Cardiac Muscle

Question 1

Describe the structure of cardiac muscle

Answer 1

Between adjacent z disks- all proteins required for muscle contraction are within the sarcomere

Regular lattice structure to striated cardiac muscle

The T tubule is when the myofiliaments are bound by cell membranes-dives down into the depths of every z-line
These are necessary in larger cells in order to bring the AP to the centre of the cell. THIS ALLOWS FOR SYNCHRONOUS COORDINATED CONTRACTION .

Either side of the T-tubule lie Terminal Cisternae of the sarcoplasmic reticulum which are tanks of Calcium.

Cardiac Muscles- Dyads ( SR and T tubule appear as double structures)
Skeletal- Triads

Large relative mitochondria

Question 2

Comment on the excitability of the cardiac muscle

Answer 2

(see graph)

Systole begins when the membrane is depolarised allowing the Ca++ to enter.
The Ca++ leaves the cell as it builds up- needs to be removed before the next action potential arises.

At resting potential- there is a high concentration of K+ - the high permeability to K+ leaves behind negative Cl- as K+ leaves down its concentration gradient. Cell therefore become more negative. However a point will be reached where the force pulling K+ back in equals the force of the concentration gradient.

Ions cannot pass through the bilayer so ions will move through
Ion Channels (can be specific to stimulus- open and close due to voltage) 
Ion Pumps ( driven across using metabolic energy) 
Ion exchangers/symports

Question 3

Comment on the cardiac action potentials

Answer 3

As the HR increases, the action potential duration decreases
The refectory period- is the period where no other action potential can be triggers if another stimulus comes along
The relative refectory period means that if a stimulus is big enough and AP can be triggered

The AP is long- this means that tetany is prevented and also prevents extra beats being triggered hence protecting against re entrant arrhymias

Question 4

Contractions-

Answer 4

Cardiac Muscle requires Ca influx to INITIATE MUSCLE CONTRACTION- as when Ca is removed contraction of the myocyte is completely blocked within milliseconds.

AP is what triggers the Ca transient and therefore contraction ( see graph - delay between the contraction and peak of intracellular Ca)

Question 5

Where does the Calcium come from ?

Answer 5

Comes from the extracellular Ca moving into the cell

Comes form the release of Ca from the intracellular stores in the SR

Question 6

Describe how excitation brings about Ca release from intracellular stores in cardiac muscle

Answer 6

1. AP arrives at the the L type channel on the T tubules
2. Ca++ ions move through the L-type Ca channel to bind the RyR channel, having diffused across the dyadic cleft to the SR ( NB the RyR has a high affinity to Ca)
3. This allows Ca++ to move out of the SR into the cytoplasm- therefore binding to the troponin

THEREFORE Cardiac muscle has calcium induced calcium release

4. Sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA associated with PLB) takes Calcium back up against the concentration gradient
5. The Na/Ca exchanger removes Ca from the cell

Question 7

Describe how excitation brings about Ca release from intracellular stores in skeletal muscle

Answer 7

when AP arrives at the L type channel- it changes shape pulling a plug on the release channel letting the Ca++ out

Question 8

Mechanism by which Ca activates contraction

Answer 8

Ca binds to the troponin C pulling tropomyosin out the way.

The more Ca++ is present- the more cross bridges can be formed which increases the tension.

Question 9

Describe the length tension relationship

Answer 9

Increasing the sacromere length will increase the Ca sensitivity which will increase the force.