Topic 5 – The propagation of the action potential in nerve fibers (compound

action potential, Erlanger-Gasser and Lloyd-Hunt axon classifications, saltatoric

conduction)

Propagation

In unmyelinated axons, action potentials propagate as an interaction

between passively spreading membrane depolarization and voltage-gated

sodium channels. When one patch of cell membrane is depolarized enough

to open its voltage-gated sodium channels, sodium ions enter the cell by

facilitated diffusion. Once inside, sodium ions "nudge" adjacent ions down

the axon by electrostatic repulsion and attract negative ions away from the

adjacent membrane. As a result, a wave of positivity moves down the axon.

The process repeats itself down the length of the axon, with an action

potential regenerated at each segment of membrane.

Speed of propagation

Action potentials propagate faster in axons of larger diameter.

They typically travel from 10-100 m/s.

Saltatory conduction

In myelinated axons, saltatory conduction through the

nodes of Ranvier is the process by which an action

potential propagates along the length of an axon.

Saltatory conduction increases nerve conduction velocity

without having to dramatically increase axon diameter.

General classification scheme (Erlanger-Gasser,

1939):

– A fibers: Myelinated

• Subtypes: α, β, γ, δ

• Fastest conducting and largest diameter – α

(120 m/sec, 20 m)

• “A” often dropped: alpha motor neuron

– B fibers: Slower myelinated (seldom used)

– C fibers: Unmyelinated

• Slower conducting and smallest diameter (0.5 m/sec, 0.5 m)

Sensory nerve classification (Lloyd-Hunt):

– I, II, III fibers: Myelinated

• Subtypes: Ia, Ib

• Fastest conducting + largest diameter – Ia

– IV fibers: Unmyelinated

• Slower conducting than IIIs and smallest diameter

Erlange

r-Gasser

Lloyd

-Hunt

Sensory nerve classification

Aα Alpha motor neuron. The

fastest – 120 m/sec

Discrimnative touch

Ia

Ib

Muscle spindle – annulospiral

Golgi tendon organ

Aβ Touch & pressure II Muscle spindle – flower spray

cutanoues mechanoreceptors

Aγ Efferent of muscle spindle

(γ-loop)

Fast pain & temp (cold)

-

Aδ Pressure and pain afferent III Myelinated,

deep pressure sensors in muscle

B Pre-ganglionic fibers of the

ANS

-

C Unmyelinated.

Post-ganglionic fibers of the

ANS.

Slow pain & temp (warm)

IV Unmyelinated.

Dorsal Root

pain fiberes

Compund action potential – contain mixed fibers. Due to changes in conduction velocity the the signal

will reach the sight of measurment.