What happens when action potential reaches axon terminal? Forms a synapse with another neuron or muscle Two types: Electrical and Chemical

Chemical Synapse

1. action potential arrives at terminal 2. voltage-gated Ca channels open 3. Ca triggers exocytosis of vesicles 4. neurotransmitter is re- leased, binds to re- ceptor 5. ligand-gated Na or K 6. neurotransmitter broken down, taken up 7. synaptic vesicles re- constituted 8. neurotransmitter stored, awaits next impulse Action potential Synaptic vesicle active uptake Ca2+ voltage- gated Ca channel Ca2+ docking protein K+ Postsynaptic cell Na+

Neuromuscular Junction acetylcholine X botulinum toxin X curare (antagonist) nicotine (agonist) acetylcholinesterase Nicotinic cholinergic synapse

Muscarinic cholinergic synapse acetylcholine X atropine (antagonist) muscarine (agonist)

Post-Synaptic Events Graded vs Action Potentials Excitatory vs. Inhibitory PSPs Pre- vs. Post-Synaptic Inhibition Divergence vs. Convergence

Receptor binding results in opening of agonist-gated K and Cl channels e.g. muscarinic acetylcholine receptor of heart

Receptor binding results in opening of agonist-gated Na channel e.g. nicotinic acetylcholine receptor of neuromuscular junction

One synapse is generally not sufficient to generate 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 axonal hillock 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 of postsynaptic neuron Axon terminals of presynaptic neuron Dendrite of postsynaptic neuron Glial cell processes Axon

Spatial summation

excitatory inhibitory A D E B C electrode threshold A A+B A+B+C D D+E A+B+C+E

Postsynaptic Inhibition

Presynaptic Inhibition

Divergent circuit Allows for amplification of a signal:

single pyramidal cell in the motor cortex 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 Important for summing, correlating and sorting information in the CNS