Nervous System Basics VI: PURPOSEFULNESS

Angevine's fifth basic organizing principle, purposefulness:

"The Purposefulness of Neural Components
Every part of the nervous system has at least one function, often many more. Small parts of the CNS may play crucial roles, as in the extensive distribution and profound influence of axons from inconspicuous brain centers. The locus ceruleus ("blue spot") on each side of the fourth ventricle contains about 12,000 large melanin-pigmented neurons. These synthesize norepinephrine and release it in the cerebral cortex, cerebellum, and almost every other part of the CNS. Electrically, they are almost silent in sleep, hypoactive in wakefulness, and hyperactive in watchful or startling situations. They serve vigilance and attention to novel stimuli. They contribute, indirectly but no less crucially, to perceptual and cognitive functions. By contrast, immense structures make large but expensive contributions, as in the cognitive and motor abilities afforded us by the billions of neurons in our cerebral and cerebellar cortices."

I never have heard such attributes associated with the locus ceruleus before. Fascinating. Another tidbit on locus ceruleus, from Kandel, p. 483:

"...other descending inhibitory systems that suppress the activity of nociceptive neurons in the dorsal horn originate in the noradrenergic locus ceruleus and other nuclei of the medulla and pons. These descending projections block the output of neurons in laminae I and V by direct and indirect inhibitory actions. They also interact with endogenous opioid-containing circuits in the dorsal horn..."

So, locus ceruleus is involved in descending inhibition of pain. Doubly fascinating.

On another topic expanding from this organizing principle, i.e., preconscious genesis/control of conscious thought or action, of ordinary activities we "imagine" to be of our own "free will", much research has demonstrated that, in fact, non-conscious areas of the brain truly run all the decision making activities and simply provide us a grand illusion that we somehow have choice in what we are going to "do" in any given moment.

This can pose a problem if one's concept of the brain is
1. it is monolithic and singular, or
2. if one identifies conscious awareness with the brain itself
3. if one's experience is that when one wants to pick up one's hand, one can, and that's all there is to it.

It may seem odd that nonconscious parts of one's own brain control the behavior and timing of the "I" construct, instead of the other way round. Yet, this is more like how things actually are.

Antonio Damasio's book, The Feeling of What Happens, helps this all fall into place. Reading this book helped my own concept of the brain to change completely from thinking of it as some big homogenous blob up on the top of my body, to an appreciation of the brain as a community of discrete parts that communicate intensely and continuously, a predictor and simulator.

After reading this book, my image of the brain changed to one in which a main, nonconscious "brain", operating autonomously but with my best interests first and foremost, exists in space with two parts attached, a large mobile body attached to the back end, and something called "conscious awareness" affixed (sort of like a miner's head lamp, but easily swiveled) to the front end. The "brain" in the middle can coordinate these two parts easily. (It's a simplistic image but it works for me. In PT, it will take quite awhile before all of us switch from regarding the brain as that blob at the top of the body that is none of our business, to seeing the body as merely the big blob behind the brain, and the brain as the main focus of our interventions.)

There is a trail of research on the timing of conscious awareness as being an after-the-fact phenomenon leading back to Benjamin Libet's Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential): the unconscious initiation of a freely voluntary act. Note the extensive citation list.

Deric Bownds spoke of it recently on MindBlog. Here is a more recent paper he mentioned: Unconscious determinants of free decisions in the human brain.

Additional reading:

1. Books by Benjamin Libet
2. Review of Mind Time, one of the books
3. Publisher comment on another Libet book, The Volitional Brain
4. An analysis of Libet's work by John McCrone

Nervous System Basics V: SPECIALIZATION

Here is Angevine's 4th vantage point:

"Specialization
Reflecting its diverse tasks, the nervous system is specialized, from the single neuron to each brain region. Specialized subsystems analyze sensations. They differ in some ways, but data processing is progressive and networked in all. Neurons and the neuroglia have special shapes and roles, but both enjoy all criteria for cells and work in concert. Less obvious but equally specialized are subsystems for other functions: sleep-wakefulness, alertness, attention, affect, collating pages of a report, reading out loud from a book, self-awareness, brain damage control, and so on ad infinitum.

Ubiquitous specializations include those for high nerve conduction velocity (large axon diameter, thick myelin sheath), space-saving bundling (small-axon diameter, thin myelin sheath, shared sheaths), short latency response (monosynaptic reflex), staggered, persistent latencies (parallel side chaining of long-axoned neurons), dependability (neuron redundancy), feature analysis (parallel processing), effect monitoring (feedback circuits), and force multiplication (feed-forward circuits). The neurons performing such tasks and the neuroglia backing them up are as specialized as these many diversified services. For neurons and the neuroglia, form indeed reflects function."

I think each of these features listed in the second paragraph could be a book in itself; I will list them out again:
1. high nerve conduction velocity (large axon diameter, thick myelin sheath)
2. space-saving bundling (small-axon diameter, thin myelin sheath, shared sheaths)
3. short latency response (monosynaptic reflex)
4. staggered, persistent latencies (parallel side chaining of long-axoned neurons)
5. dependability (neuron redundancy)
6. feature analysis (parallel processing)
7. effect monitoring (feedback circuits)
8. force multiplication (feed-forward circuits)

Specialization also applies to microglia.

Additional reading from Scholarpedia:
1. neuron
2. neuronal cable theory
3. Rall model on cable properties of dendritic trees

Nervous System Basics IV: CENTRALIZATION

Angevine's third basic organizing 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."

This is a very instructive passage, particularly in its clear explanation of the peripherality of the axon reflex, but I would be so bold as to quibble with Angevine over his use of the term, "pain receptors." Some pain researchers part company with this terminology, preferring instead to refer to peripheral receptors that register chemical, mechanical and temperature stimuli which could be harmful (but aren't necessarily), as nociceptors, not "pain receptors." They are quite clear that strictly speaking, incoming information to the CNS is not "pain" until the brain decides it is, at which point it will make it so. It may seem a small point, but depending on context, the brain may choose to ignore nociception entirely to deal with a completely different, but from its perspective, more pertinent or immediate threat. Numerous examples of this are in the pain literature dating back to the Civil War. Also, the brain is capable of making "pain" in the absence of any noxious input (Derbyshire 2004).


Additional reading

For axon reflex:

1. Axon Reflex (3-page pdf)
2. Excerpts from book, Clinical Motor Electroneurography: Evoked Responses Beyond the M-wave on axon reflex
3. Axon reflex as discussed in book, Biology of Skin
4. Caselli A; Validation of the nerve axon reflex for the assessment of small fibre dysfunction JNNP 2006 (abstract)

For pain without nociception:

5. Derbyshire SW Cerebral activation during hypnotically induced and imagined pain 2004 (10-page pdf)

For pattern generators:

6. Hooper, SL Central pattern generators, 2000: 16-page pdf

Nervous System Basics III: UNITY

Here is the second organizing principle of the human nervous system;

"Unity
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.

References:
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)

Nervous System Basics: Part II: UBIQUITY

There are 8 considerations presented by the author on how to contemplate the nervous system. They are,
1. Ubiquity
2. Unity
3. Centralization
4. Specialization
5. Purposefulness
6. Uniformity with Versatility
7. Plasticity
8. Chemical Message Coding

This is the first. From p. 331, Vol III, Encyclopedia of the Human Brain, author Jay B. Angevine:

"Ubiquity
With 100,000 miles of nerve fibers the nervous system rivals the vascular system. Both pervade the body and function in harmony. By nerve impulses or circulating red and white cells, glucose, hormones and immune principles, they integrate body activity, protect the body, enhance its performance to met stress or demand, promote its growth and nutrition, and maintain its tone and vigor. The trunk and branches of both systems reflect body form. If either system and no other part of a person were visible, he or she would be recognizable. Density of innervation varies as the value of parts to sensory discrimination or motor control. In well-innervated areas (lips, fingertips) stimuli are sharply discriminated as to modality, intensity, and location, but in sparsely innervated areas (flanks, legs) these are less defined. Similarly, muscles vary in the ratio of motor neurons to muscle fibres. The higher the ratio, the more precise the control of the muscle and the movement it serves (a motor neuron may excite 2000 muscle fibers in a limb muscle or as few as 5 in extrinsic ocular muscles)."

I don't know what else to say. To me this is a beautiful image of a filamentous system which comprises only 2% of our physicality, but which regulates 100% of our function.

Nervous System Basics: Part I

In this series of posts I intend to bring out information found in one (just one) section of the 4-volume Encyclopedia of the Human Brain, 2002, edited by V.S. Ramachandran.

The section in question is written by Jay B. Angevine at the U. of Arizona, and begins page 313 in Vol. 3. He states the nervous system has:

*100 billion neurons of 10,000 types,
*1-10 trillion neuroglial cells,
*100 trillion chemical synapses,
*160,000 km. of neuronal processes,
*thousands of neuronal clusters and fiber tracts,
*hundreds of functional systems,
*dozens of functional subsystems,
*7 central regions, and
*three main divisions.

One hundred and sixty thousand kilometers, or about 100,000 miles of nerve fiber: the Bodyworlds Exhibit states that there are 72.5 kilometers (45 miles) of nerves, which are macro bundles of many fibers.. and that seemed a big number...

Angevine says, "... all of these parts form a coherent, bodily pervasive, diversified, complex epithelium with interdependent connectivity of neurons", most of which are interneurons rather than sensory or motor. The key organizing principles are centralization and integration (although there are many others as we will find out).

The nervous system performs the dual roles of regulation and initiation.

"In the first, it counteracts: responsively and homeostatically, gathering stimuli from outside and inside the body (including the brain), assessing their short-term and long-range significance, generating activity from faster breathing to stock trading, even to functional plasticity in learning or after brain damage.

In the other, it acts: endogenously, not so homeostatically, replacing one state of neural activity with another, generating activity from doing nothing at all to creative thinking and extraordinary achievement, even taking steps toward understanding how itself, the nervous system, works."

Angevine examines the overall organization of the nervous system from a number of perspectives in this section that runs 58 pages, and here I will delve into the third of ten sections he outlines, basic organizing principles, of which there are 8. I want to give each one of these principles some time and thought here.

Transcript For BrainScience Podcast #31

I have prepared a transcript of Dr. Ginger Campbell's Episode #31, Brain Rhythms with Györgi Buzsáki, with her permission.

You can read it here: Synchrony and Oscillation in the Brain.

The transcript was written to assist my own learning of the material in both the book and podcast. Here is a link to Brain Oscillations: Ten Part Series, on the same topic.

The ideas in the podcast are much easier to follow if one can read along as one listens; the intention of publishing this transcript is for it to be a listening/learning aid for anyone who wishes to dig deeper into understanding the presentation, and the book upon which it is based.