1 Session 5 The Neuron II: Synaptic Transmission PS111: Brain & Behaviour Module 1: Psychobiology

ElectrotonicAction Potential Synaptic Signal Transmission

Direction of Signal Transfer A B Postsynaptic Neuron (receives signal) Presynaptic Neuron (sends signal) Axon Terminal Dendritic Spine Synapse

Direction of Signal Transfer A B Postsynaptic Neuron (receives signal) Presynaptic Neuron (sends signal) Axon Terminal Dendritic Spine Synapse

Synaptic Cleft Pre- synaptic Membrane Postsynaptic Membrane

Vesicles (filled with Neurotransmitter) Ion Channels Ca ++ Ion Channels

Ca ++ channels open

Ca ++ ions enter

Vesicles fuse with presynaptic membrane

Chemical Synapse fuse with pre- synaptic membrane Vesicles ?

Membrane outside (usually more positive) inside (usually more negative) Ion channel (a very complex protein)

Yet another chemical soup: neurotransmitter Membrane! Ca ++ ions

Chemical Synapse fuse with pre- synaptic membrane Vesicles

release NT into synaptic cleft Vesicles

Ion Channels open bind with postsynaptic ion channels NT empty vesicles are removed

Ions enter post- synaptic cell New Vesicles arrive

Post-Synaptic Potential (PSP) generated

NT are removed Ion Channels close

Synaptic Membranes & Protein Channels l Why do ion channels open? membrane outside (usually more positive) inside (usually more negative) Ion channels (proteins)

Ion channels come in two types: Voltage-gated channels Transmitter-gated channels l Why do ion channels open? Synaptic Membranes & Protein Channels

l Voltage-gated ion channels Examples of voltage-gated channels: Na + - and K + -channels in the axon hillock and the axon Ca ++ -channels in the axon terminal Synaptic Membranes & Protein Channels

l Transmitter-gated ion channels Not (or not only) affected by voltage changes! Respond to chemicals (neurotransmitter) Receptor site Neuro- transmitter molecule Synaptic Membranes & Protein Channels

Receptors come in two basic types: Ionotropic receptors open their channel directly Metabotropic receptors open it indirectly Synaptic Membranes & Protein Channels

Receptor l Transmitter-gated ion channels Receptors come in two basic types: Ionotropic receptors open their channel directly Metabotropic receptors open it indirectly G-Protein second messengers activate channel Synaptic Membranes & Protein Channels

l Transmitter-gated ion channels All ion channels in the post-synaptic membrane are transmitter- gated But different channels respond to different neuro-transmitters! Synaptic Membranes & Protein Channels

l Key-Lock Principle All ion channels in the post-synaptic membrane are transmitter- gated But different channels respond to different neuro-transmitters! Synaptic Membranes & Protein Channels

l Key-Lock Principle All ion channels in the post-synaptic membrane are transmitter- gated But different channels respond to different neuro-transmitters! Synaptic Membranes & Protein Channels

l Key-Lock Principle All ion channels in the post-synaptic membrane are transmitter- gated But different channels respond to different neuro-transmitters! Synaptic Membranes & Protein Channels

l Key-Lock Principle All ion channels in the post-synaptic membrane are transmitter- gated But different channels respond to different neuro-transmitters! Synaptic Membranes & Protein Channels

l Key-Lock Principle Which type of ion enters the cell depends on the type of channel that opens Which type of channel opens depends on the type of neurotransmitter that binds to the receptor site Synaptic Membranes & Protein Channels ` Door Lock Key

l Excitation and Inhibition: Whether a synapse is excitatory or inhibitory depends on the type of neurotransmitter released by the axon terminal ion channel & receptor present in the post-synaptic membrane Remember: For an AP to be triggered, membrane potential at the axon hillock must depolarise beyond threshold (~ -50 mV) At an inhibitory synapse, negative ions enter (hyperpolarisation; IPSP), making it less likely that an AP will be triggered at the axon hillock At an excitatory synapse, positive ions enter (depolarisation; EPSP), making it more likely that an AP will be triggered at the axon hillock Synaptic Transmission

? < Threshold?  no AP > Threshold?  AP!

E E E I

E E E I R T 0

E E E I R T 0

This is ‘Information Processing’ in the NS These integration & transformation processes are the basis for ALL behaviour! Post-synaptic summation Electrochemical changes in post-synaptic neurons triggered by a single AP insufficient to generate new AP Generator potential build-up in post-synaptic cell is slow and graded (unlike AP!) GP integrates (‘sums’) changes caused by several APs at one post-synaptic neuron: Temporal: Combines PSPs occurring in rapid succession; Spatial: Combines PSPs from different synapses (of one post-synaptic neuron) Generator Potential

Electrochemical change in receptor cell Voltage change at dendrites of sensory neuron Decaying generator potential spreads towards axon hillock Action potential actively transported down the axon, without decay Voltage change at dendrites of post-synaptic neuron At terminal buttons, action potential triggers release of neurotransmitters into synaptic cleft If membrane at axon hillock depolarises beyond threshold, action potential is triggered Neurotransmitter bind to receptor molecules at post-synaptic membrane Electrochemical changes inside muscle/gland cell Information Transfer in the Nervous System (a simplified schematic) (channels open) (Or not! In that case, everything stops here!)

Neurotransmitter Removal Neurotransmitter degradation & re-uptake: Neurotransmitters do not change when they bind to a receptor; have to be actively removed to stop their influence on post- synaptic cell. Two types of NT removal: Degradation: Special enzymes in the synaptic cleft break down (inactivate) NTs. Components are (partly) ‘recycled’ to make new NTs. Re-uptake: Receptor molecules at pre-synaptic axon terminal take up NTs, return them into pre-synaptic cell.

And that’s it for now!