Multicellular Systems - Molecular Theory of the Living Cell - page 556

motions (a slow process ), the brain fires the right number of the right neurons in
the motor cortex (a fast process ), innervating the right set of muscle cells that
then generate the needed mechanical force to be subsequently amplified through the
upward causal mechanism discussed above.
The brain consists of approximately 10 12 neurons which are organized into
functional cortical areas. For example, the motor cortex constitutes 6.3% of the
total cortical area of the human brain or about 100 cm 2 (Cook 1986, p. 69). There is
experimental evidence (Cook 1986, pp. 61-73) that the neocortex is organized in
terms of column -like structures arranged in grid-like formation, each consisting
of ten to a hundred thousand neurons. The motor cortical column is about 500 m m
in diameter and contains 30,000 pyramidal cells, and there are a maximum of 10 6
such columns per cerebral hemisphere (Cook 1986, p. 63). Each cortical column is
thought to possess a specific computational function, for example, the processing of
the information from a specific whisker in a rat's mustache. Thus, the cortical
column may be viewed as a basic computational unit of the cortex.
The downward causation of mind over molecule begins with somatic nerves that
originate in the motor cortex and form the neuromuscular junctions (or end plates)
on the surface of target muscle cells. Each muscle cell is innervated by one efferent
somatic neuron and one such neuron can synapse with tens of thousands of muscle
cells, an arrangement that seems ideal for synchronizing the activation of many
muscle cells for the purpose of amplifying force and distance of myosin action.
When activated, these nerves release the neurotransmitter, acetylcholine (Ach),
at the neuromuscular junction, causing the depolarization of the postsynaptic
muscle cells by opening their Na ++ and K + ion channels in sequence which in
turn leads to the release of intracellular Ca ++ from the sarcoplasmic reticulum. The
rise in the Ca ++ concentration in muscle cells activates a series of intracellular
events, resulting in the generation of mechanical force in myosin molecules
coupled to ATP hydrolysis, most likely through the conformon mechanism
(see Chap. 8 and Fig. 11.34 ) . The downward arm of the reciprocal causation of
mind and molecules (Fig. 15.17 ) begins in the motor cortex and ends at the level of
neuromuscular junction as schematically depicted in Fig. 15.20 . Strictly speaking
the downward causation does not implicate any molecule directly but only indi-
rectly through depolarized cells and hence should be referred to as mind-cell
coupling rather than mind-molecule coupling which should be reserved for the
upward causation. In other words, the motor neurons in the motor cortex do not
communicate directly with myosin molecules but only indirectly through muscle
cells which control myosin and associated molecules involved in contraction .
As alluded to above, the downward causation also implicates coupling two
partial processes - one slow and the other fast . It is here postulated that the slow,
endergonic partial process underlying the downward causation is the thermal
fluctuation-induced random and transient contact formation (or assembling)
and detachment process (or disassembling) among cortical columns in the motor
cortex and the fast, exergonic partial process is identifiable with membrane depo-
larization of assembled columns. Here it is assumed that cortical columns possess
structures (such as specific axon terminals) that can actively explore potential