We experience all—or at least everything pertaining to the material world—through the agency of specialized, irreplaceable cells called neurons, 100 billion of them in the brain alone, that channel information throughout the body and within the vast supporting cellular milieu of the central nervous system (the brain and spinal cord). This is all accomplished by only three kinds of neurons: sensory neurons, which carry the flow of sensation from the peripheral nervous system (by definition all parts of the nervous system excepting the brain and spinal cord) into the central nervous system and consciousness; motor neurons, which carry instructions from the brain and spinal cord into the peripheral nervous system, and from there to muscles and glands; and interneurons, or association neurons, which are interposed between the sensory neurons and the motor neurons, and which transmit our will and volition to the motor neurons. The sensory information is carried into the dorsal horn of the spinal cord by way of dorsal roots, and the motor information is carried out of the ventral horn of the spinal cord by way of ventral roots. The dorsal and ventral roots join to form mixed (motor and sensory) spinal nerves that in turn innervate structures throughout the body (figs. 1.3-9).
[Technical notes: Because this is a book correlating biomedical science with yoga, which many consider to be a science of mind, a few comments are required on a subject of perennial, although possibly overworked, philosophical interest—the nature of mind vis-a-vis the nervous system. Speaking for neuroscientists. I think I can say that most of us accept as axiomatic that neurons are collectively responsible for all of our thinking, cognition, emotions, and other activities of mind, and that the totality of mind is inherent in the nervous system. But I also have to say as a practicing yogi that according to that tradition, the principle of mind is separate from and more subtle than the nervous system, and is considered to be a life principle that extends even beyond the body.l
I How and whether these questions become resolved in the third millennium is anyone's guess. They are topics that are not usually taken seriously by working scientists, who usually consider it a waste of time to ponder non-testable propositions, which are by definition propositions that cannot possibly be proven wrong. Such statements abound in new age commentaries, and are a source of mild embarrassment to those of us who are trying to examine older traditions using techniques of modern science. This says nothing about the accuracy of such proposals. It may be true, for example, that "life cannot continue in the absence of prana." The problem is that short of developing a definition and assay for prana, such a statement can not be tested—it can only be accepted, denied, or argued ail infinitum.1
ITliig approach to experimentation and observation doesn't require a lot of brilliance. It simply stipulates that you must always ask yourself if the nature and content of a statement make it potentially refutable with an experimental approach. If it's not, you will be accurate 90^ or the time if you conclude that the idea is spurious, even though it may sound inviting or may even appear self-evident, as did the chemical theory of phlogiston in the niid-l8th century. To give the benefit of the doubt to the purveyors of such statements, it's rare that they are outright fabrications. On the
other hand, one should always keep in mind that all of us (including scientists) have a huge capacity for deceiving ourselves when it comes to defending our ideas and innovations. The problem is that it's often impossible to distinguish fantasy, wishful thinking, mild exaggeration, and imprecise language from out-and-out fraud. What to do? In the end it's a waste of time to make a career of ferreting out errors—one can't get rid of bad ideas by pointing them out. On the other hand, if we turn our attention to propositions that can be tested, the creative attention this requires sometimes brings inspiration and better ideas, which in turn disposes of bad ideas by displacing them. Lavoisier discredited the theory of phlogiston by pointing to brilliant experiments (many of them carried out by others), not by crafting cunning arguments. I
[One last concern: if your complaint is that you can't understand a particular concept and do not feel competent to criticize it, don't assume that the problem is your own lack of intelligence or scientific background. More than likely, the idea wasn't presented in a straightforward manner, and it usually happens that this masks one ventral
right dorsal root ganglion with cell bodies of sensory ventral root containing motor neuronal axons that course out to the mixed spinal nerve (right side)
dorsal root right dorsal root ganglion with cell bodies of sensory ventral root containing motor neuronal axons that course out to the mixed spinal nerve (right side)
ventral mixed spinal nerve (motor and sensory: left side)
Figure 1.3. Microscopic section of dorsal root ganglion (above), and three-dimensional view of the first lumbar segment (L1) of the spinal cord, showing paired dorsal and ventral roots and mixed (motor and sensory) spinal nerves (from Quain).
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