The Nervous System Overview

The nervous system is the body's rapid communication network, enabling perception, cognition, voluntary movement, and homeostatic regulation. It is structurally divided 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.

The CNS is protected by the skull and vertebral column, the meninges (dura, arachnoid, and pia mater), and the blood-brain barrier — tight junctions between CNS endothelial cells reinforced by astrocyte end-feet that restrict the passage of most molecules. The PNS includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. It is further divided into the somatic nervous system (voluntary motor and sensory) and the autonomic nervous system (ANS), which regulates involuntary functions via sympathetic, parasympathetic, and enteric divisions.

Neurons are the fundamental signalling units. Structurally, a typical neuron has a cell body (soma), branching dendrites that receive input, and a single axon that transmits output to target cells. Neurons are classified by function: sensory (afferent) neurons convey information from receptors to the CNS; motor (efferent) neurons carry commands from the CNS to muscles and glands; interneurons (the vast majority in the CNS) integrate signals between neurons. Axons are insulated by myelin — produced by oligodendrocytes in the CNS and Schwann cells in the PNS — which dramatically increases conduction velocity via saltatory conduction between nodes of Ranvier.

The resting membrane potential of a typical neuron is approximately −70 mV, determined primarily by the selective permeability of the membrane to K+ (via leak channels) and the electrochemical gradients maintained by the Na+/K+-ATPase (3 Na+ out, 2 K+ in per cycle). At rest, the membrane is more permeable to K+ than Na+; K+ diffuses outward down its concentration gradient, leaving net negative charge inside.

An action potential is an all-or-nothing electrical event that propagates along the axon. When a depolarising stimulus raises the membrane potential to the threshold (~−55 mV), voltage-gated Na+ channels open rapidly, causing an influx of Na+ and rapid depolarisation to ~+30 mV (upstroke). Voltage-gated Na+ channels then inactivate; delayed-rectifier K+ channels open and K+ efflux repolarises the membrane (downstroke). Brief hyperpolarisation (afterhyperpolarisation) occurs as K+ channels close slowly, after which the membrane returns to rest. The absolute refractory period (when Na+ channels are inactivated) prevents backward propagation and limits firing frequency.

The CNS is organised into distinct functional regions. The cerebral cortex processes sensation, perception, language, reasoning, and voluntary movement. The cerebellum coordinates movement and balance. The brainstem (midbrain, pons, medulla) controls vital functions including respiration, cardiovascular regulation, and consciousness. The diencephalon contains the thalamus (sensory relay station) and hypothalamus (homeostatic control centre for temperature, hunger, thirst, circadian rhythms, and endocrine regulation via the pituitary). The limbic system (hippocampus, amygdala, cingulate cortex) is involved in memory formation and emotional processing.

Synaptic transmission enables neuron-to-neuron communication. An action potential arriving at a presynaptic terminal triggers voltage-gated Ca2+ channel opening; Ca2+ influx induces fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft. Neurotransmitters bind to postsynaptic receptors, generating either an excitatory postsynaptic potential (EPSP — depolarisation, e.g., glutamate via AMPA receptors) or an inhibitory postsynaptic potential (IPSP — hyperpolarisation, e.g., GABA via GABA-A receptors). Temporal and spatial summation of EPSPs and IPSPs determines whether the postsynaptic neuron fires.

The autonomic nervous system controls smooth muscle, cardiac muscle, and glands. Sympathetic ("fight or flight") responses include increased heart rate and contractility, bronchodilation, reduced GI motility, and pupil dilation; noradrenaline is the key postganglionic transmitter. Parasympathetic ("rest and digest") responses include decreased heart rate, increased GI motility, and glandular secretion; acetylcholine is the postganglionic transmitter.