Action Potential revisited
When a stimulus reaches threshold level,
Sodium channels open up and Sodium
rushes into the axon along the
concentration gradient. depolarization
Once a membrane potential of 40mV is
reached, the sodium channels close, and
Potassium channels open. This causes a
rush of Potassium ions to the outside of the
cell, and the cell is repolarized.
The potassium gates close relatively slowly,
therefore, hyperpolarization occurs, and the
cell is said to be in a refractory period (toilet
flushing)
The Sodium-Potassium pump moves ions
back across the membrane against the
concentration gradient, and resting potential
is restored.
The refractory period helps to ensure that
stimulus only flows in one direction.
Neurotransmitters
Neurotransmitters are chemicals that
alter the membrane potentials of
postsynaptic neurons.
Acetylcholine is a neurotransmitter found
in the end plates of many nerve cells.
It acts as an excitatory neurotransmitter
on many postsynaptic neurons by
opening Sodium ion channels.
What would happen if the sodium channels
remained open?
The presynaptic neuron releases the
enzyme cholinesterase which breaks down
acetylcholine, allowing the sodium
channels to close, and repolarization to
take place.
Why is this important to neuron function?
Some neurotransmitters act on the
postsynaptic membrane by making the
membrane more permeable to Potassium.
This causes the cell to go through a state of
hyperpolarization.
What effect do you think this has on the
neuron?
These are called inhibitory
neurotransmitters. The inhibitory
neurotransmitter GABA is the most
abundant neurotransmitter in the brain.
Summation refers to multiple presynaptic
neurons acting at one time on a
postsynaptic neuron.
When do you think summation would be
necessary?
Now, lets look at how different drugs act
on the synapse.