What Is the Nervous System and Why Do We Need It?
Your body is a city of 37 trillion cells. For this city to function, information must travel at lightning speed — when you touch a hot pan, your hand must pull away before your brain even consciously registers the heat. That’s the nervous system doing its job.
The nervous system is the body’s electrical communication network. It detects stimuli (changes in the environment), processes the information, and coordinates responses. Without it, your heart wouldn’t know to beat faster when you run, your pupils wouldn’t adjust in bright light, and you couldn’t learn, remember, or feel anything.
For CBSE Class 10 boards and NEET, this chapter carries significant weightage. The neuron structure, impulse transmission, reflex arc, and differences between voluntary and involuntary actions are the most frequently tested areas.
Key Terms & Definitions
Neuron (nerve cell): The structural and functional unit of the nervous system. A neuron is specialised to receive, process, and transmit electrical signals.
Nerve impulse: An electrochemical signal that travels along a neuron. At rest, the inside of a neuron is negatively charged; during an impulse, a wave of positive charge (depolarisation) travels along the axon.
Synapse: The junction between two neurons. The electrical signal cannot jump directly — it is converted to a chemical signal (neurotransmitter), crosses the synaptic cleft, and triggers a new electrical signal in the next neuron.
Reflex arc: The pathway of nerve impulse during a reflex action — receptor → sensory neuron → spinal cord → motor neuron → effector.
CNS (Central Nervous System): Brain and spinal cord — the control centre.
PNS (Peripheral Nervous System): All nerves outside the CNS — carries signals to and from the CNS.
Structure of a Neuron
The neuron has three main parts — understanding each part’s function is essential.
Dendrites
Short, branched projections that receive signals from other neurons or from sensory receptors. Think of them as antennas. A single neuron can have hundreds of dendrites, allowing it to receive input from many sources simultaneously.
Cell Body (Cyton/Soma)
Contains the nucleus and most organelles. The cell body integrates incoming signals. If the total stimulation crosses a threshold, an impulse is generated.
Axon
A long, single projection that transmits the impulse away from the cell body toward the next neuron or effector (muscle/gland). Axons can be up to 1 metre long (the sciatic nerve in your leg).
Many axons are covered by a myelin sheath — a fatty insulating layer produced by Schwann cells. The sheath has gaps called nodes of Ranvier. The impulse “jumps” from node to node (saltatory conduction), which dramatically increases speed. This is why nerve impulses travel at up to 120 m/s in myelinated fibres.
Resting potential: Inside of neuron = −70 mV (negative relative to outside)
Action potential: A stimulus causes Na⁺ ions to rush in → inside becomes +40 mV (depolarisation) → K⁺ ions rush out → repolarisation back to −70 mV.
This cycle of depolarisation-repolarisation IS the nerve impulse — a self-propagating wave of ion movements.
How Impulses Cross a Synapse
The synapse is the critical junction. Here’s what happens step-by-step:
- The impulse reaches the pre-synaptic terminal (end of the axon)
- This triggers release of neurotransmitters (e.g., acetylcholine, dopamine, serotonin) from vesicles into the synaptic cleft
- Neurotransmitters diffuse across the 20–40 nm cleft
- They bind to receptors on the post-synaptic membrane (dendrite of next neuron)
- This opens ion channels → new impulse generated in the next neuron
- Neurotransmitters are broken down by enzymes (e.g., acetylcholinesterase) to stop the signal
Many drugs and toxins work at synapses. Caffeine blocks adenosine receptors (keeping you awake). Serotonin-related drugs treat depression. Understanding this mechanism connects biology to pharmacology — a favourite NEET MCQ link.
Reflex Actions and the Reflex Arc
A reflex action is a rapid, involuntary response to a stimulus that does NOT require the brain to process the signal first. The decision is made at the spinal cord level.
Why is this useful? Speed. Sending a signal from your foot to the brain and back takes about 0.5 seconds. The spinal cord reflex takes <0.1 seconds — enough to pull your hand away from a flame before serious burns occur.
The Reflex Arc (must memorise this pathway)
Stimulus → Receptor → Sensory neuron → Spinal cord (integration) → Motor neuron → Effector (muscle/gland) → Response
For the knee-jerk reflex (patellar reflex):
- Tap below knee → stretch receptor in quadriceps stimulated → sensory neuron carries signal to spinal cord → motor neuron carries signal back to quadriceps → leg kicks forward
CBSE boards frequently ask “What is a reflex arc? Give one example.” For NEET, they test whether you know where the processing happens (spinal cord, not brain) and what type of neurons are involved. A well-drawn diagram of reflex arc with all five components labelled is worth full marks.
Human Brain — Regions and Functions
The brain is divided into three major regions:
Forebrain:
- Cerebrum — largest part, 85% of brain weight. Controls voluntary movements, conscious thought, memory, language, emotion, sensory perception.
- Thalamus — relay station; routes sensory signals to appropriate cortex regions
- Hypothalamus — controls body temperature, hunger, thirst, sleep-wake cycle, links nervous and endocrine systems
Midbrain: Controls visual and auditory reflexes, eye and head movements
Hindbrain:
- Cerebellum — coordinates movement, balance, fine motor skills (damage causes tremors, loss of coordination)
- Medulla oblongata — controls involuntary functions: heart rate, breathing, blood pressure, vomiting, swallowing
- Pons — connects different parts of the brainstem, involved in sleep regulation
Solved Examples
Easy — CBSE Class 10 Level
Q: Name the parts of a neuron. What is the function of the myelin sheath?
Solution: A neuron has three parts — dendrites (receive signals), cell body/cyton (integrates signals), and axon (transmits signals). The myelin sheath is a fatty insulating layer around the axon that speeds up impulse transmission (saltatory conduction) and prevents signal loss.
Medium — CBSE Class 12 / NEET Level
Q: Explain the ionic basis of a nerve impulse.
Solution: At rest, the neuron membrane maintains a potential of −70 mV (inside negative) due to unequal distribution of Na⁺ and K⁺ ions maintained by the Na⁺/K⁺ ATPase pump. When a stimulus exceeds threshold, Na⁺ channels open → Na⁺ rushes in → inside becomes +40 mV (depolarisation). Then K⁺ channels open → K⁺ rushes out → membrane repolarises to −70 mV. This cycle propagates along the axon as the nerve impulse.
Hard — NEET Level
Q: A person touches a hot object and withdraws the hand. Later, they feel pain. Explain the neural pathway for both events.
Solution:
- Withdrawal (reflex): Stimulus → pain receptor in skin → sensory neuron → dorsal root → spinal cord → motor neuron → ventral root → flexor muscle contracts → hand withdraws. This happens via the spinal cord WITHOUT brain involvement.
- Pain perception (conscious): Simultaneously, the sensory signal is also relayed upward via ascending tracts to the thalamus → sensory cortex → person consciously feels pain. This takes slightly longer, which is why withdrawal happens before you fully feel the pain.
This question combining reflex arc and conscious perception was asked in NEET 2019. The key point is that both pathways start from the same receptor simultaneously — the spinal reflex just completes faster.
Exam-Specific Tips
CBSE Class 10:
- Focus on: neuron diagram with labels, reflex arc with diagram, difference between voluntary and involuntary actions, parts of brain and functions
- Marks split: 1-mark recall + 3-mark explanation + 5-mark diagram-based
CBSE Class 12:
- Ionic basis of nerve impulse (action potential) is now in the syllabus
- Synapse mechanism and neurotransmitters are tested in 3-mark questions
NEET:
- 3–4 questions from this chapter typically appear each year
- High-frequency topics: saltatory conduction, location of reflex processing (spinal cord), neurotransmitter names, cranial nerves (12 pairs), brain parts and functions
NEET 2023 asked about which part of the brain is responsible for maintaining posture and balance — answer: cerebellum. NEET 2022 asked about the role of medulla oblongata. Memorise the brain region-function mapping precisely.
Common Mistakes to Avoid
Mistake 1: Writing “the brain controls reflex actions.” Wrong — spinal cord controls most reflex actions. The brain only receives a copy of the signal (for conscious perception, which comes later).
Mistake 2: Confusing sensory and motor neurons. Sensory neurons carry signals FROM receptors TO the spinal cord/brain (afferent). Motor neurons carry signals FROM the spinal cord/brain TO muscles/glands (efferent). Mnemonic: SAME = Sensory-Afferent, Motor-Efferent.
Mistake 3: Saying “impulses travel faster in non-myelinated fibres.” It’s the opposite — myelinated fibres are faster because of saltatory conduction (impulse jumps between nodes of Ranvier).
Mistake 4: Drawing the synapse incorrectly — students often show the synapse as a direct connection. There is always a physical gap (synaptic cleft) between pre- and post-synaptic membranes. Chemical transmission (neurotransmitters) bridges this gap.
Mistake 5: Confusing voluntary and involuntary actions. Voluntary actions are controlled by the cerebrum (conscious decision). Involuntary actions are controlled by medulla oblongata and spinal cord (automatic). Reflex actions are a subset of involuntary actions.
Practice Questions
1. What is the function of dendrites?
Dendrites are short, branched projections of the neuron that receive signals (nerve impulses) from other neurons or from sense organs. They carry the impulse toward the cell body.
2. Where exactly does the processing happen in a spinal reflex?
In a spinal reflex, processing occurs in the spinal cord (specifically in the interneurons/relay neurons of the grey matter). The signal does not travel to the brain for the reflex response itself, though a copy is simultaneously sent upward for conscious perception.
3. What are neurotransmitters? Give two examples.
Neurotransmitters are chemical messengers released from the pre-synaptic terminal into the synaptic cleft. They bind to receptors on the post-synaptic membrane and trigger a new electrical impulse. Examples: acetylcholine (most common at neuromuscular junctions), dopamine (reward, movement), serotonin (mood), norepinephrine (alertness).
4. Why is the myelin sheath white?
The myelin sheath is white because it is composed primarily of lipids (fats) and proteins produced by Schwann cells (in PNS) or oligodendrocytes (in CNS). The white matter of the brain and spinal cord consists of myelinated axons; grey matter consists of cell bodies and unmyelinated fibres.
5. What is the all-or-none principle?
The all-or-none principle states that once a stimulus exceeds the threshold level, a nerve impulse of full magnitude is generated. A weaker-than-threshold stimulus generates NO impulse, and a stronger-than-threshold stimulus generates the SAME magnitude impulse as a threshold stimulus. The strength of the response is coded by the frequency of impulses, not the size of each impulse.
6. Name the cranial nerve that is purely sensory.
The optic nerve (CN II, for vision) and the olfactory nerve (CN I, for smell) are purely sensory cranial nerves. There are 12 pairs of cranial nerves total; some are purely sensory, some purely motor, and some mixed.
7. What is the role of the cerebellum?
The cerebellum coordinates voluntary movements, maintains balance and posture, and fine-tunes motor activity. Damage to the cerebellum causes ataxia (loss of coordination), tremors, and difficulty with precise movements. It does NOT initiate movements — that role belongs to the motor cortex of the cerebrum.
8. Explain why a person who is drunk staggers.
Alcohol particularly affects the cerebellum, which is responsible for coordination, balance, and fine motor control. When the cerebellum is impaired by alcohol, the person loses precise muscle coordination — hence the staggering gait, slurred speech, and inability to perform tasks requiring fine motor skills.
FAQs
Q: Is the spinal cord part of the CNS or PNS? The spinal cord is part of the CNS (along with the brain). The PNS includes all the nerves that branch out from the brain (cranial nerves) and spinal cord (spinal nerves) to reach all parts of the body.
Q: How many neurons does the human brain have? Approximately 86 billion neurons, of which about 16 billion are in the cerebral cortex. Each neuron can form thousands of synaptic connections, giving the brain roughly 100 trillion synaptic connections total.
Q: What is the difference between a nerve and a neuron? A neuron is a single nerve cell. A nerve is a bundle of many axons (from many neurons) wrapped together by connective tissue — like a cable containing many wires. The optic nerve, for example, contains over 1 million axons.
Q: Can damaged neurons regenerate? Peripheral nervous system (PNS) neurons can regenerate slowly (about 1 mm/day) if the cell body is intact. CNS neurons (brain and spinal cord) have very limited regeneration capacity, which is why spinal cord injuries are often permanent.
Q: What is multiple sclerosis? Multiple sclerosis is an autoimmune disease where the immune system attacks and destroys the myelin sheath. Without proper myelin, nerve impulses travel slowly or incorrectly, causing muscle weakness, vision problems, and loss of coordination.