Advertisement

Physiologic Inferences Based on a New Structural Concept of the Spinal Cord

  • Santiago Ramón y Cajal
Chapter

Abstract

We have just seen from the structural study of the spinal cord, that this organ represents only the site of concurrence and articulation of four classes of neurons. I st. The primary or sensory neuron exemplified by the spinal ganglion cell. 2nd. The secondary and tertiary sensory neurons, i.e. the uncrossed and crossed funicular cells of the gray matter. 3rd. The primary motor neuron or ventral radicular cell. And 4th, the secondary motor neurons, represented by both, pyramidal cells of the motor area of the cerebral cortex which form the pyramidal pathway, and [cells of the intrinsic cerebellar nuclei] which continue as the descending cerebellar fibers of Marchia.

Keywords

Motor Nucleus Motor Pathway Sensory Pathway Superior Cerebellar Peduncle Middle Cerebellar Peduncle 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

Footnotes

  1. 1.
    The existence of these cellulifugal impulses in the principal stem, and their propagation to the cord by the central branch is another argument against the opinions of Van Gehuchten and Lugaro concerning the arrival of the sensory excitation to the soma. If this were the case, the principal stem would be frequently the site of two currents of opposite direction, unless it were proven that the stem contains separate cellulipetal and cellulifugal conductors, which has never been demonstrated, nor does it reconcile with the results of histogenesis. Furthermore, the purely cellulifugal nature of the principal process is also demonstrated by its having connections at the origin with special neural arborizations, which so far have been found only in the soma and dendrites. The fact, indicated by Lugaro, that fibrils of the peripheral process, as a rule, do not pass directly to the central branch, continuing in the principal process instead, has lost all of its strength since the demonstration of the transversal conductivity of nerve fibers.Google Scholar
  2. 2.
    Marquez (1897) also explains the convulsions of visceral origin frequently occurring in infants, by links established between sympathetic and sensory cells.Google Scholar
  3. 3.
    We are not mentioning here, because of their illusive nature, certain influences which must modify this intensity. Conductors length, perhaps increases proportionally more than its consumption of neural energy, and the degree of excitability may vary in different parts of the conductor.Google Scholar
  4. 4.
    This author adopts the formulation of the law of electric currents derived from Bequerel and Kirchoff as follows: the intensity of each derived current is in inverse proportion to the length, and in direct proportion to the section of the respective conductor. This approximation between electric and neural conductors is illuminating in certain respects, but can not be accepted in all of its parts. It must be recalled that the neural conductor, not only propagates energy, but also generates it, as proved by the well known phenomenon of motor avalanche (the farthest from the muscle is the stimulated nerve, the greater the energy of muscle contraction).Google Scholar
  5. 5.
    Helmholtz already demonstrated in the frog, that the time taken by the sensory impulse originated in the skin to become a muscle movement, is twelve times greater than the conduction velocity of nerves. The reflex time, i.e. the amount that must be added to that of the velocity of the impulse in nerve fibers, is 0.008 to 0.015sec for short unilateral reflexes. This figure must be increased by one third in crossed and diffuse reflexes (Landois). As can be appreciated, such a considerable delay can not be attributed solely to the greater length of the conductors.Google Scholar
  6. 6.
    According to Goldscheider (1897), the route followed by the sensory excitation could vary somewhat within the determinism of the structure, as a consequence of the state of fatigue or hyperexcitation of each neuron due to a previous work. Thus, an electric discharge in nerves of the face paralyze one side and causes hyperexcitation in the opposite side; the excitability of the neuron is increased after weak stimulations and depressed after strong stimulations. In this way, the impulse arriving from a spot in the skin may change its route in the spinal cord, within certain limits, because the path of least resistance will be different according to the functional state of neurons along the route.Google Scholar
  7. 7.
    We offer this formulation only as a more or less likely and approximate assumption, being probable that the intensity of the impulse decreases less rapidly than the diameter of the conductors due to the phenomenon of avalanche; but this would have little effect on our hypothesis.Google Scholar
  8. 8.
    The experiments of Belmondo and Oddi (1890) are also in favor of this tonic action of neural foci over other neural foci,. They found decreased excitability of spinal ventral roots by application of cocaine to sensory roots thus preventing the arrival of peripheral impulses. Similarly, Tomasini (1894) observed a major decrease in the excitability of the motor cortex after cutting one or many dorsal roots of the opposite side.Google Scholar

Annotations

  1. a.
    The existence of direct cerebellospinal pathways has been disputed since its original description by Marchi and the subsequent adoption by Cajal. It is now accepted that severing the restiform body in the original degeneration experiments, caused unwanted damaged to the lateral vestibular (Deiters) nucleus, and that observed degenerative changes in the spinal cord represented the vestibulospinal, and not the cerebellospinal pathway [For detailed review see Van Gehuchten (1904) Névraxe 6: 19-73].Google Scholar
  2. b.
    See annotation i this Chapter.Google Scholar
  3. c.
    See annotation a in Chapter XVI for discussion on intraepidermic terminations.Google Scholar
  4. d.
    See annotation J in Chapter V for discussion on propagation of the nerve impulse in ganglion cell processes.Google Scholar
  5. e.
    See annotation d in Chapter IV for discussion of pericellular arborizations on ganglion cells.Google Scholar
  6. f.
    See annotation f in Chapter XII for actual role of recurrent collaterals of motor axons.Google Scholar
  7. g.
    Cajal considers here the possibility of an inhibitory influence of the pyramidal pathway on spinal reflexes, but he attributes such function to overstimulation beyond a certain limit.Google Scholar
  8. h.
    Here is a clear enunciation of the axon sprouting phenomenon and the neurotropic theory guiding connectivity.Google Scholar
  9. i.
    This sentence, taken from the Histologie, indicates that in the interim period between Textura and Histologie, Cajal accepted the thalamus as an interposed network in the sensory pathway, as attested in the subsequent paragraph. Apparently, however, he does not make the distinction as yet between sensory and motor cortices.Google Scholar
  10. J.
    It is now known that most of the ascending axons of this pathway derive from cells in laminae VI, VII and VIII on the side of entry of the first order sensory fiber, the axons of which cross the midline in the white commissure and form the lateral spinothalamic tract, a component of the classic fascicle of Gowers. See annotation h in Chapter X.Google Scholar
  11. k.
    See annotation c in Chapter XII for actual termination of pyramidal pathway fibers.Google Scholar
  12. l*.
    The concept of the secondary decussation of motor pathways in generally accepted today as derived from the ancestral coiling reflex of primitive vertebrates (larval stage of the salamander) which required the crossing of the sensory pathway (second order neuron) to activate the ganglion chain of the opposite side in order to escape the stimulus [Coghill (1929) Anatomy and the problem of behavior. Cambridge Univ Press, Cambridge, pp 113]. In higher vertebrates, the withdrawal reflex occurs on the same side as the noxious stimulus and, therefore, it requires either the elimination of the ancient sensory crossing or the occurrence of a new motor crossing. Nature has opt apparently for the second solution.Google Scholar
  13. m.
    Is the ventral spinocerebellar tract an exception to the rule of the non-recrossing of pathways? The subject shall be discussed in annotations referred to the cerebellar peduncles (Volume II, Chapter XXII).Google Scholar
  14. n.
    There is still no general agreement as to the termination of the ventral corticospinal tract which is present only in higher primates and merely reaches upper thoracic levels. Some fibers apparently end in the ipsilateral ventral horn, and others cross over through the ventral white commissure to terminate in the contralateral horn [Kuypers (1981) Handbook of Physiology 2. Am Physiol Soc, Bethesda, pp 597-666, 627-631].Google Scholar
  15. o.
    Fig. 205.—C, termination of the ascending branch in the dorsal horn.Google Scholar
  16. p.
    Fig. 208.—H, thalamus; I, thalamocortical pathway.Google Scholar
  17. q.
    Fig. 209.—H, unidentified.Google Scholar
  18. r.
    Regardless of whether pyramidal fibres provide collaterals to the pontine nuclei, it is clear that the bulk of corticopontine fibers takes origin in widespread cortical areas outside the sensory-motor cortex. This fact was eventually accepted, at least in part, by Cajal, as stated in the last sentence taken from the Histologie. The internal organization of the pons will be discussed in corresponding annotations (Volume II, Chapter XVI).Google Scholar
  19. s.
    See annotation a in this Chapter for discussion of the cerebellospinal pathway.Google Scholar
  20. t.
    It is evident at this point that the concept of inhibition as an active process, escaped Cajal completely.Google Scholar
  21. u.
    The link between spinal motor nuclei and the cerebellum via Gower’s fascicle may be less farfetched than it sounds when considering the origin of ventral spinocerebellar fibers in large neurons of the ventral horn, the so-called border cells [Cooper and Sherrington (1940) Brain 63: 123-134].Google Scholar

Copyright information

© Springer-Verlag Wien 1999

Authors and Affiliations

  • Santiago Ramón y Cajal

There are no affiliations available

Personalised recommendations