218 Neuroanatomy: Draw It to Know It
Cranial Nerve 5: Tracts ( Advanced )
Here, we will draw the trigeminothalamic tracts and the
somatotopic organization of the spinal trigeminal
nucleus. Th e central sensory aff erents of the trigeminal
nerve relay to the cortex through two diff erent trigemi-
nothalamic pathways: anterior and posterior. Draw a
coronal view of the brainstem. Th en, draw the trigemi-
nal ganglion and show that the small fi ber sensory
modality trigeminal nerve fi bers project to the spinal
trigeminal tract and nucleus and that the large fi ber sen-
sory modality trigeminal nerve fi bers project to the prin-
cipal sensory nucleus. Note that although the principal
sensory nucleus is oft en considered the trigeminal func-
tional equivalent of the gracile and cuneate nuclei and the
spinal trigeminal nucleus is oft en considered the trigemi-
nal functional equivalent of the anterolateral system (ie,
spinothalamic tract), postsynaptic interconnections
within the trigeminal system make this an imperfect rela-
tionship and we are unable to use it, clinically.^3
Now, let’s draw the trigeminothalamic projections.
First, show that the anterolateral portion of the principal
sensory nucleus projects via the anterior trigeminotha-
lamic tract to the contralateral ventroposterior medial
nucleus of the thalamus, and then show that the postero-
medial portion of the principal sensory nucleus projects
via the posterior trigeminothalamic tract to the ipsilat-
eral ventroposterior medial nucleus of the thalamus.
Th en, indicate that the spinal trigeminal nucleus projects
via the anterior trigeminothalamic tract to the contralat-
eral ventroposterior medial nucleus of the thalamus.
Note that in regards to the somatotopy of the principal
sensory nucleus, the mandibular division of the trigemi-
nal nerve synapses posteriorly within the principal sen-
sory nucleus and the ophthalmic division synapses
anteriorly; the maxillary division synapses intermediately
between the mandibular and ophthalmic divisions.^3
Next, let’s draw the spinal trigeminal nucleus. It runs
medial to the spinal trigeminal tract, has an onionskin
somatotopy, and divides into three diff erent cytoarchi-
tectural regions. In sagittal view, show that the spinal
trigeminal nucleus spans from the pons to the upper cer-
vical spinal cord. Indicate that within the spinal trigemi-
nal nucleus, pars oralis is the superior-most subnucleus:
it spans from the pons to the mid-medulla; show that
pars interpolaris is the middle subnucleus: it lies in the
mid-medulla; and, fi nally, indicate that pars caudalis is
the inferior-most subnucleus: it spans from the lower
medulla to the upper cervical spinal cord. Its inferior
extent is variably listed as anywhere from C2 to C4.
Next, show that the somatotopic features of the face in
the spinal trigeminal somatotopic map are stretched and
distorted to fi t into the proportions of the long , colum-
nar spinal trigeminal nucleus (imagine pulling a rubber
mask off of your face to visualize the distortion). Th e
superior features of the face (eg , the eyes) lie anterior and
the inferior features (eg, the jaw) lie posterior; the most
superior portion of the pars caudalis subnucleus receives
the lips and perioral area and the most inferior compo-
nent receives the outer ears. Note that our somatotopic
discussion of the spinal trigeminal nucleus refers to the
pars caudalis subnucleus, only; therefore, the most supe-
rior area is the inferior medulla and the most inferior
area is the upper cervical spinal cord. Clinically, when we
discuss the spinal trigeminal nucleus, we generally are
referring to the pars caudalis subnucleus of the spinal
trigeminal nucleus, only.
Now, let’s draw the onion-skin layers of the spinal
trigeminal somatotopic map: the central sensory pro-
cessing map for pain/temperature information in the
face. Draw an undistorted face. Show that the lips and
perioral area constitute the outermost layer of the
onion — they lie within the most superior area of the pars
caudalis subnucleus of the spinal trigeminal nucleus; the
next innermost layer, moving inferiorly within pars cau-
dalis, comprises the nose, eyes, and outer oral areas; then,
continuing downward, lies the cheeks and forehead;
then the vertical band just in front of the ears; and fi nally
lies the partial spinal trigeminal sensory coverage of the
external ear (from cranial nerves 7, 9, and 10).^1 – 6 , 8 , 10