Definition

The Neurons / the nerve cells transmits signal from the surroundings to the central nervous system, among different regions of the central nervous system, and from the central nervous system back to various organs / the periphery. Very small molecules called “Neurotransmitters” mediate the signals transmitted.

“Neurotransmitters are chemicals allowing the movement of information from one neuron to its adjacent neurons through the gap between it. The release of neurotransmitters from one area of a neuron and the recognition of the chemicals by a receptor site on the adjacent neuron causes an electrical reaction that facilitates the release of the neurotransmitter and its movement across the gap.”  (neurotransmitter)

Criteria for Neurotransmitters

  • Synthesis – The molecule is synthesized in presynaptic neurons

  • Localization – The molecule is present in the presynaptic terminal

  • Release – Upon stimulation, the molecule is released into the synapse

  • Response Mimicry – When experimentally applied, the molecule produces a response that is similar to the endogenous release of neurotransmitter

  • Inactivation – A specific mechanism exists to remove the molecule from the synaptic cleft or to degrade it

Types of Neurotransmitters

Otto Loewi, an Austrian scientist, in 1921 discovered the first neurotransmitter. He termed this compound as "vagusstoff," during his experimentation with the vagus nerve of frog hearts. Presently, this compound “vagusstoff” is popularly called acetylcholine.

Generally, the neurotransmitters can be categorized into two, either (1) excitatory or (2) inhibitory. The first category - Excitatory neurotransmitters increases while the second category - inhibitory neurotransmitters decreases the activities of the postsynaptic neurons. The role of neurons varies from recognizing, integration, to passing on the signals conveyed by the neurotransmitters. Example: Some neurons repeatedly fire at a definite rate and can be either excited / inhibited in reply to environmental changes while other neurons generally are at rest. As a result, any alteration in their activities must occur in the shape of excitation. Accordingly, this neuronal excitation demonstrates an essential role in controlling brain performance and brain operation and substances that interfere with excitatory neurotransmitter systems can dramatically affect brain function and behavior. Alcohol is one such substance; it appears to interfere with the signal transmission mediated by the neurotransmitter glutamate.

Neurotransmitter

Function

Effect of Deficit

Effect of Surplus

Acetylcholine (Ach)

Excitatory: It produces muscle contractions and is found in the motor neurons; in the hippocampus, it is involved in memory formation, learning and general intellectual function

Paralysis; A factor associated with Alzheimer’s disease: levels of acetylcholine are severely reduced associated with memory impairment.

Violent muscle contractions

Dopamine

Excitatory: involved in voluntary muscle movements, attention, learning, memory, and emotional arousal and rewarding sensations

Muscle rigidity; A factor associated with Parkinson’s disease: degeneration of neurons producing dopamine in the substantia nigra.

One factor associated with schizophrenia-like symptoms such as hallucinations and perceptual disorders,

Serotonin

Inhibitory or excitatory: involved in mood, sexual behavior, pain perception, sleep, eating behavior, maintaining a normal body temperature and hormonal state

Anxiety, mood disorders, insomnia; One factor associated with obsessive-compulsive disorder and depression

Autism

Endorphins

Inhibitory: regulates pain perception and concerned with positive emotions linked with aerobic exercise, sexuality, pregnancy, and labor - the brains natural opiates.

Body experiences pain

Body may not give adequate warning about pain

Norepinephrine

Excitatory and inhibitory: involved in increasing heartbeat, arousal, learning, memory, and eating

One factor associated with depression.

Anxiety

GABA (gamma aminobutyric acid)

Inhibitory: communicates messages to others (neuron), assists to balance & offset excitatory messages. It is also involved in allergies

Destruction of GABA-producing neurons in Huntington’s disease produces tremors and loss of motor control, as well as personality

Sleep and eating disorders

Neurotransmitters have different actions while some neurotransmitters have different effects too which depends upon which receptor they bind. As an example, acetylcholine can act as stimulatory when linked to one receptor while it can act as inhibitory when linked to some other receptor.

How does Neurotransmitters Work?

The spread of information from one to other neurons depends upon the ability of the information to pass through the microscopic gap, called the synapse, which can be located in between the terminal end of a neuron and the receptor end of another adjacent neuron. This transfer is accomplished by neurotransmitters. The cell generating the signals is called the presynaptic cell whereas the recipient cell is known as the postsynaptic cell.

Neurotransmitters are produced in the cell body of a neuron. From the cell body of the neuron, these neurotransmitters are sent to the terminal end of the neuron. Here these neurotransmitters are enclosed in vesicles (small membrane-bound bags). The neurotransmitters are left out from the terminal area when the vesicle membrane combines with the neuron membrane to respond to a potential signal. Then, the neurotransmitter chemical then spreads across the synapse.

After the release of neurotransmitter into the synapse, they chemically fuse with highly specific protein molecules (receptors), rooted in the surface membranes of the postsynaptic cell. A particular receptor may then be capable of linking to a neurotransmitter. The binding is selective and not all neurotransmitters can be linked to all receptors.

Whenever a particular receptor site distinguishes a neurotransmitter, this site becomes activated and results in depolarization / hyper-polarization. This generally acts straight on the affected neurons or activates another molecule (second messenger) which in due course changes the flow of information between the neurons.

“Depolarization” kindles the discharge of the neurotransmitter from the terminal end of the neuron while hyper-polarization creates difficulties in the release process. Whenever a neuron is found in resting state, neuron’s voltage is around -70 millivolts. The excitatory neurotransmitter changes the membrane of the postsynaptic neuron, which allows the ions to move across the neuron's membranes; increasing the neuron's voltage towards zero. If sufficient excitatory receptors have been activated, then the postsynaptic neuron fires, i.e., it generates a nerve impulse which triggers the neurotransmitters to be discharged into the next synapse. The Inhibitory neurotransmitter makes various ions to pass through the postsynaptic neuron's membrane that lesser the nerve cell’s voltage to -80 or -90 millivolts. The postsynaptic cell will not fire due to this significant drop in the nerve cell’s voltage.

In case the postsynaptic cell is found to be a muscle cell, then the excitatory neurotransmitter makes the muscle to contract. In case the postsynaptic cell is found to be a gland cell, then the excitatory neurotransmitter causes the cell to secrete its contents.

In case of degradation by a specific enzyme, the neurotransmitters can also be inactivated (for example: acetyl cholinesterase degrades acetylcholine). Cells called astrocytes can eliminate neurotransmitters from the receptor area. Eventually, some neurotransmitters (such as norepinephrine, dopamine, and serotonin) can be reabsorbed into the terminal region of the neuron.

Most of the neurotransmitters work together with their receptors initiating a new nerve impulse which energizes / inhibits the adjoining cell. While other neurotransmitters communications do not initiate / restrain nerve impulses. As an alternative, these neurotransmitters cooperate with another type of receptor which alters the internal chemistry of the postsynaptic cell. This alteration takes place by either causing / blocking the formation of chemicals known as second messenger molecules. The second messenger molecules control the postsynaptic cell's biochemical processes and facilitate it to carry out the maintenance essential to carry on synthesizing neurotransmitters and conducting nerve impulses.

The neurotransmitters have been pushed into the synapses and passed on their chemical signals. During this event, the presynaptic neuron clears the synapse of neurotransmitter molecules. As an example: the acetylcholine is crashed by acetyl cholinesterase enzyme to choline and acetate. Few neurotransmitters such as dopamine, gamma amino butyric acid and serotonin are removed by a physical process known as “Reuptake”. In “Reuptake” process, a protein in the presynaptic membrane takes the role of a sponge which causes the neurotransmitters to reenter the presynaptic neuron. In the presynaptic neuron, they can be crashed by enzymes / repackaged for reuse.

How Drugs Can Affect Synaptic Transmission

  1. Drugs can mimic specific neurotransmitters. Nicotine is chemically similar to acetylcholine and can occupy acetylcholine receptor sites, stimulating skeletal muscles and causing the heart to beat more rapidly.

  2. Drugs can mimic or block the effects of a neurotransmitter by fitting into receptor sites and preventing the neurotransmitter from acting. For example, the drug curare produces almost instant paralysis by blocking acetylcholine receptor sites on motor neurons.

  3. Drugs can affect the length of time the neurotransmitter remains in the synaptic gap, either increasing or decreasing the amount available to the postsynaptic receptor.

  4. Drugs can increase or decrease the amount of neurotransmitters released by neurons.

Acetylcholine

Nicotine: increases the release of acetylcholine

Curare: blocks the receptor sites of acetylcholine

Botulin: poisons found in improperly canned food, blocks the release of acetylcholine resulting in paralysis of the muscles

Nerve gas: continual release of acetylcholine

Scopolamine: blocks ACh receptors and impairs learning and even at low doses causes drowsiness, amnesia and confusion

Dopamine

L-dopa: converts into dopamine in the brain

Pheneothaizine: reduces dopamine in the brain

Amphetamines: Increases dopamine and norepinehrine, and to some extent serotonin and activates the sympathetic nervous system

Serotonin

Serotonin LSD: Impairs the reuptake of serotonin, making more serotonin available.

Prozac: Prevents the reuptake of serotonin, making more serotonin available

MDMA (ecstasy): Destroys serotonin nerve cells in animals with moderate and large doses.

Cocaine: Affects norepinephrine and serotonin, and prevents the reuptake of dopamine in the synapse and activate the sympathetic nervous system.

Endorphins

Opiates: Increases the production of endorphins

Naloxone: blocks endorphin receptor sites

Norepinephrine

Caffeine: Reduces the ability of the brain to produce adenosine, the “brakes” of the brain and CNS. Doses of 700 mg can contribute to panic attacks (200 mg is two strong cups of coffee, Mountain Dew is 54 mg).

Cocaine: Affects norepinephrine and serotonin, and prevents the reuptake of dopamine in the synapse and activate the sympathetic nervous system.

Amphetamines: Increases dopamine and norepinephrine, and to some extent serotonin and activates the sympathetic nervous system

GABA (gamma

aminobutyric acid)

Valium, Xanax, Depressants, GBH, easy lay and alcohol work by increasing GABA activities, which hold back the action possibility and slows brain activities.

Order now

Related essays