Myocardinal Flashcards
What is excitation contraction coupling
What is the myocyte plasma membrane like
The myocyte is excited and then a few milliseconds later there is contraction.
T tubules and intercalated discs rapidly transmit action potentials in the myocardium and makes the coupling faster.
Sarcolemma has thousands of invaginations and inwards folding forming T tubules.
This allows the action potential to stimulate all parts simultaneously and cause a faster rate of contraction.
What is the sarcoplasmic reticulum
Fluid filled membranous sac surrounding each myofibril.
It is a Ca store.
It forms a triad of a T tubule and two cisterns
What happens in a relaxed muscle
What are Ca levels like
Titin
Calsequestrin
Tropomyosin
Amount of Ca in compartments
Ca levels are low due to the Ca pumps that mop up Ca from the sarcoplasm and store it in the sarcoplasmic reticulum.
Titin tethers myosin and actin in the sarcomere and stop the muscle falling apart when tension is applied.
Calsequestrin binds to Ca in the sarcoplasmic reticulum and makes the pumps think the Ca conc is lower than it is. The pumps will now work harder to bring Ca into the sarcoplasmic reticulum.
Tropomyosin hides the myosin head binding site and prevents contraction.
Ca in sarcoplasm is 0.1um
Ca in SR is 10nM
What happens to cause contraction
Action potential travels down the T tubules and activates the voltage gated Ca channels in the sarcoplasmic reticulum.
Ca enters the sarcoplasm and binds to troponin to expose myosin head binding site allowing contraction
ATP is required for cross bridge cycling and muscle shortening.
PICTURE
Optimum length for sarcomeres
The more cross bridges that form the more muscle tension can be generated.
The number of cross bridges formable is dependant on the sarcomere length.
This means there must be an optimum length for maximum tension generation.
Short sarcomeres have overlapping thin filaments so less tension generation.
Long sarcomeres have reduced areas for cross bridge formation so less tension generation.
Length tension relationship measurements.
A papillary muscles can be stimulated using an electrode causing it to twitch.
A thread connects the muscle to a force transducer which measures the force of the twitch.
What happens when the muscle is short
What is the optimum sarcomere length that gives the most force
Why can’t it be longer
All sarcomeres are bunched up and can’t bind.
2.2-2.6um.
There will be a decreased force produced and they will be too far apart to bind.
picture
What stops muscles going beyond their optimum lengths
Skeletal muscle is constrained by bones and tendons
Heart muscle is not connecting to anything constraining so it can go beyond the optimum length.
This can cause weak contraction and blood left in the heart which will stretch it even more and cause heart failure.
What is the passive force in the heart
Elastic forces help contraction
What occurs in the sarcomere at 2.2um
When is this length occurring in the heart
Maximum tension and the maximum number of cross bridges formed.
When there is 10-12mmHg filling pressure.
This is the pre systolic pressure. Blood is filling left ventricle.
This is the optimal pressure in the heart required to produce the optimal length tension relationship in the cardiac myocyte.
How do skeletal muscles increase their strength compared to cardiac muscles.
Skeletal muscles recruit more muscle fibres. This takes time so contraction is slower.
Cardiac muscles don’t. They increase the force that each myocyte is producing.
This is very fast. One heart beat can be weak and the next can be changed to strong.
What is isotonic contraction
The tension of the muscle remains constant throughout the whole movement.
The muscle will shorten
What is the pre load and the after load
Blood entering the heart from the vena cava is the pre load and it stretches the heart muscle.
The heart will then reach the EDV (end diastolic volume) and the sarcomeres will be stretched to the optimum length 2.2um.
This will cause contraction.
The after load is the force the ventricles have to act against to eject blood.
The heavier the load in an insotonic contraction…
The slower the contraction.
What is the link between force and velocity
What happens if you increase the pre load
The harder you squeeze (force)
The faster the muscle squeezes (velocity)
This gives a more maximal force of contraction.
It causes an increase in velocity.
