Main modes of excitation conduction
There are two basic mechanisms by which a nerve impulse propagates along excitable tissues: continuous and saltatory. Both rely on the generation of local currents, but proceed in completely different ways.
1. Continuous conduction This type of transmission is characteristic of unmyelinated nerve fibers (UM) and all types of muscle tissue. The process proceeds sequentially, involving new areas of the cell membrane step by step:
- In the zone of initial excitation, depolarization occurs (the "+" charge on the outside of the membrane changes to "-").
- A potential difference arises between this excited area and the adjacent one, which is still in a resting state.
- This difference generates local currents (LC), which flow sequentially along the membrane.
- Local currents affect the adjacent unexcited area, causing its partial depolarization.
- As soon as the charge reaches the critical level, pores for sodium ions (Na+) open in the membrane.
- A full-fledged action potential arises, and the entire cycle repeats for the next section of the membrane.
2. Saltatory conduction This mechanism is inherent only to myelinated nerve fibers (M). They possess a myelin sheath, which acts as an electrical insulator.
- Here, excitation propagates in a saltatory manner, exclusively via the nodes of Ranvier—small areas of the membrane free of myelin.
- Local currents close between the nodes, literally "jumping over" the insulated zones.
- The excitation is so strong that it simultaneously covers 3 to 5 subsequent unexcited nodes.
- The main advantage of this mechanism is that the propagation velocity of excitation is significantly higher than during slow continuous conduction.
Laws of excitation conduction in nerves
Physiology distinguishes four basic laws that govern the propagation of impulses in neural structures:
- Law of anatomical and physiological integrity (morphological and functional continuity). For successful signal conduction, complete preservation of the anatomical structure of the nerve (absence of transections) and the normal functional state of its membrane are required. If function is impaired (e.g., under the action of anesthetics), conduction is blocked. However, a reliability factor exists: excitation can "jump over" a blocked section if its length is less than the distance between 3–5 nodes of Ranvier.
- Law of bidirectional conduction of excitation. When a stimulus is applied to any point of a nerve fiber, the resulting impulse always propagates from the stimulation point (P) in both directions symmetrically.
- Law of isolated conduction of excitation. A signal traveling along one specific fiber within a common nerve trunk is not transmitted to adjacent fibers. Isolation is reliably provided by myelin and connective tissue sheaths.
- Law of relative non-fatiguability of the nerve. Under prolonged rhythmic stimulation, a nerve is able to conduct impulses without signs of fatigue significantly longer than muscles or synapses can.
Compound nerve action potential
Anatomically, a nerve trunk is a bundle of numerous diverse nerve fibers. If the electrical activity of an entire nerve is recorded, the shape of the resulting graph will heavily depend on the distance to the stimulation site.
- Near the stimulation site: Excitation starts almost simultaneously in all nerve fibers. A single powerful, unified action potential peak is recorded on the graph.
- Far from the stimulation site: The so-called dispersion (splitting) of the signal into separate peaks occurs. Multiple waves appear on the graph, designated by letters (alpha, beta, gamma, delta, B, C).
The reason for this splitting is that different fiber groups have different excitation conduction velocities. The highest velocity is characteristic of group A fibers, and the lowest velocity is for group C. During the journey from the stimulation point to the remote electrode, "fast" impulses manage to run ahead, while "slow" ones lag behind, forming a series of separate peaks.
Briefly: answers to topical questions
What classes of nerve fibers are distinguished according to the Erlanger-Gasser classification?
According to the Erlanger-Gasser classification, fiber groups Aα, Aβ, Aγ, Aδ, B, and C are distinguished.
- Group A (Aα, Aβ, Aγ, Aδ) — myelinated fibers (e.g., type Aα forms afferent fibers from muscle proprioceptors).
- Group B — myelinated (lightly myelinated) preganglionic fibers.
- Group C — unmyelinated ("gray") postganglionic fibers, which are the slowest and thinnest.
What factors cause a violation of the law of physiological integrity of the nerve?
A violation of the physiological integrity of the nerve is caused by factors that disrupt the functions of the nerve fiber membrane while preserving its anatomical structure.
These factors include:
- Anesthesia (action of anesthetics).
- Cooling.
- Pressure (mechanical compression, e.g., in an uncomfortable posture when a person falls asleep on a limb).
To remember fiber types: Myelinated — Lightning-fast (conduct saltatory and rapidly), Unmyelinated — Slower (conduct the signal continuously and sequentially).
Class and exam questions
Why is saltatory conduction faster than continuous conduction?
In myelinated fibers, local currents do not pass through every micrometer of the membrane sequentially. They "leap" over the myelin-insulated segments, closing only at the nodes of Ranvier and spanning 3–5 nodes at once, which enormously saves time.
What happens if a nerve is exposed to an anesthetic while preserving its anatomical integrity?
A functional block will occur, and impulse conduction will be disrupted according to the law of physiological integrity. However, if the blocked area is short enough (less than the distance between 3–5 nodes), the signal will be able to jump over it thanks to the reliability factor.
Why are several peaks recorded instead of one at a distance from the nerve stimulation site?
Because a whole nerve consists of different fiber groups with unequal conduction velocities (the fastest are group A, the slowest are group C). Over a long distance, this velocity gap leads to the splitting of a single signal into several waves.
What is the role of sodium ions (Na+) in the conduction of excitation?
Upon the membrane reaching the critical level of depolarization, specialized pores open. Through them, sodium ions rush en masse into the cell, which leads to the generation of a new action potential in the adjacent area.
What else to study on this topic
- Mechanisms of potential difference and local current generation
- Classification of nerve fibers by conduction velocity (groups A, B, C)
- Molecular structure of the nodes of Ranvier and concentration of Na channels
- The role of myelin and connective tissue sheaths in isolated conduction
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Conduction of excitation along nerve fibers" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the section "Excitation Conduction and Synaptic Transmission"
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.