162 Neuroanatomy: Draw It to Know It
Major Sensory Projections
Th ere are three main somatosensory pathways within
the brainstem: the medial lemniscus tract, the spinotha-
lamic tract of the anterolateral system, and the trigemi-
nothalamic tract. Th e medial lemniscus carries input
from the large sensory fi bers from the body, which
transmit vibration, two-point discrimination, and joint
position sensory information. Th e spinothalamic tract
carries input from the small sensory fi bers from the body,
which transmit pain, itch, and thermal sensory informa-
tion. Th e trigeminothalamic tract projects sensory infor-
mation from the face. All three pathways ascend the
brainstem separately and then bundle within the ventro-
posterior thalamus before they project to the somatosen-
sory cortex. Th e position of the spinothalamic tract is
largely unchanged throughout its spinal cord and brain-
stem ascent, so we will not draw it here, and we draw the
trigeminothalamic tracts in detail where we discuss the
trigeminal nucleus, itself (see Drawing 13-3), so we will
not draw them here either. Here, we will focus on the
medial lemniscus tract; specifi cally, we will draw the ori-
gins of the medial lemniscus tract in the gracile and
cuneate nuclei, the internal arcuate decussation of the
gracile and cuneate projection fi bers, the medial lemnis-
cus brainstem ascent, and the thalamocortical projec-
tions to the parietal lobe.
First, draw the lower medulla and show that the
leg fi bers of the medial lemniscus tract originate in the
gracile nucleus in the posteromedial medulla and, then,
that the arm fi bers of the medial lemniscus tract origi-
nate in the cuneate nucleus in the posterolateral medulla.
Th e gracile nucleus receives aff erents from the spinal
cord posterior column gracile tract and the cuneate
nucleus receives aff erents from the posterior column
cuneate tract. Next, indicate that the gracile and cuneate
fi bers decussate (ie, cross midline) as internal arcuate
fi bers in the lower medulla; in the process, they rotate
their somatotopic orientation from medial–lateral to
anterior–posterior: the leg fi bers go from medial to ante-
rior and the arm fi bers go from lateral to posterior. Post-
decussation, these fi bers bundle as the medial lemniscus
tract.
Next, show that as the medial lemniscus ascends
the pons and midbrain, it fl ips back to medial–lateral
orientation but with the arm fi bers medial, leg fi bers lat-
eral. Indicate that within the thalamus, the body fi bers
(arms and legs) project to the ventroposterior lateral
nucleus and the facial fi bers project to the ventroposte-
rior medial nucleus. Next, draw the twisting thalamo-
cortical sensory projections to the cerebral cortex. In
their ascent, the sensory fi bers again reverse their orien-
tation: the leg fi bers project medially to terminate in the
posterior paracentral g yrus, the arm fi bers project lateral
to the legs and terminate in the upper convexity of the
postcentral g yrus, and the facial fi bers project lateral to
both of them and terminate in the lower lateral postcen-
tral g yrus.
Now, demonstrate this multi-step rotational ascent
with your hand for easy reference. Place your right hand
in front of you, palm down, with your fi ngers pointing to
your right. Your thumb represents the leg fi bers and your
little fi nger represents the arm fi bers. To demonstrate the
decussation of the internal arcuate fi bers, move your
hand across midline and bring it in towards you. In the
process, rotate your hand into anterior–posterior orien-
tation with your little fi nger behind your thumb: arm
fi bers posterior to leg fi bers. Next, to demonstrate the
orientation reversal that occurs during the medial lem-
niscus ascent to the thalamus, rotate your hand back into
medial–lateral orientation (the little fi nger [the arm] is
medial and the thumb [the leg ] is lateral) and simultane-
ously raise it. Th en, to demonstrate the thalamocortical
projection to the somatosensory cortex, twist your hand
over so that the thumb (the leg ) is medial and the little
fi nger (the arm) is lateral.^1 – 6