How does the conduction of an impulse in a myelinated neurone differ from that in an unmyelinated neurone?

Myelinated neurones use saltatory conduction, where the action potential 'jumps' between nodes of Ranvier. Unmyelinated neurones use continuous conduction, where the wave of depolarisation must travel along the entire length of the axon membrane, making it much slower.

Why does a larger axon diameter result in a faster nerve impulse?

A larger diameter increases the volume of the cytoplasm, which reduces the internal resistance to the flow of ions. This allows local currents to spread faster and further, triggering the next action potential more rapidly.

What is the relationship between the refractory period and the maximum frequency of nerve impulses?

The maximum frequency is inversely proportional to the duration of the refractory period. Because the neurone cannot fire again until the refractory period ends, a shorter recovery time allows for more impulses to be sent per second.

What common error do students make when describing the 'jumping' of an action potential?

Students often mistakenly state that the action potential jumps between different neurones. In reality, saltatory conduction refers to the impulse jumping between nodes of Ranvier along the axon of a single neurone.

Why might a calculation of maximum frequency be incorrect if the refractory period is $2 ms$ and the student calculates $1/2 = 0.5$?

The student failed to convert milliseconds to seconds. The correct calculation should use $0.002 s$, resulting in a frequency of $1 / 0.002 = 500 Hz$.

How does temperature affect the speed of diffusion during an action potential?

Higher temperatures increase the kinetic energy of ions. This causes them to move and diffuse across the membrane and through the cytoplasm more quickly, speeding up the depolarisation process.

Define 'Saltatory Conduction'.

It is the method of nerve impulse transmission in myelinated neurones where the action potential jumps from one node of Ranvier to the next, bypassing the insulated myelinated sections.

What role do Schwann cells play in impulse conduction?

Schwann cells wrap around the axon to produce the myelin sheath. This sheath acts as an electrical insulator that prevents ion leakage and enables saltatory conduction.

What is the formula for calculating the maximum frequency of action potentials?

The formula is $\text{Frequency} = 1 / \text{refractory period duration}$. The time must be in seconds to give the answer in Hertz (Hz) or impulses per second.

Why does temperature have a limit in increasing conduction speed in mammals?

While heat increases kinetic energy and enzyme activity, excessively high temperatures can denature the proteins and enzymes (like the $Na^+/K^+$ pump) required for maintaining the resting potential, eventually stopping conduction.

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Speed of Impulse Conduction

Summary

The velocity at which a nerve impulse travels along an axon is not constant across all biological systems; it is determined by specific structural and environmental factors. By optimizing the physical properties of the neurone—such as insulation via myelin and increasing internal diameter—organisms can significantly enhance the efficiency and speed of their nervous coordination.

1. Definition & Core Concepts

Conduction Velocity: This refers to the speed at which an action potential propagates along the length of an axon, typically measured in meters per second ( $m/s$ ). Faster conduction allows for more rapid responses to stimuli, which is a critical evolutionary advantage for survival.
Nerve Impulse: An electrical signal consisting of a wave of depolarisation and repolarisation that moves along the axon membrane. The speed of this wave is influenced by how quickly ions can move and how well the electrical charge is maintained.
Determinants of Speed: The three primary factors that dictate the speed of conduction are the presence of a myelin sheath, the diameter of the axon, and the ambient temperature of the organism's environment.

Graph showing the relationship between axon diameter and conduction speed for myelinated and unmyelinated neurones.

2. Myelination & Saltatory Conduction

Myelin Sheath: This is an insulating layer composed of fatty substances produced by Schwann cells that wrap around the axon. It prevents the leakage of ions across the membrane, effectively forcing the electrical current to travel further down the axon before needing regeneration.
Nodes of Ranvier: These are small gaps in the myelin sheath where the axon membrane is exposed to the extracellular fluid. Because the myelin acts as an insulator, action potentials can only occur at these specific points where ion channels are concentrated.
Saltatory Conduction: In myelinated neurones, the action potential appears to 'jump' from one node of Ranvier to the next. This is significantly faster than continuous conduction because the depolarisation process only needs to happen at intervals rather than along the entire length of the membrane.

3. Axon Diameter & Resistance

4. Temperature Effects

5. Calculating Maximum Impulse Frequency

6. Key Distinctions

7. Exam Strategy & Tips