164. The Autonomic Nervous System: The Sympathetic System

  1. Introduction: the autonomic nervous system (ANS) involves those processes that are normally beyond voluntary control and, for the most part, beneath consciousness. In this way, it differs from the voluntary central nervous system. However, it is under the control of centers in the central nervous system and cannot function as an independent unit
    1. THE AUTONOMIC NERVOUS SYSTEM is composed of 2 major portions which are anatomically and physiologically distinct: the sympathetic (thoracolumbar) and parasympathetic (craniosacral) systems. These systems are essentially motor systems since the sensory afferent nerves, with but a few exceptions, follow the ordinary sensory pathways. They are also essentially a 2-chain system of pre-and postganglionic fibers
  2. The sympathetic nervous system
    1. CELLS FROM THE NEURAL CREST and ventral portion of the neural tube of the thoracic region migrate on either side of the spinal cord, toward the region just behind the dorsal aorta, at about week 5 of development. These are to become the sympathetic neuroblasts or future sympathetic cells
      1. Some detach themselves from the tube and arrange themselves along the motor root
    2. THE MIGRATING CELLS form 2 chains of sympathetic ganglia on either side of the vertebral column
      1. The ganglia are segmental or metameric, but in contrast to the spinal ganglia, they are interconnected to each other by longitudinal nerve fibers or axons of some of the cells. The resulting interconnected ganglia form the lateral vertebral sympathetic chains
      2. From their thoracic portion, the neuroblasts migrate and extend the sympathetic system into both the cervical (neck) and lumbosacral region
        1. An upward extension into the neck forms the superior, middle, and inferior cervical ganglia, which exist to supply structures of the head and neck
    3. SOME OF THE SYMPATHETIC NEUROBLASTS migrate even farther ventrally to form preaortic ganglia such as seen in the solar (celiac) and mesenteric plexuses, the visceral or gastrointestinal ganglia of the myenteric plexus of Auerbach, and in the submucous plexus of Meissner
      1. Still other sympathetic cells migrate to the heart and lungs where they give rise to the sympathetic organ plexuses
    4. WHILE THE GANGLIA ARE FORMING, fibers coming from the visceral motor areas of the medulla and spinal cord make synapses with the sympathetic neuroblasts of 1 of the 3 ganglionic levels to form the preganglionic fibers
      1. The preganglionic fibers are myelinated, and their paths from the spinal nerve to the sympathetic ganglia are thus called the white rami communicantes
    5. THE AXONS OF SYMPATHETIC NEUROBLASTS, found in the ganglia, constitute the unmyelinated postganglionic fibers
      1. These fibers leave the lateral chain ganglion system at 1 of its 3 levels to join the spinal nerves and are called the gray rami communicantes
      2. The postganglionic fibers innervate diffuse structures such as smooth muscle, cardiac (heart) muscle and glands
      3. The fibers innervating the eye, heart, and lungs, as well as the digestive system, originate in the 3 ganglion levels (cervical, thoracic, and preaortic, respectively)

the autonomic nervous system:  the sympathetic system: image #1