Neurotransmitters refer to certain chemicals agents which relay, modify and modulate signals from the neurons to other cells in the body (Rang, 2003). When these chemicals are secreted at terminal axon ends of the nerve cells, they diffuse across the synaptic gap thereby transmitting information to the nearby adjoining cells such as glands, neurons and muscle cells. A synaptic gap refers to the distance between one neuron and the adjacent neuron. Effective transmission of information from one neuron to the other is dependent on the ability of that particular information to transverse the synaptic gap which is found between the terminal of one neuron which is the source of the information and the receptor of an adjacent neuron which receives the information.
Characteristics of neurotransmitters.
There are many types of neurotransmitters which serve different functions in the body related to transmission of signals from one part of the body to the other. Over the years, a variety of criteria have evolved to try and distinguish neurotransmitters from other chemical substances in the body such as hormones. However, identifying a particular neurotransmitter present at a given synapse is still difficult since for many synapses especially those present in the brain, there are no clear distinctions on the nature of the neurotransmitters.
Nevertheless, there are some common characteristics which a chemical compound must posses in order to be classified as a neurotransmitter. First of all, the substance must be present in the pre-synaptic element of a neuron and its quantity must be enough to influence the activity of the post-synaptic neuron (Robert, Jack & Floyd, 2003). Moreover, the substance must be chemically able to bind specific receptors in a biochemical mechanism.
One of the most common distinctive characteristic of neurotransmitters is that they work at miniscule distances near the presynaptic site as compared to the other signaling compounds in the body such as hormones which target cells which are far from the secretion site (Bernard &George, 1999). In addition, neurotransmitters at the terminal ends bind to specific receptor ends of other neurons on the post-synaptic cell. This shows that the neurotransmitters act at very short distances which are less than one micrometer. In certain cases where the transmitters diffuse within the neuron to alter the electrical properties of multiple post-synaptic cells, the distances within which they act are still less than 100 micrometers.
Another general characteristic of neurotransmitters is that, all their actions and effects are influenced by the receptor cells to which the information is targeted. For instance, acetylcholine which was the first neurotransmitter to be discovered is a stimulator for the skeletal muscle cells transmissions but an inhibitor for heart muscle cells. In this case,the neurotransmitter acts both as a stimulator and an inhibitor depending on the receptor cell to which it binds.
However, though the distinction between neurotransmitters and hormones is quite clear, it is possible for a molecule to act as a neurotransmitter at a certain region in the brain and act as a hormone elsewhere in the body. Examples of this include oxytocin and vasopressin. These two are peptide hormones which are secreted by the posterior pituitary glands and they also act as neurotransmitters at certain central synapses.
Types of neurotransmitters.
The first neurotransmitter was discovered back in the year 1921 by an Austrian scientist known as Loewi. He discovered a certain compound in the vagus nerves of frogs and he name the chemical as vagusstoff (Rang, 2003). This compound is now known as acetylcholine. Many other types of neurotransmitters have since been discovered and some of them include:
Acetylcholine;- this is a chemical compound which is found in both vertebrates and invertebrates and is vital for the stimulation of the skeletal muscle tissues. It is the most abundant neurotransmitter in the body which is responsible for controlling the levels of hormone vasopressin. During impulse transmission, stimulation makes acetylcholine to decompose into acetate and choline compounds which are quickly absorbed back into the neuron cell body to form another acetylcholine neurotransmitter molecule. The function of acetylcholine is inhibited by a certain poison known as curare which blocks its transmission as well as some nerve gases which block its decomposition hence forming continuous skeletal muscle cells stimulations or spasms at the heart muscle cells.
Dopamine;- this is a catecholamine which is found in the brain. It is useful in the regulation of emotional responses and other critical activities which take place in the brain. However, the levels of dopamine in the brain should be controlled as any increase or decrease above or below the normal levels is likely to trigger abnormal transmissions which results in certain diseases such as Schizophrenia, Parkinson’s disease, or certain addictions.
Epinephrine and norepinephrine;- these two chemical compounds are secreted from the adrenal gland and they are responsible for increasing the heart rate and glucose production in the body. They are important when the body is confronted with a situation which requires a lot of energy or what is known as a ‘flight or fight ‘response. Norepinephrine is also responsible for varying sleeping patterns, learning and memory related activities.
Serotonin;- this is a neurotransmitter which is synthesized from trytophan amino acid. It is found widely distributed in the blood platelets, brain and the digestive tract lining. It is responsible for the contraction of smooth muscles which lead to regulation of moods, sleep, emotions, anxiety, depression and other mood disorders.
Gamma amino-butyric acid (GABA);- this is an inhibitor neurotransmitter which controls the action of excitatory neurotransmitters which may lead to anxiety disorders (Majumdar & Sephali, 1998). GABA deficiencies results in over anxiety while lack of GABA in certain regions of the brain leads to epilepsy. The levels of GABA can be increased by certain drugs such as Valium which is used to control anxiety, over-excitements and unexplained panic. Recent studies have shown that alcohol and barbiturates drugs are likely to interfere with GABA receptors in the brain leading to its deficiency (Boehm, Ponomarev, Blednov & Harris, 2006).
Aspartate;- this is an amino acid which is responsible for stimulation of neurons in the CNS (central nervous system). It is found particularly in those neurons which transmit signals to and from the cerebrum region of the brain.
Oxytocin;- this is a short peptide which is released within the brain, testes and ovaries. It is responsible for the maternal behavior in both male and females, release of milk by the mammary glands and contractions during the birth process. Oxytocin also acts as a hormone which is released from the pituitary gland.
Insulin;- this is a peptide neurotransmitter which is secreted by the pancreas and it plays a major role in the stimulation of cells to absorb glucose. Pancreas malfunction which results in under or overproduction of insulin leads to a disease known as diabetes.
Somatostatin;- this is a peptide which inhibits the secretion of growth hormones from the pituitary gland. It also inhibits the secretion of insulin and other gastrointestinal hormonal substances which are responsible for nutrient absorption in the body.
How neurotransmitters work.
The manufacturing site for neurotransmitters in the neuron is known as the cell body (Albert, Johnson, Lewis et al, 2002). From the cell body, the chemical agents are transported to the neuron terminal end where they become enclosed in membrane bags known as vesicles. Once the neurons receive any potential signal from the other parts of the body, they respond by fusing the vesicle membrane with the neuron membrane and the neurotransmitter chemical diffuses across the synaptic gap.
On the post-synaptic side, the neurotransmitter meets with the receptors which are transmembrane proteins. The neurotransmitter binds selectively to a specific receptor the same way a certain key fits into a given lock. Once the binding process has taken place, the site becomes activated and this results in either depolarization or hyper-polarization of the affected neurons. This may also lead to activation of another receptor site which receives the information and alters the signal flow between the two initial neurons.
Depolarization is responsible for the stimulation of the neurons which results in a release of the neurotransmitter from the terminal end of one neuron to the receptor end of another neuron while, hyper-polarization acts on the reverse inhibiting the release of a neurotransmitter from the terminal end of a given neuron. This two processes determine the rate and effectiveness of information transfer from one neuron to the other (Rang, 2003).
When a neurotransmitter binds to its target receptor, a biological effect is triggered and once the effect is complete, the site activation ceases and the receptor is ready to bind another neurotransmitter. Site inactivation can also result from degradation caused by certain enzymes. For instance, enzyme acetylcholinesterase is known to degrade acetylcholine neurotransmitter thus causing site inactivation and this leads to ineffective information transfer. Certain cells which are known as astrocytes are also known to remove neurotransmitters from the site where they are bound to the receptors hence causing similar effects of site inactivation.
Functions of neurotransmitters.
Neurotransmitters are best known for their functions in the transfer of information to and from the central and peripheral nervous systems. Different neurotransmitters play different roles in the body which are all related to information transfer. Some of the major functions of neurotransmitters include sensitization of pain, heat and mechanical hyperalgesia, modulatory functions in the striatum, as well as ‘flight and fight’ responses in the body. Neurotransmitters are also believed to play a major role in causing depression, addictions and other mental disorders.
To ensure proper mental and physical health, the amount of neurotransmitters in the body should be maintained at the correct levels since more often than not, any deficiency is likely to result in a certain disorder. For instance, Alzheimer’s disease and Parkinson’s disease occur due to lack of acetylcholine and dopamine in certain parts of the brain respectively. Deficiencies of serotonin and dopamine neurotransmitters results in many disorders such as anxiety, depression, eating disorders, migraines, insomnia, low libido, fibromyalgia among others. Signs of neurotransmitter deficiencies include behavioral changes, certain cravings, change of moods and attitudes towards people and things among others. Laboratory testings can also be used to verify neurotransmitter levels in the body.
High levels of neurotransmitters such as catecholamine are likely to cause excessive stimulation of the receptor cells or excited post-synaptic firing. Some of the disorders related to excess excretion of neurotransmitters include hyperactivity and anxiety related syndromes such as OCD, ADHD among others (Robert, Jack & Floyd, 2003).
Neurotransmitters are very important for transfer of information from one neuron to the other which is important for overall body coordination. They help the body to respond to certain internal and external signals such as heat, pain, physical injury, emergencies and so forth, by transmitting the necessary signals for action. For instance, when one finds himself in situation which requires self defence, the epinephrine and norepinephrine neurotransmitters act to increase the heart rate and the levels of glucose in the body in order to provide the necessary energy needed for fight or flight. This is an automatic defence mechanism which is controlled by the neurotransmitters. It is thus clear that neurotransmitters are very important in the body and their levels should always be maintained at the right levels for optimal physical and mental health.
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Boehm, S., Ponomarev, I., Blednov, Y. & Harris, R. (2006). From gene to behavior and back again: New perspectives on GABA receptor subunit selectivity of alcohol actions. Advanced Pharmacology, 54: 171–203.
Majumdar, D. & Sephali, G. (1998). GABA and several GABA inhibitors. Journal of Molecular Structure, 180, 125-140.
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