Vesicle shape transformations driven by confined active filaments.

In active matter systems, deformable boundaries provide a mechanism to organize internal active stresses. To study a minimal model of such a system, we perform particle-based simulations of an elastic vesicle containing a collection of polar active filaments. The interplay between the active stress organization due to interparticle interactions and that due to the deformability of the confinement leads to a variety of filament spatiotemporal organizations that have not been observed in bulk systems or under rigid confinement, including highly-aligned rings and caps. In turn, these filament assemblies drive dramatic and tunable transformations of the vesicle shape and its dynamics. We present simple scaling models that reveal the mechanisms underlying these emergent behaviors and yield design principles for engineering active materials with targeted shape dynamics.

Topological structure and dynamics of three-dimensional active nematics.

Topological structures are effective descriptors of the nonequilibrium dynamics of diverse many-body systems. For example, motile, point-like topological defects capture the salient features of two-dimensional active liquid crystals composed of energy-consuming anisotropic units. We dispersed force-generating microtubule bundles in a passive colloidal liquid crystal to form a three-dimensional active nematic. Light-sheet microscopy revealed the temporal evolution of the millimeter-scale structure of these active nematics with single-bundle resolution. The primary topological excitations are extended, charge-neutral disclination loops that undergo complex dynamics and recombination events. Our work suggests a framework for analyzing the nonequilibrium dynamics of bulk anisotropic systems as diverse as driven complex fluids, active metamaterials, biological tissues, and collections of robots or organisms.

Pathway-specific expression of PKC and PKA in sympathetic neurons.

We used multiple-labelling immunofluorescence, intracellular dye injection, electrophysiological recording and confocal microscopy to examine the expression of immunoreactivity to protein kinase C (PKC) and protein kinase A (PKA) in sympathetic ganglia of guinea-pigs. PKCalpha and PKCgamma were widespread in vasoconstrictor and pilomotor neurons. High levels of PKA RIIalpha and RIIbeta were restricted to neurons that lacked significant expression of PKC, including somatostatin-containing neurons projecting to the gut, and non-noradrenergic vasodilator neurons. In coeliac ganglia, most neurons with PKC contained neuropeptide Y and displayed phasic patterns of action potential firing, often with a long after-hyperpolarization. Tonically firing neurons lacked both neuropeptide Y and PKC. These results show remarkably pathway-specific expression of protein kinases in functionally identified populations of sympathetic neurons.

Synaptic organisation of lumbar sympathetic ganglia of guinea pigs: serial section ultrastructural analysis of dye-filled sympathetic final motor neurons.

The authors serially sectioned seven dye-filled neuronal somata and more than 1.6 mm of their dendrites from the lumbar sympathetic ganglia of guinea pigs and examined them ultrastructurally to determine the distribution of preganglionic synaptic inputs to their dendrites and cell bodies. Most of the surface of the neurons was covered with Schwann cells. Apposing boutons were rare, with an average density of one axosomatic bouton per 125 microm2 of somatic membrane and one axodendritic bouton per 25 microm of dendrite. Many dendritic segments that were more than 50 microm long completely lacked any apposing boutons. Although the average density of apposing boutons was low, local densities could be high, so that clusters of up to four adjacent boutons occurred on cell bodies and dendrites alike. The spatial arrangement of the apposing boutons for each of the cells examined here was not significantly different from a random distribution. Consequently, the number of apposing boutons observed for any neuron was simply proportional to the amount of neuronal surface sampled in the serial section run. About 50% of boutons directly apposing the neurons lacked any detectable presynaptic specialisations. When they were present, the presynaptic densities had a mean length of about 220 nm, with no difference between boutons that made axosomatic or axodendritic appositions. By applying these data to complete reconstructions of the dendritic trees of dye-filled sympathetic neurons at the light microscopic level, the authors estimated that few neurons in the lumbar sympathetic chain of guinea pigs would receive more than 200 synapses or apposing boutons and that many of them would receive less than 100 synapses. Up to 50% of these boutons would be predicted to make axosomatic contacts. These new observations provide a strong morphological framework for a better understanding of how sympathetic final motor neurons process their preganglionic synaptic inputs.

Synaptic organisation of neuropeptide-containing preganglionic boutons in lumbar sympathetic ganglia of guinea pigs.

Within the lumbar sympathetic ganglia of guinea pigs, the endings of different populations of neuropeptide-containing preganglionic neurons form well-defined pericellular baskets of boutons around target neurons in specific functional pathways. We have used multiple-labelling immunofluorescence, confocal microscopy, and ultrastructural immunocytochemistry to investigate synaptic organisation within pericellular baskets labelled for immunoreactivity to calcitonin gene-related peptide (CGRP), substance P (SP), or the pro-enkephalin-derived peptide, met-enkephalin-arg-gly-leu (MERGL) in relation to their target neurons. Different functional populations of neurons, identified by their neurochemical profile, showed a significant degree of spatial clustering and predicted well the distribution of specific classes of pericellular baskets. Most of the boutons in a basket were completely surrounded by Schwann cell processes and did not form synapses. The synapses that were present were made mostly onto dendrites enclosed by the Schwann cell sheath surrounding the neuron within the basket. These dendrites probably originated from neurochemically similar neighbouring neurons. Nevertheless, some of the boutons in the baskets did form synapses with the cell body or proximal dendrites of the neuron they surrounded. Occasionally, cell bodies received a relatively high number of synapses and close appositions from boutons in a pericellular basket. Synaptic convergence of two immunohistochemically distinct types of preganglionic inputs was found in baskets of SP-immunoreactive or MERGL-immunoreactive, but not CGRP-immunoreactive, boutons. Taken together, our results show that the appearance of pericellular baskets is primarily due to the packing of the target neurons. The grouping of functionally similar classes of neurons with their pathway-specific projections of peptide-containing preganglionic neurons suggests that peptides could exert their effects in relatively well-defined zones within the ganglia.

Sympathetic vasoconstrictor neurons projecting from the guinea-pig superior cervical ganglion to cutaneous or skeletal muscle vascular beds can be distinguished by soma size.

We have used a combination of retrograde axonal tracing and intracellular dye injections to determine the soma size of sympathetic vasoconstrictor neurons projecting from the superior cervical ganglion to the cutaneous vascular bed of the eartips, or to the vascular beds of the masseter muscle, of guinea-pigs. Neurons projecting to vasculature of the masseter muscle had a cross-sectional area of 956 +/- 295 microns2 (mean +/- SD; n = 45 cells) and were significantly larger than neurons projecting to the vasculature of the eartip skin (mean cross-sectional area +/- SD, 604 +/- 251 microns2; n = 39 cells). These results are consistent with physiological observations showing that muscle vasoconstrictor neurons have faster conduction velocities than cutaneous vasoconstrictor neurons. Furthermore, they suggest that muscle vasoconstrictor neurons may innervate a larger volume of vasculature compared with cutaneous vasoconstrictor neurons.

Dendritic morphology of presumptive vasoconstrictor and pilomotor neurons and their relations with neuropeptide-containing preganglionic fibres in lumbar sympathetic ganglia of guinea-pigs.

We have used intracellular dye-filling combined with multiple-labelling immunofluorescence to examine the dendritic morphology of neurons and their relations with neuropeptide-containing preganglionic terminals in the lumbar sympathetic chain of guinea-pigs. Presumptive vasoconstrictor neurons with immunoreactivity for both tyrosine hydroxylase and neuropeptide Y dendritic fields that were significantly smaller, on average, than those of presumptive pilomotor neurons containing immunoreactivity to tyrosine hydroxylase but not to neuropeptide Y. However, there was considerable variation in the sizes of the dendritic fields of the vasoconstrictor neurons. Preganglionic nerve terminals containing immunoreactivity to calcitonin gene-related peptide, but not to substance P, only surrounded cell bodies of vasoconstrictor neurons containing immunoreactivity to tyrosine hydroxylase and neuropeptide Y. In most cases, the neuropeptide-containing preganglionic terminals were not associated closely with the distal dendrites of these neurons. Few neuropeptide-containing terminals were associated closely with either the cell bodies or dendrites of the pilomotor neurons. These results show that there is a considerable range in the size of dendritic trees of sympathetic final motor neurons. Some of this variation is related to the pathways within which the neurons lie, so that presumptive pilomotor neurons generally are larger than presumptive vasoconstrictor neurons. The marked variation in size of vasoconstrictor neurons raises the possibility that there may be a size dependent recruitment of these neurons, similar to that seen in pools of spinal motor neurons. The distribution of the peptide-containing preganglionic endings suggests that they would act predominantly at the cell body and proximal dendrites of the final motor neurons.