Although the nervous system is often discussed in terms of peripheral and central components, it should be regarded as a highly integrated whole in which the central nervous system (brain and spinal cord) plays a critical information gathering and processing role. The peripheral nervous system is often divided into the autonomic and somatic components. The somatic system controls the voluntary functions of the body, like those of the skeletal muscles. The autonomic system, in contrast, is often referred to as the “involuntary” system. It regulates parts of the body where we execute little or no conscious control, such as the heart, intestines, vasculature, and other internal organs.
The autonomic nervous system is divided into the sympathetic and parasympathetic components, which typically exert opposing effects. The sympathetic system is involved in the “fight or flight” reaction (increased blood pressure and heart rate, and accommodation for increased vision, for example) that prepares the organism for stressful situations. The parasympathetic system conversely establishes a more relaxed situation, for instance, the rest period after a meal. The autonomic nervous system that is responsible for the independent control of the mechanical and secretory functions of the gastrointestinal tract is sometimes called the enteric system.
Drugs that affect the central nervous system may also have a major action in the gut. Thus, the constipating effects of opium alkaloids are exerted through this system and a number of the important withdrawal symptoms reflect the actions of the enteric nervous system. The nervous system is often regarded as a command (efferent) system that sends instructions to be executed. However, there is also a sensory (afferent) component, that receives information from innervated systems and that is vital to the overall integrated nervous response.
Despite the anatomical and functional differences between the various components of the nervous system, they share a fundamental similarity in their use of chemicals (neurotransmitters) to convey information.
The individual unit of the nervous system is the neuron, a specialized cell that both receives and transmits information.
The nervous system contains more than 100 billion neurons and is a major user of metabolic energy in the human body. It is also a region particularly susceptible to injury from toxic chemicals, lack of oxygen, and other assaults. Depending on the nervous region in which they reside, neurons may have different anatomical features and may use different chemical transmitters. Neurons communicate with each other and with their end organs by these chemical signals, which are released from the nerve terminal and interact with specific receptors on adjacent neurons or cells.
The chemical transmitters may be small molecules—notably acetylcholine, norepinephrine, epinephrine, serotonin, dopamine, or histamine. Acetylcholine and norpeinephrine are the dominant neurotransmitters in the parasympathetic and sympathetic nervous systems, respectively.
Dopamine and serotonin are employed primarily in the central nervous system. Neurotransmitters may also be more complex peptides (small proteins) such as substance P, vasopressin, endorphins, and enkephalins. The latter agents are of particular importance to our considerations of opium since they represent the “endogenous” opiates—agents that exist within the body whose actions are mimicked by exogenous, or outside, agents such as morphine, heroin, codeine, and so on. These neurotransmitters serve to convey information between neurons across the synaptic cleft (the junction where two neurons meet) or at the neuroeffector junction (the site between neuron and an innervated organ such as muscle or secretory gland).
Each neuron has specific synthetic machinery that enables it to both synthesize and eliminate a specific neurotransmitter.
For example, neurons of the sympathetic nervous system employ norepinephrine and epinephrine as their transmitters. Other neurons, particularly in the central nervous system, employ dopamine as their transmitter. Dopamine is a particularly important transmitter for a variety of neuronal functions. Its loss is associated with Parkinson disease, and it is a critical agent in the mediation of pleasure and reward processes. Dopamine, due to its association
with pleasurable sensations, is widely implicated in the actions of a number of drugs of abuse, including cocaine, opiates, and methamphetamines.
The autonomic nervous system is divided into the sympathetic and parasympathetic components, which typically exert opposing effects. The sympathetic system is involved in the “fight or flight” reaction (increased blood pressure and heart rate, and accommodation for increased vision, for example) that prepares the organism for stressful situations. The parasympathetic system conversely establishes a more relaxed situation, for instance, the rest period after a meal. The autonomic nervous system that is responsible for the independent control of the mechanical and secretory functions of the gastrointestinal tract is sometimes called the enteric system.
Drugs that affect the central nervous system may also have a major action in the gut. Thus, the constipating effects of opium alkaloids are exerted through this system and a number of the important withdrawal symptoms reflect the actions of the enteric nervous system. The nervous system is often regarded as a command (efferent) system that sends instructions to be executed. However, there is also a sensory (afferent) component, that receives information from innervated systems and that is vital to the overall integrated nervous response.
Despite the anatomical and functional differences between the various components of the nervous system, they share a fundamental similarity in their use of chemicals (neurotransmitters) to convey information.
The individual unit of the nervous system is the neuron, a specialized cell that both receives and transmits information.
The nervous system contains more than 100 billion neurons and is a major user of metabolic energy in the human body. It is also a region particularly susceptible to injury from toxic chemicals, lack of oxygen, and other assaults. Depending on the nervous region in which they reside, neurons may have different anatomical features and may use different chemical transmitters. Neurons communicate with each other and with their end organs by these chemical signals, which are released from the nerve terminal and interact with specific receptors on adjacent neurons or cells.
The chemical transmitters may be small molecules—notably acetylcholine, norepinephrine, epinephrine, serotonin, dopamine, or histamine. Acetylcholine and norpeinephrine are the dominant neurotransmitters in the parasympathetic and sympathetic nervous systems, respectively.
Dopamine and serotonin are employed primarily in the central nervous system. Neurotransmitters may also be more complex peptides (small proteins) such as substance P, vasopressin, endorphins, and enkephalins. The latter agents are of particular importance to our considerations of opium since they represent the “endogenous” opiates—agents that exist within the body whose actions are mimicked by exogenous, or outside, agents such as morphine, heroin, codeine, and so on. These neurotransmitters serve to convey information between neurons across the synaptic cleft (the junction where two neurons meet) or at the neuroeffector junction (the site between neuron and an innervated organ such as muscle or secretory gland).
Each neuron has specific synthetic machinery that enables it to both synthesize and eliminate a specific neurotransmitter.
For example, neurons of the sympathetic nervous system employ norepinephrine and epinephrine as their transmitters. Other neurons, particularly in the central nervous system, employ dopamine as their transmitter. Dopamine is a particularly important transmitter for a variety of neuronal functions. Its loss is associated with Parkinson disease, and it is a critical agent in the mediation of pleasure and reward processes. Dopamine, due to its association
with pleasurable sensations, is widely implicated in the actions of a number of drugs of abuse, including cocaine, opiates, and methamphetamines.
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