explain the importance of the refractory period in determining the frequency of impulses

Published by Patrick Mutisya · 14 days ago

Control and Coordination in Mammals – Refractory Period

Control and Coordination in Mammals

Objective

Explain the importance of the refractory period in determining the frequency of impulses.

Key Concepts

  • Action potential: A rapid, self‑propagating change in membrane potential that carries a nerve impulse.

  • Refractory period: The time after an action potential during which a second impulse cannot be generated (absolute) or can be generated only with a stronger stimulus (relative).

Types of Refractory Period

  1. Absolute refractory period (ARP): No new action potential can be initiated, regardless of stimulus strength.

  2. Relative refractory period (RRP): A new action potential can be initiated, but only if the stimulus exceeds the normal threshold.

Why the Refractory Period Limits Impulse Frequency

The total time required before a neuron can fire another action potential is the sum of the absolute and relative refractory periods. This interval sets an upper limit on how rapidly impulses can be generated.

The maximum possible frequency (\$f_{\text{max}}\$) of impulses is therefore given by:

\$f_{\text{max}} = \frac{1}{\text{ARP} + \text{RRP}}\$

Where ARP and RRP are expressed in seconds. A longer refractory period results in a lower \$f_{\text{max}}\$, reducing the speed at which information can be transmitted.

Factors Influencing the Refractory Period

  • Ion channel kinetics (e.g., speed of Na⁺ inactivation and K⁺ activation).

  • Temperature – higher temperatures generally shorten the refractory period.

  • Myelination – myelinated fibres have shorter refractory periods than unmyelinated fibres.

  • Pathological conditions – certain toxins or diseases can prolong the refractory period.

Example Calculation

Consider a typical mammalian motor neuron with:

  • Absolute refractory period = 0.8 ms

  • Relative refractory period = 0.4 ms

Convert to seconds: 0.8 ms = \$8 \times 10^{-4}\$ s, 0.4 ms = \$4 \times 10^{-4}\$ s.

Maximum frequency:

\$f_{\text{max}} = \frac{1}{8 \times 10^{-4} + 4 \times 10^{-4}} = \frac{1}{1.2 \times 10^{-3}} \approx 833\ \text{Hz}\$

This illustrates how even small changes in the refractory periods can markedly affect the highest possible firing rate.

Summary Table

Neuron TypeAbsolute Refractory Period (ms)Relative Refractory Period (ms)Maximum Frequency (Hz)
Fast‑twitch motor neuron0.70.3\overline{1250}
Slow‑twitch motor neuron1.00.5\overline{667}
Unmyelinated sensory fibre2.51.0\overline{286}

Suggested diagram: A graph showing the relationship between refractory period duration and maximum impulse frequency, with separate curves for absolute and relative periods.

Key Take‑aways

  • The refractory period ensures unidirectional propagation of the impulse and prevents overlap of successive action potentials.

  • It sets a physiological ceiling on how fast a neuron can fire, directly influencing the frequency of nerve impulses.

  • Understanding the refractory period is essential for explaining muscle contraction rates, sensory processing speed, and the effects of drugs that modify ion channel behaviour.