139. Metameric Organization of The Nervous System

  1. Introduction: each metamere* carries a pair of nerve ganglia, a cutaneous component, a group of muscles, and a number of other mesodermal derivatives, such as visceral, vascular, etc. Thus, there develops an anatomic and physiologic unit which is more advanced in organization than the diffuse structures of primitive invertebrates
    1. AS WE ASCEND THE PHYLOGENETIC SCALE of invertebrates, toward the insects, segmental independence is sacrificed for the welfare of the organism as a whole, and neighboring ganglionic pairs tend to fuse, and corresponding metameres interweave and coordinate their functions
  2. In vertebrates and humans the neural tube and neural crest systems replace what was essentially the ganglionic system of the invertebrates. Metamerization, however, is still obvious, especially in the trunk region. A metamere consists of dermatomes, ganglia, neuromere, and myotomes
    1. METAMERIZATION manifests itself by the aggregation of paraxial mesoderm into somites
      1. The first somites make their appearance in the posterior part of the cephalic region, and segmentation, from that area, progresses toward the caudal end
    2. THE NEURAL CRESTS, in a parallel manner, break up and produce the ganglionic primordia which correspond, one to one, to the lateral somites
    3. THE CUTANEOUS AREAS opposite the somites contribute the dermatomes
    4. THE SPINAL CORD itself does not divide; however, the cord can be considered to be virtually segmented since a medullary level or neuromere corresponds functionally to each ganglion and somite level
  3. Medullary metamerization
    1. EACH NEUROMERE receives sensory fibers from a specific cutaneous area, the dermatome, and sends out motor fibers to a specific group of muscles, the myotome. Afferent sensory fibers also pass to the viscera, and the response from the latter is made via the motor pathways of the autonomic nervous system
    2. THE VARIOUS ELEMENTS OF THE METAMERE and its organization are embryologically interdependent
  4. Variations in segmentation: in humans and in mammals segmentation persists in the sensory pathways, but is less well defined in the motor pathways
    1. SENSORY PATHWAYS: the dermatome is a cutaneous zone which corresponds precisely to the sensory root that it innervates. There are a number of clinical applications related to this relationship
      1. In herpes zoster, the ganglion level affected can be determined from the pattern of the cutaneous lesion
      2. In cases of medullary compression by a tumor, the level of compression can be determined by the loss of sensitivity in all the subjacent dermatomes.
    2. MOTOR PATHWAYS: segmentation persists and is clearly seen in the intercostal spaces. It is much less clear in the limbs and the girdles since the primordia of these structures are not located opposite a single metamere, but opposite several. Thus, the same muscle group may be innervated by motor nerves coming from different levels
      1. Functionally, segmental medullary activities, such as osteotendinous or micturition reflexes, are progressively controlled by the higher centers as one ascends the evolutionary scale of the vertebrates. Man can regulate many of these activities at will
      1. Segmentation is diminished in the involuntary pathways
      2. Organs are innervated by nerves from various levels, as in most of the somatic muscle groups
    4. *A metamere is one of a series of homologous segments in the body. Metameric organization is seen phylogenetically for the first time in annelids.

metameric organization of  the nervous system: image #1