Nervous System: Synaptic Transmission Flashcards
How does synaptic transmission between neurons work?
It uses chemical synapses between a target and presynaptic neuron. This is also known as an axodendritic synapse
How does synaptic transmission between a neuron (motorneuron) and a muscle fibre work?
It uses a neuromuscular junction (“end plate”). Axons outside the CNS connect to muscle fibres at neuromuscular junctions/end plates
What types of synapse is a neuromuscular junction/endplate
it is excitatory synaptic transmission
The neurotransmitter in neuromuscular junctions/endplate
Acetylcholine (Ach)
What is the ionic mechanism of End-plate potentials (EPPs)
The transient opening of ion channels selective to BOTH Na+ and K+ (also known as non-selective cationic channels). Membrane potential will go between each of their equilibrium potentials
What is EPP always?
Suprathreshold i.e. always triggers an AP as it’s depolarisation is sufficiently larger to reach threshold
What is the process of synaptic transmission at a neuromuscular jucntion/endplate?
- An action potential arrives from the cell body of a motor neuron which is transmitted to the PREsynaptic terminal. Once AP has invaded terminal it causes the opening of voltage gated Ca channel.
- There is a lot more Ca outside the cell membrane than inside so when channel is open there is an influx of Ca through the presynaptic terminal/button
- Entry of Ca into the PREsynpatic terminal causes fusion of synaptic vesicles (small structures storing neurotransmitters , Acetylcholine) with the synaptic membrane
- Content of vesicles diffuses to synaptic cleft, neurotransmitters bind to receptors in POSTsynaptic membrane (ligand/chemically gated channels)
- Ligand gated-ion channels open and ions move into cell
- Leading to depolarization in POSTsynaptic mem (EPP - form of synaptic potential, local depolarization of the mem, current spreads passively in both directions in loops)
Types of channels involved in synaptic transmission at a neuromuscular junction/endplate
voltage gated Ca channels
ligand gated ion channels
Length of synaptic delay
0.5ms (short)
The two types of chemical synapses in the CNS
Excitatory and inhibitory synapses
What is a excitatory postsynaptic potential (EPSP)
the depolarisation of the post synaptic membrane
What is the inhibitory postsynaptic potential (IPSP)
the hyperpolarization of the postsynaptic membrane
The neurotransmitter in excitatory postsynaptic potentials (EPSP)
Mainly Glutamic acid (glutamate) and also ACh
Ionic mechanism of EPSPs
The transient opening of non selective channels that are permeable to Na+, K+ and sometimes Ca2+ which causes a depolarisation of the membrane somewhere in-between the ions equilibrium potentials
The neurotransmitter in inhibitory postsynaptic potentials (IPSP)
Mainly GABA or glycine
Ionic mechanism of IPSPs
the transient opening of ligand gated K+ channels (has to be ligand gated because it is receptor changes in permeability). it causes a hyperpolarisarion that doesn’t last for long
The two types of neurotransmitters
Small molecule neurotransmitter/classical neurotransmitters AND neuropeptides/neuromodulators
Properties of Small molecule neurotransmitter/classical neurotransmitters
fast action (within milliseconds) and act directly on postsynaptic receptors
Examples of small molecule neurotransmitter/classical neurotransmitters
Amino acids (glutamate, GABA, glycine), Acetylcholine (ACh), Amines: serotonin (5-HT), noradrenaline, dopamine
Examples of neuropeptides/neuromodulators
Neuropeptide Y (NPY), Substance P, Kisspeptin, Enkephalin
Properties of neuropeptides/neuromodulators
they are larger molecules that act indirectly (metabotropic) that act on postsynaptic receptors and modulates the action of other neurotransmitters. It is slow (seconds to minutes)
The three factors determining synaptic action
- Type of neurotransmitter / neuromodulator
- Type of neurotransmitter receptor/ channel complex expressed in the postsynaptic membrane
- Amount of neurotransmitter receptor present in the postsynaptic membrane
What is Synaptic plasticity
Amount of neurotransmitter receptor present in the postsynaptic membrane. There are two types of receptors
LTP or LTD (receptors are not static in membrane/dynamic)
What is LTP
Long-term potentiation
What is LTD
Long-term depression
What are the three main subtypes of glutamate receptors
- AMPA Receptor
- NMDA Receptor
- Kainate Receptor
Properties of the glutamate AMPA receptor
when activated by glutamate, it opens and is non-selective for Na and K = depolarises
Properties of the glutamate NMDA receptor
permeable to Na, K and Ca, is very important if permeable to Ca, too much in nerve cells = Excitotoxicity
Stages of neurotransmitter inactivation and recovery
- Diffusion away from the synapse
- Enzymatic degradation in synaptic cleft
- Re-uptake (most aa and amines) + recycling
- Transporters in presynaptic membrane
Why is the re-uptake of neurotransmitters important
Nerve cells don’t have to make neurotransmitters from scratch
Enzyme that degrades ACh
Achase
Examples of specific neurotransmitter transporters in the presynaptic membrane (in nerve and glia cells)
glutamate transporter, dopamine transporter or serotonin transporter
How do neurotransmitter transporters in the presynaptic membrane take in neurotransmitters
Chemical connects to transporter, change in shape of transporter, shift of molecule across membrane and released on other side (usually against conc gradient) - taking in chemicals like dopamine and serotonin
Voltage of an individual synapse received by neurons in their axon initial segment in the postsynaptic membrane
0.1mV (very small)
What needs to happen to EPSPs in order to depolarise the initial segment to threshold to generate an AP
They need to be enhanced or they will be ignored as they do not reach the threshold
Temporal Summation of postsynaptic potential at axon initial segment
influx of Ca (not enough time to remove) so greater release of neurotransmitter. A single presynaptic neuron stimulates the creation of action potentials in a postsynaptic neuron when it fires at a fast-enough rate that the sum of EPSPs in the postsynaptic neuron moves above threshold.
Spatial Summation of postsynaptic potential at axon initial segment
E 1 and 2 coming through two synapses are activated at same time/almost same time, postsynaptic will summate and reach threshold
