Surely this has happened to you before: you see a glass fall out of the corner of your eye and catch it reflexively in a split second before it shatters into hundreds of pieces on the ground. We owe this lightning-fast reaction to the saltatoric excitation conduction. The saltatoric excitation conduction is a particularly fast way of transmitting stimulus between nerve cells. It takes place on myelinated nerve fibers.
Excitation Conduction – Foundation
Before you clicked on this article, a corresponding signal was created in your brain. In the next step, this signal must first reach your fingers. Once the signal has reached your fingers, the corresponding muscles can contract there and in this way carry out the command of your brain. The transmission of such signals happens in nerve and muscle cells in the form of electrical stimuli. For this purpose, a so-called action potential is generated at the axon hillock of a nerve cell. This travels along the axon and can then be transferred to a subsequent nerve or muscle cell.
Check out our action potential explanation if you want to know more about this topic.
Depending on the type and process, there are two types of transmission of electrical signals:
- The continuous excitation line
- The saltatory conduction
Figure 1: Process of saltatory conduction
Saltatory conduction principle
In order for a neuron to conduct saltatory conduction, it needs a myelin sheath. Such myelinated nerve cells are also referred to as marrow-containing neurons.
The myelin sheath insulates the nerve cell much like a rubber sheath insulates the conductors of a power cable. In contrast to cables, however, the insulating layer of the axons is interrupted every 0.5 to 2 mm. These non-sheathed sections will Ranvier’s nodes called, while the covered areas in between are called internodes.
The myelin sheath is formed by so-called glial cells. In the CNS, these are oligodendrocytes. In the peripheral nervous system, the Schwann cells This task.
glial cells are specialized cells of the nervous system that are responsible for the electrical insulation and the nutrition of the nerve cells. They also form the supporting tissue in the nervous system.
This structure is crucial for the enormous advantage that myelinated neurons have over myelinated neurons. This is because, while the action potential has to be passed on along the entire axon in the case of continuous excitation conduction, the action potential in myelinated neurons can jump from one node of Ranvier to the next.
the saltatory conduction is therefore a way of conducting excitation to nerve cells in which the action potential does not have to be transported continuously via the axon.
saltars is Latin and means jump. This describes this form of excitation conduction very well:
The internodes are used in the formation of action potentials skippedso action potentials from node to node jump. As a result, the forwarding can take place much faster than with the continuous excitation conduction.
The majority of nerve cells in the body of vertebrates use this principle of stimulus transmission.
Drainage of the saltatory excitation conduction
In the following sections you will get to know the process of saltatoric excitation transmission step by step.
triggering of the action potential
In order for an excitation transmission to take place, there must first of all be a stimulus. This must be strong enough to attach to the axon hillock of the nerve cell threshold potential from -40 to -50 mV. Now find one potential reversalr instead. This means that sodium channels in the membrane open. Due to the influx of sodium ions, the inside of the axon, which is negative in the resting state, becomes positive (up to approx. +30 mV) in comparison to the outside environment. This change in membrane potential is called an action potential.
transmission of excitement
Due to the formation of the action potential, the front axon section has a more positive charge than the neighboring, still unexcited axon section (there is a resting potential of approx. – 70 mV). So there is a charge difference between the excited and not yet excited area. This charge difference now causes ions to flow between the two sections of the axon to compensate for the difference. the Compensation current theory (or stream theory for short) is based on the assumption of such balancing ionic or circular streams.
Due to the equalizing circular currents on both sides of the membrane, ions are drawn from «axon downwards», whereby the resting potential there becomes more and more positive. This depolarization is ultimately sufficient to open Na+ channels and create a new action potential. However, the Na+ channels are found almost exclusively at Ranvier’s nodes, so that the ion currents have to flow to the next node and only there do the Na+ channels open due to the depolarization and a new action potential is created.
Now the interior of this node of Ranvier is more positively charged than the next segment of the axon, so the whole thing repeats itself. And this continues until the terminal button of the axon is reached. Each action potential is the trigger for the emergence of new action potentials at the adjacent node of Ranvier.
If the constantly newly formed action potentials are created by ionic currents, why are they always directed towards the end of the head? Why don’t they flow backwards?
This is due to the so-called refractory period: after an action potential has expired, the membrane cannot be excited for a short time (absolute refractory phase) because the channels close and no longer open for a while. Even with stimuli above the threshold, the channels do not open. As a result, the action potential can only continue in the direction of the points where the channels are still open (towards the end of the axon). It also limits the duration of the action potential.
Transmission of excitation to downstream cells
Arriving at the terminal button, the excitation can be transmitted – depending on the neuron – either to a subsequent nerve cell or a muscle cell. This happens at the so-called synapses. Here the electrical signal is converted into a chemical signal. If it is a neuromuscular synapse, the transmission of the stimulus ultimately leads to the contraction of a muscle cell.
If you want to know more about what happens when the action potential reaches the end of the axon, read the synapse and motor endplate explanation.
Benefits of saltatory excitation conduction
The saltatory is the more efficient method compared to the continuous excitation conduction. This is due to the following points:
- High reaction speed: A higher conduction velocity allows the signal to reach the appropriate muscle fibers faster, so that the planned movement can also be carried out faster. These faster reactions constitute a survival advantage.
- Material & space savings: Since the excitation conduction itself is already abruptly fast, the diameter of the axons can be reduced. That means you don’t have to use up so much building material for nerve fibers and space.
- energy reduction: In the internodes of myelinated axons there are almost no Na+ channels and Na+-K+ pumps, which would consume a lot of energy. These can be found almost exclusively on the lacing rings. This limitation also saves energy in the saltatory excitation conduction.
Salatorial conduction – internode length
As you already know, there are almost no Na+ channels in the internodes. As a result, the ion currents must flow from Ranvier’s node 1 to Ranvier’s node 2 in order to be able to trigger a new action potential. The ion current on its way becomes weaker and weaker as the distance increases. So that an above-threshold depolarization can still occur at the second node of Ranvier, the distance between the nodes must not be too large.
On the other hand, the internodes save energy because there are almost no energy-consuming Na+-K+ pumps there. For reasons of energy, one would therefore prefer internodes that are as long as possible.
As so often in life, a compromise is needed! As a result, a maximum distance of 2 mm between the nodes of Ranvier has evolved. Because at this distance you can save energy and the incoming ion current is just sufficient to trigger an above-threshold depolarization.
Saltatory conduction – influencing factors
There are a few factors that affect the speed of excitation conduction:
- myelination: As you now know, myelinated axons undergo saltatory transmission. Since the excitation line «jumps» from node to node here, the transmission of the electrical signal is quite fast. In contrast, impulses are continuously transmitted in unmedullated axons. New action potentials are constantly being produced, which means that it takes longer for the electrical signal to reach the end of the axon.
- fiber diameter: The larger the nerve fiber diameter, the greater the conduction velocity. This reduces the internal resistance. You can imagine it like a door: the wider it is, the faster the class can run out because more students can fit through it.
- Temperature: There is also an optimum temperature range for optimum excitation conduction. Above and below the speed is reduced.
Unlike vertebrates, squid do not have myelinated nerve fibers. Nevertheless, they are able to quickly transmit stimuli within their nervous system. This is possible because they use a different principle to increase the speed of stimulus transmission: they have so-called giant axons. These are axons that can be up to 1 mm thick in cross-section. The increased diameter reduces the internal resistance in the axon. Due to the lower resistance, transmission can take place much faster than on a thin axon.
Saltatory vs. continuous conduction
In the following table you can see the characteristics of the two forms of arousal transmission so that you can compare them at a glance:
Figure 3: Comparison of continuous and saltatory excitation conduction