describe and explain the rapid transmission of an impulse in a myelinated neurone with reference to saltatory conduction

Published by Patrick Mutisya · 8 days ago

Control and Coordination in Mammals – Saltatory Conduction

Control and Coordination in Mammals

Rapid Transmission of an Impulse in a Myelinated Neurone

In mammals, the speed at which an electrical impulse travels along a neurone is crucial for timely responses. Myelination dramatically increases this speed through a process called saltatory conduction. The following notes describe the structural basis, the mechanism, and the factors influencing this rapid transmission.

1. Structure of a Myelinated Neurone

  • Axon: Long, cylindrical process that carries the impulse away from the cell body.

  • Myelin sheath: Layers of lipid‑rich membrane formed by Schwann cells (PNS) or oligodendrocytes (CNS) that wrap around the axon.

  • Nodes of Ranvier: Small gaps (\overline{1} µm) between adjacent myelin segments where the axonal membrane is exposed.

  • Internodes: Myelinated sections between two nodes; typically 100–200 µm long in peripheral nerves.

Suggested diagram: Cross‑section of a myelinated axon showing nodes of Ranvier and internodes.

2. Mechanism of Saltatory Conduction

Saltatory conduction means “jumping” of the action potential from one node to the next. The sequence is:

  1. At a node, voltage‑gated Na⁺ channels open, depolarising the membrane and generating an action potential.

  2. The depolarisation creates an electric field that spreads rapidly along the interior of the axon beneath the myelin.

  3. Because the myelin sheath is an excellent electrical insulator, the current does not leak out; it travels longitudinally to the next node.

  4. When the depolarising current reaches the next node, it reaches the threshold, opening Na⁺ channels there and regenerating the action potential.

  5. The process repeats at successive nodes, giving the appearance that the impulse “jumps” along the axon.

The speed of conduction (\$v\$) can be approximated by the relationship:

\$v \approx \frac{L}{\sqrt{Rm Cm}}\$

where \$L\$ is the length of an internode, \$Rm\$ is the membrane resistance (increased by myelin), and \$Cm\$ is the membrane capacitance (decreased by myelin). The increase in \$Rm\$ and decrease in \$Cm\$ together raise \$v\$.

3. Key Factors Affecting Conduction \cdot elocity

FactorEffect on \cdot elocityExplanation
Myelin thicknessIncreasesThicker myelin raises membrane resistance, reducing current loss.
Internode length (L)Increases up to an optimumLonger internodes allow the impulse to travel farther before regeneration, but if too long the depolarisation may fall below threshold.
Axon diameterIncreasesLarger diameter reduces internal resistance, facilitating faster longitudinal current flow.
TemperatureIncreasesHigher temperature speeds the kinetics of ion channels, shortening the refractory period.

4. Comparison with Unmyelinated Axons

In unmyelinated axons, the action potential must be regenerated continuously along the membrane, leading to slower conduction. The differences can be summarised as:

  • Conduction speed: Myelinated ≈ 50–120 m s⁻¹; Unmyelinated ≈ 0.5–2 m s⁻¹.

  • Energy consumption: Myelinated axons require fewer Na⁺/K⁺‑ATPase cycles because fewer ions cross the membrane per unit length.

  • Signal fidelity: Saltatory conduction reduces the chance of signal degradation over long distances.

5. Summary

Saltatory conduction is a highly efficient method of impulse transmission in myelinated neurones. By insulating most of the axonal membrane and concentrating voltage‑gated channels at the nodes of Ranvier, mammals achieve rapid, energy‑efficient signalling. The key determinants of speed are myelin thickness, internode length, axon diameter, and temperature, all of which modify the electrical properties (\$Rm\$, \$Cm\$, internal resistance) governing the propagation of the depolarising current.