1. Neurons, Synapses, and Communication
Neurons communicate with each other and other cells through specialized junctions called synapses, where plasma membranes of two cells come close together.
Two types of synapses occur, electrical and chemical.
Electrical synapses allow ionic currents to pass directly from one cell to the other through gap junctions so the action potential in the presynaptic neuron can directly trigger an action potential in the postsynaptic cell.
Electrical synapses have a very short delay and are common in many invertebrates.

2. Two principal kinds of synapses: electrical and chemical

3. Electrical synaptic transmission.

4. Gap junctions are formed where hexameric pores called connexons connect with one between cells

6. Electrical synapses are built for speed

7. Electrical coupling is a way to synchronize neurons with one another

8. Electrical synapses are not presently considered to be the primary means of communication between neurons in the mammalian nervous system, but they may prove to be more important than presently recognized

9. Rectification and uni-directionality of electrical synapses
…not just simple bidirectional bridges between cells
Conductance through gap junctions may be sensitive to the junctional potential (i.e the voltage drop between the two coupled cells), or sensitive to the membrane potential of either of the coupled cells
Glial cells can also be connected by gap junctions, which allows synchronous oscillations of intracellular calcium

10. Electrical Synapses
Gap junction-type communication important for rapidly synchronizing syncytia of cells as is observed in astrocytes, heart, and developing brains. Present in invertebrates to promote rapid defensive secretions.
Problems with electrical: difficult to modulate gating of channels (exceptions exist cAMP, pH).
Can't change sign, i.e. charge always flows "down hill.“
Electrical synaptic transmission requires that the presynaptic cell or terminal be larger than the postsynaptic cell for it to inject considerable charge, no real amplification mechanism.

11. Chemical Synaptic Transmission.
Definition: Communication between cells which involves the rapid release and diffusion of a substance to another cell where it binds to a receptor (at a localized site) resulting in a change in the postsynaptic cells properties.

12. Chemical transmission.
Contrary to electrical transmission multiple steps are required to release transmitter chemicals and for them to act on postsynaptic receptors, resulting in a time delay (can be as short as 0.1 msec).
Directional, select localization of release machinery to presynaptic terminals and receptors to postsynaptic specializations.
Can change sign by release of inhibitory transmitter.
Highly modulatable as it has many steps presynaptic terminal and at the postsynaptic sites.

13. Steps to chemical synaptic transmission
First need to bring the presynaptic neuron to threshold at axon hillock
Conduction down axon, length, R*C dependent
Opening of voltage gated Ca channels
Diffusion and action of Ca at release machinery
Exocytosis and diffusion of transmitter in cleft
Activation of postsynaptic receptors

14. Chemical synapses: the predominant means of communication between neurons

15. Types of Synapses CNS Synapses Axodendritic: Axon to dendrite
Axosomatic: Axon to cell body
Axoaxonic: Axon to axon
Dendrodendritic: Dendrite to dendrite
Gray’s Type I: Asymmetrical, excitatory
Gray’s Type II: Symmetrical, inhibitory

17. Synapse structure like real estate location, location, location!!
Multiple release sites NMJ

18. Criteria for a chemical transmitter
The transmitter substance must be synthesized in the presynaptic neuron.
It must be present in the presynaptic terminal and released in amounts sufficient to result in the level of response produced by the endogenous transmitter.
When applied exogenously the substance should mimic the effect of the endogenous transmitter.
A specific mechanism must exist for removing the transmitter from the synaptic cleft.

19. Criteria that define a neurotransmitter:
Must be present at presynaptic terminal
Must be released by depolarization, Ca++-dependent
Specific receptors must be present

20. Neurotransmitters may be either small molecules or peptides
Mechanisms and sites of synthesis are different
Peptides, or neuropeptides are synthesized in the endoplasmic reticulum and transported to the synapse, sometimes they are processed along the way. Neuropeptides are packaged in large dense-core vesicles
Small molecule transmitters are synthesized at terminals, packaged into small clear-core vesicles (often referred to as ‘synaptic vesicles’

21. Neurotransmitter is released in discrete packages, or quanta

22. Failure analysis reveals that neurons release many quanta of neurotransmitter when stimulated, that all contribute to the response
Quantal content:
The number of quanta released by stimulation of the neuron
Quantal size:
How size of the individual quanta

23. Quanta correspond to release of individual synaptic vesicles
EM images and biochemistry suggest that a MEPP could be caused by a single vesicle
EM studies revealed correlation between fusion of vesicles with plasma membrane and size of postsynaptic response

24. Quantal aspects of release at the neuromuscular junction
At the neuromuscular junction small spontaneous potentials (depolarizations) termed miniature end plate potentials are observed.
The amplitude of evoked responses (due to calcium influx) is always an integer multiple of the unitary response.
Shown that calcium increases the probability of observing a unitary response.
These data suggested the existence of transmitter quanta or packets.
Evoked transmission mediated by the release of ~ 150 quanta over a 1-3 ms period. Each quantum leads to about 0.5 mV depolarization.

25. Calcium influx is necessary for neurotransmitter release
Voltage-gated calcium channels

26. Calcium influx is sufficient for neurotransmitter release

27. Synaptic release II
The synaptic vesicle release cycle
Tools and Pools
Molecular biology and biochemistry of vesicle release:
Docking
Priming
Fusion
Recovery and recycling of synaptic vesicles

28. The synaptic vesicle cycle

29. How do we study vesicle dynamics? Morphological techniques
Electron microscopy to obtain static pictures of vesicle distribution; TIRFM (total internal reflection fluorescence microscopy) to visualize movement of vesicles close to the membrane
Physiological studies
Chromaffin cells
Neuroendocrine cells derived from adrenal medulla with large dense-core vesicles. Can measure membrane fusion, or direct release of catecholamine transmitters using carbon fiber electrodes
Neurons
Measure release of neurotransmitter from a presynaptic cell by quantifying the response of a postsynaptic cell
Genetics
Delete or overexpress proteins in mice, worms, or flies, and analyze phenotype using the above techniques

30. Synaptic vesicle release consists of three principal steps: Docking
Docked vesicles lie close to plasma membrane (within 30 nm)
Priming
Primed vesicles can be induced to fuse with the plasma membrane by sustained depolarization, high K+, elevated Ca++, hypertonic sucrose treatment
Fusion
Vesicles fuse with the plasma membrane to release transmitter. Physiologically this occurs near calcium channels, but can be induced experimentally over larger area. The ‘active zone’ is the site of physiological release, and can sometimes be recognized as an electron-dense structure. .

31. In CNS neurons, vesicles are divided into
Reserve pool (80-95%)
Recycling pool (5-20%)
Readily-releasable pool (0.1-2%; 5-10 synapses per active zone)
A small fraction of vesicles (the recycling pool) replenishes the RRP upon mild stimulation. Strong stimulation causes the reserve pool to mobilize and be released

32. Vesicle release requires many proteins on vesicle and plasma membrane

34. Priming
Vesicles in the reserve pool undergo priming to enter the readily-releasable pool
At a molecular level, priming corresponds to the assembly of the SNARE complex

35. Synaptotagmin functions as a calcium sensor, promoting vesicle fusion

36. Synaptic vesicles recycle post-fusion

37. Principles of Chemical Synaptic Transmission
Neurotransmitter Receptors and Effectors
Ionotropic: Transmitter-gated ion channels
Metabotropic: G-protein-coupled receptor

38. Principles of Chemical Synaptic Transmission
G-Protein-Coupled Receptors
Steps of neurotransmitter action
Bind to receptor proteins
Activate small proteins
Activate “effector” proteins
Second messengers
Metabotropic receptors: Metabolic effects
Same neurotransmitter: Different postsynaptic actions
ACh Effect: Heart, skeletal muscle

40. Principles of Chemical Synaptic Transmission
Autoreceptors
Presynaptic receptors sensitive to neurotransmitter released by presynaptic terminal
Act as safety valve to reduce release when levels are high in synaptic cleft (autoregulation)

41. Principles of Chemical Synaptic Transmission
Neurotransmitter Recovery and Degradation
Diffusion: Away from the synapse
Reuptake: Neurotransmitter re-enters presynaptic axon terminal
Enzymatic destruction inside terminal cytosol or synaptic cleft
Desensitization: e.g., AChE cleaves Ach to inactive state

42. Post synaptic potentials
IPSP: Transient hyperpolarization of postsynaptic membrane potential caused by presynaptic release of neurotransmitter
EPSP:Transient postsynaptic membrane depolarization by presynaptic release of neurotransmitter

43. Principles of Synaptic Integration
EPSP Summation
Allows for neurons to perform sophisticated computations
Integration: EPSPs added together to produce significant postsynaptic depolarization
Spatial: EPSP generated simultaneously in different spaces
Temporal: EPSP generated at same synapse in rapid succession

45. Principles of Synaptic Integration
Excitable Dendrites
Dendrites of neurons of voltage-gated sodium, calcium, and potassium channels
Can act as amplifiers (vs. passive)
Dendritic sodium channels: May carry electrical signals in opposite direction, from soma outward along dendrites

46. Principles of Synaptic Integration
Inhibition
Action of synapses to take membrane potential away from action potential threshold
Exert powerful control over neuron output

47. Principles of Synaptic Integration
IPSPs and Shunting Inhibition
Excitatory vs. inhibitory synapses: Bind different neurotransmitters, allow different ions to pass through channels
Membrane potential less negative than -65mV = hyperpolarizing IPSP
Shunting Inhibition: Inhibiting current flow from soma to axon hillock

48. Principles of Synaptic Integration
The Geometry of Excitatory and Inhibitory Synapses
Excitatory synapses
Gray’s type I morphology
Clustered on soma and near axon hillock
Inhibitory synapses
Gray’s type II morphology

49. Type I Excitatory
Type II Inhibitory

50. Principles of Synaptic Integration
Modulation
Synaptic transmission that modifies effectiveness of EPSPs generated by other synapses with transmitter-gated ion channels
Example: Activating NE β receptor
Suggest inserting figure 5.21 of NE example