Overview of the nervous system

The human nervous system is the body's command and communication centre. It coordinates sensory perception, movement, body functions and cognition. Three functional subsystems are distinguished:

1. Central Nervous System (CNS)

Components:

Brain (encephalon) — cerebrum, cerebellum, brainstem.
Spinal cord (medulla spinalis).

Function: processing of all sensory input, decision-making, voluntary movement control, consciousness, language, memory.

Sensitivity to oxygen lack:

The brain is about 2% of body mass but consumes about 20% of total oxygen — the most O₂-sensitive organ.
Within 4 seconds without O₂ at 30 000 ft a pilot loses consciousness (TUC).

Relevant for pilots:

Hypoxia affects the brain first — confusion, slow thinking, slow reactions.
Alcohol, drugs, fatigue impair central processing.
G load can reduce cerebral blood flow → grey-out, G-LOC.

2. Peripheral Nervous System (PNS)

Components:

Sensory nerves (afferent): convey stimuli from sensory receptors (eyes, skin, ears, muscles, joints) to the CNS.
Motor nerves (efferent): convey commands from CNS to muscles.

Function: physical link between brain and body periphery — voluntary movement and conscious sensation.

Relevant for pilots:

Proprioception (limb position sense) is part of the PNS.
Hand–eye coordination (flying the controls, reading instruments) depends on healthy PNS.

3. Autonomic Nervous System (ANS)

The ANS regulates involuntary functions — heart rate, breathing, digestion, sweating, pupil response, blood pressure.

Two antagonists:

Sympathetic

Activates the "fight-or-flight" response in stress.
Effects:
- Heart rate increases,
- Breathing faster and deeper,
- Pupils dilate,
- Blood redirected from digestion to muscles,
- Adrenaline release,
- Sweating.

Parasympathetic

Activates the "rest-and-digest" response in relaxation.
Effects:
- Heart rate decreases,
- Digestion runs,
- Pupils constrict.

Relevance to pilot practice

Stress and the sympathetic system

In an emergency (engine failure, IMC entry) the sympathetic activates the body.
Positive: heightened alertness and reaction readiness.
Negative: at very strong activation — tunnel vision, "black-out" of conscious thinking, motor tension, wrong reactions.
Training and routine reduce sympathetic over-activation.

Sensory illusions

The ANS and PNS respond to misleading inputs (e.g. felt turn without actual turn) → spatial disorientation.
See dedicated lessons: Spatial disorientation, The vestibular system.

Fatigue and the nervous system

Fatigue affects CNS functions first: attention, decision, vision.
Reflex and PNS functions remain longer — dangerous, because the tired pilot "still feels fit" but is cognitively impaired.

Alcohol and the nervous system

Alcohol depresses the CNS → reaction time ↑, judgement ↓, coordination ↓.
Even small amounts: as little as 0.3 ‰ measurably degrades pilot performance.

Visual illusions

The visual cortex interprets images — with unusual stimuli (black-hole effect at night, sloping horizon) misperceptions arise.
These cannot be switched off by willpower — the pilot must trust the instruments, not the gut.