Convergence and Divergence

Now, there are certain general principles which apply to the simultaneous conduction of response impulses along adjacent paths. In the first place, there is a marked tendency for them to converge or to condense upon one another as they proceed. This convergence is due to the manner of concatenation of the nerve fibres and can occur functionally only at the synaptic points, where there is a transfer of the excitations from one set of conductors to another; but there is also a geometrical convergence of the efferent nerve fibres in their paths from all parts of the body to the central nervous system. Functional convergence involves the passage from a larger to a smaller number of conducting elements. This principle is exemplified in Sherrington's conception of the "final common path," which is ordinarily a motor nerve, in the case of simple reflexes. Such a common path can be energized from a considerable number of different sensory stations. However, in the more complex forms of response, the convergence point is to be looked for in the brain, rather than in the efferent nerves.

In some cases of response we are also concerned with a principle of divergence, since the higher nerve centers exercise a simultaneous control over a very large portion-if not the whole--of the bodily musculature in typical cases of voluntary response. Nerve currents, emanating from a relatively small section of the brain, spread out fan-like to all parts of the organism. In some cases, sensory control of this fan of motor impulses may be found in a very restricted "initial common path." Consider, for example, the act of striking at a biting mosquito in the dark. Very frequently, if not usually, in the case of response which is controlled through the higher nerve centers, we are concerned simultaneously with convergence and divergence. A large number of nerve impulses are focused at one time upon the center and regulate another complex outflow of excitations therefrom. We may picture this process in terms of a double fan, resembling the passage of a cone of light rays through a focal point.