Remodelling neurons during metamorphosis

The lifestyle and behaviour of the adult moth are quite different from those of the larva, and the two stages have completely different locomotory and sensory systems. The larva moves by crawling movements involving stubby pro-legs on the abdomen and thorax, and these legs and their muscles disappear during the pupal stage. Almost all of the larval muscles die during metamorphosis, and are replaced by new muscles that move adult structures such as wings and thoracic legs. The adult has more sophisticated sense organs than the larva, such as compound eyes and antennae. Rather than being dismantled and rebuilt, the central nervous system is extensively remodelled. A few new neurons are born and some die, but many are remodelled and incorporated into new circuits.

One neuron that is remodelled is MN1, a motor neuron found in each abdominal ganglion (Levine & Truman, 1982). In the larva, the muscle it controls bends the abdomen sideways. This muscle dies during metamorphosis, and MN1 comes to innervate a new muscle that bends the abdomen upwards. This means that the left and right MN1 neurons are often used in opposition to each other in the larva, but are used together in the adult. Associated with the change in target muscle are changes in the morphology of MN1 and its connections with other neurons (Fig. 9.2). In the larva, dendrites of the neuron are nearly all on the same side of the ganglion as its axon, but it sprouts new dendrites during the pupal stage, and has a separate dendritic region on each side of the ganglion in the adult.

In the larva and in the adult, MN1 is excited by a stretch receptor which responds whenever the muscle controlled by MN1 is stretched. This stretch receptor probably makes a direct, excitatory connection with MN1 (the unbroken arrow in Fig. 9.2) because each sensory spike evokes an EPSP in MN1 after a short delay. Therefore, there is a short feedback pathway that excites MN1 whenever its muscle is stretched. The stretch receptor on the other side of the segment also participates in a circuit with MN1, but this

Figure 9.2 Changes in the morphology and central connections of an identified motor neuron, MN1, in Manduca. The MN1 drawn innervated a muscle on the left side of an abdominal segment but its cell body is on the right side of the ganglion, which is a relatively unusual arrangement in an insect. Vertical dotted lines indicate the midline of the ganglion. Arrows beneath each diagram indicate connections made by identified stretch receptor cells on the left and right sides of the animal. (Drawings of larva, pupa and adult modified after Levine & Weeks, 1990; MN1 modified after Truman & Reiss, 1988; reproduced with permission; copyright Society for Neuroscience.)

Figure 9.2 Changes in the morphology and central connections of an identified motor neuron, MN1, in Manduca. The MN1 drawn innervated a muscle on the left side of an abdominal segment but its cell body is on the right side of the ganglion, which is a relatively unusual arrangement in an insect. Vertical dotted lines indicate the midline of the ganglion. Arrows beneath each diagram indicate connections made by identified stretch receptor cells on the left and right sides of the animal. (Drawings of larva, pupa and adult modified after Levine & Weeks, 1990; MN1 modified after Truman & Reiss, 1988; reproduced with permission; copyright Society for Neuroscience.)

circuit alters during metamorphosis. In the larva, a spike in the right stretch receptor causes an IPSP in the left MN1. The delay between the spike and the IPSP is longer than the delay for the EPSP, so the circuit probably includes an interneuron, and is indicated by a broken arrow in Fig. 9.2. The different inputs from the two receptors help to ensure that the larva tends to straighten its abdomen if it becomes bent, and that the left and right MN1 neurons act in opposing ways.

During metamorphosis, the stretch receptor continues to make connections with the left and right MN1 motor neurons. It continues to excite the MN1 on the same side of the segment, but its inhibitory connection with the opposite MN1 becomes weaker and is then replaced with a direct excitatory connection. What is almost certainly occurring is that the new den-drites of the motor neuron come into contact with the right stretch receptor, and excitatory synapses form between the two neurons. In the adult, therefore, the left and right stretch receptors are excited whenever the abdomen is bent downwards, and act in a new proprioceptive reflex that excites the left and right MN1s. Up and down movements of the abdomen occur during flight-steering manoeuvres, during mating and during egg laying.

The growth of new dendrites of MN1 occurs at the same time as a rapid rise in the level of ecdysone within the body during the pupal stage (Truman & Reiss, 1988). Experiments on diapausing pupa, in which the emergence of the adult can be delayed for months in response to environmental stress, have shown that it is ecdysone that causes the motor neuron to grow new dendrites. In diapausing pupae, the concentration of ecdysone is unusually low, and MN1 does not grow new dendrites. Injecting ecdysone into a diapausing pupa causes MN1 to sprout its new dendrites.

The effect that ecdysone has on MN1 is regulated by another hormone, a turpenoid called juvenile hormone. If juvenile hormone is present when ecdysone levels rise, MN1 retains its larval characteristics. The same combination of rising levels of ecdysone and low levels of juvenile hormone has widespread effects on the central nervous system and elsewhere in the body (Streichert & Weeks, 1995; Levine, Morton & Restifo, 1995). It causes many of the larval muscles and some of the motor neurons to die, as well as triggering the development of new dendrites in motor neurons that survive, such as MN1. However, not all of the events involved in remodelling the nervous system are triggered by rising ecdysone levels. For example, the inhibitory connection from the right abdominal stretch receptor to the left MN1 disappears before the rise in ecdysone, and its cause is not yet known.

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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