1. You are playing in the very first intragalactic all-humanoid basketball tournament, against the people of the planet Garf. The Garfians are very similar to Earthlings, except they average about a foot taller in height, so their team towers over the tallest team that Earth can muster. Their stamina is amazing. A Garfian can run at a fast pace all day long without stopping to rest. To your surprise, although no one on your team could block a shot from a Garfian and your whole team was worn out by the end of the game, you beat them to every position, clobbered 'em on fast breaks, and they were too slow to block even the most obvious passes. You won easily.
Extensive physical and neurophysiological exams were conducted on humans (oops-not politically correct - Earthlings) by the Garfians when contact was established, and we, of course, examined them. We found the physiology of Earthlings and Garfians to be nearly identical, however there was a major difference between the axons of the motor neurons of the two species. Based on the basketball results and the neurophysiological exam, why do you suspect the Garfians were slower, and what difference in axon properties could be responsible?
gfxxykk-ahhh-CHOO! That means close, but incorrect in Garfian. The results of the game do suggest that Garfian motor neurons conduct action potentials more slowly than those of Earthlings. Partial depolarization of a membrane does affect conduction of an action potential. However, in the Garfians a lower potential is normal. The ion channels that are responsible for an action potential would be reset in the resting state, just as they are in Earthlings. If anything, a lower normal resting potential in Garfians would result in the potential reaching threshold sooner than for Earthlings. The result would be a faster conduction velocity, and the Garfians, with their superior height, would have "smoked" us.
phlbbb! That means "bingo" or correct in Garfian. Garfians apparently react mentally just as fast as we do, but the signal simply takes longer to reach the synaptic cleft. That is, conduction velocity of their motor neurons is probably slower. The smaller axon diameter is the most likely reason. Fiber diameter and nerve conduction velocity have been demonstrated to be well correlated in Earth vertebrates. Here are a couple of likely reasons.
First, the passive wave, that is, the "jolt" that is felt down the length of the axon when a region is depolarized, is better preserved in a large diameter fiber because the electrical resistance of a conductor is inversely proportional to its diameter. Less resistance means more efficient conduction of the passive wave, so a threshold signal is felt further down the nerve. Conduction in myelinated nerves (motor neurons are myelinated) is facilitated by the insulation provided by the myelin sheath, which is interrupted periodically by the nodes of Ranvier. A larger diameter fiber can "jump" more nodes.
Second, the thickness of the sheath and distance between nodes appears to be reduced proportionally in smaller diameter fibers. Since smaller diameter fibers have more nodes per unit length, a stimulus must move ions through proportionally more sites to generate an action potential. Therefore, a stimulus of a given magnitude is less effective on a small diameter axon than on a large diameter axon.
For a small diameter axon the stimulus that triggers an action potential would be dissipated more quickly, and it would be less efficient, all resulting in a slower rate of conduction of an action potential t the effector organs. From brain to foot, the distance i Earthlings and Garfians is on the order of 2 meters. At 100 meters/sec, it takes 0.02 sec to conduct the information, plus any delays at the synapses. If Garfians lose a hundreth of a second to conduction time with each and every muscle movement, they don't stand a chance.
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[Obscene noise] Any obscene noise means "no" on the planet Garf - they are not known as a polite people. One might think that reducing the rate of transport of neurotransmitter substances from the cell body to the synaptic cleft might slow communication between the nerve and muscle. However, if sufficient neurotransmitter is there in order to deliver the signal, then it will happen. Neuromuscular transmission in vertebrates is "all or none", this is, either the signal is delivered or it can't be delivered. Reduced availability of neurostransmitter would result in faster fatique of the neuromuscular junction. The problem with the Garfians wasn't that they couldn't respond. They simply couldn't react fast enough.