Next, I want you to understand a very important functional aspect of the thalamus.
The thalamus is a very important relay center for sensory information. All sensory information
is relayed through thalamic connections up then to the cerebral cortex with the
exception of olfaction. This slide is simpler in that it’s not overwhelmed with labels.
But it is demonstrating the thalamic radiations so these would be the connections that exist
between the thalamic nuclei and the cerebral cortex. Thalamic fibers are shown in through here,
these darker-colored structures. This is a more anterior view of those thalamic radiations.
This is a more posterior view of thalamic radiations. The thalamic radiations travel
in the internal capsule. So they do accompany the motor axon fibers that are descending
within the internal capsule. The motor fibers are shown here in the lighter color descending
through the internal capsule. The thalamus may be considered the gate to consciousness.
Here, we see the oval thalamic nuclei. These are bilateral. Each one of these bilateral
structures contains multiple nuclear components that make this a very complex structure.
Then here in the axial view, we can see various thalamic nuclei and their relationships
to one another. I don’t mean to overwhelm you at the beginning here but the purpose of this
slide is just to provide you a menu of the various thalamic nuclei. We are going to take a look
at each one of these in greater detail, so I’m not going to read all these off to you at this moment.
But let’s now take a look at the anterior thalamic nucleus. The anterior thalamic nucleus
is shown in through here and has been highlighted for you. When we think about thalamic nuclei,
in this case the anterior one, we need to think about input, output. So what kind of information
is coming in to this anterior thalamic nucleus? Well, the input here is from the mammillary body
and the hippocampal formation. Now that the anterior thalamic nucleus has received this input,
where is it going? What’s its output? Well, the output will be to the cingulate gyrus which is
in the cerebral cortex. The function of your anterior thalamic nucleus is that it’s involved
in the limbic pathway. The limbic pathway is discussed in detail in another lecture.
Next, we have the ventral posteromedial nucleus or simply VPM and it is highlighted here.
It too would have an input consideration and an output consideration. Input to the VPM
is sensory from the face and taste, gustation. The output from this nucleus is going to be
to the somatosensory cortex. Consequently, this nucleus is involved in relaying somatosensory
cranial nerve inputs, cranial nerve inputs and taste or gustation to the cortex. We also have
for your consideration, the ventral posterolateral nucleus of the thalamus, VPL and it is
highlighted here. Its input is going to be sensory from the body and the limbs. This is going to
process a lot of information coming into the thalamus. As it processes it, it’s going to
output it, send it to the somatosensory cortex so we can perceive various sensations:
touch, pressure, vibration for example. Again, it’s going to relay somatosensory spinal inputs
up to the somatosensory cortex so that we can perceive those various senses.
Next on our list is the ventral anterior/lateral thalamic nuclei, the VA/L. These are highlighted
in the image, so they can be seen here. Their input is going to be from the basal ganglia
as well as from the cerebellum. Output from this collection of two nuclei is going to be
to the motor, premotor, and supplemental motor cortices. These are located in the frontal lobe.
Consequently, this is the relay then between the basal ganglia and cerebellar inputs
to the motor cortical areas. Another thalamic nucleus is the mediodorsal nucleus.
This is not shown in the plane of section that we have in the image. The inputs here
are from the amygdala and olfactory inputs as well as limbic basal ganglia, so multiple
inputs here. But the output is all to the frontal cortex. As a result, one of the functions here
is for the limbic pathway then to allow a major relay to the frontal cortex allowing for that
interconnectedness. This area can be damaged, can be lesioned and result in
Wernicke–Korsakoff syndrome. Next is the pulvinar nucleus. The pulvinar nucleus is shown here
in a light blue color. It’s receiving input from the visual pathway, auditory pathway and then
there are other sensory pathways that will contribute input into this nucleus. Output
is going to be to the parietal-temporal-occipital association area. This is going to allow us
to orient ourselves toward visual, auditory, and other sensory stimuli. Here’s another thalamic
nucleus that I want you to think about and remember. This is the medial geniculate body, MGB.
It’s labeled, a little slender portion of it right along in through here. The input into the medial
geniculate body is going to be from the inferior colliculus. Its output is going to be to the
auditory cortex of the temporal lobe. Thus, it functions as a relay for auditory input
to the cortex. If we have a medial geniculate body, it stands to reason that we have a lateral
geniculate body, LGB and that we can see it right in this particular area, not much there in the
illustration or image. But this is the area that’s going to receive information from the retina,
so that’s the input. Output then will be to your primary visual cortex in the occipital lobe.
So this is a very important relay nucleus then of visual input to the cortex for the perception
of vision. This thalamic nuclear collection is the intralaminar/midline thalamic nuclei.
They are shown in this general area on the medial side, this collection of thalamic nuclei.
These are involved in arousal. As a result of that, they will activate the reticular,
will be part of the activation of the reticular activating system. Here, we could focus on the lower
portion of the slide. We’ll just come right in here to this center box. Right below it, you see
various sensory inputs: vision, audition, olfaction for example. Let’s just say you have a
visual stimulus. Now, let’s take a look at what happens next in response to the visual stimulus.
This will have excitatory output to the reticular activating system. We can see that excitatory
input from the visual stimulus into the reticular activating system that is found in the brainstem.
From here, there is input that’s excitatory to thalamic nuclei. And so, we see the excitatory
input to nonspecific thalamic nuclei. Then from here, you relay to the cortex to activate,
to stimulate it. That’s the connectedness, the circuitry that exists between thalamic nuclei
and the reticular activating system.