21 Lab 3: Comparative Neuroanatomy
Erin Mazerolle and Sherry Neville-MacLean
Learning Objectives
- Compare and contrast the location and appearance of the 12 cranial nerves on the sheep and human brains.
- Identify commonly preserved cranial nerves on a sheep brain specimen.
- Compare and contrast sheep and human coronal sections.
- Start considering changes in some structures as you move anterior to posterior in coronal cuts.
In this lab, we will explore the similarities and differences between the gross anatomy of the sheep and human brain. We will use your sheep brain specimen as well as models and images of the human brain.
The groups at tables at the back of the lab will start with the cranial nerves. The materials you need for this activity are at the back of the room.
The groups at tables at the front of the lab will start with the coronal sections. The materials you need for this activity are at the front of the room.
We will switch after about 30 minutes. This will give us time to clean at the end of our lab section.
If the materials you need are currently being used by another group, please read ahead as there will be other activities you can complete while you are waiting.
Cranial nerves
Examine the left hemisphere of your sheep brain specimen. Consider the image below. Are any of the cranial nerves on your specimen still intact? Which ones? If you have not already performed the coronal sectioning, you can also compare the right and left hemispheres. Does one hemisphere have different cranial nerves still intact than the other?
Only a few nerves will still be visible on your specimen. The important information is approximately where they are and what they do, rather than to be able to locate them on your specimen. Be familiar with their location/order, as well as their general function (fibre type), and specific function(s).
Next, examine sheep brain specimen with the meninges still intact (at the back of the room). Compare this specimen with yours. Which cranial nerves are still visible/intact on the specimen with the meninges still intact? You can compare to the sheep brain atlas as well.
Next, consider the cranial nerves visible on the ventral surface of the human brain. What similarities and differences do you observe between the sheep and human cranial nerves in terms of location, morphology (shape), and size?
Think about the functions of each cranial nerve. Do you expect any differences in the functions between the two species? Why or why not?
Review your homework from last week – the cranial nerve testing you developed.
Coronal sections
For this activity, be sure to section only the right hemisphere! (On the sheep’s right!)
Remember to read through all the instructions before starting!
Procedure
- Place the right hemisphere of your sheep brain specimen in the dissection tray. You should not be holding the sheep brain in your hands as you cut.
- Recall the slicing procedure from the midsagittal section you performed in a previous week. All dissection cuts should be made in a single fluid motion, and each section might require multiple passes of the scalpel. If the cuts are not clean, it is more difficult to identify all the structures.
- If possible, have a different group member perform the cutting this week.
- To begin the first coronal slice, lay the medial surface of your specimen on the dissection tray so that it rests mostly flat on the tray (The hemisphere should rest steadily, rather than rocking). Position your specimen so you can comfortably cut using an inferior to superior motion (or superior to inferior motion). In a single fluid motion, make a small incision 12-18 mm (½ to ¾ of an inch) from the tip of the frontal lobe. Your first slice should not contact the corpus callosum.
- Now, make a cut that is another 3-5-mm from the anterior portion of your specimen (think the width of a processed cheese slice). Thinner slices may make it easier to identify all relevant structures. Remember, if you miss a structure, you will have to rely on other groups’ specimens as examples. You should now be able to see the genu (anterior end) of the corpus callosum.
- Repeat steps 4 & 5 to make and study 2-3 more cuts.
- What structures are visible on your sections? If the brain model at the front of the room is available, bring your specimen up and compare it to the brain model. Can you find the part of the brain model that is most similar to your section?
- Make a few more cuts until you can see the same structures on your specimen as you can see on the first two coronal sections of the brain model. You can use the human brain model manual to help identify structures.
- Select two coronal slices, take one photo showing both of them, and be prepared to submit the photo with labelling for homework. The photo must be in .jpeg, .jpg, .png, or .pdf format.
More questions to consider
Identify gray matter, white matter, sulci, and gyri on all your sections. Which sections can you view the corpus callosum? Which sections can you view the thalamus? Which sections can you view the ventricles? What do you notice about the views of these structures (corpus callosum, thalamus, and ventricles) in the coronal slices you have made as you move posteriorly through the slices?
Consider this image of sheep brain lobes. Which lobes have you cut through?
When you have finished, please rinse the dissection tray and clean your tools in the designated sink. If you are in an 8:30 am or a 10:00 am lab, please stack the trays in opposite directions (rather than sitting one inside of another) at the center of your tables, and place your cleaned scissors, scalpels, probes, and/or forceps on a clean, dry paper towel. If you are in the 11:30 am lab, please stack the trays in opposite directions (rather than sitting one inside of another)at the back of the lab, and place your cleaned scissors, scalpels, probes, and/or forceps in the appropriate wooden tool box.
Lab Assignment Instructions
Complete the quiz on Moodle.
