What Is the Craniosacral Division?
The craniosacral division is an anatomical term for the parasympathetic nervous system, named for the regions of the central nervous system where its nerve fibres originate: the cranial region (brain stem) and the sacral region (lower spinal cord). This division of the autonomic nervous system is responsible for “rest and digest” functions that generally oppose the activating effects of the sympathetic (thoraco-lumbar) division.
Relevance to Polygraph Testing
The craniosacral (parasympathetic) division influences polygraph testing primarily through its effects on cardiovascular function. The vagus nerve — the most important cranial parasympathetic nerve — slows the heart rate and reduces cardiac output. During polygraph testing, the balance between sympathetic activation (which increases heart rate and blood pressure) and parasympathetic modulation (which counteracts these increases) shapes the cardiovascular response patterns visible in the cardiograph tracing.
Anatomical Origin
Cranial parasympathetic fibres exit through cranial nerves III, VII, IX, and X (the vagus). They innervate the pupils, salivary glands, heart, lungs, and upper digestive tract. Sacral parasympathetic fibres exit through sacral spinal nerves S2–S4 and innervate the lower digestive tract, bladder, and reproductive organs. The craniosacral designation distinguishes this division from the thoraco-lumbar (sympathetic) division, whose fibres originate from the thoracic and lumbar segments of the spinal cord.
Contrast with Thoraco-Lumbar Division
The craniosacral and thoraco-lumbar divisions work in a complementary fashion, typically with opposing effects on target organs. During the fight-or-flight response associated with deception, the thoraco-lumbar (sympathetic) division dominates — producing the increased arousal the polygraph measures — while the craniosacral (parasympathetic) division is suppressed.