Laboratory 7 Brain Anatomy

Transcription

1 Laboratory 7 Brain Anatomy Goals: Describe the organization & function of the three meningeal layers. Explain where cerebrospinal fluid is found in the central nervous system Identify the major structures of the human brain using models. Identify the major structures of the sheep brain by dissection. Identify selected cranial nerves on the human and sheep brains. 1. Coverings of the Brain: The brain and spinal cord are protected by multiple layers of tissue. Most external is the skin, followed by the skull. Underneath the skull are three layers of connective tissue membranes collectively referred to as meninges. The outer most layer is called the dura mater (literally, hard or tough mother). It is a tough, double- layered membrane. The most superficial layer of the dura mater forms the inner periosteum of the cranial bones. The middle layer, the arachnoid mater (think spider) has a delicate spider web- like appearance and helps to cushion the brain. The innermost layer, the pia mater (literally soft mother), is a thin, waterproof layer that adheres to the outer surface of the brain and spinal cord. Identification: On the sheep brain, identify the three layers of the meninges. Identify & remove the dura and arachnoid maters. 2. Regions of the Brain: The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer with many bumps and grooves called the cerebrum. The visible part of the cerebrum is the neocortex. Interior to the cortex is the midbrain. Underneath the cerebrum lies the brainstem, resembling handle underneath the main portion of the brain. At the rear of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a much finer bumps than the cerebrum. The same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain. During development, the brain develops from three expansions of the embryonic neural tube referred to as the forebrain (also called the proencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The forebrain forms the largest region of the human brain form forming the telencephelon or cerebrum, and the diencephalon. The diencephalon becomes the thalamus and hypothalamus controlling movement and autonomic/endocrine function, respectively. The midbrain is the smallest region and is located between the forebrain and hindbrain. The midbrain contains a number 91

2 of nuclei involved in relaying auditory and visual information and in regulating voluntary muscle movement. The hindbrain is closest to the spinal cord and forms the cerebellum, pons, and medulla oblongata. Figure 2. Developmental brain regions in the mammalian brain. Some brain systems transverse multiple brain regions. For example, voluntary movement begins in the neocortex, but normal voluntary movement also requires input from the cerebellum and midbrain and signals are relayed in the thalamus. Identification: On the sheep brain, identify the telencephalon (or cerebrum), cerebellum, pons, and medulla oblongata. The diencephalon and midbrain will not be visible. On a human brain model, identify all six brain regions. 3. Surface anatomy of the brain Cerebral lobes The cerebrum is forms the most superior portion of the brain and is divided by a prominent longitudinal fissure into right and left hemispheres. These hemispheres are further dived into five cerebral lobes. The four cerebral lobes on the surface of the brain are named after the skull bones under which they lie: frontal, parietal, temporal, and occipital. The insular lobe can be found interior to the lateral sulcus separating the parietal and temporal lobes. 92

3 Note that the surface of the cerebral hemispheres has ridges of tissue called gyri (singular gyrus) separated by grooves called sulci (singular sulcus). Deeper sulci are referred to as fissures that separate the cerebrum into five cerebral lobes. Forming the border between the frontal and parietal lobes, the central sulcus is perpendicular to the longitudinal fissure on each lateral aspect of the brain. The lateral sulcus separates the temporal lobe from the parietal and frontal lobes. The transverse fissure separates the neocortex from the cerebellum. Importantly, the precentral and postcentral gyri are anterior and posterior to the central sulcus. The precentral gyrus is also known as the primary motor cortex. Neurons in this cortical region are responsible for initiating voluntary movement. The postcentral gyrus is also known as the primary somatosensory cortex and the region responsible for conscious sensation from our tactile senses for the body, Connecting the right and left cerebral hemispheres, and visible from the dorsal (superior) aspect if you gently pull apart the hemispheres along the longitudinal fissure is the corpus collusum. This structure is comprised of white matter tracts that allow communication between the two cortical hemispheres. Identification: On the sheep brain and a human brain model, identify gyri, sulci, the 4 primary lobes of the cerebral hemispheres and the major grooves & fissures. 93

4 4.Surface anatomy of the brain Ventral aspect: From the ventral or inferior aspect of the brain, the frontal and temporal lobes are apparent as well as the midbrain, pons, medulla oblongata. All twelve cranial nerve are also visible from the ventral view. The cranial nerves and their locations are described in the next laboratory. The olfactory bulbs, the neural tissue responsible for the sense of smell, are found on the underside of the frontal lobes. These give rise to the first set of cranial nerves, the olfactory nerve. The optic nerves form an X at the optic chiasm and continues as the optic tract until the synapsing in the thalamus. The rest of the cranial nerves can be seen along the pons and brainstem. Just posterior to the optic chiasm, you can see the infundibular stalk which connects to the pituitary gland and the mammillary bodies. Posterior to the hypothalamus are two large tracts called the cerebral peduncles and this represents the part of the midbrain viewable from this aspect. The medial gyrus on the temporal lobe is the parahippocampal gyrus. Immediately deep to this gyrus is the hippocampus, part of the limbic system. 94

5 5. Anatomy of the Midsaggital Section: The midsaggital section is a view of the brain bisected along the longitudinal fissure. From this view, the frontal, parietal and occipital lobes are seen. Two white matter tracts are readily apparent: the larger corpus collosum which allows communication between the hemisphere and the fornix which is the white matter tract leaving the hippocampus. White matter in the brain consists of myelinated axons as compared to gray matter that consists of cells bodies and unmyelinated neurites. The opening seen between the two white matter structures is the lateral ventricle. The third ventricle is the space medial to the hypothalamus connected to the fourth ventricle between the cerebellum and brainstem by the cerebral aquaduct. Immediately inferior to the fornix is a largish rounded area of the thalamus. Anterior and slightly inferior is the triangularly shaped hypothalamus. The infidibulum is a stalk projecting from the anterior point of the hypothalamus and is attached to the pituaitary. Immediately caudal to the hypothalamus is the rounded mammillary body. The midbrain, pons, and medulla oblongata are seen as the cerebellum with the cerebellar white matter tracts known as the arbor vitae. Identification: On the sheep brain and a human brain model, identify as many of the surface structures shown in the above illustrations as possible. 95

6 6. Cerebellum: The cerebellum is located on the caudal aspect of the brain, with the large mass of the cerebral cortex above it and the portion of the brainstem called the pons anterior. The cerebellum is divided into right and left halves with the vermis (latin for worm) protruding midline. The cerebellum, responsible for fine control of voluntary movement, is composed of a gray matter cortex and a white matter arbor vitae. 7. Ventricles: There are a series of hollow spaces within the brain called ventricles. The ventricles are continuous with each other as well as the central canal of the spinal Figure 7. Posterior view of the cerebellum. cord through a series of canals. The two large, C- shaped lateral ventricles encircle the two thalamic lobes and connect medially (via interventricular foramina) to a centrally located third ventricle. Arising from the posterior- ventral wall of the third ventricle, the cerebral aqueduct runs inferior to the corpora quadrigemina of the midbrain to the fourth ventricle, which lies inferior to the cerebrum. The central canal of the spinal cord begins at the base of the fourth ventricle. Cerebrospinal fluid (CSF) is produced by special vascularized patches of tissue within the ventricles called choroid plexus, fills the ventricles, ducts, central canal. CSF flows from the ventricles to the central canal, which is continuous with the subarachnoid space surrounding the entire central nervous system. Once the CSF returns to the head, it is drained through specialized shunts called arachoid villi and returned to the circulatory system. Cerebral Aquaduct Identification: On the sheep brain and a human brain model, identify the choroid plexus, lateral ventricles, third ventricle, cerebral aqueduct & fourth ventricle. 96

7 8. Sheep Brain Dissection Use the following link for guidance on sheep brain dissection: Use a digital camera or cell phone camera, take pictures of the sheep brain as you dissect it. You should have at least (but are not limited to) one image from each of the different primary views (inferior, superior, lateral & midsagital section), Place images into a Powerpoint or Google presentation (ASK YOUR INSTRUCTOR IF THEY HAVE A PREFERENCE) and label as many of the following structures as you can in each image (you should be able to label all the structures on the list at least once): Cerebrum Frontal lobe Temporal lobe Parietal lobe Occipital lobe Precentral gyrus Postcentral gyrus Central sulcus Lateral sulcus Transverse fissure Longitudinal fissure Corpus collosum Fornix Thalamus Hypothalamus Lateral ventricles Third ventricle Cerebral aquaduct Fourth ventricle Cerebellum Vermis Arbor vitae Midbrain Pons Medulla Olfactory bulb Optic nerve Optic chiasm Optic tract Attach the Word document to the back of your lab report or share and the Google Doc to your instructor. ASK YOUR INSTRUCTOR WHICH THEY PREFER. 97

8 Attributions: Figure 1. National Cancer Institute (n.d.). [Meninges of the CNS]. SEER Training Modules, The Central Nervous System. Retrieved July 12, 2012 from Figure 2. Nrets (2007). [Encephalon]. and Looie496(2008). [Vertebrate- brain- regions]. Wikimedia Coomons. Retrieved August 16, 2012 from brain- regions.pn. Figure 3. Looie496. (2011). [Vertebrate brain regions]. Wikimedia Commons. Retrieved July 12, 2012 from brain- regions_small.png. Figure 4. NEUROtiker. (2007). [Brain, lateral, medial and inferior views]. Wikimedia Commons. Retrieved July 12, 2012 from _Hauptsulci_beschriftet.svg.. Figure 5. Modified from Beal, J.A. (2005). [Human brain view on traverse temporal and isular gyri description]. Wikimedia Commons. Retrieved July 12, 2012 from description.jpg. Figure 6. Beal, J.A. (2006). [Human brain midsagittal cut description]. Wikimedia Commons. Retrieved July 12, 2012 from Figure 7. Figure 8. Modified from Was a bee. (2009). [Third ventricle]. Wikimedia Commons. Retrieved July 12, 2012 from 98

9 Lab 07 Brain CNS Name Lab Section 1. Briefly define the following terms: Meninges Hemisphere Lobe Sulcus Fissure Cerebral Cortex Ventricle (brain) Choroid Plexus 2. Name the adult brain structures that arise from each of the following embryonic regions: Prosencephalon Mesencephalon Rhombencephalon 3. Name the groove/fissure/sulcus that separates the following brain regions: Left & Right Cerebral Hemispheres Cerebrum & Cerebellum Frontal & Parietal Lobes Parietal & Temporal Lobes Parietal & Occipital Lobes 99

10 4. Describe the position of the insula: 5. Briefly differentiate between White Matter & Grey Matter 6. Supply the missing terms in the following sentences that describes the LONGEST possible route that CSF can take from its production till it reaches the central canal of the spinal cord: CSF is produced by in one of the two. From there it flows through a into the. From there it flows through the into the, and then into the central canal of the spinal cord. 7. You have likely noticed that the Sheep s olfactory bulbs are much larger (both their actual size as well as their size compared to the overall size of the brain) than the human s. Why might this be the case? 8. Attach your pictures of the sheep brain dissection or submit to your instructor online (ask your instructor which). 100