Neuromuscular Junction

Question 1

Where on the axon is the AP generated?

Answer 1

At the axon hillock:

-Hillock has lower threshold compared to other ares

Question 2

Excitatory Post Synaptic Potential

Answer 2

• Synaptic potential that can depolarize membrane above threshold
• Produce AP

Question 3

Inhibitory Post Synaptic Potential

Answer 3

• Hyperpolarizes
• Or Stabilizes an already hyperpolarized membrane
• Makes it harder to get to AP

Question 4

Electrical Synapse

Answer 4

• Direct spread of ionic current b/t pre and post synaptic cells
• Physical continuity b/t cells
• Made up of Gap junctions
  
  • Connexons: allow ions and small molecules to pass b/t cells
  • Passes depolarizing current: trigger AP
• No amplification, so must have size match

Question 5

Chemical Synapse

Answer 5

• Chemical transmitter crosses synaptic cleft
• No physical contact b/t cells
• Predominant in CNS
• Capable of amplification
• Presynaptic vesicles released when presynaptic AP triggers Ca2+ influx
• bind post synaptic receptors to trigger response
  
  • Opens/closes channels

Question 6

Role of calcium in transmitter release from the presynaptic cell

Answer 6

• AP triggers slow opening of Ca2+ channels
• Ca2+ acts as messenger
• Controls the vesicle binding & transmitter release
• Amount of Ca2+ proportional to amount of transmitter release
  
  • Consequently affects postsynaptic potential

Question 7

Ionotropic synaptic transmission

Answer 7

• Directly opes or closes a channel in the postsynaptic membrane
• Ionic current thru open channels creates EPSPs or IPSPs
• Ex: NMJ

Question 8

Metabotropic synaptic transmission

Answer 8

• Indirect: releases a messenger (G-protein)
• May Activate:
  
  • Intracellular enzymes
  • Membrane enzymes
  • Gene transcription
  • Secondary channels

Question 9

Neuromuscular Junction

Answer 9

-ACh binds postsynaptic receptors
-Produces End Plate Potential (EPP)
EPP is EPSP at NMJ
-ACh binding: opens channel permeable to K and Na
*Low specificity: allows + ions but not -
*Na enters and K leaves
*Na+ driving force is larger

Question 10

NMJ EPP

Answer 10

• Na and K conductances equal
• Na driving force greater so more enters
• Causes DEPOLARIZATION
• EPP ALWAYS excitatory
• ALWAYS produces action potential
• No inhibition
• Amplitude of 60mV

Question 11

EPSP in the CNS

Answer 11

• PSPs rarely larger than 1mV
• EPSP required to bring CNS cell to threshold
• Has EPSPs and IPSPs

Question 12

CNS breakdown of ACh

Answer 12

• Done by Acetylchlinesterase in the cleft
• splits ACh into Acetate and Choline
• Once split, can’t bind receptors
• Receptor channels close & current stops
• Acetate diffuses out into extracellular fluid
• Choline recaptured presynaptically
  
  • Co-transport in w/ Na out
  • membrane pinches off to form vesicle
  • Vesicles refilled w/ transmitter

Question 13

Action Potential v. EPSP

Answer 13

• Action Potentials:
  
  • Activated by voltage
  • Propagates
  • Separate Na and K channels
  • Sequential Na and K conductance change
  • Regenerative
  • No summation
• EPSPs:
  
  • Activated by transmitter
  • No propagation
  • Na and K pass thru same channel
  • Simultaneous conductances changes
  • Not regenerative
  • Can Sum

Question 14

CNS EPSP production

Answer 14

• Produced by transmitter increasing membrane permeability to both Na and K
• Amplitude ~1-2mV

Question 15

CNS IPSP production

Answer 15

• Produced by transmitter increasing membrane permeability to Cl- or K+
• Moves the membrane potential toward the equilibrium potentials of those ions (hyperpolarizing)
• Also occurs by stabilizing already hyperpolarized cell:
  
  • Cl- doesn’t change membrane potential, just inhibits other permeability changes from moving membrane potential
  • Decreases excitability

Question 16

CNS Metabotropic EPSP

Answer 16

• Colse normally open channel in post synaptic membrane
• Transmitter binding releases 2nd messenger that binds membrane channel
• Channel closes and stops the hyperpolarizing K+ current
• Depolarizes and makes EPSP easier
• Increases Excitability

Question 17

Presynaptic Facilitation

Answer 17

• Increases the amount of transmitter released
• 2nd cell releases transmitter on 1st cells synaptic terminal
• Blocks some volt-gate K+ channels and repolarizes 1st cells AP: increases AP duration
• Longer duration increases Ca2+ and thurs transmitter released

Question 18

Presynaptic Inhibition

Answer 18

• Cell 1 releases transmitter onto cell A’s synaptic terminal
• Blocks A’s Ca2+ channels
  
  • Less Ca2+ so less transmitter released

Question 19

Temporal Summation

Answer 19

• If same synapse is rapidly stimulated before previous sub-threshold EPSP has died away
• Add together to produce an AP
• Rapid firing of single synapse to bring cell to threshold

Question 20

Spatial Summation

Answer 20

• Multiple synapses firing individual sub-threshold EPSPs
• When all are fired simultaneously the individual synaptic currents sum spatially to produce EPSP that reaches threshold
• Simultaneous firing of many synapses to bring cell to threshold