Lecture 3 - Axons Flashcards by Jihan Haji

What do nerve cells exhibit?

Excitability

Conductibility

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What do neurons have a threshold for?

Initialisation of an action potential of about -45 to -55mv

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What produces a large but transient flow of positive charge into the cell?

Increasing the voltage from -60 to 0mv

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What is followed by a sustained flow of positive charge out of the cell?

The transient inward current

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Where are each action potential initiated in?

The initial portion of axon

Axon initial segment

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What does the initial segment of axon have?

Lowest threshold for action potential generation

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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

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What is the refractory period?

Inactivation of Na+ channels

Activation of K+ channels

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What is absolute refractory period?

Inactivation of Na+ channels

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What is relative refractory period?

Occurs during the action potential afterhyperpolarisarion

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What is the implication of refractory period?

Action potential are not allowed to “reverberate”

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What is critical for allowing impulses to propagate?

Local circuit

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Where does continuous conduction take place?

Unmyelinated fibres

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What is continuous conduction?

The wave of depolarisation travels down the length of the axon

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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

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Why is the distribution of current slightly different?

Effect of the activation of voltage gated potassium channels following the activation of sodium channel

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How does the sodium channel work?

Alters the amount of charge held on the membrane capacitance

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What flows in opposite direction?

Ionic current

Capacitance current

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How does a sodium channel depolarise?

Passing ionic current inwards which is matched by a capacity current going outward

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Why can’t you change a membrane potential?

There is no capacity current

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What is Na+ current?

Inward and brief

Activate and inactivate

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What is K+ current?

Outward and sustained

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How do ionic currents contribute to action potential?

Na+ current generate upswing of action potential

Na+ channel inactivation and K+ channels activation underlie depolarisation

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What does K+ channel activation generate?

Afterhyperpolarisation

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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?

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