The brain accounts for only 2% of body weight. How does it control body activities?

Human beings’ greatest curiosity is to explore the vast universe outside and the exquisite brain inside.

Although this mysterious tissue in the cranial cavity accounts for only 2% of the body's body weight, it consumes 20% of the body's oxygen and 25% of its glucose. About 80% of its composition is water, and the most abundant substance in the brain is fat. In this small space, there are nearly 86 billion neurons and 84.6 billion glial cells...

It seems easy to describe the brain with such a series of numbers, but simple numbers cannot tell us how it constantly regulates so many of our daily activities: our breathing, our perception of the world, and all our thinking.

What is a neuron?

Ancient Greek physician Galen believed that the brain and spinal cord controlled the whole body through chemical secretions. It was not until the 19th century that Italian scientist Camillo Golgi invented the silver staining technique that allowed people to observe the morphology of neurons under a microscope. This staining technique was called Golgi staining. Another Spanish scientist, Santiago Ramón y Cajal, used Golgi staining to discover that nervous tissue is composed of discrete nerve cells, thus laying the foundation for the neuron theory.

Interestingly, Golky was nominated for the first Nobel Prize in Physiology or Medicine in 1901, but he did not win it together with Cajal until 1906. This was the first time in the history of the Nobel Prize that two people won the prize together.

Today, the neuron theory is widely accepted. Neurons are considered to be the functional units of the brain, with an asymmetric and complex structure consisting of a cell body and neurites, which are further divided into dendrites and axons.

The nucleus of a neuron is located in the cell body. In addition, the cell body contains many organelles that are the same as those in other cells. The neurites that extend from the cell body are unique structures of neurons. Generally speaking, a neuron's cell body has many dendrites, but only one axon.

Dendrites are like the "antennae" of neurons, helping them receive information input. In addition, neurons can also receive information input through the cell body; information processing generally occurs in the cell body; axons are the output components of neurons. The starting point of the axon extending from the cell body is called the axon hillock. The length of the part of the axon that extends out varies greatly. (The length of the axon depends on the function of the neuron. The longest axon in the human body is located in the sciatic nerve, which is up to 1 meter long)

The beginning of the axon extending from the axon hillock is the starting point of signal conduction on the neuron, and the end of the axon is where the neuron outputs information. Sometimes the end of the axon has several branches.

There are layers of positive and negative electrons distributed inside and outside the cell membrane of neurons. In the resting state (when no external information is received), due to the different distribution of ions inside and outside the cell, there is a potential difference between the inside and outside of the cell membrane, and the cell is in a polarized state.

Overall, this transmembrane potential is negative inside and positive outside. The extracellular potential is usually defined as 0, so the intracellular potential is generally -60mV∽-70mV, which is called the resting membrane potential of the neuron.

When neurons receive external input, changes in the ion gradient inside and outside the membrane occur, leading to depolarization. When the potential difference between the inside and outside of the membrane exceeds the threshold, an action potential is triggered.

How neurons work

Early technology made it difficult for people to study the generation of action potentials. British physiologists Alan Hodgkin and Andrew Huxley inserted electrodes into the giant axon of the squid, which was about 1.5 mm in diameter, for recording, which advanced people's understanding of the action potential conduction mechanism. The two researchers jointly won the 1963 Nobel Prize in Physiology or Medicine.

The biggest feature of action potential is that it is an all-or-none event, that is, as long as the current that triggers the action potential exceeds the threshold, its significance is the same regardless of its size.

In addition, after the occurrence of an action potential, the neuron will regain its polarization state and reach hyperpolarization. The period from hyperpolarization to recovery to rest is called the refractory period. During the refractory period, the threshold for the occurrence of an action potential increases (it is more difficult to depolarize).

The communication between neurons depends on a structure called "synapse". The synaptic structure is composed of the presynaptic membrane, the synaptic cleft and the postsynaptic membrane. The presynaptic membrane is formed by the axon terminal, the postsynaptic membrane is usually formed by the dendrites or cell body, and the synaptic cleft refers to the extracellular space between the two.

Synapses can be divided into chemical synapses and electrical synapses. Chemical synapses are more common, where neurotransmitters released from the presynaptic membrane bind to receptors on the postsynaptic membrane to complete information transmission between neurons. In some cases, people also describe the type of neuron according to the type of neurotransmitter released by the neuron, such as dopaminergic neurons.

Another type of synapse, the electrical synapse, is less common but exists in all nervous systems. The distance between the presynaptic membrane and the postsynaptic membrane of the electrical synapse is very close, and the two transmit ion currents through paired ion channels on the membrane, which is a very fast way of information transmission.

The picture comes from Tuchong.com

Another large class of brain cells is glial cells. The mature central nervous system mainly includes three types of glial cells: astrocytes, oligodendrocytes and microglia. There is also another type of glial cell called Schwann cells in the peripheral nervous system.

The largest number of them are astrocytes, which exist between neurons. One of their very important functions is to regulate the homeostasis of neurons outside the cell, for example, they remove excess neurotransmitters in the synaptic cleft. At present, people do not fully understand the role of astrocytes. Some new studies have shown that astrocytes are also related to synaptic plasticity and participate in regulating cognitive functions such as memory.

The main function of oligodendrocytes is to form myelin sheaths to wrap part of the axons of neurons. Myelin sheaths are lipid-rich insulating layers that help action potentials to be conducted quickly on the axons.

The role of Schwann cells is to produce myelin in the peripheral nervous system.

Microglia are immune cells of the central nervous system. Their main functions are to regulate brain development, maintain the homeostasis of the nervous system, and participate in some immune processes.

At this point, you should have a good understanding of these two cells. Let's review them again: neurons, as the functional units of the nervous system, transmit information in the brain in the form of electrical and chemical signals; glial cells regulate the development of the nervous system and maintain the homeostasis of the normal operation of the nervous system.

This article is a work supported by Science Popularization China Starry Sky Project

Author: NeuroReality

Review: Tao Ning

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.