neurons in the brain (Illustration credit: Benedict Campbell. Wellcome Images images@wellcome.ac.uk: CC BY-NC-ND 2.0

Until 1887, our brain and the spinal cord was considered as composed of a continuous single network. Santiago Cajal, in 1887, using a stain developed by Camilio Golgi, showed separate cells connected through spaces. However, Camilio Golgi disputed the claim. Wilhelm von Waldeyer-Hartz named these cells as “neurons”.

Previously, we entered the neuron forest. Neurons are like trees in a forest. They are elegant, superior, and live wonderful lives. On average, our brain owns about 100 billion neurons.

What is a neuron?

A neuron is a specialized cell, only found in the brain and its connected nervous system. The following illustration describes parts of a typical neuron; however, we can find very highly specialized neurons in different parts of the brain.

Cell body

Like any other cell, a neuron owns a cell body – the main control center. It contains a nucleus and other essential structures to maintain its life such as mitochondria to produce energy and other apparatus that transport substances within the fluid inside the cell.

Dendrites

You can find dendrites as tree-like structures that are very much closer to the cell body. These branches receive messages from other neurons and bring those messages toward the cell body. In some places such as in Cerebellum, one neuron can have even about 200,000 branches.

Axon

This is the tube-like structure that propagates received signals towards the axon terminals with the aim of transmitting to other neurons or its endpoint for action. Axons’ length can vary from millimeters to a meter. For example, axons that propagate information from the spinal cord to toes should be very lengthy. Sometimes, one axon owns many terminals. As a result, these axons can send messages to many neurons at the same time. On the other hand, some neurons such as the cells in the retina where information from eyes is processed do not have axons.

Myelin sheath

The axons that transmit information for action (motor neurons) have a thick covering, called “myelin sheaths”. They act like an insulator in a wire to send information faster.

Myelin sheaths are not parts of a neuron; these are produced by its supportive cells, called, “Glial cells”.

As you can see, these sheaths are interrupted from place to place; that is to recharge the electric signals. These places are called “nodes of Ranvier” because those were first described by Ranvier.

How neurons transmit information

The following GIF explains vividly how this happens though an axon. As you can see when the axon is at rest, the inside of the cell membrane is negatively charged; exactly – 70 millivolts than the outside. It is maintained using pump. It is costly because it pumps out three sodium ions while pumping in two potassium ions. This pump uses energy using one ATP molecule at each time.

This video clip explains the above mechanism more clearly too:

How neurons communicate each other: The synapse

Once a message reaches the axon’s terminal, it has to travel through a small pond to pass it on the next neuron. It is no more an electric transmission. Rather, it is a chemical transmission.

The following diagram drawn by Thomas (the link given above) illustrates this pond and the activities that take place when an electrical message reaches the cup-shaped terminal. The other end is the beginning of a dendrite.

The lower end which faces the axon terminal is the dendritic end. The electrical impulse stimulates chemical-filled vesicles and they release their chemicals into the cleft. Finally, the receptors of the dendrites capture those chemicals; it triggers electrical impulses in the dendrites and travels until it reaches the neurons’ axon terminals.

So, the neuron – the messenger – does not go anywhere; only the message – after translating the message’s electrical form into a chemical form – travels from one neuron to another. That is why the whole neuron system consists of about 100 billion neurons.

The following video clip explains how a synapse operates:

Different message types

Although the mechanism is the same, neurons transmit different message types; either to carry on certain tasks such as moving a hand or to recognise certain sensations such as smell, touch, pain etc. The motor neurons carry messages to carry or not to carry movements. The sensory neurons bring messages about sensations – touch, pain, hearing, seeing etc.

“Neuron” by NIH-NCATS is licensed under CC BY 2.0

Our brain contains about 100 billion neurons; it looks like a neuron forest. because a neuron is more or less similar to a tree.

These are cells – a special kind of cells. At one end, it sprouts a large number of very thin short threads – “dendrites”. The ends of these receive electrical signals from other neurons via small fluid-filled ponds – “synapses”. The received signals pass along until it reaches the tree (cell) body. From there, it shoots away to the next neuron through another a thicker branch; it is named “Axon”. So, dendrites take electrical signals from previous neurons, bring those to the cell body of the neuron, and the Axon transmits those signals to the next neuron – sometimes to muscle endings and organs. We own about 100 billion neurons; So, it is, in fact, a huge “neuronal forest”.

Santiago Cajal: A Nobel Laureate

For the first time, Santiago Cajal – a Spanish medical specialist – stained neuron cells with a special stain, called gold stain. It colored only the neurons. In fact, he improved Camillo Golgi’s method. Cajal’s neuronal mapping became phenomenal which elevated him to share the prestigious Nobel prize with Camillo Golgi in 1906. At that time, people thought that the brain was an interconnected unbroken network. The Cajal Institute has been carrying his legacy since then until now in Spain through a range of academic programs dedicated to neurology.

Not only was he a medical scientist, but he was also a gifted artist. As a result, he used his artistic skill to draw various shapes of neurons. Enter into his neuron forest. now. I took the following drawings from an online teacher resource published by Weisman Art Museum for classroom activities. This excellent resource is freely available. You can access to the whole document through this link: “The Beautiful Brain: The Drawings of Santiago Ramon Y Cajal”.

A Pyramidal Neuron drawn by Cajal

Appreciate that how precisely he has drawn the following. This type of neurons exist in our brain cortex. It is called Pyramidal neuron due to the shape of the cell body. He had drawn this in 1904.

The Neuronal forest by Cajal

More recently, with the advancement of the technology, we can now see how our neuron forest look like in 3 dimensional view.

Enter into the neuron forest; enjoy the universe of synapases;