Development of the action potential in embryo amphibian neurons in vivo.
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Action Potentials↗
Biomedical subjects
N C Spitzer
Publications and source records attributed to N C Spitzer.
The development of the action potential mechanism of amphibian neurons isolated in culture.
Nerve and muscle cells differentiated morphologically, in cultures of dissociated cells prepared from amphibian neural plate and underlying mesoderm (Xenopus laevis, Nieuwkoop and Faber stage 15). Cultures were grown in a defined medium containing sterile Steinberg's salt solution and 0.1% bovine serum albumin, and maintained for periods up to 5 days.
Action Potentials↗
The ionic basis of the resting potential and a slow depolarizing response in Rohon-Beard neurones of Xenopus tadpoles.
1. Rohon-Beard cells in the spinal cord of Xenopus laevis tadpoles have been studied in animals 4-days to 2-weeks-old (Nieuwkoop & Faber, 1956, stages 45-49). These neurones have an unusually large resting membrane potential of -88 mV, in Ringer solution containing 3-0 mM K+. 2. Their resting potential (R..) depends on the concentration gradient of K+ across the cell membrane. These cells follow the prediction of the Nernst equation for a K+-selective electrode, down to external K+ concentrations as low as 1-0 mM (R.P. -118 mV). 3. The resting potentials of muscle cells in these animals exhibit the same dependence on external [K+], as has been shown previously. 4. Rohon-Beard cells can be driven antidromically, bu stimulation of the anterior end of the spinal cord with brief current pulses through a suction electrode. Antidromic action potentials fail to invade the cell body with repeated stimulation at 1Hz. 5. Even when impulses fail to invade Rohon-Beard somata, slow depolarizations can be produced by single shocks or trains of shocks which cause impulse activity in other neurones. The response can be observed to a single stimulus or to a train of stimuli. The magnitude of the depolarization is graded, depending on the number of stimuli and the frequency of stimulation. 6. Support is presented for the hypothesis that the slow depolarization in Rohon-Beard cells is mediated by the release of K+ into their environment by the impulse activity of neighbouring neurones. The slow depolarization increases in solutions containing 1-5 mM-K+, and decreases in solutions containing 6-0 mM-K+. The changes are in quantitative agreement with those anticipated by theory. 7. The slow depolarization is unlikely to be due to a conductance change produced by a synaptic transmitter, since hyperpolarization and depolarization of the Rohon-Beard cell with injected current do not change the amplitude of the response. Further, low Ca-high Mg solutions which block neuromuscular transmission do not block the response. 8. The possible role of the slow depolarizing response in the physiological activity of these neurones is discussed.
Action Potentials↗
Absence of action potentials in frog slow muscle fibres paralysed by botulinum toxin.
1. As shown previously, slow muscle fibres of the frog develop the ability to produce action potentials in 2 weeks after denervation.2. Normal transmission at the slow fibre neuromuscular junction gives a junction potential of 11 mV average (range 6-20 mV), caused by summation of potentials from several motor axons.3. Botulinum toxin injected intramuscularly into the iliofibularis blocks the neuromuscular junction of slow fibres in 6 days at room temperature. Single nerve stimuli give junction potentials of 0.05 mV average (range 0-1.6 mV). Contraction of slow fibres in response to tetanic stimulation is eliminated.4. No action potentials could be elicited in botulinum treated fibres, even 6(1/2) weeks after injection of toxin. This finding is discussed in relation to the possible involvement of a ;trophic' factor regulating the action potential mechanism in frog slow muscle fibres.
Action Potentials↗
Action of botulinum toxin on transmission from sympathetic nerves to the vas deferens.
Botulinum toxin (Type A) depressed or abolished transmission from postganglionic nerves to smooth muscle of isolated preparations of guinea-pig and mouse vas deferens. The time course of blockade was 2 to 6 times slower than that observed with the same concentration of the same batch of toxin on the rat diaphragm. Spontaneous excitatory junction potentials were still observed after 7 h exposure, indicating that the smooth muscle membrane was still able to respond to noradrenaline. Depression of transmission is probably due to a presynaptic action of unknown nature.
Action Potentials↗
Viewing neuromuscular junctions in live, unstained skeletal muscle.
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Amphibians↗
Low resistance connections between cells in the developing anther of the lily.
Low resistance junctions were demonstrated between cells in anthers from young buds of Lilium longiflorum Croft by standard electrophysiological techniques. Electrodes containing a dye were used to stain impaled cells for later histological identification. Electrical coupling is widespread; germinal cells are coupled to one another; coupling is also observed between somatic elements, and germinal and somatic cells are similarly interconnected. Cytoplasmic bridges are implicated in the first case; plasmodesmata are probably responsible for the interactions in the other two. Although the physiological role of the low resistance junctions shown here and present in embryonic animal tissues is unknown, the possible function of this form of intercellular communication in the development of the anther is discussed.
Cytoplasm↗