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With an Appexdix

C. P. Bean

1. We have investigated the possibility that individual lateral vestibulospinal tract (LVST) axons branch so as to innervate more than one spinal cord level.

2. LVST cells in Deiters' nucleus were activated antidromically by means of electrical stimulation applied through fine metal electrodes inserted into the spinal cord. Both by directly measuring the spread of effect of stimulus current, and from theoretical considerations (Appendix), we determined that in most cases an estimate of spread of effect of stimulus current was 10 µm/µA. From the magnitude of the threshold stimulus and from the location of the stimulus point we could often exclude the possibility that the stimulus was spreading to the LVST instead of activating local branches.

3. Movable stimulating electrodes, or multi-electrode arrays placed in fixed position, were used to activate 115 LVST neurones antidromically by stimulation of local branches in the lower cervical or upper thoracic cord. Of these cells, 50% were also fired antidromically by stimulation of the LVST at levels ranging from L1 to L4 (L_c cells). The remaining cells were not activated by the lumbar stimulus (C cells). An additional group of cells was only fired by the lumbar tract stimulus (L cells).

4. The distribution of locations of L_c cells within Deiters' nucleus more closely resembles that of L cells than that of C cells. In addition the median conduction velocity of L_c cells is similar to that of L cells, but higher than that of C cells.

5. Much of the information reaching the lower cervical level from neurones of the LVST is information that is also simultaneously being passed downward to the lumbar region. Such integration makes it possible for a single neurone to be used to co-ordinate widespread motor activity.

6. A theory is presented in a separate section (Appendix) to account for the spread of effect of stimulus current upon a myelinated axon submerged in an isotropic medium. The threshold for stimulation of a node by a nearby monopolar electrode is predicted to be proportional to the electrode-node spacing. The constant of proportionality is given in a closed form that depends on the electrical properties of both the neurone and the surrounding medium. The predictions of the theory are shown to be in good accord with the experimental results.

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