Lecture: Physiology 2: Neuronal signalling: Ions, action potentials and the synapse Flashcards
What is voltage (DV)?
The difference in electric potential energy per unit charge between two points.
What is voltage used to signal?
Voltage across membrane is used to signal changes, ie. ion movements across the membrane.
What is current (I)?
The flow of electric charge through a medium.
What is resistance (R)?
The opposition to the passage of an electric current.
What is electrical conductance (g)?
Inverse of resistance; measuring how easily electricity flows along a certain path.
Equation for Ohm’s Law
V = I x R
Equation for conductance (g)
g = 1/R or current/voltage
What is meant by ‘selectively permeable’?
cell membrane is a “selectively permeable membrane” whereby some ions can pass through, but others cannot
What effects ionic permeability?
Ionic permeabilities (and related conductance) depend on ion channels being open, so selectivity can vary over time.
What is transmembrane potential (Vm)?
Measurement of the overall electrical potential energy across the membrane
What determines transmembrane potential (Vm)?
The relative electrical currents and conductance’s of different ions.
What are the 4 steps of an action potential?
- At rest, K+ is going out of cell; -ve membrane potential (e.g. -70 mV)
- External factor causes the membrane to depolarise slightly.
- If the voltage reaches threshold, then Na+ enters cell and membrane potential depolarises and becomes positive (+40 mV). Positive Feedback Loop.
- Eventually Na+ conduction diminishes, K+ conductance increases; membrane repolarises, becomes slightly more negative and then returns to resting.
Describe the ‘all or nothing’ model for action potentials
An action potential is not trigger unless there is an appropriate force from a stimuli to trigger an action potential; depolarisation of the cell must reach a certain limit, otherwise the action potential will not follow through.
What is the refractory period?
Period within an action potential whereby new action potentials cannot be initiated.
What are neurotransmitters?
Neurotransmitters are endogenous chemicals that are released extracellularly by neurons across a synaptic junction.
What are the purpose of neurotransmitters in signalling?
Synapses use neurotransmitters to transfer action potentials between cells to propagate signalling.
How do neurotransmitters send signals across a synapse?
- Neurotransmitter is synthesised and moved to the cell membrane via vesicles.
- AP causes depolarisation which opens calcium channels and causes increase in free calcium ions.
- Increase in free calcium causes neurotransmitter to be released when vesicle fuses with membrane.
- Neurotransmitter diffuses across synaptic cleft and binds to and activates specific post-synaptic receptors which will typically open an ion channel.
What happens to neurotransmitters after they have triggered an action potential?
Neurotransmitter diffuse away and are either metabolised or transported back to terminal.
What is gating?
Biochemical changing of a protein to open or close channels. Channels open when there is a positive change inside the membrane.
What is the equilibrium potential of sodium?
+60 mV
What is the equilibrium potential of potassium?
-90 mV
What is the equilibrium potential of calcium?
+123 mV
What is the equilibrium potential of chloride?
-40 mV
How does the equilibrium potential dictate the movement of ions?
Membrane potential is driven towards the equilibrium potential of particular ion if there is great permeability (or conductance) for that ion
