Axon

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Axons carry electrical signals away from the cell body. Axons transmit signals to muscles, glands, and. most commonly, other neurons. Axons transmit signals through action potentials, which are generated at the axon hillock.

Squid Giant Axon

The squid giant axon was one of the first axons to be studied. It was first described by L. W. Williams in 1909.[1] Since then various studies have been conducted in order to learn more about the axon.

Because of it's large size, typically about .5 mm in diameter, early neuroscientists were able to study it easier than many other axons, because axons are generally much smaller. in 1952, Alan Hodgkin and Andrew Huxley used the squid giant axon to understand the ionic mechanisms that propagate action potentials. [2]

Action Potential

Action potentials, also referred to as "spikes", are generated in neurons to carry the electrical signals from the cell body to the synapses on the axon. Action potentials are short changes in the electrical potential across the cell membrane in an axon. Hodgkin and Huxley found that the resting potential across the membrane of the neuron was -60mV inside relative to outside. The action potential is a reversal of the polarity (the potential across the membrane becomes positive inside relative to outside) that travels down the axon to the axon terminals.[2][3]

The potential across the membrane is created by the presence of different ions on either side of the membrane. to maintain the potential, ion channels are employed to keep the concentrations stable. In order to propagate the action potential, the ion channels allow more positively charged ions to flow into the cell and negatively charged ions to flow out.[2]

Myelin Sheath

The squid giant axon is large because it increases the speed that the action potential moves along the axon. Another mechanism that has evolved to increase the action potential speed is myelin sheaths. Myelin sheaths consist of glial membranes that are wound multiple times around an axon. Myelin works by insulating the axon from the surrounding environment. Instead of the depolarization occurring at every point along the axons's length, it only happens at breaks in the myelin, called the nodes of Ranvier.[4]

References

  1. Williams, L. W. (1909) "Anatomy of the Common Squid" (American Museum of Natural History)
  2. 2.0 2.1 2.2 Hodgkin, A. L., Huxley, A. F., Katz, B., (1952) Measurement of Current-Voltage Relations in the Membrane of the Giant Axon of Loligo J. Physiol. 116, 424-448
  3. Barnett, Mark W., Larkman, Phillip M. (2007) The Action Potential Pract. Neurol. 7:192-197
  4. Hartline DK, Colman DR (January 2007). Rapid conduction and the evolution of giant axons and myelinated fibers. Curr. Biol. 17 (1): R29–35.