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Lab 1 Spinal Cord, Brain, Meninges, Cranial Nerves, and Blood Vessels Familiarity with the gross structure of the human nervous system will provide you with a frame- work to organize what you will learn about its function. In addition, because nerve cells and their processes frequently connect structures far removed from one another, even this early in the course it will help if you have at least a vague idea of where these “distant places” are located. The first two laboratory sessions will also introduce the proper nomenclature or terminology used for the various parts of the human brain. The sooner you learn this terminology and what it refers to, the easier it will be to understand the lectures and readings. Posterior Surface (Dorsal) Anterior of Cord Surface (Ventral) Anterior Posterior "Dorsal" "Ventral" Medial Surface Ventral View Dorsal View Figure 1.1: Brain orientation nomenclature. Orientation Nomenclature: As seen in the MRI in the figure above, for a person standing up, the axis of the cerebral hemispheres is roughly horizontal (parallel to ground), that through the brainstem oblique, and that of the spinal cord approximately vertical. Thus, for the spinal cord the term anterior refers to the part closest to the front of the neck, chest or abdomen, while for the cerebral hemispheres it means the part closest to the forehead. Obviously, for the spinal cord posterior means the part closest tos the back of the neck, chest, or abdomen Likewise, the base of the brain as it sits in the skull is sometimes referred to as “ventral,” while the superior portion of the brain just beneath the top of the head is “dorsal.” (If, however, you are trying to refer to progression along the neuraxis from the “higher level” of the cerebral hemispheres to the “lower level” of the spinal cord, calling the cerebral hemispheres “anterior” to the spinal cord can be confusing, since they are both anterior. The term “rostral” is commonly used to indicate this evolutionary or developmental relationship; thus the cerebral hemispheres are considered “rostral” to the spinal cord.) 2 LAB 1. SPINAL CORD, BRAIN, MENINGES, CRANIAL NERVES, AND BLOOD VESSELS 1.1 Spinal Cord 1.1.1 External Anatomy of Spinal Cord (Haines 2–1 to 2–4) Vertebral Column: The vertebral column consists of seven cervical, twelve thoracic, five lumbar, five fused sacral, as well as four (usually) coccygeal vertebrae. The relationships of these vertebrae with the spinal cord and roots were studied in the Structure of the Human Body course and can be appreciated in the sagittal MRI in Figure 1.2 below. Figure 1.2: Sagittal MRI showing relation of vertebral column and spinal cord. Can you find the herniated disc? Spinal Cord: The spinal cord measures about 42-45 centimeters in length. However, the specimens available for study are somewhat shorter, since all of them are lacking the first few upper cervical segments. The spinal cord itself lies within the vertebral canal and extends from the foramen magnum to the lower border of the first lumbar vertebra. The cord is cylindrical in shape and somewhat flattened anteroposteriorly. Two spindle-shaped swellings, the cervical and lumbar enlargements, comprise those portions of the cord which innervate the upper and lower extremities. Below the lumbar enlargement the cord rapidly narrows to a cone-shaped termination, the conus medullaris. From the conus a slender non-nervous filament, the filum terminale, extends downward to the fundus of the dural sac at the level of the second sacral vertebra. It penetrates the dura and, invested by the dura, forms the coccygeal ligament. The bundle of descending nerve dorsal and ventral roots below the conus medullaris is known as the cauda equina (“horse tail”) and is illustrated in Figure 1.3. They 3 are located in the lumbar cistern from which samples of cerebrospinal fluid are commonly taken. See Haines 2–4. Figure 1.3: Cauda equina and conus medullaris of spinal cord. Meninges: Examine the outer aspects of the dura mater, which is the outermost of the meninges. Notice the spinal ganglia and nerve roots coming out of the dural sheath along the lateral margins. Most cords will have spinal ganglia, particularly at the lower end of the specimen. With the spinal cord and its dural covering lying flat, use a pair of forceps and scissors to open the dura from the transected upper cervical end, along the midline to the lower end. Turn to the opposite surface and repeat the procedure. Do not cut the dura along the lateral margins where the nerve roots are located. When the dura is opened, find the denticulate ligaments, which are extensions of the pia, the innermost of the meninges that is applied directly to the lateral aspect of the cord, to the arachnoid, the intermediate layer of the meninges that lies just beneath the dura. The denticulate ligaments “tether” the cord in place inside the dural sac. See Haines 2–1. Blood Supply to Cord: The blood supply to the spinal cord is provided by (1) the anterior and posterior spinal arteries, which are branches of the vertebral arteries, and (2) by multiple radicular arteries, which are de- rived from segmental vessels. Roughly speaking, the anterior spinal artery supplies the anterior 2/3 of the cord, while the posterior spinal artery supplies the posterior 1/3, including the dorsal or posterior columns. See Haines 2–3. Observe the more continuous course of the anterior spinal artery on the anterior aspect of the cord compared to the plexiform arrangement of vessels on the posterior aspect of the cord (see Figure 1.4). The spinal and radicular arteries form a more or less continuous anastomosis for the entire length of the spinal cord. Holding the dural coverings open, note that the spinal cord has a several longitudinal furrows or grooves (often hard to see unless the pia is stripped off). On the anterior surface is a fairly deep anterior median fissure just beneath the anterior spinal artery. On the posterior surface is the 4 LAB 1. SPINAL CORD, BRAIN, MENINGES, CRANIAL NERVES, AND BLOOD VESSELS Figure 1.4: Top: anterior (ventral) view of spinal cord; bottom: posterior (dorsal) view of spinal cord. ID the blood vessels and roots on each picture. 5 shallow posterior median sulcus and, more laterally, the posterolateral sulcus, which is a fairly distinct furrow marking the entrance of the filaments of the dorsal roots. Above the level of T6 there is a posterior intermediate sulcus in between the two sulci just identified. This marks the border between the two bundles of fibers on each side that form the “dorsal columns”: the medial fasciculus gracilis and the lateral fasciculus cuneatus. Anteriorly, the anterolateral sulcus marks the exit of the ventral root fibers and is hard to see. See Haines 2–2. If you have trouble finding these structures on the spinal cord, use the rubber brain stem model. Study Questions Identify the following structures and answer the questions: • cervical and lumbar enlargements: Why do these develop? • conus medullaris: Which interspinous space is used for lumbar puncture in order to prevent damage to the conus? • cauda equina: Explain the formation of this structure. • filum terminale: Does this structure contain nerve fibers? • ventral nerve roots (motor): Where do these fibers emerge from the cord? How many seg- ments and how many nerve roots are there in the spinal cord? • dorsal nerve roots (sensory): Which sulcus marks the entrance of these fibers into the cord? Where are the cell bodies of the dorsal root nerve fibers? Where are the dorsal root ganglia located with respect to the vertebrae? • anterior median fissure and the posterior median sulcus: Which of these contains a blood vessel? What is the vessel’s name? • posterior intermediate sulcus: Does this extend throughout the length of the cord? • fasciculus gracilis and fasciculus cuneatus: Where are the cell bodies of these fibers? 1.1.2 Internal Anatomy of Spinal Cord (Haines 5–1 to 5–5) If not done already, With a scalpel, and being careful NOT to cut the dura, transect the spinal cord through the centers of the cervical and lumbar enlargements and at a mid-thoracic level. Examine the various levels of the spinal cord and note the presence of a central gray zone having an “H”-shaped configuration. In addition, Figure 1.5 displays myelin-stained cross sections of the major levels of the spinal cord and should also be referred to as you proceed with this laboratory exercise. Gray Matter: The gray matter will vary in its mass depending on the level studied. The gray matter consists of nerve cells, glial cells, and myelinated and unmyelinated fibers. The central canal in the center of the spinal cord is almost impossible to see with the naked eye but is visible under a microscope. Surrounding the spinal gray matter is white matter, consisting of ascending and descending myelinated and unmyelinated fibers. Those fibers traveling together and serving a similar function are referred to as tracts or fasciculi. The spinal gray matter is divided into a posterior or dorsal horn, an intermediate gray, and an anterior or ventral horn. 6 LAB 1. SPINAL CORD, BRAIN, MENINGES, CRANIAL NERVES, AND BLOOD VESSELS C2 C8 T10 L3 Sac Figure 1.5: Dorsal (posterior) view of spinal cord and cord cross sections from Atlas Anatomicum Cerebri Humani by G. Jelgersma, Scheltema & Holkema Boekhandel en Uitgeversmaatschappij N.V., Amsterdam.) 7 White Matter: The white matter is conventionally subdivided into posterior (dorsal), lateral, and anterior (ventral) columns or funiculi. The ratio of gray to white matter varies depending on the level studied. The gray matter is larger at levels providing innervation to the extremities (cervical or lumbosacral enlargements). The cervi- cal segments contain a greater total amount of white matter than lower levels because the ascending and descending pathways have more fibers in them at these levels than at lower levels. Compare a cervical and lumbar segment. With reference to your textbook or atlas study the following details. Cervical Cord (Haines 5–4): This is somewhat oval in outline, with an increase in its transverse diameter at lower cervical levels that are part of the cervical enlargement. Note that the anterior and posterior horns of the gray matter are large. There is no lateral horn as found at thoracic levels. The white matter forms a greater proportion of the transverse cross-sectional area here than at lower levels. The posterior or dorsal columns of the white matter are larger than elsewhere and are clearly subdivided into the medial fasciculus gracilis and the lateral fasciculus cuneatus, with the posterior intermediate sulcus separating them. Thoracic Cord: (Haines 5–3) This is smaller and more nearly circular than the cervical cord. The gray matter is reduced to slender posterior horns and a small rounded anterior horn. The lateral horn containing the sympathetic preganglionic neurons is now visible. This is characteristic of the thoracic region. Lumbar Cord: (Haines 5–2) This is more or less circular in outline and is larger in diameter than the thoracic cord but smaller than the cervical cord. There is much less white matter surrounding the gray, and the posterior columns show no subdivision into the medial fasciculus gracilis and lateral fasciculus cuneatus since only the fasciculus gracilis is present at lumbar levels. The gray matter is greatly swollen in both the dorsal and ventral horns. Sacral and Coccygeal Cord: (Haines 5–1) At these levels the cord contains only a thin rim of white matter surrounding a shrunken core of gray matter that exhibits little subdivision into dorsal and ventral horns. Review Exercise Label the structures listed below on Figure 1.5. Review again on the spinal cord. • cervical enlargement • lumbar enlargement • conus medullaris • filum terminale • cauda equina • spinal ganglia • nerve roots • anterior median fissure • posterior median fissure • posterior lateral sulcus • posterior intermediate sulcus • posterior horn • anterior horn • posterior funiculus • anterior funiculus • lateral funiculus Study Questions 1. What is the lumbar cistern and why is it important diagnostically? 2. What tracts comprise the posterior columns? 3. What is the motor horn? 4. Where do you find a lateral horn and what is its significance? 5. What are the major criteria for determining cord levels in cross-sections? 8 LAB 1. SPINAL CORD, BRAIN, MENINGES, CRANIAL NERVES, AND BLOOD VESSELS Cases (Ponder and then discuss with lab faculty) Aorta Surgery A 65 year old man with heart disease undergoes surgery on his thoracic aorta. Afterward he notes paralysis of both lower limbs. On examination there is loss of pinprick and temperature sensation from his umbilicus on down but preserved vibration and proprioception. Both lower limbs are flaccid (hypotonic), paralyzed, and areflexic. No Babinski signs are present. 1. What part of the spinal cord is involved and at what level? anterior 2/3 of cord at about T-10 level 2. Why are some sensory modalities preserved? the posterior columns are spared (different vascular supply from posterior spinal arteries) 3. A lesion in which tract has caused the paralysis? this is a corti- cospinal tract lesion 4. Why are there no upper motor neuron signs in the lower limbs? “spinal shock” has occurred, and no upper motor neuron signs are yet evident 5. What blood vessel could be involved and why? anterior spinal artery occlusion due to surgery or diseased aorta Breast Carcinoma A 50 year old woman has been diagnosed with breast carcinoma, which has spread to her axillary lymph nodes. For 2 weeks now, she has noticed sharp, shooting pain from the interscapular (upper back) area, radiating around her thorax into the the right nipple anteriorly. This increases with coughing or straining. Yesterday she awakened with numbness over the left leg and dragging of her right leg. Examination shows reduced tem- perature and pinprick sensation over the lower left thorax and entire left lower limb, reduced proprioception at the right toes and ankle, and re- duced vibration throughout the right lower limb. The right lower limb is mildly weak with increased right knee and ankle reflexes, and a right Babinski sign is present. 1. What localizing significance is there to her chest pain? right T4 radicular pain 2. Why does it increase with coughing or straining? Valsalva maneu- ver increases the intraspinal pressure leading to greater compres- sion/stretch/irritation 3. What structure(s) are involved to produce this pain? suspected extramedullary dorsal root compression 4. Explain her sensory and motor signs and symptoms. involvement of spinothalamic tract and posterior columns at right mid-thoracic cord level (suspected T4 level on account of pain localization); involvement of corticospinal tract at the right mid-thoracic level 5. What type of lesion do you most likely suspect? a hemicord lesion (Brown-Sequard) due to an extramedullary metastasis, probably originating from the adjacent vertebral body (approximately T4) 9 1.2 Brain, Meninges, and Sinuses (Haines 2–9, 2–15, 2–26) Use the whole brain or a brain model for the study of the dorsal (Figure 1.6), lateral (Figure 1.7), and ventral (Figure 1.9) surfaces, and the half brain for the study of the medial surface (Figure 1.8). Dissect away the meninges as necessary to visualize underlying structures, taking care not to destroy blood vessels that will be studied later. 1.2.1 Cerebrum (Haines 2–9, 2–15) The cerebrum is composed of two hemispheres, which display prominent round convolutions or gyri separated by sulci or fissures. The brain stem is a midline structure attached rostrally to the deep structures of the hemispheres, and its caudal end is continuous with the spinal cord. The cerebellum is attached directly to the brain stem. The cerebral hemispheres consist of a superficial covering of cortex only a few millimeters thick (called gray matter because that is its color in poorly fixed specimens), while the underlying nerve fibers wrapped with myelin appear white, hence white matter. Embedded deeply in the white matter of each hemisphere (and hence not visible in the gross brain) are large aggregates of gray matter known collectively as the basal ganglia. Likewise, within each hemisphere a large cavity is also found, the lateral ventricle. Using the whole brain or a brain model, note that the two hemispheres are separated from one another by the longitudinal fissure and from the brain stem and cerebellum by a transverse fissure. Much of the transverse fissure is hidden, including the portion that lies superior (dorsal) to the colliculi of the midbrain and the portion that lies between the diencephalon inferiorly and the fornix and corpus callosum superiorly. See Haines 2–27 and 2–28. Figure 1.6: Dorsal view of brain. Arrow indicates arachnoid granulations. Figure 1.7: Lateral view of brain. Arrow indicates lateral (Sylvian) fissure. On the lateral aspects of each hemisphere observe that the lateral (Sylvian) fissure separates the temporal lobe below it from the frontal and parietal lobes above. The central (Rolandic) sulcus forms the posterior boundary of the frontal lobe, marking its border with the parietal lobe. See Haines 2–9. Using the rubber brain stem model, note that the brain stem is attached to the cerebral hemi- 10 LAB 1. SPINAL CORD, BRAIN, MENINGES, CRANIAL NERVES, AND BLOOD VESSELS spheres by the crus cerebri, large bundles of fibers running on the ventral surface of the midbrain. From rostral to caudal, the brain stem is divided into four important regions: the diencephalon, the midbrain or mesencephalon, the pons, and the medulla. Overriding the brain stem at the level of the pons is the cerebellum, which is attached to the brain stem by three pairs of large fiber bundles, the cerebellar peduncles. On the ventral surface of the whole brain or a brain model, locate the optic nerves, optic chiasm, and the mammillary bodies. Lateral to the optic chiasm there is a bulge in the medial border of the temporal lobe called the uncus. See Haines 2–18. (Important clinical question: What cranial nerve courses just medial to the uncus and could be compressed if the uncus herniated?) Using both the half brain or a midsaggital MRI (see MRIs on lightboxes or Figure 1.8 below), locate on the medial surface the corpus callosum, septum pellucidum, fornix, anterior commissure, third ventricle, interventricular foramen (of Monro), mammillary body, pineal body, thalamus, hy- pothalamus, pituitary, optic chiasm, midbrain, pineal gland, cerebral aqueduct, superior colliculus, inferior colliculus, tentorium cerebelli, pons, fourth ventricle, medulla, cerebellum, and spinal cord. Where would midline dural structures such as the falx cerebri, superior sagittal sinus be found? You must learn to identify these structures on the midsagittal view of the brain MRI—they will be tested! Figure 1.8: Midsagittal views of brain. 11 1.2.2 Brain Stem and Cerebellum (Haines 2–18, 2–30, 3–10) Use the brain specimens, figures, and the rubber brain stem model to help you find and identify the structures in this part of the lab exercise. Figure 1.9: Top: ventral view of brain stem; bottom: dorsal view of brain stem with cerebellum removed to display floor of fourth ventricle, cerebellar peduncles, etc. The undersurface of the midbrain presents paired fiber bundles called crus cerebri (also called cerebral peduncles). The space between these obliquely placed peduncles is called the interpedun- cular fossa, where the oculomotor nerves (III) emerge. See Haines 2–18 and 2–21. The posterior surface of the midbrain consists of paired superior and inferior colliculi (all four together are re- ferred to as the tectum or roof of the midbrain). See Haines 2–28 and 2–32. 12 LAB