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Chapter 3

chapter 3 bio.rtf

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Wilfrid Laurier University
Bruce Mc Kay

Chapter 3: Anatomy of the nervous system General layout of the nervous system The vertebrate nervous system if composed of two divisions: the central nervous system and the peripheral nervous system. The central nervous system (CNS): is the division of the nervous system that is located within the skull and spine The Peripheral nervous system (PNS): is the vision that is located outside the spine and skull The CNS is comprised of two divisions: the brain and the spinal cord. The brain is the part of the CNS that is located in the skull The PNS is also composed of two divisions: the somatic nervous system and the autonomic nervous system. The Somatic nervous system (SNS) is the 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 nervous system and efferent nerves that carry motor signals from the central nervous system to the skeletal muscles. The autonomic nervous system (ANS) is the part of the peripheral nervous system that regulates the body's internal environment. It is composed of afferent nerves that carry sensory signals form internal organs to the CNS and efferent nerves that carries motor signals form the CNS to internal organs. Hint- going toward CNS starts with an a and going away from something begins with an e. The ANS as two kinds of efferent nerves: sympathetic nerves and parasympathetic nerves. The sympathetic nerves are those autonomic motor nerves that project from the CNS in the lumbar and thoracic regions of the spinal cord. The parasympathetic nerves are those autonomic motor nerves that project from the brain and sacral region of the spinal cord. All sympathetic and parasympathetic neurons project from 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 systems differ in that the sympathetic neurons that project from the CNS synapse on second stage neurons at a substantial distance from their target organs, whereas 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 these two systems stresses three important principles: 1. that sympathetic nerves stimulate, organize, and mobilize energy resources in threatening situations, whereas parasympathetic nerves act on 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 relaxation. Most of the nerves of the peripheral nervous system project from the spinal cord, but there are 12 pairs of exceptions: the 12 pairs of cranial nerves, which project from the brain. They are numbered in sequence from front to back. The cranial nerves include purely sensory nerves such as the olfactory nerves (1) and the optic nerves (11), but most contain both sensory and motor fibbers. The longest cranial nerves are the vagus nerves (X), which contain motor and sensory fibers traveling to and from the gut. -the brain and spinal cord are the most protected organs in the body. They are encased in bone and covered in 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 called the subarachnoid space, which contains many large blood vessels and cerebrospinal fluid: then comes the innermost meninx, the delicate pia matter which adheres to the surface of the CNS. Also protecting the CS is the cerebrospinal fluid (CSF), which fills the subarachnoid space, the central canal of the spinal cord, and the cerebral ventricles of the brain. The central canal is a small central channel that runs the length of the spinal cord; the cerebral ventricles are the four large internal chambers of the brain, the two lateral ventricles, the third ventricle and the fourth ventricle. The CSF supports and cushions the brain; the CSF is continuously produced by the choroid plexuses- networks of capillaries that protrude into the ventricles from the pia matter. The blood-brain barrier- barrier so that many toxic substances cannot get into the brain. This barrier is a consequence of the special structure of cerebral blood vessels Cells of the nervous system: -most of the cells of the nervous system are of two fundamentally different types: neurons and glial cells. 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 project from the cell body cell body- the metabolic center of the neuron; also called soma myelin- the fatty insulation around many axons nodes of Ranvier- the gaps between sections of myelin buttons- the button like endings of axon branches, which release chemicals into synapse synapses- the gaps between adjacent neurons which chemical signals are transmitted 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. neurotransmitters- molecules that are released from active neurons and influence the activity of other cells microtubules- tubules responsible for the rapid transport of material throughout neurons 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 bassi 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. A neuron with more than two processes extending form its cell body is classified as a multipolar neuron; most neurons are multipolar. A neuron with one process extending from its cell body is classified as a unipolar neuron, and a neuron with two processes extending from it cell body is a bipolar neuron. neurons with shorter 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. 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 central nervous system, clusters of cell bodies are called nuclei; in the peripheral nervous system, they are called ganglia. In the central nervous system, bundles of axons are called tracts; in the peripheral nervous system they are called nerves. Glial cells are found throughout the nervous system. oligodendrocytes- glial cells with extensions that wrap around the axons of some neurons of the CNS microglia- make up third class of glial cells. They are smaller than other glia, 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, the extensions of some astrocytes cover the outer surfaces of blood vessels that course through the brain, and they also make contact with neuron cell bodies. They allow the passage of some chemicals into CNS neurons and in blocking other chemicals. Neuroanatomical techniques and directions The Golgi stain was discovered by Camillo Golgi in the 1870s. His discovery made is possible to see individual neurons for the first time, although on in silhouette. The Nissl stain- the Golgi stain provided no indication of the number of neurons in an area of the nature of their inner structure, the Nissl stain overcame these shortcoming and was developed by Franz Nissl. Electron microscopy- a neuroanatomical technique that provides information about the details of neuronal structure is electron microscopy. Neuroanatomical tracing techniques- consists of two types: anterograde (forward) tracing methods and retrograde (backward) tracing methods. Anterograde tracing methods are used when an investigator wants to trace the paths of axons protecting away from cell bodies located in a particular area. The investigator injects the brain with several chemicals which are taken up by cell bodies and then transported forward along their axons to their terminal buttons. After a few days the brain is removed and sliced. Retrograde tracing methods with in reverse, used when an investigator wants to trace the paths of axons projecting into a particular area. The vertebrate nervous system has three axes: anterior-posterior, dorsal-ventral, and medial-lateral. Anterior means towards the nose end and posterior means toward the tail end. These same directions are sometimes referred to as rostral and caudal. Dorsal means toward the surface of the back or the top of the head, and ventral means toward the surface of the chest or the bottom of the head. Medial means toward the midline of the body and lateral mea
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