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2.5.2 Neurons and Ions

2.5.2.1 Overview

Neurons play decesive roles of the brain handling signals through ion transfer. The basic mechanism should be learned as follows.

2.5.2.2 Details

2.5.2.2.1 Neuron and Electrolytes (Ions)

In the following depiction, a neuron is shown in red-purple being deformed to be explained.
The largest body is the cell body or called soma. Its diameter is about 10 μm. It contains essential elements such as nucleus.
The long fiber is called axon. The axon transmits signals to the end of the axon. The end of the axon is subdivided into many tips. The tips are called axon terminals or synaptic terminals. The terminals touch other neurons. (in this case, the other neuron is shown in blue-green.)
The branches from the cell body are called dendrites. The dendrites have many granular warts (lumps) called dendric spine, where another neuron's axon terminal is commonly touching. The touching area of the axon terminal and the dendrite is called synapse. Dendrites originally have no dendric spines, dendric spines are formed and grow through repetition of signal transmissions at the synapses. A neuron has thousands of dendric spines on average. Accordingly, a neuron has thousands of axon terminals on average.
Length of the neuron including the axon differs from one neuron to another. A shorter neuron would be about a few millimeters and a longer neuron would be about dozens of centimeters.


Neuron's Deformed Diagram 1
*Attribution: https://en.wikipedia.org/wiki/File:Blausen_0657_MultipolarNeuron.png


Neuron's Deformed Diagram 2
*Attribution: https://en.wikipedia.org/wiki/File:Complete_neuron_cell_diagram_en.svg

A neuron sends electric signals from around dendrites and cell body to axon terminals (synaptic terminals) like below. When axon terminals of a neuron (red-purple of Diagram 1) touches other neuron's (blue-green's of Diagram 1) dendrites, the sender (red-purple of Diagram 1) is called the presynaptic neuron, the receiver (blue-green of Diagram 1) is called the postsynaptic neuron.

* https://en.wikipedia.org/wiki/File:Action_Potential.gif

The typical type of the synapse is the chemical synapse, while the other is the electrical synapse.

Chemical Synapse
*Attribution: https://en.wikipedia.org/wiki/File:SynapseSchematic_lines.svg

As far as the chemical synapse, an axon terminal of the presynaptic neuron holds a specific chemical substance called neurotransmitter such as glutamic acid. When signals are transmitted in the presynaptic neuron from the cell body to an axon terminal, the neurotransmitter such as glutamic acid in the axon terminal is released outward, into the space between the axon terminal and the postsynaptic neuron, typically between the axon terminal and the dendric spine of the postsynaptic neuron.
The postsynaptic neuron has receptors around dendrites typically at the dendric spine. When the receptors receive the neurotransmitter such as glutamic acid, Ligand-Gated Ion Channels are activated and signal transmission in the postsynaptic neuron starts.

Composition of body fluids' electrolytes (ionic composition) should be learned for clear understanding.
Body fluids here are liquids in the human bodies. In chemistry, some substances such as ethyl alcohol (CH₃-CH₂-OH) dissolve in water as they are without changing their structure, because they have sufficient electrical affinity to water. (Both ethyl alcohol and water have electrical polarity.)
In contrast, other substances such as acetic acid (CH₃-COOH) and sodium chloride (NaCl) dissolve in water partly dividing into electrically positive part and electrically negative part. For example, they dissolve in water partly dividing into H⁺ and CH₃-COO^-, and Na⁺ and Cl^- because of "weakness of the bond between COO and H, and Na and Cl" and "electrical affinity of CH₃-COO^-, Na⁺ and Cl^- to water", which has electrical polarity. (CH₃-COOH and NaCl themselves have less electrical polarity.) The substances dissolving in water with positive electric charge (positive electric property) or negative electric charge (negative electric property) are electrolytes. (As far as acids such as CH₃-COOH, the divided electrically negative part such as CH₃-COO^- would be called acid anion (or conjugate base of acid).)
* "Anion in Simple Wikipedia" https://simple.wikipedia.org/wiki/Anion
* "Conjugate Acid in Wikipedia" https://en.wikipedia.org/wiki/Conjugate_acid

Body fluids can be categorized into extracellular fluid and intracellular fluid. Intracellular fluid is the fluid within cells. Extracellular fluid is the fluid outside the cells. Electrolytes' composition of the extracellular fluid is mostly regulated by the kidneys. Electrolytes' composition of the cerebrospinal fluid (CSF), in which neurons are bathing, is quite similar to that of the extracellular fluid.
Electrolytes' composition (ionic composition) of and the concentrations of equivalence (concentration of the number of electrons or protons in reference to the electrolytes; milli mol / litter) of CSF would be like Na⁺ 138 mEq/L, K⁺ 5 mEq/L, Ca²⁺ 5 mEq/L, Mg²⁺ 3 mEq/L, Cl^- 113 mEq/L, HCO₃^- 27 mEq/L, Acid anions of organic acids (Organic acid anions) 6 mEq/L, Amino acid anions of proteins 2 mEq/L, HPO₄^2- 2 mEq, SO₄^2- 1 mEq/L, and a small amount of other electrolytes (ions).
In contrast, those of intracellular fluid and fluid in neurons would be like Na⁺ 14 mEq/L, K⁺ 140 mEq/L, Ca²⁺ 0.0002 mEq/L, Mg²⁺ 2 mEq/L, Cl^- 4 mEq/L, HCO₃^- 10 mEq/L, Amino acid anions of proteins 71 mEq/L, HPO₄^2- 70 mEq, SO₄^2- 1 mEq/L, and a small amount of other electrolytes (ions).
* "Equivalent (Chemistry) in Wikipedia" https://en.wikipedia.org/wiki/Equivalent_(chemistry)
The differences of electrolytes' concentrations (ion concentrations) between the extracellular fluid and the intracellular fluid would be mostly attributed to cell membranes and Ion Transporters (Ion Pumps). In addition, signal transmission of neurons is attributed to the differences of electrolytes' concentration, Voltage-Gated Ion Channels, and Ligand-Gated Ion Channels.

2.5.2.2.2 Electric Potential Difference

The surface of cells consists of cell membranes. Cell membranes mostly consist of phospholipid bilayer. Electric elements wouldn't penetrate the phospholipid bilayer. On the other hand, specific enzymes called Ion Transporters (Ion Pumps) and Ion Channels reside among the phospholipid bilayer connecting the inside and the outside of the cell.

As a simple explanation, the initial electric state of a cell could be assumed that the intracellular fluid's relevant electrolytes is the same as those of the extracellular. However, when Ion Transporters work being encouraged by ATPs, specific ions such as Na⁺, Ca²⁺, and Cl^- are transported to the outside against their concentration gradient. In contrast, specific ions such as K⁺ are transported to the inside against their concentration gradient, while some electrolytes such as Amino acid anions of proteins would be prepared inside through other specific processes. (In contrast, Ion Channels allow specific ions to pass through following the concentration gradient under specific conditions.)
Electric charges of the extracellular and the intracellular change through the transportation (and other specific processes) and the channel passing. Difference of electric charges between outside (extracellular fluid) and inside (intracellular fluid) is called difference of electric potential.

Na⁺ / K⁺ -ATPase (Sodium-Potassium Transporter) plays the central role to raise electric potential difference. The mechanism of the Sodium-Potassium Transporter raising the difference of electric potential (electric charges) between the extracellular fluid and the intracellular fluid would be as follows.

The central wall below is phospholipid bilayer (cell membrane). The left side is the inside of a cell. The right side is the outside of the cell. A Sodium-Potassium Transporter resides among the phospholipid bilayer. The Sodium-Potassium Transporter is a kind of enzymes.
Firstly, a simplified initial state is assumed as in (A). As an initial assumption, concentrations of Sodium ions (Na⁺: pink circles) outside and inside are the same in the same way as Potassium ions (K⁺: red-violet ovals). Other major electrolytes such as Chloride ions (Cl^-) and Hydrogen Phosphate ions (HPO₄^2-) are placed following the analyzed abundance above.
In (A), the Sodium-Potassium Transporter attracts Sodium ions.

Secondly in (B), 3 Sodium ions are housed in the Sodium-Potassium Transporter.


Thirdly in (C) and (D), the outside opens and 3 Sodium ions are released to the outside.




In (E), the Sodium-Potassium Transporter attracts outside Potassium ions.


In (F), 2 Potassium ions are housed in the Sodium-Potassium Transporter.




In (G) and (H), the inside opens and 2 Potassium ions are released to the inside.




3 Sodium ions are taken away, 2 Potassium ions are taken in, then the inside is negatively charged compared with the outside.
Consequently, as mentioned above, Outside Electrolytes become like Na⁺ 138 mEq/L, K⁺ 5 mEq/L, Ca²⁺ 5 mEq/L, Mg²⁺ 3 mEq/L, Cl^- 113 mEq/L, HCO₃^- 27 mEq/L, Organic acid anions 6 mEq/L, Amino acid anions of proteins 2 mEq/L, HPO₄^2- 2 mEq, SO₄^2- 1 mEq/L, and a small amount of other electrolytes (ions), and Inside Electrolytes become like Na⁺ 14 mEq/L, K⁺ 140 mEq/L, Ca²⁺ 0.0002 mEq/L, Mg²⁺ 2 mEq/L, Cl^- 4 mEq/L, HCO₃^- 10 mEq/L, Amino acid anions of proteins 71 mEq/L, HPO₄^2- 70 mEq, SO₄^2- 1 mEq/L, and a small amount of other electrolytes.
This is called "resting state." At this state, the inside is negatively charged compared with the outside and Electric Potential Difference of inside is about -70 mV (milli volt). This is called "resting potential."

* "Ion Transporter in Wikipedia" https://en.wikipedia.org/wiki/Ion_transporter
* "Na+K+-ATPase in Wikipedia" https://en.wikipedia.org/wiki/NaKATPase






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