Nerve Impulses

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How fast does a nerve signal travel?

Big Ideas: 
  • Upon being subjected to stimuli the brain sends electro/chemical signals along the nerves to the part of the body where a response is desired.
  • The difference in eye-foot and eye-hand reaction times can be attributed to the finite propagation speed of the nerve impulse along the nerve.

Why are eye-foot and eye-hand reaction times different?

The brain controls your movement by sending nerve impulses down the nerves to the place in the body where movement is desired. The nerves are composed of nerve cells called neurons. Figure 1 shows a signal propagating from one neuron to another. Along the neurons the signal propagates by an electrical impulse which travels along the axon. The axon from one neuron does not touch the next neuron forming a gap called a synapse. The signal propagates across the synapse by chemical diffusion and causes the next neuron to 'fire' and send the signal electrically down its axon. The typical spacing of the synapse is about 20-30 nm1.

Figure 1. The signals from the brain are relayed from one neuron to the next. The signal propagates electrically along the axon and chemically between the synapses2.

The typical time it takes for a person to respond to a visual signal with their hands (for example pressing a button upon seeing a green light) is around 0.28 s3. To respond with the feet takes longer, around 0.45 s3. A portion of these reaction times is due to the brain processing the visual signal and initially sending out the signal to move. The remainder of the time is taken for the signal to travel down the nerves to the hands/feet. Assuming the same brain processing time in both circumstances the difference between the reaction times for the eye-hand and eye-foot reaction times is due to the difference in distance the signal has to travel from the brain to the hand or foot. Based on the times given and an approximation of distances between body parts we can calculate the average velocity that the signal propagates at.

Calculations:

The distance from your brain to your hands is approximately 1 meter, and from your brain to your feet is approximately 1.6 meters. The extra distance the nerve impulse has to travel to go to your feet is 0.6 m as compared to your hand. It takes (0.45 -0.28) = 0.17s longer for the signal to reach your feet so the speed of the signal, vs propagating along the nerves is

$ v_s = \dfrac{0.6 \textnormal{ m}}{0.17 \textnormal{ s}} = 3.5 \textnormal {m/s} $

Note that this is an average speed of asignal traveling along one neuron. The actual time it takes for the signal to travel along the axon of one neuron can be greater than 25 m/s but to relay the signal to the next neuron across the synapse is about 1000 times slower1.

Summary:

The eye-foot reaction time is longer than the eye-hand reaction time due to the extra distance the nerve impulse has to travel. We calculated an approximate speed of signal propagation along the nerves to be 3.5 m/s. This is slower than the propagation of the signal along one neuron but takes into account that the signal must also cross the synapses by chemical diffusion.

Resources
Lecture Notes: 
Nerve Impulses Lecture Notes
Multiple Choice Problems: 
Nerve Impulses Multiple Choice Questions

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