Neuroscience Basics (page 1) (page 2)
1. Neuron
On one hand, neurons are remarkably similar to most other cells in a body. For example, both neurons and muscle require energy in the form of carbohydrates and lipids in order to survive. In fact, neurons can communicate with muscle and regulate contraction. On the other hand, groups of neurons that form circuits have have the amazing ability to generate memories and express behaviors. The challenge is to figure out how something as complex as behavior is regulated at the level of individual molecules within individual cells.
2. Neuron circuits
Electrical circuits of a brain share some basic similarities with the wiring of a house. Both circuits require mechanisms that regulate what is on and what is off. An individual neuron can transmit electrical current like a wall switch that is either turned on or off. In fact, some neurons are difficult to turn on, much like an old-fashioned wall switch. The most fundamental difference between neuronal circuits and house wiring is at the interface between adjoining neurons/wires. House current requires direct contact between metal surfaces of a switch or an outlet. Neurons have small gaps or synapses between neurons. Synapses could be considered a mechanism of regulating electrical communication between neurons. If a synapse is a gap that prevents direct contact, then how is electrical information transferred to an adjoining neuron? Chemical messengers called neurotransmitters are used to pass (transmit) information from a pre-synaptic neuron to the adjoining, post-synaptic neuron.
3. Neurotransmitters
An electrical signal, or more accurately, depolarization of a pre-synaptic neuron results in the release of neurotransmitters into the synapse. Neurotransmitters come in sizes ranging from small molecules such as serotonin to large peptides such as endorphin. Glutamate is the predominant excititory neurotransmitter that promotes depolarization of a post-synaptic neuron, propagating an electrical signal. In contrast, GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter that blocks depolarization of a post-synaptic neuron. Thus, it should be clear that a depolarized neuron is able to release any of several different neurotransmitters that affect neighboring neurons, as well itself. Which neurotransmitters are released is largely dependent on what neurotransmitters the neuron makes. From a nomenclature standpoint, a neuron that makes a particular neurotransmitter, and therefore has the potential to release it, ends in -ergic (e.g. GABAergic, serotonergic, etc.). It is important to note that a single neuron may make and release both small molecule and peptide neurotransmitters.
The brain would be a lot easier to understand if each neurotransmitter could be categorized as either excititory or inhibitory based on the response of a post-synaptic neuron. However, it would be extremely difficult to explain the complex functions of a human brain based on a relatively small number of neurotransmitters. Furthermore, the function of neurotransmitters is not limited to just regulating electrical properties of neurons. Neurotransmitters, as well as many drugs, have the potential to modify the response of a neuron to other molecules in its environment. The bottom line is that an effect of a neurotransmitter is dependent on neurotransmitter receptors.
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