Subthreshold membrane potential oscillations are membrane oscillations that do not directly trigger an action potential since they do not reach the necessary threshold for firing. However, they may facilitate sensory signal processing.
Neurons produce action potentials when their membrane potential increases past a critical threshold. In order for neurons to reach threshold for action potential to fire, enough sodium (Na+) ions must enter the cell through voltage gated sodium channels through membrane and depolarize the cell. The threshold is reached to overcome the electrochemical equilibrium within a neuron, where there is a balance between potassium ions (K+) moving down their concentration gradient (inside the cell to outside), and the electrical gradient that prevents K+ from moving down its own gradient. Once the threshold value is reached, an action potential is produced, causing a rapid increase of Na+ enters the cell with more Na+ channels along the membrane opening, resulting in a rapid depolarization of the cell. Once the cell has been depolarized, voltage-gated sodium channels close, causing potassium channels to open; K+ ions then proceed to move against their concentration gradient out of the cell.
However, if the voltage is below the threshold, the neuron does not fire, but the membrane potential still fluctuates due to postsynaptic potentials and intrinsic electrical properties of neurons. Therefore, these subthreshold membrane potential oscillations do not trigger action potentials, since the firing of an action potential is an "all-or-nothing" response, and these oscillations do not allow for the depolarization of the neuron to reach the threshold needed, which is typically around -55 mV; an "all-or-nothing" response refers to the ability of a neuron to fire an action potential only after reaching the exact threshold. For example, figure 1 depicts the localized nature and the graded potential nature of these subthreshold membrane potential oscillations, also giving a visual representation of their placement on an action potential graph, comparing subthreshold oscillations versus a fire above the threshold. In some types of neurons, the membrane potential can oscillate at specific frequencies. These oscillations can produce firing by joining with depolarizations. Although subthreshold oscillations do not directly result in neuronal firing, they may facilitate synchronous activity of neighboring neurons. It may also facilitate computation, particularly processing of sensory signals. All in all, although the subthreshold membrane potential oscillations do not produce action potentials by themselves, through summation, they are able to still impact action potential outcomes.
Overview
Neurons display, beyond synaptic and action potentials, rhythmic subthreshold membrane potential oscillations (a particular type of neural oscillations). These oscillations, which resembled sinusoidal wave forms, were originally discovered in the mammalian inferior olive nucleus cells. The functional relevance of subthreshold oscillations concerns the nature of the intrinsic electrical properties of neurons; that is, the electrical responsiveness are not derived from interactions with other cells. These properties define the dynamic phenotype independently from form or connectivity. Subthreshold oscillation frequency can vary, from few Hz to over 40 Hz, and their dynamic properties have been studied in detail in relation to neuronal activity coherence and timing in CNS, in particular with respect to the 10 Hz physiological tremor that controls motor execution, Theta rhythm in the entorhinal cortex,<ref name=":3">Alonso, A. <gwmw class="ginger-module-highlighter-mistake-anim ginger-module-highlighter-mistake-type-1" id="gwmw-15693837406294585512759">and</gwmw> Llinas, R. (1989) "Subthreshold Na+-dependent theta-like rhythmicity in entorhinal cortex layer II stellate cells". Nature, 342: 175-177., gamma band in cortical inhibitory interneurons</ref> and gamma band activity in cortical inhibitory interneurons and in thalamus neurons. They have also been described and studied in layers V of the entorhinal cortex, the inferior olive in vivo, the olfactory bulb and the dorsal cochlear nucleus. These neurons have been a major input into the cerebellum, as well, and have been found to contribute to the overall generation of movement patterns. The dynamic aspects of such oscillations have been defined using mathematical modeling.