The nervous system is the body's most rapid communication network, allowing detection of internal and external stimuli and coordinating appropriate responses within milliseconds. It is organized into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), comprising 12 pairs of cranial nerves and 31 pairs of spinal nerves (8 cervical + 12 thoracic + 5 lumbar + 5 sacral + 1 coccygeal).

The Neuron and Neural Structure The neuron is the structural and functional unit of the nervous system. It consists of a cell body (cyton or soma) containing the nucleus and Nissl granules (rough endoplasmic reticulum responsible for protein synthesis), multiple short branching dendrites that receive incoming signals, and a single long axon that transmits impulses away from the cell body. Axons in the peripheral nervous system are wrapped in a myelin sheath formed by Schwann cells (in the CNS, by oligodendrocytes). The gaps between successive Schwann cells are called nodes of Ranvier, which are critical for saltatory conduction.

The Nerve Impulse At rest, the neuron maintains a resting membrane potential of approximately -70 mV (inside negative relative to outside), established by the Na+/K+ ATPase pump (which transports 3 Na+ out and 2 K+ in per cycle) and K+ leak channels that allow continuous passive efflux of K+. When a stimulus of threshold intensity (~-55 mV) reaches the membrane, voltage-gated Na+ channels open rapidly and Na+ rushes into the cell, shifting the membrane potential to approximately +30 mV — this is depolarization. Subsequently, Na+ channels inactivate and voltage-gated K+ channels open, allowing K+ to flow out and restoring the negative potential (repolarization). A brief hyperpolarization follows before the resting potential is fully restored. The action potential obeys the all-or-none principle — it fires at full magnitude at threshold or does not fire at all. In myelinated neurons, the impulse jumps from one node of Ranvier to the next (saltatory conduction), enabling conduction velocities of 15-120 m/s, far faster than the 0.5-2 m/s of unmyelinated fibres.

The Synapse At the synapse, electrical signals are converted to chemical ones. When an action potential reaches the axon terminal, voltage-gated Ca2+ channels open and Ca2+ influx triggers exocytosis of synaptic vesicles, releasing neurotransmitters (commonly acetylcholine or norepinephrine) into the 20 nm synaptic cleft. Neurotransmitters diffuse across and bind to postsynaptic receptors, generating a new impulse. The signal is terminated by enzymatic degradation (acetylcholinesterase breaks down ACh) or neurotransmitter reuptake.

Brain Organization The brain is protected by the cranium and three meninges (dura mater, arachnoid mater, pia mater) and divided into three regions. The forebrain includes: the cerebrum (largest, with frontal, parietal, temporal, and occipital lobes — responsible for higher cognition, memory, speech, and critically, INITIATING voluntary movement), the thalamus (relay centre for all senses except smell), and the hypothalamus (regulates thermoregulation, hunger, thirst, circadian rhythms, and controls the pituitary gland — the crucial link between nervous and endocrine systems). The midbrain contains relay centres for visual and auditory reflexes. The hindbrain comprises the cerebellum (coordinates voluntary movements and maintains balance and posture — critically does NOT initiate movement), the pons (relay between cerebrum and cerebellum; houses the pneumotaxic centre that modulates respiration), and the medulla oblongata (controls vital involuntary functions: cardiovascular centre, respiratory rhythmicity centre, vomiting, and swallowing reflexes).

PNS and Autonomic System The PNS is divided into the somatic nervous system (voluntary control of skeletal muscles via a single motor neuron) and the autonomic nervous system (involuntary control of visceral organs via a two-neuron preganglionic-postganglionic chain). The ANS has two antagonistic divisions: sympathetic ("fight or flight" — increases heart rate, dilates pupils and bronchi, inhibits digestion, releases adrenaline from adrenal medulla; uses norepinephrine at postganglionic synapses) and parasympathetic ("rest and digest" — decreases heart rate, constricts pupils and bronchi, stimulates digestion; uses ACh at all synapses). All preganglionic neurons in both divisions use ACh.

Sense Organs: Eye The eye functions as a camera. Light enters through the cornea (the primary refracting surface providing ~2/3 of total optical power), passes through the pupil (regulated by the iris), is focused by the lens (which can change curvature via ciliary muscles — accommodation: more convex for near vision), and forms an inverted image on the retina. The retina contains two types of photoreceptors: rods (approximately 120 million, containing rhodopsin derived from Vitamin A, responsible for dim-light/scotopic vision, distributed across the peripheral retina) and cones (approximately 6-7 million, containing iodopsin, responsible for colour/photopic vision, concentrated at the fovea). Common visual defects include myopia (image forms in front of retina; corrected by concave lens), hypermetropia (image behind retina; corrected by convex lens), presbyopia (age-related loss of lens elasticity; corrected by bifocal lenses), and astigmatism (irregular corneal curvature; corrected by cylindrical lens).

Sense Organs: Ear The ear serves both hearing and balance. Sound waves enter the outer ear, vibrate the tympanic membrane, and are transmitted through three ossicles — malleus (hammer) → incus (anvil) → stapes (stirrup, the smallest bone in the human body) — which amplify vibrations approximately 20-fold. The stapes transmits vibrations to the oval window of the cochlea in the inner ear, where fluid movements stimulate hair cells in the organ of Corti, generating nerve impulses carried by the auditory nerve to the brain. The vestibular apparatus (three semicircular canals detecting rotational movement, plus the utricle and saccule detecting linear acceleration and head position) maintains balance and equilibrium.

The most NEET-critical concepts are: resting potential = -70 mV; Na+ in = depolarization; K+ out = repolarization; cerebrum initiates voluntary movement while cerebellum only coordinates it; rods contain rhodopsin for dim light while cones contain iodopsin for colour at the fovea; and the ossicle order MIS (Malleus-Incus-Stapes) with stapes being the smallest bone connecting to the oval window.