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Voltage sensitive calcium entry in frog motoneurones
Abstract:
1. The electrical properties of motoneurone membrane were investigated in the isolated and hemisected spinal cord of frogs, using intracellular recording techniques. 2. TTX (1 x 10(-6) g/ml.) blocked action potentials produced either by intracellular depolarizing current pulses or ventral root stimuli. Voltage--current relations from these cells showed a diminishing slope for depolarizing current pulses of increasing intensity. 3. If TEA (5--10 mM) was added to the media containing TTX, intracellular depolarizing pulses elicited prolonged regenerative depolarizations characterized by a peak of variable amplitude and a repolarizing phase preceded by a prolonged plateau of variable duration. 4. During the plateau of the response, the membrane conductance was increased above its resting value. 5. The response was shortened during repetitive stimulation and could be curtailed by applying a hyperpolarizing pulse during the plateau. 6. The response depended on the presence of external Ca2+ and increased in size and duration with increasing Ca2+ concentration. Sr2+ substituted effectively for Ca2+. Sr2+-dependent responses were considerably longer than the Ca2+-dependent ones. Ca2+ or Sr2+ dependent responses persisted in Na+-free media containing isotonic TEA, and were abolished by addition of Co2+. 7. Ca2+ or Sr2+-dependent regenerative responses were followed by a hyperpolarization which could last several seconds. The current responsible for this after-hyperpolarization was TTX and TEA resistant. 8. It is concluded that the TTX-resistant regenerative response is probably generated in the soma-dendritic membrane, and is due to influx of Ca2+ or Sr2+ through voltage sensitive channels different to those through which Na+ permeates during generation of 'normal' action potentials. In addition it is shown that the hyperpolarization following 'Ca spikes', and which might be due to an increase in K+ conductance can also be triggered by Sr2+.
Insights
This study reveals that frog motoneurone membranes generate TTX-resistant regenerative responses dependent on calcium or strontium influx through voltage-sensitive channels. These findings shed light on novel neuronal excitability mechanisms.
Area of Science:
- Neuroscience
- Electrophysiology
- Cellular Biology
Background:
- Motoneurone electrical properties are crucial for motor control.
- Understanding neuronal excitability mechanisms is fundamental in neuroscience.
Purpose of the Study:
- To investigate the electrical properties of frog motoneurone membranes.
- To characterize TTX-resistant regenerative responses and their ionic basis.
Main Methods:
- Intracellular recording techniques in isolated frog spinal cord preparations.
- Application of tetrodotoxin (TTX) and tetraethylammonium (TEA) to block sodium and potassium channels, respectively.
- Voltage-clamp analysis to study ionic currents.
Main Results:
- TTX blocked normal action potentials, while TTX + TEA revealed prolonged regenerative depolarizations.
- These responses were dependent on external calcium (Ca2+) or strontium (Sr2+) influx.
- The responses persisted in Na+-free media and were blocked by Co2+, indicating Ca2+/Sr2+ channel involvement.
- A TTX/TEA-resistant hyperpolarization followed Ca2+/Sr2+ spikes, potentially due to increased K+ conductance.
Conclusions:
- Frog motoneurones exhibit TTX-resistant regenerative responses mediated by Ca2+ or Sr2+ influx through distinct voltage-sensitive channels.
- These findings expand our understanding of neuronal excitability beyond Na+-dependent action potentials.
- The study identifies a novel mechanism for generating neuronal electrical activity involving divalent cations.