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Chapter 3

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 3: Anatomy of The Nervous System GENERAL LAYOUT OF THE NERVOUS SYSTEM • Divisions of the Nervous System • The vertebrate nervous system is composed of two divisions: • The central nervous system (CNS) - the division of the nervous system that is located within the skull and spine • The peripheral nervous system (PNS) - the division that is located outside the skull and spine • The PNS is also composed of two divisions: • The somatic nervous system (SNS) • Part of the PNS that interacts with the external environment • It is composed of afferent nerves that carry sensory signals from the skin, skeletal muscles, joints, eyes, ears, and so on, to the CNS and efferent nerves that carry motor signals from the CNS to the skeletal muscles • The automatic nervous system (ANS) • Part of the PNS that regulates the body’s internal environment • It is composed of afferent nerves that carry sensory signals from internal organs to the CNS and efferent nerves tat carry motor signals form the CNS to internal organs • The ANS has two kinds of efferent nerves: • Sympathetic nerves - those autonomic motor nerves that project from the CNS in the lumbar and thoracic regions of the spinal cord • Parasympathetic nerves - those autonomic motor nerves that project from the brain and sacral region of the spinal cord • All sympathetic and parasympathetic nerves are two-stage neural paths: The sympathetic and parasympathetic neurons project form the CNS and go only part of the way to the target organs before they synapse on other neurons that carry the signals the rest of the way • However, the sympathetic and parasympathetic 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 • The conventional view of the respective functions of the sympathetic and parasympathetic systems stresses three important principles: 1. That sympathetic nerves stimulate, organize, and mobilize energy resources in threatening situations, whereas parasympathetic nerves act to conserve energy 2. That each autonomic target organ receives opposing sympathetic and parasympathetic input, and its activity is thus controlled by relative levels of sympathetic and parasympathetic activity 3. That sympathetic changes are indicative of psychological arousal, whereas parasympathetic changes are indicative of psychological arousal • Most of the nerves of the PNS project from the brain • The cranial nerves include purely sensory nerves such as the olfactory nerves (I) and the optic nerves (II), but most contain both sensory and motor fibers • The longest cranial nerves are the vagus nerves (X), which contain motor and sensory fibers traveling to and from the gut Chapter 3: Anatomy of The Nervous System • The autonomic motor fibers of the cranial nerves are parasympathetic • Meninges, Ventricles, and Cerebrospinal Fluid • The brain and spinal cord are encased in bone and covered by three protective membranes, the three meninges •The outer meninx is a tough membrane called the dura mater •Immediately inside the dura mater is the fine arachnoid membrane •Beneath the arachnoid membrane is a space called the subarachnoid space, which contains many large blood vessels and cerebrospinal fluid; •then comes the innermost meninx, the delicate pia mater which adheres to the surface of the CNS • Also protecting the CNS is the cerebrospinal fluid (CSF), which fills the subarachnoid space, the central canal of the spinal cord, and the cerebral ventricles are the four large internal chambers of the brain: •the two lateral ventricles, the third centrical, and the fourth ventricle • Cerebrospinal fluid is continuously produced by the choroid plexuses - networks of capillaries that protrude into the ventricles from the pia mater • The excess cerebrospinal fluid is continuously absorbed form the subarachnoid space into large blood filled spaces, or dural sinuses, which run through the dura mater and drain into the large jugular veins of the neck • Occasionally, the flow of cerebrospinal fluid is blocked by a tumor near one of the narrow channels that link the ventricles • The resulting buildup of fluid in the ventricles causes the walls of the ventricles, and thus the entire brain, to expand, producing a condition called hydrocephalus • Hydrocephalus is treated by draining the excess fluid form the ventricles and trying to remove the obstruction • Blood-Brain Barrier • The blood-brain barrier impedes the passage of many toxic substances from the blood into the brain • This barrier is the consequence of the special structure of cerebral blood vessels • In the rest of the body, the cells that compose the walls of blood vessels are loosely packed; as a result, most molecules pass readily through them into surrounding tissue • In the brain, however, the cells of the blood vessel walls are tightly packed, thus forming a barrier to the passage of many molecules - particularly proteins and other large molecules • The blood-brain barrier does not impede the passage of all large molecules • Some large molecules that are critical for normal brain function are actively transported through cerebral blood vessel walls • Also, the blood vessel walls in some areas of the brain allow certain large molecules to pass through them unimpeded CELLS OF THE NERVOUS SYSTEM • Most of the cells of the nervous system are of two fundamentally different types: neurons and glial cells • Anatomy of Neurons Chapter 3: Anatomy of The Nervous System • Neurons are cells that are specialized for the reception, conduction, and transmission of electrochemical signals • External Anatomy of Neurons • Cell Body - the metabolic center of the neuron; also called the soma • Cell Membrane - the semipermeable membrane that encloses the neuron • Dendrites - the short processes emanating from the cell body, which receive most of the synaptic contacts from other neurons • Axon Hillock - the cone-shaped region at the junction between the axon and the cell body • Axon - the long, narrow process that projects from the cell body • Myelin - the fatty insulation around many axons • Nodes of Ranvier - the gaps between sections of myelin • Buttons - the buttonlike endings of the axon branches, which release chemicals into synapses • Synapses - the gaps between adjacent neurons across which chemical signals are transmitted • Internal Anatomy of Neurons • Endoplasmic Reticulum - a system of folded membrane in the cell body; rough portions play a role in the synthesis of proteins; smooth portions play a role in the synthesis of fats • Cytoplasm - the clear internal fluid of the cell • Ribosomes - internal cellular structures on which proteins are synthesized; they are located on the endoplasmic reticulum • Golgi Complex - a connected system of membranes that packages molecules in vesicles • Nucleus - the spherical DNA-containing structure of the cell body • Mitochondria - sites of aerobic energy release • Microtubules - tubules responsible for the rapid transport of material throughout neurons • Synaptic Vesicles - spherical membrane packages that store neurotransmitter molecules ready for release near synapses • Neurotransmitters - molecules that are released from active neurons and influence the activity of other cells • Neuron Cell Membrane • The neuron cell membrane is composed of a lipid bilayer or two layers of fat molecules • Embedded in the lipid bilayer are numerous protein molecules that are the basis of many of the cell membrane’s functional properties • Some membrane proteins are channel proteins, through which certain molecules can pass; others are signal proteins, which transfer a signal to the inside of the neuron when particular molecules bind to them on the outside of the membrane • Classes of Neurons • A neuron with more than two processes extending from its cell body is classified as a multipolar neuron; most neurons are multipolar Chapter 3: Anatomy of The Nervous System •A neuron with one process extending from its cell body is classified as a unipolar neuron, and a neuron with two processes extending from its cell body is classified as a bipolar neuron •Neurons with a short axon or no axon at all are called interneurons; their function is to integrate the neural activity within a single brain structure, not to conduct signals from one structure to another • Neurons and Neuroanatomical Structure •In general, there are two kinds of gross neural structures in the nervous system: those composed primarily of cell bodies and those composed primarily of axons •In the CNS, clusters of cell bodies are called nuclei; in the PNS, they are called ganglia •In the CNS, bundles of axons are called tracts; in the PNS, they are called nerves • Glial Cells: The Forgotten Cells • Neurons are not the only cells in the nervous system; glial cells are found throughout the system • Glial cells do predominate in some brain structures, but overall the numbers of glial cells and neural cells are approximately equal • There are several kinds of glial cells • Oligodendrocytes are glial cells with extensions that wrap around the axons of some neurons of the CNS • These extensions are rich in myelin, a fatty insulating substance, and the myelin sheaths that they form increase the speed and efficiency of axonal conduction • A similar function is performed in the PNS by Schwann cells, a second class of glial cells • Each Schwann cell constitutes one myelin segment, whereas each oligodendrocyte provides several myelin segments, often on more than one axon • Another important difference between Schwann cells and oligodendrocytes is that only Schwann cells can guide axonal regeneration after damage • That is why effective axonal regeneration in the mammalian nervous system is restricted to the PNS • Microglia make up a third class of glial cells but smaller • They respond to injury or disease by multiplying, engulfing cellular debris, and triggering inflammatory responses • Astrocytes constitute a fourth class of glial cells • They are the largest glial cells and they are so named because they are star- shaped • The extensions of some astrocytes cover the outer surfaces of blood vessels that course through the brain; they also make contact with neuron cell bodies • Astrocytes play a role in allowing the passage of some chemicals from the blood into CNS neurons and in blocking other chemicals • Astrocytes have been shown to send and receive signals from neurons and other glial cells, to control the establishment and maintenance of synapses between neurons, to modulate neural activity, to maintain the function of axons, and to participate in glial circuits Chapter 3: Anatomy of The Nervous System NEUROANATOMICAL TECHNIQUES AND DIRECTIONS • Neuroanatomical Techniques • The key to the study of neuroanatomy lies in preparing neural tissue in a variety of ways, each of which permits a clear view of a different aspect of neuronal structure, and then combining the knowledge obtained from each of the preparations • Golgi Stain • By exposing a block of neural tissue to potassium dichromate and silver nitrate • The silver chromate created by the chemical reaction of the two substances Golgi was using invaded a few neurons in each slice of tissue and stained each invaded neuron entirely black • Golgi stains are commonly used when the overall shape of neurons is of interest • Nissl Stain • The most common dye used in the Nissl method is cresyl violet • Cresyl violet and other Nissl dyes penetrate all cells on a slide, but they bind effectively only to structures in neuron cell bodies • Thus, they often are used to estimate the number of cell bodies in an area, by counting the number of Nissl-stained dots • Electron Microscopy • A neuroanatomical technique that provides information about the details of neuronal structure is electron microscopy • An electron micrograph captures neuronal structure in exquisite detail • A scanning electron microscope provides spectacular electron micrographs in 3D but it is not capable of as much magnification as a conventional electron microscope • The strength of electron microscopy is also a weakness: because the images are so detailed, they can make it difficult to visualize general aspects of neuroanatomical structure • Neuroanatomical Tracing Techniques • Neuroanatomical tracing techniques are two types: • Anterograde tracing methods are used when an investigator wants to trace the paths of axons projecting away from cell bodies located in a particular area • Retrograde tracing methods are used when an investigator wants to trace the paths of axons projecting into a particular area • Directions in the Vertebrates Nervous System • The vertebrate nervous system has three axes: anterior-posterior, dorsal-ventral, and medial-lateral • Humans complicate this simple three-axis system of neuroanatomical directions by insisting on walking around on our hind legs • This changes the orientation o
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