Structure and Function of Neurons
The nervous system is a network of individual nerve cells or neurons. The function of neurons is complex--nothing like the easy binary of a computer with programmed "intelligence". The brain is capable of sending mixed signals, and one of the major functions of certain neurons is to temporarily stop other neuronal activity.
How Neurons Work
To see how neurons work, one needs to know the basic chemistry in a neuron. The axon of the neuron is the part of the neuron responsible for transmission of electrical charge. This charge is a bit different from the dry and alternating currents we're used to. In this case the movement of charge, or the action potential of a neuron is created when positively charged sodium enters the cell. This sodium enters at the base of the axon and cascades down the axon of the neuron. Eventually, when this movement of electricity reaches the axon terminals, neurotransmitters are released.
Chemistry Inside the Neuron
Before we dig into neurotransmitters, to understand how neurons work we need to look at the chemistry that underlies the action potential of the neuron. In the main cell body of a neuron there are many large proteins that tend to carry a negative charge. To neutralize some (but not all) of this charge, the cell absorbs positively charged potassium, selectively. That is to say the neuron contains channels that only allow potassium in. Meanwhile, sodium is kept at bay outside the cell, most importantly outside the axon of the neuron.
This resting state is chemically unstable because of diffusion. If, for example, you separate a bucket, filling one part with pure water and the other with water and food coloring, the two will blend if you don't keep them separate. The coloring will diffuse to the pure water. The situation is likewise in the cell. The concentration of potassium in the cell is attracted to the negative charge of the proteins, but is also repelled by diffusion. Meanwhile, sodium is attracted to the inside of the nerve cell, both from diffusion and attraction to the negative charge. The overall effect of this system is that a neuron at rest has a negative charge of -70 millivolts. Upset this delicate order and an action potential of the neuron will fire.
The manipulation of the electircal charge in the cell bodies of neurons is the mechanism for how neurons work. In the simplest of scenarios neurotransmitters open ion channels with positive ions polarizing the cell to fire, or negative ions to hyperpolarize the cell (thus inhibiting fire). In the more complicated systems, receptor coupled proteins and neuromodulators (that modify how other messengers work) are involved.
Once the axon of the neuron fires, the action potential will cascade down the axon which branches out to a number of axon terminals. These terminals connect neurons to many other neurons to the receiving end of a nerve cell, the dendrites of the neuron. But the electrical charge does not jump from the axon terminal to the dendrites of the axon. Instead there is a gap called the synaptic cleft, where certain bran chemicals, neurotransmitters, are realized. These neurotransmitters affect the neuron through neurotransmitter receptor sites located in the dendrites of the neuron.
Anatomy of a Neuron
Neurotransmitters in the Brain
The number of neurotransmitters in the brain, and the litany of receptor sites for those many neurotransmitters is beyond the scope of this basic review. Four of the most prominent are the glutamate, GABA (gamma-aminobutyric acid), serotonin and dopamine neurotransmitters.
Glutamate Neurotransmitters, the main excitatory system:
Glutamate neurotransmitters are the most common in the nervous system. They consist of the simple amino acid glumate, and these neurotransmitters are generally excitatory. They are the brains basic on switch. When the glutamate neurotransmitter is blocked (by an antagonist), certain effects begin to manifest, noticably hallucinations and anathesia. Well known drugs that block glutamate activity include PCP and ketamine. Over stimulation (or agonzing) of the glumate system can lead to excitotoxicity, or cell death.
GABA, the main inhibiting neurotransmitter
It's just as important for the brain to stop certain neural activity as it is to excite it. In the extreme, a lack of inhibition will lead to seizure. Uncontrolled body movements are common in such conditions as Huntington's disease. More generally, however, a lack of inhibition by GABA results in high levels of anxiety. Panic attacks and insomnia may occur. Therefore any drug that activates (agonizes) GABA can have both an anti-anxiety and sedative affect. many well known drugs agonize GABA receptors: barbituates, benzodiazapenes (such as Xanax or Valium), and alcohol. All of these drugs come with a risk of dependency, addiction, and/or overdose (usually when CNS depression is so complete breathing fatally slows). Moreover, all these drugs activate different sites on the GABA receptor so that taking these drugs has a cumulative effect, making them dangerous in combination.
Dopamine receptors, the reward system
Dopamine is an excitatory neurotransmitter. Dopamine is present in many areas of the brain, but perhaps the most important is in the reward system (called the mesolimbic pathway). When you feel rewarded for having done something important, the reward you feel is the result of the action of the neurotransmitter dopamine. Activation of dopamine is highly pleasurable, and the mesolimbic pathway is the source of the potent addiction that can occur from taking drugs such as cocaine and methamphetimine. Dopamine is also linked to schizophrenia, although the exact link is uncertain. A reward system for irrelevant information/connections may be the source of the often fierce delusions that come with the disease. Blocking dopamine activity has been common in the treatment of schizophrenia for over 50 years, (Recent drugs have demonstrated other factors are involved.) Blocking dopamine brings its own set of problems especially in the motor functions. At the most extreme side of blocked dopamine is Parkinson's disease, where purposeful movement becomes extremely difficult. Blocking the neurotransmitter dopamine can also cause anhedonia (complete lack of pleasure) by shutting off the reward system.
Serotonin receptors, important inhibtors
The neurotransmitter serotonin is an inhibitor and best known for its modulating effect on depression. Three generations of antidepressants: MAOI's, tricyclics, and SSRI's are all believed to work by increasing the amount of serotonin activity. How this occurs depression is mediated is still a bit mysterious, since these drugs take weeks to generate an effect. By contrast alcohol's effects will be felt in less than an hour. While increasing of the effects of serotonin neurotransmitters tends to have an overall increase in mood, it can be toxic at high enough levels though this usually requires a combination of drugs to occur. The excitation of certain serotonin receptors can cause vivid hallucinations: this is the mechanism behind the effects of LSD, mushrooms, mescaline, and DMT (agonists of serotonin receptors) along with ecstasy (which causes serotonin to break free from axon terminals without the action potential of a neuron). Decreased serotonin levels are associated with depression, but serotonin also has numerous functions outside the nervous system.
How neurons work, a review:
The nervous system is composed of a connected myriad of individual neurons. These neurons have three basic parts: the dendrites that receive signals from surrounding cells, the cell body where the signals (i.e. electric charge) accumulate, and the axon of the neuron, which produces an action potential down its length, telling the neurotransmitters in the brain to leave the axon terminal across the synaptic cleft of other neuron's dendrites. Two neurotransmitters in the brain, glutamate and GABA, are the major on and off switches in the brain. Many other neurotransmitters serve important but more specialized roles. Serotonin and dopamine are notable because they have responded so well to psychiactric medication. The neurotransmitter dopamine is especially important because its reinforcement effects have a great deal to do with how we learn to behave.