Within the Nervous System: Neurons and Synapses

The neuron, or nerve cell, is the fundamental brain unit. The brain is made up of billions of neurons that work together in complex networks, almost like a society. Neurons are constantly sensing the outside world-analyzing and communicating information.

Neurons pass information to each other through chemical signaling. When a large branch of neurons sends a signal synchronously (at the same time), the signal will travel down the tail of the branch. This tail, also called the axon, will relay the signal to other parts of the brain.

Synapses

Neuron cells have many appendages to connect them to other neurons, to pass the messages along. The space where the neurons meet is called the synapse.

Chemical signal released by one cell binds to the next, where it gets transformed to electricity, which travels along thin fibers, called axons, that reach out to other neurons. When the signal gets to the space between these two cells, it is converted into chemicals again, which bind to the next cell and create a chain reaction. The axons are covered with an insulating coat to protect the electric charge.

Within the Nervous System: Neurons and Synapses

In order to understand the workings of the nervous system, there are two main things to keep in mind: the nervous system’s main job is to pass signals between the body, the spinal cord, and the brain, and the system is made of special cells that facilitate this!

There are two types of cells within the nervous tissue: neurons and glial cells. Neurons create a communication network within the body that transmits the signals to organs and tissues. Glial cells support the neurons. The work of nervous tissue allows the body to be aware of and respond to its surroundings.

What Do Neurons Look Like?

Unlike the round pictures of cells most people imagine, neurons are long and thin. The long thin part is called an axon and is responsible for conducting electrical signals. Some sections of axons are covered with a special coating (called a myelin sheath) that acts  as insulation and speeds up the transmission of the electrical signal. At one end of the axon, or off to the side, is the cell body, which includes the cell’s nucleus. The big difference between the round cells that many envision and the cell bodies of neurons is that neuron cell bodies have long, thin projections called dendrites. These dendrites are what allow the neurons to reach out to each other and to cover distance inside the body. Both the cell body and the dendrites receive signals from other neurons. At the other end of the axon is the axon terminal, which sends the signal on to the next neuron. Signals can only travel in one direction: from the cell body, along the axon, and finally to the axon terminal.

Signals do not stop at the axon terminal. Once the signal reaches the axon terminal of a neuron, it must go to the next neuron. The place where two neurons meet is called a synapse, and the actual space between them is called the synaptic cleft. A signal moves from one axon terminal, across a synapse, and then to the cell body of the next neuron.

How Do Signals Travel Through The Body?

The signals are passed between cells. They travel electrically down an axon inside a neuron or chemically across a synapse (between neurons). This means that the signal actually changes in nature all the time! It also means there is electricity coursing through your body! But, it remains inside your neuron cells.

The electricity in the nervous system is created by electrically charged particles called ions that move in and out of neurons. As different ions move in and out of neurons, they change the overall charge on the inside and the outside of the cell. When no signal is being carried down an axon, the cell is positively charged on the outside and negatively charged on the inside. With the arrival of signals from outside the cell, more positive ions enter the cell, flipping the charges. The movement of ions at one end of the axon stimulates ion movement a little farther down, and in this way the electrical signal propagates down the axon.

A synapse is just the name for the place where the axon terminal of one neuron meets the cell body or dendrite of another neuron. Usually the two do not touch. The actual space between the two neurons is called the synaptic cleft.

When an electrical signal reaches the axon terminal of a neuron, cell structures in the terminal produce a specific kind of particle, proteins called neurotransmitters, and release them into the area outside the cell. These neurotransmitters are a chemical form of the signal. So the signal travels across the synapse in a chemical form.

Once the neurotransmitters reach the next neuron, they cause gates on the surface of the neuron to open. With these gates open, more positive ions flow into the cell, making that part more positive! This causes a cascading effect that opens more gates down the axon, which then become more positive and open the next gates, causing the signal to travel down the axon electrically. After the signal reaches the axon terminal, it must cross the synapse again.

Sometimes, it may take neurotransmitters coming from different neurons to initiate an electrical signal in one neuron! These traveling signals are what allow us to breathe, eat, move, talk, and perform thousands of other functions.

What about the other cells in the nervous tissue?

Glial cells help neurons by carrying out support functions.
There are several different types of support cells; each has specific functions within the nervous system. For instance, one type of support cell forms the myelin insulation around the neuron's axon. Another type of support cell forms a scar when the spinal tissue is damaged. These examples show that the support cells do exactly what their name implies—they support neurons.

Because neurons are specialized to carry out such a complicated process millions of times a day, it’s easy to see why they need help! In fact, there are more support cells than neurons. For ever neuron there are about ten support cells.