Tapering of human nerve fibres

To determine the tapering of human nerve fibres, rostral and caudal root pieces of cauda equina nerve roots were removed and nerve fibre diameter distributions were constructed for 4 myelin sheath thickness ranges for the two sites, and compared with each other. The reduction of the group diameter in the different alpha-motoneuron groups was 0.2 % per 13 cm. Accounting for systematic errors, there may be even less tapering. An identified single nerve fibre showed no tapering. Further, there is indication that gamma-motoneurons, preganglionic sympathetic and parasympathetic fibres and skin afferents also reduce their fibre diameter by 0.2 % per 13 cm or less. Consequently, a nerve fibre with a diameter of 10 microm would be reduced to approximately 9.8 microm at 1m from the cell soma. Preganglionic parasympathetic fibres were found to be represented in roots S1 to S5. At similar distances from the spinal cord, the mean diameter of ventral root alpha1-motoneuron (FF) axons increased from the thoracic towards the lumbo-sacral region before decreasing again in the lower sacral region. Usually no alpha1-motoneuron axons were found in S5 roots. The diameter distribution of unmyelinated nerve fibres of a ventral S5 root showed three peaks at 0.25, 0.95 and 1.2 microm. The unmyelinated fibres with diameters around 0.25 microm may represent parasympathetic fibres. In six selected areas of the ventral S5 root, 6.6 times more unmyelinated nerve fibres than myelinated fibres were found on the average.     

Electromechanical coupling between the atria and mitral valve

Anterior leaflet (AL) stiffening during isovolumic contraction (IVC) may aid mitral valve closure. We tested the hypothesis that AL stiffening requires atrial depolarization. Ten sheep had radioopaque-marker arrays implanted in the left ventricle, mitral annulus, AL, and papillary muscle tips. Four-dimensional marker coordinates (x, y, z, and t) were obtained from biplane videofluoroscopy at baseline (control, CTRL) and during basal interventricular-septal pacing (no atrial contraction, NAC; 110-117 beats/min) to generate ventricular depolarization not preceded by atrial depolarization. Circumferential and radial stiffness values, reflecting force generation in three leaflet regions (annular, belly, and free-edge), were obtained from finite-element analysis of AL displacements in response to transleaflet pressure changes during both IVC and isovolumic relaxation (IVR). In CTRL, IVC circumferential and radial stiffness was 46 ± 6% greater than IVR stiffness in all regions (P < 0.001). In NAC, AL annular IVC stiffness decreased by 25% (P = 0.004) in the circumferential and 31% (P = 0.005) in the radial directions relative to CTRL, without affecting edge stiffness. Thus AL annular stiffening during IVC was abolished when atrial depolarization did not precede ventricular systole, in support of the hypothesis. The likely mechanism underlying AL annular stiffening during IVC is contraction of cardiac muscle that extends into the leaflet and requires atrial excitation. The AL edge has no cardiac muscle, and thus IVC AL edge stiffness was not affected by loss of atrial depolarization. These findings suggest one reason why heart block, atrial dysrhythmias, or ventricular pacing may be accompanied by mitral regurgitation or may worsen regurgitation when already present.     

Selective fasciculation of nerve fibres in culture

Small aggregates of embryonic rat retina and perinatal rat sympathetic ganglia were put into culture and allowed to form axonal outgrowths. Neuritic outgrowths from adjacent sympathetic explants grew freely into one another and appeared to form common bundles; neurites from adjacent retinal explants showed a similar pattern of interaction. In contrast, when neurites from retinal and sympathetic explants confronted one another they showed a marked avoidance reaction. This response included the partial retraction of some axons, changes in the direction of their growth and, eventually, the formation of discrete bundles of a single kind of axon. In a second kind of experiment, single-cell preparations from retina and sympathetic ganglia were mixed and allowed to form aggregates. These were put into culture and the distribution of sympathetic fibres within the resulting outgrowth was detected by incubation with radioactive norepinephrine followed by radioautography. It was found that the sympathetic axons segregated from the retinal axons as they grew and formed separate bundles of predominantly one kind of fibre. It is concluded that selective fasciculation of nerve axons can occur in culture and we discuss some possible contributory mechanisms.     

Spider silk fibres in artificial nerve constructs promote peripheral nerve regeneration

Abstract.  Objective : In our study, we describe the use of spider silk fibres as a new material in nerve tissue engineering, in a 20-mm sciatic nerve defect in rats. Materials and methods : We compared isogenic nerve grafts to vein grafts with spider silk fibres, either alone or supplemented with Schwann cells, or Schwann cells and matrigel. Controls, consisting of veins and matrigel, were transplanted. After 6 months, regeneration was evaluated for clinical outcome, as well as for histological and morphometrical performance. Results : Nerve regeneration was achieved with isogenic nerve grafts as well as with all constructs, but not in the control group. Effective regeneration by isogenic nerve grafts and grafts containing spider silk was corroborated by diminished degeneration of the gastrocnemius muscle and by good histological evaluation results. Nerves stained for S-100 and neurofilament indicated existence of Schwann cells and axonal re-growth. Axons were aligned regularly and had a healthy appearance on ultrastructural examination. Interestingly, in contrast to recently published studies, we found that bridging an extensive gap by cell-free constructs based on vein and spider silk was highly effective in nerve regeneration. Conclusion : We conclude that spider silk is a viable guiding material for Schwann cell migration and proliferation as well as for axonal re-growth in a long-distance model for peripheral nerve regeneration.     

Electromechanical coupling in cardiomyocytes from transmural layers of guinea pig left ventricle

The electrical and mechanical activity of heart ventricle cardiomyocytes is known to vary depending on the spatial location of cells in the wall, in particular, transmurally from the sub-endocardial layer to the sub-epicardial one. To investigate intracellular mechanisms of the functional heterogeneity of cardiomyocytes we developed mathematical models of the electromechanical coupling in cardiomyocytes from different transmural layers across the left ventricle (LV) wall of guinea pig. It is shown that the mechanisms of both direct linkages and feedback in the electromechanical coupling contribute to differences in both the shape and duration of action potential, and speed characteristics of contraction between isolated cardiac myocytes from the sub-endocardial and sub-epicardial layers.     

Pulse evolution on coupled nerve fibres

A numerical study of the coupled nerve fibre problem is given which verifies and extends the perturbation theory of Luzader. Pulses on adjacent fibres can couple together with two possible stable pulse separations.     

On the capacity current in myelinated nerve fibres

Different equivalent circuits are presented to describe the capacity current, Ic, of potential-clamped Ranvier nodes. The corresponding transfer functions are discussed in examining the influence of the involved parameters on size and time course of Ic. The following results were obtained: i. The duration and the amplitude of Ic increased with increasing membrane capacity. ii. The amplitude of Ic increased with increasing gain or decreasing time constant of the clamp amplifier while its duration was shortened. iii. Similar effects were seen with decreasing resistance in series with the nodal membrane. iv. Qualitatively identical but stronger effects were seen with increasing bandwidth of the current measuring internode. Measurements were carried out under experimental conditions close to the calculated ones. They resembled the calculated curves satisfactorily.     

Semiquantitative estimations of quinacrine fluorescence in intestinal nerve fibres

Summary Quinacrine has been shown to bind selectively to a population of nerves and ganglion cells in the mouse, rat and guinea-pig gut. In the present report a method for semiquantitation of nervous quinacrine contents using semiquantitative estimations of fluorescence intensity and nerve fibre density is presented and evaluated. Estimation of fluorescence intensity and nerve fibre density is based on a scale with eleven steps from 0 to 5. Preparations were incubated in ¹⁴C-labelled quinacrine hydrochloride. Reliability of the scale was expressed as the correlation coefficient between two consecutive blind estimations of the same preparations with recoding and remixing specimens in between. This correlation was found to be 0.95 or higher. Validity was expressed as the correlation between the semiquantitative estimations and ¹⁴C-quinacrine uptake measurements into the same specimens. Also this correlation was found to be strongly positive.It was concluded that nervous quinacrine content and amount of quinacrine binding nerves can be reliably estimated by fluorescence microscopy.     

Mitochondrial accumulations in nerve fibres of human sympathetic ganglia

Summary The fine structure of mitochondrial accumulations in axonal swellings of human sympathetic ganglia is described. A typical swelling contained, in addition to regularly organized mitochondria, bundles of neurofilaments and vesicles as well as large dense-cored vesicles and myelin figures. Synaptic contacts between axonal swellings with mitochondrial accumulations and ganglion cells were not found.A three-dimensional model of the mitochondrial accumulation based on serial sectioning is presented. The possible degenerative and regenerative features of these accumulations are discussed. It is possible that mitochondrial accumulations are functionally active energy producers rather than results of degenerative processes.     

The distribution and origin of calcitonin gene-related peptide-containing nerve fibres in feline dental pulp. Relationship with substance P-containing nerve fibres

The origin and distribution of calcitonin gene-related peptide (CGRP)-like immunoreactivity in feline dental pulp were studied using indirect immunofluorescence. Nerve fibres with varicosities exhibiting CGRP-like immunoreactivity were observed to enter the pulp with blood vessels. Many CGRP-containing nerve fibres were found to extend along blood vessels in the central pulp, and some of these fibres exhibited a network arrangement in the walls of dental pulp blood vessels. However, some of fibres were apparently not associated with blood vessels. Some thin, CGRP-containing nerve fibres formed a part of the nerve plexus in the subodontoblastic area and penetrated into the odontoblastic layer. In animals that had undergone transection of the inferior alveolar nerve, no CGRP-containing nerve fibres were observed. Application of a double-immunofluorescence staining technique also revealed that the distribution of CGRP-containing nerve fibres is very similar to that of substance P-containing nerve fibres.