Hypothalamus: Neuroanatomy Video Lab – Brain Dissections

Hypothalamus: Neuroanatomy Video Lab – Brain Dissections

>>We’re going to study
today the yellow hypothalamus which sits underneath
the regular pink thalamus which has the third
ventricle, a narrow blue slit, separating the right
and left sides. The hypothalamus has
two principle functions. One is an endocrine function and the other is an autonomic
nervous system control function. It’s an incredibly
important square centimeter of tissue, so let’s look at it. Let’s begin looking
at the hypothalamus in this mid-sagittal section. This surface here is
the ventricular surface, the third ventricle,
and both the thalamus and the hypothalamus are
touching the third ventricle and there is a groove, sort
of an imaginary groove, called the hypothalamic sulcus
that separates the thalamus above which is all of this big
structure I’m moving my probe over from the hypothalamus
below. Now the boundaries of the hypothalamus
posteriorly are approximately where the aqueduct
is coming down here by the mammillary bodies
down here to the optic chiasm up to the anterior commissure. Here is the fornix coming from the hippocampus
and then back here. So this sort of one
square centimeter of tissue is incredibly
vital and let’s look at it on the ventral surface. This is a rather atypical
ventral surface but it allows us to see the base of the brain. Here are our optic nerves
and here is our chiasm and the optic tract,
so the optic tract and chiasm are boundaries
of the hypothalamus and if I turn it here just a
little bit more you can see the two mammillary bodies which
are the posterior extent and then this soft area here on either side this region
is called the tuber cinereum and that hole there is where the
infundibular stalk was attached but has been torn off
when the brain was removed from the skull, because there
is dura mater separating the pituitary gland from
the infundibular stalk, and if I poke my probe through
here and turn this section over what I can see is that
it’s going to be continuous with this hole here in the
bottom of the third ventricle. Here’s our third
ventricle so down here where my probe is
is the hypothalamus. Here’s the beginning
of the aqueduct. Here is our fornix. These are the boundaries:
fornix, aqueduct, the side of the ventricle and
then the widened third ventricle in this particular patient. Now let’s start and talk
about some of the functions of the hypothalamus and
some of its connections. So we’re going to look at
three coronal sections. The first one is
the most anterior. Here is our optic chiasm
and the area right above it and below the anterior
commissure represents the very front end sometimes this area
general region is called the preoptic area. What I wanted to make note of
is that there are axons coming from the optic nerve
that don’t go on back to the lateral geniculate but
instead go to nuclei near here on either side, the
suprachiasmatic, above the chiasm, nucleus
and it’s very interesting. This is important in setting
your circadian rhythms so your rhythmic activity of day
and night is controlled by input of light into this region here. Here’s another sample a
little bit farther back. These are not from the same
brain so don’t be surprised. Here is our third ventricle. This is imaginary
hypothalamic sulcus, so this is the hypothalamic
region, and we’re kind of fortunate because
there’s just a little bit of the infundibular
stalk left on here and it’s a little bit gray because there are capillaries
surrounding this stalk that form part of the
hypophysial-portal system which allows hormones produced
in the hypothalamus to circulate down to the anterior pituitary
which we’ll talk about shortly. Also out here laterally
there’s a bundle of fibers which cannot be seen called
the medial forebrain bundle. It has various sources
and destinations but the medial forebrain bundle
connects the hypothalamus back with the autonomic nervous
system in descending control of the autonomic nervous system
as well as projecting forward up into the basal forebrain area
and parts of the limbic system. This third section is at the
level of the mammillary bodies which we noted were the back
part of the hypothalamus, so here we have that and we
associate the mammillary bodies with memory consolidation
and damage here such as in Korsakoff’s disease lead
to problems with memory as does the involvement of
this dorsal medial nucleus of the thalamus so memory
consolidation for new memories in the mammillary bodies
and dorsal medial nuclei. Some other functions and
afferents to the hypothalamus, besides the retina
and the hippocampus, is input from the amygdala,
which is right here. The amygdala can easily
funnel into the hypothalamus and remember we associate
the amygdala with fear and that is a very,
very important trigger for autonomic responses. A remarkable thing about
the hypothalamus is that different neurons
and different nuclei, and we’re not going to learn
all of them, have receptors and respond to, that is they
respond physiologically, to circulating hormones
from the pituitary gland and represent a way of feeding
back and controlling the release of hormones both
from the hypothalamus and from the pituitary
gland itself and there are circulating
hormones that affect appetite. We have orexigenic ghrelin
coming from the stomach when you’re starving and
anorexigenic things like insulin or glucose or leptin so neurons
respond to these substances in the blood as well
as presence of sodium and the very delicate
balance between salt and water and of course we have to
maintain our temperature for homeostasis and there
are neurons that respond to the temperature of
the circulating blood and also respond to
prostaglandin circulating as a result of inflammatory
reactions. This is a mid-sagittal
radiograph and it shows you on the right the cerebellum at
the top, the corpus callosum, and the pituitary gland
is indicated by arrows with the anterior lobe on the
left and the posterior lobe on the right, and
you can imagine that infundibular stalk
extending down from the base of the third ventricle
which is outlined and the posterior boundary
of the hypothalamus which is the mammillary body. So now the anterior lobe and
the posterior lobe have very different functions and
connections and we want to look at those in more detail. So let’s discuss the
posterior pituitary first. The posterior pituitary is
really an outgrowth of the floor of the diencephalon
or the hypothalamus and here we can see
neurons, large neurons, magnocellular neurons,
in the periventricular and supraoptic nuclei and
these neurons make hormones. They make two very important
hormones, ADH or vasopressin and oxytocin and these
hormones then travel down through axoplasmic flow through the
hypothalamo-neurohypophysial tract to terminate as axon
terminals on blood vessels that have fenestra in them,
fenestrated capillaries, so that the large hormones
can move into the bloodstream, and these hormones are
discharged in proportion to the number of axon
impulses that are coming down carrying these packaged
hormones, so a direct output from the hypothalamus into
the circulatory system. And these two hormones,
vasopressin and oxytocin, can result in inappropriate
responses if they are not in
proper balance. For example if you were to have
trauma or surgery or a tumor and something should
interrupt the flow of these hormones you could end up with something
called diabetes insipidus when there is too low
vasopressin secretion and therefore you
pass too much water and you have a large
volume of urine and these people then are
always thirsty and want to replenish their water
supply and they manifest with what we call polydipsia. So diabetes insipidus
results from an interruption in the level of vasopressin
secretion. The anterior pituitary has a
little bit different system. It evolved as an outpouching
of the roof of the mouth, and so the connections with
the brain are a two-staged sort of system. We have small or
parvicellular neurons located in the hypothalamus in
nuclei called the arcuate or periventricular. They’re down in that soft
fleshy part of the bottom of the hypothalamus
called the tuber cinereum. And these neurons
produce hormones, but we call them releasing
hormones, because they flow down the axons but
end on capillaries in this portal system that
surrounds the median eminence and then these very short-lived
releasing hormones circulate just a short distance to
the anterior pituitary and cause the anterior pituitary
to release the hormones that it produces such as
corticotropin and thyrotropin. So it shouldn’t surprise you
that if you had a tumor that was in the hypothalamus and interrupted this circuitry
you would have hormonal problems or if you had a tumor in the
pituitary gland that grew and pushed up on the
hypothalamus you could have both visual and hormonal and
even behavioral problems, so this is a very important
and very small area. Now let’s think about
the other major function of the hypothalamus, that
is autonomic function. Hypothalamic control of the autonomic nervous
system is very direct. We’ve seen that all sorts of
systems and situations funnel into the behavior
of the hypothalamus and it shouldn’t
surprise us therefore that there is a direct
pathway from the hypothalamus down to the spinal cord and
therefore we need to control and modify both smooth
muscles and glands and this tract is called
the hypothalamospinal tract. Now you can’t really
see it but it’s there because when it’s
interrupted you see changes. So there are two neurons in the
autonomic nervous system plan. The first is the
pre-ganglionic neuron with the cell body
leaving and going out to a peripheral ganglion and
then a post-ganglionic neuron out in the periphery going to the target gland
or smooth muscle. Now let’s look at for
example the reaction when the amygdala responds in a
fearful way and sends a barrage of stimulus to the hypothalamus
to release a pattern of behavior that would be accompanying fear. So what happens when you’re
afraid, we always have that sort of trite response
called fight or flight and this is exactly
what the hypothalamus and the forebrain “decide.” Is it time to use your
brain to think yourself out of this situation or
is it time just to turn on the autonomic system and
get the heck out of there. Well, when you go for the
hypothalamic route this is the pathway that you see and
ultimately through this chain of three neurons you end up with
increasing your blood pressure, your heart rate, the
flow to the muscles so that you can move
out of there. You’ve got to open
up your bronchi so that you can breathe
heavily, you sweat and so on. Even so you also stimulate
for example the pupils. If you’re scared
your pupil enlarges and that is another part
of the sympathetic fight or flight response to fear. Lesions of this
hypothalamospinal tract can occur as you can imagine
anywhere from the hypothalamus down to the spinal cord. This example of an infarct,
some time in the past, of the medulla, a lateral
medullary lesion, is shown here where the tissue has died and where I have
put a bright red dot in the approximate position
of that descending pathway. When this happens then you have
an interruption of that response and another classical
sign of interruption of this pathway is what is
called Horner’s syndrome, and we won’t go into that
but if you’re interested in Horner’s syndrome you
can read more about it. So, in conclusion I hope you
now realize how important this square centimeter of tissue is in controlling not only your
autonomic responses but all of your hormonal interactions
not to mention things like memory and sleep
and wakefulness. So the hypothalamus gets
input from almost every region of the brain and it goes
to very many places. It’s enough to blow your mind.

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