Want to know more about synapses?

Quick summary!

A neuron is a type of cell located in the brain that transmits information to other neurons through a junction called a synapse. The communication between neurons at synapses in our brain enables us to think and carry out behaviors.

Entry:

In Latin, the word “cell” means “small room”. A cell is the smallest unit of life. Some life forms, like the bacteria Salmonella, are made up of 1 cell. Other life forms like humans have trillions of cells that work together. We can think of a human as a large skyscraper made up of trillions of rooms and each room has a location and function. A neuron is like a room located in the skyscraper in the communication department. A neuron is a specialized cell that processes and sends information throughout the body.

There are electrically charged ions located inside of the neuron and outside of the neuron in the environment.  A neuron has a more negative charge inside compared to its outside environment, in other words it is “polarized”. The resting membrane potential of a neuron is approximately -70 millivolts. The resting membrane potential is regulated by macromolecules called proteins. Neurons communicate by means of an action potential. An action potential is an electrochemical signal. An action potential is a change of the resting membrane potential of a neuron. When an action potential occurs the neuron becomes less negative and more positive. The electrical signal goes down the neuron, traveling like a wave, until it reaches the axon terminal where it becomes chemical through the release of neurotransmitters from the axon terminal. There a neuron can communicate to other cells to pass along the message or end it through a synapse. A synapse is a very small gap (can be less than 0.5 micrometers) between cells that allows for communication between the cells. There are two types of synapses. One type is called electrical synapses. There are very few of these synapses in the brain. Due to their fast communication time, these synapses are mostly utilized in cardiac muscles for example. Electrical synapses have gap junctions. Gap junctions are a connection between the cytoplasm of two cells. In electrical synapses, the action potential will go straight from one cell to another. The most common type of synapses in the brain is chemical synapses, and this is the type of synapse we will be focusing on. This type of synapse consists of the axon terminal of one cell and the dendritic spine of another cell. They communicate by sending signals called neurotransmitters when an action potential reaches the axon terminal.  The neurotransmitters are stored in vesicles. When an action potential reaches the axon terminal, the neurotransmitters are released with the help of calcium to send a message to the receiving cell.  The neuron sending the message is the presynaptic cell and the cell receiving the message is a postsynaptic cell. The presynaptic and postsynaptic cell are held together by different bridge-like structures called cell adhesion molecules. After the neurotransmitters are released by the presynaptic cell, they diffuse across the synapse towards the postsynaptic cell. The neurotransmitters will bind to proteins called receptor proteins on the surface of the postsynaptic cell. Receptors are like ears that receive the neurotransmitter message.

There are neurotransmitters that send excitatory signal promoting action potential on the postsynaptic cells, and there are neurotransmitters that are inhibitory signal preventing action potential on the postsynaptic cell. Likewise, there are excitatory and inhibitory receptors on the postsynaptic cell. The balance between excitatory and inhibitory allows for healthy communication and function of the neurons.

In addition, there are two major types of receptors: ionotropic receptors and metabotropic receptors. When neurotransmitters bind to ionotropic receptors, it will change the shape of the receptor and open an ion channel allowing particular ion types (electrically charged atoms) to flow inside of the cell. If an excitatory neurotransmitter binds to a receptor that is excitatory for that neurotransmitter then a positive charge may enter the postsynaptic cell and cause depolarization and perhaps action potential. A neuron needs to reach a certain threshold of depolarization (i.e. needs to be depolarized enough) for an action potential to start.  For example, neurotransmitter acetylcholine binds to an ionotropic receptor for that neurotransmitter. As a result, sodium enters the postsynaptic cell and depolarizes the postsynaptic cell.

However, some neurotransmitters are inhibitory neurotransmitters. For example, when gamma-aminobutyric acid (GABA) binds to ionotropic GABA-A receptors that are inhibitors, it will allow for the flow of negatively charged ions such as chloride ions to enter the cell. This will hyperpolarize the cell meaning more negative charge will enter the cell and not allow for an action potential. Notice that an excitatory neurotransmitter needs to bind to a receptor that is for that neurotransmitter and the receptor must be excitatory also. And vice versa, an inhibitory neurotransmitter needs to bind to a receptor that is for that neurotransmitter and the receptor must be inhibitory also. Neurotransmitters and receptors work together and must coordinate in order for an effect to be made in the postsynaptic cell. A postsynaptic cell is receiving signals from the presynaptic cell and undergoes “summation” which means adding up all of the signals to determine if the postsynaptic cell is depolarized enough to fire an action potential itself.

Metabotropic receptors act differently than ionotropic receptors. When a neurotransmitter binds to a metabotropic receptor, it doesn’t directly open an ion channel, but it brings about a series of changes to the postsynaptic cell through helpers inside the cell called secondary messengers. Neurons communicate all the time in our brain in a fast and effective manner which enables us to think and carry out our behaviors.

Glossary:

resting membrane potential, macromolecules, electrochemical signal, electrical synapses, chemical synapses, neurotransmitters, calcium, presynaptic cell, postsynaptic cell, receptor, excitatory signal, inhibitory signal, ionotropic receptors, metabotropic receptors, depolarization, acetylcholine, gamma-aminobutyric acid (GABA,) GABA-A receptors, hyperpolarize, secondary messengers