Neurotransmitter Acid example essay topic

Neurotransmitters Neurotransmitters are chemicals made by neurons and used by them to transmit signals to the other neurons or non-neuronal cells (e. g., skeletal muscle; myocardium, pineal glandular cells) that they innervate. The neurotransmitters produce their effects by being released into synapses when their neuron of origin fires (i. e., becomes depolarized) and then attaching to receptors in the membrane of the post-synaptic cells. This causes changes in the fluxes of particular ions across that membrane, making cells more likely to become depolarized, if the neurotransmitter happens to be, or less likely if it is inhibitory. Neurotransmitters can also produce their effects by modulating the production of other signal- molecules ('second messengers " messengers') in the post-synaptic cells (Cooper, Bloom and Roth 1996). Nine compounds -- belonging to three chemical families -- are generally believed to function as neurotransmitters somewhere in the central nervous system (CNS) or periphery. In addition, certain other body chemicals, for example adenosine, histamine, , endorphins, and epinephrine, have neurotransmitter-like properties, and many additional true neurotransmitters may await discovery.

The first of these families, and the group about which most is known, is the amine neurotransmitters, a group of compounds containing a nitrogen molecule which is not part of a ring structure. Among the amine neurotransmitters are acetylcholine, norepinephrine, dopamine, and serotonin. Acetylcholine is possibly the most widely used neurotransmitter in the body, and all axons that leave the central nervous system (for example, those running to skeletal muscle, or to sympathetic or parasympathetic ganglia) use acetylcholine as their neurotransmitter. Within the brain acetylcholine is the transmitter of, among other neurons, those generating the tracts that run from the septum to the HIPPOCAMPUS, and from the nucleus basal is to the CEREBRAL CORTEX -- both of to the CEREBRAL CORTEX -- both of which seem to be needed to sustain memory and learning. It is also the neurotransmitter released by short-axon inter neurons of the BASAL GANGLIA. Norepinephrine is the neurotransmitter released by sympathetic nerves (e. g., those innervating the heart and blood vessels) and, within the brain, those of the locus, a nucleus activated in the process of focussing attention.

Dopamine and Serotonin apparently are neurotransmitters only within the CNS. Some (i. e., dopamine-releasing) neurons run from the substantial nigra to the corpus stratum; their loss gives rise to the clinical manifestations of Parkinson's Disease (Korczyn 1994); others, involved in the rewarding effects of drugs and natural stimuli, run from the mesencephalon to the accumbent. The second neurotransmitter family includes amino acids, compounds that contain both an amino group (NH 2) and a acid group (COOH) and which are also the building blocks of peptides and proteins. The amino acids known to serve as neurotransmitters are glycine, and acids, all present in all proteins, and gamma-amino butyrin acid (GABA), produced only in brain neurons.

Glutamic acid and GABA are the most abundant neurotransmitters within the central nervous system, particularly in the cerebral cortex; acid tends to be and GABA inhibitory. Aspartic acid and glycine subserve these functions in the spinal cord (Cooper, Bloom, and Roth 1996). The third neurotransmitter family is composed of peptides, compounds that contain at least two and sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly understood: Evidence that they are, in fact, transmitters tends to be incomplete, and restricted to their location within nerve terminals, and the physiologic effects produced when they are applied to neurons.

Probably the best understood peptide neurotransmitter is substance P, a compound that transmits signals generated by pain. In general each neuron uses only a single compound as its neurotransmitter. However some neurons contain both an amine and a peptide, and may release both into synapses. Moreover, many neurons release adenosine, an inhibitory compound, along with their 'true' transmitter, for instance, norepinephrine or acetylcholine.

The stimulant effect of caffeine results from its ability to block receptors for this adenosine. Neurotransmitters are manufactured from circulating precursor compounds like amino acids, glucose, and the dietary amine choline. Neurons modify the structure of these precursor compounds through a series of enzymatic reactions that often are limited not by the amount of enzyme present but by the concentration of the precursor -- which can change, for example, as a consequence of eating (Wurtman 1988). Neurotransmitters that come from amino acids include serotonin, which is derived from tryptophan; dopamine and norepinephrine, which are derived from tyrosine; and glycine, which is derived from threonine.

Among the neurotransmitters made from glucose are glutamate, aspartame, and GABA. Choline serves as a the precursor for acetylcholine. Once released into the synapse, each neurotransmitter combines chemically with one or more highly specific receptors; these are protein molecules which are imbedded in the post-synaptic membrane. As noted above, this interaction can affect the electrical properties of the post-synaptic cell, its chemical properties, or both. When a NEURON is in its resting state, it sustains a voltage of about -70 millivolts as the consequence of differences between the concentrations of certain ions at the internal and external sides of its bounding membrane. Excitatory neurotransmitters either open protein-lined channels in this membrane, allowing extracellular ions, like sodium, to move into the cell, or close channels for potassium.

This raises the neuron's voltage. This raises the neuron's voltage towards zero, and makes it more likely that -- if enough such receptors are occupied -- the cell will become depolarized. If the post synaptic cell happens also to be a neuron (i. e., as opposed to a muscle cell), this depolarization will cause it to release its own neurotransmitter from its terminals. Inhibitory neurotransmitters like GABA activate receptors that cause other ions -- usually chloride -- to pass through the membrane; this usually hyper polarizes the post synaptic cell, and decreases the likelihood that it will become depolarized. (The neurotransmitter acid, acting via its NMDA receptor, can also open channels for calcium ions. Some investigators believe that excessive activation of these receptors in neurological diseases can cause toxic of calcium to enter the cells, and kill them.) If the post synaptic cell is a muscle cell rather than a neuron, an neurotransmitter will cause the muscle to contract.

If the post synaptic cell is a glandular cell, an neurotransmitter will cause the cell to secrete its contents. While most neurotransmitters interact with their receptors to change the voltage of post-synaptic cells, some neurotransmitter interactions, involving a different type of receptor, modify the chemical composition of the post synaptic cell by either causing or blocking the formation of 'second messenger' molecules. These second messengers regulate many of the post synaptic cell's biochemical processes, including gene expression; they generally produce their effects by activating enzymes that add high-energy phosphate groups to specific cellular proteins. Examples of second messengers formed within the post synaptic cell include cyclic adenosine mono phosphate, , and inositol phosphates. Once neurotransmitters have been secreted into synapses and been secreted into synapses and have acted on their receptors, they are cleared from the synapse either by enzymatic breakdown -- for example acetylcholine, which is converted by the enzyme acetyl cholinesterase to choline and acetate, neither of which has neurotransmitter activity -- or, for neurotransmitters like dopamine, serotonin and GABA, a physical process called re uptake. In re uptake, a protein in the pre synaptic membrane acts as a sort of sponge, causing the neurotransmitter molecules to reenter their neuron of origin, where they can be broken down by other enzymes (for example mono amine oxidase, in, , or nor adrenergic neurons) or repackaged for reuse.

As indicated above, particular neurotransmitters are now known to be involved in many neurological and behavioral disorders. For example, in Alzheimer's disease, whose victims exhibit loss of intellectual capacity (particularly short-term memory), disintegration of personality, mental confusion, hallucinations, and aggressive -- even violent -- behaviors, many families of neurons, utilizing many neurotransmitters, die (Wurtman et al. 1996). However, the most heavily damaged family seems to be the long-axon acetylcholine-releasing neurons, originating in the septum and the nucleus basal is, which innervate the and cerebral cortices. Acetylcholinesterase inhibitors which increase brain levels of acetylcholine can improve short-term memory, albeit it can improve short-term memory, albeit transiently, in some Alzheimer's disease patients.

Most drugs -- therapeutic or recreational -- that affect brain and behavior do so by acting at synapses to affect the production, release, effects on receptors, or inactivation of neurotransmitter molecules (Bernstein 1988). Such drugs can also constitute important and specific probes for understanding cognition and other brain functions. Neurotransmitters are the chemicals which account for the transmission of signals from one neuron to the next across synapses. They are also found at the axon endings of motor neurons, where they stimulate the muscle fibers to contract. And they and their close relatives are produced by some glands such as the pituitary and the adrenal glands. In this chapter, we will review some of the most significant neurotransmitters.

Acetylcholine was the first neurotransmitter to be discovered. It was isolated in 1921 by a German biologist named Otto Loewi, who would later win the Nobel Prize for his work. Acetylcholine has many functions: It is responsible for much of the stimulation of muscles, including the muscles of the gastro-intestinal system. It is also found in sensory neurons and in the autonomic nervous system, and has a part in scheduling REM (dream) sleep. The well-known poison botulin works by blocking acetylcholine, causing paralysis. The botulin derivative botox is used by many people to temporarily eliminate wrinkles -- a sad commentary on our times, I would say.

On a more serious note, there is a link between acetylcholine and Alzheimer's disease: There is something on the order of a 90% loss of acetylcholine in the brains of people suffering from that debilitating disease.

Bibliography

Bio psychology, pine l 2002.