Sheep Brain Dissection

Objectives: 

          By the end of this laboratory you should be able to do the following:

• Learn the basic anatomy and function of the different parts to the nervous system.

• Identify all regions on the dorsal surface of a sheep brain.

• Identify all regions on the ventral surface of a sheep brain.

• Identify all regions of the sheep brain following a midsagittal cut.

      Today’s laboratory involves the dissection of a sheep brain.  The laboratory will begin with a short lecture on the anatomy and function of the different parts to the nervous system.  This material will be part of the in class quizzes and examinations, so pay close attention and take good notes. After the lecture is complete we will move on to the brain dissection. Since this laboratory involves dissection of animal tissue it is important to remember to be thankful, respectful, and subsequently take your studies, and in this case, your dissections seriously. Once again you will be using extremely sharp scalpels and probes so be careful with them. Please proceed with caution and care during your dissections.  Work in groups of no more than two. Use the dull and sharp probes to try to identify all of the brain regions discussed.  Follow the instructions carefully before you cut into your brain.  A single clean slice will make viewing clearer.  I will be asking you to identify the different regions, parts, and cranial nerves of your sheep brains so study while you dissect. The brain is the most complex and fascinating organ in an animal’s body.  The anatomical structures that you see in the sheep brain are an extremely close approximation to those in your own brain.  The regions are the same, the cranial nerves are the same, but fortunately for most of you, your brains are a bit larger and somewhat more complex than that of a sheep. You will find throughout this preparation that I have stated the functions of the individual brain region or nerve in italics and in parentheses just following that item.  When you have completed your dissection you will need to do the exercise on the last page matching region or nerve to function in order to complete the laboratory.  Good Luck in your explorations.

Anatomy and Function of the Nervous System

Neuroanatomy Lecture (html)        Neuroanatomy Lecture (powerpoint)

Identify all regions on the dorsal surface of a sheep brain.

     We will begin our dissection with the dorsal surface of the intact sheep brain.  You should remember that with the sheep heart dorsal meant the back of the heart.  Since the sheep is a 4-legged animal, the top of the skull is considered the back or dorsal side.  The underside of the brain is called the ventral side.  We use the terms “rostral” and “caudal” to refer to the forehead side and spinal cord side of the brain respectively. Rostral is Latin for beak and caudal is Latin for tail.  We also use the terms “medial” and “lateral” to refer to towards the midline and away from the midline respectively.  Use the figure on the next page to identify the largest portion of the brain the cerebrum.  This region has many folds or gyri (gyrus – singular) and grooves or sulci (sulcus – singular).  Identify the central groove the medial longitudinal fissure.  This separates the right and left hemisphere of the cerebral cortex (integration of information, thought center).  Moving caudal identify the cerebellum (fine motor coordination), medulla (vital functions), and spinal cord (pathway of sensory and motor neurons).  Gently flex the brain between the cerebellum and cerebrum and peak inside to see the midbrain (sleep/arousal regulation area) region.

Identify all regions on the ventral surface of a sheep brain.

     Now gently turn your sheep brain over to expose its’ ventral surface.  The full brainstem, all or most of the cranial nerves, and temporal lobe (pyriform lobe) should be clearly visible.  Use the figures on the next two pages to identify the three regions of the brainstem: the medulla, pons (REM sleep area, nerve pathway) and midbrain.  Identify also the large lateral projection of the temporal or pyriform lobes (auditory center).  Identify the medially placed hypothalamus (center for lusts) and pituitary gland if still attached. Your last and most difficult task is to identify the 12 cranial nerves.  Nerves are bundles of axons that are either coming or going to a group of specific cells or nuclei.  Start rostrally and identify the two large olfactory bulbs and nerves - cranial nerve I (detect smells), the optic nerve - II (visual information) and optic chiasm, the oculomotor nerve – III (eye movement, pupil constriction), trochlear – IV (controls extra-ocular muscles), trigeminal – V (face senses and movement, mastication), abducens – VI (eye movement), facial – VII (facial senses, salivation), vestibulocochlear – VIII (balance and hearing), glossopharyngeal – IX (taste), vagus – X (visceral system regulator), accessory – XI (neck and shoulder muscles), and hypoglossal nerve – XII (tongue movement).  These nerves can be very hard to find.  You might want to try to clean up your brains by gently removing the meningial coverings and any blood vessels that are in the way. Blood vessels should be dark while all of the myelin coated nerves will be white.  Take your time, use the diagrams on the next two pages, and move around gently and you should be able to find most if not all of the cranial nerves. 

Identify all regions of the sheep brain following a midsagittal cut.

      It is now time to separate your brain into two equal halves.  Take a spatula or dull scalpel blade and cut in one motion through the median longitudinal fissure, through the cerebellum and spinal cord.  This is called a midsagittal cut.  Observe one half of your brain and compare it to the figures on this page and the next page.  Identify the cerebrum, corpus callosum (pathway connecting right and left hemispheres), septum pellucidum (separates the lateral ventricles), optic nerve, thalamus (great relay station), pineal gland (circadian rhythms), superior colliculus (visual information relay pathway), inferior colliculus (auditory information relay pathway), hypothalamus, midbrain, pons, medulla, cerebellum, and the spinal cord. 

Matching Exercise

Place the number of the appropriate region next to its’ appropriate function

region or nerve                                                    function

1. cerebral cortex                                           ______ center for lusts

2. hypothalamus                                             ______ circadian rhythms

3. medulla                                                       ______ taste

4. pons                                                            ______ detect smells

5. thalamus                                                     ______ balance and hearing

6. corpus callosum                                          ______ tongue movement

7. CN I                                                           ______ auditory information relay pathway

8. CN II                                                          ______ integration of information, thought center

9. CN III                                                         ______ REM sleep area, nerve pathway

10. CN IV                                                       ______ facial senses, salivation

11. CN V                                                         ______ neck and shoulder muscles

12. CN VI                                                        ______ visual information

13. CN VII                                                      ______ fine motor coordination

14. CN VIII                                                    ______ great relay station

15. CN IX                                                       ______ pathway connecting right and left hemispheres

16. CN X                                                         ______ auditory center

17. CN XI                                                        ______ sleep/arousal regulation area

18. CN XII                                                      ______ eye movement

19. cerebellum                                                ______ visual information relay pathway

20. spinal cord                                                ______ separates the lateral ventricles

21. midbrain                                                    ______  pathway of sensory and motor neurons

22. temporal lobe                                            ______ vital functions

23. septum pellucidum                                    ______  face senses and movement, mastication

24. superior colliculus                                     ______ eye movement, pupil constriction

25. inferior colliculus                                       ______ controls extra-ocular muscles

26. pineal gland                                                ______ visceral system regulator

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