18.2
Le système nerveux parasympathique
Aperçu
Le système nerveux parasympathique est l’une des deux principales divisions du système nerveux autonome. Ce système parasympathique est respons…
- Le système nerveux parasympathiquerétablit le corps dans les moments de calme,lorsqu'il n'y a aucune menace. On l'appelle souvent "le système digestion et repos". C'est l'une des principales divisionsdu système nerveux autonomequi contrôle les fonctions involontairesen régulant l'activité du muscle lissedans les organes viscéraux, le muscle cardiaque dans le cœuret certaines glandes, comme les glandes salivaires.
Lorsque le système nerveux parasympathique est activé,des signaux des neurones pré-ganglioniques,dans le tronc cérébralet la partie sacrale de la moelle épinière,sont envoyés aux ganglions,groupements de corps de cellules nerveusesdans le système nerveux périphérique, près des organesou des glandes contrôlées. Là, le message est transmis aux neurones post-ganglioniquesqui utilisent généralement le neurotransmetteur acétycholinepour réguler l'activité des tissus ciblés. Par exemple, ils abaissent le rythme cardiaque,détendant le corps, et stimulent la contraction musculairedans les intestins et la vessie,favorisant la digestion et l'excrétion.
Résultat : le système parasympathiqueaide à reconstruire les stocks d'énergieet à maintenir l'homéostasie quand le corps peut se reposer.
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Q1: What is the parasympathetic nervous system and what does it do?
The parasympathetic nervous system is a major division of the autonomic nervous system that restores the body during calm times when there are no threats. Often called the rest and digest system, it regulates involuntary functions by controlling smooth muscle in visceral organs, cardiac muscle in the heart, and certain glands. It helps rebuild energy stores and maintain homeostasis when the body can safely rest.
Q2: How does the parasympathetic nervous system transmit signals to target organs?
Preganglionic neurons in the brainstem and sacral spinal cord send signals to ganglia, clusters of neuronal cell bodies in the peripheral nervous system located near target organs. Here, messages transfer to postganglionic neurons, which typically use acetylcholine to regulate target tissue activity. This pathway allows precise control of heart rate, digestion, and other involuntary functions.
Q3: What specific effects does parasympathetic activation have on the body?
Parasympathetic activation lowers heart rate and constricts pupils, promoting relaxation. It stimulates digestion by increasing salivation and peristaltic contractions in the stomach and intestines, and promotes bladder contraction for urination. It also increases pancreatic insulin secretion to rebuild energy stores and enhances blood flow to reproductive organs.
Q4: How does the parasympathetic nervous system differ from the sympathetic nervous system?
While the parasympathetic nervous system promotes rest, digestion, and energy restoration during calm times, the sympathetic nervous system activates during threats or stress. The parasympathetic system lowers heart rate and stimulates digestion, whereas the sympathetic nervous system increases heart rate and inhibits digestion. Together, these opposing systems maintain homeostasis by responding to different body states.
Q5: What role does acetylcholine play in parasympathetic signaling?
Acetylcholine is the primary neurotransmitter used by postganglionic parasympathetic neurons to regulate target tissue activity. Released at the synapse and neurotransmitters junction, acetylcholine binds to receptors on smooth muscle, cardiac muscle, and glands to produce parasympathetic effects like lowering heart rate and stimulating digestion.
Q6: Why is proper parasympathetic signaling important for standing up?
The parasympathetic nervous system controls vasodilation of cerebral arteries, allowing blood vessels to relax and maintain adequate blood flow to the brain during postural changes. Improper parasympathetic signaling can cause orthostatic intolerance, leading to lightheadedness or fainting when standing. This demonstrates how parasympathetic control is essential even for routine movements.
Q7: How does the hypothalamus control parasympathetic nervous system activation?
The hypothalamus activates the parasympathetic nervous system by sending signals that trigger preganglionic neurons in the brainstem and sacral spinal cord. These neurons then relay messages through ganglia to postganglionic neurons, which innervate target organs and glands. This hierarchical control allows the brain to coordinate parasympathetic responses that promote rest and digestion during appropriate times.