Cell & Membrane Physiology

Question 1

What are the 4 characteristics of cardiac muscle?

Answer 1

1. automaticity (electrical activity)
2. excitability
3. conductivity
4. contractility

Question 2

What connections exist between cardiac myocytes?

Answer 2

gap junctions for electrical conductivity and desmosomes for adhesion.

Question 3

Where is the sinoatrial node (SA node) found?

Answer 3

posterior wall of the right atrium at the area of the crista terminalis

Question 4

What does the SA node do?

Answer 4

Area which generates electrical impulses and sends them through the gap junctions and desmosomes of atria where they meet the atrioventricular node (AV node).

Question 5

Where does the electrical impulse travel after the AV node?

Answer 5

Down the bundle of his to the purkinje fibers, which go around the ventricles causing them to contract.

Question 6

How often do the atria and ventricles contract?

Answer 6

about 80 times/minute (+/- 20)

Question 7

What occurs across these cardiac membranes as the heart contracts?

Answer 7

ionic fluxes

Question 8

Is cardiac muscle different from skeletal and smooth muscle?

Answer 8

YES.

Question 9

What is ischemia?

Answer 9

low blood flow

Question 10

What happens when cardiac muscle becomes ischemic?

Answer 10

it is not getting adequate blood flow, and therefore not enough O2.

Question 11

From where does the energy come for cardiac muscle?

Answer 11

mitochondria, which produce ATP

Question 12

What does the histology look like?

Answer 12

striated, muscle with intercalated discs and nuclei

Question 13

What is cardiac output (CO)?

Answer 13

Heart rate x stroke volume: (CO= HR x SV)

Question 14

What makes up a cardiac cell membrane?

Answer 14

A phospholipid bilayer with various membrane proteins. On the surface you will find peripheral proteins and carbohydrate moieties. Thus, the patient must have an intake of cholesterol, fats, lipids, and proteins to maintain these membranes.

Question 15

What is the purpose of the cell membrane and its channels?

Answer 15

To provide a barrier between the inside and outside of the cell. The channels may be uni-directional or bi-directional.

Question 16

What is an example of a situation where membrane channels can’t open leading to decreased glucose uptake?

Answer 16

Diabetes or pancreatic damage, decreasing insulin function or availability, and leading to increased glucose in the extracellular fluid (blood) = hyperglycemia!

Question 17

At physiologic pH, is the charge on the intracellular side of the cell membrane slightly positive or slightly negative, with respect to the extracellular side?

Answer 17

slightly negative, due to negatively charged proteins. These will also prevent K+ from leaving, increasing the concentration of K+ on the inside of the cell relative to the outside.

Question 18

What happens when membrane channels open?

Answer 18

Na+ rushes in due to its electrochemical gradient, changing the charge of the inside relative to the outside, and K+ will begin to flow out at faster rate than the Na+ is entering, to reestablish the electrochemical gradient.

Question 19

What is the ionic basis for how cardiac contraction occurs?

Answer 19

excitation/contraction = ionic events produce electrical events, which produce mechanical events.

Question 20

What do cardiac electrical events ultimately produce?

Answer 20

the cardiac action potential

Question 21

Does the cardiac action potential differ at the SA node, AV node, bundle of his, purkinje fibers, and ventricles?

Answer 21

YES.

Question 22

What modifies the protein membrane channels?

Answer 22

glycolipid and carbohydrate moieties.

Question 23

How can channels be affected?

Answer 23

chemically and electrically (voltage gated), affecting the permeability of some of the myocyte channels

Question 24

What is an example of altering voltage gated channels due to dysrhythmias?

Answer 24

using a defibrillator to shock a patient back into sinus rhythm.

Question 25

How long is a normal cardiac action potential (AP)?

Answer 25

300ms and looks like a fireman’s hat.

Question 26

What phase is the upstroke of the cardiac AP?

Answer 26

phase 0. This is depolarization as Na+ enters taking the membrane potential from -96 mV to +52 mV.

Question 27

What occurs at phase 1 of the cardiac AP?

Answer 27

potassium channels open slowly to allow K+ to efflux in an attempt to repolarize. Cl- also leaves too.

Question 28

What occurs at phase 2 of the cardiac AP?

Answer 28

Plateau (maintained depolarization). Na+ is trying to come in, K+ is trying to go out (closing of some K+ channels) and Ca++ is entering into the myocytes as a result of the potential change (voltage-gated slow Ca++ channels opening), which induces Ca++ release from the SR, and binds to troponin C, moving troponin/tropomyosin complex around the actin filament, exposing the myosin binding sites for myosin interaction. ATP will be cleaved allowing for the sarcomeres to get smaller = SYSTOLE.

Question 29

What occurs at phase 3 of the cardiac AP?

Answer 29

Na+ and Ca++ channels close and more K+ exits (more voltage-gated K+ channels open), repolarizing the cell.

Question 30

What occurs at phase 4 of the cardiac AP?

Answer 30

this is the resting membrane potential (-96 mV).

Question 31

What happens when the calcium enters the myocytes?

Answer 31

Ca++ induced Ca++ release for the calcium stores of the SR.

Question 32

When does the absolute refractory period occur?

Answer 32

during the plateau phase (phase 2).

Question 33

When does the relative refractory period occur?

Answer 33

during repolarization (phase 3).

Question 34

What maintains the electrical-chemical gradients?

Answer 34

the sodium potassium ATPase.

Question 35

What happens to the excess calcium as the cell repolarizes?

Answer 35

it is resequestered into the SR.

Question 36

What does the P wave of the EKG represent?

Answer 36

atrial depolarization

Question 37

What does the QRS wave of the EKG represent?

Answer 37

ventricular DEpolarization

Question 38

What does the T wave represent?

Answer 38

ventricular REpolarization

Question 39

Do cardiac APs produce EKGs?

Answer 39

YES

Question 40

What does the EKG show us collectively?

Answer 40

What is going on with all of the APs going on together in the heart. This is ELECTRICAL only!

Question 41

Do atrial and ventricular APs look a little different?

Answer 41

YES

Question 42

Will an EKG tell you in what direction the heart is depolarizing?

Answer 42

YES