Animal-to-animal variability in motor pattern production in adults and during growth

Abstract

Which features of network output are well preserved during growth of the nervous system and across different preparations of the same size? To address this issue, we characterized the pyloric rhythms generated by the stomatogastric nervous systems of 99 adult and 12 juvenile lobsters (Homarus americanus). Anatomical studies of single pyloric network neurons and of the whole stomatogastric ganglion (STG) showed that the STG and its neurons grow considerably from juvenile to adult. Despite these changes in size, intracellularly recorded membrane potential waveforms of pyloric network neurons and the phase relationships in the pyloric rhythm were very similar between juvenile and adult preparations. Across adult preparations, the cycle period and number of spikes per burst were not tightly maintained, but the mean phase relationships were independent of the period of the rhythm and relatively tightly maintained across preparations. We interpret this as evidence for homeostatic regulation of network activity.

Figure 1.

Growth of the stomatogastric ganglion in the lobster, H. americanus. A, Posterized photograph of a juvenile and an adult animal, representing the sizes used for this study. B, Volume rendering from confocal scans of the autofluorescence of a juvenile and an adult STG. C, Surface reconstructions of the same ganglia. D, Morphometric values obtained from segmented images of juvenile and adult STGs. All values were significantly larger in adults (unpaired t tests).

Figure 2.

Growth of the stomatogastric ganglion. Maximum intensity projections of confocal images obtained from PD neurons filled with Alexa 568 hydrazide in two juvenile and two adult STGs. All projections are shown at the same scale (see scale bar in bottom right corner). Note the variability of gross morphological features.

Figure 3.

Simultaneous intracellular soma recordings of PD, LP, and PY in the juvenile and the adult. Calibration: 10 mV, 1 s. The cell body membrane of STG neurons does not produce regenerative activity, and spikes are therefore considerably attenuated in soma recordings.

Figure 4.

Growth and spike conduction in the peripheral nerves of the stomatogastric nervous system.A, Schematic outlines of the peripheral nerves between STG and extracellular recording sites. Relative scaling between different nerves and between juvenile and adult represents means from measurements taken in 10 juveniles and 7 adults. B, Averaged traces of intracellular recordings of a PD neuron and extracellular pdn recordings in a juvenile and an adult. The intracellularly recorded peak of the spike was used as a trigger. Note the increase in conduction delay between soma and nerve. C, Conduction delays measured from intracellular to extracellular spikes. Indicated are numbers of measurements for the specific cell types recorded in the 10 juvenile and 7 adult STGs (unpaired t tests). D, Conduction velocities calculated from distance/delay (unpaired t tests).

Figure 5.

Quantification of the triphasic pyloric rhythm in adult preparations. A, Nerve recordings in a relatively slow rhythm. Indicated are the latency measurements taken from the onset of each PD neuron burst. B, Nerve recordings in a relatively fast rhythm. The same measurements as in A are indicated. The small amplitude signals in the pyn recording (arrow) are the action potentials of the two LPG neurons. C, The mean latencies plotted as a function of the mean cycle period from 99 adult preparations. D, The mean phase values plotted as a function of the mean cycle period. The histogram at the top of the figure shows the distribution of mean cycle periods from the 99 adult animals. The histograms at the right of the figure show the distributions of the mean phase values. Lines in B and C are linear regression fits.

Figure 6.

Comparison of the pyloric rhythm in juvenile and adult preparations. A, Nerve recordings in a juvenile preparation. Indicated are the same measurements as in Figure 5. B, Comparison of the mean cycle periods in 12 juvenile and 99 adult preparations. Unpaired t test. C, Plot of the mean phase relationships in juvenile and adult preparations. PD off: p = 0.19; LP on: p < 0.05; LP off: p < 0.001; PY on: p < 0.001; PY off: p = 0.78 (unpaired t tests). D, Distributions of the phase values that were significantly different in juvenile and adult preparations, as indicated in C. Note that all juvenile values lie within the ranges of adult preparations (bin size: 0.02). Overlap of juvenile and adult values is indicated by hatched pattern.

Figure 7.

Phase relationships within preparations, quantified from cycle-to-cycle period variability. A, Phase relationships in three different preparations with similar mean cycle periods and mean phase values. B, Distributions of slope values of phase over cycle period from 99 adult and 12 juvenile preparations (bin size: 0.05). Nonsignificant slopes are shown in light shading.

Figure 8.

Mean number of spikes per burst and mean spike frequency within bursts from 99 adult and 12 juvenile preparations.