Nervous System Organisation

All 10 Terms & Definitions

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) and produces ~95% of the body's serotonin.