Action potentials and synaptic transmission

Question 1

Define threshold potential.

Answer 1

The level of polarization at which the chance of an action potential is 50%.

Question 2

What are the steps of an action potential for voltage gated Na channel (start from closed)?

Answer 2

Closed:
Open: stimuli causes the channel to open (or not, for our case it opens) Opening of channels causes depolarization, causing more channels to open (positive feedback loop).
Inactivated: once the channel is open it’s binding site is exposed to the inactivator. This binds in less then a second. Inactivation ends after 1-2 ms spent at a more negative voltage.
Closed: After enough time at a more negative voltage, channels are in the closed configuration again.

Question 3

Does every voltage gate fire in an action potential? How many gates does a nerve cell have?

Answer 3

no.

1000’s.

Question 4

What is the Hodgkin Cycle and what does it describe?

Answer 4

Na+ influx causes membrane depolarization, which causes more gates to open increasing p(na) (permeability of na), which causes a Na+ influx.
It describes temporary transient positive feedback loop seen in action potentials.

Question 5

Define non-decrementing propagation. Does it apply to action potentials?

Answer 5

as the signal is propagated it’s amplitude doesn’t decrease. Yes.

Question 6

Name the three states of being for VGSCs.

Answer 6

Note: the pore is on the cytosolic side.
Closed: The pore is closed.
Open: the pore is open.
Inactivated: The pore is open, inactivation complex is blocking.

Question 7

Where is the voltage sensing S4 segment in each of the three states for a VSGC?

Answer 7

Closed: The positive S4 segment is pushed away from the positive voltage outside of the cell.
Open: the S4 segment is drawn towards the more negative voltage outside of the cell.
Inactivated: the S4 segment is drawn towards the more negative voltage outside of the cell.`

Question 8

Inactivation causes the unidirectional motion of an action potential. T or F?

Answer 8

T

Question 9

To ‘recover’ from inactivation what must happen? What factor dictates speed of recovery?

Answer 9

in order for the channel to go from the inactivated to tthe close conformation the channel must enter a negative potential environment ~60 mV. The more negative the environment the faster the recovery will be.

Question 10

About how long does recovery from inactivation take?

Answer 10

1-2 ms

Question 11

Tetrodotoxin (ttx) does what? Is found where?

Answer 11

It blocks the Na channel by plugging the extracellular side of the pore.
It is found in many things, including blue ringed octopus, pufferfish, and rough-skinned newt.

Question 12

What produces TTX?

Answer 12

Symbiotic bacteria, like psedoalteromonas tetradonis.

Question 13

What is the Red tide caused by? What are its implications?

Answer 13

The red tide is caused by Dinoflagellates (marine plankton bloom). Dinoflagellates produce Saxitoxin (STX), which acts like TTX. It concentrates in shellfish because they are filter feeders. It also kills fish.

Question 14

What does saltatory mean?

Answer 14

Latin for to hop to leap.

Question 15

How are Schwann cells involved in signaling?

Answer 15

They wrap around the axon, and lay myelin an isolator. This insulation makes it so that if current from a voltage appears on one end of a myelin sheath it will travel through the insulated surface to the next node.

Question 16

Schwann cells are the _____ cells of the peripheral nervous system.

Answer 16

Glial

Question 17

How does a capacitor work?

Answer 17

Two conductors are separated by an insulator (dielectric). If charge builds up on one plate it will push away similar charges are the other plate. This causes a build up of opposite charges on the two plates.

Question 18

How does capacitance effect action potentials in terms of sodium?

Answer 18

There are two way capacitance could be affecting action potentials. Either the negative resting membrane potential, pushes away negative ions on the other side, and sodium follows, to some degree, follows the flow of negative charge away from the plate, thus lowering extracellular sodium and consequently make it so action potentials are slower.

Question 19

Myelination:

• ______ electrical capacitance
• ______ electrical resistance
• ______ membrane conductance
• ______ propagation velocity
• ______ the metabolic energy expenditure of the cell (from production of Na channels and from decreased need to use the Na/K pump.

Answer 19

Decreases capacitance
Increases resistance
Decreases conductors
Increases propagation velocity
Decreases metabolic expenditure

Question 20

Multiple Sclerosis (MS) is caused by what?

Answer 20

It caused by the immune system attacking myelin sheaths.

Question 21

Where do sodium channels exist in a myelinated cell?

Answer 21

At the Nodes of Ranvier exclusively.

Question 22

Where do K+ channels predominate in the myelinated cell?

Answer 22

In the myelinated areas, with the exception of the point where the myelin attaches (which is immediately adjacent to the nodes of ranvier).

Question 23

What is the extracellular protein named Caspr? Where might it be found?

Answer 23

Caspr is essentially a molecular glue to anchor the myelin to the membrane. It is found attaching the myelin to points adjacent to the nodes of ranvier

Question 24

Given that K+ channels are found in areas myelinated by glial and schwann cells, what must these cells do?

Answer 24

These cells must absorb the K+ that comes from nerves K+ channels. (there is also some chance that they arrange the ions to be the correct concentration in this space, that’d be cool but I have no evidence)

Question 25

Action potentials occur only at the _____ in myelinated nerves. However a ________ depolarization is passively ______ from one Node to the next Node beneath the intervening myelin sheath. Define this.

Answer 25

Nodes
non-regenerating
conducted.
This means that while only the nodes will have an action potential, flow of current will occur because of the build up of charge (at the node) in front of a insulated system.

Question 26

There are two types of synapses, how do they differ and what are they.

Answer 26

Chemical synapse: involves a neurotransmitter being excreted across the synapse.
Electrical synapse: are synapses where gap junctions (membranes that connect to the next cell) directly carry the Na action potential/current(sort of current) to the next cell in a continuous motion.

Question 27

There are two types of neurotransmitter receptors, iontropic and ______. How do they work?

Answer 27

iontropic channels have an ion channel as part of the same membrane protein to which the neurotransmitter first bound.
Metabotropic Receptors: will not have an ion channel in the same protein to which the neurotransmitter first bound. They may often still end up opening a channel, but it will be done indirectly.

Question 28

Local anesthesia blocks what type of channel?

Answer 28

Voltage gated na channels.

Question 29

Describe a neuromuscular junction (synapse) from the action potential on the presynaptic cell to the release of intracellular Ca in the sarcoplasmic reticulum of the postsynaptic cell.

Answer 29

The action potential arrive via voltage gated sodium channels at the synapse. There the depolarization triggers opening of voltage gated calcium channels. Calcium signals the release of Acetylcholine from the presynaptic cell. Acetylcholine binds to a acetylcholine gated cation channel, which in turn causes the cell to depolarize, activating voltage gated sodium channels which will send action potentials down the T tubules. These action potentials will trigger voltage gated Ca channels inside the T tubule which are actually physically linked to the Ca channels in the sarcoplasmic reticulum and will cause them to open. Releasing intracellular calcium.

Question 30

What is the most common affect seen in the postsynaptic cell after a neurotransmitter release?

Answer 30

The most common affect is the depolarization of the postsynaptic cell.

Question 31

What are synaptic vesicles? where do they concentrate? How concentrated?

Answer 31

They are vesicles filled with neurotransmitter which are concentrated in the presynaptic cell, they are packed tight in there!

Question 32

What is the synaptic cleft?

Answer 32

It is simply the extracellular space between the presynaptic and post synaptic cell.

Question 33

What is the ‘synaptic density’?

Answer 33

‘The synaptic density’ is a super strange name for a series of functional proteins concentrated at the synapse which dye a very dark color.

Question 34

All neurotransmitters are enclosed by an intracellular lipid bilayer and follow regulated exocytosis but what type?

Answer 34

Lipid soluble neurotransmitters, like endogenous cannabinoid anandamide, or NO gas. This will simply go through membranes.

Question 35

What are the names of two lipid soluble neurotransmitters?

Answer 35

Endogenous cannabinoid anandamide and NO gas.

Question 36

What is a quantum of neurotransmitter?

Answer 36

A single vesicle of neurotransmitter. All vesicles within a single cell will be roughly the same size and have the same amount of neurotransmitter. Size and amount of neurotransmitter will differ between synapsis however (all cells quanta won’t be the same size).

Question 37

The magnitude of a postsynaptic response corresponds to the number of quanta. T or F.

Answer 37

T

Question 38

Reticularist believe…

Answer 38

That the membranes of the post synaptic cell and the presynaptic cell are connected. They’re wrong. Golgi was a reticularist.

Question 39

Neuronalists believe…

Answer 39

That the cells are not connected by membranes. They are pretty much right (gap junction being the exception).

Question 40

______ proved that neuronalists were right using ______, a dedicated reticularist’s dye.

Answer 40

Santiago Cajal

Golgi