Neuronal Activity

Neuronal activity is composed of individual action potentials that strongly depend on potential shifts across the cellular membranes. These can be recorded by single cell techniques or by means of summed action potentials.

Insects offer the possibility to study the development of individual neurons because they possess very large neurons with long axons and widely spread dendritic arborization. These features offer access to individual neurons even in the case of extracellular recordings. In vertebrates, this approach is limited, particularly in developing animals. In fact, the only example in which the development of an individual neuron, its physiological properties, and its importance for behavior were described was in the Mauthner cell in fish and in larval amphibians (Eaton et al. 2001, Hatta and Korn 1998). But this excellent model was, unfortunately, never considered for physiological adaptation studies in altered gravity at the cellular level.

The development of neuronal activity was determined after the spaceflight of two species, the house cricket Acheta domesticus and the clawed toad, Xenopus laevis. In both species, larval stages were exposed to microgravity and then returned to Earth to investigate their changes in neuronal activity. In both species, the neuronal activity was clearly related to development. Furthermore, in both species there was evidence that microgravity exposure caused transient but pronounced desensitization or developmental retardation after return to normal gravity.

Crickets possess a neuron that changes its activity in relation to the creature's posture (Sakaguchi and Murphey 1983). This neuron is called the Posture Sensitive Interneuron (PSI). In each developmental stage up to adulthood, there is only one PSI on each side of the nervous system. The cell body (or soma) of this neuron lies on the contralateral side with respect to its dendritic arborization, and its long axon ascends from the terminal ganglion towards the brain passing the thoracic ganglia ipsilateral to the location of the dendritic tree. The PSI receives its input from the cercal gravity receptors (see Figure 5-01).

This peculiar anatomy allows for recording its activity in an extracelluar manner at each stage. This activity is modulated by a 360-degree lateral roll tilt of the animal. The development of the mean maximal frequency modulation increases steadily between the 4th larval stage and the adult stage (Riewe 2000).

The effect of microgravity on the modulation of this activity in Acheta domesticus was investigated in two stages: one had reached the 4th stage, the other the 6th stage at the beginning of a 16-day spaceflight. Post flight recordings revealed a significant depression of the PSI's activity modulation. However, the modulation returned to normal baseline values about two weeks after the flight (Figure 5-11).

In Xenopus, the development of physiological activity at the synaptic level was performed in cultures of myocytes and embryonic neurons. This study revealed a lack of significant sensitivity to simulated microgravity at the level of ion ACh-channels (Reitstetter and Gruener 1994), despite significant maturation-related modifications in the morphology neuro-muscular synapses (Gruener and Hoeger 1990, Gruener et al. 1994) (Figure 5-06).

The development of neuronal activity was also analyzed for spinal motoneurons using the model of fictive swimming (Figure 5-12). Fictive swimming is a regular occurring rhythmic activity that can be recorded from the ventral roots during early embryonic and tadpole periods of life up to the hind limb bud stage, i.e., between stages 38 and 47 according to the standard atlas of development (Nieuwlcoop andFaber 1967).

In older tadpoles, fictive swimming disappears and is substituted by struggling activity, an irregular activity in contrast to the regular occurring burst activity during fictive swimming. During normal maturation of embryos to tadpoles the rostrocaudal delay, burst duration, and cycle length increased while episode duration decreased.

After a 10-day spaceflight, this rhythmic motor activity was considerably affected. The episodes of fictive swimming became longer, while the rostroeaudal delay was significantly depressed. Burst duration was slightly decreased. However, cycle length was not affected by development under microgravity compared to controls. For this period of life, these modifications can be defined as developmental retardation. The increase in episode duration corresponds to the increase in freely swimming duration after the flight. Normalization of fictive swimming occurred during the post flight days 3 to 6 (Böser 2003) (Figure 5-13).

Hypergravity exposure also modified this activity but in an age-related manner: young stages were sensitive after 10 days at a 3-g exposure, whereas older stages were insensitive (Böser and Horn 2006).

Figure 5-12. Swimming pattern is generated by a central oscillator. It is possible to observe the rhythmical, burst-like activity of motoneurons by extracellular recordings from ventral roots of the spinal cord in paralyzed animals. This figure shows the method used by the author for recording this fictive swimming in Xenopus laevis young tadpoles. An episode of fictive swimming induced by a mechanical stimulus is shown on the right. Three bursts from this episode, with the relevant parameters used for analysis (cycle length, burst duration, and rostrocaudal delay), are shown in the middle.

Figure 5-12. Swimming pattern is generated by a central oscillator. It is possible to observe the rhythmical, burst-like activity of motoneurons by extracellular recordings from ventral roots of the spinal cord in paralyzed animals. This figure shows the method used by the author for recording this fictive swimming in Xenopus laevis young tadpoles. An episode of fictive swimming induced by a mechanical stimulus is shown on the right. Three bursts from this episode, with the relevant parameters used for analysis (cycle length, burst duration, and rostrocaudal delay), are shown in the middle.

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