DISSECTION OF THE SHEEP'S BRAIN

PSY222: Neuropsychology

 

 

           

Introduction.

 

            The purpose of the sheep brain dissection is to familiarize you with the three-dimensional structure of the brain and teach you one of the great methods of studying the brain: looking at its structure.  One of the great truths of studying biology is the saying that "anatomy precedes physiology".  You will get sick of me saying that phrase this phrase if I teach well.  What this phrase means is that how something is put together tells us much about how it works.  My challenge to you with this exercise and throughout the term will be to examine a structure and think what this means about the operation of the brain.  Your ideas can be as valid as anyone else's who has tackled this delightfully impossible task if you think carefully

            While the course will emphasize the human brain, observation and evolution indicate that there are many similarities between the sheep brain and the human brain.  Even the differences are instructive and help us to learn about the brain.  Being able to locate important structures in the sheep brain will be of great benefit to understanding how structures are related to each other in the human brain.  If the same structure exists in both brains (and most structures are the same), they are in the same relative location.  During the course of the dissection, I will point out some of the differences between brains so that you will be better able to appreciate the development of the human brain. 

            It is extremely important for the rest of the class that you learn the structure of the sheep's brain well.  In the rest of the course, we will regularly refer to structures that we examine in this dissection. 

            Please follow the following steps in order.  All terms that you need to know are in bold italics the first time they are listed.

 

Materials and Preparation.

 

1.         Before beginning inspection and dissection of the brain you should have these materials on hand:

                        dissection pan                            dissection kit:

                        brain                                                                 scalpel

                                                                                                probe

                                                                                                scissors

 

 

2.         The brains are stored in a preservative solution.  To minimize the drying of your hands, rinse the brain under a slow stream of running water before proceeding with the dissection.  When not in use, the brains should be stored in preservative solution in the container given to you and sealed tightly.

 

3.         These steps will need to be repeated prior to each laboratory session.

  

Procedure.

 

Before beginning the dissection of the sheep brain you will need to know the terms used to specify the location and relative location of various brain structures.  You will need to find the definitions for the following terms in your book:  anterior, posterior, dorsal, ventral, lateral, and medial.  Also note the definitions at the end of this guide for frontal plane, sagittal plane, and horizontal plane.  The rest of the guide and course will use these terms so it is important that you learn them well.

            The procedure is divided into four main sections: Examination of the Exterior of the Brain, Examination of the Mid-Sagittal Plane of the Brain, Examination of one Interior Sagittal Cut, and Examination of one Frontal Cut.

            Use the online sheep brain guide to help you find structures.  There is a link to this site on the syllabus and list of class related sites.  It should be up in the lab when you come in.  For the most part, except for 1, the numbers refer to the images on the online sheep brain guide.

 

 

Examination of the Exterior of the Brain. 

The first portion of the dissection will be a detailed examination of the brain surface.  No actual cutting of the brain is required for this portion of the dissection.  As you proceed to identify the listed parts of the brain, note their structure and how they are related to other parts of the brain.  What conclusions can you make about the brain from this examination? 

 

  1. First examine the exterior of the entire brain.  You may be able to see one or two of the three layers of the meninges, the dura mater, the arachnoid layer, and the pia mater.  The meninges are the protective coverings, which enclose the brain and spinal cord.  The dura mater, the tough outer layer, will have been mostly removed when the brains were prepared for the dissection; however, some of the dura mater may remain near the base of the brain.  The arachnoid layer, the middle layer, and pia mater, the inner layer, are still likely to cover the brain.  The pia mater follows the gyri and sulci and most likely is still on your specimen and may be indistinguishable from the brain.  Blood vessels are between the arachnoid layer and the pia mater.  These vessels and the arachnoid layer will obscure your view of the sulci making the identifications below difficult and confusing.  Before proceeding with the identification of structures on the surface of the brain you will need to remove the arachnoid layer and the blood vessels.  Use your tweezers and be very careful because the brain is soft and easily damaged.
     
  2. Next locate the area referred to as the brain stem.  This area is made up of the pons, medulla, and cerebellum.  Find also the root where the pituitary gland was attached to your brain.  The pituitary gland may have been there when you first cleaned your brain.
     
  3. Examine the ventral surface of the sheep brain.  The next several steps will view this surface of the brain.  A pair of olfactory bulbs may be seen, one under each lobe of the frontal cortex.  Several important parts of the visual system are visible in the ventral view of the brain.  Muscles, other nerves and fatty tissue may surround the optic nerve on your specimen.  After inspection of these, use a scalpel to cut away this muscle tissue, leaving as much of the optic nerve as possible protruding from the ventral side of the brain.  Notice that as the optic nerves from the right and left eyes proceed towards the center of the brain, they meet in the optic chiasm (named for the Greek letter chi, C, which it resembles).  In the optic chiasm, there is a partial crossover of fibers carrying visual information.  Any time fibers in a tract or nerve cross the midline of the brain it is called a decussation.  After the optic chiasm, visual information proceeds along the optic tract toward the visual cortex.  You need to know the difference between a nerve and a tract.  On this screen also note the longitudninal fissure and the cranial nerve called the oculomotor (III) nerve which helps control eye movements.
     
  4. On the alternate image find two important structures from the midbrain, the superior colliculus and inferior colliculus.  Also find the pons.
     
  5. Find the medulla (oblongata) which is an elongation below the pons.  Among the cranial nerves you should find the very large root of the Trigeminal and the thin fibers that will make up the vagus nerve.
     
  6. From this view find the IV ventricle and the cerebellum
     
  7. From this view you can get another view of the superior, inferior colliculi, and the other end of the IV ventricle.  It is very important to move around and manipulate the brain and get a sense of the three dimensional nature of the brain.
     
  8. Review sites seen earlier.
     
  9. The superior and inferior colliculi make up the Tectum.
     
  10. On this slide, find the cerebral peduncles.
     
  11. On this screen note the Hippocampal gyrus so named because the hippocampus lies above it.  Also find the rhinal fissure which defines one boundary of the limbic system.
     
  12. Now find the four lobes of the cerebrum: frontal, parietal, temporal, and occipital.

 


Do not proceed to the next step before checking with the lab instructor.

  1.  No you will make a mid-saggital cut.  Hold the brain level and flat and cut along the longitudinal fissure.  On this screen you can find the lateral ventricles (and septum pellucidum), third ventricle, the cerebral acqueduct (which connects the third and fourth ventricle), and the tegmentum, the other part of the mid brain.
     
  2. This is a more detailed view of the mid-saggital section.  Here you can find the largest of all of the commisures (a band of fibers that connects the two sides of the central nervous system).  This is the corpus callosum.  It is so big that different parts of it get different names.  So you have the genu, splenium, and the body of the corpus callosum.  In addition note the pineal body famous from our discussion of Decarte), the hypothalamus, and the massa intermedia
     
  3. From this screen identify the cingulate gyrus, the septal area, and anterior commisureThe anterior commissure is the other primary forebrain commissure.

 

Do not proceed to the next step before checking with the lab instructor.

  1.  No you have scrapped way a little of the medial surface of the brain.  Find here the fornix, the hippocampus, and the septum pellucidum.  The septum pellucidum is a membrane that divides the two hemispheres.
     
  2. Now you are looking at the cerebellum.  Notice the pattern of grey and white matter.  To some it resembles a tree or bush and is called as a result the arbor vitae (the tree of life –ok a bit strong).

 



DISSECTION OF THE SHEEP'S BRAIN

Part 2

Frontal or Coronal Cuts

 

 

  1. Find the putamen, globus pallidus, and caudate nucleus.  These structures are collectively known as the Basal Ganglia.  In addition you should see the crossing of the anterior commissure right above the optic chiasm. While not labeled see if you can see the corpus callosum and the lateral ventricles.
  1. Find the lateral ventricle, in more than one place.  Move the structure around and see if you can see how it is positioned in the brain.  Find also the hippocampus, lateral geniculate nucleus (called lateral geniculate body on the screen), and the third ventricle.  See also how the optic tract has moved up into the brain and connects to the lateral geniculate body.  Final also the thalamus, lateral ventricles, and corpus callosum (not all of these structures are labeled on the figure). 
  1. Here you will get a great view of the hippocampus and the lateral ventricle.  You can also see again the lateral geniculate nucleus and the medial geniculate nucleus.  Notice the locatuion of the cerebral peduncles below.
     
  2. This is your last cut.  You will be cutting into the cerebellum and the brain stem.  You will see deep cerebellar nuclei (a general term) but more interestingly the trapezoid body and decussating fibers (find these ventrally).  You will also see clearly into the fourth ventricle.