#06: Heart I Continued/Heart II

Question 1

Bifurcations and Anastomoses

Answer 1

• Cardiac muscle has splitting, or bifurcations, where part of muscle may branch out and join another muscle. The entire thing is called anastomoses, which may look like a spider web of muscles. Provides many ways to get to the same point.

Question 2

Intercalated Discs

Answer 2

• Intercalated discs are present at junctions of cardiac muscle cells. Specialized type of cell to cell contact.
○ One part is desmosomes. Kinda like buttons. Hold cells together, especially needed during pressure. Each cell expresses cells called kinexins on the surface which make up the desmosome.
○ Other part is gap junctions, an electrical linkage between cells. Allows for movement of ions and signaling molecules between cells. Not only at ends of cells, but between cells at middle also.
§ Has a syncitia process going on, that makes sure all the gap junctions are acting in unison.

Question 3

Skeleltal Muscle Motor Unit

Answer 3

○ Motor unit is composed of alpha motor neuron and innervated muscle fibers.

Question 4

Skeletal Muscle Contraction

Answer 4

○ Steps of Contraction

1. Somatic motor neuron releases ACh at neuromuscular junction.
2. Net entry of Na+ through ACh receptor-channel initiates a muscle action potential.
3. Action potential in t-tubule alters conformation of DHP receptor.
4. DHP receptor opens RyR Ca2+ channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm.
5. Ca+2 binds to troponin, allowing actin-myosin binding.
6. Myosin heads execute power stroke.
7. Actin filament slides toward center of sarcomere.

Question 5

Cardiac Muscle Conscious Control

Answer 5

• Cardiac muscle is not consciously controlled and contractions are autorhythmic (self-generated).

Question 6

Cardiac Sarcomeres

Answer 6

• Like skeletal muscle, cardiac muscle has myosin and action filaments arranged in sarcomeres. Cardiac muscle generates tension by crossbridge formation and sarcomere shortening in the same manner as skeletal muscle.

Question 7

Cardiac Muscle Sarcocplasmic Reticulum

Answer 7

• Cardiac muscle has a smooth sarcoplasmic reticulum but not arranged in the same manner or as numerous as in skeletal muscle. The SR doesn’t contain dilated cisternae present is skeletal muscle. The SR comes into close association with the transverse tubules at several points. The transverse tubules are less abundant in cardiac muscle and located near Z disks.

Question 8

Arrangement of Transverse Tubules and Sarcoplasmic Reticulum in Cardiac Muscle

Answer 8

• Arrangement of transverse tubules and sarcoplasmic reticulum in cardiac muscle has functional consequences. The slower calcium delivery to cardiac muscle means that the muscle contracts slower than skeletal. But, the contraction is prolonged, because the inverse is also true and calcium is removed more slowly. Depolarization of the cardiac cell membrane is not carried out as efficiently into the myofibrils as in skeletal muscle, and calcium must diffuse a greater distance. To further contrast, a substantial amount of calcium enters cardiac muscle cells from extracellular fluid. These differences contribute to controlled beating of heart.

Question 9

Cardiac Muscle Action Potential: Ions

Answer 9

• Cardiac Action Potential is slightly different from that of skeletal muscle. Depolarization of the cardiac muscle cell membrane leads to an action potential, but the ions responsible for the cardiac action potential are different from those in neurons or skeletal muscle cells.

Question 10

Cardiac Muscle AP: T Tubule and SR

Answer 10

○ In cardiac muscle cells, there is no physical link between the T tubule and the Sarcoplasmic Reticulum. But they’re still close. The DHP channel releases a small amount of calcium into the cytoplasm which makes a big enough change in membrane potential to open receptor on sarcoplasmic reticulum to release a whole lot more calcium into cytoplasm. This is called Calcium Induced Calcium Release.

Question 11

How Can ANS System Increase Cardiac Muscle AP Strength?

Answer 11

• One way ANS system increases strength of contraction in cardiac muscle cell is by phosphorylating the DHP channel, keep it open longer to put more calcium in the cytoplasm to make even more calcium leave the SR.

Question 12

Length Tension Relationship in Muscle

Answer 12

• The longer a rubber band is, the more pressure you can put on it by pulling on it. Speaking similarly of cardiac muscle, if you put more blood into the chambers, the muscle cells will become more dilated, and thus you can more forcibly contract muscle because there’s essentially more to work with.

Question 13

Circa Pump

Answer 13

• Circa is the primary mechanism for getting calcium out of the cytosol after it’s been implemented. It is a ATP-dependent pump to pump Calcium out of cytosol.

Question 14

Na+/Ca++ Exchanger and Na+/K+-ATPase Pump

Answer 14

However, in cardiac muscle, two other pumps are also present to pump out Calcium of cytosol, Na+/Ca++ exchanger, and Na+/K+-ATPase pump. This is due to heightened importance of calcium.
○ Na+/Ca++ exchanger, when we have calcium coming out of SR, we need to be rid of it. Circa is only going to pump out the calcium that came from the sarcoplasmic reticulum, NOT the calcium that came out of the DHP channel. This channel in particular pumps sodium in and then pumps sodium out.
○ Then you have the Na+/K+-ATPase pump, which will pump the influx of Na+ OUT of the cell, while pumping K+ into the cell. So the more the Na+/Ca++ pump works, the more this pump works, but the ultimate goal and ridding of calcium is accomplished.

Question 15

SA Node

Answer 15

• Sinoatrial Node, or pacemaker. The cells that make up the SA node are autorhythmic, no need for nerve to initiate cause these conductive cells spontaneously generate an action potential, over and over again. SA node in particular does it at a greater frequency than other cells which is why it’s called the pacemaker.

Question 16

Conductive Cells: Depolarization

Answer 16

• Since conductive cells are autorhythmic, they produce action potentials a little differently. At a very negative membrane potential, the HCN channel opens and passes positive current (Na+) to make cell more depolarized. Only channel that does this. Found inside conductive cells.

Question 17

Conductive Cells: After Opening HCN Channels

Answer 17

• So after opening of HCN channel, once potential reaches -50, calcium channels open. Two types, first one to open is T-type Calcium channel. Patches small amount of calcium into cell and then closes. Once threshold is reached, L-type Calcium channel opens (long-lasting). Unlike skeletal and neurons, this rising phase is due to calcium, not sodium.
○ When it’s time for potential to fall, potassium channels open and this part is similar to that of skeletal muscle and neurons.

Question 18

You Only Need To Initiate Depolarization in SA Node To Depolarize Enter Heart

Answer 18

• The depolarization of the SA node cells is sufficient to induce the depolarization of the entire heart. This is because the electrical gap junctions between cells and the specialized electrical conduction pathway within the heart ensures that the depolarization spreads.