Cardiac ganglion of limulus: intracellular activity in the unipolar cells.

The spontaneous rhythmic beating of the heart of the horseshoe crab, Limulus polyphemus, has its origin in a cardiac ganglion located on the external dorsal surface of the heart (Carlson, 1904; 1909). The heart in Limulus is a long tubular structure divided into nine unequal segments by eight pairs of ostia. The cardiac ganglion is a cord-like structure extending almost the entire length of the heart. The microscopic anatomy of the cardiac ganglion has been recently described (Bursey & Pax, 1970). The ganglion consists of two parts, a fibre-tract portion and a cellular portion. The fibre tract extends the entire length of the ganglion, but the cellular portion reaches only as far forward as the anterior of the third heart segment. Within the cellular portion of the ganglion four different cell types can be recognized: large pigmented unipolar cells, large pigmented bipolar cells, pigmented multipolar cells and non-pigmented multipolar cells. A number of investigators have made an effort to determine the site within the cardiac ganglion where electrical activity originates, that is, the site of the pacemaker. Carlson (1905) and Edwards (1920), noting that activity generally begins first in the middle third of the heart, concluded that the pacemaker for the heart is located in this region. Garrey (1930, 1932) and Bullock, Burr & Nims (1943), however, demonstrated that the site at which activity first appears within the ganglion at each heartbeat can vary. A number of procedures, notably local heating or cooling, may shift the pacemaker as far anterior as the anterior end of the third segment or as far posterior as the eighth segment. On the basis of sectioning experiments and subsequent histological studies Heinbecker (1936) concluded that the large unipolar cells are probably the pacemakers in the ganglion. We report here the results of experiments we have performed in an effort to define more clearly the role of these large unipolar cells in the production of the spontaneous rhythm seen in the cardiac ganglion of Limulus.