Between ca.35.000 - 29.000 BCE, Neanderthals and evolved Cro-Magnon shared the Earth (ca. 10.000 BCE, Cro-Magnon gave way to the Neolithic peoples). Maybe the latter exterminated the former (Joseph, 2002, p.349). The Cro-Magnon's superiority was based on abstract thought, individuality, speech and an enhanced regulation of emotion and action as well as tool-making and the consolidation of cultural forms. In Neanderthal craniums, the importance of visual input and also a less developed frontal lobe stand out. The receding eyes of the Cro-Magnon made room for a frontal development, which was added to the overall development of the cerebrum. If the Neanderthal did not possess an angular gyrus (cf. infra), as made probable by Joseph (2002, pp.313-360), is it then not likely this ancestor had only perfected the skills of the archaic hominids (like Homo erectus) ? His mode of thought was mythical and iconical. Limbic executants are at work and rule the game.

On the one hand, the frontal lobes temper the passions of temporal-limbic spirituality by symbolizing fear and joy, as well as developing vocabularies to give "meaning" to Divine Presence and the holy. On the other hand, too much superstructures or a rejection of religion (as in atheism), may cause the communication between the "old" systems and the "new" cortex to be more unreliable or disrupted, leading to a lower level of integration of the brain. This evolutionary block puts limitations on its executant capabilities (cf. the absence of creativity and inventivity, as well as uplifting, joyous, profound, holy experiences).

The symbolization of the holy experiences computed by the amygdalean-hippocampal "God-spot", gives to their raw directness an opportunity "to leave the limbic system" and recieve a more definitive form. Ex hypothesi and from a neurospiritual point of view, a superstructure is efficient if it does not disrupt or block the feedback from the prefrontal cortex to the limbic "God-spot". In other words, symbolization is inefficient, when it disables the use of the "God-spot" beyond the limbic system, or condemns it to the unconscious, as in atheist ideologies. The experience of Divine Presence has specific neuronal executants and so the mind is called to position these experiences within the conscious field of operation of the first person phenomenology.

In mature mystics, the profound & symbolized experience of the holy is continuous, implying an efficient frontal lobe/temporal-limbic integration. The "God-spot" executes the direct experience of the Divine and the "God-circuit" lets it run through the neocortex, especially its prefrontal area and back to the limbic system. During the frontal lobe phase, volition becomes associated (cf. the impact of spiritual experiences on individual morality). The overall efficiency of this circuit is, ex hypothesi, part of the evolutionary purpose of the brain as a potentially highly complex but integrated organ, executing a variety of mental states, making the essential peace possible between, as it were, the limbic Neanderthaler and the frontal Cro-Magnon. The brain may also compute the bi-polar nature of all spiritual experience, rooted in the basal telencephalon (hippocampus, amygdala) and the limbic system (thalamus, hippothalamus), but branching out towards the neocortex, and thus enabling the expression of higher order spirituality, based on self-experience and symbolization.

Neanderthals & Cro-Magnons show the root of human spirituality to be buried in high-order, lower mammalian structures covered with low-order, higher cortical networks.

3 The mammalian brain, the democratic neuron and philogenesis.

A comprehensive description of the human brain lies outside the scope of this text and the reader is referred to Bear, Connors & Paradiso (2001) and other similar studies. Let me sketch an outline and focus on those details which are of importance in our philosophical investigation of the neuronal executants of spiritual experience. Indeed, the human brain is the most complex piece of matter on Earth. The description below is thus but a scaffold of some of its important contents & functions.

general characteristics

The human brain is the most complex organ known in the observable universe. It expends 25% of our energy intake, while consisting in only 2% of our tissue mass. In general, at any given moment, 1 to 10% of all our neurons fire, on average 100 times a second. If processing speed is measured in "FLOPS" ("floating point operations per seconds" or the number of calculations with a floating point done in one second), and each firing is one FLOP, then 1 teraflops is the upper limit of the brain's information processing speed (1 megaflop = 1 million FLOPS, 1 gigaflop = 1000 megaflops, 1 teraflops = 1000 gigaflops). Counting in the synapses brings the estimated speed up to 10 teraflops (Moravec, 1988). 

The first creatures to possess one single neuron, the first cellular brains, were able to process, store and express a billion times more information than any sensory or motor cell could (Joseph, 1993, p.13). Living beings such as bacteria, algae, mosses, fungi and plants have no brains, although they are able to communicate & interact. Sponges have nervelike tissue, concentrated around the external orifices and pores. In the nervenet of a worm, we already recognize two large pairs of clusters of similarly functioning neurons (neural ganglia) in the head region. Neurons are able to plan and create memories as communicate this to other neurons in the form of electrical and chemical messages. 

The human nervous system follows the basic mammalian plan, although -due to its extreme complexity- it is distorted as a result of the selective growth of some parts, like the human neocortex. The nervous system of all mammals has two divisions : the central nervous system (CNS, encased in bone : brain and spinal cord) and the peripheral nervous system (PNS). The latter encompasses all parts of the nervous system other than the brain and the spinal cord. Recently, neurocardiology and neurogastroenterology discovered two other neuronal ganglia, respectively situated in the heart (and Solar Plexus) and in the gut area.

In 1991, after years of extensive research, dr.J.Andrew Amour of Dalhouse University in Halifax, Canada, evidenced the existence of a functional "heart brain". This is an intricate network of neurons, neurotransmitters, proteins and support cells allowing activity independent from the "head brain". Many of the signals flowing out of the heart to the brain regulate signals flowing out of the brain to the heart, the blood vessels, glands and organs. The work of the Laceys (1970) and others made clear neural messages from the heart affect cortical activity, including the amygdala and the higher brain centers.

There are at least forty thousand neurons in the heart. This is as many as in the various subcortical areas of the brain. The heart is not merely pumping blood, but demonstrating a complex set of interactions between these neurons and those in the brain. The idea peripheral autonomic ganglia function as "little brains" dates from the XVIIIth century, and was formulated by Benigne Winslow, a Swedish anatomist working in Paris. These neuronal networks were considered to perform routine tasks required to maintain organ function, ensuring afferent data arising from each organ did not flood the spinal cord and the brain.

"Only one or two thousand nerve fibers connect the brain to the hundred million nerve cells in the small intestine. Those hundred million nerve cells are quite capable of carrying on nicely, even when every one of their connections with the brain is severed ..."
Gershon, M. : The Second Brain, HarperCollins - San Francisco, 1999, p.xiv.

When describing the "wiring" or axons of the nervous system, two important terms should be kept in mind : afferent ("carry to") and efferent ("carry from"). Axons transporting information from the PNS to the CNS are afferents or sensoric axons (incoming data). Those emerging from the CNS to innervate the muscles or glands are efferents or motoric axons (outgoing (re)actions).

The PNS is divided in two parts : the voluntary, somatic PNS and the autonomic nervous system (ANS). The former contains spinal nerves innervating the skin, the joints and the muscles. The latter consists of neurons innervating the internal organs, blood vessels and glands (it is therefore also called the visceral PNS). 

Looking down on the nervous system, we see it can be divided into two equal halves, the right side of the brain and spinal cord being the mirror image of the left side. This bilateral symmetry is valid for most structures within the nervous system, which come in pairs along the midline (as can already be seen in worms).

A side (lateral) view of the mammalian brain reveals four parts are common to all mammals :

As we shall see, the frontal lobe of the cerebrum is the "senior executive" in the computation of individuality (ego) & volition, as well as (together with the temporal lobe) abstract & creative thought. The cerebrum, especially the frontal-temporal lobes, made possible the explosion of meaning invoked by the Homo sapiens sapiens (Cro-Magnon) and his highly developed neocortex. 

the democratic neuron

Let us step back, and introduce the smallest operational unit of the nervous system : the neuron. Only members of the animal kingdom possess neurons. The neuronal kingdom appeared around 700 million years ago, likely without axons and dendrites, secreting electrical and chemical substances acting on other cells in a general, non-specific way. Then a single, long transmission fiber emerged, the axon, enabling the selective transmission of these messages to a second neuron. Lastly, insulation was invented (myelin or fatty cells wrapping around the axon, preventing leakage).

There are two broad types of cells in the nervous system : neurons and glia ("glue"). There are about 130 billion neurons in the human brain (of which ca. 20 billion constitute the cerebral neocortex) and ca. 1.000 billion glia. The latter are thought to contribute to brain function mainly by insulating, supporting and nourishing neighboring neurons.

Most neurons are in the range of 0.01 - 0.05 mm in diameter. The size of a neuron is at or beyond the limit of what can be seen with the naked eye. The central region containing the cell nucleus is the cell body (or soma), whereas the tubes radiating away from the soma are called neurites, which are either axons or dendrites. The soma usually gives rise to a single axon. Axons act as "wires" carrying the output of the neurons (they travel over great distances of a meter or more). Dendrites (extending rarely more than 2 mm in length) extend from the soma and taper to a fine point. They come in contact with many axons and somata.

It was Cajal (1852 - 1934) who correctly argued the neurites are not continuous from one cell to the next, communicating by contact. Earlier, Golgi (1843 - 1926), the histologist who soaked brain tissue in a silver chromate solution, called the Golgi stain, had maintained the opposite (both shared the Nobel Prize in 1906 but remained rivals). According to the latter, the neurites communicate by way of fusion (a continuous network or reticulum similar to the arteries and veins of the circulatory system). Cajal's neuron doctrine was finally proven with the development of the electron microscope, with a resolution of about 0.1 nm, i.e. a million time better than the unaided eye, distinguishing two points separated by more than one-tenth of a millimeter (0.1 mm).

"A handful of artificial neurons with a dozen-plus connections can learn to tell the difference between apples and bananas. A dozen with a few score connections can be taught to play Hexapawn or tic-tac-toe, perfectly, in a few seconds. Fifty or so, with about 1200 connections, have in reality done a better job of diagnosing heart attacks in an emergency room than a team of expert cardiologists. A hundred neurons, with a few thousand connections, can make money -a lot of money- in notoriously volatile markets. A few hundred can mimic the development of human speech. Of what, then, is the 20 billion neuron, multiquadrillion connection human brain capable ?"
Satinover, J. : The Quantum Brain, Wiley - New York, 2001, p.45.

The structure of the neuron is divided in soma & neurites (axons and dendrites). The internal neuronal process gives rise to two fundamental states of each living neuron : the neural membrane at rest or the action potential. The external neuronal process is summarized by synaptic transmission.

A) THE SOMA

The spherical cell body or soma of the typical neuron is 0.02 mm in diameter. A neuronal membrane separates the inside of the neuron, composed of a salty, potassium-rich fluid or cytosol and a number of enclosed structures or organelles, from the outside. The cell body of the neuron contains the same organelles as those found in all animal cells, they are : the nucleus and the cytoplasm (everything contained within the membrane excluding the nucleus). The details of what happens in the cell falls outside our scope (cf. gene expression, protein synthesis, Krebs-cycle in mitochondria, etc.).

The neuronal membrane (ca. 5 nm thick and studded with proteins), enclosing cytoplasm & nucleus, excludes certain substances floating in the fluid surrounding the neuron. It has been compared to a circus tent draped on an internal scaffolding or cytoskeleton. Its "bones" contain elements dynamically regulated and in continuous motion.

B) THE AXON

This is a structure unique to neurons. It is a highly specialized component for the transfer of electrical current or nerve impulse over distances in the nervous system. There is no protein synthesis in the axon, fed by the soma. It is the different proteins in the axonal membrane enabling it to serve as "wire". If an axon branches, it is generally at right angles (the so-called axon collaterals, which may be recurrent, i.e. return back to the original neuron). The diameter of an axon ranges from less than 0.001 mm to about 0.0025 mm. The speed of the nerve impulse is proportionate to the axonal diameter.

The beginning of an axon (the axon hillock) is differentiated from the axon proper or middle part and from its axon terminal, terminal bouton at the end. The terminal is the place where the axon comes in contact with other neurons or cells, passing the nervous impulse on to them. This point of contact is called the synapse ("to fasten together"). Axons may have many branches at their ends (the terminal arbor) and each branch may form a synapse on dendrites or cell bodies in the same region. Axons may form synapses at swollen regions or "boutons en passant" along their middle part and terminate elsewhere. When an axon makes synaptic contact with another cell it provides innervation to it.

C) THE DENDRITES

A dendrite ("tree") is a branch extending from the soma. The dendrites of a single neuron or dendritic tree has a wide variety of shapes and sizes used to classify neurons (in the cerebral neocortex there are two broad classes : pyramidal neurons -with long axons- and stellate neurons -with short axons-). The dendrites function as the antennae of the neuron and are covered with thousands of synapses. The cytoplasm of dendrites resembles that of axons. Some neurons have dendrites covered with dendritic spines regulating synaptic input.

D) THE SYNAPSE

At the synapse, the nerve impulse is transferred from one neuron to another. The usual direction of this synaptic transmission, gives rise to presynaptic and postsynaptic sides of the synaptic cleft. The presynapse is usually an axon terminal and the postsynapse may be the dendrite or the soma of another neuron.

At most synapses, an electrical impulse traveling down the axon is converted in its terminal into a chemical signal crossing the synaptic cleft. This signal is converted on the postsynaptic membrane into an electrical impulse. This chemical signal is a neurotransmitter stored and released from the synaptic vesicles within the axon terminal. This typical electrical-to-chemical-to-electrical transformation of information is one of the great advantages of synaptic activity and makes possible many of the brain's computations. Modifications of this process is involved in memory and learning. Mental dysfunctions, nerve gas and psychoactive drugs are also related to synaptic transmission, influencing the chemical balance.

In addition to anterograde transport (from soma to terminal), there is a mechanism signalling the soma about changes in the metabolic needs of the axon terminal. This is retrograde transport (from terminal to soma). The action potential of a stimulated neurite does not jump on the membrane of the next neuron, but only runs to the presynaptic side of the synaptic cleft. After a short delay (cf. the transformation of the electrical impulse in a chemical signal and back), this neuron reacts by lowering the potential of the neuronal membrane (or axonal membrane). The level of depolarization increases with the number of stimulating synapses (sum = a + b). Only if at the postsynaptic membrane the depolarization-treshold is crossed will the next neuron transmit the nerve impulse via its axon, etc.

E) REST POTENTIAL

When a voltmeter measures the difference in electrical potential between the tip of a microelectrode inside the neuron and a wire in the extracellular fluid, the inside is ca. -65 Mv with respect to the outside. This is the restpotential or equilibrium potential of the neuron (electrical potential, or voltage, is the force exerted on a charged particle, reflecting the difference in charge between anode and cathode). This electrical voltage is maintained by the neuron. A sodium-potassium pump produces and maintains a large number of K⁺ ions across the neuronal membrane, leaving the inside of the neuron negatively charged.

F) ACTION POTENTIAL

When an electrical impulse hits a neuron by means of synapses (touching the soma or the dendrites), the rest potential lowers a little (about +10 Mv) and when the threshold of depolarization is reached, electrical couter-forces are released. The neuron "fires" one action potential and this all-or-none phenomenon moves with great speed from soma to axonal terminal (between 0.5 and 120 m/s, depending on the axonal diameter). When the potential enters the axon, the permeability of the axonal membrane changes for a short period, while Na⁺ ions move to the inside of the axon. For a millisecond, the inside is positively and the outside negatively charged. After this action potential or spike (+40 Mv), the rest potential becomes higher than before (ca. -80 Mv or hyperpolarization), making the membrane refractive, i.e. uninfluenced by nerve impulses. This allows the spike to move further forwards (direction axonal terminal).

G) NEURONAL LOGIC 

The neuron computes the logical operators "and", "or" and "not" as follows :

Suppose neuron A, touched by two synapses B and C. 

In reality, a neuron is fed by hundreds of stimulating (yes) and inhibiting (no) nerve impulses. If the stimulating impulses outweigh the inhibiting ones, the neuron "fires". If otherwise, nothing happens and the neuron is "mute". This impulse-frequency can modulate between 0 and about 250 action potentials per second. The "democratic" principle is clear : majority rules. Moreover, although individual neuronal operations have a relatively high margin of error (compared to a computer), they are performed hundreds of times simultaneously. This statistical method guarantees individual errors weigh less on the final result. Another advantage is cortical plasticity : groups of neurons adapting their activity after the destruction of another group (cf. neuroplasticity).

the philogenesis of the human brain

The embryo starts as a flat disks with three layers of cells : endoderm (the internal organs), mesoderm (the skeleton and the muscles) and ectoderm (the nervous system and the skin). Changes in the ectoderm give rise to the neural plate, a flat sheet of cells.

In the neural plate a groove is formed, running from anterior (rostral) to posterior (caudal). The walls of the groove or neural folds, move together and fuse dorsally forming the neural tube. Out of the walls of this neural tube the whole nervous system develops. When folding, some neural ectoderm is pinched off lateral to the neural tube. All neurons with cell bodies in the PNS derive from this so-called neural crest. The process from neural plate to neural tube or neurulation occurs about 22 days after conception. 

The first step in the differentiation of the neural tube is the development at its rostral end of three swellings called the primary vesicles, out of which the whole brain emerges.

These swellings of the neural tube define the fundamental structure of the human brain : forebrain, midbrain and hindbrain. The two cerebral hemispheres grow and lie above and on either side of the diencephalon. The ventral-medial surfaces of these hemispheres have fused with the lateral surfaces of the diencephalon. Unlike the forebrain, the midbrain differentiates relatively little. From the hindbrain, important structures emerged. 

Out of the primary vesicles, the following brainstructure emerges :

• first primary vesicle : forebrain
  * secondary olfactory vesicles : olfactory bulbs (ventral surface of the hemispheres) - over the course of evolution, a group of primal sensory neurons formed an internally located ganglion of like-minded cells forming the olfactory bulbs, round projections part of the forebrain - from the olfactory system, the limbic system of the "old" mammalian brain evolved (ruled by the amygdala) ;
  * secondary optic vesicles : the two retina's & optic nerve are differentiated - the major target of retinofugal projection is the superior colliculus ("little hill") of the tectum ("roof") of the midbrain ;
  * secondary "telencephalic" vesicle : telencephalon ("endbrain") or cerebrum with its two cerebral hemispheres, basal telencephalon (a pair of amygdala and the hippocampus), the corpus callosum, cortical white matter and the internal capsule ;
  * secondary "diencephalic" vesicle : diencephalon ("between brain") : the thalamus and hypothalamus ;

• second primary vesicle : midbrain : tectum and tegmentum ("floor"), with the cerebral aqueduct in between ;

• third primary vesicle : hindbrain : cerebellum, pons ("bridge") and medulla (oblongata).