Age-related changes in human ciliary muscle and lens: a magnetic resonance imaging study.

PURPOSE: To use high-resolution magnetic resonance (MR) images of the eye to directly measure the relationship between ciliary muscle contraction and lens response with advancing age. METHODS: A General Electric, 1.5-Tesla MR imager and a custom-designed eye imaging coil were used to collect high-resolution MR images from 25 subjects, 22 through 83 years of age. A nonmagnetic binocular stimulus apparatus was used to induce both relaxed accommodation (0.1 diopter [D]) and strong accommodative effort (8.0 D). Measurements of the ciliary muscle ring diameter (based on the inner apex), lens equatorial diameter, and lens thickness were derived from the MR images. RESULTS: Muscle contraction is present in all subjects and reduces only slightly with advancing age. A decrease in the diameter of the unaccommodated ciliary muscle ring was highly correlated with advancing age. Lens equatorial diameter does not correlate with age for either accommodative state. Although unaccommodated lens thickness (i.e., lens minor axis length) increases with age, the thickness of the lens under accommodative effort is only modestly age-dependent. CONCLUSIONS: Ciliary muscle contractile activity remains active in all subjects. A decrease in the unaccommodated ciliary muscle diameter, along with the previously noted increase in lens thickness (the "lens paradox"), demonstrates the greatest correlation with advancing age. These results support the theory that presbyopia is actually the loss in ability to disaccommodate due to increases in lens thickness, the inward movement of the ciliary ring, or both.

A model of orientational ordering in phosphatidylcholine bilayers based on conformational analysis of the glycerol backbone region.

Molecular and conformational ordering in aqueous multilamellar suspensions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) have been examined by deuterium nuclear magnetic resonance (2H NMR) in the liquid crystalline (L alpha) phase. Motionally averaged quadrupolar splittings vQ from six sites in the vicinity of the glycerol backbone have been analyzed by a molecular frame and order matrix approach in which the usual assumption of a freely-rotating molecule is not invoked. By assuming a relatively rigid glycerol backbone region, the six vQ values are found to be consistent with a conformation of the glycerol backbone that is almost identical to that of one of the two structures in crystalline DMPC dihydrate (Pearson, R. H., and I. Pascher, 1979, Nature (Lond.) 281: 499-501). The orientation of the most-ordered axis of the DMPC molecule is found to be tilted at an angle of 27 +/- 2 degrees with respect to the long axis of the sn-1 chain in its extended all trans conformation. The ordering of the most ordered molecular axis with respect to the bilayer normal is expressed by an order parameter of Szz approximately equal to 0.6 +/- 0.1, consistent with values in analogous thermotropic liquid crystals.

Spatial modulation of water ordering in lecithin bilayers. Evidence for a ripple-ripple phase transition.

Intense motional averaging effects on the 2H nuclear magnetic resonance (NMR) spectrum of 2H2O that occur in aqueous dispersions of dimyristoyl-sn-glycero-3-phosphocholine (Myr2-PtdCho) are explained by a spatial modulation in the orientational order of the water induced by ripplelike structures. The ratio of the amplitude to the periodic length of the ripples, A/lambda, at a molar ratio of water/Myr2-PtdCho of 9.5:1, is measured by 2H NMR and found to be consistent with x-ray measurements of this ratio in the P beta phase of dipalmitoyl-sn-glycero-3-phosphocholine (Pam2-PtdCho) bilayers. The sensitivity of 2H NMR allows us to report the presence of two distinct ripple phases mediated with a discontinuous change in the value of A/lambda. This result suggests that the two ripple structures observed for several phospholipid systems in excess water by freeze-fracture electron microscopy may be associated with two different phases instead of the same phase as previously assumed.

Phase transitions in phosphatidylcholine multibilayers.

The (2)H NMR spectrum of a multilamellar dispersion of 1-myristoyl-2-[14,14,14-(2)H(3)]myristoyl-sn-glycero-3-phosphocholine with 1 mol% cholesterol in excess water has been recorded at temperatures between -15 degrees C and 36 degrees C. Motionally averaged quadrupole coupling constants nu(Q) and motionally induced asymmetry parameters eta are obtained by spectral analysis. Values of these quantities indicate that, at temperatures below -4 degrees C, any rotational motion of the molecules about their molecular long axis is slow on the NMR time scale. At temperatures immediately above the pretransition these same parameters show that a fast-rotational motion is occurring about the molecular long axis. This rotational motion is hindered in that the molecules flip about a twofold symmetry axis. Between -4 degrees C and the pretransition, spectra appear as the superposition of two powder patterns, one corresponding to the pattern observed below -4 degrees C and the other to the pattern above the pretransition. The relative contribution of the latter increases with temperature until the pretransition is reached. These data have been interpreted in two ways: either the sample between -4 degrees C and the pretransition contains two populations of rapidly and slowly rotating molecules, or there is only a single population of molecules undergoing a 180 degrees flipping motion on the time scale of the NMR measurement. The latter interpretation is more consistent with other experimental findings. At the temperature of the main transition the hydrocarbon chains melt. In the absence of cholesterol, spectra are more complex in that the line shape is reproduced by the superposition of three spectral powder patterns between -4 degrees C and the pretransition and by the superposition of two spectral patterns above the pretransition. It is postulated that these two patterns observed above the pretransition are in direct correspondence to the two ripple structures observed by freeze-fracture electron microscopy in the absence of cholesterol.