Lecture 8

Question 1

what is spatial summation

Answer 1

• occurs when the currents from nearly simultaneous graded potentials combine

Question 2

explain how summation of several subthreshold signals results in an action potential

Answer 2

1. 3 excitatory neurons fire, their graded potentials separately are all below threshold
2. graded potentials arrive at trigger zone together and sum to create a supra-threshold signal
3. an action potential is generated

Question 3

explain how one inhibitory postsynaptic potential (IPSP) sums with 2 excitatory postsynaptic potentials (EPSPs) to prevent an action potential in the postsynaptic cell

Answer 3

1. one inhibitory and two excitatory neurons fire
2. the summed potentials are below threshold, so no action potential is generated

Question 4

what is temporal summation and what causes no summation and summation

Answer 4

• occurs when 2 graded potentials from one presynaptic neuron occur close together in time
  no summation - 2 subthreshold graded potentials will not initiate an action potential if they are far apart in time
  summation causing action potential - if 2 subthreshold potentials arrive at the trigger zone within a short period of time, they may sum and initiate an action potential

Question 5

explain the steps of membrane potential changes during an AP

Answer 5

1. resting membrane potential
2. depolarizing stimulus
3. membrane depolarizes to threshold, voltage gated Na+ and K+ channels being to open
4. rapid Na+ entry depolarizes cell
5. Na+ channels inactivate and slower K+ channels open
6. K+ moves from cell to extracellular fluid
7. K+ channels remain open and additional K+ leaves cell, hyperpolarizing it
8. voltage gated K+ channels close, less K+ leaks out of the cell
9. cell returns to resting ion permeability and resting membrane potential

Question 6

what is overshoot and undershoot

Answer 6

overshoot = region above 0 mV
undershoot = region under RMP

Question 7

describe how ion permeability changes during an action potential

Answer 7

• Na+ channels open and close faster than K+ channels
• rapid rise in PNa drives rising phase of action potential
• rise in Pk drives falling phase

Question 8

explain how the rising phase is driven by positive feedback loop of Nav channel activation

Answer 8

• depolarization triggers->
• Na+ channel activation gates open rapidly ->
• Na+ enters the cell ->
• to stop cycle slower Na+ channel inactivation gate closes ->
• more depolarization

Question 9

explain how the voltage gated Na+ channel gating works

Answer 9

1. at the resting membrane potential, the activation gate closes the channel (deactivated state) (-70mV)
2. depolarizing stimulus arrives at the channel, activation gate opens (-55mV)
3. with activation gate open, Na+ enters the cell
4. inactivation gate closes and Na+ entry stops (Inactivated) (+30mV)
5. during repolarization caused by K+ leaving the cell, the two gates reset to their original positions (only repolarization can remove the inactivation gate)

Question 10

what is a refractory period

Answer 10

• limits how soon after an AP another can be triggered

Question 11

what is absolute refractory period

Answer 11

• majority of Na channels are inactivated
• no stimulus can trigger another AP
• excitability = 0

Question 12

what is relative refractory period

Answer 12

• some Na channels recovered from inactivation but K channels still open
• stronger stimulus required to account for fewer available Na channels and hyperpolarizing efflux
• excitability is recovering

Question 13

what is local current flow

Answer 13

• when a section of axon depolarizes, positive charges move by local current flow into adjacent sections of the cytoplasm
• on the extracellular surface, current flows toward the depolarized region
• charges more down electrical gradient
• biological current is defined as the movement of positive charges
• Na+ entry = inward current
• K+ exit = outward current

Question 14

explain how charge movement can be described by local current flow

Answer 14

I=V/R
- current flows faster and farther along large cells (decrease Ri) with few leak channels
- (increase Rm) + myelin

Question 15

explain AP conduction in unmyelinated axons

Answer 15

• action potential is regenerated as current moves along axon and activated Na channels
• although current can move backwards, AP does not due to refractory period
  1. a graded potential above threshold reached the trigger zone
  2. voltage gated Na+ channels open and Na+ enters the axon
  3. positive charge flows into adjacent sections of the axon by local current flow
  4. local current flow from the active region causes new sections of the membrane to depolarize
  5. the refractory period prevents backward conduction, loss of K+ from the cytoplasm repolarizes the membrane

Question 16

explain the concept of myelinated axons and saltatory conduction

Answer 16

• action potentials appear to jump from one node of ranvier to the next, only the nodes have Na+ voltage gated channels
• high density of Na+ channels at the nodes of ranvier
• no Na+ channels present along sections of axon covered by myelin sheath

Question 17

myelin is an insulator and enhances action potential propagation because

Answer 17

1. it increases membrane resistance (Rm), enhancing current flow along the axon
2. action potentials are only generated at nodes of ranvier, not slowed down by channel opening in between
3. it decreases membrane capacitance

Question 18

what is membrane capacitance

Answer 18

• a measure of how much charge needs to be separated across the membrane to produce a given voltage

Question 19

how does myelin decrease capacitance

Answer 19

• membrane is like a capacitor
• 2 layers of conducting material (ECF and ICF) separated by a thin layer of insulator (phospholipid cell membrane)
• myelin sheath increases the distance between ECF and ICF to decrease capacitance
• less current needs to be separated to cause a given voltage
• myelin stops ions from leaking out and stop ions from charging up capacitance

Question 20

what does loss of myelin result in

Answer 20

• decreased Rm
• increased Cm
• increased loss of current and voltage signal along the axon