Where are neuron cell bodies located
- CNS
- Peripheral ganglia
- intestinal walls
What is the site of action potential generation
Initial segment of the axon hillock
How much can action potentials vary in size
All or nothing
How many inputs does each neuron receive at any given moment
Likely thousands of inhibitory and excitatory inputs
Where in the neuron has the highest density of voltage gated sodium channels
Initial segment of the axon hillock
What makes the axon hillock so excitable
It has the highest density of voltage gated sodium channels
How are voltage gated sodium channels opened
Depolarisation of the membrane
What happens when the voltage gated sodium channels opens
The neuron becomes more depolarised
What has to occur for the cell membrane to reach spike threshold
Depolarising local potentials and sufficient Na+ channel depolarisation
What is spike threshold
~ -70mV but varies between neurons
What are the phases of an action potential
Depolarisation
Overshoot
Repolarisation
Hyperpolarisation
What coincides with the start of the depolarisation phase
An increase in sodium conductance
What coincides with the end of the depolarisation phase
Decline in Na+ conductance (due to channel inactivation)
At the peak of an action potential what has happened to the membrane potential
It is reversed
The cell is positive inside
Why is the increase in potassium permeability delayed
Voltage-gated potassium channels are slow opening
Why does Hyperpolarisation happen
At the AP peak, the K+ gradients are outwards, potassium leaves the cell
What happens to k+ channels before they can open again
They are inactivated by -ve voltage
What is the absolute refractory period
Impossible to generate another AP
What is the relative refractory period
The RMP is hyperpolarised and it is harder to generate an AP
When is the absolute refractory period
When Na+ channels are inactivated
What is the function of the refractory periods
-prevents backwards propagation of APS
- sets upper limit of AP firing frequency
How is information encoded in APs
- number
- frequency
- pattern
Action potential function
To trigger the release of a signalling chemical
