What Are Neurons and How Do They Communicate?
There are two kinds of cells in the nervous system - neurons and glial cells. Neurons are nerve cells that handle the information-processing function of the nervous system, while Glial Cells support neurons by providing them with the nutritional benefit that they need. Without glial cells, neuronal activity diminishes and neurons can die.
There are about 100 billion neurons in the brain (although neurons can also be found throughout the entire body); and each neuron connects to approximately 10,000 other neurons. The speed of communication between neurons is quite fast - reaching up to 330 miles per hour, or 147.5 m/s. This is the reason why it takes less than a second for us to know, for instance, what's cooking, to locate where the food is being cooked, and perhaps even think about who could probably be doing the cooking.
There are three (3) basic parts of a neuron - the cell body, the dendrites and the axon.
- The cell body houses the nucleus, which manufactures the substance needed by the neuron to maintain itself. The cell body also processes the information received from neighboring neurons.
- The dendrites act like TV antennae. They receive information coming from the neighboring neurons.
- The axon, on the other hand, conveys the information processed by the cell body to other neurons. It is about 0.0001 inch (1/10,000th of an inch), and can reach up to 3 feet long. At the end of an axon are branches that end with terminal buttons. These terminal buttons houses the synaptic vesicles or sacs that contain chemicals called neurotransmitters. The neurotransmitters, as will be discussed below in greater detail, are the chemicals released by a neuron to communicate with neighboring neurons. A neuron typically contains only one type of neurotransmitter. The axon is encased and insulated by a kind of fat called the myelin sheath for faster electrical travel (just like how electrical wires are covered with plastic). It is widely believed that encasing the axon with fat reflects the evolution of the brain for faster information-processing. However, the myelin sheath may harden (a disease called multiple sclerosis), hampering the electrochemical activity of the neuron.
Neurons are covered by a semi-permeable membrane that contains Ion Channels. These ion channels open and close to let positive ions, such as sodium (Na+) and potassium (K+), and negative ions, like chlorine (Cl-), enter the neuron. Ion channels are closed when the neuron is not transmitting information or is resting. Using a device called the oscilloscope, neuroscientists have found that the Resting Potential of a neuron (or the electrical charge of a resting neuron) is -70 millivolts (mV). Thus, an electric eel having 8,400 neurons can generate up to 588 V!
Because a resting neuron is negatively charged in relation to its positive surrounding (that is, more negative ions are present inside the cell body of the neuron), a resting neuron is said to be polarized. However, when dendrites receive neurotransmitters from neighboring neurons, the ion channels open up and let positive ions depolarize the neuron. The Action Potential of a neuron (or the electrical charge of an activated neuron) depends upon the kind of neurotransmitter housed by the neuron. The qualitative differences between neurons result to variation in their voltage threshold. A depolarized neuron that has reached its voltage threshold "fires" at a level of intensity that remains unchanged, called the All-or-None principle, and as fast as 0.001 second. The brief wave of positively charged ions inside the neuron pushes the synaptic vesicles at the end of the terminal buttons, thereby releasing the neurotransmitters to the Synapse or the Synaptic Gap, the small gap between neurons. Neurotransmitters that are released in the synapse do not move in a predictive manner; rather, they randomly wander along the synaptic gap. Different neurotransmitters have different shapes, and the shapes serve as keys for opening the receptor sites at the dendrites of the receiving neuron. After the neurotransmitter activates the receiving neuron, they are then reabsorbed back in the synaptic vesicles of the terminal buttons of the releasing neuron. The re-absorption causes the neuron to get polarized and rest once again.
In summary, the pre-synaptic neuron releases neurotransmitters that activate the post-synaptic neuron, which, in turn, releases neurotransmitters that can further activate other neighboring neurons. In this manner, electrical transmission is converted into chemical code, and back to electricity. This is the reason why the nervous system is characterized with electrochemical transmission. However, communication between neurons is not a linear path. Known as the Volley principle, a firing neuron can activate many neurons at once; vice versa, a neuron can be activated by the simultaneous firing of neurons that have different neurotransmitter types.
Neurotransmitters are the chemicals released by a neuron to communicate with another neuron. They may be excitatory or inhibitory. Excitatory Neurotransmitters are those that activate other neurons to fire, while Inhibitory Neurotransmitters are those that restrain neurons to fire. There are currently fifty (50) discovered neurotransmitters. The six (6) types of neurotransmitters featured below are those that have major effects on behavior.
- Acetylcholine or Ach is an excitatory neurotransmitter involved in muscular action, learning and memory. It is located throughout the central and peripheral nervous system. Low level of acetylcholine is associated with Alzheimer's Disease, a degenerative disease wherein neurons die at a rate faster than normal. Up to date, no treatment has been found to cure this deadly disease. Drugs that supply ACh are only used to delay or reduce the rate of neuronal death.
- Dopamine is an inhibitory neurotransmitter that controls voluntary movement, sleep, mood, attention and learning. Inhibition is a very important element for controlled action. Low level of dopamine is associated with Parkinson's Disease, wherein patients shake uncontrollably on different parts of the body. The disease is also degenerative, so that the shaking can range from mild to extremely uncomfortable. On the other hand, high level of dopamine is observed among schizophrenic patients.
- Endorphin is an excitatory neurotransmitter involved in feelings of pleasure. Endorphins are called the natural opiates in the body and serve as pain killers especially on pregnant women during labor and delivery. High level of endorphins is also observed among long-distance runners and persons shocked from a traumatic accident.
- Gamma Amino Butyric Acid or GABA is an inhibitory neurotransmitter that controls the electrochemical communication among neurons. It is located only in the central nervous system, and fills one-third (1/3) of the brain's synapses. Low level of GABA is associated with anxiety.
- Norepinephrine is both inhibitory and excitatory. It controls alertness and regulates sleep and wakefulness with ACh. As an inhibitory neurotransmitter, norepinephrine acts like GABA in preventing neurons from firing uncontrollably. As an excitatory neurotransmitter, it activates the heart muscles, the intestine and the neuro-genital tract. Low level of norepinephrine is linked with depression, while high level of norepinephrine is associated with agitation and mania.
- Serotonin is an inhibitory neurotransmitter that works with Ach and norepinephrine in regulating sleep, mood, attention and learning. Just like norepinephrine, low level of serotonin is associated with depression.
How neurotransmitters work is related to how drugs are made to function in the body. Drugs may be classified as agonist or antagonist. Agonists are drugs that mimic or increase the effects of neurotransmitters. Antagonists are those drugs that block or hinder the effects of neurotransmitters. Below are examples of drugs that behave like neurotransmitters in activating or inhibiting neuronal activity:
- Nicotine stimulates ACh receptors in the brain.
- The Black Widow Spider's venom causes ACh to gush in huge amounts between the spinal cord and skeletal muscles, resulting to violent muscular spasms.
- Curare, the drug used in the poison dart tips of South American Indians, blocks ACh receptors and causes muscle paralysis.
- Cocaine and Amphetamine activate dopamine and norepinephrine receptors, leading to feelings of excitement, alertness and elevated mood, decreased fatigue and increased motor activity.
- Morphine, an important derivative of opium, mimics endorphins, resulting to elevated feelings of pleasure.
- Wild poppies were used by the Greeks in 400 BC to induce euphoria. They stimulate endorphin receptors in the brain.
- Valium and other anti-anxiety drugs increase inhibition by stimulating receptors of GABA and norepinephrine.
- Prozac and other anti-depressant drugs are used to combat depression by increasing the level of serotonin in the brain.