### 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.