Chemical Neurotransmitters

The nerve cells which are used for the perception of external events will, upon being excited by the proper stimulus, transmit an action potential down their axons. When the electrical signal reaches the axon terminal bundle, it interacts with structures called synaptic knobs. It stimulates an influx of calcium (Ca²⁺) through voltage-gated Ca²⁺ gates. This causes the movement of vescicles toward the membranes of the synaptic knobs. Inside these vesicles are neurotransmitter chemicals. The neurotransmitters are manufactured in the cell body and travel down the axon to be stored in vesicles associated with the synaptic knobs. When a vesicle reaches the cell membrane of the synaptic knob, it fuses with the cell membrane and releases its neurotransmitter into the synaptic region. On the postsynaptic neuron are receptors that will specifically bind these neurotransmitters. The neurotransmitter will either excite or inhibit the firing of the postsynaptic neuron.

One mechanism for inhibition of the firing of the post-synaptic neuron is to cause hyperpolarization like that which follows the pulse of an action potential. This would raise the threshold for firing of the neuron.

Note that the pre- and post-synaptic neurons have been drawn identically above, but that is just out of ignorance of what the structural differences are.

Contributing author: Ka Xiong Charand

Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans.

Acetylcholine is the neurotransmitter involved in the contraction of a muscle fiber, and in the synapse for the muscle cell the acetylcholine receptor opens a gate for the influx of sodium into the cell to initiate the series of events that contracts the muscle.

The term cholinergic is used to describe parts of the body that use acetylcholine in stimulation, and anticholinergic if it is used to decrease activity. Acetylcholine is the primary neurotransmitter in the parasympathetic nervous system. It also functions as a transmitter in the sympathetic nervous system. In the brain, acetylcholine functions as both a neurotransmitter and a neuromodulator. It is one of several chemicals that act to regulate diverse populations of neurons with effects that last significantly longer than the normal short-term nerve action.

In the central nervous system, acetylcholine has many important functions in the brain. For example, it has an important role in the enhancement of alertness when we wake up, in sustaining attention, and in learning and memory.

Acetylcholinesterase

Acetylcholinesterase is an enzyme that catalyzes the breakdown of acetylcholine and of some other choline esters that function as neurotransmitters. It has an essential role in neuromuscular junctions and in chemical synapses of the cholinergic type, where its activity serves to terminate synaptic transmission. With the acetylcholine removed, the nerve cell quickly returns to its relaxed state.

Acetylcholinesterase is the primary target of inhibition by organophosphorus compounds such as nerve agents and pesticides. Such agents block the action of acetylcholinesterase so that the acetylcholine continues to call for contraction of the muscles. In blocking acetylcholinesterase these agents are acting as irreversible inhibitors.

Acetylcholinesterase has a very high catalytic efficiency, with each molecule capable of degrading about 25,000 molecules of acetylcholine.

Important Neurotransmitters

While there is an abundance of chemical species which play the role of a neurotransmitter in specific settings, there a few including acetylcholine that play major roles in human physiology.

Epinephrine

Epinephrine, also called adrenaline, is produced by the adrenal glands located above the kidneys. This neurotransmitter is responsible for your body's "fight or flight" response. Once released, it works throughout the central nervous system to increase heartrate and bring extra oxygen to your muscles quickly. It is part of your body's defense against stress, speeding up your breathing and contributing to alertness.

Norepinephrine

Norepinephrine, also called noradrenaline, has the general function of mobilizing the brain and body for action. Its release is lowest during sleep, rises during wakefulness, and reaches much higher levels in situations of stress or danger (fight or flight response). In the brain, norepinephrine increases arousal and alertness, promotes vigilance, enhances formation and retrieval of memory, and focuses attention. It also increases restlessness and anxiety. In the rest of the body, norepinephrine increases heart rate and blood pressure, triggers the release of glucose from energy stores, increases blood flow to skeletal muscle, reduces blood flow to the gastrointestinal system, and inhibits voiding of the bladder. These bodily effects are characteristic of the sympathetic nervous system and norephinephrine is the main neurotransmitter used by that system. Sympathetic activation of the adrenal glands causes the part called the adrenal medulla to release norepinephrine (as well as epinephrine) into the bloodstream, from which, functioning as a hormone, it gains further access to a wide variety of tissues.

Dopamine

Dopamine is considered in popular culture as the pleasure drug, but the pharmacological opinion is that in the central nervous system it confers motivational salience, signaling the desirability or aversiveness of and outcome and propelling one's behavior toward or away from that outcome. Outside the central nervous system it functions as a messenger. "In blood vessels, it inhibits norepinephrine release and acts as a vasodilator (at normal concentrations); in the kidneys, it increases sodium excretion and urine output; in the pancreas, it reduces insulin production; in the digestive system, it reduces gastrointestinal motility and protects intestinal mucosa; and in the immune system, it reduces the activity of lymphocytes."(Wiki) There is much discussion of dopamine in relation to drug addictions. Cocaine, methamphetamine, and other addictive drugs are seen as inhibiting the re-uptake of dopamine, leading to high dopamine levels in the brain and to many of the manifestations of drug addiction.

Serotonin

Serotonin is a neurotransmitter for which the "biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction." About 90% of the body's serotonin is located in the gastrointensinal tract where it regulates intestinal movements. About 8% of it is found in blood platelets and 1-2% in the central nervous system. Functions in the CNS include the regulation of mood, appetite, and sleep. It also has some cognitive functions, including memory and learning.

Serotonin is sometimes called the "calming chemical" and is known for its mood modulating effects. A lack of serotonin has been linked to depression and related neuropsychiatric disorders.

Glutamate

Glutamate is common in a normal diet. Glutamate is present in 90 percent of synapses, acting as the main excitatory neurotransmitter in the central nervous system. Too much glutamate can over-excite brain cells, but this is typically controlled by glutamate transporter molecules that collect the excess glutamate in the cell's environment. Glutamate is important to neuroplasticity, the capacity of the brain to rewire neural connections for new tasks. This helps us to learn and reinforces our memory.

Glutamate is the negative ion form of glutamic acid. It is synthesized from the amino acid glutamine in the central nervous system. Glutamate is used in the synthesis of GABA.

Glycine

Glycine is one of the 20 amino acids used by the body. "Glycine is used for treating schizophrenia, stroke, benign prostatic hyperplasia (BPH), and some rare inherited metabolic disorders. It is also used to protect kidneys from the harmful side effects of certain drugs used after organ transplantation as well as the liver from harmful effects of alcohol."rxlist

g-Aminobutiric acid (GABA)

GABA is primarily synthesized from glutamate. The process converts glutamate (the principal excitatory neurotransmitter) into GABA (the principal inhibitory neurotransmitter).