Low-threshold spikes

Low-threshold spikes (LTS) refer to action potentials generated from a membrane potential negative to that for sodium spike threshold (firing level).

Events

It occurs when the neuron is maintained (~150ms) in a hyperpolarised state (-70mV) state which deinactivates T-type calcium channel. Upon receiving excitatory stimulation (glutamate), this causes a small depolarisation, allowing the T-type Ca channel to open for a period (~100ms) and this influx of Ca depolarises the neuron. This results in a Ca spike with a characteristic triangular shaped depolarisation which is important for the influx of Na+ ions to further depolarise the neuron resulting in a number (between 2-8) of action potentials, with interval between each AP less than 5ms. The T-type Ca channels close after ~100ms from opening and must remain closed for at least another 100ms before another set of LTS can occur.

The functional significance of having multiple APs is so that these can 'add together' in the next neuron to allow temporal summation and a higher probability that it will activate the next neuron in the circuit

Origin

These LTSs were originally observed in inferior olive neurons^[1] and then in thalamic neurons.^[2]

Notes

1. ^ Llinas and Yarom 1981
2. ^ Llinas and Jahnsen 1982

References

• Llinas, R. and Yarom, Y. (1981) "Properties and distribution of ionic conductances generating electroresponsiveness of mammalian inferior olivary neurones in vitro". J. Physiol. (London) 315: 569-584.
• Llinas, R. and Jahnsen, H. (1982) " Electrophysiology of mammalian thalamic neurons in vitro". Nature 297: 406-408, 1982.