Hello everyone! As my blog uncovers the various branches of biology, I've decided to explore a rather underappreciated and forgotten branch of biology: Neurobiology.
The fight-or-flight response is the body’s acute stress response, a survival mechanism that has evolved to protect humans from immediate threats. This complex process integrates the nervous, endocrine, and cardiovascular systems and is mediated by intricate chemical signaling. Let’s explore this physiological phenomenon in detail.
The first step of this response is the body sensing a threat. When a potential danger is perceived, the sensory organs (eyes, ears, etc.) send signals to the brain. The amygdala assesses the significance of the threat. If the amygdala determines the situation to be dangerous, it activates the hypothalamus, the control center for autonomic and endocrine responses.
Then, the sympathetic nervous system is activated. The hypothalamus stimulates the sympathetic nervous system (SNS), a branch of the autonomic nervous system responsible for immediate, involuntary responses. The SNS acts through:
Preganglionic neurons release the neurotransmitter acetylcholine (ACh) to activate postganglionic neurons.
Postganglionic neurons release norepinephrine (NE), a key chemical messenger in the fight-or-flight response.
Norepinephrine binds to adrenergic receptors on various organs, triggering rapid physiological changes:
Increased heart rate (tachycardia): Ensures more oxygen and nutrients are delivered to muscles.
Bronchodilation: Expands airways for improved oxygen intake.
Pupil dilation (mydriasis): Enhances vision to detect threats.
Next, the body receives a hormonal surge from the adrenal glands. The hypothalamus activates the hypothalamic-pituitary-adrenal (HPA) axis:
The hypothalamus secretes corticotropin-releasing hormone (CRH).
CRH stimulates the anterior pituitary gland to release adrenocorticotropic hormone (ACTH).
ACTH signals the adrenal medulla to secrete adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream.
These hormones amplify the SNS response, causing systemic effects:
Mobilization of glucose: The liver breaks down glycogen into glucose (glycogenolysis), providing energy to skeletal muscles.
Redistribution of blood flow: Blood is diverted from non-essential systems (digestive and urinary) to muscles and the brain.
Then come the cognitive and emotional effects. The surge of norepinephrine and epinephrine heightens alertness and vigilance. Simultaneously, the hippocampus and prefrontal cortex help evaluate the threat and strategize a response. This balance between the reactive amygdala and rational prefrontal cortex can influence whether an individual chooses to fight, flee, or freeze.
Lastly, as the threat subsides, the parasympathetic nervous system (PNS) counteracts the SNS to restore homeostasis. Acetylcholine plays a role in slowing the heart rate, constricting pupils, and resuming normal digestive activity.
Now, after learning the biological process of this response, you may be wondering why it is so important to understand.
Works Cited:
Harvard Health Publishing. "Understanding the Stress Response: Chronic Activation of the Stress Response System Can Disrupt Almost All Body Processes." Harvard Medical School, www.health.harvard.edu.
National Institute of Mental Health (NIMH). "What is Stress?" www.nimh.nih.gov.
Ulrich-Lai, Y. M., & Herman, J. P. "Neural Regulation of Endocrine and Autonomic Stress Responses." Nature Reviews Neuroscience, vol. 10, no. 6, 2009, pp. 397-409.
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