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Biomedical subjects

G M Jones

Publications and source records attributed to G M Jones.

At least 163 records · Page 9Linked to original sources
PubMed1973

Degradation of glucose-metabolizing enzymes in the rat small intestine during starvation.

1. The degradation rates and half-lives of hexokinase, 6-phosphogluconate dehydrogenase, lactate dehydrogenase, pyruvate kinase, glucose 6-phosphate dehydrogenase, phosphoglycerate kinase and aldolase were calculated from measurements of the decline in activities of these enzymes in rat small intestine during starvation. 2. The half-lives of the enzymes are: hexokinase, 5.7h; 6-phosphogluconate dehydrogenase, 7.6h; glucose 6-phosphate dehydrogenase, 6.0h; pyruvate kinase, 8.9h; lactate dehydrogenase, 8.7h; phosphoglycerate kinase, 8.7h; aldolase, 5.1h. 3. The significance of the results is discussed with respect to the regulation of enzyme concentrations in response to changes in diet.

Animals↗
PubMed1971

Frequency-response analysis of central vestibular unit activity resulting from rotational stimulation of the semicircular canals.

1. The neural response of semicircular canal-dependent units in the vestibular nuclei of cats has been examined over a frequency range of sinusoidal rotation extending from 0.004 to 0.9 Hz.2. Frequency-response analysis indicates that, over the range examined, the information contained in the neural signal received by the brain stem was similar to that expected from the mechanical end-organ.3. Over the experimental frequency range, the relation between neural response and mechanical stimulus was found to be dominated by a single time constant of about 4 sec, such that two response regions can be defined above and below a stimulus frequency of (1/4) rad/sec ( approximately approximately Hz).4. Above this frequency the information content of the neural signal tends towards that of angular velocity and below that frequency it tends towards that of angular acceleration.5. It is inferred (a) that the so-called ;long' time constant of the cat's horizontal canal is about 4 sec and (b) that during most normal head movements containing frequencies below about 1 Hz the informational mode of neural signals generated in the canal and received in the brain stem probably tends towards that of head angular velocity.6. This seems appropriate for the generation of vestibulo-ocular reflex compensation for head movement and for reflex damping (negative velocity feed-back) of unintended head and body movements.7. The average neural gain of central unit responses is estimated at 1264 action potentials/sec, per degree of cupular deflexion. This high value reflects the very small angles of cupular deflexion assessed on the basis of physical characteristics of the canal.8. The results permit a rough estimate of the elastic restoring coefficient of the cupula in the horizontal canal as 2.05 x 10(-3) dyne. cm.

Acceleration↗
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↗
PubMed1970

The preparation of alcohol dehydrogenase and glyceraldehyde 3-phosphate dehydrogenase from baker's yeast.

A procedure has been developed for the preparation of alcohol dehydrogenase and glyceraldehyde 3-phosphate dehydrogenase from the same sample of baker's yeast. The two enzymes were obtained in good yield in a pure crystalline form. The method minimizes the work involved in preparing the two enzymes and would be of particular advantage for preparing the enzymes in radioactive form from yeast grown in a radioactive medium.

Alcohol Oxidoreductases↗
PubMed1970

Characteristics of neural transmission from the semicircular canal to the vestibular nuclei of cats.

1. The characteristics of the dynamic response of specifically canal-dependent neural units in cat vestibular nuclei have been examined during sinusoidal rotation of the head in decerebrate cats over the narrow frequency range 0.-1.7 Hz.2. Unit action potential frequencies were averaged on line from extracellular steel micro-electrodes stereotaxically located in the vestibular nuclei through the intact cerebellum.3. Action potential frequency was approximately in phase with stimulus angular velocity, the mean phase for forty-six units being + 11. degrees S.E. of the mean +/- 2.2 degrees ). Correction for a form of dynamic asymmetry reduced this value almost to zero.4. The mean gain of thirty-nine single unit responses was 0.76 S.E. +/- 0.08) AP/sec per degrees /sec. The gain varied as the (-0.28) power of stimulus angular velocity, for the five cells appropriately tested.5. A method was evolved for computing the spontaneous condition of cells, in terms of an equivalent spontaneous firing frequency, f(sp) irrespective of whether they were spontaneously active or silent. This f(sp) value was normally distributed about a mean value of 11.2 AP/sec (range + 70 to - 40 AP/sec).6. Directionality was examined in 116 units, of which 62% were ipsilateral (e.g. cells on left side excited by left-going rotational velocity) and 38% contralateral. Ipsilateral units proved easier to isolate and retain than contralateral ones.7. No significant differences in mean phase or gain were found in the sub-sets of spontaneously active/inactive cells and ipsilateral/contralateral cells.8. It is inferred that the cell population examined was a functionally homogeneous one in which the neural signal was closely tied to the angular velocity of canal rotation for the narrow band of sinusoidal rotational stimuli here employed. It is suggested that this signal is probably retained essentially intact in the ensemble neural message fed forwards from this region.

Action Potentials↗
PubMed1970

Response of semicircular canal dependent units in vestibular nuclei to rotation of a linear acceleration vector without angular acceleration.

1. Recent experiments have shown that rotation of a linear acceleration vector round the head can generate involuntary ocular nystagmus in the absence of angular acceleration. The present experiments examine the suggestion that adequate stimulation of the semicircular canals may contribute to this response.2. Decerebrate cats were located in a stereotaxic device on a platform, slung from four parallel cables, which could be driven smoothly round a circular orbit without inducing significant angular movement of the platform. This Parallel Swing Rotation (PSR) generated a centripetal acceleration of 4.4 m/sec(2) which rotated round the head at 0.52 rev/sec.3. The discharge frequency of specifically lateral canal-dependent neural units in the vestibular nuclei of cats was recorded during PSR to right and left, and in the absence of motion. The dynamic responses to purely angular motion were also examined on a servo-driven turntable.4. Without exception all proven canal-dependent cells examined (twenty-nine cells in nine cats) were more active during PSR in the direction of endolymph circulation assessed to be excitatory to the unit, than during PSR in the opposite direction.5. The observed changes in discharge frequency are assessed to have been of a magnitude appropriate for the generation of the involuntary oculomotor response induced by the same stimulus in the intact animal.6. The findings suggest that a linear acceleration vector which rotates in the plane of the lateral semicircular canals can be an adequate stimulus to ampullary receptors, though an explanation which invokes the modulation of canal cells by a signal dependent upon the sequential activation of macular receptors cannot be positively excluded.

Acceleration↗