Human neural system: CNS and PNS

Human neural system

In plain words: the brain and spinal cord are the head office; the nerves running out to every part of the body are the phone lines. Some lines bring news in, others carry orders out, and the orders go either to muscles you control at will or, automatically, to your internal organs.

Central and peripheral neural systems

• CNS: the brain and the spinal cord; the site of information processing and control (NCERT page 231).
• PNS: NCERT describes all the nerves associated with the CNS (brain and spinal cord) (page 231). Broader anatomy also includes peripheral ganglia and sensory endings; it is not just a set of wires (NCI SEER, Peripheral Nervous System).
• Beyond the NCERT text, the US National Cancer Institute’s training module gives the nerves’ numbers: twelve pairs of cranial nerves emerge from the brain, and thirty-one pairs of spinal nerves emerge from the spinal cord (NCI SEER Training Modules, Peripheral Nervous System).

Afferent and efferent fibres

The nerve fibres of the PNS are of two types (NCERT page 231):

• Afferent fibres carry impulses from tissues and organs to the CNS.
• Efferent fibres carry regulatory impulses from the CNS to the tissues and organs concerned.

Somatic and autonomic neural systems

NCERT describes somatic and autonomic divisions by their outgoing motor roles (page 231). Keep this distinct from the afferent/efferent classification above: sensory input exists as well as motor output.

• The somatic neural system relays impulses from the CNS to the skeletal muscles.
• The autonomic motor system regulates smooth muscle, cardiac muscle and glands (NCERT: involuntary organs and smooth muscles).
• Skeletal muscles are under voluntary control, while smooth (visceral) and cardiac muscles are involuntary (NCERT Chapter 17, pages 218–219; see types of muscle). The somatic motor system also carries involuntary reflex commands to skeletal muscle; “somatic” does not mean every action is conscious. The autonomic system regulates visceral effectors, such as heart rate (NCERT Chapter 15, page 202; StatPearls, “Physiology, Withdrawal Response”).

Sympathetic and parasympathetic neural systems

• NCERT further classifies the autonomic neural system into the sympathetic and the parasympathetic neural systems (NCERT page 231).
• Chapter 15 gives an example of the two working in opposite ways: sympathetic signals raise the heart rate and cardiac output, and parasympathetic signals lower them (NCERT page 202; see regulation of cardiac activity).

Visceral nervous system

• The visceral nervous system is the part of the PNS made of the whole complex of nerves, fibres, ganglia and plexuses through which impulses travel from the CNS to the viscera and from the viscera to the CNS (NCERT page 231).

Solved examples

Common mistakes to avoid

Quick revision summary

• CNS: brain and spinal cord, for information processing and control. PNS: all the nerves joined to the CNS.
• PNS fibres: afferent (to the CNS) and efferent (from the CNS).
• Motor output: somatic (to skeletal muscles, voluntary and reflex actions) and autonomic (to smooth muscle, cardiac muscle and glands). NCERT names sympathetic and parasympathetic autonomic divisions.
• The visceral nervous system links the CNS and the viscera both ways.

Questions students ask

What is the difference between the visceral and the autonomic nervous system?

In NCERT’s definitions, the visceral nervous system and the autonomic neural system differ in direction: the visceral system carries impulses from the CNS to the viscera and from the viscera to the CNS, while the autonomic system carries them one way, from the CNS to the involuntary organs and smooth muscles (NCERT page 231). So in NCERT’s terms the visceral system is the wider one: it also takes in the sensory fibres from the viscera, called visceral afferents beyond the NCERT text (MeSH, “Visceral Afferents”). Outside NCERT the two names often mean one system, and MeSH lists “Visceral Nervous System” as another name for the autonomic nervous system, so for NEET use NCERT’s definitions.

What is the difference between the somatic and the visceral nervous system?

The somatic neural system serves the skeletal muscles, while the visceral nervous system serves the internal organs, the viscera: NCERT says the somatic system relays impulses from the CNS to the skeletal muscles, while the visceral system carries impulses from the CNS to the viscera and from the viscera back to the CNS (page 231). Beyond the NCERT text, somatic sensory pathways serve skin, muscles and joints, while somatic motor pathways activate skeletal muscles in both voluntary and reflex actions, while autonomic nerves, the outgoing part of the visceral system, connect the CNS to visceral organs such as the heart, stomach and intestines and mediate unconscious ones (NCI SEER Training Modules, Peripheral Nervous System).

How do the sympathetic and parasympathetic systems work together?

Sympathetic and parasympathetic effects depend on the target organ; in some cases they oppose one another: in NCERT’s example, sympathetic signals increase the rate of heart beat and parasympathetic signals decrease it (NCERT Chapter 15, page 202). Beyond the NCERT text, many visceral organs get fibres from both, one stimulating and the other inhibiting, a balance that helps maintain homeostasis (NCI SEER Training Modules, Peripheral Nervous System). The sympathetic system helps mobilise resources during stress, while parasympathetic activity often supports maintenance and digestion. They are not universal on/off opposites: some targets, including most blood vessels and sweat glands, are mainly sympathetically supplied (McCorry, “Physiology of the Autonomic Nervous System”, 2007).

How can you remember afferent and efferent?

An easy way to remember afferent and efferent is that afferent impulses arrive at the CNS and efferent impulses exit it: NCERT’s afferent fibres carry impulses from tissues and organs to the CNS, and its efferent fibres carry regulatory impulses from the CNS to them (page 231). Beyond the NCERT text, the words themselves name a direction, towards a centre or away from it (MeSH, “Afferent Pathways”, “Efferent Pathways”), and the kidney uses the same pair for blood vessels: the afferent arteriole brings blood into the glomerulus and the efferent arteriole carries it away (NCERT Chapter 16, page 207).

Is the visceral nervous system in the NEET syllabus?

The visceral nervous system is in the NEET (UG) 2026 syllabus: Unit 5, Human Physiology, lists “Nervous system in humans-central nervous system, peripheral nervous system and visceral nervous system” under Neural control and coordination. NCERT gives it a single sentence, in Section 18.2 (page 231), which makes it the part of the peripheral nervous system by which impulses travel from the CNS to the viscera and from the viscera to the CNS. Learn that sentence closely, since the NCERT chapter says nothing more about it.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which part of the neural system is the site of information processing and control?

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   The CNS, the brain and spinal cord (NCERT page 231).

2. Q2. Into which two systems is the autonomic neural system classified?

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   The sympathetic and parasympathetic neural systems (NCERT page 231).

3. Q3. What does the visceral nervous system consist of?

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   The whole complex of nerves, fibres, ganglia and plexuses by which impulses travel between the CNS and the viscera (NCERT page 231).