The Action Potential & Impulse/Signal Propagation

Learning ObjectiveBe able to describe what a synapse is.

Be able to describe how an action potential causes the activation of synapses.

Be able to explain how signals cross the synapse.

• A synapse is the point where neurons connect together.

• An action potential travels across an axon and arrives at the synapse.

• The synapse is the opposing membranes of two neurons separated by a gap.

• Because of this gap signals must be passed on chemically

A Synapse

• The synapse is made of several parts.

• The Pre-Synaptic membrane is on the neurone where the signal is coming from.

• The Post-Synaptic membrane is on the neurone where the signal is going to.

• Between these two membranes is the synaptic cleft

Synapse Depolarisation

• On the pre synaptic membrane are Calcium pumps which pump calcium out of the cell

• When the action potential arrives at the synapse is causes a voltage gates calcium channel to open

• This causes calcium to flood into the cell

Neurotransmitter Secretion

• As the intracellular levels of calcium rise this triggers special synaptic vesicles containing neurotransmitters to fuse with the pre-synaptic membrane

• This causes neurotransmitters to be released (by exocytosis) into the synaptic cleft

• This exocytosis process uses ATP derived from mitochondria

Synapse Mechanism

Neurotransmitter Receptors

• Receptors on the post-synaptic membrane detect the neurotransmitters

• Sodium channels are then opened which cause a sodium to enter the cell creating a threshold potential

• This then sets off an action potential along the next membrane

• To stop the neurotransmitters effects, enzymes (e.g. acetylcholinase) break them down. This also stop neurone cross firing