Sheep Brain Dissection: Lab 1

The Meninges. In situ (in place) the living brain and spinal cord are encapsulated by three membrane sheaths called the meninges. The outermost membrane is the dura-mater, tough, fibrous, and translucent. There may be some dura attached to your specimen, but most of it was likely removed at the time that the brain was extracted from the skull. As shown in the schematic drawing, the dura coats the outer surface of the brain.The middle layer of the meninges is the arachnoid; it is more delicate than the dura and has a cobweb-like appearance, when the brain is intact. The arachnoid is not visible to you at this time, because the process of preservation produces shrinkage of the tissue. The subarachnoid space is filled with cerebrospinal fluid. The innermost meningeal layer, the pia-mater, is an extremely delicate membrane that covers the surface of the CNS. It extends into every sulcus and fissure. In contrast, the dura mater wraps the the surface of the CNS without projecting into the crevices formed by the mounds of the gyri. Cerebrospinal fluid (CSF), found in the subarachnoid space, that space between the pia and the arachnoid, is manufactured in specialized tissue called choroid plexus, which is located at several sites in the CNS, some of which you will observe later. Functional importance of the meninges. CSF and the meninges are designed to protect the brain and spinal cord from mechanical shock. Without them, or a functionally similar system, it is conceivable that slight, otherwise insignificant bumps on the head, or even a mild shaking of the head back and forth, would induce a state of unconsciousness as the brain was thrust against one side of the skull cavity and then the other. CSF can be of diagnostic use to neurologists and neurosurgeons. When a spinal tap is conducted, that is, when CSF is withdrawn from the spinal cord, several characteristics of the CSF are assayed. Under normal conditions it should have no color, and it should have no cloudiness or turbidity. Deviations from these normal conditions are clinically significant (e.g., a slightly pink and cloudy sample might indicate minute intracerebral hemorraghing). Next step: remove the dura and pituitary gland. The meninges, especially the dura mater, greatly impede visual appreciation of the external features of the brain; so, you must carefully remove them (but not yet!). The pia mater is very delicate and adheres to the brain's surface tenaciously; it will take considerable patience and care to remove it. Wait for instructions from the instructor or lab assistant on how to remove the dura mater from your specimen, because several cranial nerves pass through the dura and you want to avoid accidentally removing them when you remove this protective sheath. Once you are certain about how to proceed, begin to remove the dura from the dorsal surface of the brain. The instructor or lab assistant will help you locate the pituitary gland on the ventral surface of the brain. It is the prominent and protruding mass of tissue lying on the ventral midline, just anterior to the brainstem. It will be necessary to remove the pituitary in order to have an unobstructed view of some of the features present on the ventral surface of the brain. It is likely that a variety of tissues will be found proximal to the pituitary. The tough, transparent material overlaying the pituitary is dura mater, while the dark brown, porous, and hairlike matter located along the gland's lateral surfaces is a capillary bed, which is an appropriate accompaniment considering the operation of the pituitary. Why do you think this is so? Carefully lift the pituitary mass from its caudal end. You will see two large, flat fiber bundles that are attached to the ventral surface of the brain and to the dura. These fiber bundles are the oculomotor nerve, cranial nerve III. Keeping the pituitary mass lifted away from the ventral surface of the brain, use a pair of scissors to sever the two branches of the III nerve close to its attachment to the dura, that is, leave as much of the nerve attached to the brain stem as you are able. As you lift the pituitary, notice a small, thin tubular structure, the infundibulum or infundibular stalk, which joins the pituitary to the base of the brain. It is located on the midline, anterior to the oculomotor nerves. The stalk will rupture as you remove the pituitary and will leave a small hole, which represents the point at which the infundibulum attached the pituitary to the hypothalamus. Carefully cut any other connective tissue present, lift the pituitary away, and set it aside. (Do not throw it away; you will be using it in a later session.) If you perform these actions carefully, you should be able to preserve many of the cranial nerves that emerge from the ventral surface of the brain. You will also have to remove the dura from the optic nerve (II) and the optic chiasm. The optic nerve and the chiasm (which means crossing) can be identified as the large structures that form an "X" on the ventral surface of the brain. Proceed cautiously, or you may break off the nerves or damage the chiasm. Nerves and Features of the Ventral Surface of the Brain If your dura and pituitary gland have been removed, you are ready to begin to locate and identify the cranial nerves that remain on your specimen. One basis for classifying nerves is by location. Spinal nerves appear entering or exiting the spinal cord and cranial nerves are found entering or exiting from the brain. A second basis for classification is by function, that is, whether the nerve is sensory (afferent) or motor (efferent). Students often have difficulty remembering whether it is sensory or motor information that is afferent or efferent and whether it is the dorsal or ventral root that carries afferent or efferent information. The word, SAME, and the name, DAVE, are good mnemonics for keeping this information straight. For example, the letters of the word SAME can be used to recall that sensory information is afferent and motor information is efferent. Similarly, the name, DAVE, can help keep the organization of the spinal roots clear. Sensory Dorsal Afferent (information going to the brain) Motor Efferent Afferent Ventral (information exiting brain and spinal cord going to muscles and glands) Efferent This is a good place to mention the Law of Roots (also known as the Law of Bell and Magendie). The Law of Roots states that sensory information enters through the dorsal root of the cord and motor information exits through the ventral root. There are 31 spinal nerves. Each of the 31 dorsal roots carry incoming (afferent) sensory information, while the axons in all 31 ventral roots carry out-going(efferent) motor information. The dorsal and ventral roots join to form mixed spinal nerves that contain both sensory and motor fibers. Ask the instructor or lab assistant to show you the model of the human spinal cord, or, better yet, request to see the preserved human spinal cord tissue to obtain a clear appreciation of this spinal organization. Identification of the Cranial Nerves There are 12 pairs of cranial nerves customarily numbered with the Roman numerals, I to XII. They appear in an approximate rostral to caudal sequence along the ventral surface of the brain. Refer to Table 1, for an outline of the functions of all 12 cranial nerves and some mnemonics to help you remember their names and numbers. You will find this Table useful in your studying. On the practicum, you will be expected to be able to indicate whether a given nerve is sensory, motor, or both. In addition, you must know what functions each of the 12 nerves serve. You will have to identify, by location, only cranial nerves I through VI. On the ventral aspect of your specimen, at the very anterior limit, locate the two padlike flaps of tissue that are the olfactory bulbs, the second-order neurons of cranial nerve I. Caudal to the olfactory bulbs, along the midline, note the "X" formed by two fairly substantial fiber groups. The fibers anterior to the intersection of the "X," are optic nerve fibers. The intersection of the 'X,' is the optic chiasm, while fibers caudal to the optic chiasm are optic tract fibers. Follow the link for information about the optic chiasm and the optic nerve. Take time now to consider the facts about the optic chiasm that were presented in the table. Do you understand • where the optic fibers originate • how some fibers crossover as they begin their journey to occipital (visual) cortex? Stop, now, and draw a figure in the margin of your dissection guide that demonstrates how these fibers project to occipital cortex. Do not proceed unless you know : • the difference between the optic nerve and optic tract • whether the optic nerve or optic tract contains ipsilateral, or contralateral fibers, or both). Just caudal to the optic tracts are two very large bundles of fibers running parallel to the lateral surfaces of the brain. These large fiber bundles are called the cerebral peduncles. The cerebral peduncles are large bundles of axons coming from cell bodies located in motor cortex. These cell bodies are called pyramidal cells. As the pyramidal cell axons leave motor cortex, they are arranged in a large crescent-shaped structure called the internal capsule. As the axons make their descent to their final destination, the spinal cord, they merge into the two, tight bundles of fibers seen on the ventral surface of the brain, where they are called the cerebral peduncles. The axons continue to course downward and disappear beneath the pons. The fiber bundle reappears as striations on the external surface of the medulla, where they are called the pyramidal tract. Once these fibers reach the spinal cord, they are called the cortico-spinal tract. (Remember the rule regarding the naming of nerves and tracts? The first part of the name indicates where the fibers begin (or are coming from) and the second half of the name specifies the destination.) The axons in this long pathway from motor cortex to internal capsule, internal capsule to cerebral peduncles, cerebral peduncles to pyramidal tract and spinal cord comprise one of the major motor pathways in the CNS. This motor system, called the pyramidal motor system, is largely responsible for intentional movement. Christopher Reeves, the actor who suffered a severe spinal cord injury, is paralyzed, because the axons in his cortico- spinal tract were severed in his accident. As a result, the signals from his motor cortex to his spinal cord have been disrupted; his pyramidal motor system axons no longer communicate with the muscles of his body to produce movement. There are two other important components that are involved in the production of the many complex movements we can exhibit: the extra-pyramidal system and the cerebellum. We'll learn more about the contributions of these two aspects of our motor systems later. On the surface of each peduncle, you should find cranial nerve III, the oculomotor nerve, which supplies four eye muscles. Two muscles are marked by the letters: a and d. As you can surmise, these muscles move the eye up and down. A third muscle, marked 'c' in the figure, is attached to the medial surface of the eyeball and produces inward movements of the eye. You activate this muscle when you cross your eyes. The fourth muscle supplied by the oculomotor nerve moves the eye down and laterally. This fourth muscle is not shown. Looking postero-laterally, between the cerebral hemispheres and the cerebellum, find a very thin, thread-like nerve, the trochlear nerve, cranial nerve IV. It exits the dorsal surface of the brainstem, but courses immediately ventralward; it is often hidden by the bulk of the the trigeminal nerve, cranial nerve V. The muscle marked 'e' in the figure is supplied by the trochlear nerve which moves your eye up and to the side, that is, up and diagonally. At the anterior limit of the medulla (that section of the brain located caudal to the pons and anterior to the spinal cord) you will find several cranial nerve processes aligned in a near transverse manner. The most medial of these is cranial nerve VI, the abducens, which serves the external rectus muscle of the eye that allows you to move your eyes laterally, that is, side to side. The external rectus of the eye is marked by the letter 'b' in the Figure. Note that the nerve has been cut in the figure to reveal the muscle marked 'c'. Now locate cranial nerve V, the trigeminal; it will be seen as a relatively thick fiber bundle near the lateral-most junction of the pons and medulla. It may be that the trigeminal nerve has been torn away from your specimen; if this is so, locate it using this ventral view of the brain. Cranial nerves VII through XII were quite likely stripped from your sheep brain. Therefore, you will not be responsible for locating these nerves during the practicum, but you will need to know whether these nerves are sensory or motor, or mixed, and what functions they subserve.