5 - Cardiovascular Control 1 Flashcards
What is the Nernst equation used for?
to determine equilibrium potential (EK)
What are some features of the resting membrane potential?
- depends on flow of K+ out of cell
- if membrane only permeable to K+ at rest then potential will equal K+ equilibrium potential
- changes depending on relative permeabilities of membrane to various ions
Other than the Nernst equation, what other equation is used to determine equilibrium potential and what does it take into account?
- Goldman-Hodgkin-Katz equation
- takes into account relative permeabilities of several ions simultaneously
What is the absolute refractory period?
time during which no action potential can be initiated regardless of stimulus intensity
What is the relative refractory period?
period after ARP where an AP can be elicited but only with stimulus strength larger than normal
What is the potassium hypothesis?
- device added to measure chemical conc.
- 2 chambers with different KCl concs. separated by impermeable barrier
- membrane only permeable to K+
- no movement of Cl- ions
- positive charge accumulation and electrical gradient builds up
- when electrical gradient = chemical gradient, equilibrium achieved
- driving force = 0 at equilibrium
What is the duration of an action potential equal to and what is a feature of it?
- duration of contraction
- slower contraction better and more efficient
What causes a refractory period and what is a feature of a cardiac muscle refractory period?
- caused by Na+ channel inactivation
- long refractory period in cardiac muscle (not possible to re-excite muscle until process of contraction underway)
- means cardiac muscle can’t be tetanised
What are the 5 phases of an action potential?
- phase 0: upstroke
- phase 1: early repolarisation
- phase 2: plateau
- phase 3: repolarisation
- phase 4: resting membrane potential
What causes the characteristic notch of phase 1 in an action potential?
brief increase in K+ permeability
What are the labels of the action potential?
a. phase 0: depolarisation
b. phase 1: early repolarisation
c. phase 2: plataeu
d. phase 3: repolarisation
e. phase 4: resting membrane potential
1. membrane potential (mV)
2. time (ms)
What are the features of the sino-atrial node?
- most channels exist in node
- very little Na+ influx
- upstroke caused by Ca2+ influx
- no stable resting membrane potential (no IK1 channels present)
- pacemaker current
What is different in the activation of T-type and L-type Ca2+ channels?
-type calcium channels activate at more negative potentials than l-type
Where is the SA node?
- just below epicardial surface
- at boundary between right atrium and superior vena cava
- specialised cells comprising node mark start of conduction pathway
What are some features of pacemaker cells?
- modulated by SNS and PNS thorugh vagus nerve
- nerves from SNS and PNS synapse with SA node
- SNS innervation increases heart rate and contractility
What are the differences in adrenaline and acetylcholine in the speed of heart rate?
- adrenaline (SNS): released faster so faster heart rate
- acetylcholine released slower so slower heart rate
What are the features of the cardiac conduction system?
- SA node: specialised cluster of autorhythmic cells
- internodal fibres: rapid conduction tracts to stimulate atrial myocardium
- AV 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 impulse across ventricular myocardium
What are the electrics of the heart?
- heart is muscle (so vital it contracts)
- contraction of myocytes initiated by electrical impulses (AP)
- doesn’t require neurones (unlike skeletal muscle)
- heart generates own electrical stimulation (intrinsic electric properties)
- heart can beat when removed from body
What is impulse propagation?
- due to combination of passive spread of current and existence of threshold which, when reached, causes cell to generate own AP
- gap junctions greatly reduce membrane resistance allowing current to easily leak from one cell to a neighbouring cell
