Much of our understanding of neuron function was based on studies of the squid giant axon. Squid move through the water by contracting the muscles of the mantle. This compresses water inside the mantle that is forced out the siphon. Squid can change the direction of movement by directing the flow from the siphon either forward or backward (relative to the head or anterior end).
1. In which direction would the squid move if water flow from the siphon was directed toward the head end of the organism?
2. When squid are startled or in danger, they can simultaneously contract all muscles of the mantle to jet water forcefully out of the siphon and escape rapidly. Assume the mantle of the squid is 30 cm in length (about 12 inches). The brain sends a signal to major nerve ganglia in the mantle, which relay the signals to axons innervating the
mantle muscles. For all muscles of the mantle to contract simultaneously, all nerve signals sent along these axons must reach all parts of the mantle at the same time.
a. On the diagram of the squid above, draw and number three neurons. Assume all are simultaneously stimulated by a single motor neuron from the brain. Neuron 1 innervates the mantle muscles nearest the brain. Neuron 2 innervates the muscles in the midregion of the mantle. Neuron 3 innervates the muscles at the tail end of the mantle.
b. In invertebrates, like the squid, how must the nervous system be structured to allow both the muscles nearest the brain and those farthest from it to contract simultaneously? Modify your drawing to indicate any differences in the size or structure of the three neurons that would be required. Explain your reasoning.
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