From Tom Bourbon (930512.1050)
A few days ago, in his long (but not exhaustive) list of misconceptions about
PCT, Rick Marken mentioned the old chestnut in which certain people who
should know better (and who probably do) claim that cutting the sensory roots
of spinal nerves (rhizotomy, or deafferentation) eliminates all sensory
connections to the spinal cord but has no effect on behavior, therefore
sensory feedback is not essential to behavior, therefore PCT is a crock.
Aside from the indisputable fact that behavior is not the same after
deafferentation, there is another interesting problem with that old argument:
Cutting the so-called sensory roots of spinal nerves does not eliminate
sensory connections to the cord. Just a few days ago, I discovered that my
recently nomadic career path has led me to a department chairman (Guy
Clifton) who, in spite of looking too young to have dome so, produced some
of the physiological data that, along with anatomical observations (some of
them very old), comprise the complete refutation of the venerable
Bell-Magendie Law (aka the Law of Spinal Roots). That law asserts that the
fibers in dorsal roots of spinal nerves are sensory; those in ventral roots,
motor. In spite of its near-universal acceptance, the "Law" is false.
I will not present all of the details, but for those who have previously
expressed interest in this topic (Gary C., I know you have), I will
summarize the evidence against Bell-Magendie\Roots and will give a few
citations that might be useful the next time a motor-plan\program(me) person
tries to back a lone PCTer into the corner and shout, "deafferentation!"
First the anatomy. As long ago as 1894, C. S. Sherrington described
evidence that there are about 5 to 40 large myelinated sensory fibers in
each ventral (putatively motor) root. Not very many, but a start at
chipping away at the exclusionary "Law." The count of myelinated sensory
fibers in ventral roots has held nearly constant in studies on mammalian
species ever since. (Sherrington CS. On the anatomical constitution of
nerves of skeletal muscles with remarks on recurrent fibers in the ventral
spinal root. Journal of Physiology 1894;17:211-258) (There were similar
reports even earlier than Sherrington's.)
All through the late 1800s and early 1900s, there were reports of "recurrent"
(that is, afferent or sensory) fibers in ventral roots -- a veritable lost
literature that refutes the treatment of spinal roots in every text from
which I ever studied or taught. (There are many citations of this
literature in the more recent references I am about to describe. People
interested in this topic should take an afternoon and browse through some of
the many citations. It is an eye-opening experience.)
By the late 1960s to mid 1970s, the evidence was in that there are far more
sensory fibers than Sherrington found. Most of them are small, unmyelinated
fibers that were not seen in some of the earlier procedures, or were
discounted when they were seen. For over 25 years now, study after study has
shown that from 25% to 30% of ventral root neurons are small and
unmyelinated and that half of those are sensory. Therefore, about 12% to 15%
of ventral root neurons are small, unmyelinated sensory fibers. (And
there are the 5 to 40 large myelinated ones.) There is a large (and largely
unknown) literature in which people suggest that these ventral root sensory
neurons probably explain the common failure of rhizotomy (the cutting of
dorsal roots) as a treatment for chronic pain and other disorders. Meanwhile,
the cutting continues.
By the early to mid 1970s, there were good physiological data that identified
the effective stimuli for many ventral sensory neurons, and that mapped out
their receptive fields. Many of these neurons serve the connective tissues
and the viscera. (So what could they possibly have to do with overt
skeletomuscular behavior, your critics will ask? And you will know how to
reply.) Guy was a young medical student in those days and worked in a lab at
the University of Texas Medical Branch in Galveston, where I recently stopped
off for a while in my wanderings. I will give a few citations of their work.
They were good scholars -- they included many references to the extensive
earlier literature.
Clifton GL, Vance WH, Applebaum ML, Coggeshall RE, Willis WD. Responses of
unmyelinated afferents in the mammalian ventral root. Brain Research
1974;82:163-167.
Coggeshall RE, Clifton GL, Vance WH, Applebaum ML. Unmyelinated axons in
the ventral root. Anatomical Record 1975;181:334.
Applebaum ML, Clifton GL, Coggeshall RE, Coulter JD, Vance WH, Willis WD.
Unmyelinated fibres in the sacral 3 and caudal 1 ventral roots of the cat.
Journal of Physiology 1976;256:557-572.
Clifton GL, Coggeshall RE, Vance WH, Willis WD. Receptive fields of
unmyelinated ventral root afferents in the cat. Journal of Physiology
1976;256:573-600.
I stumbled across this literature recently while looking for material to
refute some invocations of "The Law" and of deafferentation that some
motor-programme people were using to muddy up the neurophysiological
literature. My first thought was that it must have been Guy's father.
When I asked him if he were old enough to have done the work, he was
startled. To paraphrase only lightly, "You mean you read that? I don't
think anyone else ever did."
Along the line of refuting classic assumptions, someone might want to look
in The British Medical Journal for July 17, 1909, pages 125-132. That is the
text of an address by Sir Victor Horsley, The Lincare Lecture on The
Function of the So-Called Motor Area of the Brain, a lecture delivered
before the master and fellows of St. John's College, Cambridge, May 6, 1909.
He reports on his work from as early as 1885. I think you can get the
general theme from the title.
Classic mistakes die hard. Sometimes it seems they never will.
Until later,
Tom Bourbon