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D G Watt

Publications and source records attributed to D G Watt.

34 records · Page 2Linked to original sources
PubMed1979

Doppler detection of thresholds for decompression-induced venous gas emboli in the awake rat.

Awake, unrestrained rats with chronic implants of a Doppler probe on the vena cava, were saturated in a small plexiglass chamber at a maximum pressure (P1), rapidly decompressed to a predetermined pressure (P2) and then monitored for the presence of intravascular bubbles. If there were no indication of bubbles within 1 h, the decompression was considered bubble free. For each saturation pressure, an increasing pressure difference (deltaP = P1 - P2) was tried on each exposure until the threshold for bubble detection was found. The relationship deltaP = a P1 + b, where a and b are constants defining the threshold for bubble detection, was experimentally determined at pressures from 3-10 ATA. The depth-dependent linear relationship deltaP = 0.38P, + 1.29 (r = 0.97, P is less than 0.001) was found for first exposures and deltaP = 0.62 P1 + 0.16 (r = 0.99, p is less than 0.001) for repeat exposures. The chronic Doppler preparation allows a more sensitive threshold determination than past methods for small animals and also allows repeat pressure exposures without the effects of a severe decompression stress.

Animals↗
PubMed1976

Referencing procedure for location of lumbosacral alpha-motoneurons.

This report presents a simple referencing procedure to facilitate microelectrode positioning within a desired motor nucleus in the cat spinal cord. The procedure is used in conjunction with the conventional technique of interpreting microelectrode position in the ventral gray column on the basis of the relative amplitude of antidromically or orthodromically evoked field potentials from select motor nuclei. The procedure optimizes the use of a single surface anatomical landmark such that the nuclei can be quantitatively referenced both horizontally and sagitally.

Animals↗
PubMed1976

Responses of cats to sudden falls: an otolith-originating reflex assisting landing.

A short-latency electromyographic response has been demonstrated in the human gastrocnemius during unexpected falls (11). The hypothesis that it is a form of otolith-spinal reflex has been tested in the cat. Electromyographic activity was measured in four muscles while suddenly and unexpectedly dropping a series of six cats from 50 cm above the ground. In normal animals, electrical activation of all muscles tested occurred in two distinct phases. An early burst commenced as early as 15 ms from the instant of release, was usually spindle shaped, and often but not always disappeared by 100 ms. This burst was totally and permanently abolished by bilateral labyrinthectomy, but not by selective inactivation of the relevant semicircular canals. A later phase of muscle activity commenced 75-140 ms after release and gradually built up and then subsided, rougly centered about the movement of landing on the ground. This activity was not greatly altered by labyrinthectomy. The present results appear to confirm that the early electromyographic response to sudden falls is due to an otolith-spinal reflex. A later component is nonlabyrinthine in origin, however. Smooth landings from short falls are not possible in the absence of either phase of the response.

Animals↗
PubMed1976

Analysis of muscle receptor connections by spike-triggered averaging. 1. Spindle primary and tendon organ afferents.

1. The synaptic connections of 44 single identified muscle spindle Ia afferents and of 21 Golgi tendon organ (Ib) afferents from medial gastrocnemius (MG) were studied in 46 cats by the spike-triggered averaging of synaptic noise in 803 motoneurons of various types. 2. The well-known monosynaptic Ia excitatory connections were confirmed and their characteristics examined in 113 cells. The method was used at greater sensitivity than before and revealed that, in addition to the larger EPSPs of the order of 300 muV, there were many below the previously reported lower 17-muV limit. 3. By studying the Ia disynaptic inhibitory pathway with quick stretch-evoked Ia volleys and by spike-triggered averaging (STA), it was shown that the latter method can reveal disynaptic and possibly trisynaptic excitatory connections. This is believed to depend on having continuous activity in the relevant interneurons. 4. Latencies of individual connections showed broad distributions and arguments are advanced for setting working limits to mono- and disynaptic paths for Ia excitation and inhibition. Monosynaptic EPSP latency from cord entry was 0.4-1.1 ms and disynaptic inhibition was 1.2-2.4 ms. It was recognized that the boundaries are not rigid and monosynaptic Ia EPSPs may have latencies up to 1.5 ms. 5. Rise times of disynaptic PSPs were, on average, significantly longer than monosynaptic, but individual disynaptic responses could have values within the monosynaptic range. 6. A small diphasic wave shortly preceding the monosynaptic EPSPs was interpreted as a presynaptic spike. Its timing was consistent with this and, as such, permitted estimates to be made of central conduction time. 7. An early negative wave (latency less than or equal 1.1 ms) of small emplitude was sometimes detected in antagonist motoneurons when triggering from Ia afferents. It was found tha extracellular fields could be detected due to single Ia afferent excitations and efforts were made to see if the early negative wave could be explained by this. In a few cases there was evidence that a very short-latency IPSP might be occuring. This evidence and its implications are discussed with attention to the new factors which have to be considered in using the spike-triggered averaging method at very high sensitivity. 8. Ib effect were di- or trisynaptic. They were excitatory to 18% of synergists and to 28% of antagonists. They were inhibitory to 41% of synergists and to 19% of antagonists. The Ib IPSPs were larger than the EPSPs.

Animals↗
PubMed1976

Analysis of muscle receptor connections by spike-triggered averaging. 2. Spindle group II afferents.

1. The spike-triggered averaging (STA) method has been used to study synaptic connections of nine spindle group II afferents from medial gastrocnemius to 151 motoneurons of leg muscles in the cat. 2. EPSPs were found in 40 cells, predominantly of triceps surae with latency from cord entry ranging from 0.3 to 4.2 ms. Those with latency less than or equal to 1.4 ms were deduced to be monosynaptic in confirmation of Kirkwood and Sears (13). Mean amplitude for MG-LGS cells was 30.1 muV and mean rise time 1.0 ms (compared with 65.4 muV and 1.0 ms for monosynaptic Ia EPSPs from the preceding report (26)). It is argued that monosynaptic latency for spindle group II afferents could be as large as 1.65 ms. 3. The occurrence of a presynaptic spike permitted the division of EPSP latencies into central conduction time and synaptic delay components. Sindle group II central conduction times were significantly longer than those of Ia afferents, while there were no differences in the synaptic delays associated with the two afferent types. 4. EPSPs of longer latency were judged to be di- or trisynaptic. They were smaller and had longer rise times than the monosynaptic effects. Evidence is presented to show that short rise time of an individual PSP does not guarantee that it is monosynaptic. Rise times are different only on a population basis. 5. Inhibitory responses were found with latencies and mean rise times appropriate for di- and trisynaptic connections. Their mean amplitude was 4.6 muV. 6. The distribution of EPSPs and IPSPs was generally consistent with their exerting stretch reflex effects similar to that of Ia afferents and inconsistent with the inclusion of them in the grouping known as "flexor reflex afferents."

Animals↗
PubMed1976

System for averaging physiological data recorded on film or paper.

A simple electro-mechanical system has been constructed to convert photographically-recorded data into a form that can be readily handled by a small, special-purpose averaging computer. Manual tracing of a series of records results in the production of an immediate and highly reproducible average response. The method has proven to be particularly useful in the analysis of electromyograms obtained during various form of locomotion.

Computers↗
PubMed1976

Passive mechanical factors governing head and neck movements at the onset of sudden falls in the cat.

The head and neck movements of 16 cats undergoing sudden, unexpected falls have been assessed as a study preliminary to the investigation of reflex responses to the same stimulus. Six of the cats were studied while normal, and similarly tested at a later date to examine for any changes induced by making these animals pitch and roll insensitive, or by labyrinthectomy. The head and neck always described a sterotyped pattern of movement, consisting of progressive dorsiflexion of the neck without rotation of the head in space. This was unaltered by partial or complete loss of eighth nerve function. These findings are consistent with the head and neck movements of a falling cat being predominantly passive in origin, at least for the first 200 msec or so of an unexpected fall.

Animals↗
PubMed1971

Observations on the control of stepping and hopping movements in man.

1. The presence of a form of stretch reflex, previously described in the arm by other authors, has been confirmed in the gastrocnemius muscle of the human leg. The electromyographic (e.m.g.) manifestation of this reflex occurred 120 msec (S.E. of mean = 3.5 msec) following a sharply applied, and maintained, dorsiflexing force to the foot. This form of response is referred to in this article as the Functional Stretch Reflex (FSR).2. To determine the contribution of the FSR to the control of normal leg movement, the e.m.g. activity in the above muscle was monitored during single downward steps of 12.7, 25.4 and 38.1 cm and during repetitive, rhythmic, hopping movements on one foot.3. It was found that e.m.g. activity associated with steps to the ground began 141 msec (S.E. of mean = 8.5 msec) before contact with the ground and ended 131 msec (S.E. of mean = 7.6 msec) after contact, when the e.m.g. usually became temporarily inactive.4. It is inferred from these results that the muscular deceleration associated with landing was brought about by the release of a pre-programmed pattern of neuromuscular activity which was inaccessible to reflex activity resulting from the mechanical event of landing, rather than by a stretch reflex.5. It was found that subjects chose their preferred frequency of hopping with great accuracy and consistency. The mean value obtained was 2.06 Hz (S.E. of mean = 0.02 Hz).6. At the preferred frequency, e.m.g. activity began 84 msec (S.E. of mean = 9.6 msec) before and terminated 263 msec (S.E. of mean = 10 msec) after contact with the ground.7. It is inferred that in rhythmical hopping and perhaps also in running, each landing is effected, as in single steps, by a predetermined pattern of neuromuscular activity. However, when hopping at the preferred frequency, the take-off phase of muscular activity is timed to make maximal use of the FSR, i.e. between 120 and 260 msec after initial contact.8. The results emphasize the importance of pre-programming complex muscular contractions suitable for opposing sudden passive stretching forces, and of initiating them prior to the onset of these forces.

Adult↗
PubMed1971

Muscular control of landing from unexpected falls in man.

1. It was previously shown that the controlled landing from single steps to the ground is typically brought about by accurately timed motor activity, commencing before the actual landing, and completed before time would permit the participation of a useful stretch reflex response.2. To investigate further the validity of this conclusion, subjects were dropped from an electromagnetic suspension at unexpected moments. Their gastrocnemius electromyographic (e.m.g.) responses and the forces applied to their feet were recorded throughout.3. No useful contribution of a stretch reflex response was detected. Indeed, it was shown that a functionally effective reflex resulting from the mechanical event of landing would occur far too late to contribute to the muscular deceleration of the fall.4. It was also found that a consistent muscular response occurred, commencing 74.2 msec (S.E. of mean = 1.4 msec) after starting the fall, independent of height.5. It is suggested that this response in the leg musculature is a reflex originating in the otolith apparatus. In addition, a possible mechanism for the control of repetitive hopping, and perhaps running, movements, involving the above reflex, is suggested.

Adult↗
PubMedDate not supplied

"Torso rotation" experiments; 1: Adaptation to motion sickness does not correlate with changes in VOR gain.

Following a 30-minute exposure to an unusual motor strategy called "Torso Rotation" (TR), the signs and symptoms of motion sickness appear along with perceptual illusions during movement, gaze and postural instability, and a significantly reduced vestibulo-ocular response (VOR) gain. With repeated exposure to TR, the motion sickness disappears and gaze instability seems to be reduced. Is this apparent improvement in gaze stability associated with a reduction of the transient change in VOR gain? Motion sickness (subjective questionnaire) and VOR gain (passive step rotations in darkness) were measured before and repeatedly after TR on 7 consecutive days. Despite a complete loss of symptoms in 3 to 4 days, the transient, daily change in VOR gain remained unaffected. Furthermore, there was no increase in the use of compensatory saccades. It is concluded that adaptation to TR-induced motion sickness is not the result of a change in VOR's sensitivity to TR.

Adaptation, Physiological↗
PubMedDate not supplied

"Torso rotation" experiments; 2: Gaze stability during voluntary head movements improves with adaptation to motion sickness.

Following a 30-minute exposure to an unusual motor strategy called "Torso Rotation" (TR), the signs and symptoms of motion sickness appear along with perceptual illusions during movement, gaze and postural instability, and a significantly reduced vestibulo-ocular response (VOR) gain. With repeated exposure to TR, motion sickness symptoms disappear and gaze instability seems reduced, but without any concomitant change in VOR gain. Is the reduction in gaze instability a perceptual illusion or a real, measurable phenomenon? Velocity gain (eye velocity/ head velocity) was evaluated during voluntary head shaking in the light over the frequency range 0.3 to 3.0 Hz. A significant improvement was seen after 3 days of testing (P < 0.01). Furthermore, the time course of improvement in gaze stability was correlated with the loss of motion sickness symptoms reported in the previous study (1). We suggest that adaptation to motion sickness could be related to an overall change in sensori-motor strategy, perhaps including a de-emphasis of a vestibular reference.

Adaptation, Physiological↗
PubMedDate not supplied

"Torso rotation" experiments; 3: Effects of acute changes in vestibular function on the control of voluntary head movements.

"Torso Rotation" (TR) produces an acute, reversible change in human vestibular function. Experiments were performed to determine if repeated exposure to this technique would result in long-term adaptive modifications. In one experiment, VOR gain was evaluated. Measurements were obtained before and 3 times after 30 minutes of TR, on 7 consecutive days. VOR gain dropped each day after TR, returning to normal within about 20 min. In a separate experiment with different subjects, eyes-open gaze stability was measured during voluntary head shaking between 3.0 and 0.3 Hz. The same test schedule was used. Analysis of gaze stability (limited to frequencies between 1.0 and 2.0 Hz) was complicated by an unexpected finding. Despite careful instructions, head displacement increased each day after TR, also returning to normal within about 20 min. Surprisingly, subjects were unaware of this change. Combining the 2 experiments, VOR gain and head amplitude were averaged across all 7 days, separately for the 4 daily tests. Head amplitude was plotted against VOR gain for these 4 averages. Amplitude was greater when VOR gain was reduced, with a remarkably high correlation (R2 = 0.996). These findings confirm that vestibular feedback plays an important role in the control of voluntary head movement. Furthermore, the data suggest that instability of the visual scene reported by subjects shaking their heads after TR resulted not only from a lower VOR gain, but also from the inadvertent use of higher head velocities.

Female↗