21.5
Axe hypothalamique-hypophysaire
La réponse au stress — qu’il soit physique ou psychologique, aigu ou chronique — implique l’activation de l’axe hypothalamo-hypophyso-surrénalien (HHS…
Notre réaction au stress,physique ou psychologique, commence dans le cerveauavec l'activation du système neuroendocrinien,l'axe hypothalamo-hypophyso-surrénalien, ou axe HPA. L'hypothalamus, le H, libère une corticotrophinehormone de libération, ou CRH, qui activel'hypophyse ou glande pituitaire, le P, dans l'axe. Bien que cette glande neuroendocrineait de nombreuses fonctions,pendant les périodes de stress, elle libère les hormonescorticotropes surrénales, ou ACTH, dans le sang.
L'ACTH stimule les corticosurrénales, le A,de petites structures à la surface des reins. Les surrénales libèrent deux types d'hormones. Il y a d'abord la norépinéphrine et l'épinéphrine,deux composés sympathomimétiques responsablespour exciter le corps à l'actionen initiant le système nerveux sympathiqueet provoquant une augmentation de la fréquence cardiaqueet de la circulation sanguine.
Deuxièmement, les glucocorticoïdes,en particulier le cortisol. Ils activent les systèmes de l'organismeconçus pour soutenir les réactions de stressen mobilisant les réserves d'énergiepar le biais d'une augmentation de la glucogenèseet en supprimant des processus non vitauxcomme le système immunitaire. Quand le taux de cortisol est élevé,une boucle de rétroaction négative surl'hypophyse et l'hypothalamus est initiée.
Cette réaction arrête la libération de l'ACTH et de la CRH,ce qui met fin à la réaction de stress.
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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.