Spontaneous and Reflex Activities: General Characteristics

Abstract

As used in this chapter, the term “spontaneous activity” refers to electrical potentials that are not induced directly by the investigator; “reflex activity,” to potentials induced by special afferent stimulation. It should be noted, however, that spontaneous activity is also usually controlled by afferent input, even if it is not specially induced by the latter. For example, the cardiac rhythm generated by spontaneous sympathetic activity is due to the baroreceptor control of sympathetic centers. Furthermore, one can observe the spontaneous activity in sympathetic ganglion neurons only if their preganglionic fibers remain connected to their centers or if their afferent fibers of peripheral origin remain connected to appropriate peripheral organs. The neurons with the autogenic type of intracellular spontaneous activity resulting from the properties of their soma, similar to the spontaneous activity observed in invertebrate pacemaker neurons (cf. Tauc, 1955; Hagiwara and Bullock, 1957), have not been found so far in the vertebral autonomic ganglia, except the enteric system (see Chapter 6; Wood, 1975; Wood and Mayer, 1978) and the inferior mesenteric ganglion (Jule and Szurszewski, 1983; Jule et al., 1983). In all cases, the spontaneous intracellular spikes recorded from the neuronal soma appear to be triggered by excitatory postsynaptic potentials (EPSPs), by electrotonic invasions of the soma by spikes originating in the processes of the cell, or by an endogenous depolarizing mechanism (see below).