Synaptic Transmission Flashcards
What are the two types of synapse?
Electrical and chemical
Which synapse type is more prevalent?
Chemical
Which synapse type is more biologically flexible?
Chemical
What is a synapse?
The interface between the pre-synaptic part and post-synaptic part of a neuron.
Which type of synapse is faster?
Electrical are faster
Why are chemical synapses more flexible?
Huge range of NTs - and different expressions of the receptors - means that there is variability as to their response.
What is a gap junction and what is it made from?
A physical connection between two neurons - is made from a protein called Connexon 36. (CX36)
How many connexin subunits for the connexon channel?
6
How do gap junctions work?
When an AP arrives - it undergoes conformational change - allowing ions and small molecules to flow through it.
In which type of synapse are the cells physically connected together?
Gap junction - electrical synapse
What is the difference between a pre-synaptic potential and a post-synaptic potential (in terms of voltage).
Pre-synaptic = huge change in voltage - from -60 to approx +34/40 mV
Post-synaptic - very small change in voltage transferred from pre-synaptic - only about 1mV
Given the difference between pre-synaptic voltage and the post-synaptic potential that is received - what does this mean for generating an AP in the post-synaptic neuron?
As the post-synaptic neuron only receives a small depolarisation from the pre-synaptic neuron (via the gap junction) - it means the post-synaptic neuron needs lots of depolarisations in order to generate an AP in it.
How do multiple depolarisations happen in the post-synaptic neuron?
They are often innervated by multiple cells = neural integration & summation of signal.
What do chemical synapses use to propagate a signal?
They use neurotransmitters
What is the site of NT release on a neuron called?
The active zone
Where are NTs stored?
In synaptic vesicles
Are the cells connected in a chemical synapse?
No - there is a space between them (30nm wide)
What happens when the NT crosses the synaptic cleft? How does it cause an AP?
NT crosses the cleft, binds to a post-synaptic receptor in the membrane - this enables the post-synaptic receptor to undergo conformational change, which allows the induction of ions into the post-synaptic neuron, causing depolarisation and generating potential required for an AP to occur.
What is the name of a synapse between an axon and a dendrite?
Axodendritic synapse
What is the name of a synapse between an axon and a soma?
Axosomatic synapse
What is the name of a synapse between the axons of neurons?
Axoaxonic synapse
What are the names of these synapses?
A). Axodendritic synapse
B). Axosomatic synapse
C). Axoaxonic synapse
Where are NTs synthesised?
In neuronal main cell body (usually in the RER).
What are NTs?
Molecules that cross the synaptic cleft, binding to receptor targets on the postsynaptic membrane.
Why do NTs need to be removed from the cleft?
To prevent constant stimulation of the post-synaptic neuron.
What is transport of NTs from the cell body to the synapse called?
Axonal transport
How are NTs transported in the neuron?
In vesicles, along microtubules
Where are NTs packaged into vesicles?
In the Golgi apparatus
What is the direction of travel of NTs from cell body to synapse called?
Which protein is used to facilitate this?
Anterograde direction
Protein = Kinesin
What is the direction of travel of NTs from the synapse to the cell body called?
Which protein is used to facilitate this?
Retrograde direction
Protein = Dynein
What happens to NTs when an AP arrives at the pre-synaptic membrane?
AP arrives and depolarises the pre-synaptic neuron.
This causes Ca voltage-gated ion channels in the active zone to open - allowing an influx of Ca into the neuron.
The Ca influx changes proteins on vesicles & the presynaptic membrane (snaps and snares) - these changes bring about docking of the vesicle - and then the contents are released into the synapse via exocytosis.
What is the name of the process by which NTs are released into the synaptic cleft?
Exocytosis
What is the lifecycle of a vesicle in the pre-synaptic membrane?
Made by Golgi - contains NT
Travels to pre-synaptic membrane
Docking
Priming
AP arrives, Ca channels open - causes conformational change of snaps & snares on vesicle leading to Exocytosis
The excess membrane is budded back into the cell via endocytosis
The empty vesicle then joins an endosome
Eventually reused and filled with NTs again
How are docked vesicles held in position on the membrane?
By specialised proteins
Name 4 ways in which NTs are removed from the synaptic cleft?
1). Degradation by enzymes in the cleft
2). Uptaken into glial cells nearby (and then broken down with enzymes or recycled and repackaged into vesicles)
3). Reabsorbed into the pre-synaptic cell for repackaging
4). Reabsorption into the pre-synaptic cell for degradation
What ensures that the amount of NT released is proportional to the signal that is received by the post-synaptic neuron?
The removal of NTs from the cleft in a speedy manner
By which process do glial cells absorb NTs from the cleft?
Via pinocytosis
How is ACh removed from the cleft?
What happens to the products?
It is removed by acetylcholinesterase in the cleft (broken into down into acetate and choline). Choline is then reabsorbed by the pre-synaptic neuron and used to make more ACh.
Do motor nerves axons have single branches or multiple branches?
Motor nerve axons have multiple branches - when they reach skeletal muscle they branch out.
Are motor neuron axons myelinated?
They are myelinated by Schwann cells until they get close to the motor end plate. Then they are myelinated very thinly by a Schwann cell but only as protection from the fluid in the area, rather than for conduct of the impulse.
What increases the surface area in the motor end plate?
Subneural clefts
What do subneural clefts do?
Increase the surface area upon which ACh can act.
Which ion initiates exocytosis in the neuromuscular junction?
Calcium
What is another name for the active zone?
The dense bar
What type of receptor is the AChR?
Ligand-gated ion channel.
How many subunits are in the AChR?
5 - 2 x ⍺, β, 𝛾 & 𝛅
Which subunits does the ACh bind to?
The ⍺ units
Which ions can pass through the AChR channel when it is open?
Large amounts of Na+
and
Much smaller amounts of K+ & Ca+
What is the polarisation of the end plate known as?
The end plate potential
How does opening of the AChR channels affect the end plate potential?
Opening of the AChR = influx of Na+ = local depolarisation of the end plate potential - this opens the remaining Na channels and an AP is initiated.
What happens when the end plate is depolarised and an AP is initiated?
Muscle contracts
