10 Neuroanatomy: Draw It to Know It
Divisions & Signs
Here, we will learn the central and peripheral divisions
of the nervous system and the meaning of upper and
lower motor neuron signs; note that we exclude the
autonomic division of the nervous system, here, for sim-
plicity. First, let’s draw the central nervous system. Draw
a brain, brainstem and cerebellum, and spinal cord. Next,
in order to label the upper motor neuron and nerve com-
ponents of the central nervous system, draw a cerebral
cortical neuron and label it as an upper motor neuron,
and then show a white matter tract descend from it, and
label the tract as an upper motor neuron fi ber.
Now, let’s draw the peripheral nervous system. From
the brainstem, draw a cranial nerve and from the spinal
cord, draw a spinal nerve. Next, in order to label the
lower motor neuron and nerve components of the
peripheral nervous system, draw a spinal motor neuron
and label it as a lower motor neuron and label the spinal
nerve as a lower motor neuron fi ber. Note that a common
point of confusion is when the lower motor neuron lies
within the central nervous system, as it does here. For
this reason, it’s easier to determine whether a neuron is
upper motor or lower motor, if we think about the fi ber
type the neuron projects rather than the location of the
neuron, itself. Generally, cerebral cortical pyramidal cells
are upper motor neuron whereas cranial nerve nuclei
and spinal motor neurons are lower motor neuron.
Next, we will use three diff erent locations of nervous
system injury to learn the exam fi ndings in upper and
lower motor neuron lesions. Let’s start by creating a small
table. Across the top, write the words: injury type, muscle
tone, muscle stretch refl exes, and pathologic refl exes
(eg, the Babinski sign). Now, for the fi rst injury type,
show that injury to the cerebral cortex, such as from a
stroke, disrupts the brain and the related white matter
tracts. Indicate that it causes an upper motor neuron pat-
tern of injury: there is spastic muscle tone, hyperactive
muscle stretch refl exes, and the presence of pathologic
refl exes. Second, show that injury to the spinal nerve
fi ber, such as from neuropathy, causes a lower motor
neuron pattern of injury: there is fl accid muscle tone,
hypoactive muscle stretch refl exes, and the absence of
pathologic refl exes.
Th e third pattern of injury, spinal cord injury, is
mixed. Transect the cervical spinal cord. Th en, indicate
that above the level of lesion, the patient is normal. Below
the level of lesion, there is damage to the upper motor
neuron fi bers, so show that below the level of lesion,
there is an upper motor neuron pattern of injury. Th en,
at the level of lesion, the spinal motor neuron and its
emanating fi bers are aff ected, so show that at the level of
lesion, there is a lower motor neuron pattern of injury.
Note that upper motor neuron fi ndings oft en evolve
over hours to weeks. Initially, in a spinal cord injury, for
instance, the observed pattern of defi cit below the level
of injury may be more characteristic of lower motor
neuron injury than upper motor neuron injury — there
may be loss of muscle tone and arefl exia; however, over
hours to weeks, the patient’s tone and refl exes will
become pathologically increased and take on a more
typical upper motor neuron injury pattern. Th is initial
phase is called spinal shock in acute spinal cord injury
and cerebral shock in acute brain injury.^1 – 4 , 6 , 7 , 9