178 Neuroanatomy: Draw It to Know It
Spinal & Cranial Nerve Origins
To understand the origins of the spinal and cranial nerves,
we will draw the developing embryo. First, draw a coro-
nal section through the dorsal half of the embryo. Label
the outer layer as the ectoderm; it derives the epidermis
and neuroectoderm. Th en, draw endoderm at the bottom
of the diagram; it comprises most of the ventral half of
the embryo and it forms the gut and respiratory contents
of the developing embryo. Next, draw the notochord and
indicate that it is mesoderm-derived. We have thus estab-
lished the three main layers of the trilaminar developing
embryo: the ectoderm, mesoderm, and endoderm.
Th e developing embryo forms around the notochord
and the notochord eventually degenerates into the jelly-
like substance of the intervertebral discs, called nucleus
pulposus. If the notochord persists in its primitive state,
it is considered chordoma — notochord tumor. Th e
notochord induces the overlying ectoderm to develop
into the neural plate. Its neural folds then invaginate to
become the oval-shaped neural tube, which has a long ,
narrow cerebrospinal fl uid space in its center, and, as
mentioned, is ectoderm-derived. We will draw the details
of the neural tube later.
Dorsolateral to the neural tube, draw the neural crests,
which are neural tube derivatives that give rise to,
amongst other things, the peripheral nervous system.
Next, draw somite tissue masses lateral to the notochord;
they, like the notochord, are mesoderm-derived. Th e
neural crest cells and somite tissue masses develop early
in embryogenesis and the somite masses derive scleroto-
mal, myotomal, and dermatomal cells. We will include
the somites in our diagram along with their derivatives,
albeit an anachronism, for simplicity.
Draw three arrows from one of the somite masses
as follows. Direct one to the notochord and label
it sclerotome: the sclerotome diff erentiates into bone.
Indicate that the sclerotome around the notochord
becomes vertebral bone. Next, draw another arrow dor-
sally to the surface of the embryo and label it dermatome,
which derives the dermis: the skin layer underlying the
epidermis. Th en, direct the last arrow laterally to the
myotomal masses, which derive skeletal muscle.
An artery and nerve pair corresponds to each somite
segment, and wherever the products of that somite go,
the nerve and artery pair follows. Th is is the anatomic
basis of how we systematically assess dermatomal and
myotomal levels during the neurologic exam. From the
peripheral distribution of the defi cit, we localize the
rostral–caudal level of the lesion.
With that embryogenesis as a background, let’s take
a closer look at the origin of the cranial and spinal
nerves. Separate the tissue within the neural tube into a
ventral-lying basal plate and a dorsal-lying alar plate.
Along the horizontal meridian, label the sulcus limitans:
a small sulcus, which cuts into the lateral walls of the
central-lying cerebrospinal fl uid space and separates
the basal and alar plates. Somite nerve pairs emanate
from these neural plates as nerve roots: the ventral roots
carry motor fi bers and the dorsal roots carry sensory
fi bers (in accordance with the law of Bell and Magendie).
Th is ventral–dorsal functional division applies to the
neural tube, as well. Th e basal plates, which produce the
ventral roots, house motor cell columns, and the alar
plates, which receive sensory roots, house sensory cell
columns. Th e ventral–motor/dorsal–sensory division
that exists early in embryogenesis in the neural tube per-
sists throughout development. Th is division provides
the basis for the organization of the spinal and cranial
nerves.^1 – 4