NSB 3. Synaptic Transmission Flashcards
What is the definition of synaptic transmission (neurotransmission)?
- process by which neurotransmitters are released by pre-synaptic neuron, bind to and activate the receptors of the post-synaptic neuron
- neurotransmission is essential for communication betwen 2 neurons
- it is either excitatory or inhibitory (the neurotransmitters)
What is there a lot of in the neuron? (very vague question)
mitochondria!
- because the process requires a lot of energy
What type of gradient is neurotransmission dependent on?
electrochemical gradients
What ions are involved in creating the electrochemical gradients required for neurotransmission? (just list them)
[1] Na+
[2] K+
[3] Cl-
[4] Ca2+
What is the concentration of Na+ ions extracellularly and intracellularly?
Extracellular: 145 mM
Intracellular: 15mM
What is the concentration of K+ ions extracellularly and intracellularly?
Extracellular: 4.5 mM
Intracellular: 120mM
What is the concentration of Cl- ions extracellularly and intracellularly?
Extracellular: 116mM
Intracellular: 20mM
What is the concentration of Ca2+ ions extracellularly and intracellularly?
Extracellular: 2mM
Intracellular: 20 to 100 nM
What are the major ions in the extracellular space? [3]
[1] Na+
[2] Cl-
[3] Ca2+
What are the major ions in the intracellular space? [1]
[1] K+
Why is Ca2+ important in neurotransmission?
it is important to allow for the release of neurotransmitters
In a typical neuron, what is the resting membrane potential?
-70 mV
What are 2 examples when the membrane potential will be equal to zero?
[1] when the membrane separating 2 fluids is NOT permeable and so, does not allow for the movement of ions
[2] when the membrane separating 2 fluids is permeable to both ions and so there will be the same # of ions in each compartment - reaches an equilibrium
What type of membrane helps to create a membrane potential?
selectively permeable (semi-permeable) membrane - e.g. only allowing K+ to move through -- allows K+ to leave the cell and create a neg. charge inside the cell (from the residual Cl-)
What pump creates the electrochemical gradient in a cell?
Na+/K+ pump
- it pumps 3 Na+ out and 2K+ in
- the K+ can then diffuse outwards anyways
What is the difference between voltage-gated and ligand-gated ion channels? Give examples of some.
Voltage-Gated Channels:
- they open in response to a voltage (like when the cell gets depolarized)
Ligand-Gated Channels:
- they open in response to ligand binding to them (some chemical signal)
Examples:
- K+ Channel
- Na+ Channel
- Ca2+ Channel
- Cl- Channel
What is equilibrium?
the electrical force balances the chemical force, leading to NO net transport
Explain what the chemical driving force and electrical driving force are in relation to K+.
Chemical Driving Force:
- chemical gradient acts as a driving force for diffusion out of the cell
Electrical Driving Force:
- residual neg. charge draws the K+ back into the cell
What is the Nernst Equation?
E = 61 x log (Co/Ci)
Co = concentration of ion outside Ci = concentration of ion inside 61 = Faraday constant, gas constant, absolute temp., valence ions
What is the equilibrium potential for K+? What does this mean?
-87mV/-90mV
- this means that there is less K+ outisde the cell
- there are more K+ ions inside the cell
- this drives the K+ out of the cell
What is the equilibrium potential for Na+? What does this mean?
+60mV
- this means that there is less Na+ inside the cell
- there are more Na+ ions outside the cell
- this drives Na+ into the cell
What is an EPSP?
excitatory post-synaptic potential
In an axon, what is the direction of the impulse?
they go from the dendrites to the axon and the terminal branches - onto the next neuron
What are post-synaptic densities?
- is a network of proteins within and adjacent to the postsynaptic membrane
- they consist of anchoring and scaffolding molecules, signaling enzymes and cytoskeletal components
- they spatially and functionally organize the neurotransmitter receptors at the synapse
List and describe the process of neurotransmitter release once an action potential reaches the axon.
[1] action potential reaches + depolarizes the axon terminal
[2] depolarization activates voltage-gated, pre-synaptic Ca2+ channels (N-type + P-type)
[3] localized Ca2+ entry triggers the release of nearby vesicles containing NTs through the activation of Ca2+-sensitive fusion proteins
[4] NTs diffuse into synaptic cleft and activate NT receptors on post-synaptic membrane
[5] NT may also activate pre-synaptic NT receptors (positive/negative feedback)!!
[6] recycling/uptake of NTs are done through Glutamine Synthetase [GLUL]
What are the types of pre-synaptic Ca2+ channels that are activated when an action potential depolarized the axon terminal?
N-type or P-type
What aids in the recycling/uptake of NTs?
Glutamine Synthetase - GLUL
What is a tripartite synapse?
- refers to the functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding glia
- astrocyte processes are in close contact with the synapses
What are the functions of astrocytes in tripartite synapses?
- they recycle NTs (through glutamine synthetase)
- they secrete neurotransmitters and gliotransmitters (glutamate, D-serine, TNF-alpha etc.)
- they buffer extracellular K+
What are the 2 types of neurotransmitter receptors? (just list them)
[1] Receptor with Intrinsic Ion Channel
- Ionotropic Receptors
[2] G-Protein Coupled Receptors
- Metabotropic Receptors
Explain how ionotropic receptors and g-protein coupled receptors work.
Ionotropic Receptors:
- fast synaptic transmission
- NTs bind to ligand-gated channels which increase permeability to ions
- – e.g. nicotinic receptor for Na+
G-Protein Coupled Receptors:
- slower synaptic transmission
- activated receptor triggers activation of G protein that either: __ inside cell that modifes ion channels
- —- (a) directly modifies function of ion channels or
- —- (b) triggers production of chemical second messenger (e.g. cAMP)
- mediates short term + long term effects (e.g. gene expression)
Which NT receptor allows for fast synaptic transmission?
iontropic receptors
Which NT receptor leads to slower synaptic transmission?
g-protein coupled receptors
What are the main excitatory neurotransmitters in the CNS?
[1] glutamate
[2] acetylcholine
What are the main inhibitory neurotransmitters in the CNS?
[1] GABA
[2] Glycine
Which receptors does the neurotransmitter, glutamate, act on? [3]
[1] AMPA
[2] NMDA
[3] Kainate receptors
What are some other classical NTs (other than the excitatory and inhibitory ones)? [3]
[1] Serotonin
[2] Dopamine
[3] Noradrenaline
What can the excitation of inhibitory neurons lead to?
an inhibitory response
What is co-transmission?
It is when classical, small molecule NTs are co-released with larger, peptide neurotransmitters
– small NT + large, peptide NT
What is the difference in function between the small NT and the large, peptide NT?
small NT:
- fast response is mediated by this
peptide NT:
- a neuromodulatory, slow response is mediated by this
What is an IPSP?
inhibitory post synaptic potential
How does an inhibitory synapse work?
the NT binding increases the permeability to either K+ or Cl- causing the “holding” of resting membrane potential or small hyperpolarisation
– membrane potential moves farther away from the action potential threshold
- if Cl- enters the cell, membrane potential becomes more negative
- if K+ leaves the cell, membrane potential becomes more negative
What is summation? What are the 2 different types?
Summation is the addition of EPSPs together that allows for an action potential threshold to be reached and teh action potential to be fired.
2 Types:
[1] Temporal Summation
[2] Spatial Summation
What is temporal summation?
- the EPSPs last for about 15msec
- therefore, high frequency activation of a single pre-synaptic terminal can cause summation if the interval between intervals is less than 15msec
What is spatial summation?
EASY DEFINITION: the effect of triggering an action potential in a neuron from one or more presynaptic neurons
- most synapses terminate on dendrites (there is a loss of current due to membrane leakage + EPSP declines with distance)
- some synapses end on soma (very low resistance [due to large diameter] and local EPSP or IPSP can spread through soma
- SYNAPSES NEAR SOMA HAVE MORE EFFECT THAN SYNAPSES ON DENDRITES on the potential of the soma
- activation of multiple pre-synaptic terminals on dendrites can cause summation
What happens when the axon reached a threshold of -50mV?
- this is the threshold potential
- when depolarization reaches the threshold it leads to the activation of voltage-gated Na+ channels in the axon hillock
What is the refractory period? How long does the refractory period normally last for? What happens during repolarization?
refractory period: period of time in which the membrane is depolarized and another action potential cannot be sent through
- – usually lasts for about 0.001 to 0.002 seconds
- – this means that around 500 to 1000 impulses can be sent per second
repolarisation occurs through:
[1] opening of K+ channels and facilitated diffusion of K+ out of cell
– I guess they are brought back in through the Na+/K+ pump
[2] Na+ actively transported out of cell
Why is the glutamate receptor important in neurotransmission and sodium channel gating?
- glutamate is an excitatory NT
- it allows for the excitation of the axon and the opening of the Na+ channels
