Neural Control of Heartbeat and Respiration in Crustaceans and Insects

Crustaceans and insects, having evolved from a common ancestor, developed distinct respiratory and circulatory systems as they adapted to various habitats throughout their evolutionary history. Crustaceans, which breathe through gills, use their circulatory system to supply oxygen to the entire body, similar to us vertebrates. In contrast, insects, having evolved the tracheal system, have freed their circulatory system from the role of oxygen transport, allowing it to regulate hemolymph pressure and distribute body heat. This post describes how the rhythmic muscular movements involved in respiration and circulation are produced, controlled, and coordinated in these two groups of animals.

Figure 1. Hearts of Three Crustaceans and a Cockroach

 

 

Crustacean Heartbeat Rhythm

Crustaceans have diverse cardiovascular systems (Figure 1). Their hearts contain the cardiac ganglion, which is located on the dorsal side and consists of a small number of neurons that function as the pacemaker, electrically exciting the heart at a regular rhythm (Figure 2). The electrical excitation spreads throughout the heart via axons of the cardiac ganglion neurons and excites the myocardium via neuromuscular synapses (Figure 3), producing myocardial contraction.

Figure 2. Cardiac Ganglia in a Decapod, an Isopod, and a Cockroach

 

CG: cardiac ganglion, CGC: cardiac ganglion cell, LC: large cell, O: ostia, SC: small cell. (Modified from Refs. [1,2,5])

 

 

Figure 3, Myocardial Membrane Potential Activities in Crayfish and Fruit Fly

 

Crustacean myocardial activity (upper trace), driven by the cardiac ganglion, exhibits a steep-rising waveform. In contrast, the myocardium of insects (lower trace) displays a slow, sine wave-like activity.

 

    The number of neurons in the cardiac ganglion varies among species, with one cell in ostracods and 16 cells in crayfish [2,3]. Branchiopods, such as tadpole shrimp and brine shrimp, do not have cardiac ganglion, and their myocardia excite spontaneously [4]. In decapods, the cardiac ganglion neurons are differentiated into small and large cells, with the small cells acting as the rhythm generator and the large cells serving as motoneurons that innervate the myocardium [1,2]. In stomatopods and isopods, all the cardiac ganglion neurons have endogenous oscillatory properties and are electrically connected to each other by gap junctions, functioning as if they were one giant cell [2].

    The pacemaker function of cardiac ganglion cells is attributed to ion channels in the cell membrane, which allow Na+ and K+ ions to pass through and are slowly activated and deactivated in response to changes in membrane potential, causing them to oscillate periodically. In addition to these channels, fast voltage-gated cation channels that open and close in sequence produce rhythmic bursts of action potentials [2].

 

Crustacean Respiratory Rhythm

Oxygen is taken up into the hemolymph through the gills. In species where part of the swimmerets acts as gills, such as tadpole shrimp and mantis shrimp, the rhythmic back-and-forth movement of the swimmerets facilitates respiration. In decapods, the gills are located in the gill chambers of the thorax, and the scaphognathites at the anterior openings of the gill chambers draw water into the gills with a flapping, fan-like motion. The rhythmic movements of the swimmerets and the scaphognathites are produced by central pattern generator circuits in the ganglia of the ventral nerve cord [1].

Insect Heartbeat Rhythm 

A major difference between the hearts of insects and crustaceans is that insect hearts beat through spontaneous electrical excitation of the myocardium (Figure 3), whereas crustacean hearts use the cardiac ganglion as the pacemaker. Insects have nerve networks near the heart (Figure 2) that serve as regulatory elements, controlling the frequency and direction of beating rather than generating rhythmic beats like the cardiac ganglia of crustaceans [5].

    Another major characteristic of insect hearts is the heartbeat reversal, where the direction of the heartbeat reverses back and forth. The heartbeat reversal changes the pressure in the body cavity at the front and back of the body separately and promotes gas convection in the tracheae. It also helps warm up the flight muscles and disperse the heat generated by flight movement, thus regulating body temperature. The constriction between the thorax and abdomen enhances the efficiency of pressure differences and thermoregulation. Additionally, the heartbeat reversal aids in localized body expansion during molting and hatching [5].

    Auxiliary hearts are present at the bases of protrusions such as antennae, proboscis, wings, and legs. These specialized compartments of blood vessels supply hemolymph to the extremities of the body. The auxiliary hearts beat through the movement of muscles attached to them and the body wall muscles [5].

 

Insect Respiratory Rhythm 

In insects, oxygen is delivered directly to tissues throughout the body via the tracheae. Gas diffusion and convection in the tracheae are driven by rhythmic contractions of the body wall and flight muscles and by pressure changes in body cavities caused by the heartbeat reversal. The tracheal openings (spiracles) are equipped with tracheal valves and associated muscles that control their closure, which are centrally regulated by motor nerves [6]. The opening and closing of the tracheal valves occur cyclically. When oxygen concentration in the tracheae decreases, the valves open and close repeatedly, resembling wing flapping, and remain open for tens of seconds to several minutes when carbon dioxide accumulates. This intermittent opening of the tracheal valves is believed to help reduce water loss associated with gas exchange.

Coordination of Respiration and Circulation 

In crustaceans, respiratory and cardiac rhythms are generated by separate neural circuits but are regulated by the central nervous system. Both respiration and heartbeat are abruptly halted by the central nervous system when danger is detected. This response is a defensive strategy to avoid detection by predators, such as sharks, that are sensitive to myoelectric potentials underwater. The localization of the pacemaker in the cardiac ganglion facilitates this rapid control of the heartbeat [1].

    In insects, tracheal respiration involves two types of centrally controlled rhythms: skeletal muscle contraction and the opening and closing of spiracles. The heartbeat reversal is centrally regulated and closely coordinated with tracheal respiration. This complex interplay between respiration and heart rate likely contributes to insects' ability to adapt and thrive in harsh environments and supports their advanced locomotor capabilities. The diversity of respiratory and circulatory rhythms reflects the evolutionary history of insects and crustaceans.

 

 

References 

[1] J. L. Wilkens, Amer. Zool., 39, 199-214 (1999).

[2] I. M. Cooke, Biol. Bull., 202, 108-136 (2002).

[3] Y. Ando et al., Zool. Sci., 18, 651-658 (2001) 

[4] H. Yamagishi et al., Biol. Bull., 193, 350-358 (1997).

[5] T. A. Miller, Gen. Pharmacol.: The Vascular System, 29, 23-38 (1997).

[6] M. J. Klowden, Physiological systems in insects, (Elsevier Science & Technology, Oxford, England, 2013).