Action Potentials

Question 1

Resting Membrane Potential and Concentration Gradients

Answer 1

• Resting membrane potential of -40 - -90mV
• [Na] is higher outside the cell, [K] is higher inside the cell
• cell membrane is super permeable to [K] due to potassium leakage channels
• maintains gradients through K/Na pumps, ATP-ase pumps 3 Na out, 2 K in

Question 2

Sodium Voltage-Gated Channels: Gates and States

Answer 2

• Gate-m (activation gate) is normally closed, but opens when the cell becomes positive
• Gate-h (deactivation gate) is normally open, but closes when the cell becomes very positive

States:

1) Deactivated: closed, @resting potential
2) Activated: open when threshold is reached
3) Inactivated: closed as neuron depolarizes and h-gate swings shut

Question 3

Potassium Voltage-Gated Channels: Gate and Function

Answer 3

-The n-gate is normally closed, but opens when the cell is depolarized

Question 4

Stages of an Action Potential

Answer 4

0) A triggering depolarizes the cell body
- usually the result of neurotransmitter-gated ion channels
1) Depolarization: cell becomes less polar
- threshold is reached, and V-gated Na channels open
2) Repolarization: brings cells back to resting potential
- inactivation of sodium channels via closing of h-gate
- K channels open, K flows outside cell
3) Hyperpolarization
- K channels lag, staying open a bit past resting potential
- fixed by Na/K pumps

Question 5

Graded Potentials vs Action Potentials

Answer 5

Question 6

Cation and Anion Gradients

Answer 6

• Cations
  
  • Potassium, K (inside)
  • Sodium, Na (outside)
  • Calcium, Ca (outside)
• Anions
  
  • Chloride, Cl (outside)
  • Organic Anions, OA (inside)
    
    • the membrane is impermeable to OA’s

Question 7

Potassium Leakage/Gradient/Equilibrium

Answer 7

• Equilibrium occurs at around -70mV, with the chemical gradient pushing it outside and the electrical gradient retaining it inside

Question 8

Chloride Gradient and Maintenence

Answer 8

• Resting membrane potential is the dominant force for the movement of chloride, so at resting potential (-60mV) there is a low intracellular concentration of Cl
• Chloride-Potassium Symporter uses the gradient-based diffusion of potassium to move chloride out of the cell

Question 9

Maintenence of Intracellular Calcium Conentrations

Answer 9

• Sodium/Calcium Exchanger uses the gradient-driven diffusion of sodium into the cell to expell calcium
• The equilibrium potential of calcium is ~+120mV, but permeability of calcium is VERY low

Question 10

Types of Action Potential Firing Patterns

Answer 10

1. Activity-dependent: excitatory input leads to action potentials. Large, sustained input leads to a train of action potentials
2. Steady-firing neurons: differences in leakage channels leads to rythmic firing of action potentials, excitatory input –> increased frequency, inhibitory input –> decreased frequency
3. Steady-firing burst neurons: fire bursts of action potentials at steady frequency, input changes both frequency and number of action potentials per burst

• Steady firing neurons can relay both excitatory and inhibitory information

Question 11

Pre-Synaptic Neurotransmitter Release Mechanism

Answer 11

1. AP travels down axon and depolarizes the axon terminal, opening V-gated calcium channels
2. Calcium activates Synaptotagmin (vesicle protein)
3. Synaptotagmin binds and activates Complexin (vesicle protein that prevents vesicle fusion in deactivated state)
4. Activated Complexin promotes SNARE-formation, leading to vesicle fusion and neurotransmitter release

Question 12

Post-Synaptic Response: Direct and Plasticity

Answer 12

• Direct: mainly ligand-gated ion channels, which can be either excitatory or inhibitory depending on which ion they are permeable to.
• Plasticity: indirect, the activated receptor sets off a cascade that can lead to the insertion or removal of receptors into the membrane (up and down-regulation, respectively)

Question 13

Two Types of Synapses

Answer 13

1. Chemical Synapse: has a gap in between the terminals, and uses receptors to pass signal.
2. Electrical Synapse: uses gap junctions to directly pass ions into the post-synaptic neuron

Question 14

Two Categories of Neurotransmitter Receptors

Answer 14

1. Ionotropic: ligand-gated ion channels
2. Metabotropic: NT binding leads to activation of secondary messengers

• the effects are slower than ionotropic receptors, but can be bigger, more widespread, and last longer

Question 15

Neurotransmitter Removal

Answer 15

1. Diffusion (passive)
2. Enzymatic: enzymes break down NT in synapse
3. Pre-Synaptic Reuptake Pumps
4. Astrocyte Endfeet: pump NT out of synapse and into astrocyte

Question 16

Neuroplasticity

Answer 16

Synaptic: changes in synapses or in response to input

• Presynaptic: changes in the amount of NT released (more=potentiation, less=depression)
• Postsynaptic: change in receptors

Structural: occurs on the level of entire cells

• Changes in the number of either presynaptic terminals or postsynaptic dendrites