The action potential of spinal axons in vitro

Despite the trauma of dissection and special metabolic requirements, the physiological properties of funiculi of the mammalian spinal cord can be studied in vitro. They are adequately oxygenated by diffusion at 0.88 atm. pO(2) and remain in a functionally normal state for over 12 hours. The internal consistency of several kinds of data presented in this and the foregoing papers (5, 38) serves to characterize certain properties of central myelinated axons whether excised or in situ. (1) Spinal tracts support a large spike potential in vitro whose form, duration, and velocity are comparable to those of alpha fibers in vitro and spinal tracts in vivo. (2) Properties consistent with a large L fraction are found in central axons whether excised or in situ. (3) Following conduction there has been identified post-spike supernormality with exponential time course (7.5 msecs. half-time) which is the result of activity intrinsic to parent fibers of dorsal columns. The supernormality is similar in form and magnitude both in excised and intact funiculi. (4) In excised funiculi the action potential of parent axons includes a large negative after-potential whose form and duration correspond satisfactorily with this supernormality. This potential appears not to result from activity arising in broken collaterals. (5) Central axons, excised or intact, fire spontaneously in the presence of citrate ion, and when synchronized by stimulation develop periodic oscillations at about 400 C.P.S. but show no such behavior in the presence of excess potassium ion. Certain characteristics peculiar to central axons indicate that they occupy an extreme position in the spectrum of properties encountered in conducting tissues. Dorsal column myelinated axons differ from their peripheral counterparts, even though they are parts of the same cell, in the following ways. The maintenance of the column spike potential is more critically dependent on CO(2) and the entire tissue mass has a higher oxygen consumption. The negative after-potential is much larger and the positive after-potential, non-existent following a single volley, is more difficult to develop by repetitive stimulation. Unlike peripheral nerve, central axons are not incited to spontaneous activity by manipulation of certain constituents normally present in their environment. However, when induced by the application of citrate the resulting rhythmic behavior has twice the frequency of that in peripheral nerve. In general, the recovery process in central axons is more invariant than that in peripheral axons when they are subjected to similar changes in their artificial environments.     

Development and relationships of locomotor, feeding, and oxygen consumption rhythms in house crickets

ABSTRACT. Locomotor, feeding, drinking, and oxygen consumption rhythms in adult virgin Acheta domesticus L. all appear to peak in the first half of the scotophase, be entrained cophasically by a LD 14:10 h cycle, have a lights-off Zeitgeber and persist in LL with a π c. 25 h for the locomotor rhythm and c. 23 h for the oxygen consumption rhythm. There is no evidence of these rhythms in last instar larvae. The onset of the locomotor rhythm requires 3 days at 30°C but 5–7 days at 25–28°C after the final ecdysis in virgins, indicating a temperature related development of the locomotor rhythm. Oxygen consumption rhythms are lacking in 2-day-old virgins but present in 8-day-old virgins. Feeding rhythms can be recorded in virgins as young as 2 days (before locomotor rhythm developed). Both oxygen consumption and locomotor rhythms persist during starvation. The results suggest that a central brain oscillator drives both feeding and locomotor rhythms independently, but that the oxygen consumption rhythm is derived from the metabolic demands associated with the other rhythms.     

Ca−K Bi-ionic action potential in squid giant axons

Summary Under intracellular perfusion with a solution containing K⁺ as the sole cation species, squid giant axons were found to be capable of developing all-or-none action potentials when immersed in a medium in which CaCl₂ was the only electrolyte. The adequate range of ion concentration for demonstrating this capability was mentioned. The reversal potential level measured by the voltage-clamp technique varied directly with the logarithm of the concentration of extracellular Ca-ion; the proportionality constant was close toRT/2F. The action potential observed under this Ca–K bi-ionic condition could not be suppressed by addition of tetrodotoxin or saxitoxin to the external medium. The external Ca-ion could be replaced with Co- or Mn-ion without eliminating the capability of the axons to develop action potentials. D-600 could not suppress the inward current observed under the voltage-clamp condition, but 4-aminopyridine could suppress it. The experimental findings were interpreted based on the current channel hypothesis and on the macromolecular theory.     

Some quantitative relationships between leaf area index and canopy nitrogen content and distribution

In a previous study (Yin et al. 2000. Annals of Botany 85: 579-585), a generic logarithmic equation for leaf area index (L) in relation to canopy nitrogen content (N) was developed: L=(1/ktn)1n(1+ktnN/nb). The equation has two parameters: the minimum leaf nitrogen required to support photosynthesis (nb), and the leaf nitrogen extinction coefficient (ktn). Relative to nb, there is less information in the literature regarding the variation of ktn. We therefore derived an equation to theoretically estimate the value of ktn. The predicted profile of leaf nitrogen in a canopy using this theoretically estimated value of ktn is slightly more uniform than the profile predicted by the optimum nitrogen distribution that maximizes canopy photosynthesis. Relative to the optimum profile, the predicted profile is somewhat closer to the observed one. Based on the L-N logarithmic equation and the theoretical ktn value, we further quantified early leaf area development of a canopy in relation to nitrogen using simulation analysis. In general, there are two types of relations between L and N, which hold for canopies at different developmental phases. For a fully developed canopy where the lowest leaves are senescing due to nitrogen shortage, the relationship between L and N is described well by the logarithmic model above. For a young, unclosed canopy (i.e. L < 1.0), the relation between L and N is nearly linear. This linearity is virtually the special case of the logarithmic model when applied to a young canopy where its total nitrogen content approaches zero and the amount of nitrogen in its lowest leaves is well above nb. The expected patterns of the L-N relationship are discussed for the phase of transition from young to fully developed canopies.     

The relationships between dream content and physical health, mood, and self-construal.

Research is presented that examines the relationship among dream content, physical health, mood, and self-construal. Participants were 27 undergraduate students who completed the Medical Outcomes SF-36 Health Survey (SF-36), the Profile of Mood States Scale (POMS-SF), and the Self-Construal Scale (SCS). Each participant handed in four dream reports, which were analyzed according to the Hall and Van de Castle (1966) system of content analysis. Multiple significant correlations were observed between dream content and the SF-36, the POMS-SF, and the SCS. Most notable were the findings between physical health and dream content. Participants displaying poor physical health reported more bodily misfortunes, injuries and illnesses, medical themes, and body parts in their dreams. Findings support continuity between dreams and waking life physical and mental functioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Isolated epithelial cells of the toad bladder. Their preparation, oxygen consumption, and electrolyte content

Epithelial cells of the toad bladder were disaggregated with EDTA, trypsin, hyaluronidase, or collagenase and were then scraped free of the underlying connective tissue. In most experiments EDTA was complexed with a divalent cation before the tissue was scraped. Q _OO2, sucrose and inulin spaces, and electrolytes of the isolated cells were measured. Cells disaggregated by collagenase or hyaluronidase consumed O₂ at a rate of 4 µl hr^-1 dry wt^-1. Q _OO2 was increased 50% by ADH (100 U/liter) or by cyclic 3'',5''-AMP (10 mM/liter). Na⁺-free Ringer''s depressed the Q _OO2 by 40%. The Q _OO2 of cells prepared by trypsin treatment or by two EDTA methods was depressed by Na⁺-free Ringer''s but was stimulated relatively little by ADH. Two other EDTA protocols produced cells that did not respond to Na⁺ lack or ADH. The intracellular Na⁺ and K⁺ concentrations of collagenase-disaggregated cells were 32 and 117 mEq/kg cell H₂O, respectively. Cation concentrations of hyaluronidase cells were similar, but cells that did not respond to ADH had higher intracellular Na⁺ concentrations. Cells unresponsive to ADH and Na⁺ lack had high sucrose spaces and low transcellular membrane gradients of Na⁺, K⁺, and Cl^-. The results suggest that trypsin and EDTA disaggregation damage the active Na⁺ transport system of the isolated cell. Certain EDTA techniques may also produce a general increase in permeability. Collagenase and hyaluronidase cells appear to function normally.     

A model for thermal exchange in axons during action potential propagation

Several experiments have shown that during propagation of the action potential in axons, thermal energy is locally exchanged. In this paper, we use a simple model based on statistical physics to show that an important part of this exchange comes from the physics of the effusion. We evaluate, during the action potential propagation, the variation of internal energy and of the energy associated with the chemical potential of the effusion of water and ions to extract the thermal energy exchanged. The temperature exchanged is then evaluated on the area where the action potential is active. Results give a good correspondence between experimental work and this model, showing that an important part of the thermal energy exchange comes from the statistical cooling power of the effusion.     

Mitochondrial coenzyme Q content and aging.

The main objective of this study was to determine the nature of the relationship between aging and mitochondrial coenzyme Q (CoQ) content. Mitochondria in the heart, skeletal muscle, kidney and brain of the mouse varied in both the amount of total CoQ (CoQ9 + CoQ10) content as well as in the ratio of the CoQ9 to CoQ10. CoQ content declined with age only in the skeletal muscle. Caloric restriction (CR) resulted in an increase in the amount of CoQ9 in skeletal muscle mitochondria. This effect was partially reversible upon termination of the caloric restriction regimen. Results suggest that a decrease in mitochondrial CoQ content is an integral aspect of aging in skeletal muscle.     

Effects of arterial oxygen content on peripheral locomotor muscle fatigue.

The effect of arterial O2 content (Ca(O2)) on quadriceps fatigue was assessed in healthy, trained male athletes. On separate days, eight participants completed three constant-workload trials on a bicycle ergometer at fixed workloads (314 +/- 13 W). The first trial was performed while the subjects breathed a hypoxic gas mixture [inspired O2 fraction (Fi(O2)) = 0.15, Hb saturation = 81.6%, Ca(O2) = 18.2 ml O2/dl blood; Hypo] until exhaustion (4.5 +/- 0.4 min). The remaining two trials were randomized and time matched with Hypo. The second and third trials were performed while the subjects breathed a normoxic (Fi(O2) = 0.21, Hb saturation = 95.0%, Ca(O2) = 21.3 ml O2/dl blood; Norm) and a hyperoxic (Fi(O2) = 1.0, Hb saturation = 100%, Ca(O2) = 23.8 ml O2/dl blood; Hyper) gas mixture, respectively. Quadriceps muscle fatigue was assessed via magnetic femoral nerve stimulation (1-100 Hz) before and 2.5 min after exercise. Myoelectrical activity of the vastus lateralis was obtained from surface electrodes throughout exercise. Immediately after exercise, the mean force response across 1-100 Hz decreased from preexercise values (P < 0.01) by -26 +/- 2, -17 +/- 2, and -13 +/- 2% for Hypo, Norm, and Hyper, respectively; each of the decrements differed significantly (P < 0.05). Integrated electromyogram increased significantly throughout exercise (P < 0.01) by 23 +/- 3, 10 +/- 1, and 6 +/- 1% for Hypo, Norm, and Hyper, respectively; each of the increments differed significantly (P < 0.05). Mean power frequency fell more (P < 0.05) during Hypo (-15 +/- 2%); the difference between Norm (-7 +/- 1%) and Hyper (-6 +/- 1%) was not significant (P = 0.32). We conclude that deltaCa(O2) during strenuous systemic exercise at equal workloads and durations affects the rate of locomotor muscle fatigue development.     

Relationship between cytosolic calcium and oxygen consumption in isolated rat hearts.

Maximum oxygen consumption was attained in isolated perfused rat hearts using high perfusate calcium and/or isoproterenol, or phenylephrine. The amplitude of calcium transients was directly related to oxygen consumption until oxygen consumed per beat reached maximum. At saturating oxygen consumption the amplitude of [Ca2+]i transients continued to increase, indicative of a calcium overload. In all cases +dP/dt correlated proportionately with +dCa2+/dt. Augmented developed pressure, related to isoproterenol-induced increase in cytosolic cAMP, cannot be attributed totally to elevated levels of [Ca2+]i transients. Adenosine (10(-5) M) added to the medium containing isoproterenol (10(-6) M) negated the isoproterenol-induced increase in cAMP and returned cardiac performance, oxygen consumption, and amplitude of [Ca2+]i transients to control state.     

Relationship between record time and maximal oxygen consumption in middle-distance running.

The relationship between record time (tr) and maximal oxygen uptake (VO2max) has been examined in 69 male physical education students who had taken part in 800-m and 1500-m footraces. It was found that tr and VO2max were inversely related. The relationships tr = f(VO2max) have been fitted by two exponential equations: tr (1500 m) = 698e-0.0145VO2max; tr (800 m) = 272e-0.011VO2max; P less than 0.001. A mathematical formulation of the energy conservation principle in supramaximal running, based on the exponential increase of the oxygen uptake as a function of time with a rate constant of 0.025 s-1 has been applied to the tr calculation from VO2max. As calculated tr were highly correlated to measured tr (P less than 0.001), it was concluded that the relationships tr = f(VO2max) can be interpreted on the basis of the model described in this study.     

Stochastic simulations on the reliability of action potential propagation in thin axons

It is generally assumed that axons use action potentials (APs) to transmit information fast and reliably to synapses. Yet, the reliability of transmission along fibers below 0.5 μm diameter, such as cortical and cerebellar axons, is unknown. Using detailed models of rodent cortical and squid axons and stochastic simulations, we show how conduction along such thin axons is affected by the probabilistic nature of voltage-gated ion channels (channel noise). We identify four distinct effects that corrupt propagating spike trains in thin axons: spikes were added, deleted, jittered, or split into groups depending upon the temporal pattern of spikes. Additional APs may appear spontaneously; however, APs in general seldom fail (<1%). Spike timing is jittered on the order of milliseconds over distances of millimeters, as conduction velocity fluctuates in two ways. First, variability in the number of Na channels opening in the early rising phase of the AP cause propagation speed to fluctuate gradually. Second, a novel mode of AP propagation (stochastic microsaltatory conduction), where the AP leaps ahead toward spontaneously formed clusters of open Na channels, produces random discrete jumps in spike time reliability. The combined effect of these two mechanisms depends on the pattern of spikes. Our results show that axonal variability is a general problem and should be taken into account when considering both neural coding and the reliability of synaptic transmission in densely connected cortical networks, where small synapses are typically innervated by thin axons. In contrast we find that thicker axons above 0.5 μm diameter are reliable.     

Dopamine modifies oxygen consumption and mitochondrial membrane potential in striatal mitochondria

Dopamine is a neurotransmitter that has been related to mitochondrial dysfunction. In this study, striatal intact mitochondria and submitochondrial membranes were incubated with different dopamine concentrations, and changes on mitochondrial function, hydrogen peroxide, and nitric oxide production were evaluated. A 35% decrease in state 3 oxygen uptake (active respiration state) was found after 1 mM dopamine incubation. In addition, mitochondrial respiratory control significantly decreased, indicating mitochondrial dysfunction. High dopamine concentrations induced mitochondrial depolarization. Also, evaluation of hydrogen peroxide production by intact striatal mitochondria showed a significant increase after 0.5 and 1 mM dopamine incubation. Incubation with 0.5 and 1 mM dopamine increased nitric oxide production in submitochondrial membranes by 28 and 49%, respectively, as compared with control values. This study provides evidence that high dopamine concentrations induce striatal mitochondrial dysfunction through a decrease in mitochondrial respiratory control and loss of membrane potential, probably mediated by free radical production.     

Levels of myo-inositol in normal and degenerating peripheral nerve

—Free inositol was measured in peripheral nerves of the monkey, rabbit, rat, frog and lobster; levels in mammalian nerve were similar, and two to three times greater than in the other species. Concentrations of myo-inositol in rabbit tibial nerve increased from proximal to distal segments; in optic nerve the concentrations decreased with greater distance from the retina. In the early stages of Wallerian degeneration rabbit tibial nerve contained 25 per cent less free myo-inositol, rat nerve 50 per cent less. Rabbit nerves were analysed at 2 and 5 weeks after section; by 5 weeks levels of myo-inositol had increased to 50 per cent above normal. Similar changes were found in degenerating rabbit optic nerve. The combination of galactose feeding and nerve section resulted in reduction of the myo-inositol in rat sciatic nerve to one-fifth of the control value; galactitol in the nerve decreased by 50 per cent after section. The evidence suggests that myo-inositol in nerve is located mainly in Schwann cells or glia.