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PubMed2004

Scleral structure, organisation and disease. A review.

Although disease of the sclera is unusual, when it occurs it can rapidly destroy both the eye and vision. However, normally the sclera provides an opaque protective coat for the intraocular tissues and a stable support during variations in internal pressure and eye movements, which would otherwise perturb the visual process through distortion of the retina and the lens/iris diaphragm. This stability, which is vital for clear vision is made possible by the organisation and viscoelastic properties of scleral connective tissue. Microscopically, the sclera displays distinct concentric layers including, from outside, Tenon's capsule, episclera, the scleral stroma proper and lamina fusca, melding into underlying choroid. Two sites exhibit specialised structure and function: the perilimbal trabecular meshwork, through which aqueous filters into Schlemm's canal, and the lamina cribrosa, which permits axons of the optic nerve to exit the posterior sclera. Throughout, sclera is densely collagenous, the stroma consisting of fibrils with various diameters combining into either interlacing fibre bundles or defined lamellae in outer zones. Scleral fibrils are heterotypic structures made of collagen types I and III, with small amounts of types V and VI also present. Scleral elastic fibres are especially abundant in lamina fusca and trabecular meshwork. The interfibrillar matrix is occupied by small leucine-rich proteoglycans, decorin and biglycan, containing dermatan and dermatan/chondroitin sulphate glycosaminoglycans, together with the large proteoglycan, aggrecan, which also carries keratan sulphate sidechains. Decorin is closely associated with the collagen fibrils at specific binding sites situated close to the C-terminus of the collagen molecules. Proteoglycans influence hydration, solute diffusion and fluid movement through the sclera, both from the uvea and via the trabecular meshwork. As the sclera is avascular, nutrients come from the choroid and vascular plexi in Tenon's capsule and episclera, where there is an artery to artery anastomosis in which blood oscillates, rather than flows rapidly. This predisposes to the development of vasculitis causing a spectrum of inflammatory conditions of varying intensity which, in the most severe form, necrotising scleritis, may destroy all of the structural and cellular components of the sclera. Scleral cells become fibroblastic and the stroma is infiltrated with inflammatory cells dominated by macrophages and T-lymphocytes. This process resembles, and may be concurrent with, systemic disease affecting other connective tissues, particularly the synovial joints in rheumatoid arthritis. Current views support an autoimmune aetiology for scleritis. Whilst the role of immune complexes and the nature of initial pro-inflammatory antigen(s) remain unknown, the latter may reside in scleral tissue components which are released or modified by viral infection, injury or surgical trauma.

Aging↗
PubMed1996

Self-reinnervated cat medial gastrocnemius muscles. II. analysis of the mechanisms and significance of fiber type grouping in reinnervated muscles.

1. The technique of glycogen depletion was used to determine whether regenerating motor axons reestablish the normal regionalization of motor units (MUs) in the cat medial gastrocnemius (MG) muscle, 2) whether the extent of clumping between MU fibers and/or type grouping of muscle fibers progressively increases with a decrease in reinnervated MU numbers, and 3) whether the pattern of innervation can explain why MUs fail to increase significantly in size when the cut nerve is sutured directly to the muscle, even when few axons make functional connections. 2. Distributions of MU fibers were analyzed in 5 normal and 14 reinnervated cat MG muscles 4.5-16 mo after sectioning of its nerve and suturing of the proximal end to the distal nerve sheaths (N-N suture) or directly to the muscle fascia (N-M suture). Muscle unit distributions were quantified according to location, territory size, density, and extent of clumping between fibers from the same MU. 3. Normal MU fibers were regionalized within five regions along the muscle's longitudinal and transverse axes. Reinnervated MUs were located within similar regions, indicating that regenerating axons follow the major proximal nerve branches to restore normal compartmentalization. 4. Muscle unit fibers were diffusely scattered within discrete MU territories in normal muscles. Territory size tended to increase with MU size, whereas density of muscle unit fibers within the territory decreased. 5. Territories increased with MU size after N-N suture but were smaller and showed little size variation after N-M suture. The extent of muscle unit fiber clumping was inversely related to the number of reinnervated MUs. On average, the extent of clumping was substantially higher in muscles reinnervated after N-M suture. These results indicate that distal nerve sheaths facilitate proximal axon branching, which establishes MU territory size. Once the territory is established, motor axons branch distally to increase MU size, which in turn compensates for reduced MU numbers. 6. Muscles reinnervated by < 80% of the MUs exhibited fiber type grouping of type I fibers, and on average the extent of clumping was substantially higher in muscles reinnervated after N-M suture. With less innervation, type grouping increased inversely with the number of reinnervated MUs. However, for a similar number of MUs, type I fiber type grouping was substantially higher in muscle reinnervated after N-M suture. Type grouping therefore reflects muscle unit fiber clumping under conditions where MU size increased (N-N suture) or MU territory size decreased (N-M suture).

Animals↗
PubMed1993

Fasciculation-myokymic activity and prolonged nerve conduction block. A physiopathological relationship in radiation-induced brachial plexopathy.

Fourteen radiation-induced brachial plexus neuropathies in 12 patients suffering from cancer were studied. Neurophysiological evaluation showed a diffuse neurogenic lesion with muscular denervation signs associated with motor and sensory nerve conduction impairment of axonal type in the distal segments of the arm. Somatosensory evoked potentials were frequently abnormal, with absence of N9 in 9 out of the 10 extremities explored. The most characteristic findings were, however, the presence of fasciculation potentials--single and grouped--and myokymic discharges in 78.5% of cases (11 out of 14 plexuses), and a motor nerve conduction block on proximal stimulation, at the supraclavicular as well as cervical spine levels, in all of the cases. Both phenomena showed a high correlation when analyzed in the same neuromuscular territory. The 5 muscles with no voluntary activity and complete--or nearly complete--motor nerve conduction block were the ones with the most intense ectopic activities. The conduction blocks were present after long periods of illness in all cases and, in 2 of the cases, they persisted in successive explorations at intervals of 9 months and 2 years respectively. These data would support a probable cause-effect relationship between a persistent and prolonged motor nerve conduction block and the presence of fasciculation-myokymic type activities. One could even postulate that the infrequent neuropathies, in which both findings have been described as relevant features, have a similar physiopathological mechanism.

Action Potentials↗
PubMedDate not supplied

[The otoneurological symptoms in diffuse axonal brain lesion from the age aspect].

Since in the acute period of diffuse axonal craniocerebral trauma (CCT) the patient is comatose or, less frequently, soporous, only objective otoneurological signs (spontaneous nystagmus, altered caloric nystagmus, and traumatic damage to the otorhinolaryngological organs) may be revealed. The caloric nystagmus usually possesses all the signs characteristic of coma of different depth (absence of the caloric nystagmus in the absence of all reflexes, respiratory and cardiac disorders, total or partial loss of the rapid phase of the caloric nystagmus with the eye drifting and stopping in the direction of the slow phase of the nystagmus, the duration of such a reaction increases 3-4 fold on both sides, the vital functions are usually not disturbed in this syndrome). Both syndromes are manifested, as a rule, particularly distinctly in increasing brain edema. In the soporous state hyperreflexia of the caloric nystagmus from 2 sides was encountered with its sharp tonicity, occasional drifts of the eyes in the direction of the slow phase of the nystagmus at the peak of the caloric reaction, or hyperreflexia and tonicity of the caloric nystagmus in one direction was revealed in loss of the slow phase of the caloric nystagmus in the other direction. The last named was encountered in predominant edema of one of the cerebral hemispheres. Distinct correlations were noted as a rule between the features of the caloric nystagmus, the level of consciousness, and the degree of brain edema judged according to the findings of CT.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute Disease↗