Special Properties of Reflex Centers

In general, we do not find it possible to explain either the spatial, or the temporal structures of the reaction by reference merely to those of the afferent portion of the response process. In the case of the scratch reflex, the stimulus may be substantially continuous in its action, whereas the innervation of the leg muscles is intermittent, so as to produce the scratching movements. Consequently, we must suppose that the central or motor elements are equipped with a device for generating intermittent or alternating action, on the basis of a continuous innervation from the afferent side. Sometimes, however, the explanation lies in the fact that the first phases of the motor process bring new afferent impulses into existence which combine with the original ones in a systematic manner to modify, or control, the course of the reaction. In the phenomenon known as after-discharge, which is characteristic of reflexes, the reaction continues for some time after the stimulus, and presumably the afferent nerve current, have ceased.

The properties of reflexes which we have considered above distinguish them to a considerable extent from simple nerve conduction of a non-reflex character. This latter type of conduction is that which occurs between points in a single neurone, or fibre, a process which is not observable in isolation in any of the complex organisms, but which must nevertheless be regarded as a component feature of any response, however complicated. Consequently, the distinguishing characteristics of the reflex must be attributed to some constituent of the reflex arc which is additional to the individual neurones which enter into it. Sherrington believes that this feature consists in the synapses or junction points between the afferent and efferent conductors. As compared with plain nerve fibres or "nerve trunks," synapses must therefore be supposed to possess the following properties which are shown by reflex mechanisms: (1) slow speed of response, especially in the form of a prolonged initial delay (latent period), (2) extensive afterdischarge, (3) lack of correspondence of rhythm of stimulus and effect, (4) lack of correspondence between the intensity gradations of stimulus and effect, (5) resistance to a single stimulus, combined with a summation of the effects of successive small stimuli, (6) conduction only from afferent to efferent, (7) relatively great fatiguability, (8) relatively great general variability of the threshold, (9) very long refractory periods, (10) great dependency upon oxygen and (11) relatively great susceptibility to the action of drugs. These characteristics may be summarized by saying that the synapse appears to exhibit greater inertia, and resistance to the passage of a nerve current than does a continuous single nerve fibre, or group of such fibres operating in parallel.

The conception of a synapse as consisting merely of a junction point between an afferent and an efferent conductor, is undoubtedly altogether too simple. It is highly probable that such junction points can possess peculiar and complex properties which differentiate them from the conjoined neurones. In particular, we can readily understand how they can exhibit an increased resistance to the passage of nerve impulses, since there is an apparent break in the continuity of the conducting tissue at the synaptic points, so that the impulse has--so to speak--to leap across a gap. However, it is almost certain that in the majority of reflexes, special central nerve mechanisms are concerned which involve centrally located nerve cells as well as junction points. Nevertheless, such central mechanisms may be regarded as being interpolated between the afferent and efferent structures and as being logically separable from either of the latter. The most convenient general term for this central mechanism, including the synapse. It will of course be realized that in any concrete reflex a very large number of synapses must be operative simultaneously, all of these with their complex interrelations being comprised in the adjustor process or structure.