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Biomedical subjects

P R Burgess

Publications and source records attributed to P R Burgess.

At least 19 recordsLinked to original sources
PubMed2002

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort.

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80 degrees at 10, 20 and 60 degrees/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.

Adult↗
PubMed1998

Neural gain changes subserving perceptual acuity.

When small and large objects of equal weight are lifted, the small object feels heavier than the large one (the size-weight illusion) and requires more effort to lift (the size-effort illusion). It has been suggested that these illusions result from neural gain changes designed to maintain acuity under different working conditions. If this suggestion is correct, a given mass should produce a larger increase in perceived weight or effort when added to the small object. This was found to be the case.

Adolescent↗
PubMed1997

Perceptions of effort and heaviness during fatigue and during the size-weight illusion.

Previous work has shown that force perception and the sense of motor effort are different attributes of sensorimotor function. This study explores the hypothesis that one reason force and effort perceptions are distinct is to inform an individual of impaired motor function when muscular force lags effort. This hypothesis predicts that effort and force perceptions will dissociate when motor function is impaired by fatigue but not during the size-weight illusion. All subjects reported a distinct increase in effort when lifting a standard test weight as fatigue developed. When fatigue was sufficiently marked so that they could barely lift the test weight, they rated their effort as similar to that required to lift a maximal weight in the unfatigued state. The perceived heaviness of the test weight also increased as fatigue developed, but this fatigue-weight illusion was smaller than the increase in effort for all subjects and displayed greater variability. In contrast, both the perceived weight of a small object and the effort required to lift it increased in parallel when small and large objects were lifted sequentially. The size-weight and size-effort illusions appear to be examples of a common phenomenon in which perceptual experience is rescaled to maintain acuity under different working conditions. The fatigue-weight illusion also has the effect of increasing perceptual acuity as the subject's weight lifting range decreases due to fatigue.

Adolescent↗
PubMed1995

The sense of effort and two models of single-joint motor control.

Two sets of experiments were carried out. In the first set, human subjects were asked to make the same effort with the elbow flexors at different joint angles under isometric conditions. In some experiments, the subjects were standing with the arm in a vertical (parasagittal) plane; in others, they were seated with the arm in a horizontal (transverse) plane. When muscular torque at a given effort level (ordinate) was plotted as a function of elbow joint angle (abscissa), the resulting isoeffort torque-angle profiles tended to be flat or negatively sloping over a range from 45 degrees to 135 degrees, and they were often nonmonotonic. Increases in effort up to near-maximal levels caused the isoeffort torque-angle profiles to shift upward with little alteration in shape. In the second set of experiments, seated subjects with the arm horizontal resisted baseline torques produced by a motor that acted to extend the elbow joint. Unexpected increases and decreases in torque were superimposed on the baseline torque. The subjects either were instructed to intervene and return the elbow to the initial (90 degree) position, or were told, "Do not intervene voluntarily; let the motor move your arm." Effort was reported both under baseline conditions and after the changes in torque. It was found that changes in effort were a function of the changes in torque opposed by the elbow flexors, and were similar whether the subject had repositioned the arm or allowed it to be moved by the motor. In the latter case, the arm came to rest after displacements that were a function of the size and direction of the torque change. For individual subjects, the largest angular displacements ranged from +/- 10 degrees to +/- 20 degrees for changes in torque of +/- 10 N.m. There was no evidence for any angular dependence of the effort judgements at a given torque over this angular range. Depending on whether effort is primarily an efferent perception proportional to voluntary motor activity or also has a significant afferent (involuntary) component, different models of motor control are supported by these data.

Adult↗
PubMed1993

A computerized expert system for handling the output of the Technicon H1 haematology analyser.

A computer-based expert system is described which handles the output of the Technicon H1 in the haematology laboratory of a large teaching hospital. Using patient request data, analyser results, error and morphology flags, the expert system decides; whether to validate the main indices plus differential; whether a blood film is required for manual review; and which abnormal results require phoning to the requesting doctor. The results of this computer-assisted analysis are sent to a printer adjacent to the analyser. The print-out details any further action required of the operator before release of results to the main hospital computer, and serves as a log of all samples run on that analyser. Benefits of the expert system include greatly simplified interpretation of the large array of analyser flags, and consistency in sample handling for all operators over all shifts.

Algorithms↗
PubMed1989

A second look at consequences for medical education of problem solving in science and medicine.

This article reports that most freshman and senior medical students in one medical school were not able to solve a written problem case, concerning back pain, that required the examinee to recognize that foot drop was a key physical finding. The students' responses were not influenced by the fact that half of the examinations listed the foot drop finding first and the other half listed it seventh in a list of eight physical findings. The authors conclude that the outcome of this sample problem hints at a fault in medical education: the failure of medical students to learn the skill of formulating overall hypotheses and subhypotheses before choosing treatment options.

Adult↗
PubMed1988

Teaching basic science: Dr Fox in the physiology chicken coop.

In a variation of the Dr Fox study, an educational specialist delivered a lecture in the first-year course on physiology. A highly expressive teaching style was used for content which was determined by physiology teachers. The aim was to determine whether students would note a difference. According to student evaluations, this was the highest rated lecture in the 1984 course. The lecture was repeated in the 1985 and 1986 courses with positive, but less high ratings. No students detected that the lecturer was not a physiologist.

Attitude of Health Personnel↗
PubMed1986

Joint angle signaling by muscle spindle receptors.

Nerve impulses were recorded from sensory fibers supplying the tibialis anterior and soleus muscles of anesthetized cats as the ankle joint was moved from one end of the flexion-extension axis to the other and back again in steps of 6-7 degrees. The rate of movement from one position to the next was 40 deg/s and each position was held for 16-18 s. Plots were made of receptor discharge frequency as a function of ankle joint angle during joint movement (dynamic input-output (I-O) functions) as well as 2 and 15 s after movement terminated (2 and 15 s static I-O functions). Only receptors with a sustained (5s) static response within the physiological range were studied. A total of 229 tibialis anterior receptors met this criterion, of which 11 were identified as tendon organs. One hundred and five soleus receptors were studied, of which 6 were tendon organs. Thus tendon organ activity accounted for only a small part of the muscle afferent signal under passive conditions. The spindle receptors in soleus and tibialis anterior divided the ankle flexion-extension range about equally between them, those in soleus signaling over the flexion half of the range and those in tibialis anterior over the extension half. At angles where the receptors in a particular muscle did not signal joint angle, the tendon of the muscle was observed to be slack. Thus the total muscle afferent discharge in a relaxed animal is high at one end of the range, declines progressively as the ankle is displaced to an intermediate position, and then increases again as the joint moves toward the opposite end of the range. The spindle receptors within an individual muscle were recruited rather early as the muscle came under tension so that over most of a muscle's signaling range joint angle could have been coded by changes in receptor discharge frequency but not by which spindle receptors were active. To evaluate the information signaled by individual muscle spindle receptors, the following measurements were made from plots of impulse frequency vs joint angle: dynamic response, defined as the frequency difference between the dynamic and 2 s static I-O functions during muscle lengthening; adaptation, defined as the frequency difference between the 2 and 15 s static I-O functions during muscle lengthening.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1986

Classification of muscle spindle receptors.

The conduction velocities of muscle spindle afferent fibers have a bimodal distribution, and classifications of spindle receptors based on afferent fiber diameter have therefore divided these receptors into two groups, the well known primary and secondary endings. However, measures of spindle function that are likely to be important for kinesthetic sensibility such as dynamic response, adaptation and linear directionality (hysteresis) are distributed rather uniformly. Therefore, from this functional perspective it might be argued that muscle spindle receptors should not be subdivided at all. On the other hand, different receptors demonstrate these properties to varying degrees, and there are simple, linear correlations among log (dynamic response), log (adaptation), linear directionality and conduction velocity. Thus, the receptors can be divided into as many as 5-10 different subpopulations that differ significantly in one or more of these properties.

Adaptation, Physiological↗
PubMed1984

Rapid displacements of the skin lack clear positional information.

Rapid skin indentations can produce strong tap-like sensations that contain little information about skin indentation depth. Although slower stimuli produce weaker sensations, subjects can more accurately identify how the position of the skin surface changes with respect to the deeper tissues. This dissociation between intensity and depth information suggests that intensity and depth are served by different neural circuits.

Humans↗
PubMed1984

Ascending spinal axons that signal the position of the hindlimbs under static conditions: location and receptor input.

Psychophysical experiments have shown that signals from slowly adapting subcutaneous receptors are used to sense limb position under static conditions (i.e., when the joints are stationary). The ascending collaterals of the slowly adapting primary sensory neurons supplying the deep tissues of the hindlimb do not project to the brain via the fasciculus gracilis. In experiments on cats, we have found a population of axons in the lateral fasciculus that signal the position of the ipsilateral hindlimb with a slowly adapting discharge. In the lower thoracic cord these fibers lie between the spinocervical tract and the ventral roots. Although plentiful in the lower thoracic cord, they are sparse or absent below L3. In addition, a few position signaling axons with crossed input were found in the ventral part of the lateral white matter and in the ventral columns. Since the clinical evidence suggests that the spinal pathway for position sense is uncrossed, we propose that information used for conscious judgments of limb position when the joints are stationary initially ascends via the dorsal columns and then relays to the lateral fasciculus on the same side. These slowly adapting signals also may be used to judge limb position when the joints are moving. To determine whether this slowly adapting discharge originates from muscle or joint receptors, the tendons crossing the ankle joint were exposed but left in continuity and then pulled on while the joint was stationary. In this way individual lateral fascicular axons that signaled ankle flexion, extension, abduction or adduction could be shown to receive a strong excitatory input from muscle receptors. After the muscle tendons crossing the ankle joint were cut, tract fibers signaling ankle flexion, extension, abduction or adduction could no longer be found in this portion of the spinal white matter. Axons signaling clockwise or counterclockwise twist of the ankle were reduced in number but a few were still present. These results suggest that muscle receptors provide the predominant signal used to sense ankle flexion, extension, abduction and adduction and that receptors in articular tissues may signal ankle twist.

Afferent Pathways↗
PubMed1984

Signaling of ankle joint position by receptors in different muscles.

Plots were made of multiunit activity versus ankle joint position for receptors in each of the 12 muscles crossing the cat ankle joint, except peroneus tertius, by recording from populations of afferent fibers in muscle nerves. The discharge was measured 15 or 30 sec after terminating the movements that altered the position of the joint. These recordings were dominated by large-spike activity that would be expected to originate mainly from primary spindle endings. Seven of the 12 muscles also cross other joints. Their responses at a given ankle joint position were so altered by changes in the position of the knee or toe joints that they could not reliably signal the position of the ankle joint. As judged from multiunit recording, receptors in each of the five muscles specific to the ankle joint were influenced by more than one axis of ankle joint displacement. Single-unit recording from dorsal root filaments was used to determine whether primary or secondary spindle receptors in soleus and tibialis anterior could selectively signal one axis of ankle joint rotation. Individual soleus receptors were tested both on the flexion-extension axis and with a combined adduction-eversion movement. For 38 of the 70 soleus receptors examined (54%), firm adduction-eversion produced a level of activity greater than that caused by 10 degrees of flexion, and for 77% the level of activity was greater than that caused by 5 degrees of flexion. For 168 of the 184 tibialis anterior receptors studied (91%), firm abduction-inversion produced a level of activity greater than that caused by 10 degrees of extension. Thus few receptors were found that responded exclusively to one axis of rotation. One way in which the position of the ankle joint could be specified in the face of multiaxial receptor activity is by examining the receptor discharge from more than one muscle. A suggestion for how the nervous system might do this is given in the discussion.

Afferent Pathways↗
PubMed1984

The neural signal for the intensity of a tactile stimulus.

The effect of indenting the skin at different rates on the perceived intensity of the stimulus was studied by indenting the skin of the fingertip with two triangular waveforms, given as a pair. The subjects were asked to judge which member of the pair was more intense. Perceived intensity was found to increase both with the depth and the speed of the indentation. In contrast, changes in the rate of skin indentation had little influence on perceived skin indentation depth. This suggests that intensity and depth are different attributes of tactile sensibility. Since the skin is viscous, a rapid indentation is more forceful than a slow indentation of the same depth, raising the possibility that perceived intensity is related to stimulus force. Even though intensity judgments were more closely correlated with the force of a stimulus than with the indentation it produced, a rapidly increasing force was felt as more intense than one that increased more slowly but attained the same final magnitude. When mechanoreceptors in the palmar aspect of the monkey's hand were excited with triangular stimuli like those used in the psychophysical studies, their discharge frequency increased with the rate of skin indentation. However, the receptors were distinctly more rate sensitive than the human judgments of stimulus intensity, suggesting that impulse summation in the central nervous system summates (integrates in the mathematical sense) the receptor input so as to enhance, relatively, the perceived intensity of the slower stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1983

The neural signal for skin indentation depth. II. Steady indentations.

The glabrous skin of the monkey's hand was stimulated with a waveform that indented the skin at a rate of 0.4 mm/sec, held the skin steadily or nearly steadily indented for 12 sec or longer, and then retracted back to the starting position. Recordings were made of activity in single afferent fibers in response to these stimuli. The average discharge frequency of 21 slowly adapting mechanoreceptors declined 38% during the first 12 sec of a steady indentation when the amplitude of the displacement was 0.65 mm and 36% when the displacement was 1.3 mm. When the plateau was not steady but the indentation depth gradually decreased by 15% during the 12-sec plateau period, the average decline was 47% for the 0.65-mm indentation and 46% for the 1.3-mm stimulus. When the indentation depth gradually increased by 15% during the 12-sec plateau, the discharge declined an average of 26% during the 0.65-mm indentation and 22% during the 1.3-mm displacement. To determine the effect of receptor adaptation on the perception of skin indentation depth, 13 human subjects had the skin of their fingertips indented 1 mm with similar trapezoidal waveform and were asked whether the indentation depth increased or decreased during the plateau portion of the stimulus. Ten of the 13 subjects thought that the indentation depth was increasing when the plateau was steady. The method of limits was then used to determine how much the stimulus had to change for the subject to feel the depth during the plateau as unchanging; i.e., a "perceptual zero."(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1983

Boring's formulation: a scheme for identifying functional neuron groups in a sensory system.

In 1935 Edwin Boring proposed that each attribute of sensation reflects the activity of a different neural circuit. If this idea is valid, it could facilitate both psychophysical and neurophysiological research on sensory systems. We think it likely that Boring's formulation is correct for three reasons: 1) Different sensory attributes reflect conscious information about different parameters of a stimulus. To be measured by any device, each of these parameters must be individually computed. Different neural circuits would appear to be necessary for the nervous system to carry out these different computations. 2) Perceived information about different sensory attributes can be made to diverge by appropriate manipulations of the stimuli. If there is a rigorous relationship between conscious sensory experience and neural activity, such a divergence implies that different sensory attributes are served by different neural circuits. 3) Accurate information about a sensory attribute requires that a human observer's attention be focused on that attribute. Changes in direction of attention are thought to involve a process of switching from one neural circuit to another, and provide another way to cause perceived information about different sensory attributes to diverge.

Fingers↗
PubMed1983

The neural signal for skin indentation depth. I. Changing indentations.

Psychophysical tests on human subjects showed that judgments of skin indentation depth made when the fingertip was indented at rates from 0.2 to 16 mm/sec were quite insensitive to changes in indentation velocity. Similar results were obtained on the forearm at indentation velocities of 0.4 to 16 mm/sec. Recordings were made from mechanoreceptors in the monkey's hand that were able to respond over the same range of velocities and at comparable depths to determine how skin indentation depth might be signaled (coded) at the receptor level and to examine the rate sensitivity of the possible depth codes. It was found that most of the receptors with foci under the stimulator were recruited relatively early during an indentation, especially at velocities of 1.6 mm/sec and higher, making it improbable that the full range of indentation depths is signaled by the "subsurface" recruitment of different receptors at different indentation depths. A subsurface recruitment code involving subcutaneous receptors is not likely since subjects could feel virtually none of the stimuli after skin anesthesia. Progressive recruitment with depth of receptors whose foci lie further and further away from the stimulator ("lateral" recruitment) was considered an unlikely depth code because changing the area of the stimulator had little effect on its perceived depth. Also, it was shown that subjects could sense the curvature of the indentation (the profile of the depth at right angles to the skin surface), which requires information about the depth of individual patches of skin beneath the stimulator. There is no obvious way that a lateral recruitment code can provide this information. Thus it is probable that the discharge rate of some or all of the receptors excited by the indentation is involved in indicating its depth. Both impulse frequency and receptor recruitment at any given depth increased as the velocity of the indentation increased. The demonstrated reliability of information about skin indentation depth in humans indicates that the central neural circuitry responsible for judgments of skin indentation depth is able to compensate for the rate-sensitive receptor signals.

Adult↗