Nervous System Organisation

The nervous system is divided anatomically into the central nervous system (CNS) — comprising the brain and spinal cord — and the peripheral nervous system (PNS) — comprising all neural tissue outside the CNS, including the 12 pairs of cranial nerves, 31 pairs of spinal nerves, and their associated ganglia. This anatomical division is defined by location relative to the meningeal layers and bony enclosures, not by function. The CNS is the site of neural integration, processing, and higher cognitive function; the PNS connects it to every tissue in the body via afferent (sensory) and efferent (motor) pathways.

The PNS is further divided functionally into the somatic and autonomic systems. The somatic nervous system serves voluntary, conscious motor control — a single lower motor neuron carries motor commands from the ventral horn of the spinal cord directly to skeletal muscle, releasing acetylcholine at the neuromuscular junction — and carries somatic sensory information (touch, pain, temperature, proprioception) to the CNS via afferent fibres with cell bodies in dorsal root ganglia. The autonomic nervous system (ANS) regulates involuntary functions — cardiac muscle, smooth muscle, and exocrine glands — using a two-neuron chain: a preganglionic neuron synapses in an autonomic ganglion, and a postganglionic neuron continues to the effector organ. All preganglionic neurons are cholinergic (ACh on nicotinic receptors); sympathetic postganglionic neurons release noradrenaline (adrenergic receptors); parasympathetic postganglionic neurons release ACh (muscarinic receptors).

The two ANS divisions have distinct anatomical origins and opposing functional effects. The sympathetic division ("thoracolumbar") has preganglionic neurons originating from T1–L2, with ganglia in paravertebral chains or pre-aortic plexuses. It prepares the body for emergency action ("fight or flight"): increasing heart rate and cardiac output, dilating bronchioles, constricting blood vessels in skin and viscera (redirecting blood to skeletal muscle), stimulating hepatic glycogenolysis (raising blood glucose), dilating pupils, and inhibiting digestion. The parasympathetic division ("craniosacral") has preganglionic neurons originating from cranial nerves (CN III, VII, IX, X — the vagus being dominant) and sacral cord (S2–S4), with terminal ganglia close to or within the target organ. It promotes rest-and-digest: slowing heart rate, stimulating gastrointestinal motility and secretion, constricting pupils, and promoting urination.

The blood-brain barrier (BBB) is a critical feature of CNS organisation. Formed primarily by tight junctions between cerebral endothelial cells, with contributions from astrocyte end-feet and pericytes, it selectively controls substance passage from blood into the CNS — allowing glucose (GLUT1), O2, CO2, and lipid-soluble molecules, while blocking most pathogens, large proteins, and hydrophilic compounds. Efflux transporters (P-glycoprotein) actively remove many substances. The BBB is a fundamental challenge in CNS pharmacology. Finally, the enteric nervous system — 200–600 million neurons in two plexuses in the gut wall — constitutes a semi-autonomous "second brain" that can regulate gastrointestinal function independently of the CNS while communicating bidirectionally via the vagus nerve.