354 Neuroanatomy: Draw It to Know It
Hypothalamus: Tracts
Here, we will draw the prominent pathways of the hypo-
thalamus and in the process learn select highlights of its
physiolog y. Draw a sagittal view of the hypothalamus
and pituitary gland, midbrain, and thalamus. Th en
divide the pituitary gland into the adenohypophysis,
anteriorly, and neurohypophysis, posteriorly. Next, label
the paraventricular and supraoptic nuclei (a more accu-
rate depiction of their anatomic positions is shown in
Drawing 20-5 ). Now, within the neurohypophysis, draw
a capillary plexus. Th en, show that the magnocellular
neurons of the paraventricular and supraoptic nuclei
project to the capillary plexus of the neurohypophysis
via the hypothalamohypophysial tract, and then show
that the neurohypophysis releases hormones into the
general circulation via the hypophysial veins. Indicate
that through this pathway, the paraventricular and
supraoptic nuclei release the hormones vasopressin (aka
antidiuretic hormone [ADH]) and oxytocin. Vasopressin
causes increased reabsorption of water in the collecting
tubules of the kidneys. Oxytocin causes increased uter-
ine and mammillary gland contraction. Note that the
hypothalamohypophysial tract is also referred to as the
combined supraopticohypophysial and paraventriculo-
hypophysial tracts.
Now, within the median eminence of the hypothala-
mus, draw the primary capillary plexus. Th en, within the
adenohypophysis, draw the secondary capillary plexus.
Next, connect the primary and secondary plexuses with
the hypothalamohypophysial portal venous system. Th en,
show that the epithelial-cell–fi lled adenohypophysis
releases the following hormones into the general circula-
tion via hypophysial veins: follicle-stimulating hormone,
luteinizing hormone, adrenocorticotropic hormone, thy-
roid-stimulating hormone, prolactin, growth hormone, and
melanocyte-stimulating hormone. Note that the fi rst fi ve
hormones form the quirky yet memorable acronym
FLATPiG. Th e hypothalamus controls the release of
these hormones via a variety of parvocellular neurons,
which release chemicals (peptides, most notably) into
the primary capillary plexus that serve as either releasing
hormones or release-inhibiting hormones; the most
notable source of these neurochemicals is the arcuate
(aka infundibular) nucleus. Oft en the name of the releas-
ing hormone contains the name of the hormone itself;
for instance, growth hormone-releasing hormone pro-
motes the release of growth hormone.^20 , 21 One clinically
important release-inhibiting hormone is dopamine,
which inhibits the release of prolactin; therefore, exoge-
nous dopamine agonists, such as bromocriptine, are used
to help control prolactin levels and reduce tumor size in
prolactin-secreting pituitary tumors (prolactinomas).^22
Now, let's show some additional hypothalamic tracts.
Show that the amygdala, the emotional center, projects
fi bers that pass over the thalamus to the bed nucleus of
the stria terminalis, septal nuclei, and hypothalamus via
the stria terminalis.^23 Th en, show that the fornix sends a
descending column through the hypothalamus to the
mammillary nuclei and that the mammillary nuclei proj-
ect to the anterior thalamic nuclei via the mammillotha-
lamic fasciculus; these two projections form a key portion
of the Papez circuit, an important limbic system path-
way for memory consolidation (see Drawing 21-4).
Next, indicate that the mammillary nuclei project via
the mammillotegmental fasciculus to the brainstem teg-
mentum. Most texts indicate that the mammillotegmen-
tal fasciculus terminates in the midbrain, but select texts
indicate that it also projects to the pons.^24 , 25 Th en, show
that the retino-hypothalamic pathway, which is funda-
mental for the regulation of the light–dark circadian
cycle, synapses in the suprachiasmatic nuclei. Now, show
that diff use nuclei of the limbic system, including those
of the septal nuclei and the olfactory and periamygdaloid
areas, project through the lateral hypothalamus to the
midbrain tegmentum as the medial forebrain bundle.^20 , 25
Finally, show that the dorsal longitudinal fasciculus proj-
ects from the hypothalamus into the brainstem; it
coalesces most prominently in the periaqueductal gray
area (in the midbrain).^6 , 7