In this lab, we will return to the sheep brain we started dissecting in Lab 1: Gross Anatomy and Lab 2: Comparative Neuroanatomy. We will identify regions of the brain that produce some of the neurotransmitters covered in Chapter 5 of the Open Neuroscience Initiative (textbook on Perusall).  

As you may recall, you dissected the sheep brain to view the midsagittal structures during the Gross Anatomy lab. At that time, you focused on the corpus callosum and its job in connecting the hemispheres of the brain. This week, return to the left (or remaining intact) half sheep brain and acquaint yourself with some other structures. 

Remember that, during development, the rhombencephalon divides into the myelencephalon and the metencephalon. The myelencephalon develops into the medulla oblongata. The mesencephalon develops into the pons and the cerebellum. Together, the pons and medulla are referred to as the brainstem.

Let’s move into the structures that are formed from the mesencephalon (aka the midbrain). The midbrain is the most anterior part of the brainstem   in the sheep brain. In the human brain, you would say that the midbrain is superior to the pons.

Within the midbrain, we can find the tegmentum and the substantia nigra.

Recall from class that while there are few patches of neurons in the brain that produce dopamine, most of those we do find exist in the midbrain. Two areas where those neurons are concentrated are the ventral tegmental area and the substantia nigra.

(We will return to our discussions of dopamine-producing neurons as we continue with the dissections in this lab.)

In the Comparative Neuroanatomy lab, one of your two tasks was to make a few coronal dissections on the right hemisphere of your sheep brain specimen. With the second dissection cut you made, you should have cut through the genu of the corpus callosum. We will consider this slice to be Coronal Slice 1. Although you are working on one hemisphere, we will use photos in the lab manual that show both hemispheres. The following photo depicts Coronal Slice 1.

While they are difficult to distinguish in this specimen and without staining, the caudate nucleus and the putamen are parts of this dissection slice. The caudate nucleus and the putamen are divisions of the striatum. The striatum is a subcortical area that is also involved in dopamine production.   We will be able to see the caudate better on Coronal Slice 2b.

In the Comparative Neuroanatomy lab, you continued to make slices about 2-3 mm posterior to each slice. Look for a dissection slice that was made about halfway between Coronal Slice 1 and the optic chiasm (you may want to look at the ventral aspect to find the chiasm). This dissection slice will be referred to as Coronal Slice 2a.

Find another slice that was made near the middle of the optic chiasm. This dissection slice will be referred to as Coronal Slice 2b.

You should notice the corpus callosum and the lateral ventricles are more clearly distinguished. To find the head of the caudate nucleus look at the structure dividing the ventricles. This structure is called the septum pellucidum. Starting at the septum pellucidum (or the medial aspect of the lateral ventricle if the septum pellucidum is not present), move laterally across the ventricle. The next bit of brain matter you encounter is the head of the caudate nucleus.

The entire caudate nucleus is a relatively large, curved structure (highlighted in red in the human brain below).

The head of the caudate nucleus is the more anterior curve.

The putamen can be found if you look more laterally and inferiorly. The head of the caudate nucleus and the putamen are separated by the internal capsule.  The internal capsule is a large white matter tract that connects the cortex to subcortical regions, the brain stem, and the spinal cord. On this slice, you can see striations (stripes) running between the head of the caudate nucleus and the putamen through the internal capsule. The thin curve of white matter on the lateral/inferior side of the putamen is called the external capsule.

If you have a dissection slice that was made just near the posterior edge of the optic chiasm, the third ventricle may be visible in it.   We will call this Coronal Slice 3a.

Although the anterior portion of the thalamus, that important relay station, is visible in this slice, the thalamus appears larger in more posterior slices, including Coronal Slice 3b which is found if you made your dissection slice near the center of where the pituitary gland could be found. (Remember, you may not have seen a pituitary gland on your specimen, but you can estimate its location.)

Although structures in this slice may be difficult to distinguish without something to aid in our observations (for example, a stain that might help to highlight the details in the slice), you should become familiar with the general location of these structures:

Cerebral aqueduct (the channel that connects the third and fourth ventricles): Considering the superior-inferior plane (i.e., top to bottom of this photograph) of the midbrain portion of this slice, the cerebral aqueduct is the small hole about one-thirds of the way. It appears as a diamond shape in this image.

The cerebral aqueduct is surrounded by the periaqueductal grey (PAG). Recall from class and the textbook that the receptors for neuropeptides are heavily expressed in this area of the midbrain.

Also found near the most inferior portion of this coronal slice is the substantia nigra. It was indicated previously that we would return to our discussions of dopamine-producing neurons. This is the second of the two most notable areas comprised of dopamine-producing neurons. You cannot visualize the borders of the substantia nigra very well on this image.  Check out the substantia nigra in a human brain.

The fourth ventricle is visible in this dissection slice. On this specimen, it appears as a u-shaped gap (you can probe gently) near the bottom third of the slice.

Much like the problem we encounter with distinguishing structures in Coronal Slice 4 with the naked eye, we encounter problems with distinguishing structures in Coronal Slice 5. Again, we encourage you to think more in terms of general location. The Raphé nuclei can be found near the medial aspect and inferior to the fourth ventricle. Recall that there are few brain areas where serotonin is synthesized, but the Raphé nuclei, found in the brainstem, is one area where serotonin-producing neurons can be found.

Having trouble finding a structure?

Check out the brain atlases at the back of the lab, or the other resources linked on Moodle.

Homework

See Moodle.