Biomedical subjects

H F Ross

Publications and source records attributed to H F Ross.

At least 37 records · Page 2Linked to original sources

PubMed1989

Quantifying capillary distribution in four dimensions.

Analysis of spatial distribution using numerical (O-D) distribution is limited to global estimates, while linear (1-D) separation of capillaries may be used to describe unrealistic spatial patterns. Intramuscular diffusion is best viewed as an integration of all distances between surrounding capillaries, or area (2-D) of influence for individual vessels. True planar analysis also accommodates other forms of heterogeneity, and may be extrapolated to give the volume (3-D) of tissue supplied by capillaries. Temporal (4-D) heterogeneity in functional spacing may then be quantified.

Animals↗

PubMed1989

Influence of muscle phenotype on local capillary supply.

The general method used to compute a local capillary fibre ratio for each fibre is as follows. The selected area of the section is covered by a tessellation of domains. For each fibre, the contribution of each capillary whose domain intersects the fibre is computed as the proportion of the domain area which overlaps the fibre. The sum of all the contributions from overlapping domains is taken to be the effective number of capillaries contributing to that fibre, and is called here the local capillary fibre ratio (LCFR). This parameter may be normalised by dividing by fibre area to give an index, independent of fibre size, which is capable of identifying the differential capillary supply to fibre types within a mixed muscle.

Animals↗

PubMed1988

Collation of student results in practical class experiments in physiology, using a BBC ECONET computer network.

Funding was provided to Queen's University by the Department of Education and by the Industrial Development Board for Northern Ireland to provide microcomputers for undergraduate use. An allocation from the grant to the Department of Physiology enabled the purchase of 20 BBC Master 128 microcomputers with monitors used as student work-stations connected together by an ECONET network with a file server using a dual floppy disc drive, two printer servers and two demonstrator stations. A BASIC program was written to analyse the students' practical class measurements which they entered manually at their work-station keyboards. Class results were presented to the students in the form of frequency distribution histograms or X/Y graphs. Program modifications to suit different practicals can be made relatively easily. The time taken to analyse data has been shortened. It is easy for the students to get immediate comparison of their own results with those of the rest of the class--particularly advantageous if the student's own experiment did not work. The class can be divided into groups to study different variations of the experiment and provide the data from each group to the whole class. The students' opinions on whether the equipment had (1) improved the teaching of physiology and (2) provided helpful preparation for the use of computers in medical practice were assessed by a questionnaire which showed that a clear majority felt these aims had been fulfilled.

Computer-Assisted Instruction↗

PubMed1987

Functional analysis of a palindromic sequence required for normal replication of several staphylococcal plasmids.

Most small multicopy antibiotic-resistance plasmids of Staphylococcus aureus contain a major axis of hyphenated dyad symmetry (palA) that is required for normal replication and stability, although located outside of the minimal replicon. Rearrangements affecting palA cause plasmid instability, a marked reduction in copy number, and the accumulation of large quantities of strand-specific circular single-stranded plasmid DNA. In view of the recent observation that pT181 initiates replication by a nick and 3'-extension mechanism (S. Khan, personal communication), it is suggested that these plasmids replicate by an asymmetric rolling-circle mechanism in which the displaced plus strand remains single stranded until palA is exposed, forming a hairpin that serves as the lagging strand origin.

Anti-Bacterial Agents↗

PubMed1986

The role of reflexes in the resting tremor of Parkinson's disease.

Forces and electromyograms were recorded from patients with Parkinson's disease during imposed joint movements. Muscles which were stretched by 3 to 5 Hz sinusoidally alternating movements often showed vigorous bursts of EMG activity whose timing established that it was a reflex response to the movement. The same movements provoke no stretch reflex response from normal subjects. When resting tremor was present the driven movements sometimes entrained it, and the bursts of EMG activity then became locked to the imposed movement. On other occasions the tremor activity continued at its own rate; EMG bursts then occurred at times unrelated to the movement, and the irregular force records reflected a conflict between the movement and the muscle activity. Tremor was most consistently entrained when a large mass of muscle was driven through a large movement at a frequency that was close to the usual tremor frequency. Tremor which involved synchronous contractions of muscles at different joints was often resistant to the effects of our imposed movements, but it could sometimes also be entrained by large movements of a single joint. When tremor was entrained by a driving movement, the EMG discharge was indistinguishable from a reflex response, and the limb exerted forces on the machinery which had the timing and magnitude that would be expected of a reflex response. Spontaneous tremor in the same subjects had frequencies which altered in the predicted way with changes of mechanical load. We conclude that peripheral reflexes are more important in parkinsonian tremor than has often been supposed, although afferent activity from the moving limb probably interacts with other potentially oscillatory mechanisms.

Aged↗

PubMed1984

The ankle stretch reflexes in normal and spastic subjects. The response to sinusoidal movement.

Forces and electromyograms were recorded during sinusoidal flexion-extension movements of normal and spastic ankle joints. Spastic subjects showed relatively stereotyped responses, with evidence of a vigorous spinal stretch reflex. The responses of normal limbs were variable; there was little reflex response to the first cycles, but as the movement continued the reflex responses increased and often came to resemble the responses of spastic limbs. At some frequency between 3 and 7 Hz, the reflex response was so timed that it tended to assist rather than resist the movement; this was the frequency at which many subjects (normal, as well as spastic) exhibited spontaneous clonus if an appropriate load was attached to the foot. The frequency of this clonus changed with changes of load. It is concluded that whereas the gain of a normal stretch reflex may vary considerably, the stretch reflex of the spastic subject is set at one end of the normal range. With this high gain, the stretch reflex may support spontaneous clonus in both normal and spastic subjects.

Adolescent↗

PubMed1984

The tendon of flexor pollicis longus: its effects on the muscular control of force and position at the human thumb.

Human flexor pollicis longus tendons obtained at autopsy were subjected to repeated sinusoidal stretching movements. The associated force changes were almost in phase with the movement (force led position by less than 4 degrees), and alterations in the frequency of movement between 2 and 16 Hz had little effect on them. Examination of the thumbs of formalin-fixed cadavers demonstrated that the tendon exerts its force 7.4-8.0 mm in front of the axis of movement of the interphalangeal joint. From a knowledge of the tendon properties and the joint anatomy, one can calculate the changes in tendon length that would accompany any force change at the joint. Equipped with this information one can re-examine the responses to sinusoidal movements of the thumb interphalangeal joints of normal subjects. When the subject exerts a steady flexing force in which stretch reflexes play no important part, measurements of joint stiffness indicate that only a portion of the imposed movement reaches the muscle fibres. The extension of those (visco-elastic) muscle fibres lags behind the extension of the (elastic) tendon. Stretch reflexes contribute to the mechanical resistance of muscle fibres to low frequency (3-5 Hz) displacements, but in the presence of a compliant tendon the length of the muscle fibres does not determine the angle of the joint in any positive way. It is suggested that the compliant tendons of many thumb and finger muscles simplify the neuromuscular control of forces during gripping and handling movements.

Biomechanical Phenomena↗

PubMed1983

Response of the normal human ankle joint to imposed sinusoidal movements.

Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made of electromyograms (e.m.g.s) in calf muscles, and of the forces at the joints. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus e.m.g.s. The e.m.g. response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as one could expect of a reflex pathway that involves a delay. The results suggest that this delay is between 50 and 60 ms, and we conclude that under these circumstances spinal stretch reflexes are playing the important part. The relation of the resisting force to the movement has been displayed as a vector. As the frequency changes, this vector describes the circular path that is characteristic of a system which includes delays or lags; this path enables one to draw conclusions about the amplitude and timing of the reflex resistance to the movement. When a subject exerts a moderate flexing force against the sinusoidal movement for some minutes, the reflex response becomes progressively potentiated. A subject whose reflex responses are normally slight may then exhibit a vigorous reflex response to the movement of that ankle. This enhancement of spinal reflex activity was accompanied by an increase in the myotatic reflex response at the ankle. Reflex responses to sinusoidal movement were most clearly seen when the subject exerted a mean flexing force that amounted to about one-fifth of his maximum. Very small movements (+/- 0.5 degrees) generated little or no reflex response. With large amplitudes of movement there was more reflex activity, but at some amplitude (which varied from subject to subject and from time to time) the reflex mechanism appeared to 'saturate', and further increases in amplitude were not accompanied by comparable increases in the reflex response. With movements at 10-15 Hz the e.m.g. response often became large in alternate cycles, with less activity in the intervening cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1983

Reflex responses at the human ankle: the importance of tendon compliance.

Subjects with active stretch reflexes responded to an imposed sinusoidal movement of the ankle joint with a reflex force whose amplitude and timing varied widely with changes in the frequency of movement. At some frequency between 6 and 8 Hz, the reflex force tended to offset the non-reflex component of resistance, and thus to reduce the total resistance to movement. At this frequency the reflex response was particularly vigorous, with a deep modulation of electromyogram (e.m.g.) activity and a displacement of the joint stiffness vectors far from their high frequency values. The total resistance to movement might then be small, or it might be zero, or the reflex might actually assist the movement. As the frequency of movement was decreased through this critical range, the timing of the reflex response to movement changed rapidly with an abrupt advancement of the triceps surae e.m.g. signal, and a wide separation of the joint stiffness vectors as they passed close to the origin. This result was attributed to a changing distribution of the movement between the muscle fibres and an elastic Achilles tendon. It was assumed that at most frequencies the muscle fibres resisted extension, so that a major part of the imposed movement went into stretching the tendon; when, however, at 6-8 Hz, the reflex response was so timed as to reduce or abolish the resistance of the muscle fibres, more of the movement would take place in them. The muscle spindles would 'see' this larger movement of the muscle fibres, and generate correspondingly more reflex activity. A simplified model of the muscle-tendon combination behaves in a way that supports this view, and the available information about the human Achilles tendon indicates that it is sufficiently compliant for such an explanation. Therefore, movements imposed on the ankle joint would not necessarily be 'seen' by the muscle spindles, since they would be modified by transmission through a compliant tendon. By assuming a value for the tendon stiffness, it was possible to calculate the course of movements that actually occurred in the muscle fibres and spindles. Records of these spindle movements indicated how some non-linearities might arise.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1982

Forces generated at the thumb interphalangeal joint during imposed sinusoidal movements.

1. A method is described for driving the interphalangeal joint of the thumb through repeated sinusoidal flexion-extension movements, while immobilizing other joints of the wrist and hand.2. The joint met the sinusoidal movement with a force that fluctuated in an approximately sinusoidal manner. This paper is concerned with the relationship between the position and force sinusoids.3. When the thumb was relaxed the forces were small, but when the flexor pollicis longus was tetanically stimulated there was a large force change in response to each cycle of the movement. In either case, the maximum resistance to extension occurred during the later part of the extension movement, some 20 degrees -45 degrees in advance of maximum extension. A similar result was obtained when the subject exerted a maximal voluntary flexing force.4. The resistance to movement can conveniently be displayed as a stiffness vector, the amplitude of which is the force/displacement, and the phase is the angle by which the force sinusoid leads the position sinusoid.5. When the subject maintained a continuous, though sub-maximal, flexing effort the timing of the force fluctuations changed with changing frequency of movement in a characteristic way, and the stiffness vectors described a C-shaped or spiral path. With increasing frequency the stiffness vectors moved round this path in a clockwise direction.6. For descriptive purposes the resistance to movement can usefully be regarded as the vector sum of a reflex and a non-reflex component. Since the reflex pathway involves significant conduction delays, the reflex force can be expected to appear later in the cycle of a higher frequency movement, and give rise to a vector which moves round in a clockwise direction as the frequency increases. The non-reflex stiffness, however, changes much less with frequency.7. It is concluded that the size of the C-shaped or spiral vector path gives an indication of the strength of the reflex activity, while the position of the high frequency points gives an indication of the non-reflex resistance to the movement.

Action Potentials↗

PubMed1982

Electromyographic responses to imposed sinusoidal movement of the human thumb.

1. The interphalangeal joint of the thumb was driven through sinusoidal flexion-extension movements while electromyograms were recorded from over the flexor pollicis longus muscle. 2. When the subject relaxed his thumb the movement generated no detectable e.m.g. response. When, however, he exerted a voluntary flexing force electrical activity could be recorded from the flexor pollicis longus, the amplitude of which was modulated at the frequency of the movement. 3. As the driving frequency was increased, the maximal e.m.g. activity occurred progressively later in the cycle of movement; for frequencies above about 6 Hz the timing of the averaged e.m.g. was compatible with a reflex delay of 55-65 msec. 4. The frequency-phase plot was not, however, the perfect straight line that would arise from a simple and constant reflex delay. There were some consistent departures from linearity and some random variations. In either case, the timing of the e.m.g. and the timing of the reflex force (Brown, Rack & Ross, 1982a) changed together in ways that increased confidence in each of the measurements. 5. The amplitude of the e.m.g. signal was more deeply modulated by movements at 8-14 Hz than by higher or lower frequencies, and it was concluded that the stretch reflex responds particularly readily to signals in that frequency range.

Adult↗

PubMed1982

A range of different stretch reflex responses in the human thumb.

1. Imposed sinusoids were used to assess the resistance to movement at the thumb interphalangeal joint.2. The resistance to high-frequency movements (> 12 Hz) increased when the subject exerted a large voluntary flexing force; this increase was attributable to a greater non-reflex resistance of the contracting flexor muscles. This resistance was essentially ;visco-elastic', and the force was phase-advanced on joint position. At moderately large forces (up to half maximal), however, the resistance changed with changing frequency, and over a range 4-12 Hz the vectors which represented joint stiffness described the wide path that is characteristic of an active stretch reflex (Brown, Rack & Ross, 1982a). At frequencies between about 4 and 6 Hz the force was sometimes phase-delayed on position, and the joint exhibited a negative viscous stiffness. When the voluntary flexing force was very large the reflex contributed less to the resisting force, which was then phase-advanced on position at all frequencies of movement.3. Large amplitude movements did not generate correspondingly large reflex responses; as the amplitude of movement was increased, the reflex component of the resisting force became relatively smaller and the total resisting force was then phase-advanced on joint position at all frequencies.4. The reflex component of the resisting force (as indicated by the excursion of the joint stiffness vectors) varied from subject to subject and from time to time; the reflex usually became more active late in an experiment when the subject had exerted flexing forces against the imposed movement for some minutes. Extreme fatigue, however, diminished the amount of reflex force.5. In some subjects the joint-stiffness records indicated a particularly vigorous reflex response at 8-11 Hz, in contrast to a rather feeble response at 6 or 7 Hz. It is suggested that the reflex pathways then had a relatively low impedance to afferent signals that were modulated at 8-11 Hz, related perhaps to the firing patterns of the most recently recruited motoneurones.6. Under the conditions of these experiments, it appears that the stretch reflex has too small a gain to function as a very effective error-controlled position servo-mechanism.

Adult↗

PubMed1982

Different types of tremor in the human thumb.

1. The upper limbs of normal subjects were immobilized in a way that allowed measurement of forces and movements at the thumb interphalangeal joint without significant movement elsewhere in the limb. 2. When the subject attempted to maintain a steady flexing force at the joint against a rigid stop, the actual force showed the irregular 8-11 Hz fluctuations characteristic of a 'physiological tremor'. This force fluctuation increased when the mean flexing force increased. 3. If the subject exerted his flexing force against a light complaint spring, there was an analogous irregular 8-11 Hz movement at the joint. 4. When, however, an extra inertial load was added to the terminal phalanx, flexion against a complaint spring was often accompanied by a different type of tremor. This was a more regular oscillation, of lower frequency (3-6 Hz), and of much larger amplitude. 5. The precise frequency and amplitude of this type of tremor depended on the characteristics of the added inertia and spring, in a way that could have been predicted from the responses of the joint to an imposed sinusoidal movement (Brown, Rack & Ross, 1982a). The movement appeared to arise from re-excitation within stretch reflex pathways. 6. The irregular 8-11 Hz tremor at this joint could not be attributed to reflex re-excitation, since the responses to sinusoidal movement indicated a stretch reflex whose timing would not support a movement at that frequency. It is, however, emphasized that other joints of the hand and fingers may behave in different ways.

Adult↗

PubMed1979