Signal Transmission and NTs

Question 1

What are the two types of synapses?

Answer 1

Electrical and Chemical

Question 2

Define an electrical synapse.

Answer 2

Passive transfer of the current to the adjacent cell.

Question 3

Define a chemical synapse.

Answer 3

Release of neurotransmitters that effect a change in the adjacent cell

• ionotropic
  
  • directly opens ion channels
• metabotropic
  
  • indirectly opens ion channels via metabolic changes n the adjacent cell

Question 4

What are the characteristics of an electrical synapse?

Answer 4

• adjacent cells are physically connected
• current flows from one cell directly to the next
  
  • changes membrane potential
• current flow is instantaneous
  
  • often not polarized

Question 5

What are gap junctions?

Answer 5

• proteins bridge the gap between the two cells
• low resistance path for current flow
• voltage in adjacent cell is decreases - there is some resistance in the gap junction

Question 6

What are the characteristics of chemical synapses?

Answer 6

• Discontinuity between cells - synaptic cleft
• Presynaptic electrical impulse transduced into a chemical signal
• Neurotransmitter rapidly diffuses to post-synaptic membrane
  
  • generates a cheical or electrical response

Question 7

What is a synaptic cleft?

Answer 7

• membrane-dense areas on both sides of the synaptic cleft
  
  • presynaptic (vesicles)
  • ​postsynpatic (accumulation of receptors and scaffolding proteins)

Question 8

How does signal transfer from cell to cell occur?

Answer 8

• presynaptic neuron releases NT in response to AP
• NT released by Ca²⁺ dependent exocytosis
• NT diffuses across synaptic cleft
• NT binds to postsynaptic receptor
  
  • usually results in a change in the membrane potential

Question 9

How does AP trigger a Ca²⁺ influx?

Answer 9

• Depolarization of the axon terminal by the AP
• Opens the voltage-gated Ca channel
• Influx of Ca²⁺
  
  • Intracellular Ca²⁺ concentration< extracellular Ca²⁺ concentration
  • E_Ca = +130mV
  • Membrane potential ~ -70mV
  • Concentration gradient and electrical gradients favour calcium influx

Question 10

How does an AP affect NT release?

Answer 10

• Single AP
  
  • Ca²⁺ enters the cell
    
    • Quickly bound by intracellular buffers or removed via Ca ATPase pumps
• ​APs arrive at high frequency
  
  • Ca²⁺ influx > Ca²⁺ binding/removal
    
    • Increases the internal Ca²⁺ signal
    • Stronger signal for SV exocytosis

Question 11

How does the Ca²⁺ influx triggers exocytosis of the synaptic vesicle?

Answer 11

• Ca²⁺ triggers fusion SV to the presynaptic membrane
• Release of NT into the synaptic cleft

Question 12

What does it mean that NT release is quantal?

Answer 12

The release of NT is in multi-molecular packages

Question 13

What are the two theories of fusion/retrieval of the SV?

Answer 13

• Classical
  
  • SV fuses with plasma membrane
  • New vesicle retrieed by endocytosis
  • Uses clathrin and dynamin
• “Kiss and Run”
  
  • Partial fusion
  • Vesicle re-internalized

Question 14

What are Dale’s Principles?

Answer 14

• candidate NT must be present in the presynpatic terminal
• candidate NT must be released, upon presynaptic stimulus, in amounts sufficient to effect a response in the postsynaptic cell
• when added to extracellular fluid, the candidate NT should induce the same changes as the endogenous NT
• a mechanism for removal should exist
• effects of drugs on transmission at the synapse must be similar for both endogenous and exogenous NT

Question 15

What is the most common exitatory NT?

Answer 15

Glutamate

Question 16

What is the most common inhibitory NT?

Answer 16

GABA

Question 17

Define an agonist/antagonist NT.

Answer 17

Question 19

How do inhbitory neurotransmitters work?

Answer 19

• Cause hyperpolarization of membrane
  
  • Inhibitor postsynaptic potential
• Make postsynaptic cell less likely to generate an AP

Question 20

How do excitatory NTs work?

Answer 20

• Cause depolarization of membrane
  
  • Excitatory postsynaptic potential
• Make post synaptic cell more likely to generate an AP

Question 21

Describe glutamate.

Answer 21

• used at most fast excitatory synapses in brain and spinal and cord
• used at “modifiable” synapses
  
  • important in memory and learning

Question 22

Describe GABA.

Answer 22

• used at majority of fast inhibitory synapses in the brain
• many sedatives enhance GABA effects
• glycine has a similar role in the spinal cord
• binds to GABA receptor
• opens Cl^- channels
• “locks” the membrane at a value more hyperpolarized than the threshold

Question 23

Describe dopamine.

Answer 23

• important in “reward” neural circuits
• dysfunctional in Parkinson’s Disease and schzophrenia

Question 24

Describe serotinin.

Answer 24

• monoamine NT
• ~90% is produced in the intestine; remainder in CNS
• regulates sleep, appetite, memory and learning
• SSRIs (ex. Prozac)

Question 25

What are the 4 different mechanisms for the inactivation of NTs?

Answer 25

• Diffusion
• Enzymatic cleavage
• Uptake by astrocytes
• Reuptake by presynaptic terminal