M&R 4.2 Conduction of the nerve impulse Flashcards
Name one disease state affecting AP conduction in the CNS and one in the PNS
CNS = Multiple sclerosis (affects all CNS nerves) PNS = Guillain-Barre syndrome
Conduction velocity of APs _________ with fibre diameter
Increases
fastest = A-alpha fibres, slowest = C fibres
How are AP’s propagated along an axon?
By local currents
Resting membrane is negative inside and positive outside
Region of membrane undergoing an AP becomes positive inside and negative outside
This induces local currents which raise adjacent regions of membrane to threshold for AP firing (and therefore propagating the AP along the membrane)
Conduction velocity depends on ___________ and ___________
Resistance and capacitance
How does resistance relate to conduction velocity?
The lower the resistance, the higher the conduction velocity (e.g. larger diameter axons have lower axoplasmic resistance and therefore higher conduction velocities)
What is conduction velocity?
The speed with which an AP propagates along an axon
How does capacitance relate to conduction velocity?
The higher the capacitance, the lower the conduction velocity (because the membrane is storing the charge, so there is less available to depolarise other regions of the membrane)
[the lower the capacitance, the higher the conduction velocity]
What makes AP conduction directional?
After an AP has fired, that region becomes refractory
Can’t fire another AP due to:
Inactivation of VG Na+ channels (mainly)
[And also because of slow closing of K+ channels leads to transient hyperpolarisation, pushing MP further from threshold]
Therefore the impulse cannot enter a region of axon that has just fired, so propagates directionally and cannot go backwards
Which glial cells are responsible for myelination in the CNS?
Oligodendrocytes
Which glial cells are responsible for myelination in the PNS?
Schwann cells
What is the internode?
The length of node wrapped by the myelination of one glial cell (~1mm)
What is a node of Ranvier?
The unmyelinated region between 2 internodes
What effect does myelination have on capacitance, and how does this affect conduction velocity?
Myelination reduces the capacitance at the internode - so much that electrical activity only occurs at the nodes of Ranvier.
Therefore impulse conduction occurs in a saltatory manner (jumps from node to node) which increases conduction velocity
How are Na+ channels distributed in myelinated vs unmyelinated nerves? What happens in demyelination?
Unmyelinated - Even distribution of Na+ channels across membrane
Myelinated - Na+ channels exclusively located at Nodes of Ranvier
Demyelinated - Na+ redistribute from the nodes of Ranvier and instead become evenly distributed
When d/D=0.7 , a myelinated axon will have optimum conduction velocity. What are d and D?
d = diameter of axon D = diameter of axon + myelin
