Ecdysis in a hawk moth

Towards the end of each developmental stage, the larva grows a new, soft exoskeleton underneath the old one, and the behaviour of ecdysis loosens the new skin from the old and then allows the old skin to be shrugged off. Ecdysis lasts for about six hours, and it provides an excellent example of how particular neurohormones can orchestrate a co-ordinated behaviour at a particular time during an animal's life history. The activity of a small number of neurons commits the whole animal to ecdysis.

Manduca has five larval stages, each one almost identical in form to the last but slightly larger, until it becomes a pupa. This final larval ecdysis is the best understood. The first event is inflation of tracheal air sacs, particularly in the head, with air. Next, during pre-ecdysis, muscles contract and then relax synchronously in all segments of the abdomen once every 5-10 s (Fig. 9.1a; Copenhaver & Truman, 1982; Miles & Weeks, 1991). At the same time, abdominal appendages called pro-legs are retracted and re-extended. Pre-ecdysis loosens the old cuticle from the new and takes about half an hour. The rhythm for pre-ecdysis is generated by a network of inter-neurons in the last abdominal ganglion, and a single left-right pair of long

Figure 9.1 Ecdysis and its control in the hawk moth (Manduca sexta). (a) Representation of the patterns of activity that occur in pre-ecdysis and in ecdysis. Times when the same muscle in the third (A3) and fourth (A4) abdominal segments contract are shown. (b) A flow diagram to illustrate the actions of ecdysis-triggering hormone, released from Inka cells, and eclosion hormone, released from four brain cells. The pre-ecdysis central pattern generator (cpg) is activated by low concentrations of eclosion-triggering hormone, and the pre-ecdysis rhythm is communicated to segmental motor neurons (mn) by a pair of co-ordinating interneurons (int). Eclosion is switched on by crustacean cardioactive peptide, released from CCAP cells throughout the central nervous system, and this peptide also switches off pre-ecdysis.

Figure 9.1 Ecdysis and its control in the hawk moth (Manduca sexta). (a) Representation of the patterns of activity that occur in pre-ecdysis and in ecdysis. Times when the same muscle in the third (A3) and fourth (A4) abdominal segments contract are shown. (b) A flow diagram to illustrate the actions of ecdysis-triggering hormone, released from Inka cells, and eclosion hormone, released from four brain cells. The pre-ecdysis central pattern generator (cpg) is activated by low concentrations of eclosion-triggering hormone, and the pre-ecdysis rhythm is communicated to segmental motor neurons (mn) by a pair of co-ordinating interneurons (int). Eclosion is switched on by crustacean cardioactive peptide, released from CCAP cells throughout the central nervous system, and this peptide also switches off pre-ecdysis.

interneurons communicates the rhythm to other abdominal ganglia, ensuring that contractions occur synchronously in all segments (Novicki & Weeks, 1995). Ecdysis uses the same muscle as pre-ecdysis, but in a different motor pattern in which a wave of contraction progresses anteriorly from segment to segment (Fig. 9.1a; Weeks & Truman, 1984). The neurons responsible for generating the ecdysis motor program have not yet been located.

Two polypeptide hormones are responsible for triggering and orchestrating pre-ecdysis and ecdysis (Fig. 9.1b; Zitnan et al., 1996; Gammie & Truman, 1997). First, ecdysis-triggering hormone is released into the blood from cells called Inka cells which are situated near to the openings of the respiratory tracheal system to the outside. The pattern generator for pre-ecdysis is very sensitive to ecdysis-triggering hormone and is initiated as soon as the concentration of this hormone starts to rise. Another effect of ecdysis-triggering hormone is to excite four cells in the brain that release the second polypeptide, eclosion hormone. These cells were identified with a stain for the RNA for eclosion hormone. Their axons extend right down the nerve cord to the hind end of the animal and then run anteriorly in nerves that lie alongside the gut. Activation of these cells depends on levels in the blood of the steroid hormone ecdysone, the hormone that acts as a master switch for the genes that instruct the production of new body structures during metamorphosis. Eclosion hormone is released both within the central nervous system and into the blood, and the Inka cells are excited by it, so there is a feed-forward system between the Inka cells and the eclosion hormone-releasing brain cells. Once activated, this mutual excitation is unlikely to stop, ensuring the animal is fully committed to ecdysis. The Inka cells and the brain cells discharge all of their hormone within about half an hour.

In the central nervous system, the targets for eclosion hormone are a set of 50 nerve cells, scattered fairly uniformly among the segmental ganglia. The effect of eclosion hormone is to excite these cells, by way of an intracellular messenger pathway involving cyclic GMR and to cause them to release another peptide hormone (called crustacean cardioactive peptide). This peptide ensures the correct change in motor program by activating the pattern generator for ecdysis and at the same time switching off the pattern generator for pre-ecdysis. Ecdysis is sustained after the initial intense release of ecdysis-triggering and eclosion hormones because the increase in cyclic GMP is quite slow in the cells that release crustacean cardioactive peptide and they maintain a steady level of discharge. These cells are also excited by sensory cells that signal the continued presence of the old cuticle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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