nerves by k9zFa8A

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									                                                                H. Wayne Lambert, Ph.D.
                                                                09 February 2001
                                                                Cell and Tissue Biology
                                                                Lecture #17

References: Ross et al. Pages 256-267; 273-276
           Wheater’s Functional Anatomy (4th ed.) Pages 116-133; 140-142

                                       Nerves
Since you will study the structure and function of neurons extensively next semester, this
lecture, therefore, designed to superficially review some material covered previously in
other classes. The main purpose of this lecture is to be able to identify components of the
nervous system in the periphery. This includes identifying nerves (primarily axons),
ganglion, and their sensory receptors in tissue sections.

I. The Neuron
       A. Neurons are similar to all other cells in the body, except that they are
          specialized for receiving and conducting information; these functions are
          reflected in their polarized nature (as shown in this nerve in Figure 1).




               1. Cell body (also called the soma) – consists of the nucleus and the
                  perikaryon, the cytoplasm around the nucleus. Constitutes 10% of
                  receptive area of the neuron.

                      a. prominent structures in the perikaryon can be visualized with
                         “Nissl” stains, basic dyes (developed by Franz Nissl).

                      b. Nissl stains RNA and DNA; neurons stain prominently because
                         they contain more RNA in their cytoplasm than any other cell
                         type.

                      c. At the EM level, Nissl bodies can be shown to be stacks of
                         rough endoplasmic reticulum. Neuron cell bodies are
                         essentially “protein” factories
       d. The nucleolus of neurons and stains densely with Nissl stains

2. The Axon – single process extending from the cell body; the axon is
   not just an extension of the cytoplasm of a neuron, but it is a
   specialized structure

       a. Though it stains throughout the perikaryon, there is NO
          staining of the axon hillocks or axon with Nissl stains; axons
          have no ribosomes and no protein synthesizing capabilities. All
          proteins necessary in axons or at presynaptic terminals must
          be synthesized in the neuron cell body and transported.

       b. A highly organized microtubule and microfilament network
          transfers proteins to the axon terminals and provides a highly
          organized cytoskeleton that provides internal support.

       c. The axon is the conducting portion of the neuron; it conducts in
          an all-or-none fashion.

       d. In man, most axons are myelinated by concentric layers of the
          Schwann cell plasma membrane. This Schwann cell insulation
          of the nerve and unmyelinated areas between the myelin called
          “Nodes of Ranvier” help increase the speed of the nerve
          impulse.

       e. The terminal endings of axons, or telodendria, are
          unmyelinated. At their endings, they have specialized pre-
          synaptic dilations called boutons. This is where synaptic
          vesicles containing chemical neurotransmitters are released
          into the synapse. In the periphery, the neurotransmitter is
          almost always acetylcholine (Ach). Postganglionic
          sympathetic nerve fibers do utilized norepinephrine (NE).

3. Dendrites – a neuron has many dendrites that receive information.

       a. processes are shorter and thicker than an axon

       b. unmyelinated

       c. form extensive arborization called the dendritic tree

       d. no sharp delineation between cell body of neurons, unlike axon

       e. has similar organelles as the cell body, except Golgi bodies
          which are located in proximity to the prominent nucleolus
II.    Nerve fibers are organized similar to muscle fibers.
       A. Endoneurium – delicate packing of loose vascular supporting tissue surround
          the plasma membrane of the Schwann cell

       B. Perineurium – condensed layer of robust collagenous tissue invested a layer
          of flat epithelial cells. The perineurium invests nerves fascicles.

       C. Epineurium – loose collagenous tissue that binds the fascicles together to
          form the strong cylindrical sheath of the peripheral nerve

III.   Neurons can be divided into a number of structural and functional types,
       including the number of processes arising from the cell body. As shown in Figure
       2, neurons can be pseudounipolar, bipolar, or multipolar.

       A. Multipolar neurons are the most common type as numerous dendrites project
          from the cell body.

       B. Bipolar neurons are rare and have only a single dendrite that arises from the
          cell body opposite the origin of the axon. Seen in neurons for the senses of
          smell, sight, and balance.

       C. Pseudounipolar neurons have a single dendrite and the axon arising from a
          common stem of the cell body. Seen in primary sensory neurons (think dorsal
          root of the spinal nerve).

IV.    Schwann Cells and the Myelin Sheath
       A. Myelinated axons are surrounded by a lipid-rich layer called the myelin
          sheath, which is produced by Schwann cells that wrap in a spiral around a
          short segment of myelin on an axon. The lipid component of the myelin
          sheath gives nerves a vacuolated appearance in standard histological stains.

              1. The myelin sheath is external and contiguous with the neurilemma, or
                 sheath of Schwann, which is a layer of Schwann cell cytoplasm that
                 contains the nucleus and most of the organelles of the Schwann cell.

              2. Only the axon hillock and terminal arborizations where the axon
                 synapses with its target cells are completely devoid of the myelin
                 sheath.

              3. The myelin sheath is segmented because it is formed by numerous
                 Schwann cells arrayed sequentially along the axon. At the junction
                 between two Schwann cells, the axon is unmyelinated and called the
                 node of Ranvier.
            4. The myelin sheath is formed once the axon is initially surrounded by a
               Schwann cell. The Schwann cell begins to wrap around the axon
               forming multiple Schwann cell layers. Oligodendrocytes are
               responsible for the CNS myelin.


V.   What you can identify in peripheral tissue in this course!!!

     A. Peripheral Nerves – look for the zigzag pattern of Schwann cells elongated
        along the longitudinal axis.

            1. Found often near arteries and veins in the neurovascular bundles.

     B. Sympathetic Ganglia – many cell bodies surrounded by irregularly placed
        satellite cells. These satellite cells are simple cuboidal cells that surround the
        cell bodies and provide metabolic and structural support. They are much
        more distinct in spinal ganglia.

     C. Free Nerve Endings – the simplest form of sensory receptor (dendrite).

            1. Best visualized with silver impregnation where you see the terminal
               branches of these unmyelinated, afferent nerve fibers.

            2. Found along the junction of the dermis and epidermis in skin.

     D. Meissner’s Corpuscles –small, encapsulated sensory receptors found in the
        dermis of skin

            1. Involved in perceiving light discriminatory touch.

            2. Concentrated in the papillary layer of the skin’s dermis, particularly in
               the fingertips, the soles of the feet, eyelids, lips, and genitalia

            3. A mass of plump, oval cells surrounded by a delicate collagenous
               tissue

     E. Pacinian Corpuscles - large, encapsulated sensory receptors found in the
        deeper layers of the skin, ligaments, and in joint capsules.

            1. Looks like an onion; concentric lamellae of flattened cells

            2. Responsive to pressure, vibrations, and tension.


     F. Remember neuromuscular spindles and neuromuscular junctions have
        already been covered in this class.

								
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