Physiology - The Heart

Question 1

Succession of beats is caused by…

Answer 1

…cardiac rhythmicity - transmitting action potentials throughout the cardiac muscle to cause the heart to beat

Question 2

Cardiac Muscle Fibers

Answer 2

3 types - atrial muscle, ventricular muscle, (both contract like skeletal muscles) and conductive muscle (contraction not as strong but rhythmic)

Interconnecting - fibers arranged in a lattice network

Striated with myofibrils (actin and myosin) = similar to skeletal muscle

Cells are individual but electrically connected by intercalated disks = gap junctions (rapid diffusion of ions

Question 3

Due to the electrically connected cells of the cardiac muscle, it is known to be…

Answer 3

…a syncytium = an action potential spreads quickly from one cell to the rest of the network (latticework)

Question 4

2 Syncytiums of the Heart

Answer 4

Atrial Syncytium = walls of the atria

Ventricular Syncytium = walls of the ventricles

Signal from the atria don’t spread to the ventricles = kept separate
This allows for the atria to beat slightly before the ventricles (specialized conductive system is used instead)

Question 5

Cardiac Muscle Action Potentials are caused by the opening of…

Answer 5

…2 types of channels:
Voltage-gated sodium channels (similar to those in skeletal muscle) = fast opening
Voltage-gated L type calcium channels (calcium-sodium channels) - slow opening but remains open for longer = Plateau Phase (0.2-0.3 seconds) = both calcium and sodium rushes in

Question 6

What causes the plateau phase in cardiac muscle?

Answer 6

Opening of voltage-gated L type calcium channels that are slow to open and slow to close = longer influx of sodium and calcium

Decrease in potassium permeability once these channels is opened = membrane cannot be repolarized = longer action potential
Permeability increases rapidly when calcium channels close

Question 7

Ventricular Muscle Action Potential

Answer 7

RMP = -85 to -95 mV

1) Depolarization = Voltage-gated sodium channels open and cause an influx of sodium (some calcium enters as well)

2) Plateau Phase = Voltage-gated L type calcium channels open = large influx of calcium along with sodium flows in
Membrane becomes less tolerable to potassium

3) Repolarization = Channels close and voltage-gated potassium channels open

Question 8

Excitation of Cardiac Muscle Cells is Controlled by…

Answer 8

…Calcium

Question 9

Calcium in Muscle Cell Excitation

Answer 9

2 Sources:
Extracellular - Voltage-Gated L Type Calcium Channels
Intracellular - Calcium release channels (Ryanodine) on Sarcoplasmic reticulum (induced by the presence of extracellular calcium)

Question 10

Relaxation of Ventricular Muscle

Answer 10

Caused by a decrease in cytoplasmic calcium = caused by:

1) L-type calcium channels and ryanodine receptors closes
2) SERCA = Sarcoplasmic Reticulum calcium ATPase pumps calcium back into the sarcoplasmic reticulum
3) Sodium calcium exchanger on cell membrane uses secondary (counter) transport to exchange sodium for calcium

Question 11

What are the durations of muscle contractions within the heart

Answer 11

Atrial = 0.2 sec

Ventricular = 0.3 sec

Question 12

Why does the action potential need to plateau in the heart?

Answer 12

B/c the heart needs long sustained contractions, not twitches

Question 13

Refractory Period in Cardiac Muscle

Answer 13

Sodium channels are inactivated

Cell may or may not be activated again at this time = when the cells is closer to RMP = more normal contraction