5. Control of the Cardiovascular system 1 Flashcards
What does membrane potential depend on?
Flow of K+ out of cells
What equation is used to predict what a potential will be across a semi-permeable membrane?
Nernst equation
If the membrane is only permeable to K+ at rest (diastole)…
Then the potential across it will equal the K+ equilibrium potential, (EK)
Which equation can account for relative permeabilities of several ions simultaneously? Why is this a better indicator of membrane potential?
Goldman-Hodgkin-Katz equation
Membrane is never uniquely selective to 1 ion
Briefly describe an AP in a nerve
Increase in Na+ permeability
Na+ floods in
Depolarises cell
Moves towards Na+ equilibrium potential
(Doesn’t get there as Na+ channels close)
Gradual increase in K+ permeability taking +charge out of cell to repolarise it and restore membrane potential
Time duration of an AP in a nerve
2 ms
Time duration of a cardiac AP
200-300 ms
Very long compared with nerves
What does the duration of a cardiac AP control?
Controls the strength and duration of contraction of the heart
Long, slow contraction is required to produce an effective pump
Absolute refractory period (ARP)
time during which no AP can be initiated regardless of stimulus intensity
Relative refractory period
period after ARP where an AP can be elicited but only with stimulus strength larger than normal
Why is the relative refractory period useful in cardiac cells?
Useful as can’t re-stimulate too quickly
Allows heart to fill before next stimulus
What are refractory periods caused by?
Na+ channel inactivation
As membrane becomes more and more negative, more and more Na+ channels become available (recover) to be activated if re-stimulated
Na+ channels recover from inactivation as the membrane repolarises
Tetanic stimulation
sustained contraction following a series of stimuli
Describe repolarisation in skeletal muscle
Repolarisation occurs very early in the contraction phase making re-stimulation and summation of contraction possible
Tetanic stimulation can be produced
What is the consequence of a long refractory period in cardiac muscle?
Long refractory period means it is not possible to re-excite the muscle until the process of contraction is well underway
Hence cardiac muscle cannot be tetanised
Describe the phases of an action potential in the ventricles
Phase 0: Upstroke: Caused by Na+ channels opening, influx of Na+ and depolarisation towards Na+ equilibrium potential
Phase 1: Early repolarisation (increase in K+ conductance of membrane)
Phase 2: Plateau (Ca2+ channels open)
Phase 3: Repolarisation (K+ channels opening)
Phase 4: Diastole, Resting membrane potential
Action potential profiles in the heart
Different parts of the heart have different AP shapes
Caused by different ion currents flowing and different ion channel expression in cell membrane
Describe the electrical properties of the heart
Intrinsic
Capable of independent spontaneous generation and coordinated propagation of electrical activity
Specialised conduction system
Can beat independently even if separated from its nerve supply
Describe the extrinsic nerve supply to the heart
Comes from the autonomic nervous system
Serves to modify and control the intrinsic beating established by the heart
What is the resting membrane potential in SAN?
No resting membrane potential in SAN
Always oscillating
What channels exist in SAN? What is the consequence of this?
Most channels exist in SAN
Exception is IK1: there is no IK1 (So membrane potential is never very stable)
What is the upstroke produced by in the SAN? What is the consequence of this?
Upstroke produced by Ca2+ influx
So upstroke is quite slow
What types of Ca2+ channel are in the SAN?
L-type (cause main upstroke)
T-type (activate at more negative potentials)
What leads to repolarisation in the SAN?
Inactivation of Ca2+ channels
Na+ influx in SAN
Very little Na+ influx
In a different type of Na+ channel so only small depolarisation
What effect does increased sympathetic stimulation to the SAN have? What neurotransmitter is involved?
Noradrenaline
SAN depolarise much more quickly and reaches threshold potential more quickly
Fire off AP more quickly
Heart rate increases
What effect does increased parasympathetic stimulation to the SAN have? What neurotransmitter is involved?
ACh
Slows diastolic depolarisation in SAN, slower to reach threshold potential, slows heart rate
Chronotropy
Describes changes in heart rate
Positive chronotropic effect
Faster heart rate
Iontropy
Describes cardiac contractility
Positive inotropic effect
Increase in contractility
How do the cardioregulatory centre and vasomotor centres in the medulla modulate intrinsic heart rate?
PNS innervation inhibits heart rate (Vagus nerve)
SNS innervation increases heart rate and contractility (sympathetic nerves)
Location of sinoatrial node
Lies just below the epicardial surface at the boundary between right atrium and superior vena cava
What does spontaneous depolarisation in the SAN allow?
Allows the heart to generate its own rhythm (autorhythmicity)
Internodal fibres
Rapid conduction tracts to stimulate atrial myocardium
Atrioventricular node
Specialised cells to delay wave of excitation and insulate from superior ventricular myocardium
Bundle of his
Rapid conduction cells to transport an insulated wave of excitation
Ventricular fibres
Propagate the impulse across the ventricular myocardium
Describe the cardiac conduction system
Start at SAN
Excitation passes along internodal fibres: rapid conduction across atria
Blood pushed into ventricles
Conduction slowed at AVN: delay wave of excitation from moving down into ventricles to allow filling of ventricles from contraction of atria
Excitation moves down bundle of His which possesses rapid conduction fibres (slightly insulated) to apex
Wave of excitation spreads up base causing ventricular excitation
What is the impulse propagation of the cardiac AP due to?
a combination of passive spread of current and the existence of a threshold which, once reached, causes the cell to generate its own AP.
What do gap junctions allow in reference to a cardiac cell?
Greatly reduce membrane resistance allowing current to easily leak from one cell to a neighbouring cell.
What do intercellular communication and impulse conduction from one cell to the next relies on?
Gap junctions
Where do gap junctions form in the heart?
At intercalated discs
What are there many of in gap junctions? What do these do?
Formed of connexons
Connexons consist of connexins
Form a tube allowing a low resistance conduction pathway from one cardiac cell to another
