Physiology 3120 Lecture Notes - Sliding Filament Theory, Intercalated Disc, Sinoatrial Node

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indigoweasel955

26 Nov 2011

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Function of myocardial cells: looking at microscopic level (cellular, molecular, etc. , heart beat = most important life-sustaining event; heart is first organ formed during embryogenesis, three types of muscle fibres, atrial*, ventricular*, excitatory & conductive. In skeletal muscle, muscle fibres function independently of each other; cardiac muscle fibres are interconnected & form a system (i. e. syncytium) Where two fibres meet, membrane folds in region called intercalated disc. Two functional syncytium in heart: atrial syncytium, contract before the ventricles because of the impulse generated in the sa node (must conduct down into ventricles, ventricular syncytium. **have contractile properties like skeletal muscle (i. e. sliding filament theory; however, there are major differences [i. e. troponin])** Impulse generated at sinus node (located just under vena cava in right atrium) Impulse generation in this area occurs faster than elsewhere in the heart. Special properties (all of which work to depolarize the sa node cells) K permeability declines during diastole (more positive charges inside cell)

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Membrane Capacitance and Conductance. For cells having simplemorphological shapes, the
intracellular voltage is nearly uniform throughout the cell suchthat any change in the membrane
potential acts equally on all parts of the membrane. If wehyperpolarize the cell or chemically block
excitable ionic conductances, the membrane potential vm ispassively related to the incoming current i
by:
Cm
dvm
dt
+Gm (vm - El ) = i (1)
where Cm is the whole-cell membrane capacitance, Gm is thewhole-cell membrane conductance, and
El is the leak potential. Most biological membranes have acapacitance of about 1 ÂµF per square
centimeter of membrane area, which allows one estimate cell size bymeasuring capacitance.
a) One way to measure the capacitance is to inject a sinusoidalcurrent i = Ie j?t and to measure the
sinusoidal perturbation in the membrane potential vm = Ve j?t . Inthe frequency domain, both the
current I and the voltage V are complex-valued numbers. Derive aformula for the cell capacitance in
terms of I and V.
b) Another way to measure cell capacitance and conductance is touse a step current injection. In this
case, the current is suddenly increased to a constant i = Io andthe voltage responds with an exponential
form. Measurement of the voltage for a known current impulseprovides a means to estimate the
capacitance and conductance. Toward this aim, find the solution ofEq. 1 for a step increase in
current. Once you have this solution, how would you use the voltageand current measurements to
estimate the passive capacitance and conductance?

A heart dysfunction that can cause death is ventricular fibrillation. This is an uncoordinated quivering of the heart as opposed to regular beating. An electric shock to the chest can cause momentary paralysis of the heart muscle, after which the heart will sometimes start organized beating again. A defibrillator is a device that applies a strong electric shock to the chest over a time of a few milliseconds. The device contains a capacitor of several microfarads, charged to several thousand volts. Electrodes called paddles, about 8 cm across and coated with conducting paste, are held against the chest on both sides of the heart, their handles are insulated to prevent injury to the operator, who calls clear and pushes a button on one paddle to discharge the capacitor through the patient's chest.
Assume that the capacitor in the defibrillator is 10 uF and is charged to 5,000 volts.
a. how much charge is stored in the capacitor before it is discharged?
b. how much energy is released when the capacitor is discharged?
c. if the capacitor completely discharges in 2.5 ms, what is the average current delivered by the defibrillator?
d. what is the average power delivered?

aquamarinedog663

In research in cardiology and exercise physiology, it is often important to know the mass of blood pumped by a person's brain in one stroke. This information can be obtained by means of a ballistocardiograph. The instrument works as follows: The subject lies on a horizontal pallet floating on a film of air. Friction on a pallet is negligible. Initially, the momentum of the system is zero. When the heart beats, it expels a mass m of blood into the aorta with speed v, and the body and platform move in the opposite direction with speed V. The speed of the blood can be determined independently (e.g., by observing an ultrasound Doppler shift). Assume that the blood's speed is 50.0 cm/s in one typical trial. The mass of the subject plus the pallet is 54.0 kg. The pallet moves at a speed of in 0.160 s after one heartbeat. Calculate the mass of the blood that leaves the heart. Assume that the mass of blood is negligible compared with the total mass of the person. This simplified example illustrates the principle of ballistocardiography, but in practice, a more sophisticated model of heart function is used.

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Physiology 3120 Lecture Notes - Sliding Filament Theory, Intercalated Disc, Sinoatrial Node

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