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The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. [1] [2] The autonomic nervous system is responsible for regulating the body's unconscious actions.
The somatic nervous system directs all voluntary movements of the skeletal muscles, and can be sub-divided into afferent and efferent neuronal flow. The autonomic nervous system is divided primarily into the sympathetic and parasympathetic nervous systems with a third system, the enteric nervous system, receiving less recognition. [2]
A parasympathomimetic drug, sometimes called a cholinomimetic drug [1] or cholinergic receptor stimulating agent, [2] is a substance that stimulates the parasympathetic nervous system (PSNS). [3] [2] These chemicals are also called cholinergic drugs because acetylcholine (ACh) is the neurotransmitter used by the PSNS.
Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system. Most are small terminal ganglia or intramural ganglia, so named because they lie near or within (respectively) the organs they innervate. The exceptions are the four paired parasympathetic ganglia of the head and neck.
The autonomic nervous system is a component of the peripheral nervous system and comprises two branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). The SNS controls the more active responses, such as increasing heart rate and blood pressure.
Because ganglionic blockers block both the parasympathetic nervous system and sympathetic nervous system, the effect of these drugs depends upon the dominant tone in the organ system. [2] The opposite of a ganglionic blocker is referred to as a ganglionic stimulant. Some substances can exhibit both stimulating and blocking effects on autonomic ...
Increased parasympathetic outflow prominently affects the heart, reducing the heart rate or, in extreme cases, completely halting heart beats. This is a common cause of fainting. [3] The parasympathetic nervous system is dominant in situations of rest and relaxation, it has an activating effect on digestive organs and a relieving one on the heart.
The reflex, controlled by the parasympathetic nervous system, involves three responses: pupil constriction, lens accommodation, and convergence. A near object (for example, a computer screen) subtends a large area in the visual field, i.e. the eyes receive light from wide angles. When moving focus from a distant to a near object, the eyes converge.