Cardiac Muscle Contraction

Question 1

Cardiovascular System

Answer 1

Heart

Blood Vessels

Question 2

Cardiovascular System function

Answer 2

Deliver blood to all organs and tissues of the body
Provide nutrients & oxygen to the tissues
Remove wastes
Deliver hormones
Deliver immune system cells to sites of infection

Question 3

Heart

Answer 3

Pump blood throughout the cardiovascular system – pulmonary & systemic circulation

Question 4

Blood vessels

Answer 4

are highly specialized tubes

Question 5

Arteries

Answer 5

• Handle high blood pressures & pulse pressure (= SP-DP) (physical exertion)
• Regulate mean arterial pressure (70-110 mm Hg) (Resistance vessels)
• Regulate tissue perfusion

Question 6

Capillaries

Answer 6

• Transfer of O2 and nutrient molecules to the tissues from blood
• Transfer of wastes from the tissues to the blood
• Transfer of hormones to the tissues from blood
• Transfer of immune system cells to the tissues from blood

Question 7

Veins

Answer 7

• Require low pressures (in order for blood to flow through capillaries)
• May have valves to insure one-way flow to the heart

Question 8

Cardiac Muscle

Answer 8

Only in the heart
Cells = cardiac myocytes
Striated, like skeletal, but involuntary
○autonomic NS control
Cardiac myocytes are short, irregular in shape, and sometimes branched with a central nucleus
~10 mm in diameter
Ultrastructure is very similar to skeletal muscle (i.e., sarcomere, myofibrils, t-tubules, SR)
Lots of capillaries
Lots of mitochondria

Question 9

The cardiac myocytes are connected in series via

Answer 9

intercalated discs

Question 10

Intercalated disc

Answer 10

combination of gap junctions and desmosomes

Allows for electrical coupling (“functional syncitium”), passing of AP from cell to cell

Question 11

Conducting System of Heart

Answer 11

Heart contains a specialized conduction system for:
1)Setting the rhythm of the heart beat – autorhythmicity
2)Transferring the electrical signal to various parts of the heart musculature
Note: These are all cardiac muscle cells NOT nerve cells

1) S-A node fires AP first
2) AP spreads throughout the Atria (Atria contract – blood flows from atria to ventricles)
3) AP conduction is delayed at the A-V node
4) AP is carried to the apex of the ventricles via conducting fibers (Bundle of His)
5) AP is carried to the ventricular muscle via Purkinje fibers
6) AP spreads throughout the ventricles (ventricles contract – blood flows out of ventricles to systemic and pulmonary circuits)

Question 12

S-A Node

Answer 12

Cells in the SA node set the rhythm of the heart beat, since they fire (AP) first and all other cells will follow in succession.

Autorhythmic – fire an AP spontaneously
Pacemaker cells

NOTE other cells in the heart are also auto- rhythmic, but the activity of the S-A node normally sets their pace and the pace of the heart rate.

Question 13

Pacemaker cells of S-A Node

Answer 13

Have a high resting potential (Vm) = ~ -65 mV

Other cardiac myocytes are at ~ -90 mV

Question 14

Have a high resting potential (Vm) = ~ -65 mV

Other cardiac myocytes are at ~ -90 mV

Why?

Answer 14

Leaky to Na+, and have a lower K+ permeabilty, thus Vm is closer to ENa+

Also have very few voltage-gated Na+ channels

Question 15

How do cardiac myocytes fire an AP?

Answer 15

1)The pacemaker cells “spontaneously” depolarize (Pacemaker Potential)
•Due to Na+ leaking in (called If for “funny” current) through “pacemaker channels”.
•Some Ca2+ leaks in too.
2)Once threshold is reached → fire AP
3)AP is due to Ca2+ entering through voltage gated Ca2+ channels (L- & T-type)
4)Repolarization is due to opening of voltage-gated K+ channels
The L & T-type Ca2+ channels become inactivated leading to a refractory period

Question 16

Why Atrial & Ventricular myocytes have a low resting potential?

Answer 16

Highest permeability at rest is to K+, thus Vm is close to EK+

Also have many voltage-gated Na+ channels
And voltage gated Ca2+ channels

Question 17

Cardiac Myocyte contraction

Answer 17

Similar to skeletal muscle
Thick and Thin Filaments, sarcomeres,….
The AP
a)Causes an influx of Ca2+
b)Causes release of Ca2+ from the SR

Two sources of Ca2+ for cardiac muscle
○Extracellular
○Sarcoplasmic reticulum

Question 18

How can heart rate be changed?

Answer 18

Modify the Pacemaker Potential

a) Epinephrine & Autonomic Sympathetic stimulation speeds up the pacemaker potential. Increases If. Opens pacemaker channels. Due to cAMP. (Top Panel)
b) Autonomic Parasympathetic stimulation slows down the pacemaker potential. Decreases If and increases IK+ by opening K+ channels. (Bottom Panel)