4 Synaptic Transmission Flashcards
What is a synapse?
Synapses are junctional zones between neurons
- they are unidirectional - signals go one way
- there is an inevitable delay - time taken for the released neurotransmitter to diffuse across the synaptic cleft (around 20nm)
from presynaptic side diffusing to the postsynaptic side
Describe synaptic transmission
- It is the transmission of info from neuron to target (e.g. motor neuron to skeletal muscle via NMJ)
- Synapses transform info from electrical to chemical signal (from AP to neurotransmitter)
Name the different types of axons:
- Axo-dendritic – axon to dendron
- Axo-axonic – axon to axon
- Axo-somatic – axon to the cell body (soma)
What are neurotransmitters?
They are endogenous chemicals (made by the body) that enable neurotransmission
Different types:
- Peptides
- Amino acids: e.g. Glutamate, GABA
- Catecholamines: e.g. Noradrenaline
- Other: serotonin, acetylcholine (NMJ)
Describe the presynaptic events during synaptic neurotransmission
- Action potential comes down the axon
- It reaches voltage-gated Ca2+ channels, which open
- This lets Ca2+ move into the cell, down its concentration gradient (high [Ca2+] outside cell, created by Ca2+ pump)
- This causes vesicles to bind to the pre-synaptic membrane
- And releases neurotransmitter molecules across the synaptic cleft by exocytosis - which diffuses and binds to receptors on the postsynaptic membrane
Describe Ionotropic receptor (direct)
A receptor is a protein (with a binding site)
- specific, sensitive, saturated
It is a ligand-gated ion channel
The neurotransmitter (ligand) binds to and opens ion channels in the postsynaptic membrane - this changes the ionic permeability of the postsynaptic cell membrane and hence the Vm of the postsynaptic cell
This is very rapid (1ms)
Describe the depolarisation/excitatory events that follow, after Neurotransmitter release
- Once neurotransmitter binds to receptors on the postsynaptic membrane, it opens the ion channel
- This lets in Na+ ions, which causes the postsynaptic membrane to depolarise
- This causes an action potential to be produced in the postsynaptic cell
Describe Metabotropic channels (indirect)
In indirect gating, the channel is inward-looking, responding only to chemical messages from inside the cell
- Link with the outside world is provided by a second protein that straddles the membrane + responds to transmitters/stimuli on the outside by triggering off a chemical response on the other side of the membrane, resulting in the required message being sent to the channel
This intracellular communication may involve one intermediate or a cascade of several of them
- very often the first link in the chain is formed by a G-protein Guanosine triphosphate (GTP)-binding protein
Describe Excitatory postsynaptic potentials and Inhibitory postsynaptic potentials
If the neuron is excitatory, transmitter binding to the postsynaptic membrane causes transient depolarisation as:
- Excitatory postsynaptic potential (EPSP)
- (+ve ions enter, or -ve ions leave postsynaptic membrane)
Inhibitory neurons release neurotransmitters that cause transient hyperpolarisations:
- Inhibitory postsynaptic potentials (IPSPs)
EPSP and IPSP amplitudes are graded with incoming signals(s) strength;
- so if there is more neurotransmitter
- more depolarisation
- threshold potential is reached
- action potential is produced
Describe how temporal summation is achieved using EPSP
Post-synaptic events for EPSP:
- more stimuli/neurotransmitter
- this creates a larger depolarisation
- known as temporal summation
Describe filtering, with respect to synaptic transmission, and postsynaptic potentials
Most of the information received by neurons at dendrites represents sensory ‘noise’
- Postsynaptic potentials (PSPs) are passive responses that degrade rapidly as they travel toward the cell body
- degradation is enhanced by the dendrite’s inherent electrical leakiness + lack of myelin
- So a small PSP may never reach the cell body
PSPs generated by a strong presynaptic activity will activate VG-ion channels across the length of the dendrite
- these enhance the signals + increase their likelihood of reaching the cell body
Describe the integration of synaptic signal transmission
Signals may integrate at the dendritic level
- PSPs may combine with PSPs arriving from other synapses as they travel toward the soma - known as Summation
Name the 2 types of summation
2 types:
- Spatial
- Temporal
Describe temporal summation
Occurs because the potential produced by a brief synaptic current fall relatively slowly
- possible to get a summation of the effects of repeated stimulation if the frequency of firing of the afferent fibre is high enough
BUT
- many EPSPs are needed for one AP to be produced out at the axon hillock
- many in for one out
Describe the events that occur in inhibitory ionotropic receptors (hyperpolarisation)
- An inhibitory neurotransmitter (Like GABA)
- opens Cl- ligand-gated Cl-channels
- allowing Cl- ions to enter
- hyperpolarising the postsynaptic membrane
