HUBS1403 Lecture Notes - Lecture 25: Saltatory Conduction, Exocytosis, Schwann Cell
8 Jun 2018
School
Department
Course
Professor
Action Potentials
Action Potentials
Sequence of depolarization and repolarization along an axon to send the signal to the axon terminal
• You never get action potentials that are half, they are complete or not at all
Active propagation:
• The pulse changes the membrane in front and behind the pulse to give propagation on 1
direction
• Digital signal - less affected by interference
• No integration - very simple, only report integration from elsewhere
Afferent - conveys info towards brain
Efferent - conveys info away from the brain
1. In touch-sensitive neurons, opening of mechanically gated ion channels alters membrane ion
permeability
At the initial segment, a depolarising local potential triggers the opening of voltage- gated Na+
channels of Vm goes above threshold for that channel
• If the local potential is large enough it sets off an action potential
• Local - signals stay the same
• Action - membrane becomes more permeable to Na
Different Neurons
1. Typical
mechanically-
sensitive sensory
neuron innervating
skin ("unipolar
neuron")
• Mechanically gated sodium-permeaable channels located on dendrites
• Increased PNa triggers depolarising local potential ("receptor potential")
• Receptor potential spreads to initial segment (shrinking as it spreads)
Note: "mechanically gated" = "stretch gated"
Receptor channels can be odorants or temperature dependent, not voltage
sensitive
2. Typical inter-neuron
in the spinal cord
("multipolar
neuron")
• Receive large numbers of synaptic inputs from other neurons
• Possess ligand-gated ion channels at synaptic input sites
Note: "ligand-gated" = "chemically-gated"
Can have positive or negative stimulus
Typical inter-neuron in
the spinal cord
("multipolar neuron")
• Synaptic potential can be depolarising or hyperpolarising, depending on the
ion selectivity of the gated channel
• In this particular example (the withdrawal reflex arc) the synaptic input from
the sensory neuron would result in a depolarising potential
• AP triggered if adequate depolarisation at initial segment
*
*Cl - channels make hyper- polarisation
Na- channels make de-polarization
Many inputs means integration of information
Descending inhibition at input of interneuron, can provide conscious inhibition of responses
An AP can be triggered by a depolarizing local potential reaching the initial segment - if its large
enough
• Time course of AP is relative to the equilibrium potentials for K and Na
• Tigered by local potentials (don’t be concerned by the value of the threshold)
• Produces all or none of the voltage pulse driven by Na ions
Red section - is the absolute refectory period where a second AP is not possible
• Whole cycle is over in 6ms
Refractory Period: Time the membrane is no longer responsive to a stimulus
• Relative - can be stimulated, but by a very big stimulated
• Absolute - will not be stimulated by anything at all
The sequence of Events that Underpin an AP
1.
o At start, leak channels open but the V-dep channels are closed
o Cite gates on channels
o Ligand or mechano gated Na channels cause local depolarising potentials
o As PD reaches threshold the V-dep channel opens
2.
o Na flows in --- depolarization ---- more opening (activation)----- more Na flow
o Will keep happening until very positive
o Example of positive feedback
o During this time v-dep K is closed
3.
o At the peak of AP, 2 things happen:
• Na inactivated -- time dependent, conformation dependent
• K channel opens -- like Na it is voltage depended but it is slow
o Refectory because of inactivation makes Na channel unavailable
o Bal and chain structure
4.
o Rate of down stroke depends on K flow (slow in heart)
o Once voltage drops 2 things happen in Na channels (channel stays closed)
o Leaved ABS-refractory phase because Na channels are now available
o Hyperpolarization because of large K flow, moves closer to K equilibrium (K slow to close)
• This is the REL-refractory phase
o V-dep K closes, return to rest PD refractory period ends
5.
• Rising - Na ions coming into cell
• Falling - K ions leaving the cell
The Action Potential Refractory Period(s)
Absolute refractory period (ARP)
• Impossible to trigger another AP due to Na+ channel activation/inactivation
Relative refractory period (RPR)
• Larger than normal stimulus required mainly because of high K+ permeability
• Also there is some residual Na channel inactivation
Action Potential propagation - unmyelinated axons
AP at one point triggers AP in the next section
• APs propagate because they are self-triggering
• Membrane space constant means that AP can depolarise the adjacent membrane to trigger an
AP there
