1.12 Synaptic Integration & Action Potential Propagation Flashcards
Describe how the AP propagates along an axon
An excitable membrane is one the can carry an AP and therefore
contains voltage-gated channels
If there is an open Na+ channel in the membrane, current will leak into the cell
This will cause the membrane around this channel to open more adjacent Na+ channels for a certain distance, which is proportional to the length constant. If the length constant is greater, the depolarisation will propagate further down the membrane
Where the influx of Na+ is sufficient to increase Vm above threshold (again for some distance proportional to lambda), the next few voltage gated Na+ channels along the membrane will open
This cycle continues until the AP propogates to the end of the membrane (usually to the axon terminal, which signals NT release)
Why does the action potential only travel in one direction?
It isn’t really confined to one direction, however, the inactivation gates, keep the Na+ channels closed for enough time so that the AP does not propagate backward from where it came from. By the time the inactivation gate has opened, the newly depolarised membrane is too far to reactivate the Na+ channels.
What factors influence conduction velocity?
The further down the membrane that Vm is increased above threshold, the quicker the AP will propagate down the axon
By increasing membrane resistance relative to internal resistance (of the axon), the depolaristion current can travel further down the axon –> by increasing membrane resistance relative to the internal resistance, the AP will propagate faster
What factors influence internal resistance?
The larger the diameter the less resistance along the axon.
What factor can influence membrane resistance?
It can be altered by insulating the holes in the membrane. This is achieved by specialised insulating cells called
myelinating cells.
Describe the Myelin Sheath and how it effects AP propagation?
Myelin sheath is produced and maintained by Oligodendrocytes (CNS) and Schwann cells (PNS)
Nodes of Ranvier are the gaps between two adjacent myelin sheaths. They contain a higher concentration of Na+ channels, making them more sensitive to the threshold potential, and therefore more likely to fire when at threshold.
The depolarisation jumps between the nodes of Ranvier, which are spaced apart sufficiently to sense changes in Vm when a depolarisation occurs from the preceding node.
Describe the concept of Neuronal Integration
If each synaptic activation initiated an AP in the postsynaptic cell, the neuron would only serve to relay information, and therefore would not perform any sophisticated neuronal computation
The soma of a neuron serve to integrate all synaptic inputs; the EPSPs and the IPSPs to make it decision
Describe Synaptic Integration
Synaptic integration is the process by which these competing inputs (EPSPs and IPSPs) are processed. Synaptic integration is a neuronal computation that integrates (sums) these multiple synaptic inputs to produce a single output state (fire or not to fire).
The sum of EPSPs and IPSPs must bring
the resting membrane potential above a threshold before an action potential is generated
What determines whether or not the axon will fire?
The decision to fire, or not to fire, is made at the axon hillock
The axon hillock is more sensitive to depolarisation
This is due to a larger number of Na+ channels at that region, allowing more Na+ to enter for each increment in Vm
Once the hillock has been discharged, it brings the axon to its threshold, and also the soma and its dendrites to their threshold (which is about -35mV)
The axon hillock is sometimes called the trigger zone, as it is where the firing commences
Describe temporal summation
As a single EPSP is insufficient to initiate an AP, several EPSPs must add up to bring the axon hillock to threshold. This summing of EPSPs occurs spatially or temporally.
Recall the length constant (lambda) which governs the decay of the spread of Vm as a function of distance.
If lambda is larger –> the Vm resulting from the EPSP will spread over a longer distance. In neurons with a larger lambda, the depolarisation will spread further and therefore spread to the trigger zone with minimum decay –> Facilitates special summation.
Spatial summation is a way of achieving an action potential in a neuron with input from multiple presynaptic cells. Spatial summation is the algebraic summation of potentials from different areas of input, usually on the dendrites. Summation of excitatory postsynaptic potentials allows the potential to reach the threshold to generate an action potential, whereas summation of an inhibitory postsynaptic potentials can prevent the cell from achieving an action potential.
Describe Temporal Summation
As a single EPSP is insufficient to initiate an AP, several EPSPs must add up to bring the axon hillock to threshold. This summing of EPSPs occurs spatially or temporally.
Temporal summation is where a high frequency of action potentials in the presynaptic neuron elicits postsynaptic potentials that overlap and summate with each other. The effect is generated by a single neuron as a way of achieving action potential. Duration of the postsynaptic potentials are a longer duration than the interval between action potentials.
The time constant (τ), tells us the decay of the Vm as a function of time
If τ is larger –> more time is required for Vm to decrease
In neurons with a larger τ, there is more time for a second EPSP to add to the preceding EPSP –> facilitates temporal summation
What are the three synaptic configurations and what are their functional roles?
Axodendritic: axon to dendrite (often excitatory)
Axosomatic: axon to cell body (often inhibitory to shunt current)
Axoaxonic: axon to axon (often modulatory by controlling
synapse release
Dendrodendritic and somasomatic: not common
Synaptic contacts near the cell body and trigger zone will have greater influence (less decay) on the outcome compared to axodendritic.
