Search PubMed⌕ Search

Biomedical subjects

J N Davis

Publications and source records attributed to J N Davis.

At least 145 records · Page 8Linked to original sources
PubMed1975

Segmental analysis of spinal cord monoamines after thoracic transection in the dog.

The noradrenaline, dopamine and serotonin content from 10 segments of the dog spinal cord before and after a T7 transection were measured. The levels of amines were highest in the conus medullaris. However, when calculated per gram cord grey matter, noradrenaline was found to be concentrated in the midthoracic and sacral cord, while serotonin was concentrated in the mid-thoracic region. Levels of dopamine were very low throughout the spinal cord. Up to 3 days after a T7 transection, the levels of all 3 amines did not change in the distal cord segments. Ten days after transection the level of all 3 amines were less than twice tissue blank distally. This segmental analysis of dog spinal cord shows some potentially important differences from previous studies of cat and rat.

Animals↗
PubMed1975

Interrelationships of the volume and time components of individual breaths in resting man.

1. The volume and time components of individual breaths have been investigated under 'steady-state' conditions during air-breathing in fifteen subjects and, in a further six subjects, also during the addition of 1-5 and 3% CO2 to the inspired gas. 2. A computer-assisted method has been used to derive from the air flow record the individual breath values of tidal volume (VT), inspiratory duration (TI), expiratory duration (TE) and cycle duration (TC = TI + TE) for a sequence of breaths in the various steady-state conditions. 3. When the subjects were breathing room air, for breath sequences of over 200 breaths (n = 228-365), mean values of TI (TI) mostly lay between 1 and 2-5 sec, TE between 2-0 and 3-5 sec and VT between 0-4 and 0-91. The distributions of VT, TI and TE were in general unimodal. 4. Significant negative correlations between VT and F, and positive correlations between VT and TC, have been confirmed. 5. In all fifteen subjects, a highly significant positive correlation existed between VT and TI (mean r = +0-704), which was stronger than that between VT and TC (mean r = +0-533). Weaker positive correlations were demonstrated between VT and TE, and between TI and TE (mean r = +0-359 and +0-381 respectively). 6. The intercept of the regression of VT on TI passed close to the origin, typically slightly positive on the VT axis (mean = +0-0991.). This, coupled with the strong positive correlation between VT and TI, indicates that the mean inspiratory flow rate (VI = VT/TI) for each breath is held relatively constant from breath to breath. 7. In the six subjects studied during CO2 inhalation, the mean % contributions of VT and F to the increase in ventilation associated with breathing 3% CO2 were 75 and 25% respectively. 8. At the three different levels of ventilation, neither VT, TI nor TE showed a wholly consistent trend, although VT tended to increase, TE to decrease and TI to be unchanged. In contrast, the average values of VI (VI) consistently increased as the chemical stimulus was raised. 9. It is concluded that the previously observed tendency for ventilation to be held constant from breath to breath during steady-state breathing depends predominantly on the tendency for VI to be held constant. Close restraints are evidently not imposed on the individual values of VT and TI under these conditions. The neural mechanism generating breathing appears to control ventilation principally by regulating the rate of inspiratory air flow and secondarily TE.

Adult↗
PubMed1975

The response to stretch of human intercostal muscle spindles studied in vitro.

1. The discharge properties of human muscle spindles have been studied in vitro in a preparation based on the biopsied external intercostal muscle. 2. The static and dynamic responsiveness of thirty-six endings in twenty visualized and histologically identified spindles have been investigated using amplitudes and velocities of stretch likely to encompass those occurring in vivo. 3. The dynamic index, measured at a stretch velocity of 3 mm/sec, ranged from 3 to 40 impulses/sec and was distributed bimodally, consitent with the presence of primary and secondary endings. 4. The relationship between the dynamic index and the velocity of stretch was approximately linear both for primary and secondary endings up to the maximum velocity tested (10 mm/sec). 5. The frequency/extension relationship was approximately linear for both primary and secondary endings. The mean values of the slope for primary and secondary endings were 16-1 +/- 8-3 S.D. of the observation and 12-1 +/- 6-5 impulses/sec per five per cent extension. 6. The slopes of the frequency/extension relationship for endings lying in the same spindle were positively correlated, significant at the 10% level. 7. It was estimated from the results in vitro that the position sensitivity of human intercostal spindles in vivo ranges from 2 to 21 impulses/sec per millimetre.

Action Potentials↗
PubMed1975

Adaptation of brain monoamine synthesis to hypoxia in the rat.

Oxygen is a substrate in the synthesis of the neurotransmitters, norepinephrine, dopamine, and serotonin. Changes in environmental oxygen appear to cause corresponding alterations in brain monoamine synthesis in vivo. The effect of chronic hypoxia was studied by exposing rats to 10% oxygen for up to 36 h. Brain monoamine synthesis, estimated in vivo, decreased initially and then returned to control levels, despite continued exposure to 10% oxygen. During this apparent adaptation to hypoxia, there were no changes in the concentration of brain serotonin, norepinephrine, dopamine, or tryptophan, while brain tryosine increased after 24 h of exposure. Tyrosine hydroxylase activity in vitro was not altered by the exposure to 10% oxygen. Evidence of hypoxic adaptation in these rats, a rightward shift of their hemoglobin dissociation curves, was found after 24 h of exposure. The adaptation of brain monoamine synthesis to hypoxia appeared to correlate with adaptive changes in brain tissue oxygen rather than any change in the intraneuronal regulation of amine synthesis.

5-Hydroxytryptophan↗
PubMed1973

Brain tryptophan hydroxylation: dependence on arterial oxygen tension.

The accumulation of cerebral 5-hydroxytryptophan after decarboxylase inhibition was decreased in rats maintained at arterial O(2) tensions below 60 mm-Hg. In contrast, brain lactate was stable above 40 mm-Hg and brain adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate were unchanged above 30 mm-Hg. There was a linear correlation of brain 5-hydroxytryptophan accumulation to cerebral venous O(2) tension. Cerebral tryptophan hydroxylase appears to have a poor affinity for oxygen and to be affected by slight hypoxia. The resultant decreases in monoamine neurotransmitter metabolism may explain the behavioral changes of mild oxygen deprivation.

5-Hydroxytryptophan↗