Basic Training Series: Nervous System Principle #3: Centralization

A PRINCIPLE is defined as "a comprehensive and fundamental law, doctrine or assumption." These principles are necessary to have a simple perspective of a very complex system. As I have started to learn more about the Nervous System, I have found that remembering these 8 Principles has made it easier to get through complexity to the simplicity of it all.

Here is Dr. Angevine's Third Principle of the Nervous System:

CENTRALIZATION

"The key feature of the nervous system is centralization. It offers few circuits for local interactions of body parts. The CNS is almost always involved even if the distance, as from thumb to index finger, is slight. Intercession of the brain and spinal cord ensures integrated and coordinated activity. 

Exceptions are instructive. The local cutaneous response to irritating stimuli (raking a blunt probe over the skin) has three components: local reddening (vasodilation from injury), wheal formation (transient edema from tissue fluid extrusion), and ensuing vasodilation (flare) with lowered thresholds and increased sensitivity to pain (pinprick). The flare and hyperalgesia represent an axon reflex. Nociceptive (pain) nerve endings are activated by substances released by injured tissue cells, and nerve impulses are conducted a short way centrally along nociceptive axons and then distally over branches of these axons to nearby arterioles, causing them to dilate. Advanced or primitive (it is sluggish, starting in about 20 sec. and developing fully in around 3 min), this reflex involves local nerve fibers only, not the CNS.

The "triple response" illustrates three concepts. Pain receptors sense chemical, as well as mechanical and thermal stimuli. Their sensitivity is increased by substances accumulating in the damaged area. Their response includes a neuroeffector component. They release substances (peptides) that initiate further events, providing further protection and favoring local tissue repair. 

Studies in invertebrate neural systems show extensive local control of visceral function. Exceptions to central control are also found in the mammalian ANS. Near-normal interaction of bowel segments persists in the absence of CNS innervation. Sensory fibers from the gut exert feedback in intramural autonomic ganglia on visceral motor neurons regulating smooth muscle in the intestinal wall. The nervous system has pattern generators, both central and peripheral: systems with cellular, synaptic, and network properties (cyclic firing rhythms, reciprocal inhibition of cell pairs, leader and follower cells) that provide automated mechanisms for generating rhythmic movements (breathing, walking) or periodic activities (sleeping, waking). Regulated by neural (sensory feedback, volitional override) or neuroendocrine influences, pattern generators are pithy examples of neural endogenous activity." 

Okay, looking at the above passage, I was a little confused for a bit but simplicity bit me in the backside again. Here's what I got out of it:

1) MOSTLY everything gets reported to and controlled by the central nervous system with a few exceptions which are important and "instructive" as Dr. Angevine says.

2) Regardless of distance in between two segments, the CNS is still involved.

3) The exception of when the CNS is not involved is something like a scratch to an arm that creates a local response of reddening, swelling, warming all which creates a stimulus to the nociceptors which CAN create the experience of pain. Since there is a local "axonal response," there is no requirement of integration and permission from the CNS. Given the givens, I can see why this is evolutionarily necessary. The body is going to, first and foremost, deal with survival as efficiently as possible, so including the CNS in an injury like that would be less than necessary initially. Eventually, it gets to the CNS though. Please don't forget that.

4) The enteric or "gut" nervous system does not require CNS innervation to function.

5) The nervous system has pattern generators that allow it to function efficiently and effectively that are regulated by neural or neuroendocrine influences.


That's enough for now. Stay tuned for the Principle #4: SPECIALIZATION.

I'd love to hear from you. Please let a comment or question.

In mind, body and spirit,

Will