1. The Action Potential & Impulse/Signal Propagation Learning Objective Be 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.

2. 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

3. 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

5. 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

6. 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

7. Synapse Mechanism

8. 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