Neuronal Noise - Sources

Sources

Noise present in neural system gives rise to the variability in the non-linear dynamical systems, but a black box still exists for the mechanism in which noise affects neural signal conduction. Instead, research has focused more on the sources of the noise present in dynamic neural networks. Several sources of response variability exist for neurons and neural networks:

• Thermal noise: Johnson–Nyquist noise occurs due to the thermal motions of ions and other charge carriers, producing voltage fluctuations proportional to temperature. This source of noise is attributed to the third law of thermodynamics, stating that kinetic energy of molecules increases with a raise in temperature. Thermal noise is the weakest source of noise and can be considered negligible.

• Ionic conductance noise: Ion channels in the membrane undergo spontaneous changes in conformation between different states and can open (or close) due to thermal fluctuations. The transmembrane embedded protein channels are made up of small subunits that undergo conformational changes and are affected by thermal fluctuations. When temperature drops below 33°C, the rate at which the channel becomes active or inactive decreases. In contrast when the temperature is increased above 33°C, the rate at which the channel becomes active or inactive increases.

• Ion pump noise: Membrane embedded ATPase ion pumps produce fluctuating potentials by transporting ions against their concentration gradient. The multistep process in which ions are transported across their gradient requires ATP. The steps involved in active transport have a net forward direction, but small stochastic steps still exist in the conformational process that move backwards. These backward steps contribute to neuronal noise present in all dynamic neuronal circuits.

• Ion channel shot noise: The number of ions that migrate through an open ion channel are discrete and random. In synapses, the number of calcium ions that enter the postsynaptic side after a spike is on the order of 250 ions, potentially making potentiation processes noisy. This noise is also associated with thermal fluctuations affecting the protein channels, as previously mentioned. This is not to be confused with shot noise, which is noise produced by the random generation of action potentials in neurons.

• Synaptic release noise: Generally, action potentials are transferred down a neuron, which then are converted to either electrical or chemical signals between neurons. Chemical synapses are not deterministic, which means that every action potential produced does not result in the release of neurotransmitters. Rather, the release of vesicles containing neurotransmitters are probabilistic in nature. The number of vesicles released by a single synapse is random in response to a specific input signal and is further influenced by the firing history of the pre- and post-synaptic neurons. This means that neurotransmitters can be released in the absence of an input signal.

• Synaptic bombardment: The large number of incoming spikes add a fluctuating amount of charge to the cell, which depends on the structure of the incoming spike trains and affects the cell's excitability.

• Chaos: Chaotic dynamics can occur in single cells (due to periodic inputs or bursting due to intrinsic currents). Simple networks of neurons can also exhibit chaotic dynamics. Even if the chaos is deterministic, it can amplify noise from the other sources to macroscopic levels due to sensitive dependence on initial conditions.

• Connectivity noise: Noise that arises from the number of connections and non-uniformity that a neuron has with other neurons within a neuronal network. There is a stronger presence of sub-threshold noise when the interconnectivity is strengthened, or the number of connection to other neurons is increased. The opposite remains true, too. If the interconnectivity of the neurons is decreased so then is the level of sub-threshold noise.

• Environmental Stimuli: Noise can be produced on a larger scale due to fluctuations in CO₂, which lead to variations in blood flow. The level of CO₂ in the blood allows for either vasoconstriction or vasodilation, which can encroach, or expand, into nearby neural networks producing noise.