Signal Transmission and NTs Flashcards
What are the two types of synapses?
Electrical and Chemical
Define an electrical synapse.
Passive transfer of the current to the adjacent cell.
Define a chemical synapse.
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
What are the characteristics of an electrical synapse?
- adjacent cells are physically connected
- current flows from one cell directly to the next
- changes membrane potential
- current flow is instantaneous
- often not polarized
What are gap junctions?
- 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
What are the characteristics of chemical synapses?
- 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
What is a synaptic cleft?
- membrane-dense areas on both sides of the synaptic cleft
- presynaptic (vesicles)
- postsynpatic (accumulation of receptors and scaffolding proteins)
How does signal transfer from cell to cell occur?
- 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
How does AP trigger a Ca2+ influx?
- Depolarization of the axon terminal by the AP
- Opens the voltage-gated Ca channel
- Influx of Ca2+
- Intracellular Ca2+ concentration< extracellular Ca2+ concentration
- ECa = +130mV
- Membrane potential ~ -70mV
- Concentration gradient and electrical gradients favour calcium influx
How does an AP affect NT release?
- Single AP
- Ca2+ enters the cell
- Quickly bound by intracellular buffers or removed via Ca ATPase pumps
- Ca2+ enters the cell
- APs arrive at high frequency
- Ca2+ influx > Ca2+ binding/removal
- Increases the internal Ca2+ signal
- Stronger signal for SV exocytosis
- Ca2+ influx > Ca2+ binding/removal
How does the Ca2+ influx triggers exocytosis of the synaptic vesicle?
- Ca2+ triggers fusion SV to the presynaptic membrane
- Release of NT into the synaptic cleft
What does it mean that NT release is quantal?
The release of NT is in multi-molecular packages
What are the two theories of fusion/retrieval of the SV?
- Classical
- SV fuses with plasma membrane
- New vesicle retrieed by endocytosis
- Uses clathrin and dynamin
- “Kiss and Run”
- Partial fusion
- Vesicle re-internalized
What are Dale’s Principles?
- 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
What is the most common exitatory NT?
Glutamate
Define an agonist/antagonist NT.
Describe glutamate.
- used at most fast excitatory synapses in brain and spinal and cord
- used at “modifiable” synapses
- important in memory and learning
Describe GABA.
- 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
