3 Mechanical Properties of the Heart I Flashcards
Q: What are single ventricular cells? Shape? Can be stimulated to? Process? (3)
A: can dissociate myocytes into single ventricular cells
rod shaped
stimulated to contract (process)
- Electrical event
- Calcium Transient (the amount of calcium in the sarcoplasm has increased for a short period of time)
- Contractile event
Q: How long and wide is a typical ventricular cell? What’s on surface? Role?
A: -Length: 100 micrometres
-Width: 15 micrometres
T-tubules= finger like invaginations from the cell surface
carry surface depolarisation: allow excitation from surface to be conducted down into middle of cell
Q: What does the heart not contract without? skeletal muscle?
A: won’t beat without external calcium
skeletal muscle can contract without external calcium
Q: What size are T tubule openings? How far are they spread? reason?
A: their openings can be up to 200 nanometres in diameter
about 2 micrometres apart so that a T-tubule lies alongside each Z-line of every myofibril
Q: What is the structure of a ventricular cell? (4)
A: -sarcoplasmic reticulum: doesn’t take up large proportion of cell
- mitochondria: very energy demanding cell + takes up around 30% of cell space
- myofibrils: take up 50% of cell
- low calcium levels are maintained in normal conditions
Q: How does the sarcoplasmic reticulum relate to myofilaments?
A: terminal part of SR wraps around myofilaments
Q: Describe the process of Excitation-Contraction Coupling in the Heart. (9)
A: 1. cardiac AP-> depolarisation (membrane potential)
- sensed by the L-type calcium channel (LTCC)
- calcium from outside enters the cell
- Some of this calcium can directly cause contraction
- rest of calcium binds to Ryanodine Receptors (also called Sarcoplasmic Reticulum Calcium Release Channel)
- causes release of calcium from the sarcoplasmic reticulum
- more contraction
- after it has had its effect, some of the calcium is taken back up into the SR by Ca ATPase channels (also called SERCA - SARCO/ENDOPLASMIC RETICULUM CALCIUM ATPase) =uses ATP
- same amount of calcium that came into the cell is effluxed by a Sodium-Calcium Exchanger
Q: How much energy does the efflux of calcium out of the cell (after Excitation-Contraction Coupling) require? Calcium taken back up by SR?
A: does NOT need energy - it uses energy from the concentration gradient of sodium (high to low) to expel calcium form the cell
needs ATP as uses ATPase channels (also called SERCA - SARCO/ENDOPLASMIC RETICULUM CALCIUM ATPase)
Q: What is an important ion channel in a cardiomyocyte?
A: L-type calcium channel
Q: What is the relationship between force production and intracellular calcium concentration? Draw a cytoplasmic calcium concentration (x)- force (y) graph.
A: force-calcium relationship is SIGMOIDAL
Around a 10 micromolar intracellular concentration of calcium is sufficient to produce maximum force
force= y= % conc= x= micromolar (μM)= logorhythmic
Q: What is the Length-Tension Relation in Cardiac Muscle? (3)
A: This is ISOMETRIC CONTRACTION = muscle doesn’t shorten - just pulling on the force transducer (in experiment set up)
An increase in muscle length causes an increase in force
As you keep stretching the muscle, you get to a point where further stretching DOES NOT generate more force - this is because there is not enough overlap between the filaments to produce force
Q: How do you produce a graph to represent the Length-Tension Relation in Cardiac Muscle? Draw the resulting graph (muscle length X by force Y). Additional plotted line?
A: cardiac muscle cell/tissue is attached to force tranducer (measures) and some stimulating electrodes -> when stimulated produces a rise and fall in force
- non stretched
- stretched preparation = longer but everything else is the same (=produces a slightly larger force)
- stretch a little more (=produces even larger fore)
- > this relationship= active force production= due to the formation of cross bridges= steeper than x=y then peaks and dips
=plotted baseline= passive force line= stretch in preparation caused by elasticity= less that x=y and remains straight
Q: What is the length-tension relation in cardiac and skeletal muscle? (show on graph)
A: REFER. similar curve but cardiac has steeper passive line which makes total force higher
Skeletal muscle has much less passive force produced but there is still a bell-shaped curve
Q: How does cardiac and skeletal muscle differ? (2) Why?
A: -cardiac muscle is less compliant than skeletal muscle
-cardiac muscle is much more resistant to stretch= CM exerts more passive force
=> due to properties of the extracellular matrix and cytoskeleton
Q: What happens if you overstretch muscle? skeletal muscle? Cardiac muscle? in relation to graph?
A: get a decrease in force - this is what happens in skeletal muscle when you pull a muscle
can’t overstretch cardiac tissue because it’s contained within the pericardium -> only ascending limb of the relation is important for cardiac muscle as all cardiac function takes place on that part of the force-length curve
Q: What is passive forced based on?
A: resistance to stretch of the muscle
Q: Why does the descending limb of the length-tension graph not occur in physiological conditions?
A: pericardium restricts the stretching
Q: What are the 2 forms of contraction that the heart uses?
A: ISOMETRIC contraction resists the high pressure - there is NO CHANGE IN LENGTH but there is a change in tone
ISOTONIC contraction is the shortening of fibres (no change in tension) when blood is ejected from the ventricles
