Speeding Up Action Potentials

Question 1

How do large axons speed up conduction velocity?

Answer 1

Just like water flows more easily through a drainpipe than a straw

Electric current flows more easily down a large axon than a small axon (axial resistance is lower)

Allows the Na+ channels to be more spaced out along the membrane, depolarisation can spread further

Question 2

How does myelination speed up conduction velocity?

Answer 2

Schwann cells in the PNS and oligodendrocytes in the CNS
Wrap layers of myelin layers around axons

Increased membrane resistance (less current leaks out membrane)- depolarisation going to spread much further

Decreased membrane capacitance (less current wasted charging up the membrane)

Action potential spreads passively from node to node and still reach threshold
Known as saltatory conduction

Question 3

Consequences of demyelination?

Answer 3

Multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS

Both demyelinating diseases that attack the myelin sheath

Decreased membrane resistance (more current leaks out membrane)

Increased membrane capacitance (more current wasted charging up the membrane)

Conduction fails

Question 4

What different kinds of axons do we have ?

Answer 4

We have small and large unmyelinated and myelinated axons

Several classifications which are confusing

These all conduct at different velocities
This generates a compound action potential

Question 5

What are the different types of axons and their functions? (fastest to slowest velocity)

Answer 5

Aa-largest myelinated- proprioception, motor neurons
AB-large myelinated-touch, pressure
Ay-small myelinated- motor neurons of muscle spindles
Adelta- smallest myelinated- touch, cold, “fast”pain
C- Unmyelinated- warmth, “slow” pain

Question 6

How do we measure compound action potential?

Answer 6

take a whole nerve and place one electrode at one end (will stimulate nerve) and place another electrode that will make recording

Question 7

What will happen when stimulate nerve?

Answer 7

will cause all neurons to depolarize to threshold and fire action potentials

Aalpha fibres will fire action potentials the quickest

Question 8

How would we measure single action potential?

Answer 8

Intracellular recording
Microelectrode through membrane
Relative to outside the cell

Question 9

How would we measure compound action potential?

Answer 9

Extracellular recording
Electrodes outside axons
Relative to earth
Each action potential very small but add up to large waves