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Ch 44 Neurons and Nervous Systems, notes part I                Davison BI 102 Lecture Notes 2/27 ‘03 

I.  Diversity of Nervous Systems (see fig. 46.1)

  1. Nerve net, a nervous system without a brain, simply a network of nerves running throughout the animal body.  This type of nervous system lacks centralization and is possessed by radially symmetrical animals such as hydra and other cnidarians.
  2. Ladder-like arrangement of nerve cords with a simple brain as possessed by planarians.
  3. Brain and ventral, solid nerve cord characterizes most invertebrates.
  4. Brain and a dorsal, hollow nerve cord (spinal cord) characterizes all vertebrates.

II.  Human (mammalian) Nervous System 

The brain and spinal cord comprise the Central Nervous System (CNS)

Sensory and motor neurons comprise the Peripheral Nervous System (PNS) 

Note:  The term “nerve” is usually in reference to “vein-like” structure in the PNS where each “vein” rather than being hollow contains a bundle of nerve cell axons and their supporting cells (see fig. 46.2 and the un-numbered illustration top of p. 834).  A single nerve carries both sensory and motor neurons and thus signals are being conducted both to and from the CNS simultaneously within a single nerve.

 

A.  Neuron Structure

      Neurons are typically elongate cells with three distinct parts:

1.      cell body, contains the nucleus

2.      dendrites, the branched nerve endings that receive signals and transmits the nervous signal towards the cell body.  Dendrites transmit signals away from their terminal ends.

3.      axon, a long, unbranched portion of a neuron that continues to transmit the nervous signal away from the dendrites.  Axons transport nervous signals towards their terminal ends.  Note:  the terminal ends of axons are usually branched, though branched this terminal portion is considered part of the axon as defined by the direction of nervous impulse transmittance (i.e., towards its terminal end).

 

B.  Three Basic Types of Neurons (see fig. 46.3)

  1. Sensory Neurons – transmit nervous signals from sensory receptors to the CNS; the dendrites of sensory neurons are located in sense organs (e.g. eyes, ears, nose, skin) and the terminal ends of every sensory neuron’s axon are joined to the CNS.
  2. Motor Neurons – transmit nervous signals from the CNS to muscles and glands; the dendrites of motor neurons are joined to the CNS and the terminal ends of every motor neuron’s axon are joined to cells in muscle or glands.
  3. Interneurons – transmit nervous signals between neurons; both axon and dendrite ends of interneurons are joined to other neurons in the following arrangement:  an interneuron’s dendrite is joined to the terminal end of another neuron’s axon terminus (this other neuron may be another interneuron or it may be a sensory neuron); an interneuron’s axon terminus is joined to another neuron’s dendrite or cell body (this other neuron may be another interneuron or it may be a motor neuron).

 

C.  Supporting Cells of Motor and Sensory Neurons

 Each axon of sensory and motor neurons is insulated with a myelin sheath formed of Schwann Cells.  Each Schwann cell is wrapped around, encircling part of an axon.  There are uninsulated gaps between Schwann cells called Nodes of Ranvier.  See figs. 46.3 & 46.8.