The brain has a complex information processing system. This text offers a general introduction of the role neurons play in this fascinating communication system.
The brain weighs around 3 pounds and its length is only around 15 cm, however, it is the most sophisticated organ in the human body. The brain has a complex information processing system, which is responsible for everything that we do, everything that we think, and everything that defines who we are.
Neurons and glial cells are the main characters in this procedure. This text offers a general and brief introduction of the role neurons play in this fascinating system.
Let’s begin clarifying some important concepts.
Neurons are brain cells responsible for collecting, integrating, processing and transmitting information. They have three main components: the soma, the dendrites and the axon. The soma is the body of the cell. The dendrites are small extensions that spread from the soma and are responsible for receiving messages from other neurons. The axon is a long fibre that also spreads from the soma and is responsible for sending messages to other neurons. In its end, the axon has branches called axon terminals, which store neurotransmitters. The axon is covered with segments of myelin, divided by gaps called nodes of Ranvier, and both are essential in speeding up the message transmission. Figure 1 shows the diagram of a neuron.
The information travels by electrical transmission within the neuron, and by chemical transmission between neurons.
Now, let’s describe how the electrical transmission works.
First, it is important to understand what resting potential means. Resting potential is a non-active state in which the cell is at equilibrium, it is ‘resting’. In this phase, ion channels in the cell membrane are closed, and the inner zone of the neuron is slightly more negative than the outer zone. This is explained by the presence of potassium ions (K+) and negatively charge proteins (A-) inside the cell, and the presence of sodium ions (Na+) in its outer space. Figure 2 shows how a neuron in resting potential looks like.
When the dendrites receive signals from other cells, the action potential, or electrical transmission, begins. The channels in the cell membrane open, sodium ions enter the soma, and when the amount of sodium reaches its threshold, the ions channels close. Now, the charge inside the neuron is positive and the charge outside the neuron is negative, this is called depolarization. This process will repeat many times as the action potential travels through the axon of the neuron. The ion exchange boosts the action potential, and it will be sped up by a process called saltatory conduction. During this process, the action potential jumps from one node of Ranvier to the next, skipping the segments of myelin, and travelling faster. After the action potential has passed by, ion channels open again, and this time, potassium exits the neuron in a process called repolarization or hyperpolarization. Figure 3 illustrates depolarization and hyperpolarization, and figure 4 illustrates saltatory conduction.
Finally, the sodium-potassium pump restores the equilibrium of the neuron, bringing potassium ions back in and taking sodium ions out. The neuron goes into a short phase in which it recovers and cannot fire again, called refractory period, and then enters a resting potential state again.
Now, let’s describe how chemical transmission works. In this process, neurotransmitters adopt the main role. It consists of the following steps:
❖ Synthesis Each neuron can synthesize only one type of neurotransmitter, though, it can receive different types of neurotransmitters. The precursor of the neurotransmitter is present in the presynaptic neuron, and acts with a specific enzyme that transforms the precursor into the neurotransmitter.
❖ Storage Neurotransmitters are encapsulated in vesicles and stored in the terminal buttons of the presynaptic neuron.
❖ Release As the action potential travels through the presynaptic neuron and reaches the terminal buttons, synaptic vesicles fuse with the membrane of the presynaptic neuron and release their contents into the synaptic cleft.
❖Receptor binding Neurotransmitters bind with a receptor located in the post-synaptic neuron. Each type of neurotransmitter can bind with a specific type of receptor. In general, there are two types of receptors: Ionotropic Receptors and Metabotropic Receptors. Ionotropic Receptors are the most common types of receptors, they are fast, and they are ion channels that allow sodium and calcium enter the cell if the neurotransmitter is excitatory, firing the action potential, or prohibiting sodium and calcium from entering the cell if the neurotransmitter is inhibitory, stopping the action potential. On the other hand, Metabotropic Receptors are slower because they need G proteins and ATP (energy) to stimulate the opening of ion channels that will fire or stop the action potential. So, it can be said that neurotransmitters are the keys that open the ion channels. Figure 5 illustrates ionotropic and metabotropic receptors.
❖ Inactivation Neurotransmitters can be inactivated by three mechanisms: re-uptake of the left neurotransmitters by proteins, and then reabsorbed by the presynaptic neuron; degradation of the left neurotransmitters by enzymes in the synaptic cleft; uptake of the left neurotransmitters by specific proteins or glial cells in the synaptic cleft.
Figure 6 shows the whole process of chemical transmission.
Understanding how the communication between neurons works does not only
enable us to learn about the normal functioning of the brain, but also allow us to comprehend a variety of illnesses and how to intervene.
References
Jones, J.R. (2011). Introductory Psychology: Neurons, Neurotransmitters, and Neuromodulators, from https://www.cuyamaca.edu/people/jr-jones/intro- psych/Unit_3_Handout.pdf
Mason, J & Staff of ACS Distance Education. Brain Chemistry & Electronics. The Brain and Behaviour, ACS Distance Education. Australia: 25-35.
Wood, S.E., Wood, E.G. & Boyd, D. (2015). Biology and Behavior, Mastering the World of Psychology. United States of America: 47-85, from http://www.macmillanlearning.com/Catalog/uploadedFiles/Licht1e_Ch02_Final_hr8.pd f