Image-based assessment of microvascular function and structure in collagen XV- and XVIII-deficient mice.

Collagen XV and XVIII are ubiquitous constituents of basement membranes. We aimed to study the physiological roles of these two components of the permeability barrier non-invasively in striated muscle in mice deficient in collagen XV or XVIII by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Structural information was obtained with transmission electron microscopy (TEM). MR data were analysed by two different analysis methods to quantify tissue perfusion and microcirculatory exchange parameters to rule out data analysis method-dependent results. Control mice (C57BL/6J Ola/Hsd strain) or mice lacking either collagen XV (Col15a1(-/-)) or XVIII (Col18a1(-/-)) were included in the study. MR images were acquired using a preclinical system using gadodiamide (Gd-DTPA-BMA, molecular weight 0.58 kDa) as a tracer. Exchange capacity (permeability (P)-surface area (S) product relative to blood flow (FB)) was increased in test mice compared to controls, but the contributions from P, S, and FB were different in these two phenotypes. FB was significantly increased in Col18a1(-/-), but slightly decreased in Col15a1(-/-). PS was significantly increased only in Col18a1(-/-) even though P was increased in both phenotypes suggesting S might also be reduced in Col15a1(-/-) mice. Immunohistochemistry and electron microscopy demonstrated alterations in capillary density and morphology in both knockout mouse strains in comparison to the control mice. Both collagen XV and XVIII are important for maintaining normal capillary permeability in the striated muscle. DCE-MRI and the perfusion analyses successfully determined microvascular haemodynamic parameters of genetically modified mice and gave results consistent with more invasive methods.

Increased microvascular proliferation is negatively correlated to tumour blood flow and is associated with unfavourable outcome in endometrial carcinomas.

BACKGROUND: We aimed to study the angiogenic profile based on histomorphological markers in endometrial carcinomas in relation to imaging parameters obtained from preoperative dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and diffusion-weighted imaging (DWI) and to explore the potential value of these markers to identify patients with poor outcome. METHODS: In fifty-four surgically staged endometrial carcinoma patients, immunohistochemical staining with factor VIII and Ki67 allowed assessment of microvessel density (MVD) and microvascular proliferation reflecting tumour angiogenesis. In the same patients, preoperative pelvic DCE-MRI and DWI allowed the calculation of parameters describing tumour microvasculature and microstructure in vivo. RESULTS: Microvascular proliferation was negatively correlated to tumour blood flow (Fb) (r=-0.36, P=0.008), capillary permeability surface area product (PS) (r=-0.39, P=0.004) and transfer from the blood to extravascular extracellular space (EES) (Ktrans) (r=-0.40, P=0.003), and was positively correlated to tumour volume (r=0.34; P=0.004). High-tumour microvascular proliferation, low Fb and low Ktrans were all significantly associated with reduced progression/recurrence-free survival (P<0.05). CONCLUSION: Disorganised angiogenesis with coexisting microvascular proliferation and low tumour blood flow is a poor prognostic factor supporting that hypoxia is associated with progression and metastatic spread in endometrial carcinomas.

Increased microvascular permeability in mice lacking Epac1 (Rapgef3).

AIM: Maintenance of the blood and extracellular volume requires tight control of endothelial macromolecule permeability, which is regulated by cAMP signalling. This study probes the role of the cAMP mediators rap guanine nucleotide exchange factor 3 and 4 (Epac1 and Epac2) for in vivo control of microvascular macromolecule permeability under basal conditions. METHODS: Epac1-/- and Epac2-/- C57BL/6J mice were produced and compared with wild-type mice for transvascular flux of radio-labelled albumin in skin, adipose tissue, intestine, heart and skeletal muscle. The transvascular leakage was also studied by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using the MRI contrast agent Gadomer-17 as probe. RESULTS: Epac1-/- mice had constitutively increased transvascular macromolecule transport, indicating Epac1-dependent restriction of baseline permeability. In addition, Epac1-/- mice showed little or no enhancement of vascular permeability in response to atrial natriuretic peptide (ANP), whether probed with labelled albumin or Gadomer-17. Epac2-/- and wild-type mice had similar basal and ANP-stimulated clearances. Ultrastructure analysis revealed that Epac1-/- microvascular interendothelial junctions had constitutively less junctional complex. CONCLUSION: Epac1 exerts a tonic inhibition of in vivo basal microvascular permeability. The loss of this tonic action increases baseline permeability, presumably by reducing the interendothelial permeability resistance. Part of the action of ANP to increase permeability in wild-type microvessels may involve inhibition of the basal Epac1-dependent activity.

Uptake of D-aspartate and L-glutamate in excitatory axon terminals in hippocampus: autoradiographic and biochemical comparison with gamma-aminobutyrate and other amino acids in normal rats and in rats with lesions.

High affinity uptake sites for 3H-labelled amino acids were studied in synaptosome-containing homogenates processed biochemically or in surface autoradiograms of incubated slices of hippocampus. D-aspartate and L-glutamate had apparently identical distributions. In normal rat hippocampus the highest uptake was in the terminal fields of axons from the pyramidal cells of regio inferior and hilus fasciae dentatae, while there was a moderate uptake in the terminal fields of the medial and lateral perforant paths, slight uptake in the mossy fibre layer and no uptake in the terminal fields of the basket cells. Uptake sites for gamma-aminobutyrate were concentrated in the latter fields, and in the most superficial cortical layers. The present method shows no uptake in cell bodies. The uptake activities were strongly inhibited by recognized blockers of (neuronal) high affinity uptake of glutamate or gamma-aminobutyrate. Autoradiographically, several other amino acids showed negligible uptake. The uptake of D-aspartate was reduced by 80% in stratum oriens and stratum radiatum of regio superior 4-14 days (70% at 3 days) after transection of the afferent pyramidal cell axons from the ipsi-and contralateral regio inferior. The reduction was in the number of uptake sites, not in their affinity. Uptake of gamma-aminobutyrate was not reduced. Lesions affecting regio superior caused a loss of D-aspartate uptake in subiculum at a site known to receive hippocampal afferents. Autoradiographically, the uptake of D-aspartate was strongly reduced in the inner zone (i.e. the target zone), but increased in the middle zone of the dentate molecular layer after lesions of the hilus fasciae dentatae. At 4 days and longer after transection of the entorhinal afferents, there was a conspicuous reduction of D-aspartate and L-glutamate uptake in the target zones of both the medial and lateral contingent of these fibres. In the same animals, the terminal zone of afferents from hilus fasciae dentatae had an increased radioactivity and was slightly wider than normally. Concomitantly, the gamma-aminobutyrate uptake was increased in the target zones of the degenerating perforant path fibres. The results demonstrate that uptake sites for D-aspartate and L-glutamate are highly localized in axon terminals of regio inferior pyramidal cells and in perforant path afferents. The latter category of terminals has a lower density of acidic amino acid uptake sites than the former. Uptake sites for gamma-aminobutyrate are localized in terminals of intrinsic neurones, including the axosomatic terminals of basket cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The corticopontine projection: axotomy-induced loss of high affinity L-glutamate and D-aspartate uptake, but not of gamma-aminobutyrate uptake, glutamate decarboxylase or choline acetyltransferase, in the pontine nuclei.

The corticopontine fibres were severed in the crus cerebri in rats and mice by a stereotaxically operated retractable wire-knife. The pontine nuclei were microscopically dissected from fresh slices of rats and synaptosome-containing homogenates were prepared. The high affinity uptake of radiolabelled L-glutamate (L-Glu) and D-aspartate (D-Asp) was heavily reduced five days after the lesions. The uptake was further reduced after bilateral (-75% for D-Asp and -65% for L-Glu) than after unilateral lesions (-55% for D-Asp and -45 to 50% for L-Glu on the lesioned side.) The molar ratio of the uptakes of D-Asp and L-Glu was consistently lower in pons after transection of the cortical afferents than normally (-28% after bilateral lesions). gamma-Aminobutyrate uptake and glutamic acid decarboxylase were not changed. Choline acetyltransferase was increased (+53%) after unilateral lesions, but not altered after bilateral lesions. Autoradiograms of slices from mice, incubated with tritium-labelled amino acids and fixed in glutaraldehyde, showed high affinity uptake sites for D-Asp to be enriched in the pontine nuclei, compared to neighbouring structures. After partial lesion of the crus cerebri the uptake was reduced in the area with degenerated corticopontine afferents. gamma-Aminobutyrate uptake sites were relatively less concentrated in the pontine nuclei than D-Asp uptake sites. The results indicate, along with the previous demonstration of Ca-dependent K-induced release of D-[3H]aspartate from the corticopontine terminals, that glutamate and/or aspartate may be transmitters in this pathway. The results also suggest that acidic amino acid uptake sites may differ in their relative transport rates for aspartate and glutamate.

Multispectral analysis of the brain using magnetic resonance imaging.

The authors demonstrate an improved differentiation of the most common tissue types in the human brain and surrounding structures by quantitative validation using multispectral analysis of magnetic resonance images. This is made possible by a combination of a special training technique and an increase in the number of magnetic resonance channel images with different pulse acquisition parameters. The authors give a description of the tissue-specific multivariate statistical distributions of the pixel intensity values and discuss how their properties may be explored to improve the statistical modeling further. A statistical method to estimate the tissue-specific longitudinal and transverse relaxation times is also given. It is concluded that multispectral analysis of magnetic resonance images is a valuable tool to recognize the most common normal tissue types in the brain and surrounding structures.

Volume distribution of cerebrospinal fluid using multispectral MR imaging.

The goal of this study was to design a reliable method to quantify and visualize the anatomical distribution of cerebrospinal fluid (CSF) intracranially. The method should be clinically applicable and based on multispectral analysis of three-dimensional (3D) magnetic resonance images. T1-weighted, T2-weighted and proton density-weighted fast 3D gradient pulse sequences were used to form high resolution multispectral 3D images of the entire head. Training on single 2D slices, the Mahalanobis distances between the resulting multivariate tissue-specific densities were studied as functions of the feature vector composition and dimension. Multispectral analysis was applied to the images of four human brains. One feature vector with three components gave CSF volumes that were in the normal range and corresponding anatomical distributions that largely agreed with general anatomical knowledge. The exception was CSF missing around the basal parts of the brain due to signal artifacts. These artifacts were almost certainly due to the coil effect and magnetic field inhomogeneities induced by the imaged head. Such misclassifications could probably be reduced by bias field estimation and proper image restoration. Most CSF voxels formed large connected components that were found automatically, so the manual post-processing of the classified 3D image to locate CSF voxels was moderate. It is concluded that some of the fast, high resolution 3D gradient echo pulse sequences that have become available on conventional clinical scanners can be used to obtain good estimates of brain cerebrospinal fluid anatomical distribution and volume.

Two-dimensional phase unwrapping using a block least-squares method.

We present a block least-squares (BLS) method for two-dimensional (2-D) phase unwrapping. The method works by tessellating the input image into small square blocks with only one phase wrap. These blocks are unwrapped using a simple procedure, and the unwrapped blocks are merged together using one of two proposed block merging algorithms. By specifying a suitable mask, the method can easily handle objects of any shape. This approach is compared with the Ghiglia-Romero method and the Marroquin-Rivera method. On synthetic images with different noise levels, the BLS method is shown to be superior, both with respect to the resulting gray values in the unwrapped image as well as visual inspection. The method is also shown to successfully unwrap synthetic and real images with shears, fiber-optic interferometry images, and medical magnetic resonance images. We believe the new method has the potential to improve the present quality of phase unwrapped images of several different image modalities.

Three-dimensional blind deconvolution of ultrasound images.

Three-dimensional ultrasound images are blurred by the ultrasound pulse through the convolution between the 3-D tissue signal and the 3-D pulse. The blurring reduces the spatial resolution of the 3-D ultrasound images and, consequently, their diagnostic value. This paper presents a method for 3-D blind homomorphic deconvolution of medical 3-D ultrasound images to improve their spatial resolution. The blind estimate of the 3-D pulse is necessary because the pulse changes in spatial extent and frequency composition as it passes through the tissues and because the pulse is not separable in its spatial dimensions. The method was tested on a 3-D image of a phantom with anechoic spheres of known size in a uniform diffuse scattering matrix. The spheres were clearly better defined and had volumes much closer to the true volume in the deconvolved image than in the original image.

Two-dimensional noise-robust blind deconvolution of ultrasound images.

This paper presents a new method for 2-D blind homomorphic deconvolution of medical B-scan ultrasound images. The method is based on noise-robust 2-D phase unwrapping and a noise-robust procedure to estimate the pulse in the complex cepstrum domain. Ordinary Wiener filtering is used in the subsequent deconvolution. The resulting images became much sharper with better defined tissue structures compared with the ordinary images. The deconvolved images had a resolution gain along the order of 3 to 7, and the signal-to-noise ratio (SNR) doubled for many of the images used in our experiments. The method gave stable results with respect to noise and grey levels through several image sequences.

Noise robust one-dimensional blind deconvolution of medical ultrasound images.

Recently, several blind cepstral deconvolution methods for medical ultrasound images were compared experimentally. The results indicated that the generalized cepstrum or the complex cepstrum with phase unwrapping give the blind homomorphic deconvolution algorithms with the best performance. However, the frequency domain phase unwrapping for pulse estimation, which is an essential part of both methods, is sensitive to the sensor noise when the values of the spectrum are small due to the randomness of the tissue response. The noise introduces abrupt changes in the phase. The phase degradation due to the noise causes variable spatial and gray scale resolution in image sequences following deconvolution. This paper introduces a noise robust Bayesian phase unwrapping method using a noncausal Markov random chain model. The prior regularizing term accounts for the noise and smoothes the phase. The phase unwrapping is formulated as a least mean square optimization problem. The optimization is done noniteratively by solving a difference equation using the cosine transform. The resulting improvement in radial and lateral blind deconvolution is demonstrated on six short ultrasound image sequences recorded in vitro or in vivo.

Cross-innervation of fast and slow-twitch muscles by motor axons of the sural nerve in the mouse.

The extensor digitorum longus (EDL) or soleus muscles of adult mice were cross-innervated by the sural nerve (SN) and deprived of their original innervation. The number and sizes of motor units and the location of endplates in these muscles were studied 1.5 to 16 months later. In the EDL muscle, the SN cross-innervated the original endplates. Very few ectopic endplates were seen, even when the nerve was implanted well outside of the original endplate area. Only 3% of the fibres were polyneuronally innervated. In the soleus muscle, however, the SN formed large numbers of ectopic endplates whether the nerve was implanted in the original endplate zone or outside of it. In addition, 20% of the muscle fibres were polyneuronally innervated. The SN cross-innervated both EDL and soleus muscles completely. There was no preference for a particular group of the SN motoneurones since all the cross-innervated muscles were innervated by all SN motor axons and the motor unit sizes of the SN were similar in the cross-innervated EDL and soleus muscles. It is concluded that intrinsic properties of a muscle determine the ability to form ectopic synapses. The distribution of the motor unit sizes is determined by the particular pool of motoneurones which innervates the muscle.

Local and systemic effects of tetrodotoxin on the formation and elimination of synapses in reinnervated adult rat muscle.

1. Polyneuronal innervation of normal and reinnervated fourth deep lumbrical muscle fibres was studied with tension measurements and intracellular recordings. From the tenth day after a complete crush of the muscle nerve, some of the reinnervated muscles were completely paralysed for up to 15 days by local application of tetrodotoxin (TTX) to the sciatic nerve. Other animals received only systemic infusion of TTX during the muscle reinnervation.2. Measurements of tetanic-tension overlap suggested that about 6% of the muscle fibres in the normal lumbrical muscle were polyneuronally innervated, while intracellular recordings suggested that the percentage was as high as 25%. This discrepancy was mainly due to the presence of one small, sub-threshold end-plate potential (e.p.p.) and one large, suprathreshold e.p.p. in almost all polyneuronally innervated muscle fibres.3. Intracellular recordings during muscle reinnervation showed that the extent of polyneuronal innervation reached a maximum of 50% 10-15 days after denervation and that by 16-20 days this had decreased to a level similar to that found in normal muscle.4. After a week of total muscle paralysis the extent of polyneuronal innervation had increased to about 80%, estimated by both tension measurements and intracellular recordings. Subsequently, there was no sign of any net elimination of the polyneuronal innervation, even in muscles paralysed for up to two weeks. Many of the polyneuronally innervated fibres were innervated by at least two motor axons. each producing suprathreshold e.p.p.s.5. In muscles contralateral to the paralysed muscles, the extent of polyneuronal innervation reached a maximum of 50% 10-15 days after denervation as in reinnervated muscles not exposed to TTX. But in contrast to the subsequent decrease in the extent of polyneuronal innervation in animals which received no TTX, this level of polyneuronal innervation persisted in muscles contralateral to the paralysed muscles. The same was true for reinnervated muscles in animals which only received TTX systemically.6. The increased level of polyneuronal innervation after TTX application was not caused by differences in the number of motor units or in number of muscle fibres.7. Paralysed muscles relaxed much more slowly than non-paralysed muscles at the end of a fused tetanic contraction. The tetanus/twitch ratio of these muscles was also smaller than in contralateral control muscles and the rise time of the twitch was greater.8. It is concluded that a substantial fraction of the fibres in the normal lumbrical muscle of young rats is polyneuronally innervated. After reinnervation, the normal innervation pattern is re-established, but no net elimination of the polyneuronal innervation occurs unless either nerve or muscle or both are active. A net elimination of synapses is also prevented when TTX is present systemically in low concentrations.

Motor unit numbers, motor unit sizes and innervation of single muscle fibres in hyperinnervated adult mouse soleus muscle.

The sural and the lateral plantar nerves were implanted simultaneously into the denervated soleus muscle of adult mice. Each of these nerves contained approximately the normal number of soleus motor axons. This procedure therefore allowed a study of how an initial excessive number of motor axons provided by two different, foreign nerves and terminating into the soleus muscle affected the final pattern of muscle innervation. In muscles examined two months or more after the implantation of the foreign nerves all muscle fibres were innervated. The fraction of the muscle innervated by either nerve varied widely from one preparation to another. However, all the motor axons which were implanted into the muscle appeared to make permanent synapses. Moreover, the distribution of motor unit sizes of each foreign nerve relative to the total number of muscle fibres innervated by that nerve was similar to the distribution of motor unit sizes in muscles cross-innervated by that nerve alone, although the absolute motor unit sizes were reduced. Estimated by intracellular recording, 20-30% of the muscle fibres were polyneuronally innervated. A similar fraction of teased muscle fibres stained for acetylcholinesterase had more than one end-plate.

Performance evaluation of two-dimensional phase unwrapping algorithms.

We present a performance evaluation of eight two-dimensional phase unwrapping methods with respect to correct phase unwrapping and execution times. The evaluated methods are block least squares (BLS), adaptive integration (AI), quality guided path following (QUAL), mask cut (MCUT), multigrid (MGRID), preconditioned conjugate gradient (PCG), Flynn's (FLYNN), and Liang's (LIANG). This set included integration- (path following), least-squares-, L(1)-, and model-based methods. The methods were tested on several synthetic images, on two magnetic resonance images, and on two interferometry images. The synthetic images were designed to demonstrate different aspects of the phase unwrapping problem. To test the noise robustness of the methods, independent noise was added to the synthetic images to yield different signal-to-noise ratios. Each experiment was performed 50 times with different noise realizations to test the stability of the methods. The results of the experiments showed that the congruent minimum L(1) norm FLYNN method was best overall and the most noise robust of the methods, but it was also one of the slowest methods. The integration-based QUAL method was the only method that correctly unwrapped the two interferometry images. The least-squares-based methods (MGRID, PCG) gave worse results on average than did the integration- (or path following) based methods (BLS, AI, QUAL, MCUT) and were also slower. The model-based LIANG method was sensitive to noise and resulted in large errors for the magnetic resonance images and the interferometry images. In conclusion, for a particular application there is a trade-off between the quality of the unwrapping and the execution time when we attempt to select the most appropriate method.

Tentative localization of glutamergic and aspartergic nerve endings in brain.

1. The locations of the high affinity uptakes of glutamate, aspartate and GABA were studied autoradiographically and microchemically in slices of hippocampus and septum in vitro. 2. In hippocampus the distributions of the uptake sites for glutamate and aspartate were very similar, with much higher uptake in zones containing pyramidal cell terminals than in other zones. A reciprocal distribution was found for GABA uptake, which was in agreement with that of GAD. 3. Cutting pyramidal cell axons to CAl reduced the uptake of aspartate and glutamate in the target area in CAl by 80%. 4. Autoradiographically the uptake of aspartate was very high in the dorsal part of the lateral septum, moderately high in nucleus accumbens septi and neostriatum, and very low in the medial septum. GABA uptake was lower in the medial than in the lateral septum, but very high in a narrow transitional zone and in the insula Cajella magna. 5. Transecting the axons from hippocampus and subiculum to septum, gave a 70% reduction in the uptakes of aspartate and glutamate in the lateral septum, but no reduction in the medial septum. 6. Literature data on uptake, content and release of glutamate and aspartate in nerve endings in brain are briefly reviewed.

Estimation of ultrasound attenuation coefficient using log-spectrum domain processing.

Ultrasound attenuation coefficient is an important diagnostic parameter in medical ultrasonography. Furthermore, it is a parameter of a component related to the attenuation process of the space-variant point spread function, which can be used to improve the spatial resolution of ultrasound images through deconvolution. A recently published approach to the estimation of the ultrasound attenuation coefficient from B-scan radiofrequency data is extended and explained in a detail. First, a parametric image of the mean attenuation coefficients between the probe and a given pixel position is computed by applying linear regression to log-spectra of short segments of radiofrequency signals. Three methods of forming the parametric image are presented. As a second step, the local tissue-specific attenuation coefficients are estimated in small regions of the obtained parametric image. The method has been tested on synthetic radiofrequency data and on radiofrequency data recorded from a tissue-mimicking phantom. A fairly good correlation with the known reference values was achieved.

Repression of inactive motor nerve terminals in partially denervated rat muscle after regeneration of active motor axons.

The fourth deep lumbrical muscle in the hind foot of adult rats was partially denervated by crushing the sural nerve (s.n.). The denervated muscle fibres became completely reinnervated by sprouts from lateral plantar nerve (l.p.n.) motor axons. By about 20 days after the nerve crush, s.n. motor axons started to reinnervate the muscle. In control muscles, a small proportion of the muscle fibres--about 2.5% of the muscle per motor unit--was reinnervated by s.n. motor axons over the following 20 days. Hence the regenerating terminals were able to re-establish functional synapses, despite the fact that all the muscle fibres were functionally innervated by l.p.n. terminals. When nerve impulse conduction in the l.p.n. was blocked with tetrodotoxin for up to 2 weeks, starting from the time when s.n. axons returned to the muscle, s.n. motor axons retrieved a much larger proportion of the muscle fibres--about 6.5% of the muscle per motor unit. There was a concomitant decrease in the tension produced by the sprouted l.p.n. motor axons. Intracellular recordings showed that many muscle fibres became innervated exclusively by regenerated s.n. motor nerve terminals. Measurements of end-plate potentials suggested that l.p.n. sprouts and the original nerve terminals were eliminated non-selectively. These results suggest that regenerating, active motor nerve terminals have an additional competitive advantage in reinnervating innervated muscles, if the intact terminals are inactive. When the l.p.n. was cut, rather than blocked, extensive reinnervation by the s.n. occurred-about 30% of the muscle per motor unit. This suggests that the absence of an intact nerve terminal in the motor end-plate provides a stronger stimulus than inactivity for synapse formation by regenerating motor axons.

Motor unit size and synaptic competition in rat lumbrical muscles reinnervated by active and inactive motor axons.

The size of motor units has been measured in adult rat muscles reinnervated by active and inactive motor axons. The results suggest that active nerve terminals have a competitive advantage over inactive terminals during neuromuscular synapse elimination. The experiments were done using the fourth deep lumbrical muscle in the rat hind foot, which receives its motor innervation from the lateral plantar nerve (l.p.n.) and the sural nerve (s.n.). The muscles were denervated by a nerve crush close to the muscle. Five to ten days later, nerve impulse conduction in the l.p.n. was blocked for 1-2 weeks by chronic superfusion of the nerve with tetrodotoxin (Betz, Caldwell & Ribchester, 1980 b). After 2 weeks of l.p.n. block, the isometric tetanic tension of s.n. motor units increased about two-fold, compared with contralateral control muscles. This was due to an increase in the number of muscle fibres innervated by s.n. motor axons. Intracellular recordings showed that more fibres were innervated by the s.n. than in normal muscles. In some animals, the blocked l.p.n. was cut 1-2 weeks later. The l.p.n. terminals were allowed to degenerate for 1-2 days. There were more s.n. terminals in zinc iodide-osmium stained preparations of these muscles than in normal muscles. Calculation of tetanic tension overlap between l.p.n. and s.n. motor units, and the amount of mono-neuronal innervation seen in intracellular recordings suggested that a larger fraction of the muscles was innervated only by s.n. motor nerve terminals than in controls. This fraction increased with time, ultimately reaching about 14% of the muscle per s.n. motor unit. The expansion of the s.n. motor units appeared to take place by terminal and preterminal sprouting of motor axons. The l.p.n.-evoked tetanic tension decreased in parallel with the increase in the s.n. tetanic tension. The decrease in the l.p.n. twitch tension did not parallel the increase in the s.n. twitch tension. At least part of this discrepancy was due to repetitive firing of the regenerated, inactive l.p.n. terminals when the nerve was stimulated electrically. The results support the notion that modifications in connectivity between pre- and post-synaptic cells can come about by growth or withdrawal of terminals, as a result of differences in the level of activity in the presynaptic cells (Hebb, 1949).

Neonatal treatment with 6-OH dopamine and the elimination of polyneuronal innervation of skeletal muscle in the rat.

The polyneuronal innervation of skeletal muscles of newborn rats was eliminated completely in spite of virtually complete destruction of the sympathetic nervous system with 6-OH dopamine (6-OHDA). There was, however, a moderate delay in the elimination of redundant nerve terminals which is probably a secondary effect of the treatment.