18.2
副交感神経の働き
概要
副交感神経は、自律神経系の2大部門のうちの1つです。この副交感神経系は、心拍数や消化など、多くの無意識の機能を調節する役割を担っています。この副交感神経系は、脳と末梢神経系の両方に存在するニューロンで構成されており、その軸索を対象となる筋肉、器官、腺に送ります。
休息と消化のシステム
副交感神…
副交感神経は 脅威がなく落ち着いた状態の時に、体を回復させるもので 休息・消化システムとも呼ばれています。これは自律神経系の主要な部分であり 内臓器官の平滑筋や 心臓の心筋、または唾液腺など 特定の腺の活性を制御することで 不随意機能をコントロールします。副交感神経系が活性化すると 脳幹と脊髄仙骨部の節前神経からのシグナルが 神経節に送られ 器官または腺の近くの末梢神経系の 神経細胞体の塊が制御されます。このように、節後神経にメッセージが送られますが 通常、神経伝達物質であるアセチルコリンが使用され 標的組織の活性化を制御します。例えば、心拍数を下げたり 体をリラックスさせたり また、腸や膀胱の筋収縮を刺激して 消化や排泄を促します。その結果として、副交感神経系は体が休息している時に エネルギー貯蔵を回復させて、ホメオスタシスを維持します。
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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.