W3: Neurones, Transmitters, Synapses And Receptors Flashcards
What do dendrites do?
Input from other neurones
What does the axon hillock do?
Axon potential generation
What does the axon do?
Impulse conduction/action potential propagation
What does the axon terminal/synaptic bouton do?
Release neurotransmitter
Roles of glial cells
Physical and metabolic support
Electrical insulation
Signalling
What are the properties of a gap junction (electrical synapse)?
- fastest channel
- direct transfer of ionic current
- bi-directional transfer of ions
- allows synchronous activity between cells
- glia-neurone, glia-glia communication
- found in cardiac myocytes
Properties of a chemical synapse
- unidirectional transfer of information - presynaptic to postsynaptic
- presynaptic terminal releases neurotransmitter to interact with postsynaptic membrane receptors
What is the process of neurotransmission dependent on?
Action potential dependent
Calcium dependent
Process of neurotransmission
- Action potential invades the nerve terminal
- Depolarisation of the presynaptic membrane triggers voltage gated calcium channels to open, leading to an influx of calcium ions.
- Synaptic vesicles fuse with the presynaptic membrane and neurotransmitter is released by exocytosis
- Diffusion across the synapse and neurotransmitter binds to the receptors on the post-synaptic membrane, depolarising it
- Rapid termination of the signal occurs
How is the signal at the synapse terminated?
Re uptake of neurotransmitter or enzymatic breakdown
What is a neurotransmitter?
Chemical messengers at the synapse
Amino acid neurotransmitters
Glutamate (major excitatory transmitter in the CNS)
GABBA (major inhibitory transmitter in the CNS)
Monoamine neutransmitters
Noradrenaline
Dopamine
5-HT
Where is ACh neurotransmitter more present in?
The PNS
What are receptors?
Recognition sites for neurotransmitters, causing an intracellular signal when bound to. Membrane spanning protein molecules. Specific to a neurotransmitter.
What is an ionotropic receptor?
Receptor operates/ligand gated ion channels
What happens when a transmitter binds to an ionotropic receptor?
It causes a conformational change in the receptor which opens the channel and creates a pore to allow ions through it.
Do ionotropic receptors have fast or slow transmission?
Fast (not as fast as gap junctions though)
What happens at an excitatory ionotropic synapse?
- neurotransmitter binds to receptor protein which changes shape and opens a pore
- sodium influxes into the post synaptic membrane causing a membrane depolarisation
- a single synaptic event will cause a small excitatory post synaptic potential, EPSP
- EPSP lasts a short amount of time and quickly decays away
Examples of excitatory ionotropic receptors
ACh nicotinic receptor
What happens at an inhibitory ionotropic synapse?
- neurotransmitter binds allowing the channel to open
- Cl- ions will influx into the postsynaptic membrane, causing a membrane hyperpolarisation
- causes a small inhibitory postsynaptic potential, IPSP
What happens when a neurotransmitter binds to a metabotropic receptor?
When neurotransmitter binds it causes a conformational change which activates a G-protein. This activates ‘effector systems’, which have indirect effects on excitability of the postsynaptic cell.
Do metabotropic receptors have slow or fast transmission?
Slow, but longer lasting effects.
What can an activated G-protein do in metabotropic receptors?
- open or close ion channels
- stimulate or inhibit enzymes/secondary messenger systems
Examples of metabotropic receptors
ACh muscarinic receptors
GABA’B
Monoamine receptors (except 5-HT)
What synaptic inputs are most influential?
Synaptic inputs which are close to the axon hillock are much more influential over the activity/excitability of a neurone than input from distant dendrites.
What happens after a single excitatory ionotropic synaptic event?
A small EPSP is produced
What is a small EPSP?
It makes the neurone more excitable (more likely to fire an AP) for a brief period of time.
Caused by the ionotropic receptor allowing sodium to enter cells, NOT voltage gated channels
When is an action potential produced?
When the level of depolarisation is high enough (reaches threshold potential) such that there is more sodium entering the cell than potassium leaving the cell.
What happens if there are e.g. 2 synaptic excitatory events at an ionotropic synapse?
A bigger synaptic event will occur
An increased EPSP is produced through spatial summation
What is spatial summation?
The summing of post synaptic potentials generated at separate synapses
What happens if multiple excitatory inputs occur at an ionotropic receptor?
Summation occurs
Threshold is reached by the ionotropic channels causing more sodium to enter the cell than potassium leaving
Depolarisation occurs, opening voltage gated channels
What happens if there is a single inhibitory ionotropic event?
A small inhibitory post synaptic potential IPSP is produced
What happens if an inhibitory and excitatory input occurs at the same time?
Spatial summation occurs
No change in the membrane potential because magnitude or the depolarisation and hyperpolarisation are approximately equal, so cancel each other out.
What happens if an individual synaptic input occurs at a high frequency?
Instead of one synaptic event, before it can decay away, another synaptic event depolarises the membrane further and adds to the first. Quick succession with little time to decay away. This can depolarised up to threshold. It causes an increased EPSP due to temporal summation.
What is temporal summation?
The summimg of post-synaptic potentials generated at the same synapse, if they occur in rapid succession.
