Heart Muscle

Question 1

What are the characteristics of cardiac muscle?j

Answer 1

Branching cells
1-2 nuclei/cell
Striated, interconected with intercalated discs
Ca++ source= SR and ECF
Tropomyosin+ Troponin
Involuntary
Medium speed contractions
Abundant mitochondria
No motor United
Extensive system of T-tubules

Question 2

Where does the Ca++ in cardiac muscle come from?

Answer 2

Terminal cisternae contains less ca++, mainly by influx from extracellular space

Question 3

What is intercalated discs?

Answer 3

Membrane junctions that hold adjacent cells together and transmitts the contraction force to each cell

Question 4

Why is gap junctions important?

Answer 4

Allow interchange and communication between the sarcoplasm of connected cardiac muscle cells

Question 5

What are the three major types of cardiac muscle?

Answer 5

Atrial muscle
Ventricular muscle
Specialized excitatory and conductive muscle fibers

Question 6

What are the two types of cells within the heart?

Answer 6

Autorythmic cells (pacemaker)
Contractile cells

Question 7

What is the features of autorythmic cells?

Answer 7

1% of cardiac cells
Acts as a pacemaker (sets the rhythm of electrical excitation)
Form the conduction system of the heart
SAN-AVN-Bundle of His-R&L bundle branches-Purkinje fibers
AUTORHYHMICITY-EXCITABILITY-CONDUCTIVITY-REFRACTORITY

Question 8

What are the features of contractile cells?

Answer 8

99% of cardiac cells
Makes the muscular walls of the atrium and ventricles
CONTRACTILITY-EXTENSIBILITY

Question 9

What does Autorhythmicity mean?

Answer 9

The ability to spontaneously generate and discharge an electrical impulse without external stimulation

Question 10

What does exitability mean?

Answer 10

The ability to respond to an electrical impulse

Question 11

What does conductivity mean?

Answer 11

The ability to transmit an electrical impulse from one cell to the next one

Question 12

What does refractority mean?

Answer 12

The inability of cardiac cell to initiate another AP for some duration of time

Question 13

What does contractility mean?

Answer 13

The ability of the cell to contract in response to stimulation

Question 14

What does extensibility mean?

Answer 14

The ability of the cell to stretch

Question 15

What are the unions of intercalated discs?

Answer 15

Gap junctions (allow free diffusion of ions and AP between adjacent fibers-electrical union)
Desmosomes (mechanical union)
Fascia adherence (mechanical union)

Question 16

What is the meaning of functional syncytium?

Answer 16

That the heart are an unique functional unit

Question 17

What are the parts of the functional syncytium?

Answer 17

Atrial syncytium (the walls of the two atria)
Ventricular syncytium (the walls of the two ventricles)

Question 18

What is the function of having two syncytiums?

Answer 18

The all or None response
Either it contracts or not
Each syncytium contracts individually, never at the same time
No gap junctions between atria and ventricles!!

Question 19

What is the sequence of the conducting system?

Answer 19

Sinoatrial Node SN
Atrioventricular Node AN
Bundle of His
R&L bundle branches
Purkinje fibers

Question 20

What are the “rhytms” of pacemaker cells?

Answer 20

SA Node- 60-100/min
AV Node and AV bundle- 40-60/min
Bundle branches and Purkinje fibers- 20-40/min

Question 21

What pacemaker cells acts as the heart pacemaker and why?

Answer 21

SA Node due to having the fastest rate of generating AP

Question 22

What happens if SA Node fails?

Answer 22

The other pacemaker cells can take over but at a slower rate

Question 23

Describe the contraction cykle of pacemaker cells

Answer 23

Does not maintain constant RMP
RMP starts at -60mV and gradually depolarizing until -40mV (TP)
Spontaneous depolarization due to slow continous influx of Na+
Depolarization starts with fast Ca++ voltage gated channels open, peaks at -10mV
Repolarization when Ca++ voltage gated channels close and K+ voltage gated channels open and K+ ions rapidly leaves the cell
When MP reaches -60mV a new cykle starts

Question 24

What are the RMP of cardiac cells?

Answer 24

Atrial muscle cells and His/Purkinje system= -80/-90mV

SA and VA nodes= -65/-50mV

Question 25

What are the phases of AP in contractile fibers?

Answer 25

Phase 0- rapid depolarization, opening of fast Na+ vgc leading to AP
Phase 1- start of repolarization, rapid closure of Na+ vgc, exit of K+ through cardiac transient outward potassium current channels (CKto) with membrane permiability for K+ five times lower
Phase 2(plateau)- opening of slow Ca++ vgc
Phase 3- repolarization closing of slow Ca++ vgc and exit of K+ via K+ vgc thus the permiability for K+ rapidly increasing
Phase 4- restoring of RMP

Question 26

Why is repolarization in cardiac cells slower and AP 100 times longer?

Answer 26

Due to slow Ca++ channels and membrane permiability for K+ decreased by five times

Question 27

Describe the spread of cardiac excitation

Answer 27

Impulse originates at SA Node
AP spreads throughout left and right atria
Impulses passes from atria into ventricles through AV Node
AP is delayed by AV Node and then rapidly travels interventricular septum by bundle of His
Impulse rapidly disperses throughout myocardium by Purkinje fibers
Rest of Ventricular cells activated by spread of impulse via gap-junctions

Question 28

Describe the effect of the refractory period of cardiac muscle

Answer 28

It has a long redractory period compared to skeletal muscle
The refractory period lasts longer than muscle contraction to prevent tetanus
Heart gets time to relax after each contraction= prevents fatigue
Allows time for the Chambers to fill during diastole before next contraction