Cognitive Connie
Nervous System Organisation
The nervous system is the body's primary rapid-communication network, translating external and internal stimuli into coordinated responses. Understanding its organisation — anatomically into CNS and PNS, and functionally into somatic and autonomic divisions — is foundational to all of biopsychology.
Key figures
Charles Bell & François Magendie
1774–1842 / 1783–1855Their combined work established the Bell-Magendie law: dorsal spinal nerve roots are sensory (afferent); ventral roots are motor (efferent). The priority is contested: Bell's 1811 private pamphlet described only the anterior (motor) roots; Magendie's 1822 published paper was the first experimental demonstration of both sensory and motor limbs, establishing the law in its full form. This was one of the first demonstrations of functional specialisation within the nervous system.
John Newport Langley
1852–1925British physiologist who coined the term "autonomic nervous system" (1898) and established its division into sympathetic and parasympathetic components. He mapped the thoracolumbar and craniosacral origins of these divisions and described the two-neuron (preganglionic/postganglionic) architecture through meticulous pharmacological and anatomical experiments.
Key concepts
Central nervous system (CNS)
The brain and spinal cord. The primary integration and processing centre of the nervous system, enclosed within the skull (brain) and vertebral column (spinal cord) and protected by three meningeal layers (dura mater, arachnoid mater, pia mater) and cerebrospinal fluid (CSF). The CNS receives sensory input, integrates it, and generates motor output; it also mediates all higher cognitive functions.
Peripheral nervous system (PNS)
All neural tissue outside the CNS: the 12 pairs of cranial nerves, 31 pairs of spinal nerves, and their associated ganglia (clusters of neuronal cell bodies outside the CNS). The PNS connects the CNS to the rest of the body via afferent (sensory) and efferent (motor) pathways. The PNS is subdivided into somatic and autonomic divisions.
Somatic nervous system
The division of the PNS that serves voluntary, conscious functions: carries somatic sensory information (touch, pain, temperature, proprioception) from the body surface, muscles, and joints to the CNS, and carries motor commands from the CNS to skeletal muscle. The motor pathway uses a single lower motor neuron running directly from the spinal cord (or brainstem) to skeletal muscle, releasing acetylcholine at the neuromuscular junction.
Autonomic nervous system (ANS)
The division of the PNS that regulates involuntary visceral functions: cardiac muscle (heart rate, contractility), smooth muscle (blood vessels, airways, digestive tract, bladder, pupil), and exocrine glands. Operates largely outside conscious awareness. Uses a two-neuron chain: preganglionic neuron (CNS → autonomic ganglion) and postganglionic neuron (ganglion → effector). Divided into sympathetic and parasympathetic divisions.
Sympathetic division ("thoracolumbar")
Prepares the body for emergency action ("fight or flight"). Preganglionic neurons originate in the thoracic (T1–T12) and upper lumbar (L1–L2) spinal segments. Effects: increases heart rate and cardiac output, dilates bronchioles, constricts blood vessels in skin and viscera (redirecting blood to skeletal muscle), raises blood glucose (glycogenolysis), dilates pupils, reduces digestive activity. Postganglionic neurons release noradrenaline on adrenergic receptors; the adrenal medulla releases adrenaline and noradrenaline into the bloodstream.
Parasympathetic division ("craniosacral")
Promotes rest, digestion, and energy conservation ("rest and digest"). Preganglionic neurons originate from cranial nerves (CN III, VII, IX, X) and the sacral spinal cord (S2–S4). The vagus nerve (CN X) provides the majority of parasympathetic outflow, reaching the heart, lungs, and abdominal viscera. Effects: reduces heart rate, stimulates gastrointestinal motility and secretion, constricts pupils, promotes urination. Postganglionic neurons release acetylcholine on muscarinic receptors.
Afferent and efferent pathways
Direction of signal flow relative to the CNS. Afferent (from Latin ad = toward): sensory information travelling from peripheral receptors toward the CNS — includes pain, temperature, touch, proprioception, and visceral sensory signals. Efferent (from Latin ex = away from): motor commands travelling from the CNS to effectors. In spinal nerves, dorsal roots are afferent (cell bodies in dorsal root ganglia) and ventral roots are efferent — the Bell-Magendie law.
Blood-brain barrier (BBB)
A selective permeability barrier between the systemic bloodstream and the CNS, formed by tight junctions between cerebral endothelial cells, with contributions from astrocyte end-feet and pericytes. Blocks the passage of most pathogens, toxins, large proteins, and hydrophilic molecules; allows glucose (via GLUT1), O2, CO2, and lipid-soluble molecules (including many drugs, alcohol, steroid hormones) to cross. Efflux transporters (P-glycoprotein) actively remove many substances. The BBB is a major challenge in CNS pharmacology.
Autonomic ganglion
A cluster of postganglionic neuronal cell bodies located outside the CNS. In the sympathetic division, ganglia are located in two chains running parallel to the spinal cord (paravertebral sympathetic chain) or in pre-aortic plexuses. Preganglionic neurons synapse here, releasing acetylcholine onto nicotinic receptors on postganglionic neurons. In the parasympathetic division, ganglia are located close to or within the target organ (terminal ganglia).
Enteric nervous system (ENS)
A large, semi-autonomous neural network embedded in the wall of the gastrointestinal tract, containing 200–600 million neurons in two plexuses: myenteric (Auerbach's, controls motility) and submucosal (Meissner's, controls secretion and blood flow). The ENS can regulate gastrointestinal function independently of the brain or spinal cord. It communicates bidirectionally with the CNS via the vagus nerve (gut-brain axis) The gastrointestinal tract as a whole produces ~95% of the body's serotonin, primarily via enterochromaffin cells in the gut epithelium — which are distinct from ENS neurons, though the ENS modulates this serotonergic signalling.
Test your knowledge
Frequently asked questions
What is the difference between the CNS and PNS?+
The CNS (brain and spinal cord) integrates and processes information; the PNS connects it to the rest of the body via cranial nerves, spinal nerves, and their ganglia. The CNS is protected by bone, meninges, and CSF; PNS tissue lies outside these enclosures. PNS damage can often regenerate (Schwann cells produce nerve growth factor); CNS regeneration is largely blocked by inhibitory factors in the CNS environment, which is why spinal cord injuries are often permanent.
What distinguishes the somatic from the autonomic nervous system?+
The somatic nervous system serves voluntary, conscious functions — carrying sensory information from skin, muscles, and joints to the CNS, and motor commands to skeletal muscle via a single lower motor neuron releasing acetylcholine at the neuromuscular junction. The autonomic nervous system regulates involuntary visceral functions (heart, smooth muscle, glands) via a two-neuron preganglionic/postganglionic chain, operating largely below conscious awareness.
How do the sympathetic and parasympathetic divisions differ?+
The sympathetic division ("thoracolumbar"; T1–L2 origins) mobilises the body for emergency action — increasing heart rate, dilating airways, diverting blood to skeletal muscle, raising blood glucose, releasing adrenaline from the adrenal medulla. The parasympathetic division ("craniosacral"; CN III, VII, IX, X and S2–S4 origins) promotes rest, digestion, and energy conservation — slowing the heart, stimulating digestion, constricting pupils. The two divisions often act antagonistically on the same target organs but are not simply opposites: both can be co-activated (e.g. during sexual arousal).
What is the Bell-Magendie law?+
The Bell-Magendie law states that dorsal spinal nerve roots are sensory (afferent) and ventral roots are motor (efferent). The law bears both names because the priority is contested: Bell's 1811 private pamphlet described only the anterior (motor) roots as motor; Magendie's 1822 published paper was the first full experimental demonstration of both limbs. It was one of the first demonstrations of functional specialisation within the nervous system. Practically, dorsal root damage (e.g. shingles affecting the dorsal root ganglion) causes sensory loss; ventral root damage causes motor paralysis. The dorsal root ganglia contain the cell bodies of sensory neurons — they sit outside the CNS, making them PNS structures.
Sources
Last reviewed September 2026- 1.
Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (2021). Principles of Neural Science (6th ed.). McGraw-Hill.
+About this source
Comprehensive reference for the anatomy and physiology of the CNS and PNS, somatic and autonomic divisions, and the blood-brain barrier.
- 2.
Langley, J. N. (1921). The Autonomic Nervous System, Part 1. Heffer.
+About this source
Langley's foundational monograph defining the autonomic nervous system, its sympathetic and parasympathetic divisions, and the two-neuron preganglionic/postganglionic architecture.
- 3.
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W. C., LaMantia, A. S., Mooney, R. D., Bhatt, D. L., & White, L. E. (2018). Neuroscience (6th ed.). Sinauer Associates.
+About this source
Standard undergraduate neuroscience textbook covering nervous system organisation, afferent and efferent pathways, somatic and autonomic divisions.
- 4.
Bear, M. F., Connors, B. W., & Paradiso, M. A. (2016). Neuroscience: Exploring the Brain (4th ed.). Wolters Kluwer.
+About this source
Widely used undergraduate neuroscience textbook with clear treatment of nervous system organisation, CNS vs PNS, somatic and autonomic divisions, and spinal cord anatomy.