Biology Reference
In-Depth Information
of
coordination dynamics
, we may conveniently describe the transition of the motor
cortex from the state where cortical columns are undergoing quasi-random explor-
ative motions to the state where the input signal-induced depolarization of a
particular configuration of cortical columns has occurred in terms of the transition
from
the metastable state
to
bi-table (or multi-stable) state
. This state transition is
suggested to be the result of coupling the slow column rearrangement and the fast
axonal depolarization obeying the generalized Franck-Condon principle or the
Principle of Slow and Fast Processes (Sect.
2.2.3
).
Based on the above mechanisms, it is possible to estimate the force generated in
the muscle when one cortical column in the motor cortex is activated as a result of
the input of some external stimuli such as visual signals (see Fig.
15.21
) through
Steps 1-4 described below:
1. The activation of the efferent motor neurons constituting a cortical column in the
motor cortex causes an almost simultaneous activation of the muscle cells
innervated by the motor neurons.
2. The number of the muscle cells activated by one motor column is equal to nr,
where n is the number of motor neurons contained in one motor column
(estimated to be 10
4
; see below) and r is the number of the muscle
cells innervated by one motor neuron which is assumed to be 10
3
, leading to
nr
10
7
, the number of the muscle cells that can be activated
synchronously by one column in the motor cortex.
3. We assume that the number m of the myosin molecules contained in one muscle
cell is approximately 10
4
. Hence the number of myosin molecules activated by
one motor column would be nrm or (10
7
)(10
4
)
10
4
10
3
¼
¼
10
11
.
4. Since one myosin molecule can generate force f in the range of 10
12
N (see
Fig.
15.21
), the force generated by activating one motor column would be
nrmf
¼
10
1
N.
5. The diameter of the cortical column is 5
(10
11
)(10
12
N)
¼
¼
10
6
m and the area of the motor cortex
is 6,817 mm
2
(or approximately equal to a circle with 8
10
2
mindiameter)
(Cook 1986, pp. 63-66). Hence the number of the columns contained in the motor
cortex is approximately [(8
10
8
.
6. Therefore, the number of the motor columns that needs be activated synchro-
nously to generate 1 N of force in the muscle to lift, say, a cup of tea or an apple
10
2
)/(5
10
6
)]
2
10
4
]
¼
[1.6
¼
3
10, which is small
compared to the total number of cortical columns present in the motor cortex of
the human brain, 3
¼
10
8
.
The force (F), distance (D), and time (T) amplification by increasing mass
(FDTABIM) is necessary for the upward causation of the mind-molecule coupling
(Figs.
15.17
,
15.19
), ultimately because force originates at the molecular level and
the objects to be moved are at the muscle level. But why is the FDTABIM necessary
for the downward causation (Figs.
15.17
,
15.20
)? In other words, why is it neces-
sary to amplify the molecular processes at the ion channel level to the macroscopic
electrical activities at the level of cortical regions such as motor cortex (Fig.
15.20
)?
One possible answer may be suggested as follows:
