Lecture 3 - Axons Flashcards
What do nerve cells exhibit?
Excitability
Conductibility
What do neurons have a threshold for?
Initialisation of an action potential of about -45 to -55mv
What produces a large but transient flow of positive charge into the cell?
Increasing the voltage from -60 to 0mv
What is followed by a sustained flow of positive charge out of the cell?
The transient inward current
Where are each action potential initiated in?
The initial portion of axon
Axon initial segment
What does the initial segment of axon have?
Lowest threshold for action potential generation
Once a spike is initiated, where does the action potential propagate?
Down the axon to the synaptic terminal where it releases a transmitter to modulate intracellular processes
What is the refractory period?
Inactivation of Na+ channels
Activation of K+ channels
What is absolute refractory period?
Inactivation of Na+ channels
What is relative refractory period?
Occurs during the action potential afterhyperpolarisarion
What is the implication of refractory period?
Action potential are not allowed to “reverberate”
What is critical for allowing impulses to propagate?
Local circuit
Where does continuous conduction take place?
Unmyelinated fibres
What is continuous conduction?
The wave of depolarisation travels down the length of the axon
How does continuous conduction work?
Current flows when a patch of membrane is active
There is inward ionic current through sodium channels all around the circumference of the axon
Why is the distribution of current slightly different?
Effect of the activation of voltage gated potassium channels following the activation of sodium channel
How does the sodium channel work?
Alters the amount of charge held on the membrane capacitance
What flows in opposite direction?
Ionic current
Capacitance current
How does a sodium channel depolarise?
Passing ionic current inwards which is matched by a capacity current going outward
Why can’t you change a membrane potential?
There is no capacity current
What is Na+ current?
Inward and brief
Activate and inactivate
What is K+ current?
Outward and sustained
How do ionic currents contribute to action potential?
Na+ current generate upswing of action potential
Na+ channel inactivation and K+ channels activation underlie depolarisation
What does K+ channel activation generate?
Afterhyperpolarisation
In squid axon, when does afterhyperpolarization occur?
After action potential
What happens when you excite a nerve somewhere along its length?
Impulses can be propagated away in both directions
What happens when action potential is initiated naturally?
Propagates only in one direction because of the refractory period
Following an action potential, what happens when sodium channels are inactivated?
Another impulse cannot be generated
Not all nerve fibres are…
The same
What can be observed in a mixed nerve in a vertebrate (frog)?
Different fibre groups
What is A alpha?
Largest motor axon
What is A beta?
Largest sensory axon
What is A Gamma?
Gamma motor neurons
What has the highest conduction velocity?
Myelinated fibres
What is B fibre?
Preganglionic synthesis
What is C fibre?
Unmyelinated
Smallest diameter
Slowest conduction
What is saltatory conduction?
A way of increasing conduction velocity in axon withoout having to make the diameter bigger
What are 2 factors that determine conduction velocity?
Large diameter
Myelination
What does large diameter show?
How far the potential can spread passively in axon
What does efficient and rapid propagation of action potentials in myelinated axons?
Molecular specialisation of the nodes of Ranvier
What does nodal regions have?
Unique set of ion channels
Cell adhesion molecule
Cytoplasmic adaptor proteins
What are Schwann cells?
Satellite cells in the peripheral nervous system
Why does sodium channel in the phase contrast image glow red?
Presence of selective antibody which is carrying fluorescent tag
What does green fluorescence show?
Terminal loop of myelin attaching to the axon
What is expressed where terminal loop of myelin touches the axon?
Casper
What is the only place generating inward current?
Node of Ranvier
What does current contour map show?
How a functionally isolated myelinated axons work
What is present in the internode ?
Outward current (large capacity current)
What is a nerve axon?
Series of element which have resistances and capacitance
What is present outside the axon?
Resistance through the extracellular fluid
What happens when current flow outside?
Potential difference occurring
What is present inside the axon?
Axial resistance
What is present across the membrane?
Resistance which can change
The ability to pass ionic currents
What are properties of C fibre?
Very thin
High axial resistance
Space constant is short (0.5 micrometer diameter)
A fibre
Local circuit is present all the way along the internode (10micrometer)
What do large axons have?
Low R axial resistance
Easier to push a current along
What did Ruston WAH calculate?
Small axons are not myelinated
Large axons are myelinated
What is the relationship between myelinated and unmyelinated fibres?
As the diameter of the fibre is increased, myelinated axons will conduct faster
What did Tasaki 1959 show?
Air is an insulator
Air gaps block any current flow
There is stimulator to initiate action potential
Action potential will propagate through nerve bath along axon
What do air gap force?
The current flow outside the axon to go through Tasaki’s wire
What does amplifier measure?
Potential changes across the resistor
V=I/R
What is an example of outward current?
capacitance/capacity
Conduction velocity in a large myelinated axon is around…
50 ms-1 (~ 60 ms-1)
An action potential at a single point lasts close to …
0.5 milliseconds at body temperature
What is a part of cytoskeleton?
anKg
What is expressed in the paranode where there are terminal loops of myelin?
Caspr and Contactin
What is Juxtaparanode?
Sodium channels expressed underneath the myelin
What is an important cell adhesion molecule in the myelin?
P0
Where is P0 expressed?
On extracellular faces of myelin membrane
What is myelin made up of?
Turns of cell membrane
Why is axon-satellite cell interaction crucial?
Formation of nodes of Ranvier
What is important factor in Na+ channel clustering?
Interaction of gliomedin in Schwann cells and NF-186
What does myelinated axon membrane incorporate?
Domain typically expressing certain ion channels and cell adhesion meolcules (CAMs)
What does the sheath contain?
characteristic CAM
E.g. P0
P0 stabilise myelin
