Axons Amplify Somatic Incomplete Spikes into Uniform Amplitudes in Mouse Cortical Pyramidal Neurons

BACKGROUND: Action potentials are the essential unit of neuronal encoding. Somatic sequential spikes in the central nervous system appear various in amplitudes. To be effective neuronal codes, these spikes should be propagated to axonal terminals where they activate the synapses and drive postsynaptic neurons. It remains unclear whether these effective neuronal codes are based on spike timing orders and/or amplitudes. METHODOLOGY/PRINCIPAL FINDINGS: We investigated this fundamental issue by simultaneously recording the axon versus soma of identical neurons and presynaptic vs. postsynaptic neurons in the cortical slices. The axons enable somatic spikes in low amplitude be enlarged, which activate synaptic transmission in consistent patterns. This facilitation in the propagation of sequential spikes through the axons is mechanistically founded by the short refractory periods, large currents and high opening probability of axonal voltage-gated sodium channels. CONCLUSION/SIGNIFICANCE: An amplification of somatic incomplete spikes into axonal complete ones makes sequential spikes to activate consistent synaptic transmission. Therefore, neuronal encoding is likely based on spike timing order, instead of graded analogues

Quantal-like current fluctuations induced by odorants in olfactory receptor cells

Many sensory systems have evolved signal detection capabilities that are limited only by the physical attributes of the stimulus1. For example, \u2018hair\u2019 cells of the inner ear can detect displacements of atomic dimensions2. Likewise, both in vertebrates and in invertebrates photoreceptors can detect a single photon3, 4. The olfactory stimulus also has a quantal unit, the single odorant molecule. Insects are reportedly able to detect a single pheromone molecule5, whereas quantal responses in vertebrate olfactory receptor cells have not been reported yet. Psychophysical measurements indicate that a minimum of 50 odorant molecules are necessary for human olfactory detection, suggesting that an individual receptor may be activated by a single odorant molecule6. We report here measurements of current fluctuations induced by odorants that suggest a quantal event of about 0.3-1 pA, presumably triggered by the binding of a single odorant molecule. \ua9 1995, Nature Publishing Group. All rights reserved