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D L Morgan

Publications and source records attributed to D L Morgan.

At least 163 records · Page 9Linked to original sources
PubMed1986

Aftereffects in the responses of cat muscle spindles.

Responses have been recorded from primary endings of muscle spindles in the cat soleus muscle. Changes in spindle responsiveness were measured following a period of conditioning that consisted of a series of rapid stretches or of tetanic ventral root stimulation. In the testing procedure the response of a single spindle afferent was recorded to stimulation of a dynamic fusimotor axon during a slow stretch. Changes in gross afferent discharge coming from the muscle were measured by integrating the activity recorded in dorsal roots. If, after conditioning stretches, the muscle was immediately returned to its initial length, the spindle responded to the test fusimotor stimulation with a high-frequency burst of afferent impulses. If the muscle was held stretched for 3 s after conditioning the response to the brief test tetanus was small or "depressed." It has been suggested that conditioning stretches result in detachment of stable crossbridges in intrafusal fibers and that these bridges then reform over the next few seconds at whatever length the muscle happens to have at the time. When it is long, shortening the muscle back to the initial length leads to the development of slack in intrafusal fibers because of the passive stiffness they have acquired from the presence of the stable bridges. Under these conditions a brief test fusimotor tetanus will lead to a depressed response because the slack must first be taken up before a full response can be generated. It was possible to reverse the depression by interposing an extrafusal contraction during the period between the conditioning and test sequences. It is suggested that lateral compression from the contracting extrafusal fibers and the stretch they impose as they relax reduces any intrafusal slack and thereby reduces the depression. A more quantitative measure of intrafusal slack than the test for depression is to determine the delay in onset of the afferent response to a longer fusimotor tetanus. The delay was short a long initial muscle lengths where, if the muscle was left undisturbed, it soon disappeared completely and spontaneously. It is suggested that at long lengths passive tension in the muscle tends to remove any slack in intrafusal fibers and therefore removes any after effects. The rise in resting discharge of muscle afferents after a conditioning tetanus applied to the ventral root ("postcontraction sensory discharge") can be accounted for by the same hypothesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1985

The discharge of cat tendon organs during unloading contractions.

Tendon organs respond preferentially to contractions of a select set of motor units. If a tendon organ is given a resting discharge by raising the passive tension in the muscle, other motor units can be identified which on contraction interrupt the discharge, presumably by unloading the passive tension. These experiments investigate the possibility that stimulation of motor units with unloading effects can reduce the response of a tendon organ to a loading contraction. We find that over most of the range of muscle lengths the unloading contraction produces only a transient lowering of the firing rate. Only at short muscle lengths where further shortening is accompanied by a steep fall in tension is the unloading contraction able to induce a sustained reduction of afferent discharge.

Action Potentials↗
PubMed1985

After-effects of stretch on the responses of cat soleus muscle spindles to static fusimotor stimulation.

Mammalian muscle spindles show persistent after-effects following conditioning stretch or fusimotor stimulation. Most previous observations have been carried out on primary endings of spindles and using dynamic fusimotor stimulation. We report here observations on after-effects produced either by conditioning stretch or by static fusimotor stimulation on the responses of primary and secondary endings to a slow test stretch during which a brief burst of static fusimotor stimulation is applied. We find that the response to the test burst is large if the muscle is kept short after conditioning but it becomes depressed if the muscle is held stretched for 3 s following conditioning. We attribute these effects to the presence of stable cross-bridges between actin and myosin filaments in intrafusal fibres. We conclude that, qualitatively, after-effects using static fusimotor testing are the same as with dynamic fusimotor testing and this must be taken into account when providing an explanation for the phenomenon.

Animals↗
PubMed1985

Action of single dynamic fusimotor neurones on cat soleus Ia afferents during muscle shortening.

The ability of single dynamic fusimotor (gamma d) fibres to sustain the firing of muscle spindle primary (Ia) afferents during shortening was investigated in soleus muscles of anaesthetised cats. Of 11 gamma d fibres, 10 could maintain Ia firing during 10 mm/s shortening. Of the 7 tested at greater velocities, 5 could maintain Ia firing during shortening at velocities greater than 50 mm/s. This ability was, however, critically dependent upon the timing of the stimulation. In particular, it rapidly reduced with increasing duration of stimulation before the onset of shortening. Furthermore, if appreciable stretch occurred between the onset of gamma d stimulation and the onset of shortening, this could greatly reduce the ability of gamma d fibres to sustain Ia discharge. If gamma d neurones are on occasion phasically activated during voluntary shortening movements, their action could be an important determinant of Ia firing, even in the presence of weak gamma s action. Therefore in chronic recordings, observation of Ia firing during muscle shortening is not an adequate criterion for inferring gamma d activity.

Action Potentials↗
PubMed1985

Erythrocytes from ozone-exposed mice exhibit decreased deformability.

Injury from short-term exposure to ozone (O3) was detected by a simple test of red blood cell (RBC) filterability. This test measures changes in the ability of the RBC to deform--as occurs during passage through small capillaries. Male CD-1 mice were exposed to 1.0, 0.7, or 0.3 ppm O3 for 4 hr, and blood samples were obtained by heart puncture. RBCs were suspended in Tris-HCl buffer, pH 7.4, containing 10 mg/dl glucose. After incubation in air for up to 6 hr, the time required for 2 ml RBC suspension to pass through a 3-micron-pore-size polycarbonate filter was determined. A significant increase in the 6-hr filtration time for O3-exposed (1.0 ppm) mice over unexposed mice and a lack of protection by vitamin E were shown. The increases in RBC filtration times for O3-exposed mice appeared to be dose related. Ozone exposure (1.0 ppm) caused a significant increase in the hematocrit of both vitamin E-deficient and -supplemented mice. Vitamin E supplementation appeared to partially prevent this increase in hematocrit. Measurement of lipid peroxidation by the thiobarbituric acid (TBA) test revealed no detectable levels of TBA-reactive material in RBC from O3-exposed mice. These results suggest that measurement of RBC filterability may be feasible as a clinical test for short-term injury from exposure to oxidant gases.

Animals↗
PubMed1985

Free radical species mediating the toxicity of ozone for cultured rat lung fibroblasts.

We have reported a model system with which to study factors influencing the toxicity of O3 for cultured rat lung cells. This continuing investigation was to determine the relative contributions of free radical species to the toxicity of O3 as measured by 51Cr release and the use of free radical scavengers. Toxicity from modes of O3 exposure favoring stable free radicals (stationary cultures or added O3-exposed medium) was prevented by the H2O2 scavenger, catalase. Toxicity produced by exposing cells to O3 through only a thin film of medium (rotated cultures) was partially prevented by either catalase or superoxide dismutase. Their combination completely prevented toxicity, suggesting that both H2O2 and O2 were major toxic species. Mannitol, an OH scavenger formed from H2O2 and O2, was partially protective with all models while the 1O2 scavengers histidine, tryptophan and xanthine were ineffective.

Animals↗
PubMed1985

Cytotoxic action of rat alveolar macrophages for ozone-exposed lung fibroblasts.

The objective of this study was to determine if exposure of cultured rat lung fibroblasts to O3 would trigger a subsequent attack by rat alveolar macrophages (AM). Fibroblasts labeled with [3H]proline (3HPR) were exposed to O3 (0, 0.74, 1.35, 2.46 or 4.48 ppm) for 1 h. AM were added (5:1, AM: fibroblast) and after 2, 24, 48, 76 and 96 h of co-culture, loss of 3HPR was measured as an index of AM-induced injury. Fibroblasts exposed to O3 and co-cultured with AM lost significantly more 3HPR than without AM co-culture. The results suggest that O3 injury to the lung may be mediated in part through an attack by AM upon O3-injured fibroblasts.

Animals↗
PubMed1985

Stimulation rate, potentiators, and sarcomere length-tension relationship of muscle.

Isometric tetani of single muscle fibers of Rana temporaria were studied as a function of stimulation rate, sarcomere length (1.7-2.3 micron), twitch-to-tetanus ratio, and exposure to twitch potentiators (Zn2+ and NO3-) at 20 degrees C. As the stimulation rate was decreased below a maximal level, tension generation decreased. This depression in tension generation was more pronounced at shorter sarcomere lengths. Therefore the magnitude and shape of the sarcomere length curve was dependent on stimulation rate. Although the depression in tension generation was always accompanied by a noticeable ripple in the tension record in fibers with large twitch-to-tetanus ratios, it could be observed even during well-fused tetani in fibers with low twitch-to-tetanus ratios. In all fibers, however, high stimulation rates or exposure to potentiators resulted in maximum tension generation at each length, and the sarcomere length-tension curve followed that found by Gordon, Huxley, and Julian. This indicates that the fall in tension between sarcomere lengths of 2.0 and 1.7 micron is not due to length-dependent activation but is more likely to be the result of mechanical interference in the force-generating interaction between cross bridges and thin filament sites.

Animals↗
PubMed1985

Site of impulse initiation in tendon organs of cat soleus muscle.

A continuing controversy surrounds the question of whether Golgi tendon organs are examples of receptors in which impulses may be generated at more than one site. This paper reports a systematic examination of a number of models incorporating single or multiple impulse generators and of the compatibility of their predictions with experimental observations. Two phenomena, in particular, that must be accounted for are nonlinear summation and cross-adaptation. When two motor units each with a direct effect on the tendon organ are stimulated together, the rate of discharge is greater than either individual rate but is less than their sum. In cross-adaptation a conditioning response elicited by one motor unit contraction produces adaptation of the discharge associated with stimulation of a second motor unit. A model with a central impulse generator can be modified to account for nonlinear summation by postulating a nonlinear transformation in the generator current-to-impulse rate conversion. Experiments measuring summation of responses to stimulation of three inputs produced results that did not support this model. Another variation of the model, which had a nonlinearity in the tension-to-current step and cross-connections (mechanical or neural) between tendon strands stressed by contracting muscle fibers, was able to account for the observations. A second model that provided the right predictions was a multiple impulse generator with cross-connections. Which of the two models best fits the experimental observations can be decided by comparing the calculated summation coefficients and cross-adaptation coefficients. A central impulse generator predicts a negative correlation, the multiple impulse generator a positive correlation. All of the observations were made using tendon organs of cat soleus muscle. Responses were recorded to stimulation of filaments of ventral root. In a comparison between 20 pairs of responses from six tendon organs the correlation between summation and cross-adaptation coefficients was found to be significantly positive. We conclude that the tendon organ model that accurately predicts all of the experimental observations incorporates multiple generators.

Adaptation, Physiological↗
PubMed1985

From sarcomeres to whole muscles.

The series connection of sarcomeres in a muscle raises the possibility of instabilities due to some sarcomeres being capable of greater isometric tension than others, due to non-uniform activation, cross-sectional area or sarcomere length. Such instabilities are heavily damped by the force-velocity curve and stabilized by passive tension. The variation of active tension with sarcomere length may act to stabilize or destabilize the situation. Such instabilities can affect tension in a complex manner, causing both increases and decreases in different situations. Some examples of situations where sarcomere nonuniformities have been observed or inferred are given, and their mode of action explained.

Animals↗
PubMed1984

Can fusimotor activity potentiate the responses of muscle spindles to a tendon tap?

Experiments on the cat soleus muscle have determined the effect of selective activation of the fusimotor system on the responses of muscle receptors to a simulated tendon tap. Primary endings of spindles responded in the passive muscle with an average 4.3 impulses at a mean instantaneous rate of 502 impulses/s. Static fusimotor stimulation at 100 pulses/s increased the number of impulses during the tap to 4.8 but dropped the mean instantaneous rate to 400 impulses/s. Dynamic fusimotor stimulation increased the number of impulses to 6.3 and the instantaneous rate to 557 impulses/s. Combined stimulation of the two axons gave intermediate values. We consider these effects as rather feeble. The tendon jerk in man shows a large increase in reflex amplitude following a reinforcement manoeuvre (Jendrassik manoeuvre). Based on our animal experiments we conclude that such increases cannot be accounted for simply in terms of selective engagement of the fusimotor system.

Action Potentials↗
PubMed1984

Non-linear summation of tension in motor units of toad slow muscle.

Tension developed by single motor units and bundles of motor units has been recorded from the slow, non-twitch portion of the iliofibularis muscle of the toad Bufo marinus. Evidence was sought for the presence of individual muscle fibres within a motor unit which remained less than fully activated when the axon was maximally stimulated, i.e. at a rate and duration sufficient to produce maximum tension. When two motor units were stimulated separately and together, one at a low rate and the other at a maximal rate, the tension recorded on combined stimulation was greater than the sum of individual tensions, i.e. combined stimulation acted to 'facilitate' tension in a proportion of fibres. Adding the anticholinesterase eserine to the solution bathing the muscle produced a large increase in individual motor unit tensions but a rather smaller increase during combined stimulation of several axons. This suggested that in normal solution, maximally stimulating a single axon left many muscle fibres less than fully activated. When the local anaesthetic lignocaine was added to the bathing solution, tension in response to nerve stimulation fell, presumably because of impulse blockade in small terminal branches of the axon. However tension fell more steeply for single motor units than for combinations of motor units, again, we propose, because single motor units contain a larger fraction of incompletely activated fibres whose tension output depends critically on the level of activation. Overlap between two motor units is a measure of the proportion of fibres innervated by both axons. When overlap was measured over a range of different muscle lengths and using contractions of different durations, evidence was obtained in support of the notion that tension output of some muscle fibres was less than maximum not only because the fibre received too sparse an innervation to achieve a sufficient over-all level of depolarization but because they had been non-uniformly activated, leading to internal motion within the fibre and a consequent further drop in tension.

Animals↗
PubMed1984

Mechanical properties of toad slow muscle attributed to non-uniform sarcomere lengths.

Tension changes have been measured during shortening or stretching movements applied to actively contracting motor units of the tonus bundle of the iliofibularis muscle of the toad Bufo marinus. During a slow, constant-velocity release tension fell, initially rapidly and then more slowly. The size of the fall, particularly later in the movement, depended on a number of factors including the duration of the isometric contraction before the onset of shortening, the amount of tension developed by the motor unit and the length of the muscle. When an isometrically contracting motor unit was rapidly shortened, the rate of rise on re-development of tension following the release was significantly slower than at the onset of the contraction. This effect was more marked if the release was preceded by a longer period of isometric contraction, if the experiment was carried out at shorter muscle lengths or if a smaller motor unit was used. If, following a period of isometric contraction, stimulation was interrupted, and a release-stretch movement applied to quickly bring the level of force down to near zero, and then stimulation recommenced , the final level of re-developed tension was less than that immediately before the release. The size of the tension deficit following re-development was larger for small motor units and at short muscle lengths. When the duration of the contraction before release was increased the size of the deficit also increased. A deficit could be prevented if the muscle was allowed to relax passively before the shortening movement was commenced. Stretch of actively contracting slow muscle produced an initial steep tension rise followed at times by a transient fall before tension slowly rose again. The transient fall became larger at short muscle lengths, and after long-duration contractions before stretch. Its tension dependence was less easy to establish because of complications involving changes in the relative series compliance. All of the above observations could be accounted for by an explanation based on the development of sarcomere non- uniformities in slow muscle fibres, produced as a result of non-uniform activation of the fibre membrane through the distributed nerve supply.

Animals↗
PubMed1984

The after-effects of stretch and fusimotor stimulation on the responses of primary endings of cat muscle spindles.

These experiments explore the after-effects of repetitive movements and dynamic fusimotor stimulation on the responses of primary endings of soleus muscle spindles in the anaesthetized cat. If immediately following a series of conditioning stretch and shortening movements, the muscle was held at the stretched length for 3 s before being returned to its rest length, the subsequent response to a brief dynamic fusimotor tetanus given during a slow test stretch produced only a small increase in spindle firing. If, on the other hand, the muscle was returned to its rest length immediately after the movements, the fusimotor tetanus evoked a much larger afferent burst. This difference in the size of the burst could only be observed if the fusimotor tetanus was given soon after onset of the test stretch. If it was delayed and given at a time when the test stretch passed through the length at which the muscle had been held stretched after the movements, there was no difference in the size of the afferent burst. If following the movements the muscle was held stretched for less than 3 s, the response to the subsequent tetanus was not fully depressed. Once the depressed condition had been achieved, the muscle had to be left undisturbed for up to half an hour before the response had recovered its original fully undepressed size. Conditioning repetitive stimulation of the fusimotor fibre was just as effective as using alternating movements in producing the effects. If the test tetanus, which was normally ten shocks in 50 ms, was made longer, the change in size of the impulse burst became a change in latency of onset of the response to the tetanus. Holding the muscle stretched at the end of the conditioning movements/tetanus produced a delay in onset of the response to the test tetanus without significantly altering its size. These observations have been interpreted as arising from development of stable cross-bridges between actin and myosin filaments in the intrafusal fibres. During repetitive movements or fusimotor stimulation, stable bridges become detached and during the subsequent 3 s they re-attach, at the length at which the muscle is being held after conditioning.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Stiffness of cat soleus muscle and tendon during activation of part of muscle.

Experiments have been carried out on the soleus muscle and its tendon in the anesthetized cat. Measurements of isometric tension and muscle stiffness were made during contraction of whole or part of the muscle in response to stimulation of ventral root filaments. In an attempt to determine the distribution of tension in different portions of the tendon during activation of only part of the muscle, the free tendon of insertion was split longitudinally into two halves and a strain gauge attached to each piece. From a large number of measurements, it was found that the mean fraction of tension recorded in one-half of the tendon remained about the same, over a wide range of tensions. However, the scatter of values, which increased as the portion of muscle contracting was reduced, was greater than expected if muscle fibers were randomly distributed throughout the muscle. Measurements of muscle and tendon stiffness were made from length and tension changes during stretch of the actively contracting muscle. Ventral root stimulation that engaged 20% or more of the muscle yielded a value for tendon compliance (0.09 mm/N), which was the same as for stimulating the whole muscle. This result suggested that for contraction of portions as small as 20% of the muscle, fibers were effectively attached to the whole tendon, indicating that tendinous attachments of individual muscle fibers ran independent of one another over only a short distance and were bound together over most of their remaining course. It was concluded that groups of muscle fibers selected by stimulation of ventral root filaments are not entirely randomly distributed throughout the muscle. However, for groups representing larger fractions of the total tension, (greater than 20%) the distribution is uniform enough and the connections between their tendinous attachments firm enough for the force applied by such a group to act through a tendon compliance, which is the same as that seen by the whole muscle.

Achilles Tendon↗
PubMed1983

In vitro inhibition of alveolar macrophage phagocytosis by ozone: absence of a role for serum or mode of ozone administration.

Cell culture conditions which protected rat lung fibroblasts from O3-induced injury were tested for their protection potential in retaining the phagocytic activity of O3-exposed rat alveolar macrophages. Conditions which protected fibroblasts from O3-related loss of 51Cr were ineffectual in preventing O3-related loss of macrophage phagocytic activity. It is suggested that the macrophage phagocytic function may be especially sensitive to H2O2 generated by the O3.

Animals↗