Human Neural System: CNS and PNS

NCERT grounding

NCERT Class 11 Biology, Chapter 18 — Neural Control and Coordination, Section 18.2 — opens with a single, examinable sentence: "The human neural system is divided into two parts: (i) the central neural system (CNS) and (ii) the peripheral neural system (PNS)." NCERT then specifies that the CNS contains the brain and spinal cord and is the site of information processing and control, while the PNS comprises all nerves of the body associated with the CNS. NIOS Biology, Chapter 17 (Section 17.4), reinforces the same map and adds the cranial-nerve / spinal-nerve counts and the sympathetic-parasympathetic table that NEET examiners draw on year after year.

"The CNS includes the brain and the spinal cord and is the site of information processing and control. The PNS comprises of all the nerves of the body associated with the CNS."

NCERT Class 11 Biology · Chapter 18 · Section 18.2

CNS and PNS — the full architecture

The neural system is a wired communication network. Every signal in the body either originates from a sensory receptor, travels into the CNS for integration, and then leaves as a motor command — or it is generated entirely within the CNS and dispatched outward. The CNS / PNS distinction is the first organising cut, because it separates the processing core from the delivery network. Once that cut is made, the PNS is split again by the direction of impulse traffic (afferent versus efferent) and again by the type of effector controlled (skeletal versus visceral). Four splits, two answers each — that is the entire NEET-relevant map.

The CNS occupies the dorsal cavity of the body. The brain sits inside the cranium, protected by three cranial meninges (dura mater, arachnoid, pia mater); the spinal cord runs inside the vertebral canal, wrapped by the same three layers. Cerebrospinal fluid fills the ventricles and the subarachnoid space and cushions the soft neural tissue against shock. Information processing — perception, motor planning, emotion, memory, autonomic set-points, reflexes — happens here. The CNS is not just a relay; NCERT explicitly calls the brain the "command and control system."

The PNS is everything else: 12 pairs of cranial nerves leaving the brain, 31 pairs of spinal nerves leaving the spinal cord, the ganglia that punctuate their courses, and the autonomic chains that flank the vertebral column. Functionally, every PNS fibre is either an afferent (sensory) fibre carrying signals from tissues into the CNS, or an efferent (motor) fibre carrying regulatory signals from the CNS out to muscles and glands. NCERT names these as the two fibre types in the PNS, and uses the afferent/efferent vocabulary again when describing reflex arcs.

Cranial nerves, spinal nerves and afferent / efferent fibres

Every wire in the PNS is bundled into a nerve. Cranial nerves emerge directly from the brain — twelve pairs in humans, numbered I to XII (olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, hypoglossal). Some are purely sensory (the olfactory, optic and vestibulocochlear nerves), some are purely motor (the oculomotor, trochlear, abducens, accessory and hypoglossal nerves), and several are mixed nerves carrying both afferent and efferent fibres. NCERT does not require you to memorise the names but does require the count — 12 pairs — and the cranial / spinal contrast.

Spinal nerves emerge segmentally from the spinal cord — 31 pairs in humans (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). Every spinal nerve is mixed. It enters the cord through two roots: the dorsal (posterior) root carrying afferent sensory fibres whose cell bodies sit in the dorsal root ganglion, and the ventral (anterior) root carrying efferent motor fibres whose cell bodies sit in the grey matter of the cord. This anatomy is the substrate for the spinal reflex arc (NCERT Section 21 and NIOS Section 17.7).

Somatic versus autonomic outflow

Now follow the motor fibres (the efferents) outward. NCERT splits them by the type of effector they reach. The somatic neural system relays impulses from the CNS to skeletal muscles — the striated, voluntary muscles attached to your bones. Reach down to lift a pencil and the motor cortex commands a somatic motor neuron in the ventral horn; its axon leaves through a ventral spinal root, runs without interruption to the neuromuscular junction at the biceps, releases acetylcholine, and the muscle contracts. One neuron, one synapse, one effector — that is the somatic plan.

The autonomic neural system transmits impulses from the CNS to involuntary organs and smooth muscles — the heart, blood vessels, bronchi, gut, urinary bladder, sweat glands, salivary glands, iris and so on. Autonomic outflow uses two neurons in series: a preganglionic neuron whose cell body sits in the CNS, and a postganglionic neuron whose cell body sits in an autonomic ganglion outside the CNS. The pre-ganglionic axon ends in the ganglion; the post-ganglionic axon goes the rest of the way to the smooth muscle or gland. This two-neuron chain is the structural signature of the autonomic system.

Sympathetic versus parasympathetic divisions

The autonomic system is itself a two-arm system. NCERT names the arms sympathetic and parasympathetic, and assigns them complementary roles. NIOS Section 17.4 makes the physiology explicit: "Sympathetic nervous system prepares the body for facing emergency situations and the parasympathetic nervous system reestablishes the normal conditions once the emergency is over." Almost every visceral organ receives both inputs, and the balance between them sets the moment-to-moment state of the body.

The two arms differ in origin, in chemistry and in effect. Sympathetic preganglionic fibres leave the CNS from the thoracic and lumbar segments of the spinal cord — the thoracolumbar outflow. They synapse in a paired chain of sympathetic ganglia that runs alongside the vertebral column. The post-ganglionic fibres are long and release noradrenaline at their targets. The functional theme is fight-or-flight: dilate the pupil to admit more light, accelerate the heart and force more cardiac output, dilate the bronchioles to move more air, redirect blood from gut and skin to skeletal muscles, mobilise glucose, and inhibit digestion.

Parasympathetic preganglionic fibres leave the CNS from the brain (through several cranial nerves, especially the vagus) and from the sacral segments of the spinal cord — the craniosacral outflow. They synapse in ganglia located inside or near the effector organ, so post-ganglionic fibres are short. Both pre- and post-ganglionic parasympathetic fibres release acetylcholine. The functional theme is rest-and-digest: constrict the pupil, slow the heart, constrict the bronchioles, stimulate salivation and gastric secretion, increase peristalsis and empty the bladder.

The standard NIOS Table 17.1 lists the antagonistic effects organ by organ, and NEET reuses it directly. Pupil: sympathetic dilates, parasympathetic constricts. Heart: sympathetic accelerates, parasympathetic slows. Bronchioles: sympathetic dilates, parasympathetic constricts. Gut peristalsis: sympathetic decreases, parasympathetic increases. Salivary secretion: sympathetic decreases, parasympathetic increases. Urinary bladder detrusor: sympathetic relaxes (storage), parasympathetic contracts (voiding). Adrenal medulla: sympathetic stimulates adrenaline release, parasympathetic has no effect. The 2025 NEET medulla-oblongata question (Q.136) is a direct application — sympathetic accelerates the heart, parasympathetic slows it, and the medullary cardiovascular centre arbitrates between the two.

The enteric and visceral nervous system

NCERT adds one more box at the bottom of the map: "Visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system." The visceral nervous system is therefore the PNS branch dedicated to the viscera; structurally it overlaps with the autonomic system, but it includes both sensory and motor pathways to and from the internal organs.

Embedded inside the wall of the gastrointestinal tract is the enteric nervous system — an extensive intrinsic network of neurons (often quoted as roughly 100 million, comparable to the spinal cord) arranged into the myenteric and submucosal plexuses. It is sometimes described as a "third division" of the autonomic system because it can regulate gut motility, secretion and local blood flow even when its connections with the CNS are cut. Sympathetic and parasympathetic inputs modulate the enteric system; they do not replace it. The 2020 cockroach question (Q.50) hints at the same principle — much of the cockroach nervous system is decentralised along the ventral nerve cord, so the body lives on after decapitation.

Worked examples

Common confusion & NEET traps