1. Neural Zones

2. How Neurons connect

3. The Synapse A functional connection between surfaces
Signal transmission zone
Synapse – synaptic cleft, presynaptic cell, and postsynaptic cell
Synaptic cleft – space in between the presynaptic and postsynaptic cell
Postsynaptic cell – neurons, muscles, and endocrine glands
Neuromuscular junction – synapse between a motor neuron and a muscle

4. * * * * The Synapse Axon terminal: found in motor neurons
Axon varicosities: ie swellings. Arranged like beads on a string and contain neurotransmitter containing vesicles
En passant synapse: CNS. Consists of a swelling along the axon
Spine synapse: presynaptic cell connects with a dendritic spine on the dendrite of the postsynaptic cell * * * *

5. The Synapse
Axodentritic: between axon terminal of one neuron and the dendrite of another
Axosomatic: between the axon terminal of one neuron and the cell body of another
Dendrodendritic: between dendrites of neurons (often are electricla synapses)
Axoaxonic: between an axon terminal of a presynatpic neuron and the axon of a postsynaptic neuron. * * * *

6. Diversity of Signal Conduction
So far:
Electrotonic
Action potentials
Saltatory conduction
Chemical and electrical synapses

7. Diversity of Synaptic Transmission

8. Electrical and Chemical Synapses
Electrical synapse
Chemical synapse
Rare in complex animals
Common in complex animals
Common in simple animals
Rare in simple animals
Fast
Sloooooow
Bi-directional ↔
Unidirectional →
Postsynaptic signal is similar to presynaptic
Postsynaptic signal can be different
Excitatory
Excitatory or inhibitory

9. Electrical synapses cells connect via gap junctions
membranes are separated by 2 nm
gap junctions link the cytosol of two cells
provide a passageway for movement of very
small molecules and ions between the cells
gap junction channels have a large conductance
NO synaptic delay (current spread from cell to cell is instantaneous) - important in some reflexes
chemical synapses do have a significant delay ie slow
commonly found in other cell types as well i.e. glia
can be modulated by intracellular Ca2+ , pH, membrane voltage, calmodulin
clusters of proteins that span the gap such that ions and small molecules can pass directly from one cell to another  

10. More about electrical synapses
cells connect via gap junctions
- made up of 6 protein subunits arranged around a central pore, made up of the connexin protein
- the two sides come together to make a complete unit of 12 proteins around the central pore

11. Chemical Synapse Diversity
Vary in structure and location

12. Chemical Synapse most common type of synapse
electrical signal in the presynaptic cell is communicated to the postsynaptic cell by a chemical (the neurotransmitter)
separation between presynaptic and postsynaptic membranes is about 20 to 30 nm
a chemical transmitter is released and diffuses to bind to receptors on postsynaptic side
bind leads (directly or indirectly) to changes in the postsynaptic membrane potential (usually by opening or closing transmitter sensitive ion channels)
the response of the neurotransmitter receptor can depolarizes (excitatory postsynaptic potential; epsp) or hyperpolarizes (inhibitory postsynaptic potential; ipsp) the post-synaptic cell and changes its activity
significant delay in signal (1 msec) but far more flexible than electrical synapse  

13. More about chemical Synapses
Some types of chemical synapse include
Excitatory - excite (depolarize the postsynaptic cell
Inhibitory - inhibit (hyperpolarize the postsynaptic cell)
Modulatory - modulates the postsynaptic cells response to other synapses

14. General sequence of events* * * * * * *

15. General sequence of events
1. Nerve impulse arrives at presynaptic terminal
2. Depolarization causes voltage-gated Ca 2+ channels to open - increases Ca 2+ influx, get a transient elevation of internal Ca 2+ ~100 mM
3. Vesicle exocytosis - increase in Ca 2+ induces fusion of synaptic vesicles to membrane - vesicles contain neurotransmitters
4. Vesicle fusion to membrane releases stored neurotransmitter
5. Transmitter diffuses across cleft to postsynaptic side
6. Neurotransmitters bind to receptor either: i) ligand-gated ion channel or ii) receptors linked to 2nd messenger systems
7. Binding results in a conductance change - channels open or close or - binding results in modulation of postsynaptic side
Cont…….

16. General sequence of events
8. Postsynaptic response - change in membrane potential (e.g. muscle contraction in the case of a motorneuron at a neuromuscular junction)
9. Neurotransmitter is removed from the cleft by two mechanisms i) transmitter is destroyed by an enzyme such as acetylcholine esterase ii) transmitter is taken back up into the presynaptic cell and recycled e.g. - acetylcholine esterase, breaks down acetylcholine in cleft, choline is recycled back into the presynaptic terminal

17. Neurotransmitters Characteristics Synthesized in neurons
Released at the presynaptic cell following depolarization
Bind to a postsynaptic receptor and causes an effect

18. Neurotransmitters, Cont.
More than 50 known substances
Categories
Amino acids
Neuropeptides
Biogenic amines
Acetylcholine
Miscellaneous …..
Neurons can synthesize many kinds of neurotransmitters

19. Neurotransmitters

20. Neurotransmitters cont.

21. Neurotransmitter Action
Inhibitory neurotransmitters
Cause hyperpolarization
Make postsynaptic cell less likely to generate an AP
Excitatory neurotransmitters
Cause depolarization
Make postsynaptic cell more likely to generate an AP

22. Amount of Neurotransmitter
Influenced by AP frequency which influences Ca2+ concentration
Control of [Ca2+]
Open voltage-gated Ca2+ channels  [Ca2+]
Binding with intracellular buffers  [Ca2+]
Ca2+ ATPases  [Ca2+]
High AP frequency  influx is greater than removal  high [Ca2+]  many synaptic vesicles release their contents  high [neurotransmitter]

23. Influenced by neurotransmitter amount and receptor activity
Signal Strength
Influenced by neurotransmitter amount and receptor activity
Neurotransmitter amount: Rate of release vs. rate of removal
Release: due to frequency of APs
Removal
Passive diffusion out of synapse
Degradation by synaptic enzymes
Uptake by surrounding cells
Receptor activity: density of receptors on postsynaptic cell

24. Ca2+ Regulates Neurotransmitter Release

25. Graded Potentials via Neurotransmitters
Vary in magnitude depending on the strength of the stimulus
e.g., more neurotransmitter  more ion channels will open
Can depolarize (Na+ and Ca2+ channels) or hyperpolarize (K+ and Cl- channels) the cell

26. Graded Potentials

27. Graded Potentials Travel Short Distances

28. Neurotransmitter Receptor Function
Ionotropic
Ligand-gated ion channels
Fast
e.g., nicotinic ACh
Metabotropic
Channel changes shape
Signal transmitted via secondary messenger
Ultimately sends signal to an ion channel
Slow
Long-term changes

29. Second Messenger again
When activated by a ligand the catalytic domain starts a phosphorylation cascade
Named based on the reaction catalyzed

30. Second Messengers to know

31. Neurotransmitter receptors
Different types of neurotransmitter receptors
Functional Type Ligand Ion Channel
Excitatory Receptors Acetylcholine Na+/K+ 
Glutamate Na+/K+; Ca2+ 
Glutamate Na+/K+ 
Serotonin Na+/K+
Inhibitory Receptors Aminobutyric acid, GABA Cl- 
Glycine Cl-

32. Removal of Neurotransmitter
broken down by enzyme
- acetylcholine esterase breaks down acetylcholine in the synaptic cleft
- many nerve gases and insecticides work by blocking acetylcholine esterase – Yikes!
- prolongs synaptic communication
b) recycled by uptake
- most neurotransmitters are removed by Na+/neurotransmitter symporters
- due to a specific neurotransmitter transporter
- recycled by uptake into presynaptic terminal or other cells   (glial cells will take up neurotransmitters)
c) diffusion: simple diffusion away from site  

33. Neurotransmitters - stages
Synthesis
- all small chemical neurotransmitters are made in the nerve terminal
- responsible for fast synaptic signalling
- synthetic enzymes + precursors transported into nerve terminal
- subject to feedback inhibition (from recycled neurotransmitters
- can be stimulated to increase activity (via Ca2+ stimulated phosphorylation)
2. Packaging into vesicles
- neurotransmitters packaged into vesicles
- packaged in small "classical" vesicles
- involves a pump powered by a pH gradient between outside and inside of vesicle
- pump blocked by drugs and these block neurotransmitter release

34. Presynaptic vesicles
Two groups  i) low molecular weight, non-peptide  e.g. acetylcholine, glycine, glutamate
 ii) neuropeptide (over 40 identified so far and counting…..)  

35. Presynaptic vesicles There are 2 types of secretory vesicles
We will only talk about small chemical synaptic vesicles
Neuropeptides are made and packaged in the cell body and transported to synapse)
Small chemical neurotransmitter vesicles
responsible for fast synaptic signaling
store non-peptide neurotransmitters,   e.g. acetylcholine, glycine, glutamate
enough vesicles in the typical nerve terminal to transmit a few thousand impulses
exocytosis only occurs after an increase of internal Ca 2+ (due to depolarization) and at active zones (regions in the presynaptic membrane adjacent to the cleft)  

36. Presynaptic vesicles * * * * * * * *

37. Vesicle Exocytosis
A group of 6 to 7 proteins work together to respond to Ca 2+ influx and regulate vesicle fusion
after exocytosis the synaptic vesicle membranes are reinternalized by endocytosis and reused (reloaded with neurotransmitter by a transmitter transporter system)
vesicles are also transported from the cell body to the nerve terminal - transmitter is synthesized in the terminal and loaded into the vesicles - enzymes and substrates necessary are present in the terminal - i.e. acetylcholine, acetyl-CoA + choline used by choline acetyltransferase 

38. Vesicle Exocytosis non-peptide transmitters
exocytosis only occurs after an increase of internal Ca 2+ (due to depolarization)
at active zones (regions in the presynaptic membrane adjacent to the synaptic cleft)
peptide-transmitters (same as for non-peptide transmitters except:)
exocytosis is NOT restricted to active zones
exocytosis is triggered by trains of action potentials  

39. SNARE hypothesis
The SNARE Hypothesis for Transport Vesicle Targeting and Fusion
SNARE is an acronym for SNAP receptor (SNAP stands for soluble N-ethylmaleimide-sensitive factor attachment proteins).
SNARES are involved in the mediation of protein transport between
various plant organelles by small membrane vesicles.
Two families:
i) V-SNARE - vesicle membrane proteins ii) T-SNARE - target membrane proteins

40. SNARE hypothesis
Vesicle docking occurs between the V-SNARE and T-SNARE proteins
The combined proteins act as a receptor for an ATPase that utilizes ATP to generate the "docked" form
One of the proteins is a Ca2+ sensor such that when Ca2+ enters the synapse the vesicle fuses with the plasma membrane and releases its contents
The membrane and proteins are then recycled through endocytosis (clatharin coat and dynamin etc.) and reused.

41. Synaptic Plasticity
Change in synaptic function in response to patterns of use
Synaptic facilitation –  APs   neurotransmitter release
Synaptic depression –  APs   neurotransmitter release
Post-tetanic potentiation (PTP) – after a train of high frequency APs   neurotransmitter release

42. Long-term potentiation

43. Diversity of Signal Conduction
So far:
Electrotonic
Action potentials
Saltatory conduction
Chemical and electrical synapses
Also:
Shape and speed of action potential
Due to diversity of Na+ and K+ channels

44. Ion Channel Isoforms Multiple isoforms Encoded by many genes
Variants of the same protein
Voltage-gated K+ channels are highly diverse (18 genes encode for 50 isoforms in mammals)
Na+ channels are less diverse (11 isoforms in mammals)

45. Higher density of voltage-gated Na+ channels
Channel Density
Higher density of voltage-gated Na+ channels
Lower threshold
Shorter relative refractory period

46. Voltage-Gated Ca2+ Channels
Open at the same time or instead of voltage-gated Na+ channels
Ca2+ enters the cell causing a depolarization
Ca2+ influx is slower and more sustained
Slower rate of APs due to a longer refractory period
Critical to the functioning of cardiac muscle

47. Chemical synapses: post-synaptic mechanisms

48. Postsynaptic Membranes and ion channels
Ligand gated ion channels – a review
Resting K+ channels: responsible for generating the resting potential across the membrane
Voltage- gated channels: responsible for propagating action potentials along the axonal membrane
Two types of ion channels in dendrites and cell bodies are responsible for generating electric signals in postsynaptic cells.
c. Has a site for binding a specific extracellular neurotransmitter
d. Coupled to a neurotransmitter receptor via a G protein.

49. More things to know about Ion channels
All the ion channels in question have a common feature
A pore that allows the ion(s) in question to flow across the lipid bilayer
The pore is specific to a certain ion or ions
Leak K+ channel only allows K+ ions to flow across the membrane
Examples: Acetylcholine (ACH) receptor allows Na+ to flow and the glycine receptor allows Cl- to flow through the channel
Ligand gated ion channels are different than voltage-gated ion channels in that they are chemically gated ie via neurotransmitters
Binding a small chemical triggers the opening of the ion channel
i) Na+ channels - excitatory (generates an excitatory postsynaptic potential) ii) Cl- channels - inhibitory (generates an inhibitory postsynaptic potential)  
Important: The specificity of a transmitter response is a function of the receptor type NOT the transmitter itself. (i.e. Ach can be excitatory when binding to one type of AchR (NMJ)) and inhibitory when binding to another type of receptor

50. Acetylcholine
Primary neurotransmitter at the vertebrate neuromuscular junction

51. Acetylcholine – general info
Motor neuron transmitter at the neuromusccular junction (NMJ) in vertebrates
Present in brain (10% of synapses)
Packaged in high numbers in vesicles 1,000 to 10,000 molecules per vesicle at the NMJ
Like all small chemical transmitters Ach is synthesized and packaged into vesicles in the synapse
The NMJ pre-synaptic side is packed full of vesicles in the axon terminal
Many vesicles are released per action potential to ensure a large safety margin so that the muscle fiber (i.e. the postsynaptic cell) will depolarize to beyond threshold.

52. Acetylcholine – receptor
Officially called the nicotinic ACH receptor (nAChR) because nicotine binds to this receptor and activates it
Ligand gated ion channel
has a depolarizing effect because Na+ is the dominant ion through these channels

53. Acetylcholine – receptor
generates an excitatory postsynaptic potential which at the NMJ (motor end plate) is often called an "end plate potential“

54. EPP - end plate potential
Aka Excitatory Junctional Potential (EJP)
End plate potentials (EPPs) evoked by stimulation of a motor neuron are
normally above threshold and therefore produce an action potential
in the postsynaptic muscle cell.

55. nACHR – a closer look
Most of the mass of the protein protrudes from the outer (synaptic) surface of the plasma membrane
The M2 alpha helix (red) in each subunit is part of the lining of the ion channel
Aspartate and glutamate side chains at both ends of each M2 helix form two rings of negative charges that help exclude anions from and attract cations to the channel. The gate, which is opened by binding of acetylcholine, lies within the pore.
Aspartate

56. The Neuromuscular junction

57. The Neuromuscular junction

58. The Neuromuscular junction
Arrival of an action potential at the terminus of a presynaptic motor neuron induces opening of voltage-gated Ca2+ channels
subsequent release of acetylcholine, which triggers opening of the ligand-gated nicotinic receptors in the muscle plasma membrane
The resulting influx of Na+ produces a localized depolarization of the membrane
leading to opening of voltage-gated Na+ channels and generation of an action potential * * *

59. Synapses in the brain or central nervous system (CNS)
A single synapse on a target is seldom found in brain
Large neurons in the brain typically receive many inputs (1000 to 80,000 per cell)
The inputs are integrated in the receiving neuron such that a "decision" is made to pass on the information onto other cells  - this "decision" is often whether or not to generate an action potential
each synaptic input usually only gives only a small depolarization, so many inputs must cooperate (summate) to reach threshold to fire an action potential

60. EPSP
An excitatory impulse, an excitatory post-synaptic potential raises the membrane potential above rest
An excitatory impulse at a synapse on the soma causes a depolarization of the whole soma including the beginning of the axon. The beginning of the axon is also known as the spike initialization zone or axon hillock and is packed with Na+ channels, an epsp of +15 to +20 mV triggers an action potential in the zone
2. Due to the absence of voltage-gated Na+ channels in the soma and dendrite of most neurons it is very unlikely that an action potential will be generated in these regions  

61. IPSP
An inhibitory impulse is called an ipsp (inhibitory post-synaptic potential) and lowers the membrane potential below rest (hyperpolarizes)
Synaptic transmission triggers the opening ligand gated Cl- channels or indirectly through other mechanisms the opening of K+ channels
Cl- flows into the cell
K+ flows out of the cell
Both increase the negative charge within the cell, hyperpolarizes the soma
Brings membrane potential further away from threshold and so it is harder to trigger an action potential therefore inhibitory
An ipsp on the dendrite will have less effect due to current loss than an ipsp in the soma 

62. CNS Major ligand gated ion channels and their neurotransmitters
Glutamate - amino acid
Most common excitatory neurotransmitters in central nervous system
Neurotransmitter of NMJ in invertebrates (locust, giant axon of squid)
Glutamate receptor - at least 3 different ligand gated ion channel receptors for glutamate - all generate epsps as Na+ is the dominant ion that flows after the channel is open  
GABA - aminobutyric acid
Major inhibitory neurotransmitter in the brain
In some areas of cortex 1 in 5 neurons are GABAergic
GABA receptors - again many different types of receptors - the more common GABA receptors are Cl - channels - usually inhibitory causes an inhibitory postsynaptic potential (IPSP) - reversal potential is the same as ECl - usually around - 70 mV
Note: reversal potential is synonymous with equilibrium potential  
cont…..

63. CNS Glycine - simplest amino acid
Major inhibitory neurotransmitter in the brainstem and spinal cord
Glycine Receptor - major receptor is a Cl - channel - inhibitory - like GABA receptor in that usually causes IPSPs - blocked by strychnine (rat poison) which literally causes convulsions and death as now the motor neurons are not inhibited and the muscles contract without control. Yikes! 

64. Cable properties again
Dendrites extend 0.5 to 1 mm in all directions from soma and receive signals from a large area
80-90% of all presynaptic terminals terminate on dendrites
Most can't produce action potentials (too few or no Na+ channels)
Transmit current by passive spread down dendrites to the soma
Therefore the membrane potential decreases as move along dendrite due to current loss thanks to our friends ri, rm and cm
Dendrites have no voltage gated Na+ channels and cell bodies ie soma have little or no voltage-gated Na+ channels current flow is solely dependent on the Cable Properties of the dendrites and soma

65. Things to remember
1) loss of current across membrane (leaky membranes)
dependent on the internal resistance (ri) and the membrane resistance (rm)
the length or space constant describes this property
2) loss of current (charge) due to capacitance properties of the membrane
cell membrane acts as a capacitor
it takes time and current (charge) to charge the membrane capacitor
the time constant describes this effect   τ = Rm x Cm  
details are in the lecture on cable properties 

66. Summation - CNS
The postsynaptic effects of most synapses in the brain are not as large as those at the neuromuscular junction
In the CNS the postsynaptic potentials are usually far below the threshold for generating postsynaptic action potentials
Neurons in the central nervous system are typically innervated by thousands of synapses, and the postsynaptic potentials produced by each active synapse can summate together (in space and in time) to bring the membrane to threshold for firing an action potential

67. Motor neurons and summation
Each motor neuron synapses with multiple muscle fibers
The motor neuron and the fibers it contacts defines the motor unit

68. Summation
Summation of multiple epsps to bring the membrane potential to threshold
for an action potential.

69. Summation
A microelectrode records the postsynaptic potentials produced by the activity of two excitatory synapses (E1 and E2) and an inhibitory synapse (I)
Electrical responses to synaptic activation
Stimulating either excitatory synapse (E1 or E2) produces a subthreshold EPSP, whereas stimulating both synapses at the same time (E1 + E2) produces a suprathreshold EPSP that evokes a postsynaptic action potential
Activation of the inhibitory synapse alone (I) results in a hyperpolarizing IPSP
Summing this IPSP with the EPSP produced by
one excitatory synapse (E1 + I) reduces the
amplitude of the EPSP, while summing it with the
suprathreshold EPSP produced by activating
synapses E1 and E2 keeps the postsynaptic
neuron below threshold, so that no action
potential is evoked.

70. Summation