Sheep Brain Dissection Picture Guide Figure 1: Right Hemisphere of Sheep s Brain Figure 2: Underside of Sheep s Brain
Figure 3: Saggital cut of Sheep s Brain to reveal subcortical structures Figure 4: Coronal cut of Sheep s Brain to reveal grey and white matter
Name: Section: Date: Sheep Brain Dissection Worksheet In this Neuroscience Module you will learn how our neurons work together to form circuits in the brain. You will also learn how these circuits function to regulate complex behaviors and how these behaviors can be disrupted due to damage or disease. We will start our exploration of the nervous system by dissecting a sheep s brain to familiarize ourselves with key brain structures. However, there s no avoiding the fact that a sheep is, well, a sheep. Can you think of two brain regions found in humans that are unlikely to be well represented in sheep? (Hint: think about things that we do that sheep don t.) 1. 2. How about a brain region that might be better represented in sheep than humans? 1. The Coverings of the Brain The sheep brains we are dissecting have been preserved in fixative. A fresh brain is much more jelly-like and susceptible to damage. The living brain is encased in fluid filled membranes or meninges that cushion the brain within the skull and help to protect it from damage. The meninges contain blood vessels that supply the brain with oxygen. Keeping the brain supplied with oxygen is critical for it to function properly. When the blood supply to the brain is disrupted, strokes occur. The meninges also contain neurons that sense pain. Pain neurons are not found within the brain itself. Migraine headaches occur when the blood vessels in the meninges dilate (swell) so that fluid leaks out. The pain neurons sense the fluid and mount an inflammatory response that causes pain. Take your scalpel and carefully strip off the meninges from the cortex. Be careful not to dig into the cortex itself.
The Overall Organization of the Central Nervous System Lay your sheep brain with the right side up then compare it with the photograph of the right side of a sheep s brain in the Sheep Brain Dissection Picture Guide. Examine the gyri and the sulci. Are there similar numbers of gyri and sulci in your sheep brain as in the one pictures? Are they identically placed? What does this suggest in terms of the relation of the gyri and sulci to functional areas of the brain? For homework, you read about how the central nervous system is organized. Make sure you can identify each part visible on the surface of the brain. Next explain what each area does in the table below, and label each area on your sketch above. Brain Region 1. Spinal Cord 2. Cerebellum 3. Frontal lobe 4. Parietal lobe 5. Temporal lobe 6. Occipital lobe 7. Central Sulcus What does it do? The Sheep Brain Dissection Picture Guide identifies the boundaries between the lobes in a cortex the longitudinal fissure and the central sulcus. Can you identify the boundaries between each of the lobes in your sheep s left cortex? If you can, draw them on your sketch. How did you decide where to put them? If not, explain why you couldn t identify the boundaries.
Lay your brain with the bottom side up. Then compare it with the photograph of the underside of the sheep s brain in the Sheep Brain Dissection Guide. Can you see any of the cranial nerves? If so, label the cranial nerves you can see on your sketch above. The Sensory and Motor Cortices We have discussed how sensory information travels up from the periphery and how motor responses travel down to the periphery. In your homework, you saw how that information is organized in the cortex in the sensory and motor homunculi. Sheep sensory and motor information is represented in a similar way in their cortices. Identify the central sulcus on your sheep brain, and then identify the sensory and motor cortices. Mark them on your sketch of the right side of the sheep s brain. Remember that the sensory cortex is within the parietal lobe behind the central sulcus, and the motor cortex is within the frontal lobe in front of the central sulcus. A Sagittal Cut to Reveal the Subcortical Organization of the Hemispheres. Although the brain has two hemispheres, the left and right cortices are neither structurally nor functionally symmetrical. Carefully turn the brain so that the left hand side is up note that the pattern of gyri and sulci are not the same as the right side. The structural differences are reflected in functional asymmetries between the two hemispheres. We can t detect these function differences just by looking at the brain. However, researchers have shown that in humans, the areas associated with hearing and speaking words are located on the left hand side of the brain, while the area associated with the emotional component of language is situated on the right side. (We will look at how we study the regulation of language later on in this unit.)
There are a number of ways that we can cut into the brain to reveal its inner structures. Right now we will separate the two hemispheres in half lengthwise. This is called a sagittal cut. First, identify the cerebellum. The cerebellum is not divided into two hemispheres and you will need to actually cut it in half. Make sure you can see the midline where you will cut. Beginning between the left and right frontal lobes, use your scalpel to make a careful cut to separate the two hemispheres. As you move backwards from the cortex to the cerebellum use the midline of the cerebellum to make sure you divide the brain exactly in two. Once you have finished cutting, open the brain like a book to reveal the subcortical structures. Put the left side of the brain to one side. Place the right side of the brain in front of you with the subcortical structures facing up. Then compare it with the photograph of the sagittal cut of the sheep s brain in the Sheep Brain Dissection Guide. Make sure you can identify each part visible within this cut of the brain. Next explain what each area does in the table below, and label each area on your sketch above. Brain Region 8. Corpus callosum 9. Thalamus 10. Hypothalamus 11. Cerebellum 12. Midbrain 13. Pons 14. Medulla 15. Spinal Cord What does it do?
Both sensory and motor neurons cross from one side of the body to the other as they ascend and descend respectively. In fact both sensory and motor neurons cross from one side to the other in the medulla. Therefore, if you damage your sensory neurons above the medulla, will you lose sensation on the same side of your body as the damage or on the opposite side? What about if you damage your sensory neurons below the medulla, will you lose sensation on the same side of your body as the damage or on the opposite side? A Coronal Cut to Reveal the Grey and White Matter of the Hemispheres. Take your scalpel and in the frontal lobe, make a careful coronal cut to reveal the grey and white matter of the cortex. Once you have finished cutting, examine the interior of the cortex and compare it to the picture of a coronal section in the Sheep s Brain Dissection Guide. The grey matter contains the neuron s cell bodies, and the white matter contains the myelinated axons traveling to and from the cortex. Label the grey and white matter in your sketch above.