Stress Physiology: The Biological Basis of Stress Responses

Stress Physiology

The Nervous System and Stress

The nervous system plays a central role in the body's response to stress, coordinating rapid and long-term physiological changes. Two major divisions are involved: the autonomic nervous system and the endocrine system.

• Autonomic Nervous System (ANS): Controls involuntary functions by innervating organs, vessels, and glands.

• Parasympathetic Division: Promotes 'rest and digest' activities, restoring the body to a calm state after stress.

• Sympathetic Division: Activates the 'fight or flight' response during stress, increasing heart rate and mobilizing energy.

• Hypothalamus: Brain region that initiates the stress response by activating the sympathetic nervous system.

Fight or Flight Response: Mobilizes resources to prepare for immediate action. Key effects include:

• Increased heart rate

• Dilated bronchioles

• Decreased gut activity

Hormonal Regulation of Stress

Hormones are chemical messengers released by glands that travel through the bloodstream to regulate body functions. The stress response involves several key hormones and glands:

• Adrenal Glands: Located above the kidneys, these glands release stress hormones.

• Adrenal Medulla: The inner part of the adrenal gland; releases epinephrine (adrenaline) and norepinephrine during acute stress.

• Adrenal Cortex: The outer part of the adrenal gland; releases cortisol in response to longer-term stress.

• Pituitary Gland: The 'master gland' that releases hormones to stimulate other glands, including the adrenal cortex.

Two Major Stress Response Pathways

The body has two main physiological responses to stress, each with distinct mechanisms and timelines:

Pathway	Mechanism	Main Hormones	Effects
Sympathetic-Adrenal-Medullary (SAM) System	Immediate neural activation via the sympathetic nervous system	Epinephrine, Norepinephrine	Increased heart rate, blood flow, energy mobilization
Hypothalamic-Pituitary-Adrenal (HPA) Axis	Hormonal cascade: hypothalamus → pituitary → adrenal cortex	Cortisol	Increased glucose availability, suppressed immune function, longer-term adaptation

Pathway Details

• SAM System: Fast response to acute stress; prepares body for immediate action.

• HPA Axis: Slower, sustained response to chronic stress; regulates metabolism and immune function.

Example Applications

• Acute Stress (e.g., sudden danger): Sympathetic nervous system and adrenal medulla are activated, releasing epinephrine.

• Chronic Stress (e.g., ongoing exams, long-term threats): HPA axis is activated, leading to cortisol release from the adrenal cortex.

Key Terms and Definitions

• Sympathetic Nervous System: Division of the ANS that prepares the body for action.

• Parasympathetic Nervous System: Division of the ANS that calms the body and conserves energy.

• Hypothalamus: Brain region controlling the stress response and hormone release.

• Pituitary Gland: Releases hormones that regulate other endocrine glands.

• Adrenal Medulla: Releases epinephrine and norepinephrine for rapid stress response.

• Adrenal Cortex: Releases cortisol for prolonged stress adaptation.

• Cortisol: A steroid hormone that increases blood glucose and suppresses immune function during stress.

Sample Pathways (from examples)

• HPA Axis Activation: Hypothalamus → Pituitary → Adrenal Cortex → Cortisol release

• Sympathetic Activation: Hypothalamus → Sympathetic Nervous System → Adrenal Medulla → Epinephrine release

Relevant Equations

• Cortisol Release Pathway:

$\text{Hypothalamus} \xrightarrow{CRH} \text{Pituitary} \xrightarrow{ACTH} \text{Adrenal Cortex} \xrightarrow{} \text{Cortisol}$

• Epinephrine Release Pathway:

$\text{Hypothalamus} \xrightarrow{} \text{Sympathetic Nervous System} \xrightarrow{} \text{Adrenal Medulla} \xrightarrow{} \text{Epinephrine}$

Summary Table: Stress Response Systems

System	Activation	Main Hormone	Duration
SAM (Sympathetic-Adrenal-Medullary)	Immediate	Epinephrine	Short-term
HPA Axis	Delayed	Cortisol	Long-term

Example: If you are startled by a loud noise, your sympathetic nervous system and adrenal medulla are activated, releasing epinephrine. If you experience ongoing stress, such as repeated exams or chronic danger, your HPA axis is activated, releasing cortisol.

Additional info: Chronic activation of the HPA axis can lead to health problems such as immune suppression, increased risk of illness, and metabolic disturbances.