Neuron Doctrine

  • The neuron is the fundamental structural & functional unit of the brain.

  • Neurons are discrete cells and not continuous with other cells.

  • Information flows from the dendrites to the axon via the cell body (soma).

Idealized Neuron

An idealized neuron is showed as following:

A neuron is nothing but a “leaky bag of charged liquid”. Contents of the neuron enclosed within a cell membrane. Cell membrane is a lipid bilayer that is impermeable to charged ion species such as \(\text{Na}^{+}\), \(\text{Cl}^{-}\), \(\text{K}^{+}\). Ionic channels embedded in membrane allow ions to flow in or out.

Ionic Channels

Each neuron maintains a potential difference across its membrane. Typically, inside is about \(-70 mV\) relative to outside. The difference is that \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) are higher outside; \(\text{K}^{+}\) and \(\text{A}^{-}\) are higher inside. Ionic pump maintains \(-70 mV\) difference by expelling \(\text{Na}^{+}\) out and allowing \(\text{K}^{+}\) in.

Ionic channels in membranes are proteins that are selective and allow only specific ions to pass through. For example, one ionic channel may only pass \(\text{Na}^{+}\) but not \(\text{K}^{+}\) or \(\text{Cl}^{-}\). Ionic channels are gated, and there are typically 3 types of gated ionic channels:

  • Voltage-gated: probability of opening depends on membrane voltage.

  • Chemically-gated: Binding to a chemical causes to open.

  • Mechanically-gated: Sensitive to pressure or stretch.

Gated channels allow neuronal signaling. Inputs from other neurons causes chemically-gated channels open and then changes in local membrane potential is occurred.

This in turn causes opening/closing of voltage-gated channels in dendrites, body and axon, resulting depolarization (positive change in voltage) or hyperpolarization (negative change in voltage). Strong depolarization causes a spike or “action potential”.

Action Potential

Voltage-gated channels causes action potentials. Strong depolarization open \(\text{Na}^{+}\) channels, causing rapid \(\text{Na}^{+}\) influx and more channels to open, until they inactivate. \(\text{K}^{+}\) outflux restores membrane potential then.

The spike propagates along the axon.

Myelin due to oligodendrocytes (glial cells) wrap axons and enable fast long-range spike communication. Action potential “hops” from one non-myelinated region (node of ranvier) to the next (saltatory conduction). This “active wire” allows lossless signal propagation.