What is the mechanism of nerve conduction?
When a neuron is stimulated by a stimulus (such as a chemical, light, heat, mechanical distortion of the
plasma membrane) a disturbance in its membrane potential (MP) occurs. This response typically begins
at a dendrite, spreads through the soma, travels down the axon and ends at the synaptic knob before
stimulating the adjacent neuron.
This signal produced by the change in MP can be of two types: graded and non-graded.
Graded or local potentials (LP) are a result of small changes in MP and are produced by ligand-regulated
or mechanically regulated gates on the dendrites and soma. They are graded and their signal travels over
a short distance and are decremental. For the purposes of nerve conduction, the action potential (AP) is
the key signal responsible. LP are not powerful enough to travel down the axon but they are considered
as the ‘forerunner’ for AP initiation.
Non-graded or action potentials (AP) are characterized by a more dramatic changein MP and are
produced by voltage-regulated ion gates which are located in high density on the trigger zone of the
neuron.
If an excitatory LP spreads all the way to the trigger zone, it can open these gates and this generates an
AP which is a rapid up-and-down shift in membrane voltage (as seen on the graph below).
Steps in the Generation of an Action Potential
1. Resting state (before stimulation)
Na+ and K+ voltage- gated ion channels (primarily located in axon hillock and axon) are closed
2. Beginning of stimulation
Na + arrives at axon hillock, depolarizing the membrane at that point (steadily rising LP). For anything
more to happen, this LP must rise to a critical voltage called the threshold (about -60mV) in order to
open voltage gates at trigger zone.
3. Depolarisation Phase:
At threshold the neuron “fires” producing an AP.
Here voltage-gated Na + gates open quickly while K+ gates open more slowly.
Positive feedback were more and more Na+ gates open, causing membrane voltage to rise rapidly.
3. Repolarisation Phase:
As rising MP passes 0 mV, Na+ gates begin to slowly close and by time they are fully closed the voltage
peaks at +35 mV. Membrane now positive on inside and negative on out (reversed to RMP).
By this peak, the Na+ gates are closed and the slow K+ gates are fully open and there is an efflux of K+ as
they are repelled by the positive ICF. This causes the repolarisation (shifting back to negative numbers)
4. Hyperpolarisation:
K+ gates stay open longer than Na+ gates, so the amount of K+ leaving is greater than amount of Na+
that entered. Thus membrane voltage drops 1-2 mV. This is restored by Na+/K+ pump and simple
diffusion of K+ back into the cell.
Signal conduction in nerve fibers
Now, if a neuron is to communicate with other cells, this signal has to travel to the end of the axon to
the synaptic knobs in order for it to be conducted to another neuron or to effect a muscle. This
conduction occurs differently depending on type of neuron fibers. Two types include; continuous and
salutatory.
Continuous
 Continuous conduction occurs along unmyelinated axons. It is a step=by-step depolarization
where the resulting depolarization excites voltage-regulated gates immediately distal to the AP.
Na+ and K+ gates open and close just like they did at the trigger zone, exciting membrane
immediately distal to that. Chain reaction continues until the travelling signal reaches the end of
axon
 Note: AP itself does not travel down axon but it stimulates the production of a new AP. This is
like a domino effect.
 Conduction velocity of unmyelinated neurons = approx 1 meter / sec (depends on diameter)
Saltatory
 This occurs in neurons that are myelinated. These neurons have gaps in between the myelin
sheaths called Nodes of Ranvier. These gaps contain the highest concentration of voltage-gated
Na+ and K+ channels. Therefore, impulse jumps from node to node.
 This increases conduction velocity by 10-100 times. conduction velocity of myelinated neurons =
approx 100 meters / sec (depends on diameter)