21.5
Hipotalamik-Hipofiz Ekseni
Strese tepki (fiziksel veya psikolojik, akut veya kronik olsun) Hipotalamik-Hipofiz-Adrenal (HPA) aksının aktivasyonunu içerir. HPA aksı nöroendokrin…
- [Anlatan] Strese tepkimiz, fiziksel veya psikolojik tepkimiz nöroendokrin sisteminin, hipotalamik-pituiter-adrenal veya HPA ekseninin aktivasyonuyla beyinde başlar. Hipotalamus (H) bir kortikotropin salgılama hormonu veya CRH salgılar, bu hormon da eksendeki pituiter bezi (P) aktifleştirir. Bu nöroendokrin bezin birçok fonksiyonu vardır, stresli zamanlarda kana adrenal kortikotropik hormon veya ACTH salgılar.
ACTH adrenal kortekslerini (A), böbreklerin yüzeyindeki küçük yapıları uyarır. Adrenaller 2 tip hormon salgılar: İlk hormon tipi norepinefrin ve epinefrindir, bunlar sempatik sinir sistemini çalıştırarak ve nabız ve kan akışı artışına yol açarak bedeni harekete geçmesi için canlandırmaktan sorumlu iki sempatomimetik bileşiktir. İkincisi glukokortikoidlerdir, bilhassa kortizoldür.
Bunlar vücutta, artan glukogenezle enerji kaynaklarını seferber ederek ve bağışıklık sistemi gibi hayati olmayan prosesleri baskılayarak stres reaksiyonlarını sürdürmek üzere tasarlanmış olan sistemleri aktifleştirirler. Kortizol düzeyleri yükselince, pituiter ve hipotalamusa negatif geri bildirim döngüsü başlatılır. Bu reaksiyon ACTH ve CRH salgılanmasını durdurur ve stres tepkisini sonlandırır.
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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.