Our body has a huge number of neurons. How many, you ask? Well, the human brain alone boasts around 86 billion neurons. That’s a very big number! These neurons are a part of our nervous system, which plays a very important role in our lives. It receives signals, processes them, and then decides and executes the appropriate outcome. As any system does, the nervous system has its own functional unit – a neuron. These neurons run throughout our body, intercepting signals and passing them to the brain or spinal cord.
Structure of a Neuron
Neurons can seem pretty complicated, but their basic structure is quite simple. They have a cell body that contains the nucleus and from this comes 2 types of structures – dendrites and axons. Dendrites are finger-like projections that receive the signals. On the other end is the axon, which is a long, thin structure like the tail of the neuron. It is responsible for conducting or passing on the signals. A neuron will have many dendrites, but only a single axon.
This axon is covered with a myelin sheath and there are nodes present in the axon, called the nodes of Ranvier. The myelin sheath helps in the faster conduction of signals. The axon, near the end, branches off and forms axon terminals. At these terminals, a tiny bulb-like structure is present, which facilitates the passage of information between 2 neurons. More specifically, it facilitates the passage between the axon of one neuron and the dendrites of the next neuron.
The neurons lie end to end, but never actually touch each other. The juncture between the axon terminals of one neuron and the dendrites of another is known as a synapse. Given that the signal being conducted by the neurons is in the form of an electrical impulse, the synapse causes a break in the chain, thus preventing the signal from moving forward. Therefore, there must be some way for the signal to “jump” across to the dendrites of the next neuron. This is done by converting the electrical impulse to a chemical signal.
Neurotransmitters are chemicals produced and stored in the axon terminals. When an electrical signal reaches the end of a neuron, it activates the release of these chemicals. The chemicals then travel to the dendrites of the next neuron, which have neurotransmitter receptors. These convert the chemical signal to an electrical impulse again, which is conducted through the neuron until it reaches the axon terminal, where the same process is repeated.
Different electrical signals orchestrate the release of different types of neurotransmitters, which, in turn, elicit the same electrical impulse in the next neuron.
Most neurons do not release a single neurotransmitter, but rather a combination of them. Some examples of neurotransmitters are noradrenaline and epinephrine. Once these transmitters have evoked the desired response in the next neuron, they are either taken in by the axon terminal from which they were released, or they are destroyed. This is a necessary step, as it prevents the same signal from being sent over and over again, even when its stimulus ceases to exist. For example, when a person steps out into bright light, their eyes automatically shut. If, at this stage, the neurotransmitters weren’t destroyed, the person wouldn’t be able to open their eyes again! Therefore, it is essential to destroy or retract the neurotransmitter once it has completed its job.
Synapses are present throughout our body. Our neurons fire signals at astonishing rates, and thus our synapses have to be very efficient at conducting the signals. Even though we perform multiple tasks at a time, our neurons and synapses never mix up the signals, thus ensuring smooth functioning of the body.
- The University of Queensland, Australia
- Illinois State University
- National Center for Biotechnology Information (NCBI)
- Indiana University Bloomington
- Washington University