Lecture 4: Synapses Flashcards
What are synapses? (3)
-Connection between two neurons or a connection between a neuron and a specalized cell
-Activation of a synapse usualy causes a graded potential in the postsynaptic cell
-The grand sum of EPSP and IPSP at the axon hillock will determine if the threshold potential is exceeded and an action potential is stimulated.
Two types of synapse
-Eletrical
-Chemical
Components of an eletrical synapse
-A gap junction (aka connexon) is made from two hemi-channels, each containing 6 connexin subunits (12 in total)
Eletrical syanpses (4)
-Electrical info passes directally between two cells
-can cause depolarization or hyperpolarization
-VERY FAST (synaptic delay of 0.1- 0.2 ms
-Other small molecules can pass through the gap junctions as well
Why have eletrical synapses (3)
-Synchronize groups of cells. Large number of neurons (or muscles) to fire at once in order to get a powerful response
* Fast responses: Escape pathways where simple sensory input elicits strong motor response
* Development: In some cases, electrical synapse develops before chemicalsynapse; synapse eventually changes to chemical. Maybe because it’s simpler to build?
Chemical Synapses (2)
-Release neurotransmitter from presynaptic cells to influence electrical activity in postsynaptic cell
-Neurotransmitter must bind to receptor
Two main types of chemical synapse receptors
ionotropic :ligand gated ion channel (FAST)
metabotropic: G-protein coupled receptor (SLOW)
Steps in a chemical synapse (5)
- AP travels down axon, causing volatge gated Ca++ channels to open
- Ca++ flows into the presynaptic terminal, causing synaptic vesicles to fuse to the presynaptic membrane
- Vesicles release neurotransmitter into the synaptic cleft
- Neurotransmitter binds to postsynaptic receptors. Time taken for neurotransmitter to travel (synaptic delay) is about 2ms
- Excess neurotransmitter is removed from the cleft by degredating enzymes or is recycled back.
Definifiton of a neurotransmitter (6)
- Must be present in a presynaptic terminal
- Released upon presynaptic stimulation
- Exogenous application of the neurotransmitter to the cleft should mimic the synaptically released transmitter
- There is a mechanism to remove the neurotransmitter from the cleft
- Agonists will mimic and antagonists will block the action of the neurotransmitter
6.Substance not meeting these criteria might be called “putative”
Types of neurotransmitter (3)
-Amino Acids like glutamate and GABA
-Biogenic amines like Acetylcholine
-Small peptides like oxytocin
Steps of a Ligand-gated ion channel (3)
- Neurotransmitter binds
- Channel opens
- Ions flow through
Steps of a G-protien coupled receptor (5)
- Neurotransmitter binds
- G-protein is activated
- G-protein subunits or intracellular messengers modulate Ion channel
- Ion channel openes
- Ions flow through
nicotinic receptors (5)
- ACh (acetylcholine) binds to niontinic receptor
- Channel opens allowing entry of Na+ and exit of small amount of K+
- Movement of + charge into the cell causes depolarization
- Depolarization causes EPSP
- FAST
GABA receptors
- GAPA binds to receptor
- Challen opens allowing Cl- to enter the cell
- movement of - charge into the cell causes hyperpolarizaion
- hyperpolarization causes IPSP
- FAST
Slow Chemical receptor example (GPCR)
- GPCR binds to ligand-gated receptor
- alpha subunit of G-protien releases GDP and binds a GTP
- Alpha subunit moves through the cell and interacts with an amplifier enzyme
- Amplifier enzyme releases secondary messengers
- Secondary messengers activiate or inhibit ion channels
Muscarinic cholinergic receptors (5)
- ACh binds to Muscarinic receptor
- G-protien is activated, which activates 2nd messengers, which activate Kinases
- Kinases cause K+ leak channels to close
- Closure of channels causes SMALL depolarization
- Small EPSP is created
Neuromuscular Junction is a special synapse (5)
- its HUGE! 1000um opposed to 0.5um
- Postsynaptic membrane is folded and has A ton of nicotinic recepors
3.The EPSP in muscle cell is also huge! 30-50 mV opposed to 0.1-5 mV. This is because A lot of ACh binding to a lot of nicotinic receptors
- High density of voltage gated Na+ channels within the post synaptic folds
- these points means that a single AP in a motor neuron will ALWAYS result in an AP in the postsynaptic muscle cell
