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A concluding look at how the nervous and endocrine systems work together during acute and chronic stress.
Why does your heart race instantly when you almost trip, yet you feel a different kind of 'burnout' after a week of final exams? Your body uses two completely different biological systems to handle these two types of pressure.
When you encounter an immediate threat, your brain doesn't have time to wait for hormones to travel through the blood. Instead, the Hypothalamus activates the Sympathetic Nervous System directly. This is known as the Sympatho-adreno-medullary (SAM) pathway. The hypothalamus sends nerve impulses down the spinal cord to the adrenal medulla, which flash-releases epinephrine (adrenaline) and norepinephrine. These chemicals act as both neurotransmitters and hormones, causing an instant increase in heart rate, dilation of pupils, and the diversion of blood toward skeletal muscles. This is an acute response designed for immediate survival, lasting only minutes before the body attempts to return to a resting state.
Consider a scenario where a car suddenly swerves into your lane. 1. Your visual cortex perceives the threat. 2. The Hypothalamus triggers the SAM pathway via neural impulses. 3. The Adrenal Medulla releases epinephrine. 4. Result: Your heart rate jumps from to in less than two seconds.
Quick Check
Which specific part of the adrenal gland is responsible for the immediate 'fight-or-flight' release of epinephrine?
Answer
The adrenal medulla.
If the stressor persists (like a high-pressure work week), the body shifts to the Hypothalamic-pituitary-adrenal (HPA) axis. This is a hormonal cascade rather than a pure neural impulse. The hypothalamus releases Corticotropin-Releasing Hormone (CRH), which signals the anterior pituitary to release Adrenocorticotropic Hormone (ACTH). ACTH travels through the blood to the adrenal cortex, stimulating the release of glucocorticoids, primarily cortisol. Unlike epinephrine, cortisol works to maintain high blood glucose levels by breaking down fats and proteins (gluconeogenesis). This provides the energy needed to 'resist' the stressor over a longer period, but it comes at a metabolic cost.
The HPA axis follows a specific sequence of activation: 1. 2. 3. This process is regulated by a negative feedback loop where high cortisol levels eventually inhibit the release of CRH and ACTH to prevent over-activation.
Quick Check
How does the delivery method of the HPA axis differ from the SAM pathway?
Answer
The HPA axis relies on hormonal transport through the bloodstream (slower), while the SAM pathway relies on direct neural impulses (faster).
While cortisol is helpful in the short term, chronic stress leads to the 'Exhaustion Phase.' Prolonged high levels of cortisol have a significant immunosuppressive effect. Cortisol inhibits the production of interleukins and reduces the activity of lymphocytes (white blood cells), making the body more susceptible to infection. Furthermore, constant high blood pressure and blood glucose can lead to allostatic load—the wear and tear on the body that results in hypertension, muscle wasting, and even structural changes in the brain's hippocampus. At this stage, the body can no longer maintain homeostasis, and organ systems begin to fail.
Imagine a student experiencing six months of severe sleep deprivation and academic pressure. 1. Persistent HPA activation keeps cortisol levels elevated: . 2. Elevated cortisol suppresses T-cell production: . 3. The student becomes sick more frequently and develops insulin resistance due to constant gluconeogenesis. 4. The negative feedback loop fails as receptors in the hypothalamus become desensitized.
Which hormone is primarily responsible for the 'Resistance' phase of the General Adaptation Syndrome?
What is the correct order of the HPA axis cascade?
Chronic stress strengthens the immune system by keeping white blood cells on high alert.
Review Tomorrow
In 24 hours, try to sketch the HPA axis from memory and list three physiological effects of cortisol versus three effects of epinephrine.
Practice Activity
Research the 'General Adaptation Syndrome' (GAS) and identify which of the three stages (Alarm, Resistance, Exhaustion) corresponds to the SAM pathway and which to the HPA axis.