What happens when action potential reaches axon terminal?

Forms a synapse with another neuron or muscle

Two types: Electrical and Chemical

Chemical Synapse

Actionpotential

Synapticvesicle

Ca2+

Ca2+dockingprotein

voltage-gated Cachannel

Postsynapticcell Na+

K+

activeuptake

1. action potential arrivesat terminal

2. voltage-gated Ca channels open

3. Ca triggers exocytosisof vesicles

4. neurotransmitter is re-leased, binds to re-ceptor

5. ligand-gated Na or K channels open

6. neurotransmitter brokendown, taken up

7. synaptic vesicles re-constituted

8. neurotransmitter stored,awaits next impulse

Neuromuscular Junction

acetylcholine

acetylcholinesterase

nicotine (agonist)

Nicotinic cholinergic synapse

X

curare (antagonist)

X

botulinumtoxin

acetylcholine

muscarine (agonist)

Muscarinic cholinergic synapse

X

atropine (antagonist)

Post-Synaptic Events

Graded vs Action Potentials

Excitatory vs. Inhibitory PSPs

Pre- vs. Post-Synaptic Inhibition

Divergence vs. Convergence

Receptor binding results in opening ofagonist-gated K and Cl channels

e.g. muscarinic acetylcholine receptor ofheart

Receptor binding results in opening ofagonist-gated Na channel

e.g. nicotinic acetylcholine receptor of neuromuscular junction

One synapse is generally not sufficient togenerate a threshold stimulus at the axonal

hillock.

Most neurons receive thousands of synaptic endings -each of which contributes a postsynaptic potential

(EPSP or IPSP) of less than 1 mV.

Each of these PSPs are conducted to the axonalhillock with decrement (decay).

An action potential will only be generated if the sum of the IPSPs and EPSPs reaching the axonal

hillock at any time exceed the threshold voltage.

Dendrite ofpostsynaptic

neuron

Dendrite ofpostsynaptic

neuronGlial cell

processesAxon

Axon terminals of presynaptic

neuron

Spatialsummation

electrode

excitatoryinhibitory

A

B

C

DE

A A+B A+B+C D D+E A+B+C+E

threshold

Postsynaptic Inhibition

Presynaptic Inhibition

Allows for amplification of a signal:

Divergentcircuit

single pyramidal cell in the motorcortex of the brain

15-20 internuncial cells in CNS

each stimulates several hundred motoneurons

each of these stimulates 100-300 muscle fibers

Divergence results in 50,000-fold amplification

Important for

summing, correlating and sorting information in the CNS

Convergent circuit