21.5
視床下部-下垂体軸
肉体的にも心理的にも、また急性であれ慢性であれ、ストレスに対する反応は、視床下部-下垂体-副腎（HPA）軸の活性化を伴います。視床下部-下垂体-副腎（HPA）軸は、神経とホルモンの両方の伝達を伴うため、神経内分泌系に属します。HPA軸の機能は、代謝、心血管、免疫などの恒常的なシステムを調節することで…
肉体的または心理的な ストレスに対する反応は 脳から始まり 神経内分泌系 視床下部 下垂体 副腎(HPA)軸の 活性化が伴います 視床下部(H)は 副腎皮質刺激ホルモン放出ホルモン(CRH)を放出し 下垂体(P)軸を活性化します この神経内分泌腺には多くの機能があり ストレスを受けると血中に 副腎皮質刺激ホルモン(ACTH)を放出します ACTHは腎臓の表面にある 小さな構造の 副腎皮質(A)を刺激します 副腎は2種類のホルモンを放出します 1つはノルエピネフリンとエピネフリンで 交感神経系を作動させて 心拍数と血流を増加させることにより 身体を興奮させる 2つの交感神経刺激化合物です もう１つはグルココルチコイドで 具体的にはコルチゾールです 増加した糖生成を通して エネルギー貯蔵を動かし 免疫系などの 重要でないプロセスを抑制することにより ストレス反応を持続するように設計された 体内のシステムを活性化します コルチゾール濃度が高くなると 下垂体と視床下部の両方への 負のフィードバックループが始まります この反応により ACTHとCRHの放出が止まり ストレス反応を終わらせます
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Q1: What is the hypothalamic-pituitary-adrenal axis and why does it activate during stress?
The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine system that regulates the body's response to stress by coordinating neuronal and hormonal communication. During stress, the hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH then signals the adrenal glands to release stress hormones that prepare the body for action and mobilize energy reserves.
Q2: What hormones does the adrenal gland release during the stress response?
The adrenal gland releases two types of stress hormones. The adrenal medulla releases epinephrine and norepinephrine, which activate the sympathetic nervous system, increasing heart rate and blood flow. The adrenal cortex releases glucocorticoids, primarily cortisol, which mobilizes glucose reserves and temporarily suppresses non-vital processes like immune function to sustain the stress response.
Q3: How do epinephrine and norepinephrine affect the body during stress?
Epinephrine and norepinephrine are sympathomimetic compounds that activate the sympathetic nervous system, causing increased heart rate, blood flow, and respiration. These hormones prepare the body for immediate action by promoting states of alertness and arousal, enabling rapid physical responses to stressors.
Q4: What role does cortisol play in the stress response?
Cortisol, the primary glucocorticoid in humans, mobilizes energy by promoting glucose production through the breakdown of fatty acids and proteins. It also temporarily inhibits immune and inflammatory responses to prioritize resources for stress management. When cortisol levels rise sufficiently, it triggers a negative feedback loop that deactivates the HPA axis and terminates the stress response.
Q5: How does negative feedback terminate the stress response?
When blood cortisol levels reach a certain threshold, they trigger a negative feedback loop that inhibits both the hypothalamus and pituitary gland. This stops the release of CRH and ACTH, effectively deactivating the HPA axis and ending the stress response, allowing the body to return to homeostasis.
Q6: What are the long-term effects of chronic HPA axis activation?
Chronic stress or repeated acute stress can lead to continuous HPA axis activation, eventually depleting cortisol reserves. This depletion disrupts cortisol's multiple functions during both stress and non-stress periods, potentially causing chronic pain, depression, infertility, and may contribute to substance abuse and addiction.
Q7: How does the HPA axis maintain homeostasis during stress?
The HPA axis regulates metabolic, cardiovascular, and immune systems to maintain homeostasis during stress. Epinephrine and norepinephrine increase cardiovascular activity for immediate response, while cortisol mobilizes energy reserves and temporarily suppresses immune function. This coordinated hormonal response enables the body to sustain stress reactions while preserving critical functions.