The nervous system is the body's electrochemical communication network. It coordinates everything from blinking to thinking. AQA splits it into two divisions: the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS) — every nerve outside the CNS.
CNS
The brain is the command centre — about 86 billion neurons. The spinal cord is a cable of axons running down the vertebral column; it carries motor commands out and sensory signals in, and produces fast, automatic reflex arcs (e.g. pulling your hand off a hot pan) without involving the brain.
PNS
The PNS splits into:
- Somatic nervous system (SNS) — voluntary control of skeletal muscles. Carries sensory information from skin/muscle to the CNS and motor commands back. This is what you use when you raise your hand or kick a ball.
- Autonomic nervous system (ANS) — involuntary control of internal organs (heart, gut, glands). It splits again:
- Sympathetic branch = arousal/expenditure ("fight-or-flight").
- Parasympathetic branch = relaxation/conservation ("rest-and-digest").
These two branches act in opposition, like an accelerator and brake.
Fight-or-flight
When the brain (specifically the amygdala) detects a threat, it signals the hypothalamus, which activates the sympathetic branch and the adrenal medulla to release adrenaline into the bloodstream. The combined effect (the acute stress response) prepares the body for action:
- Heart rate and blood pressure rise → more oxygen to muscles.
- Breathing becomes faster and shallower.
- Pupils dilate → better vision.
- Liver releases glucose → fuel for muscles.
- Digestion slows; saliva production drops (dry mouth).
- Sweat production increases.
When the threat passes, the parasympathetic branch kicks in — heart rate slows, digestion resumes — restoring homeostasis.
Cannon (1932) and modern critiques
Walter Cannon (1932) named "fight-or-flight" and argued it is a universal stress response. Modern critiques:
- Taylor et al. (2000) proposed a "tend-and-befriend" alternative more typical in females — protecting offspring and forming alliances rather than fighting or fleeing. Suggests a gender bias in early stress research.
- The response is adaptive for acute physical threats but maladaptive when triggered repeatedly by chronic psychological stressors (deadlines, social evaluation), contributing to hypertension and immune suppression.
Why this matters for GCSE
The nervous system underpins every other topic — neurons (P2.B.2), synapses, fight-or-flight, brain damage. Examiners want clear distinctions between CNS/PNS and SNS/ANS, plus a structured description of fight-or-flight including the role of adrenaline.
⚠Common mistakes— Common errors
- Confusing the somatic nervous system with the sympathetic branch — they are different splits.
- Saying adrenaline "is released by the brain" — it's released by the adrenal glands under instruction from the hypothalamus.
- Forgetting to mention the parasympathetic branch — the response must be reversed.
AI-generated · claude-opus-4-7 · v3-deep-psychology