290 Textbook chapter 4

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University of Toronto Mississauga
Amanda( Mandy) Wintink

290 Textbook Notes [Lecture 3] Chapter 3: Anatomy 1 G ENERAL L AYOUT OF THE N ERVOUS S YSTEM 1.1 D IVISIONS OF THNERVOUS S YSTEM I. The central nervous system (CNS) a. Division of the nervous system that is located within the skull and spine b. Composed of 2 divisions: i. The brain ii. The spinal cord II. The peripheral nervous system (PNS) a. The division that is located outside of the skull and spine b. Somatic nervous system (SNS) i. Interacts with the external environment ii. afferent nerves that carry sensory signals from the skin, skeletal muscles, joints, eyes, ears etc. to the central nervous system iii. Efferent nerves that carry motor signals from the central nervous system to the skeletal muscles c. Autonomic nervous system (ANS) i. Regulates the body’s internal environment ii. Composed of afferent nerves (a – arrive) that carry sensory signals from internal organs to the CNS and efferent nerves (e – exit) that carry motor signals from the CNS to internal organs iii. 2 kinds of efferent nerves: 1. Sympathetic nerves: autonomic motor nerves that project from the CNS in the lumbar (small of back) and thoracic (chest area) regions of the spinal cord 2. Parasympathetic nerves: autonomic motor nerves that project from the brain and sacral (lower back) region of the spinal cord 3. Both types of nerves are two-stage neural paths a. sympathetic and parasympathetic neurons project from CNS to go only part of the way to target organs before they synapse on other neurons (second-stage neurons) that carry the signals the rest of the way b. differ in the way that sympathetic neurons that project from the CNS synapse on second stage neurons at a substantial distance from their target organs, whereas the parasympathetic neurons that project from the CNS synapse near their target organs on very short second-stage neurons d. the conventional view of functions of the symp and parasymp systems stresses 3 important principles: i. sympathetic nerves stimulate, organize, and mobilize energy resources in threatening situations whereas parasympathetic nerves act to conserve energy ii. each autonomic target organ receives opposing sympathetic and parasympathetic input, and its activity is thus controlled by relative levels of symp and parasymp activity iii. sympathetic changes are indicative of psychological arousal, whereas parasympathetic changes are indicative of psychological relaxation e. most nerves of the peripheral nervous system project from the spinal cord f. 12 pairs of exceptions (12 pairs of cranial nerves – project from the brain) i. Include purely sensory nerves (ex. olfactory nerves) and optic nerves ii. Most contain both sensory and motor fibers iii. Longest cranial nerves – vagus nerves 1. Contain motor and sensory fibers traveling to and from the gut iv. The autonomic motor fibers of cranial nerves are parasympathetic v. Functions and locations of various cranial nerves are specific, therefore disruption of particular nerves provides excellent clues about the location and extent of tumors and other kinds of brain pathology 1.2 M ENINGES , VENTRICLES , AND CEREBROSPINAL FLUID I. CNS (brain and spine) – most protected organs in the body a. Encased in bone and covered by three protective membranes b. Three meninges i. Outer meninx – tough membrane called the dura mater (‘tough mother)’ ii. Inside the dura mater – fine arachnoid membrane (spiderweb-like membrane) iii. Beneath arachnoid membrane – subarachnoid space (contains many large blood vessels and cerebrospinal fluid) iv. Innermost meninx – pia mater (‘pious mother’) which adheres to the surface of the CNS c. Als o protecting the CNS is the cerebrospinal fluid (CSF) i. Fills the subarachnoid space, the central canal of the spinal cord, and the cerebral ventricles of the brain 1. Central canal: small central channel that runs the length of the spinal cord 2. Cerebral ventricles: 4 large internal chambers of the brain; two lateral ventricles, the third ventricle, and the fourth ventricle ii. the subarachnoid space, central canal, and cerebral ventricles are interconnected by a series of openings and thus form a single reservoir II. The CFS cushions and supports the brain i. people who had some CSF drained away often feel headaches when jerk head b. CFS is produced by the choroid plexuses (networks of capillaries – small blood vessels – that protrude into the ventricles from the pia mater c. Excess fluid is absorbed from the subarachnoid space into large blood-filled spaces (dural sinuses) which run through the dura mater and drain into the large jugular veins of the neck d. Occasionally, flow of CSF is blocked by a tumor near one of the narrow channels that link the ventricles (ex. near the cerebral aqueduct which connects 3 and 4 ventricles) i. Resulting buildup of fluid in the ventricles causes their walls, and therefore the entire brain, to expand ii. Produces condition called hydrocephalus (water head) 1. Treated by draining excess fluid and trying to remove obstruction 1.3 B LOOD -B RAIN B ARRIER I. A consequence of the special structure of cerebral blood vessels II. In the body, cells that compose walls of blood vessels are loosely packed a. As a result, most molecules pass through them into the surrounding tissue III. In the brain, the cells of the blood vessels are tightly packed a. Form a barrier to the passage of many molecules (particularly proteins and other large molecules) b. Degree to which drugs can influence brain activity depends on the ease with which they penetrate the blood-brain barrier IV. Does not impede the passage of all large molecules a. Some large molecules normal for brain function (ex. glucose) are actively transported through the walls of cerebral blood vessels 2 C ELLS OF THE N ERVOUS SYSTEM 2.1 ANATOMY OF N EURONS I. Neurons: cells that are specialized for the reception, conduction, and transmission of electrochemical signals a. come in a variety of shapes and sizes 2.1.1External Anatomy of Neurons I. Cell membrane: the semipermeable membrane that encloses the neuron II. Dendrites: the short processes emanating from the cell body; receive most of the synaptic contacts from other neurons III. Axon hillock: cone-shaped region at the junction b/w the axon and the cell body IV. Axon: the long, narrow process that projects from the cell body V. Cell body: the metabolic center of the neuron; also called the soma VI. Myelin: fatty insulation around many axons VII. Nodes of Ranvier: gaps b/w sections of myelin VIII. Buttons: buttonlike endings of axon branches which release chemicals into synapses IX. Synapses: gaps b/w adjacent neurons across which chemical signals are transmitted 2.1.2 Internal Anatomy of Neurons I. Endoplasmic reticulum: a system of folded membranes in the cell body; rough portions (those w/ ribosomes) play a role in the synthesis of proteins; smooth portions (no ribosomes) play a role in synthesis of fats II. Cytoplasm: the clear internal fluid of the cell III. Ribosomes: internal cellular structures on which proteins are synthesized; located on the endoplasmic reticulum IV. Golgi complex: connected system of membranes that packages molecules in vesicles V. Nucleus: the spherical DNA-containing structure of the cell body VI. Mitochondria: sites of aerobic (oxygen consuming) energy release VII. Microtubules: responsible for rapid transport of material throughout neurons VIII. Synaptic vesicles: spherical membrane packages that store neurotransmitter molecules ready to release near synapses IX. Neurotransmitters: molecules that are released from active neurons and influence the activity of other cells 2.1.3 Neuron Cell Membrane I. Composed of a lipid bilayer, or 2 layers of fat molecules II. Embedded in the bilayer are numerous protein molecules (basis of many of the cell membrane’s functional properties) a. Some are channel proteins (certain molecules can pass through them) b. Others are signal proteins (transfer a signal to inside of neuron when particular molecules bind to them on the outside of the membrane) 2.1.4 Classes of Neurons I. classifying neurons based on number of processes/projections emanating from their cell bodies II. More than 2 processes – multipolar neuron a. Most neurons III. Two processes – bipolar neuron IV. One process – unipolar neuron V. Interneurons – neurons w/ a short axon or no axon at all a. Function is to integrate the neural activity within a single brain structure, not to conduct signals from structure to structure 2.1.5 Neurons and Neuroanatomical Structure I. 2 kinds of gross neural structures in the nervous system: a. Those composed primarily of cell bodies b. Those composed primarily of axons II. In CNS, called nuclei (both a structure in the neuron cell body and a cluster of cell bodies in the CNS a. Bundles of axons are called tracts III. In PNS, called ganglia a. Bundles of axons are called nerves 3 N EUROANATOMICAL T ECHNIQUES AND D IRECTIONS 3.1 N EUROANATOMICAL TECHNIQUES I. Major problem w/ visualizing neurons: a. tightly packed, axons and dendrites intricately intertwined b. looking through a microscope at unprepared neural tissue reveals almost nothing II. need to prepare neural tissue in such a way that it permits a clear view of a different aspect of neuronal structure 3.1.1 Golgi Stain I. Accidentally discovered by Camillo Golgi in the early 1870s a. Was trying to stain the meninges by exposing a block of neural tissue to potassium dichromate and silver nitrate b. Noticed that the silver chromate created by reaction of the 2 substances invaded a few neurons in each slice of tissue; stained each invaded neuron entirely black II. Made it possible to see indiv neurons in silhouette III. Commonly used when overall shape of neurons is of interest IV. Provides no indication of the number of neurons in an area or the nature of their inner structure 3.1.2 Nissl Stain I. Overcame shortcomings of the Golgi stain II. Developed by Franz Nissl in the 1880s III. Most common dye used is cresyl violet a. Penetrates all cells on a slide, but bind effectively only to structures in neuron cell bodies IV. Often used to estimate number of cell bodies in an area by counting the number of Nissl stained dots 3.1.3 Electron Microscopy I. Provides information about the details of neuronal structure II. Electron micrograph captures neuronal structure in great detail III. Scanning electron microscope provides spectacular electron micrographs in 3 dimensions but isn’t capable of as much magnification as a conventional electron microscope IV. b/c images are so detailed, they can make it difficult to visualize general aspects of neuroanatomical structure 3.1.4 Neuroanatomical Tracing Techniques I. 2 types: a. Anterograde (forward) tracing methods i. Used when want to trace paths of axons projecting away from cell bodies located in a particular area ii. Inject a chemical used for anterograde tracing into the area iii. Chemical is taken up by cell bodies and transported forward along their axons to their terminal buttons iv. brain removed and sliced after few days; slices treated to reveal locations of injected chemicals b. Retrograde (backward) tracing methods i. Used when want to trace the paths of axons projecting into a particular area ii. Inject a chemical used for retrograde tracing into the area; chemical is taken up by terminal buttons and transported backward along their axon to their cell bodies iii. Brain removed and sliced after a few days; slices treated to reveal location of injected chemical 3.2 D IRECTIONS IN THE VERTEBRATE N ERVOUS S YSTEM I. directions in the vertebrate nervous system are described in relation to the orientation of the spinal cord II. 3 axes: a. Anterior-posterior i. Anterior – toward the nose end ii. Posterior – toward the tail end iii. Sometimes referred to as rostral and caudal b. Dorsal-ventral i. Dorsal – toward the surface of the back or the top of the head ii. Ventral – toward the surface of the chest or the bottom of the head c. Medial-lateral i. Medial – toward the midline of the body ii. Lateral – away from the midline toward the body’s lateral surfaces d. in humans – complicated b/c we walk on hind legs (turn brain directions clockwise for spine directions) e. terms superior and inferior are often used to refer to the top and bottom of the primate head f. proximal – ‘close’; distal – ‘far’ i. proximal means closer to the CNS and distal means farther from the CNS ii. ex. shoulders are proximal to elbows and elbows are proximal to fingers III. slices of the brain a. horizontal sections b. frontal/coronal sections c. sagittal sections d. section cut down the center b/w 2 hemispheres – midsagittal section e. section cut at a right angle to any long, narrow structure (ex. spinal cord or nerve) = cross section 4 S PINAL C ORD I. 2 different areas: a. Inner H shaped core of grey matter i. Grey matter: composed largely of cell bodies and unmyelinated interneurons b. Surrounding area of white matter i. White matter: largely myelinated axons II. 2 dorsal arms o
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