People with spinal cord injury often have circulatory disturbances below the lesion, particularly in the skin, ranging from poor temperature control to prolonged wound healing. We have examined the bundles of unmyelinated axons projecting in the sciatic nerve in rats in which the spinal cord was transected 8 weeks previously at T4 under anaesthesia with ketamine (60 mg/kg) and xylazine (10 mg/kg), i.p. Transection at this level leaves the pre- and postganglionic neurones supplying vascular effectors in the distal hindlimb disconnected from the brain stem but undamaged. Animals were maintained postoperatively for 8 weeks and then perfused with fixative under deep anaesthesia with pentobarbitone (100 mg/kg i.p.). Electron micrographs were taken of bundles of unmyelinated axons in thin transverse sections of the sciatic nerve cut from thick (100 μm) sections after pre-embedding immunohistochemical processing to demonstrate the presence of tyrosine hydroxylase (TH). The number and dimensions of all unmyelinated axons and their expression of TH were determined in bundles sampled across all parts of the sciatic nerve. The number of unmyelinated axons per area of nerve increased by 19% after spinal cord injury but their mean diameter fell from 0.57±0.01 to 0.51±0.01 μm. There was a slight increase in the number of bundles containing single axons. The percentage of axons that were TH+ was not significantly changed. These data suggest that neurones with unmyelinated axons atrophy after spinal cord transection, and that some of them may sprout within the nerve trunk, possibly reflecting the endoneurial release of neurotrophins. The loss of ongoing activity in sympathetic axons after the injury may modify the properties of both axons and Schwann cells within peripheral nerves. Supported by the NSW Spinal Cord Injury & Related Neurological Conditions Grants Program.

Endosomal trafficking is regulated by the recruitment of effector proteins to phosphatidylinositol 3-phosphate [PtdIns(3)P] on early endosomes. At the plasma membrane, phosphatidylinositol-(3,4)-bisphosphate [PtdIns(3,4)P2] binds the pleckstrin homology (PH) domain-containing proteins Akt and TAPP1. Type Ialpha inositol polyphosphate 4-phosphatase (4-phosphatase) dephosphorylates PtdIns(3,4)P2, forming PtdIns(3)P, but its subcellular localization is unknown. We report here in quiescent cells, the 4-phosphatase colocalized with early and recycling endosomes. On growth factor stimulation, 4-phosphatase endosomal localization persisted, but in addition the 4-phosphatase localized at the plasma membrane. Overexpression of the 4-phosphatase in serum-stimulated cells increased cellular PtdIns(3)P levels and prevented wortmannin-induced endosomal dilatation. Furthermore, mouse embryonic fibroblasts from homozygous Weeble mice, which have a mutation in the type I 4-phosphatase, exhibited dilated early endosomes. 4-Phosphatase translocation to the plasma membrane upon growth factor stimulation inhibited the recruitment of the TAPP1 PH domain. The 4-phosphatase contains C2 domains, which bound PtdIns(3,4)P2, and C2-domain-deletion mutants lost PtdIns(3,4)P2 4-phosphatase activity, did not localize to endosomes or inhibit TAPP1 PH domain membrane recruitment. The 4-phosphatase therefore both generates and terminates phosphoinositide 3-kinase signals at distinct subcellular locations.

Parkington, H.; Edgley, A.; Dodd, J.; Luff, S.; Worthy, K.; Coleman, H.; Tare, M.; Anderson, W. Author Information

This ultrastructural study has investigated the development of the innervation of second order mesenteric arteries from the ileum region of the rat intestine, particularly, the time course of the formation of the plexus of varicose axons around the arteries, and the formation of autonomic neuromuscular junctions. The time points studied were postnatal days-2, -4, -8 and -13. This study has revealed that the formation of neuromuscular junctions with mature structural characteristics occurred at ~2 weeks postnatal. The plexus of varicose axons developed predominantly between day-4 and day-13, which agrees with previous light microscopy studies of catecholamne containing nerves around similar vessels. At day-2 and day-4, the axons lacked varicosities and were mainly contained in large bundles located in the outer region of the adventitia. The medio-adventitial border consisted of a dense layer of extracellular matrix and fibroblasts. By day-8, there were more axons and most were distributed in smaller bundles. Some had grown through the adventitia to lie at the medio-adventitial border and axon varicosities were also observed. Some varicosities had formed rudimentary neuromuscular contacts. By day-13, there were significantly more contacting varicosities compared to day-8. They were structurally more mature, being twice the size with three times the number of synaptic vesicles and consistently contained a mitochondrion. Conversely, the neuromuscular contact areas were similar at both time points. Some organisation of the synaptic vesicles associated with the prejunctional membrane, was evident in varicosities at day-8 but there were no presynaptic membrane specialisations similar to the putative neurotransmitter release sites found at mature skeletal neuromuscular junctions. The aggregation of small vesicles at the prejunctional membrane was more pronounced in neuromuscular junctions at day-13 with some having presynaptic membrane specialisations. Comparison of the structure of developing autonomic neuromuscular junctions with that of skeletal neuromuscular junctions has revealed a number of similarities.

This review focuses on the more recent findings of the structure of sympathetic postganglionic axons and the association of their varicose terminals with vascular smooth muscle. These studies have investigated the innervation of a wide range of vessels from different regions of the vasculature in the rat, guinea pig and rabbit and have predominantly used serial sections and computerised three-dimensional reconstructions of entire varicosities. They have shown, contrary to previous studies conducted in the 1960s and 1970s, that sympathetic axon varicosities commonly form structurally specialised neuromuscular junctions with vascular smooth muscle cells of most resistance arteries and some small veins. In addition, they have shown that most axon varicosities innervating small arterioles and small mesenteric veins form neuromuscular junctions, indicating that neurotransmitter is primarily released at such neuromuscular junctions. This review discusses the structure of sympathetic neuromuscular junctions, their development, structural diversity and distribution on vessels from different regions of the vasculature. These more recent structural findings and their possible significance for our understanding of mechanisms involved in neural transmission in blood vessels is discussed.

The aim of this review is to outline the main ultrastructural features of the artery wall and the particular characteristics of the arterioles. The discussion will focus on more recent research developments and, in particular, the structural aspects of the vascular innervation. For further detailed reading, I refer you to the following classic papers and reviews.1-14

In fluorescence histochemical studies of the sympathetic innervation of the arterial vessels of the submucosa of the guinea pig ileum, we have identified clusters of varicosities overlying arteriolar branch points. These were particularly obvious on the small arterioles < 45 μm in diameter, the non-branching regions of which generally lack much other innervation. Serial reconstruction from electron micrographs of axon bundles from the region of arteriolar branch points revealed the form of the varicose axons. Branching of axon bundles sometimes involved branching of individual axons. Within a cluster, most axons had several varicosities along lengths as short as 3 μm. The varicosities were very irregular in size and shape. Most large varicosities (>1 μm in diameter) that were bare of Schwann cell covering (73%) formed neuromuscular junctions with basal lamina intervening between axon and muscle membranes. A smaller proportion (44%) of small varicosities (0.5–1.0 μm diameter) also formed junctions. Of all bare varicosities, 37% did not form contracts. In several of the large varicosities, prejunctional membrane specializations were identified over part of the neuromuscular junctions.