Neural activation of the heart of the lobster Homarus americanus.

INTRODUCTION The heart-ganglion preparation of the lobster is an independent nerve-effector system. It can be isolated from the rest of the animal and maintained for many hours, actively contracting, in a cooled saline bath. It is therefore an ideal system for studying the functional organization of a relatively simple organ a pump that is governed by a part of the nervous system composed of only a few units. The single-chambered heart, made up of striated muscle fibres ranging from 11 to 39 lira, in diameter, is enclosed within the pericardial cavity located just below the dorsal carapace of the animal (see Anderson & Smith, 1971, for details of fine structure). It is suspended from epimeral plates by elastic ligaments. Haemolymph enters the heart through two pairs of ostia and is driven out of the heart via anterior and posterior vessels. The lobster heart is neurogenic (Alexandrowicz, 1932). It contracts only in response to activity produced by nerve cells in the Y-shaped cardiac ganglion that lies on its inner dorsal wall. The ganglion contains five large follower cells which provide efferent motor output to the muscle fibres and four small pacemaker cells which provide, via synaptic contacts, input to the large cells. The electrophysiology of the lobster cardiac ganglion has been studied in detail (for reviews of early work in both Homarus and Panulirus see Maynard, i960; Hagiwara, 1961; and Bullock & Horridge, 1965; recent references: Cooke, 1966; Hartline, 1967; Connor, 1969; Hartline & Cooke, 1969; Mayeri, 1969; Rao et al. 1969; Livengood & Kusano, 1970). The small cells time the output of the events produced by the large cells (Hartline, 1967). The axons of the large cells branch profusely and innervate the muscle fibres of the heart (Alexandrowicz, 1932). They produce high-frequency bursts of impulses at regular intervals and each burst is associated with a contraction of the heart (Welsh & Maynard, 1951; Maynard, 1955). A further step in understanding the organization of the heart-ganglion system is to find out how the information processed in the ganglion is transmitted to the muscle fibres of the heart. In this paper the nerve-muscle physiology of the heart of the lobster Homarus americanus is considered. Glass microelectrodes were used to record intracellular activity from single muscle fibres. Events were recorded when the heart-