Lecture 5: Synaptic Transmission I

Front 1

What are the advantages and disadvantages of the Electrical vs. Chemical synapse? Which type is more prevalent?

Back 1

Electrical advantage: fast
Electrical disadvantage: no amplification, loss of signal, usually only excitatory

Chemical advantage: amplification, can excite or inhibit
Chemical disadvantage: kinda slow

Chemical is more prevalent

Front 2

______ signals are bidirectional and mediated by ________. __________ signals are unidirectional and mediated by ___________.

Back 2

Electrical
Gap junctions
Chemical
Presynaptic terminals and postsynaptic receptors

Front 3

Gap junctions are usually ___amers. The protein subunit is known as a _______ and is made up of ____(#) transmembrane segments. ____(#) of these subunits come together to form a _________, which is located on the ____(presynaptic or postsynaptic)____ side of the gap junction. The carboxy and amino termini are located on the _________ side. These channels are permeable to _________. Movement of ions is driven by _______.

Back 3

*Hex(amer)
*Connexin
*4
*6
*Connexon
*BOTH! (presynaptic and postsynaptic) → two hemichannels join together on each side to form the complete channel. Short extracellular loops are located at the interface of the two hemichannels to ensure proper alignment.
*Cytoplasmic (intracellular)
*It’s non-specific, so anything that is <1000 Da in size fits -- including ions and cAMP
*voltage

Front 4

What are the two main types of chemical receptors and how do they work? Which one is faster? Which one has more widespread effects on transcription and neuronal growth?

Back 4

Ionotropic (a.k.a. “direct gating”)
**Receptor and channel are the same macromolecular structure, so binding of a ligand opens the gate directly

Metabotropic
**Receptor has an indirect influence on the ion channel. The ligand binds, but an intracellular 2nd messenger (either G-protein coupled receptor or receptor tyrosine kinase) is required to open the gate.

Ionotropic is faster, but metabotropic has more widespread effects on transcription and neuronal growth.

Front 5

Describe the steps in the action at the neuromuscular junction. How long does the whole process take?

Back 5

1.) presynaptic action potential is conducted to the presynaptic axon terminal, where it INCREASES Ca++ PERMEABILITY
2.) Ca++ influx sets off the release of neurotransmitters (ACh) from the presynaptic terminal to the synaptic cleft via EXOCYTOSIS OF VESICLES.
3.) ACh DIFFUSES across cleft to postsynaptic membrane.
4.) ACh interacts with postsynaptic RECEPTORS (Nicotinic).
5.) Synaptic channels are activated. INWARD CURRENT (Na+) creates synaptic potential (the excitatory post-synaptic potential [EPSP])
6.) Postsynaptic AP leads to muscle contraction.

**Delay from presynaptic AP to postsynaptic AP is ~1-2 ms

Front 6

In the neuromuscular junction, _________ transmitters interact specifically with __________ receptors (as opposed to _______ receptors, another broad class of receptors activated by this same transmitter).

Back 6

Acetylcholine (ACh)
Nicotinic ACh receptor (nAChR)
Muscarinic

Front 7

The Nicotinic ACh receptor is a ______amer composed of ___(#) types of homologous subunits (name them). Each subunit contains one large _________ domain that serves as a binding site, four _________ domains (termed ______), and a short external __-terminus. ____(#) molecules of ACh must bind to the receptor in order for the channel to open. The _____ (#) binding sites are situated in the cleft between a ____ subunit and its ____ or ____ subunit neighbor. The ______ segment lines the ion-conducting pore.

Back 7

*Pent(amer)
*4 (2 alpha, 1 gamma, 1 delta) **Note “Homologous” means derived from the same ancestor gene.
*External
*Transmembrane
*m1-m4
*C (i.e. amino binding site)
*2
*2
*alpha
*gamma or delta
*m2

Front 8

What are two poisons that bind to the ACh receptor and prevent it from opening? Which one is reversible?

Back 8

Alpha-bungarotoxin – snake venom. Non-reversible.

d-tubocurarine – active ingredient in curare. Reversible.

***both bind to the active site on the ACh receptor and will lead to paralysis.

Front 9

Describe two diseases that involve the neuromuscular junction. (Hint: one is an autoimmune disorder and one is congenital.) Where are mutations usually found in the congenital disorder? What is the target of the autoimmune disorder?

Back 9

Myasthenia Gravis – is an autoimmune disorder that targets the extracellular N-terminus of the ACh receptor, which results in internalization of nicotinic ACh receptors. This leads to loss of muscle function over time.

Congenital myasthenic syndrome – results from a mutation in the binding domains of the ACh receptor (especially m2, the transmembrane span that forms the lining of the ion channel pore). As a consequence, the pores stay open for too long. This leads to excess Ca++ influx, which causes end-plate degeneration. The symptoms are very similar to myasthenia gravis.

Front 10

How is the NMJ post-synaptic ACh receptor different from your typical voltage-gated Na+ or K+ channel?

Back 10

It differs in its non-selectivity. The pore in the nAChR is much larger than the Na+ or K+ channel, so BOTH Na+ and K+ can pass through without discrimination. Ca++ can also pass through. Cl- will not pass through, though, due to the negative charge inside the cell.

Front 11

Describe the “plasticity” of neural cells.

Back 11

Synaptic connections and strength of the synapse are not fixed. They are constantly modified by experience, emotional states, disease, learning, etc.

Front 12

What are three instances when the body uses electrical signals instead of chemical signals?

Back 12

Anything that needs to happen quickly.

Ex:
1.) Defensive spinal cord reflexes (“escape reflexes”)
2.) Rapid eye movements (“saccadic eye movements”)
3.) Synchronization of large networks of cells in the CNS

Front 13

Which type of signal (chemical or electrical) is typically only excitatory? Which one typically leads to a loss in signal amplitude as it travels from pre-synaptic to post-synaptic?

Back 13

Electrical
Electrical

Front 14

__________ is a genetic disease resulting from a mutation in a connexin subunit, leading to demyelination from the lack of gap junctions between neurons in development.

Back 14

Charcot-Marie-Tooth Disease

Front 15

Describe the “geometric constraint” in electrical signaling?

Back 15

Since there is no signal amplification and considerable loss of signal as it travels from cell A to cell B (due to its bidirectional nature), the pre-synaptic cell must be larger than the post-synaptic cell in order for it to be able to generate a current large enough to set off an action potential.

Front 16

Though the other type of signaling is faster, this type of signal 1.) can be excitatory OR inhibitory, 2.) can maintain signal amplitude, and 3.) can occur from smaller to larger cells.

Back 16

Chemical (**note—electrical signals are only excitatory, lose signal amplitude, and are geometrically constrained to large → smaller cell signaling.)

Front 17

In _______ signals, the synapse is strongly polarized, whereas in _______ signals, there is no polarization.

Back 17

Chemical
Electrical

Front 18

A single muscle fiber is innervated by how many motor neurons? Where is the cell body of the neuron located?

Back 18

One neuron for one fiber.
Lamina IX (in the spinal cord)

Front 19

At the ______, myelination stops, but _____ cells are still surrounding. Vesicles are located in the ______, each filled with about _____(#) molecules of ACh. The ACh diffuses across the synaptic cleft and interacts with receptors located at the __________ of the motor end plate. ________ are located near the top and _________ are located deeper in the grooves. A loose collagen network lies on the __________, which does not act as a barrier or impede diffusion, but serves to cause a rapid end to the transmission of the action potential signal via the _________ housed in this network.

Back 19

End plate axon (a.k.a. NMJ)
Glial (Schwann)
Presynaptic axon terminal
5000
Junctional folds
ACh Receptors
Ion channels
Motor end plate (membrane btwn motor nerve and muscle cell)
Acetylcholinesterase

Front 20

In the 1950s, ________ did experiments to measure the membrane potential of the muscle at the point of nerve insertion. They measured a membrane voltage of ______ mV in the muscle cell relative to the outside. They then used ________ to reduce NMJ synaptic transmission in order to keep the probe from getting knocked away every time the muscle contracted. This allowed for the generation of a __________ that was not strong enough to initiate an ________. In this way, they could measure the _______ of the EPSP.

Describe what they found about the EPSP time response and propagation?

Back 20

Fatt and Katz
-90mV
Curare
Postsynaptic potential (EPSP)
Action Potential
Time course

The EPSP rapidly rose within ~1 ms, reached a peak value, then decayed slowly and passively in the next 5-10 ms back to resting potential. Further, by placing probes along the length of the muscle cell, they found that end-plate potential is NOT propagated like an action potential (it is NOT all-or-nothing). Instead, they saw “Decremental Conduction.” The charge was only actively flowing into muscle only at the site of contact (i.e. the synapse). The density of charge was greatest at the endplate (NMJ), then decreased in SIZE and SPEED along the length of the muscle cell as a result of passive leakage.

Front 21

Takeuchi & Takeuchi used _________ to study the ionic current that generates the excitatory post synaptic potential (this current is also known as ______). They found the time course of ______ is faster than the time course of ________.

Back 21

The voltage clamp
EPC (end-plate current)
Current flow
Voltage change

Front 22

Explain the bathtub analogy for the time courses of current and voltage.

Back 22

Remember V=Q/C. Current has a faster time course than voltage change because the current has to charge up the capacitor, and that takes time. The current is like the flow of water coming from the tap in a bathtub. Even though water is flowing, it takes a while for the level of water to rise (which represents voltage change). Likewise, when you pull the plug, it takes some time to empty the “capacitance” of the bathtub. The water level goes down (voltage change) slower than the water flows (current).

Front 23

Reminder: There is an equation that relates current to voltage change across a membrane (basically Ohm’s law).

Give the equation and explain it in words. What is the only variable that changes in a voltage-clamp experiment?

Back 23

I(EPSP) = g(EPSP) x [V(m) – E(EPSP)]

This equation tells you that the magnitude of current flowing through ion channels depends on conductance, which is the inverse of resistance (i.e. the number of channels that are open) AND on the driving force. The driving force is determined by the magnitude of voltage difference across the membrane [V(m)] and the chemical concentration gradient (E, equilibrium potential → the battery).

Conductance (g) is the only variable that will change. V(m) is fixed, the battery (E) is constant. So time course of the measured EPSP tells you about the opening and closing of channels.

Front 24

In a voltage-clamp study, _____ measured at the end plate shows the opening and closing of ACh receptors. Measured ______ at the end plate shows the charging and discharging of the membrane capacitors.

Back 24

Synaptic current (EPSP)
Potential (voltage)

Front 25

_____ is the electrical driving force of positive charge flow. _____ is the chemical battery, or gradient driving force.

Back 25

V(m)
EPSP

Front 26

Negative current means that positive charge __(enters or exits)____ the cell, which ___(depolarizes or polarizes)_____ the cell. In order for this to happen, there must be a driving force whereby ________ potential is more negative than _______ potential. Meahwhile, conductance is always ________. Therefore _______ is the only thing that determines the direction of the current.

Back 26

Enters
Depolarize
Membrane
Equilibrium (chemical)
Positive (or 0 for impermeant)
Driving force (i.e. Vm – E)

Front 27

The Nernst potential for sodium is around _____ mV. Hypothetically, since the membrane potential is ______, and the concentration gradient for sodium is ______, there should be a large _____ of Na+ ions when Na+ channels open (favoring the electrochemical gradient). This would result in a large _______ current until V(m) equals _____ mV, at which point net flux ceases and current direction changes (this potential is known as the __________).

Counter to what they expected, Takeuchi & Takeuchi found that the potential at which ion flux ceased was actually ____ mV. What did this tell them about he nature of NMJ channels?

Back 27

+55 mV
negative inside
more sodium outside
influx
negative (inward)
+55 mV
Reversal potential [E(rev)]
0

Since the reversal potential occurred at 0 mV instead of the expected +55 mV, the nAChR channel is not purely selective to Na+. It is also not purely selective to K+, because that reversal potential would occur around K+’s equilibrium potential, which is -80 to -90 mV. There is no ion with an equilibrium potential of 0. Thus, they must be cation non-selective channels (they let in K+ AND Na+ as well as Ca++). The equilibrium potentials for Na+ and K+ are MIXED to form the reversal potential of 0 mV.