Sensitivity of a biomechanical model of the finger to errors in experimental input.

Biomechanical models of the fingers are used to gain a greater understanding of internal loading. This can help guide the treatment of injuries and pathologies. However, to be valid these models require accurate measurement of body kinematics, external reaction forces and soft tissue architecture. This study aimed to quantify the sensitivity of one such model, to errors in these inputs. Experimental data was collected from a single subject carrying out a simple gripping activity and the experimental data altered to introduce artificial errors. We found that the correlations between errors in measurement of body kinematics and the model outputs could be used to express errors in motion capture data in terms of internal loading. However, these correlations were specific to grip type, therefore, if the grip changed significantly a new analysis would be required. Sensitivity analysis of the muscle and tendon locations indicated which parameters were most important to measure accurately; outputs were most sensitive to changes in the most highly loaded muscle-tendon units, these results were applicable across different hand orientations.

A technique for motion capture of the finger using functional joint centres and the effect of calibration range of motion on its accuracy.

A method of kinematic analysis of the fingers using stereo-photogrammetry, referred to as the phalanx transformation technique, has been proposed. Functional methods were used to define the joint axes and subsequently each finger segments' anatomical coordinate system. Thirteen subjects were tested and the accuracy of the technique assessed. The average error across the three joints of the finger was found to be 0.6 mm, which translates to a 2.2% error in predicted joint reaction force when using a biomechanical model. The subjects were required to have sufficient movement in their joints to define the joint axes functionally. Some subjects of clinical interest can have a significantly reduced mobility owing to injury or pathology, therefore, the effect of calibration range of motion on accuracy was analysed. It was found that, for a range of motion typical of a subject with rheumatoid arthritis, the errors in predicted joint reaction force were < 7%. The accuracy of this technique compared favourably with others previously proposed and, considering the other errors inherent in modelling, those found in this study were deemed to be acceptable.

Using dysprosium complexes to probe the nitrogenase paramagnetic centers.

EPR progressive power saturation techniques were used to monitor relaxation enhancement of the nitrogenase paramagnetic centers produced by interaction with Dy3+ complexes. Three models are presented for the relationship between the degree of enhancement and the distance of closest approach of Dy3+ to the intrinsic metal cluster. In the first model, the perturbing dysprosium ions are represented as a single average site. The second and third models are variations of the treatment of Innes and Brudvig [(1989) Biochemistry 28, 1116-1125] and assume the unknown protein to be spherical with Dy3+ dispersed either randomly over the surface of the protein or randomly in solution. Using these models, the distance of closest approach of the Dy3+ complex to the [4Fe-4S] cluster in the Fe-protein from Azotobacter vinelandii was determined to be 5.0-6.5 A. Similarly, the distance of closest approach to FeMoco in the MoFe-protein was determined to be 0.0-1.2 A, which, when corrected to the fact that FeMoco exists as an S = 3/2 spin state, indicates that the distance is > or = 6 A. These distances did not change when (1) either protein was in the presence of the other, (2) both proteins were cross-linked to each other, or (3) the Fe-protein from A. vinelandii was mixed with the MoFe-protein from Clostridium pasteurianum. On the other hand, formation of the inactive complex of the Fe-protein from C. pasteurianum with the MoFe-protein from A. vinelandii blocked dysprosium-induced relaxation enhancement, implying that each component protein overlaps the metal cluster in the complementing protein.

Subunit location of the iron-sulfur clusters in fumarate reductase from Escherichia coli.

The subunit location of the [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters in Escherichia coli fumarate reductase has been investigated by EPR studies of whole cells or whole cells extracts of a fumarate reductase deletion mutant with plasmid amplified expression of discrete fumarate reductase subunits or groups of subunits. The results indicate that both the [2Fe-2S] and [3Fe-4S] clusters are located entirely in the iron-sulfur protein subunit. Information concerning the specific cysteine residues that ligate these clusters has been obtained by investigating the EPR characteristics of cells of the deletion mutant amplified with a plasmid coding for the flavoprotein subunit and a truncated iron-sulfur protein subunit. While the results are not definitive with respect to the location of the [4Fe-4S] cluster, they are most readily interpreted in terms of this cluster being entirely in the flavoprotein subunit or bridging between the two catalytic domain subunits. These new results are discussed in light of the amino acid sequences of the two subunits and the sequences of structurally well characterized iron-sulfur proteins containing [2Fe-2S], [3Fe-4S], and [4Fe-4S] centers.

Morphometry of intracellularly labeled neurons of the auditory nerve: correlations with functional properties.

Single auditory-nerve fibers were injected with horseradish peroxidase after their tuning properties, characteristic frequencies, and spontaneous discharge rates were measured. From these functional properties virtually all other aspects of auditory-nerve response can be predicted. Labeled fibers were reconstructed from the point of peripheral termination on cochlear hair cells to the point at which they enter the cochlear nucleus. Several morphological properties were measured at the light-microscopic level, including axonal diameter, axonal length, internodal distances, cell-body area, and cell-body shape. All of these parameters were correlated, though some weakly, with characteristic frequency. However, only axonal diameter was correlated with spontaneous discharge rate.