Transmission Across a Synapse

An electron micrograph

Step 1

• After an action potential arrives at an axon bulb, Ca2+ enters the bulb.

• Ca2+ allow the synaptic vesicles to fuse with the pre-synaptic membrane.

• This is an example of exocytosis and requires ATP from cellular respiration in the mitochondria.

Step 1

Step 2

• Neurotransmitters are released into the synaptic cleft.

• neurotransmitters diffuse across the gap and bind to the receptor proteins on the postsynaptic membrane of dendrite.

Step 2

Step 3

• If enough neurotransmitters bind to the receptor membrane proteins in the postsynaptic membrane of the dendrite....

• the Na+ channels open and Na+ diffuse into the dendrite producing an action potential in the neuron.

• There are specific receptor proteins for specific neurotransmitters.

Step 3

What happens to the neurotransmitters?

• Enzymes can rapidly inactivate the neurotranmitter once it has performed its function.

• Acetylcholinesterase breaks down acetylcholine.OR

• The pre-synaptic membrane rapidly reabsorbs the neurotransmitters, re-packaging them into synaptic vesicles (endocytosis) or for molecular breakdown.

Some exciting neurotransmitters – and some not-so exciting ones!

• At least 25 neurotransmitters have been identified.• Excitatory

Acetylcholine – controls muscles and help regulate memory, (sometimes inhibitory)Norepinephrine – fight or flightGlutamate – most common excitatory neurotransmitter. Involved in learning and memory.

• InhibitoryDopamine – feelings of pleasure.GABA – regulates muscle tone.Seratonin – controls mood, appetitie & sensory perceptionMelatonin – helps regulate circadian rthythms, sleep-wake cycle.

Recall QuizCan you identify the numbers?

The Numbers• Postsynaptic membrane• Pre-synaptic membrane• Synaptic vesicle• Mitochondrion• Synaptic cleft• neurotransmitter• Receptor membrane protein• Ion channel• Postsynaptic dendrite• Axon bulb

Place the events in order.

• Neurotransmitters diffuse across the synaptic cleft.• Na+ diffuse through the opened channels on the

postsynaptic membrane.• The action potential arrives at an axon bulb.• Neurotransmitters bind to receptor proteins.• Ca2+ enter the axon bulb and cause synaptic

vesicles to fuse with the pre-synaptic membrane.• An action potential begins at the next neuron.