Introduction

Neurons transmit signals through electrical impulses called action potentials that typically travel from the cell body toward the axon terminal (telodendrons). However, when a nerve fiber is artificially stimulated in the middle of an axon—rather than at the beginning—the direction of impulse propagation can be quite different from normal physiological conditions. This article explores the direction of nerve impulse propagation under such experimental conditions.

Normal Direction of Nerve Impulse Propagation

• Under standard physiological conditions, the action potential originates at the axon hillock (near the cell body) and propagates unidirectionally toward the axon terminals.

• This one-way flow is ensured by the refractory period of membrane segments that have just fired, preventing backward (antidromic) propagation.old-ib.bioninja+2

Stimulation in the Middle of the Axon

• When electrical stimulation is applied to the middle of an axon experimentally, the surrounding membrane patches are both electrically excitable as neither side has undergone recent action potential firing or refractory period.

• This results in the generation of two action potentials traveling in opposite directions:

  • One impulse travels away from the stimulation site toward the cell body (antidromic conduction).

  • The other impulse travels away from the stimulation site toward the axon terminals (orthodromic conduction).wikipedia+2

• Both impulses maintain their propagation until they reach the respective terminations (cell body or synaptic end).

Explanation of Options

Supporting Experimental Evidence

• Classic neurophysiology texts and experiments have demonstrated bidirectional conduction when stimulation bypasses physiological initiation sites.kenhub+1

• The absolute refractory period and voltage-gated channel inactivation prevent backward conduction in normal physiology but are absent in both directions from the middle stimulation site.

Summary Table

Conclusion

The most accurate explanation for nerve impulse propagation when the nerve fiber is stimulated in the middle of an axon is:

(3) Nerve impulses are travelling in both the directions opposite to each other.