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Posted by on Dec 21, 2012 in Science |

Giant neurons

Giant neurons

What is the strangest cell organism? Neurons. What is the longest? Neurons. What is the maximum size of a neuron? Than 40 m In a fascinating article , Mathew J. Wedel reviewing existing data to reach such an amazing response.

The strange neurons

We have the idea of the cell as something round and small. Most of them are indeed so. But the neuron is very different. While long puzzled biologists. It was the genius of Santiago Ramon y Cajal who discovered its characteristics, including its strange shape.

It has a cell body or soma, and the remaining cells in the core which is DNA and which make most of the substances required for its operation. Two branches out of the body, dendrites (where it receives nerve impulses) and the axon (for the rising pulse). The axon is thin and extremely long. If the soma of a neuron was a soccer ball, the axon would measure 10cm diameter and length could reach 50km. . Since soma is the metabolic center, there are substances that have to make a long journey from the soma to the end of the axon and back.

Cajal pre Scientists could not see the entire axon in the microscope and therefore thought that cells were not separated but were all united in a grid. But Cajal worked hard on embryos and could decipher the riddle giving way to the neuron doctrine and revolutionizing neuroscience.

Recurrent laryngeal nerve

Evolution is not as intelligent design creationists argue. Perspective is bright and genuine jumps bungling would work better if they had been redesigned from the ground up. But this does not happen. A remarkable case is that of a nerve called the recurrent laryngeal nerve .

Giant neurons image 2

We called tetrapods to the descendants of fish including amphibians, reptiles, birds and mammals. Among other features we have four limbs.

Fish inherit the nerve called the recurrent laryngeal nerve. This nerve connects the base of the brain to the larynx, which is used to make sounds (speaking in humans) and swallow. In this nerve fish passes under uns blood vessels near the heart. This is not a problem if you’re a fish, but it becomes an inconvenience, poor design, if you’re a tetrapod.

The fish have no neck, but the rest of tetrapods. The fact is that as we evolve, the brain was separating the heart due to neck. The nerve in lower brain matter, goes under the blood vessels near the heart and goes up through the neck to the larynx. Intelligent design would have done to prevent a short circuit as long as futile journey.

Giant neurons image 3

What if we have a long neck? This is the case of the giraffe. This animal has a collar of 2.5 meters and length of the nerve, which should drop from the brain to the heart and then up to the larynx, is 5 meters. Highly inefficient.

But there were animals with longer necks as some dinosaurs. Of course they do not retain nerves, but since all existing tetrapods have the nerve, it can be assumed that they also had. The largest sauropod dinosaur had a neck of 14 meters, so that his nerve was 28 meters

The toe

The recurrent laryngeal nerve is an example of bad design and a very long nerve. But is the longest in the body? No, the longer the sensory nerve that connects the tip of the toes to the brain. Many nerves have a layover (synapses) in the spinal cord as motors and some sensory. But the fine touch and responsible for the vibration goes directly to the brain and have their cell body or soma somewhere in the column.

In a high human this nerve can have two meters. In the blue whale reaches 30 meters. And in the largest presumably could exceed 40 meters.

We often experiment with mice. Do with large animals is not ethical nor practical nor inexpensive. Not only have the nerves of the dinosaurs but also there is dissections of giraffes or whales, but assumptions seem reasonable.


Apart from the distance, does this have any other involvement? Yes, times.

The sensory nerve axon fin whale in must grow fast to accommodate the growth of the animal. Its rate is 3 cm / day. This is similar to the growth rate of cancer cells.

More important is the speed of nerve signal conduction. In vertebrate animals, the speed is increased by increasing the diameter of the axon. It is very famous the squid giant axon is seen with the naked eye and allowed to discover the mechanisms of transmission of the electrical signal (action potential).

In vertebrates, axons are covered by a myelin sheath, something like plastic coated copper wires. This reduces energy consumption and increase the transmission speed.

The transmission speed is variable between 0.5 and 100 m/s. That means that in the case of a whale can vary between the second and third of 6 seconds . Although this is the first figure, most likely, is a significant time. And related to dinosaurs, the times would be even greater. However, that is in relation to the fine nerve sensitivity. The nerve carrying pressure on the spinal synapses and thus form a reflection of an animal which react in less than one second (even a very lengthy time).

One last time interest is the transport of substances through the axon. As seen, the proteins are formed in the cell body and transported everywhere including axons. The transmission speed is slow and proteins varies between 200-400 mm / day and 0.1-1.0 mm / day. Assuming an average of 1 mm / day, most substances not arrive in time to the end of the axon in the case of a whale or dinosaur and it would take decades and the animal would have died. Moreover, the axon growth rate would be greater than the transport of substances that feed, which is impossible.

Of course it’s all speculation though well founded. The research favorite animal is the mouse, much smaller than the tiny man in relation to a whale. From dinosaurs are only bones. In any case, the giant neurons of these animals pose enromes interesting challenges.

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