Autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands.[1] The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal.[2] This system is the primary mechanism in control of the fight-or-flight response.
Autonomic nervous system
The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. Autonomic functions include control of respiration, cardiac regulation (the cardiac control center), vasomotor activity (the vasomotor center), and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.[3]
Although conflicting reports about its subdivisions exist in the literature, the autonomic nervous system has historically been considered a purely motor system, and has been divided into three branches: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system.[4][5][6][7] Some textbooks do not include the enteric nervous system as part of this system.[8] The sympathetic nervous system is often considered the "fight or flight" system, while the parasympathetic nervous system is often considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.[3]
There are inhibitory and excitatory synapses between neurons. A third subsystem of neurons has been named as non-noradrenergic, non-cholinergic transmitters (because they use nitric oxide as a neurotransmitter) and are integral in autonomic function, in particular in the gut and the lungs.[9]
Although the ANS is also known as the visceral nervous system and although most of its fibers carry non-somatic information to the CNS, many authors still consider it only connected with the motor side.[10] Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.
History[edit]
The specialised system of the autonomic nervous system was recognised by Galen.
In 1665, Thomas Willis used the terminology, and in 1900, John Newport Langley used the term, defining the two divisions as the sympathetic and parasympathetic nervous systems.[20]
Caffeine effects[edit]
Caffeine is a bioactive ingredient found in commonly consumed beverages such as coffee, tea, and sodas. Short-term physiological effects of caffeine include increased blood pressure and sympathetic nerve outflow. Habitual consumption of caffeine may inhibit physiological short-term effects. Consumption of caffeinated espresso increases parasympathetic activity in habitual caffeine consumers; however, decaffeinated espresso inhibits parasympathetic activity in habitual caffeine consumers. It is possible that other bioactive ingredients in decaffeinated espresso may also contribute to the inhibition of parasympathetic activity in habitual caffeine consumers.[21]
Caffeine is capable of increasing work capacity while individuals perform strenuous tasks. In one study, caffeine provoked a greater maximum heart rate while a strenuous task was being performed compared to a placebo. This tendency is likely due to caffeine's ability to increase sympathetic nerve outflow. Furthermore, this study found that recovery after intense exercise was slower when caffeine was consumed prior to exercise. This finding is indicative of caffeine's tendency to inhibit parasympathetic activity in non-habitual consumers. The caffeine-stimulated increase in nerve activity is likely to evoke other physiological effects as the body attempts to maintain homeostasis.[22]
The effects of caffeine on parasympathetic activity may vary depending on the position of the individual when autonomic responses are measured. One study found that the seated position inhibited autonomic activity after caffeine consumption (75 mg); however, parasympathetic activity increased in the supine position. This finding may explain why some habitual caffeine consumers (75 mg or less) do not experience short-term effects of caffeine if their routine requires many hours in a seated position. It is important to note that the data supporting increased parasympathetic activity in the supine position was derived from an experiment involving participants between the ages of 25 and 30 who were considered healthy and sedentary. Caffeine may influence autonomic activity differently for individuals who are more active or elderly.[23]