1. Grade 12 Biology THE NERVOUS SYSTEM
Part 2 – Synapses
(Passing messages from neuron to neuron)

2. How do you feel types of sensations?
Neurons either send an action potential or do not send an action potential
All-or-none response
Stronger sensations have...
more than one neuron send a signal
more signals sent in the same amount of time
neurons with high thresholds send signals
Think about it...
How can you tell the difference
between a light touch and a slap?

3. How does a neuron continue the signal to the next neuron?
Tiny space between neurons
SYNAPSE
Need to send a chemical message across this space
Neurotransmitters
Chemicals that send a message between neurons
Cause a reaction
in the next neuron
(can continue or
stop the action
potential)
Watch it happen!

4. 1 2 3 4 5 6 Action potential arrives
Vesicle fuses with plasma membrane 3
Neurotransmitter is released into synaptic cleft
Axon of sending neuron
Vesicles
SENDING NEURON
Synaptic knob
SYNAPSE
SYNAPTIC CLEFT
RECEIVING NEURON
Ion channels
Neurotransmitter molecules 4
Neuro- transmitter binds to receptor
Receiving neuron 5
Ion channel opens
Receptor
Ions
Neurotransmitter 6
Ion channel closes
Neurotransmitter broken down and released

5. Sending a message in a synapse: Action potential arrives 1 2 3 4 5 6
Sending a message in a synapse:
Action potential arrives
Vesicles with neurotransmitter
move to the membrane
Neurotransmitter is released
Into the synapse
Neurotransmitters bind to Na+
channels on the next neuron
Na+ channels open  action potential continues in the next neuron OR
Na+ channels kept closed  no action potential can happen in the next neuron
Enzymes clear away the neurotransmitter

6. Why STOP the action potential?
Examples:
Antagonist muscle pairs – one muscle must relax so the other can contract
Anaesthetic – stops a neuron from sending a pain message

7. What effect do DRUGS have on your nervous system?
Depressants
Pain Relief Medication / Alcohol
Blocks Na+ channels on the post-synaptic neuron
No action potential is sent or nervous system works slower than usual
Stimulants
Caffeine / Cocaine
Doesn’t let neurotransmitter be cleared away after the stimulation
Keeps stimulating the neuron over and over!
Neuron makes less Na+ channels... then you can’t feel “normal” without the drug
ADDICTION

8. Complete the worksheet on Synapses!
Any questions?
Complete the worksheet on Synapses!