Notes

Analogue Communication in the Retina, C. Elegans, Locust Not every single neuron of every single creature is digital and spiking. There are several examples of species (C. Elegans, Cockroaches, Locusts) and even neuron families in mammals (Photoreceptors, Horizontal Cells, Olfactory Granule Cells) where neurons don't utilize spiking. An entire textbook ("Neurones without Impulses - their significance for vertebrate and invertebrate nervous systems" - Alan Roberts) exists to cover these examples, but the lecture focuses just on one case: C. Elegans. C. Elegans have 302 neurons. None of them spike in the traditional AP generating manner. However, recently, a group found "spiking-like" activity in an AWA (olfactory sensory) neuron type.

After a series of back-and-forth arguments, this evidence proved to be inconclusive and the scientific consensus remains set on the fact that C. Elegans neurons do not generate action potentials, but the resulting "graded" potentials are quire interesting.

An Action Potential is a rapid, all-or-nothing change in the electrical potential across the membrane of a nerve cell or muscle cell. It is triggered by a threshold stimulus, and once it is initiated, it propagates along the cell membrane without decreasing in amplitude.

A Graded Potential, on the other hand, is a change in the electrical potential across the membrane of a cell that varies in amplitude and duration. They are triggered by stimuli that do not reach the threshold level required to initiate an action potential, and their amplitude decreases with distance from the point of stimulation. Graded potentials can be either excitatory or inhibitory and can summate together. In summary, an action potential is an all-or-nothing, rapid change in membrane potential, triggered by a threshold stimulus and propagates without decrease in amplitude. A graded potential is a change in membrane potential that varies in amplitude, triggered by stimuli that do not reach threshold level, and decreases with distance from the point of stimulation.

Why Spikes - from Biology? It has been demonstrated from a signal processing standpoint that digital spikes are inferior to analogue communications. It has also been demonstrated that there are insects that work perfectly well without spikes. So why use spikes? Several reasons:

• Spikes Synchronize Internal Process - A paramecium (single celled organism) uses spikes to forcefully/instantly orient its motile cilia (motors) as part of avoidance behavior.
• Spikes send local information within a Cell - The amoeba uses Mechanosensitive Calcium channels to generate a spike which causes local contraction/compression to generate movement away from an object.
• Calcium spikes regulates homeostatic processes - Ca2+ intracellular signaling cascade/pathways are the foundations of molecular biology. Seeing Ca2+ influx follows an action potential, it would be wasteful to have to come up with a different mechanism to mediate Ca2+ levels. Spiking does this as a byproduct.
• Cells send information across long distances - Any analogue wavelength would decay over time, and be vulnerable to noise fluctuations. Neurons have tricks to reduce this (myelination, increasing axon diameter in sea squids), but ultimately spikes guarantee an intact and reconstructable signal being delivered.
• Energy Efficiency - Self Explanatory. A human uses 100 Watts, the brain takes up to 20 W. Compare that to the power supply unit of a standard desktop (300W0 or a high-end rig (800W).