As in epithelium, all parts of the nervous system are physically coherent and functionally linked by nerves, tracts, and specified cell to cell contacts. Potentially each part communicates with all others. Some connections are direct (a two-neuron, monosynaptic reflex), whereas others involve myriad interposed neurons. Though complex, neural circuits offer total connectivity: fast, body-wide communication. Nerve impulses may originate in sensory nerve endings in any part of the body or anywhere in the system itself. Responsive activity complements endogenous activity, which is always evident in the human nervous system with its startling capacity to generate patterns of behavior and initiate events on its own. Sensory impulses, triggered by PNS primary sensory neurons, race over its nerves to the CNS, there diverging to clusters of secondary sensory neurons. Analysis begins. New impulses pass to central neurons on which related messages converge, which is a recombinant process providing integration. Other messages on stimulus modality, intensity, location, affective quality, body position and movement, visceral activity, fatigue, experience, and expectations are all integrated. Huge numbers of impulses are generated; untold numbers of synapses are activated. Almost instantly, nerve impulses that will elicit bodily responses stream out of the CNS to muscles and glands."
David Butler PT says in his book, The Sensitive Nervous System, p. 19;
"Each neuron is studded with approximately 5000 spines on which other neurons connect. Most of these connections will be part of feedback loops from neighboring neurons. Only a small percentage will come directly from the associated sense organs. "Every neuron is plumbed into a sea of feedback" (McCrone 1997). This gives the nervous system a recursive structure that allows the system to repeat itself again and again.This will allow a continual check/recheck on its actions.
The numbers are hard to get a feel for and popular texts are useful to try to get the message over. Kotulak (1996) based on evidence from electron microscopy research, says that there are about 350 million connections in a pinhead size speck of brain tissue. But the big numbers are just the start. It is the combination of connections possible which is awesome. Edelman (1992) reasoned that there were more possible combinations of connections than positively charged particles in the universe. There must be an extraordinary density of coding behind connections and combinations, allowing patterns of activity which can all be replayed if needed or quickly adapted for future responses. Our ultimate behavior is a result of this coding. There is surely enough space for the memories of a lifetime including all painful experiences, their contexts, the actual and possible responses at the time and future responses."
A number as big as something in the entire universe is all packed up inside the human skull, every human skull. I very much like to remember this when I find myself bogged down by some little annoyance. It makes the small stuff go back to smallness.
1. The Sensitive Nervous System (2006) David Butler PT
2. Inside the Brain, 1997, Ron Kotulak
3. Encyclopedia of the Human Brain 2002, edited by VS Ramachandran
4. The Dynamics of Brain Processing: Top-down Effects of Consciousness, 1997, John McCrone
5. Brilliant Air, Bright Fire, 1994, Gerald Edelman (lots of more recent books)