Impulse transmission by mylenated axon

Impulse transmission by mylenated axon

Saltatory condition

➢ Action potentials propagate more rapidly along myelinated axons than along 

unmyelinated axons.

➢ It occurs because of the uneven distribution of voltage-gated channels. 

➢ Few voltage-gated channels are present in regions where a myelin sheath 

covers the axolemma. 

➢ At the nodes of Ranvier (where there is no myelin sheath), the axolemma has 

many voltage-gated channels. 

➢ Hence, current carried by Na+ and K+ flows across the membrane mainly at the 

nodes.

➢ When an action potential propagates along a myelinated axon, an electric 

current flows through the extracellular fluid surrounding the myelin sheath 

and through the cytosol from one node to the next. 

➢ Action potential at the first node generates ionic currents in the cytosol and 

extracellular fluid that depolarize the membrane to threshold.

➢ It opens voltage-gated Na+ channels at the second node.

➢ The resulting ionic flow through the opened channels constitutes an action 

potential at the second node. 

➢ Then, the action potential at the second node generates an ionic current that 

opens voltage-gated Na+ channels at the third node, and so on.

➢ Flow of current across the membrane only at nodes of Ranvier has two 

consequences.

○ The action potential appears to “leap” from node to node as each nodal 

area depolarizes to threshold, thus the name “saltatory.”

○ Opening a smaller number of channels only at the nodes represents a 

more energy-efficient mode of conduction. 

Factors that affect the speed of propagation:

Amount of myelination

➢ Action potentials propagate more rapidly along myelinated axons than along 

unmyelinated axons.

Axon diameter

➢ Larger-diameter axons propagate action potentials faster than smaller ones 

due to their larger surface areas. 

Temperature

➢ Axons propagate action potentials at lower speeds when cooled.

Encoding of stimulas intensity

➢ How can your sensory systems detect stimuli of differing intensities if all nerve 

impulses are the same size? 

➢ Why does a light touch feel different from firmer pressure?

➢ The main answer to this question is the frequency of action potentials.

➢ A light touch generates a low frequency of action potentials. 

➢ A firmer pressure elicits action potentials that pass down the axon at a higher 

frequency. 

➢ In addition to this “frequency code,” a second factor is the number of sensory 

neurons recruited (activated) by the stimulus. 

➢ A firm pressure stimulates a larger number of pressure-sensitive neurons than 

does a light touch.