Lecture 7: action potential conduction Flashcards
AP propagation along unmyelinated axons
- as an action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV
- as the sodium ions entering at spread away from the open voltage gated channels, a graded depolarization quickly brings the membrane in segment 2 to threshold
- an action potential develops in segment 2
- the initial segment begins repolarization (and is now refractory) - as the sodium ions entering at segment 2 spread laterally, a graded depolarization quickly brings the membrane in segment 3 to threshold
- the actin potential can only move forward, not backward, because the membrane at the initial segment is in the absolute refractory period of repolarization
absolute refractory period
no matter how large the stimulus, another AP cannot be generated since VG Na channels are inactivated
relative refractory period
an AP can be generated, but only in response to a very large stimulus because depolarization of stimulus dampened by rel, high K permeability at this time
how are excitable cells modelled off electrical circuits
- there is resistance to current flow across the membrane
- there is resistance to current flow along the axon
- the distance that electronic spread travels is a function of the magnitude of these two resistances in relation to one another
- if internal resistance is low in relation to transmembrane resistance, electronic spread will be further along the axon
what happens if internal resistance is high
there will be current leaking out across the membrane and not spreading very far
how to make AP transmission faster
- current in solution is carried by ions
more ions = lower resistance = greater spread = faster conduction
do bigger diameter axons increase or reduce the longitudinal resistance
- bigger diameter encloses more ions and thereby reduce the longitudinal resistance
what is the impact of adding myelin / increasing axon insulation
reduces the “current leak” across the axon membrane
myelin
a lipid rich membrane sheath that wraps around axons and provides electrical insulation by limiting current leakage across the membrane
what are the small uninsulated gaps in myelin called
nodes of ranvier
density of ion channels in myelin
high = at the node
low = under the myelin
what is the transmembrane resistance like under the myelin
high
action potentials propagation in myelinated axons
- an action potential develops at the initial segment
- a local current produces a graded depolarization that brings the axolemma at node 1 to threshold
- an action potential develops at node 1
- the initial segment begins repolarization (and is now refractory) - a local current produces a graded depolarization that brings the azolemma at ndoe 2 to threshold
why is AP conduction in myelinated axons described as “saltatory”
saltatory = jumping
- the AP is said to “jump” from node to node
- electrotonic spread of current along the internodal region is very fast
is AP conduction faster in myelinated or unmyelinated axons
myelinated axons
AP conduction velocity in regards to diameter
larger diameter = thicker myelin = faster CV
action potentials key details
- they are initiated at the initial segment
- all or none
- they are regenerative and propagate along axons until they reach the nerve terminals
- at the terminals they typically trigger release of neurotransmitter onto a partner cell –> synaptic transmission
- transmitters and their receptors on synaptic partners can be excitatory or inhibitory –> EPSPs & IPSPs
- action potentials therefore conduct info away from the site of origin to a distant location to influence the excitability of partner cells
