Biology for Engineers: Cellular and Systems Neurophysiology

1. Biology for Engineers: Cellular and Systems Neurophysiology Christopher Fiorillo BiS 521, Fall 2009 042 350 4326, fiorillo@kaist.ac.kr Part 4: Synaptic Transmission Reading: Bear, Connors, and Paradiso Chapter 5

2. A Single Neuron with Synapses in Yellow

3. Synapses Are Physical Contacts between Neurons that Enable Fast Transmission of Information • Types of Synaptic Contacts • Axodendritic: Axon to dendrite • Axosomatic: Axon to cell body • Axoaxonic: Axon to axon • Dendrodendritic: Dendrite to dendrite

4. Two Types of Synaptic Transmission • Chemical Transmission • 1921- Otto Loewi • Electrical Transmission • 1959- Furshpan and Potter • There was a long-lasting debate about whether transmission was chemical or electrical. Both occur, but chemical transmission is much more common.

5. Direction of Information Flow • Information usually flows in one direction • First neuron = Presynaptic neuron • Target cell = Postsynaptic neuron Postsynaptic neuron Presynaptic neuron

6. Electrical Synapses Are Composed of Gap Junctions • Gap junction are large channels • Large enough (1-2 nm) to allow all ions plus other small molecules to pass • A Connexon spans the membrane - formed by six connexin proteins • Cells are said to be “electrically coupled” • Flow of ions from cytoplasm to cytoplasm

7. Electrical Synapses • Very fast transmission • Chemical transmission has a delay • Postsynaptic potentials (PSPs) have the same form as the presynaptic potential, but are smaller • Most electrical synapses are bidirectional, but some are unidirectional

8. A Chemical Synapse • The synaptic cleft is a 20-50 nm gap between the presynaptic terminal and the postsynpatic membrane • Neurotransmitter is released into the cleft and activates postsynaptic receptors

9. Electron Micrograph of a Chemical Synapse • Synaptic Vesicles • Made of phospholipid membrane • 50 nm in diameter • Filled with molecules of neurotransmitter • Dense-core Vesicles • Contains peptide neurotransmitters • Vesicles release neurotransmitter when they fuse with the presynaptic membrane

10. Two Synaptic Morphologies • CNS Synapses (Examples) • Gray’s Type I: Asymmetrical, usually excitatory • Gray’s Type II: Symmetrical, usually inhibitory

11. Synapses Vary in Size and Strength • Larger synapses allow the presynaptic neuron to have a larger and more reliable effect on the postsynaptic neuron

12. Neurotransmitter Synthesis and Storage • Small neurotransmitters (amines, amino acids) • Synthesized in vesicles within terminal • Peptides • Synthesized within soma and transported to terminal

13. Recording Methods: Current and Voltage Clamp (This slide and the next one should have been presented in the last section on “membrane voltage.”) • “Current Clamp” • Measures voltage across cell’s membrane • A constant current is injected through the electrode • The current can be manually adjusted by the experimenter • “Voltage Clamp” • Measures current passing through the cell’s membrane • Clamps voltage across the cell’s membrane • A feedback circuit calculates, and injects through the electrode, the amount of current that is necessary to keep the voltage constant

14. Current versus Voltage Clamp: which is appropriate? • Advantages of current clamp • More physiological. Voltage is never clamped under physiological conditions • Fast and accurate voltage clamp can be difficult to achieve • Especially in the dendrites of a large neuron • Advantages of voltage clamp • Greater experimental control: it eliminates voltage as a variable • Keeps driving force constant • Better for studying voltage-gated channels • Better temporal resolution for fast channels, because it removes the effect of membrane capacitance • In general, voltage-clamp is better for studying the properties of ion channels. Current clamp is better for studying the properties of neurons.

15. Basic Steps of Chemical Synaptic Transmission • Action potential invades synaptic terminal • Depolarization-activated Ca2+ channels open • Ca2+ triggers vesicles to fuse into membrane of presynaptic terminal (exocytosis) • Neurotransmitter spills into synaptic cleft • Binds to postsynaptic receptors • Biochemical/Electrical response elicited in postsynaptic cell • Removal of neurotransmitter from synaptic cleft • New vesicles formed by endocytosis • Vesicles are filled with neurotransmitter and prepared for release

16. Removal of Neurotransmitter from the Synaptic Cleft • Removal of neurotransmitter is important in order to limit the duration of postsynaptic stimulation. This enables high frequencies of information transmission • Three Mechanisms • Diffusion • Reuptake: Transporters bind neurotransmitter and transport it to inside of presynaptic terminal • This is the most important mechanism for removing neurotransmitters • Cocaine and Prozac (fluoxetine) block reuptake of dopamine and serotonin • Enzymatic destruction in synaptic cleft • Acetylcholineesterase eliminates acetylcholine. It is the only example of this method.

17. Neurotransmitter Release is Quantal • A action potential causes the release of a discrete number of vesicles (or quanta) • Neuromuscular junction: About 200 synaptic vesicles, EPSP of 40mV or more • CNS synapse: Single vesicle, EPSP of few tenths of a millivolt • Each vesicle contains about the same amount of neurotransmitter • Quantal content (the amount of transmitter per vesicle) is not a physiologically important variable • Spontaneous release of a single vesicle causes a miniature postsynaptic potential (current) • Often called a “mini”

18. The Neuromuscular Junction • Studies of NMJ established principles of synaptic transmission • Synapses between neurons are very similar to NMJ

19. Miniature Postsynaptic Currents Are Caused by Release of a Single Vesicle • “Minis” (mEPSCs and mIPSCs) are caused by spontaneous release of a single vesicle in the absence of a presynaptic action potential • Minis can be calcium-dependent or independent • Time course of mPSCs are identical to PSCs • ~3 ms for EPSC • ~30 ms for IPSC • Amplitude of mPSC depends on postysynaptic receptors • vesicles all contain the same amount of transmitter, which can saturate postsynaptic receptors • Frequency of mPSCs depends on presynaptic factors • At most synapses, < 0.01 mPSC / second • At some synapses, > 0.1 mPSC / second Glutamate EPSC

20. Release Probability • Not every action potential evokes vesicle release • Release probability (Pr) given action potential • Some synapses release multiple vesicles, but most release just 0 or 1 vesicle • Pr depends primarily on calcium concentration in terminal’s cytosol, which depends on: • Action potential • Recent history of action potentials • Activation of neurotransmitter receptors on synaptic terminal Pr varies from one synapse to another, as shown in the histogram at right. A typical value is 0.3

21. Presynaptic [Ca2+] at PF synapse PPD and PPF at 3 synapses. 10 stimuli at 50 Hz (20 ms intervals) Paired-Pulse Depression and Facilitation • PPD and PPF are universal features of synapses. • Some synapses show PPD, some show PPF, and some show both • All synapses may have multiple mechanisms mediating both depression and facilitation • PPD and PPF are caused primarily by a decrease or increase, respectively, in vesicle release probability • Electrical stimuli (each lasting about 0.2 ms) are applied to a brain slice maintained in vitro. This evokes postsynaptic potentials (or currents, if measured in voltage clamp). • Excitatory Postsynaptic Potential (Current): EPSP (EPSC) • Inhibitory Postsynaptic Potential (Current): IPSP (IPSC) • Each stimulus evokes action potentials in many axons, and it therefore causes vesicle release from many terminals • A PSP (PSC) is caused by release of multiple vesicles (quanta) • But if a low stimulation current is used, it is possible to stimulate only a single axon, and that axon may have only one release site. In this case, some stimuli may not release any vesicles. • The amplitude of a PSP (PSC) depends on the release probability at stimulated synapses

22. PPD and PPF at 3 synapses. 10 stimuli at 50 Hz (20 ms intervals) Causes of Synaptic Depression and Facilitation • The most common cause of facilitation is an increased calcium concentration • This is due primarily to the fact that calcium is cleared slowly after an action potential • The most common cause of depression is a loss of “docked” (releasable) vesicles • Most vesicles in the terminal are “undocked,” meaning that they are not close to the membrane and bound to the vesicle-release machinery • There may be just one docked vesicle. Once it is released, it takes time for another vesicle to be docked and ready to release. • The rate of recovery from depression (docing of vesicles) is increased by calcium • There are many ways in which release probability might be modified • Changes in membrane voltage • Changes in the properties of ion channels, particularly calcium channels, that are activated during the action potential • Modification of proteins involved in vesicle release • There are probably multiple depressing and facilitating processes happening simultaneously at each synapse. Presynaptic [Ca2+] at PF synapse

23. Presynaptic [Ca2+] at PF synapse is suppressed by cannabinoid receptor activation Modulation of Release Probability by Presynaptic Neurotransmitter Receptors • Neurotransmitter receptors on presynaptic terminals act to augment or suppress release probability • These receptors therefore alter PPD or PPF • Many receptors suppress vesicle release, including “autoreceptors” • Suppression often occurs through inhibition of Ca2+ channels and activation of K+ channels Suppression of glutamate EPSCs by adenosine receptors

24. Suppression of glutamate EPSCs by adenosine receptors How can we know whether a change in amplitude of a synaptic potential is pre- or postsynaptic? • Two Easy Tests: • Paired-pulse ratio (PPF or PPD) • A change suggests a presynaptic effect • No change suggests a postsynaptic effect • Minis • A change in frequency suggests a presynaptic effect • A change in amplitude suggests a postsynaptic effect • These tests are not definitive; there are exceptions to these rules

25. Output: quantal vesicle release, usually 0 or 1 Integration medium: [Ca2+] Imaginary quantity: “Release Probability” Spontaneous release Inputs: action potential, synaptic neurotransmission, voltage-regulated ion channels Output: All-or-none action potential Integration medium: membrane potential Imaginary quantity: “Instantaneous Firing Rate” Spontaneous action potentials Inputs: synaptic neurotransmission, voltage-regulated ion channels Analogies between Presynaptic Terminals and Somatodendritic Compartment Somatodendritic Compartment Synaptic Terminal

26. Synaptic Integration • Synaptic Integration: The process by which multiple synaptic potentials sum together within one postsynaptic neuron • This occurs in the dendrites and soma • The “decision” point in most neurons is the axon hillock, where the neuron “decides” whether to emit an action potential

27. Synaptic Inhibition • Inhibition • Takes membrane potential away from action potential threshold • Excitatory vs. inhibitory synapses: Bind different neurotransmitters, allow different ions to pass through channels • Most synaptic inhibition is mediated by GABA-gated Cl- channels • ECl- is -65 mV • If membrane potential is less negative than -65mV, GABA mediates hyperpolarizing IPSP. • Two Mechanisms of Inhibition: • Hyperpolarization • Shunting Inhibition: Inhibiting current flow from dendrites and soma to axon hillock

28. Shunting Inhibition: Inhibiting current flow to axon hillock • Increasing membrane conductance will decrease membrane space constant • Therefore, opening any channel will cause an EPSP to decay over a shorter distance • This is called “shunting” inhibition. It prevents depolarizing current from reaching the axon hillock and eliciting an action potential. • By opening Cl- channels, GABA can cause a shunting inhibition even if it causes a depolarization towards ECl.

29. A Single Neuron with Synapses in Yellow

30. Synaptic Plasticity • The strength of a synapse can change; it is “plastic” • A neuron can therefore select its own synapses • Synaptic plasticity is thought to be the main mechanism of learning and memory • Synaptic plasticity has probably been the most popular topic in neuroscience for the last 30 - 50 years • Synaptic plasticity plays a critical and necessary role in many computational neuroscience models and in all artificial neural networks • We will cover synaptic plasticity from both computational and mechanistic perspectives in future lectures