«PROCEEDI NGS OF THE PHYSIOLOGICAL SOCIETY 10 February 1945 Relation between fibre diameter and action potential of single nerve fibres. BY HELGE ...»
10 February 1945
Relation between fibre diameter and action potential of single
nerve fibres. BY HELGE HERTZ. Theory of Gymnastics Laboratory,
Copenhagen and Physiological Institute, Lund, Sweden
Experiments have been made to find to what extent the propagation of the
nervous impulse is dependent on (a) ionic exchange through the nerve mem- brane and (b) processes on the surface of the membrane.
Single fibres of the frog's sciatic were stimulated with brief condenser dis- charges, and monophasic potentials were recorded with a push-pull amplifier and cathode-ray oscillograph. The diameter of the fibre was measured with an eyepiece micrometer and water-immersion objective.
Osmotic experiments. Solutions, made up to 7 times hypertonic with NaCl or glucose, or i hypotonic, were without effect on either diameter or action potential in the first 10 min. of exposure, showing the extreme water im- permeability of the nerve membrane. Later, the nerve shrinks or swells by 10 %, and the declining phase of the action potential becomes progressively slower, until in I hr., although the nerve has returned to its previous diameter, the negativity persists indefinitely after a stimulus. The recovery process had ceased on account of permanent damage to the fibre. The osmotic damage is only reversible if exposure is less than 5 min.
Effect of KCI. In contrast to the above experiments, immersion of the nerve in isosmotic solutions with a KCl content 6-10 times normal, causes an immediate and reversible reduction in the amplitude of the action potential (cf. Cowan, 1934); its shape remains normal. The effect of KCI can be only partially counteracted by an equivalent increase in the CaCl2 concentration.
An attempt to remove K+ from the nerve by Na-Permutit ion-exchanger was without effect.
Other ions. Removal of Ca++ by citrate or oxalate did not cause spontaneous activity until 3 hr. had elapsed. Magnesium chloride (200 mg./100 c.c.) was without effect over 2 hr., and so was acetylcholine chloride (1 mg./100 c.c.).
The experiments demonstrate the impermeability of the membrane around the axis cylinder, and it appears unlikely that any great exchange of ions could take place in the short time required for the action potential to reach its a
2P PROCEEDINGS OF THE PH YSIOLOGICALmaximum. The immediate effect of KCl shows that it acts on the surface, but a normal K+: Ca++ ratio within the membrane is necessary for the surface phenomenon.
REFERENCECowan, S. L. (1934). Proc. Roy. Soc. B, 115, 216.
The colour of small objects. By H. HARTRIDGE In a letter to Natu
Killing at the 10th day yields a mesoderm-free trophoblast which grows from a diameter of 0 5 mm. to one of 5 mm. at the 16th day. This ectodermal trophoblast differentiates into its three zones with a normal type of evolution including well-developed glycogen accumulation. The mesoderm-free 'placenta' is destroyed on the lYth day by rupture of maternal vessels into the glycogen zone. If the foetus is killed after the mesoderm has grown into the placenta, the latter persists to full term, growth, however, being restricted to the trophoblast. The decidua either necroses or absorbs with ovariectomy, oestrone or F.S.H. injections. This decidual destruction impairs the growth of the rest of the placenta.
REFERENCES Courrier, R. & Gros, G. (1936). C.R. Soc. Biol., Paris, 121, 1517.
Newton, W. H. (1935). J. Physiol. 84, 196.
5P SOCIETY, 10 FEBRUARY 1945 The influence of pregnant-mares' serum on the pregnant rat.
By A. ST G. HUGGETT and J. J. PRITCHARD Abortion and resorption of foetuses following implantation of fresh anterior pituitary or injection of pregnancy urine have been described by Engle & Mermod (1928) in rats and mice, by Hill & Parkes (1932) in the rabbit, and by Coco (1942) in the rat. Alternatively pregnancy was prolonged. We have found similar effects in the rat following a single subcutaneous injection of F.S.H.
derived from pregnant-mares' serum (Gestyl-Organon). The cause of foeta] death, and the subsequent changes in the placenta have been investigated.
Foetal death is most easily produced by injection at the 10th day of pregnancy. A After the 12th day, the only effect of F.S.H. is to delay parturition. If retained after the 23rd day, foetuses die. (Full term is 21 days normally.) 50 i.u. is the minimum effective dose at the 10th day. This takes 3 days to produce its maximum effect. Ovarian weight also reaches its peak in 3 days.
From 4 to 6 days after injection, induced follicles become luteinized.
Histology. Normally the allantoic circulation through the placenta is established on the 11th day. Lethal doses of F.S.H. given on the 10th day inhibit the penetration of allantoic blood vessels into the trophoblast.
The decidua basalis is abnormally thick owing to a failure of normal involution, remaining that of the date of death, is reduced in vascularity, and shows large areas of coagulative necrosis. In most cases the trophoblast survives, grows, and differentiates, in comparatively normal fashion, in spite of decidual necrosis, absence of allantoic mesoderm, and absence of foetus.
In particular, the giant cells and glycogenic trophoblast are well developed.
The labyrinthine trophoblast is partly destroyed by haemorrhage. It is probable that the decidual changes are the ultimate cause of foetal death after F.S.H.
REFERENCES Engle, E. T. & Mermod, C. (1928). Amer. J. Physiol. 85, 518.
Hill, M. & Parkes, A. S. (1932). Proc. Roy. Soc. B, 110, 180.
Coco, R. M. (1942). Amer. J. Physiol. 137, 143.
Effect of acceleration on cats, with and without water immersion.
By A. D. M. GREENFIELD I Forty cats, anaesthetized with chloralose, were centrifuged to produce accelerations up to 20 G in the head-tail axis measured at the heart. Continuous records of acceleration and of arterial, right auricular and intrapleural pressures were obtained. Without water immersion, arterial pressure showed an abrupt fall with onset of acceleration, reached zero at the head level at a3
PROCEEDINGS OF THE PHYSIOLOGICAL6P 3-4 G, and remained approximately constant during 1 min. runs. Right auricular pressures fell, but this was partly offset by the fall in intrapleural pressure. Following the run, the auricular pressure returned to its resting value, but the arterial pressure rose for 1-3 min. to a height greatly exceeding the resting value.
Immersion in water at body temperature to levels lower than 4 cm. below the cardiac apex made little difference to the response, but with the water level at the cardiac apex it required an acceleration of about 10 G to reduce the arterial pressure to zero at the head. After the first 10 sec. of the run, arterial pressure showed an increase of 60-80 mm. Hg, which was abolished by carotid sinus denervation. When this compensation had occurred, it required 15-20 G to reduce arterial pressure to zero at the head, corresponding to a pressure of about 300 mm. Hg at the heart. The right auricular pressure showed only a slight rise during the run. Following the run the arterial pressure rose above the resting value for 1-3 min. by an amount proportional to the compensation occurring during the run. This rise was also abolished by carotid sinus denervation.
Raising the water level above the cardiac apex gave only slight further improvement of the arterial pressure response, but the right auricular pressure showed a greater rise during the run.
Respiration was slowed and became shallow with occasional gasps in the non-immersed animals, but was well maintained in the water-immersed animals up to 16 G.
Following exposure to 15-20 G with water immersion for several halfminute runs, death frequently occurred, and subendocardial haemorrhages were observed in the left ventricle.
Part of the expenses of this research were defrayed by a Grant from the Governments Grants Committee of The Royal Society.
Physiological studies on animals subjected to positive G.* By H. H. JASPER and A. J. CIPRIANI. From the Neurological Institute of McGill University Experiments were conducted on cats and monkeys (M. mulatta) with or without hydrostatic protection. A small centrifuge was used having a radius of 6 ft. and yielding a maximum of about 12 G. When possible, anaesthesia was avoided, but in some experiments light nembutal was given. Pressures were measured by means of a small photoelectric manometer which was practically isometric and not itself affected by G. Recording was by electrical methods and motion pictures.
7P SOCIETY, 10 FEBRUARY 1945 In an unsuccessful search for an objective criterion of black-out, records of physiological interest were taken of brain waves, retinal potentials and the occipital cortical response to flashes of light. With the application of sufficient G for a long enough time, the E.E.G. showed an excitatory phase, followed by a train of delta activity leading to extinction of all rhythms. On recovery there was usually epileptiform activity followed by a gradual return to normal. Clinical attacks accompanied epileptiform activity. Retinal potentials showed a diminution in the amplitude of the B component which eventually disappeared. If the G was low the E.E.G. outlasted the B wave, but if the G was high and of rapid onset the brain waves were the first to disappear.
The occipital cortical response disappeared shortly before the B wave of the retinogram.
Also: (1) Intracranial pressure decreased and became negative in a linear manner with the application of positive G. (2) Carotid arterial blood pressure and pulse pressure fell rapidly to zero with sufficient application of G. (3) Venous pressures in the femoral vein obeyed hydrostatic laws, but if measured below a valve showed a time lag in relation to measurements made above the valves. (4) Moving pictures of the cerebral vessels taken through a Forbes window under G showed a blanching of the cerebral surface, retention of blood in the larger vessels and slight movement of the brain, so long as the skull was intact. When the skull was punctured the brain sank under the application of G.
Hydrostatic protection tended to preserve the status quo to an extent which depended on the level of protection and the magnitude of the applied force. In fully protected animals subjected to high G for long and repeated periods the E.K.G. indicated right myocardial failure, and pathological examination revealed haemorrhage in the muscle of the right ventricle and the bases of the lungs.
This work was supported by a grant from the Associate Committee on Aviation Medical Research, the National Research Council of Canada.
Investigations on centrifugal force. By W. K. STEWART.
From the R.A.F. Physiological Laboratory This problem has been studied in four general ways: (1) by experienced subjects in experimental aircraft, (2) by physiologists piloting various aircraft, e.g. Diringshofen, Davidson, (3) by mass study of pilots and their reactions, (4) in man-carrying centrifuges.
The major effects of centrifugal force on man result from the increased weight of his body components, especially the blood. Carfotid blood pressure and cardiac output are lowered, with impairment of circulation and of central
PROCEEDINGS OF THE PHYSIOLOGICAL8P nervous functions. Of these, vision suffers first owing to the retinal circulation being opposed by the intraocular pressure which, if artificially raised, lowers the G threshold for blacking-out. Total failure of vision may be preceded by a progressive rise of threshold and light sense may outlast visual acuity.
In a large group of pilots, large unaccountable differences in threshold occur.
The average difference between greying of vision and black-out is 0 7 G.
Central circulatory failure results in brief unconsciousness (not necessarily preceded by black-out if G is excessive and suddenly applied) followed on recovery by marked confusion or disorientation.
Raised blood sugar, benzedrine or adrenal cortical hormone scarcely influence visual impairment, and since susceptibility to the cerebral effects of G is increased during flight by 02 lack (if severe or reinforced by CO) the effects described are attributed to retinal and cerebral anoxia. Occlusion of the leg circulation raises and reactive hyperaemia lowers the black-out threshold to G. During the partial visual impairment of a prolonged manoeuvre, muscular effort (abdominal straining) may raise the blood pressure and restore normality. A pilot's resistance to the stress may depend on his general circulatory reactivity.
Duration is important. 20 G for 0.01 sec. or 12 G for 0X1 sec. causes no visual or neurological disturbance. (Tolerance of brief large forces probably depends on the structural strength of the body.) Several minutes of acceleration insufficient to impair vision cause only fatigue. In flight, the acceleration which affects. vision usually reaches its maximum before reflex compensation (autonomic or somatic) can occur.
Increased weight may immobilize the trunk, but the limbs if supported are movable at right angles to the acceleration. Rapid rotation of the head (or a turret) at about 20°/sec. under high G may cause temporary disorientation, otherwise vestibular disturbances are uncommon in experienced pilots.
Description of a centrifuge and its use for studying the effects of centrifugal force on man. By W. R. FRANKS, W. K. KERR and B. RoSE. R.C.A.F. (Toronto) In order to investigate the effects of centrifugal force on man, a centrifuge was constructed for the R.C.A.F. It is built into a circular concrete pit 12 ft.
in depth and 31-5 ft. in diameter. To the central shaft which is supported above and below, a single, highly stressed horizontal arm, 8.5 ft. in length, is attached.
The car which carries the subject and up to 200 lb. of apparatus has an inside diameter of 6 ft. 2 in. and is suspended from the distal end of the horizontal rotating arm. The centrifuge is stressed to support 15 times the weight exerted by a fully laden car at 10 G. (1 G equals the force due to the pull of gravity.