MadSci Network: Neuroscience
Query:

Re: How fast are nerve signals?

Date: Mon Jun 28 21:55:10 1999
Posted By: Dr. Pedro M. Pereyra, Secondary School Teacher, Chemistry, Biochemistry, Ecochemistry, St. Thomas Aquinas S.S.
Area of science: Neuroscience
ID: 930153732.Ns
Message:

MAD Scientist Network

June 28, 1999

How fast are nerve signals?


Nerve conduction:

Nerve cells when stimulated generate what is known as an Action Potential 
or Spike (visit simulations at http://pb010.anes.ucla.edu/ ).  This 
Action 
Potential event, in mammalian nerve cells, lasts about 0.7ms 
(milliseconds). The Action Potential is a sharp local change in the voltage 
across the plasma membrane from an initial resting potential in the order 
of -55mV (millivolts) to a momentary potential across the membrane of +40 
to +50mV. A change in the order of 100mV.  This change in potential across 
the cell membrane is brought about by a sudden flow of positive Sodium ions 
into the cell and the flow of positive Potassium ions out of the cell. The 
change in the number of positive and negative ions inside versus outside 
the cell determines the voltage across the plasma membrane at any given 
time.

After the Spike, the cell membrane falls into a refractory period (where no 
new Action Potentials are possible) that last in the order of 2ms. During 
this time the potential across the cell membrane is lower then the resting 
potential making it very difficult for the cell to generate a new Spike. 
The concentrations of ions are being restored to Resting Potential values 
during this period.	

 As the potential across the plasma membrane changes at one point on the 
cell membrane, neighbouring areas of the membrane become excited.  The 
original site goes back to its resting potential state, and the 
neighbouring sites generate an Action Potential. In all, these will produce 
a spreading pattern of  reversals of the voltage across the neuron's  cell 
membrane (spreading depolarization potential). The Action Potential thus 
propagates from its initial trigger site on the  plasma membrane towards 
the neuron's axon.   Propagation along the axon occurs only in one 
direction: Away from the neuron's nucleus. 

Axons are surrounded (or ensheathed) along their entire length by auxiliary 
nervous system cells known as Glia Cells.  These Glia Cells surround the 
axons in essentially two ways. In one of them, Glia Cells fill the space 
between cris-crossing, or between parallel arranged axons (or nerve 
bundles).  Axons in the gray matter are ensheathed this way.  Conduction 
along the axons occurs by the spreading of a depolarization potential as  
discussed above.  Nerve signals (or trains of Action Potentials) travel in 
these types of axons at  conduction speeds ranging from 2 to 10 m/s, 
depending on the diameter of the axons ( thicker axons have faster 
propagation speeds).

The second way Glia cells surround axons, is by wrapping each axon with a 
spiral layer of Glial cell membrane known as Myelin. Myelinated axons are 
located in the white matter areas of the brain and most of the nerve 
bundles or peripheral nerves that relay information either to our muscles, 
or from sensory cells associated with fast reflexes or fast volitive 
actions.  The myelin wrapping  works essentially as an effective insulator 
that allows for small exposures of the axons (known as Ranvier Nodes) where 
the reversal of voltage across the membrane takes place.The propagation of 
the nerve impulses occurs in this case in a saltatory fashion, where the 
voltage perturbation jumps from node to node.  The length of the Myelin 
sheath between nodes can be as much as one or two millimetres long.  
Saltatory conduction, as is known, is very efficient and allows axons 
conduction speeds ranging from 10m/s up to 120m/s [36km/h  to 432 km/h!]  


The auditory and visual systems have myelinated nerves that allow for a 
fast and reliable transmission of signals from the eye, or the cochlear 
sound receptors, to the bain.  The distance covered are very small ( a few 
decimeters).  Time delays for such scales, for large  myelinated axons as 
the ones involved in these systems, will be around 5ms or less.  The 
returning impulses from the brain will take at the most, another 10ms to 
reach the muscles of the hand to grasp the falling ruler.  The rest of the 
delay in the response is due to processing delays at the sensory organs and 
in the brain's processing centers.


	


References:

	Junge, D.(1976). Nerve and Muscle Excitation. Sinauer Associates, 
Inc.  Sunderland, Massachusetts.

	Ritchie, J.M. (1984).  Physiological Basis of Conduction in 
Myelinated Nerve Fibres. In Myelin. Morell, P.,  Ed.  2nd ed. Plenum Press. 
Newy York. Pp: 117 -146.

	Kuffler, S.W., Nichols, J.G., and Martin, A.R. (1984). From Neuron 
to Brain. Sinauer Associates, Inc. Sunderland, Massachusetts.  


	Bezanilla, F. (1998). ELECTROPHYSIOLOGY and The Molecular Basis of 
Excitability.  Simulation Programs.  http://pb010.anes.ucla.edu/. [Last 
visited June 28, 1999].


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