Lec 12: Electrical Signals II

Question 1

AP propagation aka

Answer 1

(conduction)

Question 2

(AP propagation/conduction)

APs move…

Answer 2

down the axon from the point of origin (usually the Axon Hillock region)

Question 3

(AP propagation/conduction)

The depolarization at one region will

Answer 3

spread passively to surrounding regions

Question 4

(AP propagation/conduction)
- The magnitude of the initial depolarization is…

• This is known as…

Answer 4

• lower at sites distant from the initial site.

- “electrotonic spread” or “passive spread” and it’s distance is determined by the length (space) constant, Lambda

Question 5

(AP propagation/conduction)
- Lambda =

• and is determined by: (2)

Answer 5

= the distance it takes for the voltage response to decay to 37% of its original magnitude

1. ) Axon diameter
2. ) Presence or absence of myelination

Question 6

(AP propagation/conduction)
- Passive electrotonic spread of the depolarization is not

• The take home message is that

Answer 6

• AP conduction, but helps to explain it

- depolarizations spread passively to surrounding regions!

Question 7

(AP propagation/conduction of an Unmyelinated Axon)

5 Steps:

Answer 7

1. ) At the start, the membrane is completely polarized
2. ) (Passive depolarization spreads) When the action potential is initiated, a region of the membrane depolarizes. The adjacent regions become depolarized
3. ) When the adjacent region is depolarized to its threshold, an action potential starts there.
4. ) Depolarization occurs due to the outward flow of K+ ions. The depolarization spreads forwards, triggering an action potential. (The AP may be able to travel in reverse?)
5. ) Depolarization spreads forward, repeating the process

Question 8

(In the 5th step of AP propagation/conduction of an Unmyelinated Axon)
- The AP does not…

• because it is…
• However, passive spread is important in…

Answer 8

• fade (decay) like the passive spread of depolarization
• regenerated (in an “all or none” fashion) at successive regions along the axon
• setting up the AP from one region to the next region

Question 9

(Anatomy of myelinated axon)
- Made up of…

• Each sheath is…
• Acts to…
  & this: (2)
• Performed by: (2)
• The gaps between sheaths are the…
• Myelination allows for…
• Note that the ____ ____ __ ______ are present only at…

Answer 9

• Multiple intermittently spaced sheaths of membranes wrapped around the axon
• spaced evenly along the axon length.
• insulate
  1. ) Increases resistance
  2. ) Decreases capacitance

1. ) Oligodendrocytes in CNS
2. ) Schwann cells in PNS

• Nodes of Ranvier
• more rapid conduction velocities
• voltage-gated Na+ channels, the Node!

Question 10

(AP propagation/conduction of an Myelinated Axon)

5 Steps:

Answer 10

1. ) In myelinated neurons, an action potential is triggered at the axon hillock, just before the start of the myelin sheath. The depolarization then spreads along the axon.
2. ) Due to myelination, the depolarization spreads passively to the next node
3. ) The next node reaches its threshold, and a new action potential is generated
4. ) The cycle is repeated, triggering an action potential at the next node
5. ) The process continues.. resulting in Saltatory Conduction

Question 11

(In the 2nd step of AP propagation/conduction of an Myelinated Axon)
- The depolarization passively spreads…

• because…
• The passive spread is sufficient to…

Answer 11

• further along the axon in a myelinated vs. an unmyelinated axon
• the insulating myelin sheath prevents current leakage and effectively increases the lambda.
• bring the next node to above threshold level allowing for the AP to “skip” from node to node

Question 12

How is the electrical signal transmitted from cell to cell?

Answer 12

Synapses (located at the terminal branches of axon)

Question 13

2 Types of Synapses:

Answer 13

1. ) Electrical (cells are “electrically coupled”)

2. ) Chemical (uses a neurotransmitter)

Question 14

All synapses have at least __ cells

& what are they?

Answer 14

• 2 cells:

1. ) Presynaptic cell
2. ) Postsynaptic cell

(with Synaptic cleft = space in between the 2 cells)

Question 15

Synaptic cleft =

Answer 15

= space in between the 2 cells

Question 16

(Electrical Synapse)
1.) Involves…

2. ) Composed of…
3. ) The channel (“hemichannel”) of each pore (connexon) is…
4. ) Each pore is composed of…
5. ) Depolarization (AP) in one cell will…
6. ) Important in…

Answer 16

1. ) Gap Junctions (minimal synaptic cleft, 3.5 nm)
2. ) pores that connect the cytoplasm of one cell to the next
3. ) matched with the channel of a pore on the adjacent cell
4. ) 6 protein subunits known as a connexin
5. ) spread to the next electrically coupled cell
6. ) cardiac muscle and some CNS synapses

Question 17

(Chemical Synapse)
1.) AP in presynaptic cell (neuron) initiates…

2. ) Neurotransmitter (chemical ligand) diffuses…
3. ) Upon binding, the receptor initiates…

Answer 17

1. ) the release of neurotransmitter (from synaptic vesicles) into the synaptic cleft (at presynaptic membrane of terminal bulb).
2. ) across the cleft and binds to a specific receptor on the postsynaptic cell membrane (neuron or effector organ) (#2 in diagram).
3. ) an electrical event (depolarization or hyperpolarization) that begins at post synaptic membrane of the postsynaptic cell (could be a multistep process) (#3 & #4 in diagram).

Question 18

4 Types Different Neurotransmitters Discussed:

• Each presynaptic cell has only…
• Thus that cell is always either…

Answer 18

1. ) Acetylcholine
2. ) Catecholamines
3. ) Amino Acids & derivatives
4. ) Neuropeptides

• one main neurotransmitter
• excitatory or inhibitory

Question 19

Acetylcholine is important in…

excitatory or inhibitory?

Answer 19

• PNS

- (excitatory)

Question 20

Catecholamines function as…

excitatory or inhibitory?

Answer 20

• adrenalin-related compounds

- excitatory or inhibitory responses

Question 21

4 Types of Amino acids & derivatives (Neurotransmitters):

Where they function?
(excitatory or inhibitory?)

Answer 21

GABA: CNS (inhibitory)
Glycine: CNS (inhibitory)
Serotonin: CNS (excitatory)
Glutamate: CNS (excitatory

Question 22

Neuropeptides are…

• with Hundreds of…
• Frequently…
  Examples: (2)

Answer 22

• small peptides
• different molecules (have long lasting effects)
• coexist with other neurotransmitters
• Ex: endorphins, enkephalins

Question 23

Neurotransmitter Steps: (8)

Answer 23

1. ) An action potential arrives at the presynaptic terminal bulb, resulting in a transient depolarization
2. ) Depolarization opens voltage-gated calcium channels, allowing calcium ions to rush into the terminal
3. ) Increasing calcium in the terminal bulb induces the secretion of some neurosecretory vesicles.
4. ) Prolonged stimulation mobilizes additional, reserve vesicles.
5. ) Neurotransmitter diffused across the synaptic cleft to the receptors on the postsynaptic cell.
6. ) Binding of neurotransmitter to the receptor alters its properties
7. ) Channels open, letting ions flow into the postsynaptic cell. Depending on the ion, channel opening leads to either depolarization or hyperpolarization.
8. ) If sufficient depolarization occurs, an action potential will result in the postsynaptic cell

Question 24

(AP induced neurotransmitter release)

At presynaptic cell: (3)

Answer 24

1. ) AP causes voltage-gated Ca2+ channels in the membrane to open at the terminal bulb
2. ) Ca2+ diffuses into the cell and causes synaptic vesicles to fuse to the presynaptic membrane
3. ) Neurotransmitter is released into the cleft

Question 25

(AP induced neurotransmitter release) | At postsynaptic cell: (2)

Answer 25

1.) Neurotransmitter binds to its receptor 2. ) Induces a change in membrane conductance - Excitatory (EPSP – excitatory postsynaptic potential) - Inhibitory (IPSP – inhibitory postsynaptic potential) - EPSPs and IPSPs are graded potentials (not APs)

Question 26

Mechanism of synaptic vesicle docking and fusion =

Answer 26

“Stimulus-secretion Coupling” Regulated Secretion

Question 27

(Mechanism of synaptic vesicle docking and fusion) (“Stimulus-secretion Coupling” Regulated Secretion) The synaptic vesicles...

Answer 27

fuse with the presynaptic membrane via Ca2+-induced exocytosis

Question 28

3 main sets of vesicles are present in the terminal bulb: Each vesicle is associated with...

Answer 28

1. ) Undocked (far from the membrane) 2. ) Docked (close to the membrane) – held by tethering proteins 3. ) Docked and primed (close to the membrane and ready for release) ...several membrane bound proteins

Question 29

Particularly important parts on the vesicle are: (2)

Answer 29

1.) v-SNARES (vesicle SNAP receptor): Synaptobrevin (VAMP) (in this example) 2.) Synaptotagmin (the putative Ca2+ sensor)

Question 30

(Vesicle Docking (tethering)) | Priming involves the interactions of: (2)

Answer 30

1. ) v-SNAREs (vesicle-SNAP Receptor) | 2. ) t-SNARES (target-SNAP Receptors)

Question 31

(Synaptic Vesicle Priming) - Synaptic vesicles associate with... - Examples of t-SNAREs include... - Upon interaction of v-SNAREs and t-SNARES the vesicles are... - v-SNAREs bind to...

Answer 31

- the pre-synaptic membrane via the v-SNAREs and the t-SNAREs (located at the presynaptic membrane). - SNAP-25 and Syntaxin (SYX) - “primed” for release - t-SNAREs which positions the vesicle very close to the cell membrane

Question 32

(Vesicle Docking, Priming & Fusion) | 4 Steps:

Answer 32

1. ) The proper vesicle is recognized and bound by particular membrane-anchored tethering proteins 2. ) A Rab GTPase bound to the incoming vesicle stimulates association of v-SNARE with t-SNARE 3. ) Membrane fusion is promoted by the interaction of v-SNARE with t-SNARE 4. ) Binding of NSF and SNAPs promotes dissociation of the SNARE complex

Question 33

(Vesicle Docking, Priming & Fusion) Explained in 4 details: 1.) Tethering proteins help... 2. ) The v-SNARE interacts with... 3. ) The primed vesicle and membrane are thought to... 4. ) In neurons, step 3 (fusion) is...

Answer 33

1. ) hold the vesicles in position (docking) 2. ) the t-SNAREs (syntaxin and SNAP25) bringing the vesicle membrane in close proximity to the presynaptic membrane (priming) 3. ) possibly create an “unstable intermediate” or a “fusion stalk” 4. ) the Ca2+-dependent step

Question 34

(Vesicle Dynamics) 1.) The AP induces... 2. ) The Ca2+ binds to... 3. ) This triggers... 4. ) The contents of the vesicle (i.e., neurotransmitter) is...

Answer 34

1. ) an influx of Ca2+ 2. ) synaptotagmin 3. ) the vesicle membrane to fuse with the cell membrane. 4. ) released into the synaptic cleft.

Question 35

Toxins that interfere with vesicle release: (3)

Answer 35

1. ) Botulinum toxin (botox) 2. ) Tetanus toxin 3. ) Presynaptic blockers (inhibitors) of neurotransmission

Question 36

(Botulinum toxin (botox)) 1.) Causes... 2. ) From... 3. ) Botulinim toxins B,D,F and G will... 4. ) Botulinum toxins A & E will... 5. ) Botulinum toxin C1 will... 6. ) Results in...

Answer 36

1. ) botulism poisoning 2. ) the bacterium Clostridium botulinum 3. ) cleave synaptobrevin (VAMP, the v-SNARE) 4. ) cleave SNAP-25 (t-SNARE) 5. ) cleave Syntaxin (t-SNARE) 6. ) paralysis

Question 37

(Tetanus toxin) 1.) From the... 2. ) Will... 3. ) Results in...

Answer 37

1. ) bacterium Clostridium tetani 2. ) Cleave synaptobrevin (v-SNARE) 3. ) rigid spasms due to effects in the CNS

Question 38

After release, the neurotansmitters...

Answer 38

diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell.

Question 39

Neurotransmitter Receptor Classes: (2)

Answer 39

1. ) Ionotropic: the receptor is a channel 2. ) Metabotropic: the receptor communicates with a channel indirectly via some second messenger system (example G-protein)

Question 40

Ionotropic Receptor Examples: (2)

Answer 40

1. ) Nicotinic acetylcholine receptor (nAchR) | 2. ) GABAa receptor

Question 41

(Ionotropic Receptor Example/Nicotinic acetylcholine receptor (nAchR) 1.) Is a large... 2. ) Normal ligand = 3. ) Can also be... 4. ) Will... 5. ) Found at... 6. ) Can be bound and inhibited by: (3)

Answer 41

1. ) ligand-gated Na+ channel 2. ) = acetylcholine (neurotransmitter) 3. ) activated by nicotine (agonist) 4. ) Induce depolarization of postsynaptic cell (EPSP), (see ENa+) 5. ) the NMJ 6. ) - a-bungarotoxin - Cobratoxin - Postsynaptic blockers or inhibitors (antagonists) of neurotransmission

Question 42

(Ionotropic Receptor Example/GABAa receptor) 1.) Is a... 2. ) Normal ligand = 3. ) Opening of... 4. ) Result in... 5. ) Found in... 6. ) Valium (Diazopam) enhances... - Resulting in...

Answer 42

1. ) Ligand-gated Cl- channel 2. ) = GABA (neurotransmitter) 3. ) the Cl- channel leads to hyperpolarization (see ECl-) 4. ) an IPSP (decreasing the chance that the postsynaptic cell will fire and AP) 5. ) the brain 6. ) the effect of GABA by binding to the GABAA receptor (allosteric interaction) - CNS depression

Question 43

Metabotropic Receptor Example:

Answer 43

GABAb receptor (GABA is neurotransmitter)

Question 44

(Metabotropic Receptor Example/GABAb receptor) 1.) Is a... 2. ) Indirectly leads to... 3. ) Results in... 4. ) Found in... 5. ) Activation results in...

Answer 44

1. ) G-protein coupled receptor 2. ) opening of a K+ channel results in hyperpolarization (see EK+) 3. ) an IPSP (decreasing the chance that the postsynaptic cell will fire and AP) 4. ) the brain 5. ) CNS depression

Question 45

(Postsynaptic Potentials) 1.) The electrical potentials induced by... 2. ) Always... 3. ) Occur usually at... 4. ) May be... 5. ) Or may be... 6.) They are additive in... (+ 2 examples)

Answer 45

1. ) neurotransmitters binding to specific neurotransmitter receptors. 2. ) graded potentials (i.e., not APs) 3. ) Dendrites or Soma. 4. ) excitatory (depolarizing towards threshold) – Excitatory Postsynaptic Potentials (EPSPs). 5. ) inhibitory (hyperpolarizing away from threshold) – Inhibitory Postsynaptic Potentials (IPSPs) 6. ) time and space (temporal spatial summation) - Spatial Summation - Temporal Summation

Question 46

The activation state of the postsynaptic cell is determined by

Answer 46

the net influence of all of the excitatory and inhibitory influences that are active.

Question 47

Each IPSP and EPSP is a

Answer 47

graded potential that summate with all other inputs in the temporal spatial manner.

Question 48

- Incoming electrical signals at the dendrites and the soma are... - Not... - May be... - They spread to... - They decay with... - However, they are... - If a sufficient level of depolarization occurs in the soma and dendrites such that the axon hillock depolarizes to the threshold value, then...

Answer 48

- graded potentials (IPSP and EPSP) - APs - fairly large - adjoining regions passively - time and distance (Length constant, lambda/Time constant, t) - additive summation - an AP will be generated there and then propagated along the axon

Question 49

Why can’t the dendrites or soma fire APs?

Answer 49

???? | Therefore, the AP is generally initiated at the Axon Hillock???

Question 50

Review Neuron Regions (Functional Perspective) chart

Answer 50

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Question 51

Review Postsynaptic Potentials vs Action Potentials chart

Answer 51

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Question 52

1. ) Neurotransmitters must be... 2. ) In order to... 3. ) Example: 4. ) Located in...

Answer 52

1. ) inactivated (or eliminated) 2. ) prevent prolonged activation of the postsynaptic cell, neurotransmitters are degraded at the synapse 3. ) Ex: acetylcholinesterase cleaves Ach into Acetate and choline 4. ) the synaptic cleft or on the extracellular surface of the postsynaptic membrane

Question 53

- GABA & Glutamate can be... Take home message =

Answer 53

- transported into neurons and glia at synapses, eleminating them from the synaptic cleft. = Neurotransmitters can be removed from the synapse by transport into cells (neurons and glia). Not all neurotransmitters are simply degraded like Ach.