Depressants and Stimulants of the Nervous Central System
The central nervous system (CNS), (i.e., the spinal cord, and the cortical and subcortical areas of the brain), processes different kinds of sensory information through highly organized neuronal structures that function as specific information-processing areas in the brain in a modular fashion. For instance, the visual, olfactory, motion, and somatic areas are well-defined functional structures, specialized in processing specific types of sensory stimuli, whereas the association areas are able to deal with multiple pre-analyzed sensations, which are received from these other areas through a close interchange with the limbic system. In this way, memory and association areas match the inputting information with stored responses, mainly through the interplay of impulses between the cortical and the limbic system. Therefore, the CNS is a complex system of interlinked structures that requires intercommunication among distant neurons (i.e., nervous cells) and peripheral nerves, in order to integrate and regulate the activity of several modular cerebral structures.
Information is transmitted from one neuron to the next under the form of nerve impulses known as synapses. There are two major types of synapses: the electrical synapses and the chemical synapses. Electrical synapses usually occur through the opening of tubular protein structures known as gap junctions in the cells that conduct electricity (ionic solutions) from one neuron to the next. However, these structures are more common in nerves connected to visceral smooth muscles, and are not the main form of signal transmission in the CNS. The prevailing form of signal transmission in the CNS are chemical synapses, through the mediation of chemical substances that occupy specific receptors in the nerve cells, known as neurotransmitters. Some neurotransmitters are hormones originated from different glands, such as the adrenal, pituitary, pineal, and the liver. Other neurotransmitters, such as serotonin and dopamine, or yet, amino acids, such as leucine and methionine enkephalins as well as other neuropeptides, are synthesized by the neurons. Among the more than forty substances presently identified as neurotransmitters, the best understood are acetylcholine, histamine, GABA (gamma-aminobutyric acid), glycine, glutamate, serotonin, norepinephrine, and dopamine. These neurochemical substances occur in three different broad types. The first one comprehends those transmitter substances that trigger synaptic transmission, such as serotonin, dopamine, and norepinephrine. The second type are the families of regulatory substances that modulate the action of the first ones, either by competing with a specific neurotransmitter for the same receptors or yet, by binding to the neurotransmitter before it reaches the receptor (inhibitory action), or by facilitating the synthesis of a given neurotransmitter (precursors). Some substances also prevent the decay of a given neurotransmitter, making them available to be reused again in the brain. The third type of neurochemical substances, known as secondary or second messengers, bind to a given primary neurotransmitter to either enhance its action or allow for its activation. Different synaptic pathways seem to be regulated by some specific neurotransmitter systems, such as those that are primarily activated by dopamine and its secondary messengers, whereas some other pathways may be independently activated by several substances, such as glutamate, norepinephrine, dopamine, and serotonin.
Behavioral responses and personality traits are the result of and are affected by the overall activity of neurotransmitters and hormonal interactions. Such interactions are accountable for the following effects upon behavioral response: long-term neurotransmitter baselines that account for differential behavioral-response patterns, changes in a baseline of neurotransmitter activity in response to external stimuli (i.e., social or environmental changes), daily behavioral and mood cycles, such as the circadian rhythm, or monthly cycles such as those involved in mood changes due to hormonal fluctuation, and short-term changes in response to specific stimuli, such as danger, pleasure, or pain.
Some neurotransmitters may have both an excitatory and inhibitory effect in different parts of the nervous system. Acetylcholine, for instance, is synthesized mainly by the large pyramidal cells of the motor cortex, although it is also produced in many other brain areas, as well as by the motor neurons that innervate skeletal muscle, among others. Although acetylcholine usually has an excitatory effect on the nervous system, in the peripheral parasympathetic nerve endings it shows an inhibitory action, such as the inhibition of the heart by the vagus nerve. Norepinephrine helps to control overall neural activity and mood such as increased wakefulness. However, in some areas of the brain norepinephrine also inhibits receptors, preventing their activation by other transmitter substances. Its action in the sympathetic nervous system activates some organs and inhibits others as well. Serotonin and norepinephrine, two excitatory transmitters of the prefrontal lobes, are directly involved with the regulation of mood and the sense of well-being, appropriate sex drive, contentment, and psychomotor balance. Low levels of either or both these substances are related to depression, loss of appetite, grief, unhappiness, and episodes of suicidal despair. Drugs that inhibit the production of serotonin and norepinephrine in the brain, such as reserpine and lithium compounds can induce depression. On the other hand, compounds that either prevent the destruction of these two transmitters or prevent their reuptake by the nerve endings usually are effective in treating persons with depression. However, an excess of these two neurotransmitters also may cause psychological imbalance, such as increased excitability or even psychotic behavior, due to their excitatory effects. Dopamine has an inhibitory effect on several portions of the prefrontal lobes and other related areas, such as medial and anterior areas of the limbic systems, which are related with the behavioral control centers. Stressful and dangerous situations increase dopamine and adrenaline levels, triggering the fight-or-flight responses. Excess of dopamine production is also associated with schizophrenia and paranoia, and can be induced by drugs that augment dopamine levels, such as L-dopa, used to treat Parkinson's disease.
Neuropeptides usually have a slow but much longer action in the nervous system. Moreover, they are a thousand or more times more potent than the smaller neurotransmitter molecules, such as serotonin, dopamine, epinephrine, glutamate, and norepinephrine. They induce prolonged changes in activation and deactivation of specific genes in the nerve cells as well as prolonged changes in the amount of excitatory and inhibitory receptors in neurons, used by other neurotransmitters, which last for days or even months. Examples of slow-action neuropeptides are: leucine and methionine enkephalins, nerve growth factor, neurotensin, and several pituitary, hypothalamic, and other hormones.
Some biochemical substances found in diet have an excitatory effect in the nervous system, such as caffeine (from coffee), theobromine (from tea), and theophylline (from cocoa). Conversely, anesthetic drugs, painkillers, and muscle relaxants do decrease synaptic transmission in many points of the nervous system, depending on the molecular characteristic of each drug.
This is the complete article, containing 1,090 words
(approx. 4 pages at 300 words per page).
