nomenclature and fiber-type composition of spinal nerves

Bob Laird rjl at S1500.NGC.PEACHNET.EDU
Fri Feb 24 16:14:18 EST 1995

Help -

As I try to help my gross students understand PNS composition I see
statements that appear contradictory re nomenclature for, and fiber-type
composition of, peripheral nerves.  Your thoughts, for their edification,
are earnestly solicited. 

For example,I have frequently encountered the position that a number of
spinal nerves are purely motor, i.e., contain no sensory fibers, therefore,
they are "motor" nerves.  For example, that assertion has been made for the
following spinal nerves:

long thoracic           (serratus anterior)
dorsal scapular         (rhomboids, plus C5 contribution to levator scapulae)
subclavian              (subclavius)
suprascapular           (supra- and infraspinatus)
upper & lower subscapular (subscapularis & teres major)
lateral & medial pectorals (pectoralis major & minor)
posterior interosseous  (9 posterior compartment antebrachial muscles)
anterior interosseous   (pronator quadratus, flexor pollicis l., 1/2 profundus)

As an alternative position I would argue that all spinal nerves contain
both motor and sensory fibers; that their branches serve distinct
territories of muscle, skin or joints, all of which territories require
both afferentation and efferentation; that appropriate termiology would
therefore express territory served, e.g., "muscular, cutaneous, or
articular" nerves.  

Furthermore, it could be argued that only cranial nerves may carry
exclusively efferent or afferent fibers (* see below).  Therefore, "motor"
is reserved for those cranial nerves with no sensory fibers, e.g., CN's
III, IV, VI, XII (all GSE only); XI with SVE only (except that GSA fibers
from dorsal roots of spinal segments C2-4 join CN XI after it exits the
jugular foramen and provide the needed afferentation for trapezius and
sternocleidomastoid); and "sensory" for those cranial nerves that carry no
motor fibers, e.g., CN I (SVA), II (SSA), and VIII (SSA).  

Otherwise, all spinal nerves and the remaining cranial nerves could be
described as initially "mixed" nerves with branches/divisions that serve
distinct geographical territories, e.g., muscle, skin and joints, ergo, the
terminology, muscular, cutaneous, and articular.

To support that argument, one might consider:(1) skeletal muscle relies
heavily upon muscle spindles to participate in motor control, and muscle
spindles contain afferents. (2) All muscles contain arteries whose tunica
media of smooth muscle requires GVE postgangs (motor).  

So, the extrapolation of that argument is: (1) muscular nerves contain GSE
to the extrafusal motor units, GSA to afferent spindles and arteries, and
GVE to efferent smooth muscle in the arteries; (2) skin obviously contains
sensory receptors (Pacinnian corpuscles, Merkel's disks, etc.) and
therefore GSA fibers in the cutaneous nerves.  But skin also contains
arteries with their innervation requirements of GSA and GVE.  And, skin
contains aerector pili muscles requiring GVE.  Therefore, could cutaneous
nerves be considered "sensory" nerves?  (3) Articular nerves serve not only
the sensory needs of the joints for nociception and proprioception (GSA),
but joints also contain arteries which still require both afferentation and
efferentation (GSA and GVE), thus producing again a nerve containing both
motor and sensory fibers.  (4) For spinal nerves, the term "mixed" could
denote that the nerve is not yet divided to serve a single territory of
either muscle or skin or joint, each of which require both efferentation
and afferentation. (5) Definitive spinal nerves all carry GSE, GVE and GSA
fibers (* see below), and nerve trunks (the immediate product of dorsal and
ventral roots, and the source of dorsal and ventral primary rami)
additionally carry GVA's conveyed to the trunk from coelomic space viscera
by communicating rami (white and/or gray).

(*)     A frequent exception is that of spinal nerve C1 which,more often than 
        not, has no dorsal root, therefore, carrys no afferentation.  For this 
        reason, dermatome maps do not show a C1 dermatome.  Instead, spinal 
        dermatomes end cranially with C2 at the vertex and the trigeminal
        rostrally from that point to the inferior margin of the mandible with 
        its three divisions (ophthalmic, maxillary, mandibular) in sequence.

Any correction, amplification or suggestion to or for the above would be
appreciated so that my students are not led too far astray.  I apologize
for the length of this posting and, again, thanks for your input.


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