The Axon

 

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The Axon is a structure that is found only in neurons. It is highly specialized for the transfer of information over distances in the nervous system. The Axon begins with a region called the Axon Hillock, which tapers to form the initial segment of the Axon proper.

Axons may extend from less than a millimeter to well over a meter and often branch. These branches are called axon collaterals. Occasionally, an axon collateral will return to communicate with the same cell that gave rise to the Axon or with the dendrites of neighboring cells. These axon branches are called recurrent collaterals.

The tip of the Axon Collaterals is where the synapses form. The synapses are an essential component of the brain but are not described further in this study. They are covered extensively in standard textbooks and classes.

Glia cells (not shown) provide layers of membrane that insulate axons. This wrapping, called myelin, spirals around axons and provide insulation to the Action Potential that travels down the Axon.

The myelin sheath does not extend continuously along the entire length of the axon. There are breaks in the insulation where ions cross the membrane, to generate action potentials. The breaks in the myelin sheath are called the nodes of Ranvier and act as relay points for the action potential as it moves down the axon chain. Voltage-gated sodium channels are concentrated in the membrane of these nodes.

The distance between nodes is usually 0.2–2.0 mm, depending on the size of the axon (fatter axons have larger inter-nodal distances).


The Axon experiment using the Neuron Simulator

Video of a the flow of a spike through three axon compartments.

This experiment shows the flow of a spike of initial current moving down the length of three Axon compartments. Four oscilloscope probes are used to monitor the flow. The video was collected from the simulation running the body plan shown below.

If you have not done so already you may install the Neuron simulator as described in the last chapter.

Steps:

  1. Start the simulator, add components and arrange them approximately as shown in the figure below
  2. Make the connections using left clicks. The simulator should start operation
  3. Right click on the Step input you have chosen (Step 5 in this example) and set it parameters as shown.
  4. Adjust the sliders on the Oscilloscope to approximately the position shown
  5. Using the markers on the Oscilloscope measure the time difference between the leading edge on the step input and the mid point of the first 'scope (red) track. Log your reading.
  6. manipulate the parameters on the Step function to see the effect of pulse width on the spike as it moves through the chain of axons.
  7. Each compartment simulates the spiking action of the neuron Soma. Right click on an axon icon and change spiking type. Try 'Chattering'.

You might find it easer to pause the 'scope to apply the marker measurements


Collected Delays for three Axon Compartment
Component Start Stop

Delay Times

Step input 10 30 20 mS
AC 1 fire point 10.2 36.5 26.3 mS
AC1 to AC2 40.0 49.0 9 mS
AC2 to AC3 49.2 56.2 7 mS

Next: The Complete Neuron