Saltatory conduction is a type of nerve impulse that helps signals get from one place to another in a fast and efficient way. This type of conduction tends to be somewhat involved, at least from a biological perspective, but in a general sense it happens in three steps: a nerve receives a signal; that signal travels the length of the nerve by hopping or jumping from node to node; and it then arrives at its destination. In most cases this “jumping” is a lot faster than actually traveling through the nerve on the inside.
This type of conduction gets its name from the Latin root word saltare, which means "to leap." Saltation saves time and improves energy efficiency in the nervous system, but it isn’t the only way to transmit signals. It’s often the most common, but a lot of this depends on what is being sent and where it’s coming from. Really rapid impulses and responses often use this sort of jumping motion while slower, more involved signals might travel along the actual core of the nerve, known as the axon.
Nerves typically consist of an axon that is surrounded by various nodes of a fatty material known as “myelin.” Myelin acts as an insulator that both protects the nerve and keeps its signals contained. The nodes don’t usually cover the entire nerve, though, and there are usually tiny spaces between each where the axon is more or less exposed. These are called the “nodes of Ranvier.” Saltatory conduction happens when signals “hop” from myelin sheath to myelin sheath, essentially skimming over the top of the axon.
Understanding Nerve Impulses
The main goal of nerves is to transmit signals from one place to another. Sometimes the message they carry relates to pain or sensation, like hot or cold; they can also help control muscle and organ responses and movements. Signals of all sorts are often called “action potentials.”
Action potentials are perhaps best understood as electrical impulses that act as signals to and from neurons and muscle cells. These nerve impulses are typically generated only in the axon of the nerve. The axon then conducts the electrical current to its final destination, which typically is a synapse. Voltage-gated ion channels along the length of the axon sustain the electrical current and keep it working.
Importance of the Myelin Sheath
The Myelin sheath acts an insulator and prevents electrical charges from leaking through the axon membrane. Virtually all the voltage-gated channels in a myelinated axon concentrate at the nodes of Ranvier. These nodes are spaced approximately .04 inches (about 1 mm) apart.
Electrical impulses cannot actually pass through the sheathed portions of the axon, which is where the saltatory process becomes really important. Instead, the current jumps rapidly from one node to the next, each time triggering another action potential. The process continues along the length of the axon until it reaches its final destination.
Not all nerve impulses use this process. Axons that don’t have myelin can’t use it, for example. Conduction is generally much slower in unmyelinated axons because voltage-gated sodium and potassium, or Na+ and K+, channels have to repeatedly regenerate the action potential at various points in order to keep the electrical current from fading out, a process known as “decaying.” Slow signals aren’t usually a problem for internal organs, the digestive system, blood vessels, and most glands. Bodily systems that typically need to react quickly, however — such as the central nervous system — rely more heavily on myelinated axons and, as a consequence, the saltatory process.
The two biggest advantages of this sort of faster conduction are increased signal velocity and improved energy efficiency. This type of conduction is typically about 30 times faster than continuous conduction. By limiting electrical currents to the nodes of Ranvier, saltatory conduction also allows fewer ions to leak through the membrane. This ultimately saves metabolic energy, which is often a significant advantage since the human nervous system tends to use about 20 percent of the body’s metabolic energy.