BMS1052 Lecture Notes - Lecture 6: Depolarization, Internal Resistance, Myocyte
5 May 2018
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WEEK 2- lecture 6
Learning objectives
Describe what determines the size of EPSPs, and their effect at the soma
Describe the mechanisms by which Cl- channels mediates inhibition
Describe the factors affecting spatial and temporal summation of PSPs
Identify ways in which the probability of vesicle release can be modified
Describe how neuromodulation affects synaptic transmission over long timescales
Getting to threshold: dendrites integrate the inputs from each synapse (simple)
Left – a single presynaptic action-potential produces a single, small EPSP – this is usually
insufficient to produce an action potential in the post-synaptic neuron
Middle – EPSPs associated with simultaneous inputs from multiple neurons sum
Right – EPSPs associated with sequential inputs from a single neuron can sum over time
• What determines the amplitude of an EPSP?
Vesicles: quantal release & mini PSPs
Intracellular recordings from post-synaptic muscle fibre in response to presynaptic electrical
stimulation
The elementary unit of neurotransmitter is a vesicle
- Each vesicle contains a similar amount of neurotransmitter and produces a similar
PSP
- Miniature PSPs (minis) are the voltage change due to a single vesicle
- Note the failures! Probability of release < 1
Assuming that there are no voltage-gated channels:
- The effect of injected current decays exponentially with distance
decays because of leakage of current along membrane
- λ = legth ostat - distance at which depolarisation decays to 1/e of max size
- λ depeds o: - internal resistance (diameter, which increases towards the soma)
- membrane resistance (density of open channels, which is variable)
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Passive conduction in dendrites
- Signals propagate bidirectionally in dendrites
- For PSPs of the same size, synapses closer to the soma have a greater influence on
spiking probability
- Synapses further from the cell body may have more neurotransmitter receptors,
allowing them to generate a larger PSP.
NOTE:
- The EPSP/IPSP needs to reach the spike initiation zone in order to affect spiking
probability.
- This trigger zoe has the highest desit of oltage gated sodiu haels, ad
therefore, the lowest spiking threshold
An axon with wider diameter will allow more passive conduction
Active conduction in dendrites (limited)
Active conduction also occurs in some dendrites
- depends on voltage-gated channels, which boost membrane depolarisations
- improve the probability that an EPSP actually reaches the soma.
- Do’t futio like aoal atio potetials – are’t as reliale ad do’t
cause as large a depolarisation
Getting to threshold: dendrites integrate the inputs from each synapse (realistic)
The synaptic current is the charge influx due to channels opening in response to
neurotransmitter release.
The synaptic potential is measured at the cell body in response to the synaptic current.
Temporal summation efficacy depends on the membrane time constant
(i.e. how quickly it leaks charge)
Spatial summation efficacy depends on the membrane length constant (i.e. how far charge
can propagate)
Spiking threshold varies across the membrane
Spiking threshold is determined by the minimum membrane potential for which an AP will
occur.
i.e. sufficient voltage-gated Na channels need to open to initiate positive feedback
The lower panel shows the minimum membrane potential that much be reached in order to
cause an AP in the axon.
The high density of NaV channels in the trigger zone gives this region the lowest spiking
threshold.
Note, axon hillock = trigger zone =
spike initiating zone
.: more densely packed NaV channels lower spiking threshold
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Describe what determines the size of epsps, and their effect at the soma. Describe the mechanisms by which cl- channels mediates inhibition. Describe the factors affecting spatial and temporal summation of psps. Identify ways in which the probability of vesicle release can be modified. Describe how neuromodulation affects synaptic transmission over long timescales. Getting to threshold: dendrites integrate the inputs from each synapse (simple) Left a single presynaptic action-potential produces a single, small epsp this is usually insufficient to produce an action potential in the post-synaptic neuron. Middle epsps associated with simultaneous inputs from multiple neurons sum. Intracellular recordings from post-synaptic muscle fibre in response to presynaptic electrical stimulation. The elementary unit of neurotransmitter is a vesicle. Each vesicle contains a similar amount of neurotransmitter and produces a similar. Miniature psps (minis) are the voltage change due to a single vesicle. The effect of injected current decays exponentially with distance. Decays because of leakage of current along membrane.
