Horseradish peroxidase study of the spatial and electrotonic distribution of group Ia synapses on type-identified ankle extensor motoneurons in the cat.

Eight functionally identified group Ia muscle afferents from triceps surae or plantaris muscles were labeled intraaxonally with horseradish peroxidase (HRP) in seven adult cats. Subsequently, HRP was injected into two to six homonymous or heteronymous alpha-motoneurons per animal (total = 22), each identified by motor unit type and located near the site of afferent injection. The complete trajectories of labeled afferents were reconstructed, and putative synaptic contacts on HRP-labeled motoneurons were identified at high magnification. Dendritic paths from each contact were also mapped and measured. A total of 24 contact systems (the combination of a group Ia afferent and a postsynaptic motoneuron) were reconstructed, of which 17 were homonymous, and seven were heteronymous. Overall, homonymous contact systems had an average of 9.6 boutons, whereas heteronymous contact systems had an average of 5.9 boutons. The average number of boutons found on type S motoneurons in homonymous contact systems was smaller (6.4, range 3-17) than in systems involving types FF or FR motoneurons (FF: 10.4, range 4-18; FR: 11.3, range 4-32). Neither of these differences were statistically significant. In contrast to earlier reports, a majority (15/24) of contact systems included more than one collateral from the same Ia afferent. The complexity (number of branch points) in the arborization pathway leading to each contact (overall mean 8.4 +/- 3.3) was virtually identical in all contact systems, irrespective of the type of postsynaptic motoneuron. The three-dimensional distribution of group Ia contacts was not coextensive with the radially organized dendrites of motoneurons: Dendrites oriented in the ventromedial to dorsolateral axis had the fewest (8%) contacts, whereas rostrocaudal dendrites had the most (63%) contacts. Nevertheless, contacts were widely distributed on the motoneuron surface, with few on and near the soma (< or = 200 microns radial distance from the soma) or on the most distal parts of the tree (> or = 1,000 microns). The boutons in individual contact systems also showed wide spatial and estimated electrotonic distributions; only 3/24 systems had all contact located within a restricted spatial/electrotonic region. The relations between these anatomical results and existing electrophysiological data on group Ia synaptic potentials are discussed.

Membrane area and dendritic structure in type-identified triceps surae alpha motoneurons.

The size and branching structure of the dendritic tree were studied in nine type-identified triceps surae alpha-motoneurons that were labeled intracellularly with horseradish peroxidase and reconstructed from serial sections in the light microscope. The average total membrane area (AN) for motoneurons of type S (slow-twitch) motor units was about 22% smaller than AN for cells of type F units (including both FF and FR motor unit types in this category) (480.1 X 10(3) microns 2 vs. 617.7 X 10(3) microns 2, respectively). Systematic correlations were found between stem dendrite diameter and three measures of dendritic size: dendrite membrane area, combined dendritic length, and number of terminations. All of these correlations were significantly different for the dendrites of F and S motoneurons. Power-function relations between stem diameter and dendritic membrane area were used to estimate AN for a sample of 79 type-identified motoneurons. Mean estimated AN values were significantly different for the F and S motoneuron groups, despite a large overlap in AN values between these groups. The branching structure of dendrites of F and S motoneurons also showed clear differences. Type S motoneuron dendrites showed less-profuse branching and a more-even radial distribution of branch points than found in type F cells. Examination of two forms of the "3/2 power rule" for the relation between the diameters of parent and daughter dendritic branches at branch points showed that the dendrites of type S motoneurons conform less well with the anatomical constraints necessary to represent binary branching trees as equivalent cylinders than do dendrites of type F cells. There was no systematic difference between F and S motoneuron dendrites in the degree of asymmetry of first-order daughter trees. The results overall indicate that the dendrites of F and S motoneuron groups are structurally different, giving rise to a systematic difference in AN between these groups. Such structural differences suggest that the F and S groups of alpha-motoneurons can be viewed as intrinsically distinct cell types and not just large vs. small variants of the same cell species.

Three-dimensional architecture of dendritic trees in type-identified alpha-motoneurons.

We have studied the spatial distribution of dendrites of type-identified triceps surae alpha-motoneurons, labeled intracellularly with HRP, using a variety of analytical approaches that were designed to quantify the ways in which dendrites occupy three-dimensional space. All of the methods indicated a strong tendency for motoneuron dendrites to project radially. However, regions dorsal and ventral to the somata contained fewer dendritic elements, and less membrane area, than expected for complete radial symmetry. Individual dendrites projecting into these regions tended to be smaller than those projecting rostrocaudally or mediolaterally. Nevertheless, the center of mass of membrane area for five of six fully analyzed cells was within 100 micron of the soma and, in all six cells, was located in the same dorsoventral plane as the cell soma. Maps of the projection of dendritic branches onto concentric shells at various radial distances from the soma showed that some regions have high concentrations of branches, sometimes with considerable overlap between branches arising from different stem dendrites, while other regions have relatively few branches, or none at all. Each motoneuron exhibited a different pattern of projection and there were no systematic differences between fast-twitch (type F, including both types FF and FR units) and slow-twitch (type S) motoneurons evident in the patterns of dendritic concentration. Assessment of the three-dimensional territories of individual dendrites showed that dendrites with larger numbers of terminal branches tended to have larger spatial territories. Despite considerable scatter, the results suggest that the density of branches tends to be approximately the same in large and small dendrites, and in F and S cell groups. The results are discussed in relation to the spatial location of synaptic input to motoneurons.

A HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons.

The dimensions of the somata and stem dendrites of 57 alpha- and three gamma-motoneurons, identified as to motor unit type and labeled by intracellular injection of horseradish peroxidase, were measured in the triceps surae and plantaris motor pools. The somata of type S motoneurons tended to be smaller (mean diameter 47.9 micrometers) than those of FF and FR units (52.5 and 53.1 micrometer, respectively) but these mean values were not significantly different and the data distributions showed considerable overlap between the unit types. The mean numbers and diameters of stem dendrites exhibited somewhat larger differences related to motor unit type and some of these were statistically significant. The total membrane area (AN) of each cell was estimated from measurements of the soma and stem dendrites, by using recent data and Ulfhake and Kellerth ('81) to calculate the membrane area of a dendritic tree from stem dendrite diameter. Mean AN varied with motor unit type in the sequence FF greater than FR greater than S (average values: 369 X 100(3) micrometers 2, 323 X 100(3) micrometers 2, and 250 X 100(3) micrometers 2, respectively). There was covariation between AN and the conduction velocity of the motor axon as well as with the force output from the muscle unit. Comparison of AN and motoneuron input resistance (RN) in 19 alpha-motoneurons suggested that the specific resistivity of the cell membrane in type S motoneurons was systematically higher than that characteristic of type FF or FR motoneurons.

Anterograde tracer and field potential analysis of the neocortical layer I projection from nucleus ventralis medialis of the thalamus in cat.

The projection of the ventromedial nucleus of the thalamus to the neocortex was studied in cat by means of anterograde and retrograde transport of horseradish peroxidase, by the depth profile of evoked thalamocortical field potentials, and by superfusion of the cortex with manganese to block transmitter release. Horseradish peroxidase injected into the ventromedial nucleus was anterogradely transported to the outer third of layer I in the neocortex, extending from the depth of the cruciate sulcus anterior to the olfactory bulb and tract. The region of projection from the ventromedial nucleus extended mediolaterally from the medial wall of the proreus gyrus to the ventral tip of the coronal gyrus. Horseradish peroxidase injections or applications in these areas of the neocortex resulted in the retrograde labeling of neurons in the ventromedial nucleus. Injections in many other cortical areas did not result in labeled neurons in this nucleus. Stimulation of the ventromedial nucleus with single pulses elicited surface-negative waves in the medial part of the precruciate region that had superficial isoelectric points. Superfusion of the precruciate area with manganese resulted in the suppression of the ventromedial-evoked wave, whereas control extracellular waves in deeper layers were unaffected. An additional additional finding was that horseradish peroxidase injections in the ventromedial nucleus led to a dense reciprocal retrograde labeling of neurons in layer VI of that part of the cortex to which the ventromedial nucleus projects. We conclude that, in cat, (1) the ventromedial nucleus projects to layer I of the cerebral cortex anterior to the cruciate sulcus and receives a dense reciprocal projection from layer VI; (2) stimulation of neurons in the ventromedial nucleus causes depolarization of structures in layer I and these neurons are responsible for recruiting responses in the anterior cortex.

Membrane potential of spinal motoneurons during natural sleep in cats.

The membrane potential of spinal motoneurons was recorded during wakefulness, NREM sleep, and REM sleep in minimally restrained, behaving cats. At the onset of sleep, the membrane potential generally increased in polarization in rough proportion to time spent asleep. During the postural atonia of REM sleep, the membrane potential of all motoneurons was tonically hyperpolarized. Antecedents of NREM sleep electromyographic suppressions, and REM sleep myoclonic twitches were seen as transient hyperpolarizations and depolarizations, respectively.

A simple and inexpensive method for 3-dimensional visualization of neurons reconstructed from serial sections.

Neurons stained by the intracellular injection of horseradish peroxidase or fluorescent dyes are often reconstructed from serial sections as a projection onto a 2-dimensional plane, with consequent loss of depth information. We have developed a simple, rapid and inexpensive method that retains depth information and permits visualization of neuronal structure in 3 dimensions, either as stereo pairs of red-green anaglyph drawings. The procedure does not require computer assistance and can be accomplished simply with pencil and paper, making it suitable for routine laboratory use. The small effort required in drawing stereo reconstructions is more than compensated for by the enhanced accuracy and understanding or morphology provided.

A linear method for the curve fitting of multiexponentials.

Two single-pass methods for fitting multiexponentials to experimental data are described. These methods rely on the construction of a matrix whose characteristic polynomial is used to determine the rates of decay. In the first method, which we call the multiple-delay method, the matrix is constructed using time delays of the experimental data. This method is fast and highly accurate even if the experimental signal contains exponential components with similar rates of decay. In the second method, which we call the successive-integral method, the matrix is constructed using integrals of the experimental data. This procedure yields good results for noisy signals and is a generalization of the method of Martin et al. ((1993) J. Neurosci. Methods, 51: 135-146). In addition, a particular instability of the multiexponential curve fitting problem is identified and a method for overcoming this instability is given.

Membrane potential and input resistance of cat spinal motoneurons in wakefulness and sleep.

An approximation of the membrane resistance - input resistance - was measured with double-barrelled micropipettes in hindlimb motoneurons during wakefulness and the states of sleep in chronically implanted, restrained cats. During postural atonia in rapid-eye-movement (REM) sleep, motoneurons had a sustained hyperpolarization of 5-8 mV and a 30% decrease in input resistance. There was little difference in membrane potential or input resistance between wakefulness and the other sleep states. The results directly support the hypothesis that motoneuron deactivation in REM sleep arises from active inhibition.

Pentobarbital and halothane hyperpolarize cat alpha-motoneurons.

The effect of anesthetics on the membrane potential of spinal alpha-motoneurons was studied in awake-intact cats that were sequentially anesthetized with either halothane or pentobarbital. Pentobarbital (35 mg/kg) and halothane (5%) both induced a 5 mV hyperpolarization and a reduction in spontaneous synaptic activity of motoneurons. We suggest that a reduction in the urinary EPSP frequencies contributes to the hyperpolarization and the membrane potential criterion of acceptable impalements of motoneurons in awake-intact preparations should be 5 mV more depolarized than that in anesthetized preparations.

Membrane potential and input resistance in alpha motoneurons of hindlimb extensors during isolated and clustered episodes of phasic events in REM sleep.

The function of motoneurons during the phasic events of REM sleep was investigated in cats. The membrane potential, synaptic activity, input resistance and rheobase were measured in hindlimb extensor motoneurons during periods of concentrated phasic events (Type II) in REM sleep in undrugged, minimally restrained cats. During Type II episodes in REM sleep, motoneurons depolarized 3 mV on the average, reduced in input resistance by 20%, and had distinct increases in spontaneous synaptic activity, as compared with periods of REM sleep between episodes (i.e. Type I). This constellation of properties is explained by increased frequencies of both unitary EPSPs and IPSPs in motoneurons. Myoclonic jerks in Type II episodes do not appear to arise because of a temporary withdrawal of the tonic motoneuron inhibition found in REM sleep. Instead, large increases in excitatory synaptic activity overcome the inhibition, giving rise to motoneuron discharges.

Motoneuron properties during electromyogram pauses in sleep.

Electrophysiological antecedents to electromyogram pauses of NREM sleep were studied intracellularly in hindlimb motoneurons. Phasic-event-related hyperpolarizing potentials (PRHP) were 0.2-7.1 mV in amplitude and 9-90 ms in duration, and coincided with dorsal neck EMG pauses and PGO waves. Motoneurons, if spontaneously discharging, were silenced during PRHPs. The discharge rate of spike trains elicited by depolarizing current reduced during PRHPs. Membrane conductance measurements indicated that PRHPs did not arise simply from soma inhibition but from either presynaptic inhibition or conductance changes in electrically-remote dendrites. EMG pauses, which normally occur spontaneously, could be evoked by stimulation of low threshold fibers in peripheral nerves. PRHPs were most prevalent in REM sleep. We concluded that EMG pauses are generated by the same cholinergic brainstem mechanism that generates spino-bulbo-spinal presynaptic inhibition of group Ia afferents. The pauses resemble the inhibitory phase of startle responses, providing evidence for the hypothesis that startle responses are spontaneously elicited in NREM and REM sleep.

Electrotonic parameters of cat spinal alpha-motoneurons evaluated with an equivalent cylinder model that incorporates non-uniform membrane resistivity.

The membrane voltage transients in response to constant intracellular current steps were analyzed in cat spinal alpha-motoneurons, using an equivalent cylinder model extended to include spatial non-uniformity in membrane resistivity. The main hypothesis was that the soma membrane resistivity did not differ significantly from the dendritic membrane resistivity. Evidence was found that the uniform-resistance cylinder model cannot be applied to motoneurons on the basis of inconsistencies in calculations of electrotonic length. Assuming a constant membrane capacitance, the membrane resistivity of the dendrites was found to be 500 times that of the soma. We conclude that there is a considerable difference in the membrane resistivity between the soma and dendrites of motoneurons, and that uniform-resistance models should not be applied to spinal alpha-motoneurons.

Method for stable intracellular recordings of spinal alpha-motoneurons during treadmill walking in awake, intact cats.

A method is described for recording from spinal alpha-motoneurons, which are identified according to muscle innervated, with an intracellular microelectrode during treadmill walking in awake, normally respiring, intact, adult cats. The procedure involves the implantation of an orthopedic-style spinal unit, the training of the subject to locomote on the treadmill with the lumbar region restrained, and the micropipette recording of motoneurons through a small laminotomy.

Overestimation of the electrical length of neuron dendrites and synaptic electrotonic attenuation.

The electrical (also termed electrotonic) length of dendrites is a key factor in determining the magnitude of the decay of a postsynaptic potential as it propagates from the dendrites to the soma. The average electrotonic length of dendrites in spinal, hippocampal, and red nucleus neurons have been estimated at 1.2 (range of 0.9-1.5), based on single (equivalent) cylinder models. Synaptic potentials evoked at the terminals of dendrites that are 1.2 space constants in length have been estimated to decay 50% during propagation to the soma, the lost energy being dissipated as heat. The present analysis was conducted because a 50% propagation loss seemed unlikely for such a widespread neuron function as passive dendritic propagation. The explicit and implicit assumptions of the cylinder model were reconsidered. It was found that the simplifying assumption of uniform dendritic electrotonic length has led to a three-fold overestimate of dendritic electrotonic length by previous investigators. The conclusion is that dendrites have a typical electrotonic length of 0.4 rather than 1.2, therefore resulting in a propagation loss of only 7% rather than 50% for distal dendritic synapses.

Effect of a transient, geographically localised economic recovery on community health and income studied with longitudinal household cohort interview method.

STUDY OBJECTIVE: The main purpose of the study was to determine whether the health or economic status of a cohort of residents in an economically troubled geographical area changed between 1990 and 1993. DESIGN: Longitudinal, single cohort, interview survey method with the key variables of health status and economic status. Quasi-experimental pre-post design with economic rebound as the intervention. SETTING: A relatively low income geographical area in a rural, mountainous region before and after an economic rebound. In 1990, the local economy and health care system collapsed because of the closure of a series of manufacturing plants; outward migration from the area peaked. Between 1990 and 1993, new industries opened, and state and private community assistance programmes intervened, resulting in an economic rebound, migration into the area, and marked growth of the health service sector. PARTICIPANTS: A 2% sample of residents of households, using a combination of random, stratified, and clustered sampling. Residents included in the study had lived within the area throughout the 1990-1993 period of the study. MAIN RESULTS: Stable, non-migrating residents had a statistically significant 7% reduction in health status between 1990 and 1993, as measured by a composite of subjective and objective measures. The non-migrating residents also had a significant decrease in average household income ($14,700 in 1990 and $12,400 in 1993 in constant 1990 dollars) during the strong economic expansion, and therefore did not participate in or receive direct economic benefit from the expansion. There was a rapid population increase during the expansion, attributable to inward migrants who were younger and healthier than existing residents. The decline in health for the non-migrating residents was tentatively attributed to either direct or indirect effects of the decline in family income. CONCLUSIONS: Local economic development accompanied by expanded health care services availability can leave existing area residents poorer and less healthy, and this problem may be masked by an abundance of healthier, wealthier inward migrants.

Contraction properties and motor nucleus morphology of the two heads of the cat flexor carpi ulnaris muscle.

Previous studies have examined the isometric contraction properties of the two heads of the cat flexor carpi ulnaris acting as a single unit. In this study, the contraction properties and fiber architecture of each head of the flexor carpi ulnaris were determined separately and related to previous reports on the histochemical characteristics of this muscle. The morphology of retrograde-labeled motor nuclei for the two heads of the muscle was also examined. The humeral head had a significantly longer contraction time (48 msec) than the ulnar head (36 msec) as well as a significantly lower tetanic fusion frequency (28 Hz vs. 35 Hz). The maximum tetanic tension per gram of muscle tissue was 71% greater in the ulnar head. Motoneurons of the flexor carpi ulnaris formed a column 12 mm long and 0.5 mm wide in the center of the ventral grey in spinal segments C8 and T1. The ulnar head had alpha-motoneurons with greater soma diameters than those in the humeral head. The smaller soma diameter, slower contraction time, and weaker contraction in the humeral head correlate with the preponderance of oxidative-metabolic muscle fiber types found in the humeral head by other workers. These correlations suggest that the humeral head plays a major role in maintaining a sustained antigravity tension that prevents the wrist from buckling during standing.

Universal precautions compliance and exposure frequency to patient body fluids in nurses employed by urban and rural health care agencies.

Previous studies have suggested that health care workers may differ with respect to universal precautions knowledge, compliance, practice setting barriers, or exposure to patient body fluids in rural and urban areas. The purpose of this study was to determine whether or not there are rural/urban differences in the degree of precaution taken by health care workers to prevent the spread of blood borne pathogens, specifically human immunodeficiency virus (HIV) and hepatitis B virus (HBV). A random sample of rural and urban registered and licensed practical nurses in Tennessee was surveyed. The respondents completed two instruments that assessed self-reported universal precautions knowledge, precautions, and practice barriers. No measurable differences in universal precautions knowledge, compliance, or barrier scores between the two groups were found; yet rural nurses were 2.7 times as likely to be exposed to patient body fluids than urban nurses (P < 0.005). The conclusion was that rural nurses were as experienced and as knowledgeable about universal precaution techniques as their urban peers, but their knowledge was not translated into practice to the same degree. Two possible explanations offered are (1) rural nurses are more likely to be acquainted with, and thus trusting of, their patients, and (2) the lower seroprevalence of human immunodefiency virus and hepatitis B virus in rural areas may lead to complacency.

Risk-adjusted in-hospital death rates for peer hospitals in rural and urban regions.

The purpose of this research project was to compare inpatient mortality rates for rural hospitals with mortality rates of urban hospitals of given sizes and ranges of service. Statistical adjustments for risk were made in the probability of death during hospitalization for 43,000 patients across 166 hospitals by age, gender, principal diagnosis, principal surgical procedure, characteristics of the secondary diagnoses, and whether or not cancer was a secondary diagnosis. Eighty-three small hospitals that had a relatively unspecialized range of services constituted the study group. Patient characteristics of this study group were moderately representative of the national population. A standardized score was calculated for each hospital using a formula based on the actual hospital death rate and the death rate expected for a given hospital with patients of the same demographic and medical characteristics. Patients admitted to hospitals in nonmetropolitan areas had a mortality rate of 0.41 percent compared with a mortality rate of 0.66 percent in peer hospitals in metropolitan areas. After mortality rates were risk-adjusted and converted to z scores, nonmetropolitan areas had an average z of +0.16, and metropolitan areas had an average z of -0.25, where positive z scores reflect a lower-than-average adjusted mortality rate. The metropolitan-nonmetropolitan (urban-rural) difference was not statistically significant, but it is meaningful in that rural hospitals tended to have a lower adjusted mortality rate than urban hospitals of the same size and type, indicating that rural hospitals had the same or lower adjusted mortality rates. The possibility of urban hospitals having riskier patients was minimized but could not be definitively ruled out. Taken together with other studies, the data are consistent with the view that small rural hospitals generally make appropriate transfer decisions for severely ill patients and provide quality care for retained patients.