EXCITABLE CELLS Flashcards
What is potential difference?
Difference in voltage between two points
What is membrane potential (Vm)?
Difference in voltage across the cell membrane (inside and outside of the cell)
What is resting membrane?
Difference in voltage across the cell membrane when a cell is a rest
What is graded potential?
Change in membrane potential that may or may not result in an action potential (leads to hyperpolarization)
What is an action potential?
A large, rapid change in membrane potential produced by depolarisation of an excitable cell’s plasma membrane to threshold
What is an equilibrium potential?
The membrane potential that results in no NET diffusion of an ion
What makes cells excitable?
Ability of a cell to be stimulated to create an electrical current that generates an action potential
(ion concentrations are extremely different inside the cell compared to the extracellular fluid)
Give examples of excitable cells (5)
- All neurones
- Skeletal muscles
- Cardiac muscles
- Some smooth muscle
- Some endocrine cells (e.g. pancreatic β cells -
- basically cells that need to transfer info quickly
Give examples of non-excitable cells
All other body cells e.g:
- fibroblasts
- epithelial cells, endothelial cells
- adipocytes
- blood cells
How do you measure resting potential?
The resting membrane can be measured using a microelectrode (recording electrode), electrode and voltmeter
What is the resting potential membrane dependent on?
- Factor 1 – conc gradient of ions
- Factor 2 - presence of ion channels in the plasma membrane
What pumps control the resting potential’s conc. gradient?
Na+/K+, H+/K+, H+, Ca2+ ATPases
What ion channels control the resting potential?
K+, Na+, Ca2
What are the three states an ion channel can be in?
- Closed (resting) - equilibrium of the equation to left
- Open (activated) - when a depolarising stimulus arrives, the equilibrium shifts to the right = opens
- Inactivated (refractory/ desensitised) - occurs during sustained depolarisation (requires repolarization)
Define Nernst (equilibrium) potential
The electrical potential that balances the chemical potential (so no net movement of ions) - for membranes with only one ion
Give the Nernst (equilibrium) potential equation
- VN = The Nernst Equilibrium Potential (mV)
- R= Ideal Gas Constant
- 8.314 J.K-1.mol-1 (Joules per Kelvin per mole).
- T = Temperature (in Kelvin)
- K = °C + 273
- RT at room temp is 61
- z = the charge of the ion (valence)
- F = Faraday’s number
- 96485 C.mol-1 (Coulombs per mole).
How to calculate the Vm (voltage inside relative to outside), when there are multiple penetrating solutes? which equation?
- Goldman-Hodgkin-Katz (GHK) equation
- Contribution of membrane potential is weighted according to permeability
- pK is the membrane permeability for K+.
- pNa is the relative membrane permeability for Na+.
- pCl is the relative membrane permeability for Cl-
What does Van Hoff equation state?
Chemical energy is proportional to solute concentration
What is the implication of the 3 equations below combined?
- Van Hoff equation
- Nernst equation
- Goldman/GHK equation
Gated ion channels can rapidly change the membrane voltage, without any change in bulk intracelluar concentrations, on a time scale as fast as ~1ms (nerve action potential)
What is the implication of the 3 equations below combined? Van Hoff equation Nernst equation Goldman/GHK equation
Gated ion channels can rapidly change the membrane voltage, without any change in bulk intracellular concentrations, on a time scale as fast as ~1ms (nerve action potential)
What does automaticity mean?
Ability of a tissue or organ to function without external control
Describe how the heart is able to be automatic?
- Cells store energy as a concentration gradient of ions across the plasma membrane (inside differs from outside)
- Membrane proteins control passage of ions across the membrane (creates electrical activity and excitability
- Electrical signal can travel rapidly to neighbouring cells - Electrical activity is coupled to mechanical action (contraction)
Describe the appearance of cardiac muscle on a cellular level
- Short (100-200um)
- Strong, branching cells held together at intercalated discs
- Electrical (gap) junctions allow cell-to-cell spread of action potentials (in 3D) without chemical synpases
Describe an action potential in contractile cells? (5)
- Resting membrane potential (maintained by open K+ channels and closed Na+/K+
- Depolarization - threshold reached = Fast Na+ channels open = rapid depolarization
- Partial repolarization -due to influx on Na+ channels, so Na+ channels close
- Plateau - Ca2+ channels move out of cell = maintains depolarization
- Repolarization - Ca2+ channels close and K+ channels reopen (back to beginning)
What four things do contractile myocytes contain?
- Actin
- Myosin
- Troponin
- Sarcoplasmic reticulum
What is the sequence of events in electrical activity of cardiac contractile cells?
- AP causes Ca2+ to enter the cell via L-type voltage-gated Ca2+ channels
- This triggers the release of more Ca2+ from the sarcoplasmic reticulum
- Contraction occurs when cytosolic Ca2+ binds to troponin (allows troponin to interact with myosin)
- Relaxation occurs when cytosolic [Ca2+] falls
- caused by the active transport of Ca2+ into the SR and extracellular fluid
- Fall in cytoplasmic [Ca 2+] renders troponin inactive = fibres dissociate
What is the action potential upstroke in a nodal cell graph due to?
Due to voltage-gated Ca2+ channels
What is the action potential downstroke due to?
(in nodal cells)
Increase in concentration of K+
What are the currents in nodal cells activated by?
sympathetic stimulation (AdRs, e.g. noradrenaline) which open Na+ and Ca2+ channels and increase their concentration
In pacemaker cells, describe how spontaneous slow depolarisation occurs
Na+ (and K+)
Transient Ca2+ channels
In pacemakers cells, describe how rapid depolarisation occurs
due to slow Ca2+ channels
How does repolarisation in pacemaker cells occur?
K+ channels
