21.5
Eixo Hipotálamo-Pituitária
A resposta ao stress—seja física ou psicológica, aguda ou crónica—envolve a ativação do eixo Hipotalâmico-Pituitário-Adrenal (HPA). O eixo HPA faz par…
Nossa resposta ao estresse, físico ou psicológico, começa no cérebro com a ativação do sistema neuroendócrino, o eixo hipotálamo-pituitária-adrenal, ou HPA. O hipotálamo, o H, libera um hormônio liberador de corticotrofina, ou CRH, que ativa a glândula hipófise, ou pituitária, o P no eixo. Enquanto esta glândula neuroendócrina tem muitas funções, durante tempos de estresse, ela libera o hormônio adrenocorticotrópico, ou ACTH, no sangue.
O ACTH estimula os córtices adrenais, o A, pequenas estruturas na superfície dos rins. As adrenais liberam dois tipos de hormônios. Em primeiro lugar estão a norepinefrina e a epinefrina, dois compostos simpaticomiméticos responsáveis por despertar o corpo para a ação iniciando o sistema nervoso simpático e causando o aumento da frequência cardíaca e do fluxo sanguíneo.
Em segundo lugar estão os glicocorticoides, especificamente o cortisol. Eles ativam sistemas no corpo projetados para sustentar reações de estresse mobilizando reservas de energia via aumento da glicogénese e suprimindo processos não-vitais como o sistema imunológico. Quando os níveis de cortisol ficam altos, um ciclo de feedback negativo tanto na pituitária quanto no hipotálamo é iniciado.
Esta reação impede a liberação de ACTH e CRH, pondo fim à resposta ao estresse.
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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.