Synapses Flashcards
What are the 3 stages of synaptic activity?
- Presynaptic activity
- Postsynaptic activity
- Neurotransmitter inactivation (so signals can be turned off)
What two types of presynaptic mechanisms are there?
- Small molecule transmitters
- Peptide/large molecule transmitters
How do small molecule transmitters of the synapse work?
- Enzyme moves down the axon to the bouton
- Enzyme activates precursor into active form and packages it into a vesicle
- Vesicles release neurotransmitters on demand at the synaptic cleft
- Precursors are reformed and reuptaken to be repacked into active enzymes
How do peptide/large molecule transmitters of the synapse work?
- Neurotransmitters/precursors are made and packaged in the cell body ( since molecules are bigger)
- They move down the axon in vesicles via the microtubule tracks
- Enzymes modify the precursors to produce peptide neurotransmitters
- Neurotransmitters are released on demand
- They are not taken up again, but they are degraded/recycled by the glial cells
What happens to the vesicles after releasing the neurotransmitters?
How do we now?
After fusion, the vesicles are recycled
In an experiment, the boutons were placed into Horseradish Peroxidase (HRP)
- Vesicles started to form in the bouton, containing HRP
- These vesicles started to fuse together to form an even bigger structure, endosome
- The endosome was then recycled into many other vesicles, so all of these then contained HRP
What are the stages of new vesicle formation?
- Budding - new vesicles form from the endosome
- Storage - vesicles get linked together by glycoproteins & they are combined to parts of the cell wall
- Docking - back-up storage in case there is a large demand for neurotransmitters
- Priming - vesicles are pulled close proximity to the end-membrane
- Calcium entry - causes the vesicle and end-membrane to fuse together
- Fusion - membranes fuse, neurotransmitters are released
Name the glycoproteins and decribe their behaviour during docking, priming and release of neurotransmitters
Synaptotagmin, Synaptobrevin, Syntaxin and SNAP-25
- Syntaxin and SNAP-25 are attached to the membrane, Synaptotagmin and Synaptobrevin are attached to the vesicle
- Synaptobrevin, Syntax and SNAP-25 join into a helical structure
- Entering Ca+ joins to Synaptotagmin
- Calcium bound Synaptotagmin catalyses membrane fusion
What are ionotropic receptors?
How do they work?
Ligand gated ion channels
- When transmitters bind to their receptors, they cause a conformational change, creating a pore at the middle of the channel
- Ion selective: selective about ion size and about ion charge
- The passage of ions in and out changes the voltage of the cell, either to depolarise or hyperpolarise
What are metabotropic receptors?
How do they work?
G-protein coupled receptors
- When transmitter binds to receptor, the inner membrane part of the receptor undergoes a conformational change
- G-protein binds to the receptor in the cytoplasm, getting rid of GDP and picking up GTP/becomes phosphoralysed and splits into two parts (active/non-active)
- GTP and G-protein can bind to the membrane bound enzyme, creating a lot of messenger molecules
- Amplification process: these messenger molecules then can activate many channels OR they can join nuclear transcription to create more proteins for channels/receptors
- GTP sheds a Phosphate, changing the conformation of the G-protein again + ion channel is closed because messengers are broken down
- G-protein dissociates from the enzyme and no more messengers are produced
- G-protein recombines with its other part and wants to join to the receptor again but can’t, because neurotransmitter is not bound to it so there is no conformational change for the receptor
What are the two post-synaptic responses?
Describe them
Excitatory (EPSP)
- A net flow of positive ions flowing into the cell that depolarises the membrane, but causing an action potential very rarely (Na+)
Inhibitory (IPSP)
- A net flow of negative ions flowing into the cell that hyperpolarises the cell (Cl-)
- both take about 8 msec because the ions leak out
- due to the long time, lots of small depolarisations can pile up to produce an action potential
Describe the 3 types of summations of neurotransmissions
Why are EPSPs able to summate?
Single EPSP
- quanta release of neurotransmitters
Spatial summation
- Simultaneous release of more than one neurotransmitter on different axons
Temporal summation
- Neurotransmitters are released over a short range of time on the same axon
They are able to summate because they have no refractory period
How do EPSPs and IPSPs work together?
- Two excitatory post-synaptic potentials will reach the threshold and will create an action potential
- One EPSP and one IPSP will cancel each other out
- Two EPSPs and one IPSP will reach the threshold but there will be no action potential
What are the three functions of the chemical synapse?
Computation
- As automatic summation of multiple PSPs is required to achieve threshold, integration acts as a decision making process based on EPSP and IPSP input
(All info merges into one)
Rectification
- As chemical inpulses transmit information in one direction only, this rectification serves to channel information (Helps channel output info into input info of another neuron)
Plasticity
- Controlled changes in the amount of transmitters released, the number of receptors present and the efficiency of the inactivation process provide a mechanism for adaptive plasticity (LTP and LTD); involved in learning and memory; (up or down regulation)
What are the roles of IPSP in the nervous system? Give two examples
Fine tuning movements
- IPSP/inhibition helps stop and control movement
Pain modulation
- Sensory pain neuron synapses with the ascending pain neuron, letting your brain know you are in pain
- To dumb down the pain, a descending pain modulation enhances and therefore increases the inhibitory effect of an inhibitory interneuron on the ascending pain neuron
Describe synaptic plasticity in memory and learning
- You are able to form memories and learn new things when you have stronger signals
- Stronger synaptic connections mean easier and better synaptic transmission
- Stronger synaptic connections come from more vesicles, greater excitability, more receptors and more parallel synapses
