Central Nervous System, neurons and nerve impulses; all are familiar yet almost mystical words because many of us understand so little about this fundamental process producing human awareness.
Human awareness occurs in incremental stages starting with the peripheral nervous system’s sensory input, delivered via neurons, to the central nervous system where neural impulses are routed into the spine and then upwards through the spinal cord and into the brain for processing and responses. Neurons are our initial source of contact with the ‘out-side’ world through our senses of sight, sound, touch, and smell.
According to David Meyers (2010, p.49) “Our nervous system has a few million sensory neurons, a few million motor neurons, and billions and billions of interneurons.” all of which have three basic structural types, or categories: Unipolar, Bipolar and Multipolar Neurons that in many regions of the body are mixed in shape within a complex and profuse web of neuronal dendrites; a single interneuron in the cortex of the brain may have thousands of connections with other neurons through their filaments of ‘dendrites’.(Carter et al, 2009)
To get an idea of how these neurons work we can follow them in action as they function, beginning with a single sense receptor of touch (a ‘mechanoreceptor’): the oval shaped Pacinian corpuscles, within which is the neuron, in our skin (for illustration 1 see [1]).
Any skin deformation like a finger’s touch or the release of the touch, distorts what are called the cell’s ‘rings of lamellae’ and creates an internal shift of positive (+) chemical ions that, when they reach what is called a ‘Threshold potential’, generate an electrical pulse. (Carter et al, 2009)
This is the creation of a nerve signal, scientifically referred to as an ‘action potential’, that races away from the neuron cell body across the membranes of tubular ‘Axons’ (which are filled and surrounded with an intracellular fluid) during an electro-chemical propagation. According to Meyers again (2010, p.50) the neuron produces the electrical ‘action potential’ in the same fashion as our car batteries:
“The chemistry-to-electricity process involves the exchange of ‘ions’, electrically charged atoms. The fluid interior of a resting axon has an excess of negatively charged ions, while the fluid outside the axon membrane has more positively charged ions.”
What Meyers is describing is the “resting potential” that precedes a “depolarization” (that produces the charge) followed by the cyclic “re-polarization” (Carter et al, 2009)
When a second neuron (for illustration 2 see [2]) receives a neurotransmitter ‘signal’ from the Pacinian corpuscle the ‘resting potential’ balance is disturbed when the positive charge inside the receiving axon increases beyond the ‘threshold level’, just as in the Pacinian Cell’s neuron; the sodium channel ‘gates’ that are ‘normally’ closed then spring open allowing a flood of positively charged sodium ions into the axon interior (Macaulay, 2008).
“With this sudden reversal of electrical charge, the sodium channels slam shut and the potassium channels open briefly. The escape of potassium ions to the outside almost instantly returns the inner surface of the membrane to its original negative state.” (Macaulay, 2008)
Picture this axon as a porous ‘tube’ with numerous ‘gated’ openings, called ion channels, at the exposed intervals between ‘Mylien’ insulated sections [3] (for illustration 3 see [4]) at the “Axon Hillock”; the point of protrusion of the axon from the cell body. These intervals are scientifically referred to as a ‘Node of Ranvier’, or the “Neuralfibral Node” (approximately 1 μm [micrometer] wide) where the electro-chemical interactions take place and regenerate the traveling electrical impulse.
Whether during the pressure upon a Pacinian corpuscle or from the arrival of a signal from another neuron, ‘depolarization’ occurs as normally closed sodium channels in an axon membrane are electrically stimulated and rapidly open, allowing (+) sodium ions to rush into the [negatively charged] axon channel, morphing the interior to a higher positive charge than the exterior (Carter et al, 2009).
This creates a reversal of [interior] electrical charge that immediately shuts the sodium channels and opens potassium channels, briefly allowing (+) potassium ions to flow outward opposing the direction of the sodium ion flow. This action returns the area (behind the sodium ions flowing away from the cell body) to a (-) negatively charged resting state while simultaneously ‘depolarizing’ the area directly ahead of the flowing (+) positively charged sodium ions (Carter et al, 2009).
Axons at the exposed Node of Ranvier are ‘semi permeable’, which is to say they allow ions of only a particular size to traverse them (Sodium ions through their custom sized channels, Potassium ions through theirs etc.), insuring a successful process as the pulse races on unidirectional toward the axon’s synaptic terminal.
But when an impulse arrives at a presynaptic terminal it is transmitted across the synaptic ‘gap’ not with an electrical ‘charge’, but with neural transmitters that are ‘structurally shaped’ molecules. (Carter et al, 2009).
A neuron’s ‘pre-synaptic’ ‘Axon terminal’ sends structurally shaped neurotransmitters across the ‘synaptic gap’ to merge into the next neuron’s [structurally shaped post synaptic] ‘receptors’, which are located on the receiving neuron’s ‘dendrites’ and ‘cell body’; these neurotransmitters are structural ‘keys’ that can only fit into the receptors they are designed for.
A receiving/sending neuron such as would receive the Pacinian corpuscle’s neurotransmitters and subsequently generate its own impulse is one particular neuron is the type called a ‘Multipolar Neuron’ (see illustration 3).
In overview, the ‘Soma’ is the cell’s ‘body’ and its postsynaptic receptors are located on the dendrites and the cell body itself with the presynaptic Axon terminals located at the opposite end of the Axon. When the appropriate activity of electrically charged ions occurs over the dendrites and/or cell body and another ‘threshold’ is attained the sodium ion gates will open at the base of this cell body where the first ‘node of ranvier’ is located (for illustration 4 see [5]).
At the node of ranvier an axon’s “action potential” ‘Jumps’ this un insulated gap (containing all the ion channels) during the polarity change between the three stages of electro-chemical action: the “resting potential”, “depolarization” and “repolarization” (Carter et al, 2009, p. 72).
And the pulse goes on in this manner and into the brain’s processing producing our sensory imagery.
This is how our individual awareness begins through neural impulses, some traveling as fast as 200 mph; about 40 billion neurons, each communicating with thousands of other neurons through as many as 400 trillion synapses creating our perceptions (Meyers 2010, p.56 [de Courten-Meyers 2005]).
References:
1. David Myers, 2010. Psychology, Holland, MI, Worth Publishers.
2. Rita Carter, Susan Aldridge, Martyn Page, Steve Parker, (2009). The Human Brain Book, New York, NY, DK Books
3. David Macaulay, 2008, Getting to Know the Amazing Human Brain, Houghton Mifflin Company, Boston, MA
Endnotes:
[1] Illustration 1 of a Pacinian corpuscle. (2010, February 27). In Wikipedia, The Free Encyclopedia. Retrieved 19:59, June 30, 2010, from: http://en.wikipedia.org/w/index.php?title=Pacinian_corpuscle&oldid=346713949 Public domain.
[2] Illustration 2 of a Chemical synapse. (2010, June 19). In Wikipedia, The Free Encyclopedia. Retrieved 01:46, July 1, 2010,from : http://en.wikipedia.org/w/index.php?title=Chemical_synapse&oldid=368984490
Freely licensed; “This work is in the public domain in the United States because it is a work of the United States Federal Government under the terms of Title 17, Chapter 1, Section 105 of the US Code.”
[3] According to Carter (2009), “Impulses travel at widely differing rates, from 3 to more than 350 ft/s (1-100 m/s), depending on the type of nerve carrying them. They are fastest in myleinated axons. Here the impulse ‘jumps’ from one gap (Neuralfibral node), to the next.”
[4] Illustration 3 of a Neuron. (2010, June 29). In Wikipedia, The Free Encyclopedia. Retrieved 21:40, July 1, 2010, from: http://en.wikipedia.org/w/index.php?title=Neuron&oldid=370862035Freely licensed; “en.wikipedia, the copyright holder of this work, has published or hereby publishes it under the following licenses: This file is licensed under the ‘Creative Commons Attribution-Share Alike 3.0 Unported license”
[5] Illustration 4 of Nodes of Ranvier. (2010, May 23). In Wikipedia, The Free Encyclopedia. Retrieved 21:42, July 1, 2010, from: http://en.wikipedia.org/w/index.php?title=Nodes_of_Ranvier&oldid=363783698 Public domain
