A&P 1 Brain & Cranial Nerves Guide - Lab Exercises

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1 A&P 1 Brain & Cranial Nerves Guide - Lab Exercises Please make sure you read the entire set of instructions on Dissection the Sheep Brain before beginning to cut. Also, please do not forget to go over the section titled Clean Up Procedures

2 STEP 1. Study the outer brain #1 Get everything you need Get a dissecting pan, 1 scalpel, 1 pair scissors, and some latex-free surgical gloves. Your group should have the following in front of you: 1. Human Brain Model 2. Sheep Brain - Whole, with Dura Mater (you will be dissecting this!) 3. Sheep Brain Whole - without dura mater (you will not be dissecting this) 4. Sheep Brain Frontal Section Human brain model Sheep brain with meninges Sheep brain without meninges Sheep brain - Frontal section We will be comparing these four at the same time.

3 #2 Study the ninges Obtain one of the preserved sheep brains that is still encased within the protective membranes known as the meninges. The meninges consist of three layers; an outer dura mater, middle arachnoid, and an internal pia mater. While looking at the "sheep brain with meninges": Examine the dura mater, a protective outer membrane composed of a tough dense connective tissue. Examine the surface of the "sheep brain whole - without dura mater". Note the thin filmy membrane covering over the actual brain tissue. This is more delicate tissue of the arachnoid and pia mater. The arachnoid will only look like a film with dark patches here and there...you may not really see it. The pia mater will look like a shiny coat directly attached to the neural tissue.

4 #3 Study the external features of the cerebrum, hypothalamus, midbrain and brain stem. A fissure is a deep groove that goes deep and separates big regions of the cerebrum. Examine the sheep brain from a superior view (see figure below). Note the two large cerebral hemispheres that constitute the cerebrum and envelop most of the brain. The cerebral hemispheres are almost completely separated by a deep longitudinal fissure. The cerebrum and cerebellum is separated by the transverse fissure. If you gently spread the hemispheres apart, you can see a band of tissue that connects the two hemispheres medially (orange arrow in photo below). This is the corpus callosum. We will be cutting through this when we dissect the brain. Next locate the cerebellum just caudal to the cerebrum. It is separated from the cerebral hemispheres by the deep transverse fissure. (see white arrow in above photo). more

5 Find these structures on the brain model and sheep brain. Do not worry exactly where things start and stop on the sheep brain (yet): A sulcus is a shallow groove, with gyri on either side. The surface of each hemisphere consists of upward folds of nervous tissue called gyri and shallow grooves called sulci. These increase the surface area of the cerebral cortex to allow more complex interaction between neurons. Look at the cerebellum. Like the cerebrum, the cerebellum has a highly convoluted surface (gyri and sulci) and lobes (see image below). Caudal and inferior to the cerebellum note the medulla oblongata which tapers out to form the more slender spinal cord. Find these structures on the model and sheep brain. Keep in mind that you have to locate any motor and sensory cortices on your wordlist: more

6 Look at the frontal section of the sheep brain, comparing it to the sheep brain without meninges. Note the grey mater surrounding the inner white matter. Note the longitudinal fissure, the sulci and gyri, the 2 hemispheres surrounding the lateral ventricles, and the corpus callosum, which is a bridge of white mater connecting the 2 hemispheres. The dorsal portion (or "roof") of the midbrain can be examined by carefully spreading apart the transverse fissure on the whole sheep brain without meninges (see figure). This exposes four rounded bulges called the corpora quadrigemina. The two superior colliculi (that word means hills ) are larger than the two inferior colliculi. If you look on the midline between and just above the superior colliculi you will see a small fingerlike extension of tissue called the pineal body or gland. Examine the brain model and sheep brain without meninges from an inferior view (see Figure below). At the cranial end of the brain note the two olfactory bulbs that lay against the undersurface of the cerebrum. The band of tissue extending caudally from these bulbs are the olfactory tracts. Caudal to the olfactory bulbs and tracts is the X-shaped optic chiasma. This is the point where the two optic nerves from the eyes meet and partially cross over before forming the optic tracts that extend back to the visual area of the cerebrum. This structure and the nervous tissue surrounding it comprise the brain region known as the hypothalamus. Note the other structures posterior to the hypothalamus on the sheep brain image below. Hypothalamus more

7 Now, examine the brain model and sheep brain with meninges Notice that it is much harder to see the optic chiasma and olfactory bulbs if the dura mater is still intact. Caudal and inferior to the optic chiasma is a small bean shaped pituitary gland. This is a major endocrine gland and is attached to the brain by a slender stalk called the infundibulum. Now look at the sheep brain without meninges. The pituitary gland may be missing from some of the preserved brains, leaving only the infundibulum. The infundibulum is rooted in a bulge of nervous tissue called the mammillary body. A portion of the pituitary gland is actually an extension of this hypothalamus. Note: The mamillary body is NOT very defined or clear on the sheep brain!! more

8 The ventral portion (or "floor") of the midbrain consists of the cerebral peduncles. These bands of nervous tissue connect the cerebrum with other regions of the brain and are located just posterior to the hypothalamus. Caudal to the midbrain is the hindbrain which consists of the pons and the medulla oblongata. The nervous tissue then tapers to become the spinal cord.

9 #4 Dissect the sheep brain Now it is time to dissect the Sheep Brain - Whole, with Dura Mater, as we want to see internal structures. You will be comparing this with the brain model, which you should now separate the 2 halves, and any pre-sectioned preserved sheep brains you want for variability (available in a bucket at the front of the room). Special Ethmoid bone instruction : Ask instructor if necessary Determine if your specimen still has a part of the ethmoid bone attached on the anterior frontal lobe. If the bone is present, slip a blunt probe between the bone and the dura to carefully separate the bone away from the specimen. Then, using your scissors, snip away any attached dura. Examine the removed ethmoid bone and identify the cribriform plates, where the olfactory bulbs were sitting, and the crista galli: crest of bone where that serves as the surface of attachment of the meninges. Also, the optic nerve may still be attached to 2 large fat bodies these are 2 pads that sit behind your eyeball. Using the scissors, cut the optic nerves to remove the pads, trying to get the brain to look like the one in the image below paragraph before cutting! Lay the brain, upside down, on the dissecting tray. Always use a scalpel like a butter knife; never saw through the tissue. Instead, pass the scalpel all the way through once, without trying to get all the way through. It will take you several passes to get through the brain. Each time you slide the scalpel through the brain, try to stay in the previous slice. Stay as close to center of the pituitary as possible. WATCH YOUR FINGERS! Use the following images, and those in your lab book, to guide you through the parts you need to know, finding the structures on both the brain model and dissected sheep brain, using the images in your text or lab book as you need. Make sure you find everything indicated on your wordlist indicated with an Asterisk (*) on the sheep brain. You are responsible for all terms on your wordlist, whether or not they are covered in the following description

10 1. Specific Parts of the Cerebrum Locate the corpus callosum, the band of tissue that connects the two cerebral hemispheres. Also locate the band of tissue below the corpus callosum known as the fornix. Between them is the opening into the lateral ventricle. Depending on the exact position of the sectioning cut you should either see a membrane linking the fornix and corpus callosum or a cavity between them. The membrane is the choroid plexus, where cerebrospinal fluid is produced and circulated. Make sure you see the corpus callosum and the lateral ventricles on the Sheep Brain Frontal Section. Third Ventricle 2. Specific Parts of the Diencephalon: Inferior to the fornix is a round structure called the intermediate mass. This is the medial portion of the two portions of the thalamus that lie within each cerebral hemisphere. If there appears to be a depressed area around the intermediate mass of the thalamus, this will be the third ventricle. 3. Specific Parts of the Brain Stem, including the midbrain: Examine again the midbrain structures; the dorsal corpora quadrigemina and pineal body, and the ventral cerebral peduncles. Also reexamine the parts of the hindbrain; the pons, the medulla, and the cerebellum. 4. Specific Parts of the Cerebellum The white matter structure in the center is the arbor vitae. Note the cerebellar cortex ( folia ). Note that there is a slender canal that passes through the midbrain to the cavity between the cerebellum and the medulla. The cavity is the fourth ventricle of the brain and the canal is the cerebral aqueduct. The cerebral aqueduct carries cerebrospinal fluid from the third ventricle to the fourth ventricle. Blockage of this passage causes the condition known as hydrocephalus.

11 On the dissected Sheep Brain - Whole, with Dura Mater, note the following structures, and how different they look on the dissected brain, versus the whole brain: Dura mater Pituitary gland (hypophysis) Optic chiasma Mamillary body (human brain model only) Central sulcus Midbrain and corpora quadrigemina Pons Pineal gland There are others, but these really stand out! Also notice how different these structures look on the sheep brain versus the human model, do to the fact that a sheep has a neck that goes straight out, whereas our has a 90 degree bend: Hypothalamus Midbrain

12 Clean Up Procedures 1. Replace the following back into the buckets from which they came: Sheep Brain Whole - without dura mater (you will not be dissecting this) Sheep Brain Frontal Section 2. Place the brain you dissected, and any parts, in the designated container. When you are ready, place your gloves in a regular trash can and NOT in the container with the dissected brains. 3. Take the dissection tray and all instruments to the sink to be washed. 4. Remove the pad from the dissection tray and wash them separately using the provided disinfectant and sponge. 5. Also spray the instruments with disinfectant and clean them using the provided toothbrushes. 6. Rinse the pad, dissecting tray, and all tools and place them in the drying tray. 7. Place the instruments SHARP END DOWN in the appropriate portion of the drying tray. 8. Spray down the area of the lab bench where the dissection was completed with disinfectant and dry using a paper towel.