ABSTRACT
Intracellular studies on the cardiac ganglion cells of invertebrates have been confined mainly to the hearts of decapod and stomatopod crustaceans. The cardiac ganglia of these animals contain relatively few neurones, thought to be divisible into large follower cells and small pacemaker cells (for reviews see Hagiwara, 1961 ; Bullock & Horridge, 1965).
Studies on follower cells of crustacean cardiac ganglia are numerous but intracellular studies on pacemaker cells, with one exception (Bullock & Terzuolo, 1957) seem to be limited to the cardiac ganglion of the stomatopod, Squilla (Watanabe et al. 1967a; Watanabe, Obara & Akiyama, 1967b, 1968, 1969). In Squilla the most anterior-cells exhibit pacemaker potentials and fire in bursts with each heart beat. However, all neurones in the ganglion are electrically coupled (Irisawa & Hama, 1965; Watanabe et al. 1967a), and pacemaker activity may also be initiated in the posterior part of the ganglion (Irisawa & Irisawa, 1957; Shibuya, 1961; Brown, 1964). The one study on. pacemaker activity in a decapod heart was made on the two largest of the posterior cells of a crab heart (Bullock & Terzuolo, 1957). These workers found that the neurones fired a burst of overshooting spikes during each heart beat. No pacemaker potentials were present but Bullock & Terzuolo (1957) concluded that these two large cells were spontaneously active since there were no excitatory postsynaptic potentials (EPSP).
The Limulus cardiac ganglion is known to be composed of many hundreds of cells (Patten & Redenbaugh, 1899; Bursey & Pax, 1970). Electrical activity of ganglion cells was studied by Heinbecker (1933, 1936) and Prosser (1943). These workers recorded extracellularly from small pieces of the ganglion in an attempt to isolate single units. Prosser (1943) successfully recorded units firing once per burst and other units firing a train of spikes superimposed on a slow wave during each burst. Intracellular studies have been limited to one investigation of the activity of the largest cells in the ganglion, the large pigmented unipolars. These cells fire a train of attenuated spike potentials’)* superimposed on a slow depolarization, similar to that found in follower cells of crustacean cardiac ganglia (Palese, Becker & Pax, 1970).
In their exhaustive morphological study of the ganglion Bursey & Pax (1970) reported that there were five types of neurones present: large unipolars, bipolars and multipolars and small bipolars and multipolars. Only the large unipolars have been studied electrophysiologically and these have been shown to be follower cells (Palese et al. 1970). A study of the remaining morphological types of neurones has therefore been undertaken in an attempt to assess their role in ganglionic activity.
