neurons in the brain (Illustration credit: Benedict Campbell. Wellcome Images firstname.lastname@example.org: 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.
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.
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.
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.
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.