Lecture #2 Heart Conduction System

Question 1

What is the histology of Cardiac muscle Tissue?

Answer 1

Cardiac muscle fibers are shorter in length–exhibit branching, striated fibers with one or two centrally located nuclei.

Question 2

Within the cardiac strip pattern, what type of function is given?

Answer 2

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

Question 3

Cardiac muscle is a _____ of many heart muscle cells.

Answer 3

Syncytium

Question 4

Which traits do the cardiac muscle and skeletal muscle share?

Answer 4

Mitochondria are larger and more numerous in cardiac muscle

Have same arrangement of actin and myosin, and the same bands, zones, and Z discs as skeletal muscle fibers.

Question 5

What are gap junctions?

Answer 5

Small channels, that allow electrical impulses to pass quickly from one cell to the next and to spread rapidly throughout heart muscle

Question 6

Gap Junctions are located in ________ which lie between adjacent muscle cells.

Answer 6

Intercalated discs

Question 7

What else does the intercalated discs contain aside from gap junctions?

Answer 7

Desmosomes: which hold the fibers together

Question 8

What components are part of the conduction system?

Answer 8

1. SA Node (pacemaker)–setting the rhythm of electrical excitation causing contraction of the heart
2. AV node
3. AV bundle (Bundle of His)
4. Right and Left bundle branches
5. Conducting Myofibrils (Purkinje fibers)

Question 9

At what rate does the pacemaker cells discharge action potential?

Answer 9

About every 0.6 second, or 100-120+ per minute.

Question 10

What about the rate of pacemaker cells at resting state?

Answer 10

Autonomic nerves slow SA node pacing to about every 0.8 second or 75 action potential per minute

Question 11

Action Potential initiated by the SA node travels along the conduction system and spreads out to excite the “atrial” and ventricular fibers is called_____.

Answer 11

Contractile Fibers

Question 12

What is the first step in the sequence of cardiac muscle excitation?

Answer 12

Depolarization of the SA Node–when contractile fiber is brought to threshold by an action potential neighboring fibers, voltage-gated fast Na+ channels open

Question 13

Does the Na+ channels depolarize or repolarize?

Answer 13

Sodium ion channels are referred to as “fast” because they open very rapidly in response to a threshold-level depolarization.

Question 14

What is the second step in the cardiac muscle excitation?

Answer 14

Plateau, a period of maintained depolarization–due to opening of voltage-gated slow Ca2+ channels in the sarcolemma

Question 15

What is the purpose of the sequence of excitation?

Answer 15

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

Question 16

What happens when the Ca2+ concentration is increased?

Answer 16

The increased Ca2+ concentration in the cytosol ultimately triggers contraction.

Question 17

What also assists Ca2+ contraction?

Answer 17

Voltage-gated K+ channels are also found in the sarcolemma of a contractile fiber–Ca2+ balances K+ outflow.

Question 18

What is the third step of cardiac muscle excitation?

Answer 18

Repolarization phase of cardiac action potential resembles, recovery of the membrane potential.

Question 19

Which Ion channels are open in repolarization stage?

Answer 19

K+ channels open, outflow of potassium restores the negative resting membrane potential -90 mV but at the same time the calcium channels in the sarcolemma and sarcoplasmic reticulum are closing.

Question 20

What is the refractory period?

Answer 20

Unresponsive or stubborn which refers to the period of time when the muscle or nerve cell is unresponsive to stimulation

Question 21

What are the names of the two refractory period?

Answer 21

Absolute refractory period: refers to the time when the cell will not respond regardless of the strength of stimulus. The absolute refractory period in skeletal and heart muscle lasts as long as action potential (250 ms in heart muscle)

Relative refractory: refers to the time when the cell will respond only if the stimulus is “suprathreshold”