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11: The Central Nervous System (Brain)

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    12531
  • The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.

    The Cerebrum

    The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum (Figure 11.1). The wrinkled portion is the cerebral cortex, and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure. It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.

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    Figure 11.1The Cerebrum The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex.

    Cerebral Cortex

    The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus (plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex.

    The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.

    The folding of the cortex maximizes the amount of gray matter in the cranial cavity. During embryonic development, as the telencephalon expands within the skull, the brain goes through a regular course of growth that results in everyone’s brain having a similar pattern of folds. The surface of the brain can be mapped on the basis of the locations of large gyri and sulci. Using these landmarks, the cortex can be separated into four major regions, or lobes (Figure 11.2). The lateral sulcus that separates the temporal lobe from the other regions is one such landmark. Superior to the lateral sulcus are the parietal lobe and frontal lobe, which are separated from each other by the central sulcus. The posterior region of the cortex is the occipital lobe, which has no obvious anatomical border between it and the parietal or temporal lobes on the lateral surface of the brain. From the medial surface, an obvious landmark separating the parietal and occipital lobes is called the parieto- occipital sulcus.

    The fact that there is no obvious anatomical border between these lobes is consistent with the functions of these regions being interrelated.

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    Figure 11.2 Lobes of the Cerebral Cortex The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions.

    The Diencephalon

    The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean).

    The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus (Figure 11.3). There are other structures, such as the epithalamus, which contains the pineal gland, or the subthalamus, which includes the subthalamic nucleus that is part of the basal nuclei.

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    Figure 11.3 The Diencephalon

    Brain Stem

    The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 11.4). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates.

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    Figure 11.4 Brain Stem

    The Cerebellum

    The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain (Figure 11.5). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.

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    Figure 11.5 The Cerebellum The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.

    LAB 11 EXERCISES 11-1

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    Cerebrum, Cerebellum, Brainstem.

    Label the (visible) lobes of the cerebrum Label the following: a Gyrus, a Sulcus,

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    rostral

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    caudal

    LAB 11 EXERCISES 11-2

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    LOBES & LIMBIC SYSTEM

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    Label the following: Pre-central gyrus, Post- central gyrus, Central sulcus, Pre-frontal cortex, Temporal lobe, Occipital lobe.

    Label the following: Amygdala, Hippocampus, Pituitary gland, Medulla Oblongata, Pons, Thalamus, Superior & Inferior colliculi.

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    LAB 11 EXERCISES 11-3

    DIENCEPHALON AND BRAINSTEM

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    Identify the following: Arbor vitae, Corpus callosum, Septum pellucidum, Fornix, Optic chiasm, Pons

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    LAB 11 EXERCISES 11-4

    VENTRICULAR SYSTEMImage_246.png

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    Identify the following: Midbrain, Lateral ventricle, 3rd ventricle, Cerebral Aqueduct, 4th ventricle, Central canal

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    LAB 11 EXERCISES 11-5

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    CORTEX & CSF CIRCULATION

    1. Label the ventricles, aqueduct and canal.

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    2. Draw two choroid plexuses (in red, if you have a red pen handy).

    Label CSF-related areas: Lateral ventricle, 3rd ventricle, 4th ventricle, Superior sagittal sinus, Straight sinus, Subarachnoid space, Cerebral aqueduct, Arachnoid granulation, Choroid plexus, Interthalamic adhesion

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    LAB 11 EXERCISES 11-6

    CEREBRAL CORTEX

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    Label the following regions of the cortex: Primary motor cortex, Primary sensory cortex, Primary auditory cortex, Primary visual cortex, Motor association area, Pre-frontal cortex, Broca’s area, Wernicke’s area

    Match the following symptoms to the brain region they are associated with:

    (A) Ataxia, (B) Inability to form words,

    Nonsensical speech,(D) Hearing loss, (E) Vision loss,(F) Poor decision making, (G) Drooping facial muscles, (H) Paralysis of fingers, (I) Phantom limb

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    LAB 11 EXERCISES 11-7

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    CNS

    Label the following: Falx cerebri, Tentorum cerebellum, Superior sagittal sinus, Straight sinus

    Label the following: Pituitary gland, Mamillary bodies, Pons, Medulla oblongata, Temporal lobe, Frontal lobe, Cerebellum.

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    LAB 11 EXERCISES 11-8

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    Identify the following: Corpus callosum, Anterior commissure, Posterior commissure, Fornix, Interthalamic adhesion, Choroid plexus, Corpora quadrigemina

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    LAB 11 EXERCISES 11-9

    CORONAL VIEW & MISC.Image_255.png

    Identify the following regions: Septum pellucidum, Grey matter, Insula, Basal ganglia, Corpus callosum, Lateral ventricle, Longitudinal fissure

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    MODELS

    3D Brains, Ventricles, Sagittal & Coronal Brain Slices, Torsos, Sagittal Head

     Cerebrum

    • Gyrus (plural = gyri)

       Pre-central gyrus

       Post-central gyrus

    • Sulcus (plural = sulci or sulcusses)

       Central sulcus

       Lateral sulcus

    • Fissures

       Longitudinal fissure

    • Lobes of cerebrum

       Frontal lobe

       Parietal lobe

       Temporal lobe

       Occipital lobe

       Insula

    • Cerebral cortex (gray matter)

    • Basal nuclei

    • White matter (tracts)

    • Corpus callosum

    • Septum pellucidum

    • Fornix

    • Choroid Plexuses

    • Olfactory bulb

    • Olfactory tract

    • Optic chiasm

    • Optic tract

       Brain stem

      • Midbrain

      • Pons

      • Medulla oblongata

         Cerebellum

      • Vermis

      • Cerebellar cortex

      • Arbor vitae

         Ventricular system

      • Lateral ventricles

      • Interventricular foramen

      • 3rd ventricle

      • Cerebral aqueduct

      • 4th ventricle

      • Central canal

         Meningeal layers

    • Dura mater

       Falx cerebri

       Falx cerebelli

       Tentorium cerebella

       Dural venous sinuses

       Straight sinus

       Superior sagittal sinus

    • Arachnoid mater

       Arachnoid granulations

    • Pia mater

       Diencephalon

    • Thalamus

       Interthalamic adhesion

    • Anterior commissure

    • Posterior commissure

    • Hypothalamus

    • Pituitary gland

       Infundibulum

    • Mamillary bodies

    • Pineal gland

    • Copora Quadrigemina (superior & inferior colliculi)

    Sheep Brain Dissection

    Background Information:

    The sheep brain is remarkably similar to the human brain. One major difference, however, is in proportion. For example, the sheep brain has a proportionately smaller cerebrum. Another difference is in orientation of the spinal cord. The sheep spinal cord is orientated anterior to posterior, as in any four-legged animal. The human spinal cord is orientated superior to inferior. This orientation difference has a major affect on the location of the brain stem. The sheep brain stem is located more towards the rear (posteriorly). The sheep skull, in order to compensate for this, has the foramen magnum located more towards the rear of the skull. In humans, since we walk upright (bipedalism), the spinal cord is in a more vertical plane, thus the foramen magnum is located centrally on the bottom of the skull (inferiorly). By observing the movement of this major skull feature from the rear (as in very early human ancestors) to the current location (modern humans), scientists have been able to determine when the human species began to walk upright on two legs.

    Dissection Instructions:

    1. Obtain a preserved sheep brain from the bucket in the front of the classroom. Place this on your dissection tray.

    2. You will need the following dissection tools to properly perform this lab:

      • scalpel

      • scissors

      • probes

    3. The sheep brain is enclosed in a tough outer covering called the dura mater. You can still see some structures on the brain before you remove the dura mater. Take special note of the pituitary gland and the optic chiasma. These two structures will likely be pulled off when you remove the dura mater.

      Brain with Dura Mater Intact

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      Removal of the Dura Mater

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    4. This image shows the ventral surface of the sheep's brain with most of the dura mater removed. The pituitary gland and the optic chiasma are still intact.

      (A = pituitary gland, B = optic chiasma, C = olfactory bulb)

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    5. On this image, the dura mater has been completely removed, you can still see the optic chiasma but the pituitary gland is missing. The infundibulum (pituitary stalk) is now visible in the center. Careful dissection also reveals two other large nerves: the oculomotor nerves (C.2). Often these two nerves are removed with the dura mater, but in this image they are still intact.

      Ventral view of the brain with dura mater removed

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      These two figures show the fissures located on the surface of the brain. Longitudinal fissure Transverse fissure

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    6. If you flip the brain over to the other side, you can see the cerebellum, it will be loosely attached to the cerebrum in most cases. If you did not carefully remove the dura mater you may have accidentally pulled the entire cerebellum away from the brain. The lobes of the brain are visible, as well as the transverse fissure, which separates the cerebrum from the cerebellum. The convolutions of the brain are also visible as bumps (gyri) and grooves (sulci).

      Dorsal View of the Sheep Brain

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    7. The gap between the cerebrum and the cerebellum at the transverse fissure can reveal some internal parts of the brain. In this image, a student is bending the cerebellum down to show the superior and inferior colliculi. Just behind the colliculi, the pineal gland is just barely visible.

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    8. Using a scalpel and the longitudinal fissure as a guide, the brain is separated into the left and the right hemispheres. Sharp scalpels work best for this procedure. Always leave the specimen in the dissecting tray when cutting it, do NOT hold it in your hand!

      If you are very careful, you will cleanly cut the brain into two halves and can see the internal structures, the most visible of them being the corpus callosum, which divides the left and right hemispheres. The cerebrum will still be visible as a wrinkled structrure, and you can even locate the "bumps" of the superior and inferior colliculi. Remember, you located those structures by pulling down the cerebellum.

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      The cerebellum, when cut will have a very distinct tree-like white area within it. This is called the arbor vitae, or the tree of life.

    9. In the image below, a probe indicates the location of the lateral ventricle.

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    10. Once the brain is cut this way, the colliculi can also be seen from the inside and the pineal gland is revealed only if you made a very careful incision.

      On this image, the pineal is pinned in yellow and the pin continues on to where the colliculi have been bisected.

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    11. Other major structures are visible, here the probe indicates the arbor vitae (tree of life) found within the cerebellum. The fissure between the cerebrum and the cerebellum is called the transverse fissure. The cerebellum only loosely connects to the rest of the brain when the dura is removed.

    12. This brain is pinned to show the pineal gland (blue pin), thalamus (red pin) and lateral ventricle (green pin).

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    13. The image below shows a cleanly separated brain with the major internal structures visible and labeled.

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    14. Finally, a section of the brain is cut to examine the difference between white matter and gray matter.

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    15. Once you have made the cut like in the above diagram, you should be able to see the difference between the white and gray matter, just like the diagram below.

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    16. The diagram below will help you understand the difference between the gyri and the sulcus.

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    Sheep brain dissection maybe downloaded at the following site: https://jb004.k12.sd.us/my%20website...P%20BRAIN%20DI SSECTION/SHEEP%20BRAIN%20LAB.htm

    LAB 11 EXERCISES 11-6

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    Sheep Brain Dissection Analysis:

    Match the structure to the description

    1. looks like a butt

    2. leathery covering over the entire brain

    3. cauliflower, the area toward the back of the brain

    4. behind the colliculi, looks like a little nub

    5. looks like a "tree"

    6. the rounded part of the brain stem

    7. shaped like an X

    8. large area under the corpus callosum

    9. space for fluid between the corpus callosum and the fornix

    10. contains nerves, connects to the far front of the brain

    1. Arbor Vitae

    2. Lateral Ventricle

    3. Optic Chiasma

    4. Superior Colliculi

    5. Dura Mater

    6. Cerebellum

    7. Pineal Gland

    8. Thalamus

    9. Pons

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    10. Olfactory Bulb

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