the cell will respond only if the stimulus is “supra threshold”

Question 26

how long does the absolute refractory period last?

Answer 26

~250 ms in heart muscle, in skeletal and heart muscle the absolute refractory period lasts about as long as the action potential

Question 27

what are pacemaker cells called?

Answer 27

autorhythmic

Question 28

what do pacemaker cells do

Answer 28

they auto initiate action potentials

Question 29

what are pacemaker potentials?

Answer 29

unstable resting membrane potentials in pacemaker cells

Question 30

what do pacemaker cells use for the rising phase of the action potential?

Answer 30

calcium influx (rather than sodium)

Question 31

what does ECG stand for?

Answer 31

electrocardiogram

Question 32

what does an ECG do?

Answer 32

records hearts electrical currents

Question 33

what is an ECG?

Answer 33

a composite record of action potentials of all active cells at points in time during a heartbeat

Question 34

what is a P wave?

Answer 34

atrial depolarization

Question 35

what is a P to Q interval?

Answer 35

conduction time from atrial to ventricular excitation

Question 36

what makes up a QRS complex?

Answer 36

atrial repolarization and ventricular depolarization

Question 37

what is a T wave?

Answer 37

ventricular repolarization

Question 38

what are the steps of the cardiac events recorded by and ECG?

Answer 38

1. depolarization of atrial contractile fibers produces P wave
2. atrial systole (contraction)
3. depolarization of ventricular contractile fibers produces QRS complex
4. ventricular systole (contraction)
5. repolarization of ventricular contractile fibers produces T wave
6. ventricular diastole (relaxation)