The nervous system is the body’s fast control network. CBSE Class 11 has a full chapter on neural control. NEET asks one to two questions a year on the resting potential, action potential and synaptic transmission. This is high-yield if you know the ion movements.
The nervous system operates on electricity — but not the kind that flows through copper wires. Neural electricity is a wave of ion movements across a membrane, travelling at speeds from 1 to 100 m/s. Every thought, every reflex, every sensation is a pattern of these electrical signals. Once you understand the ionic basis, every question on this chapter becomes a matter of tracing which ion goes where.
Core Concepts
Organisation of the nervous system
Central Nervous System (CNS) — brain and spinal cord. The brain has three major parts:
- Cerebrum — largest part, handles higher functions (thinking, memory, speech, voluntary movement). Divided into two hemispheres, each with four lobes (frontal, parietal, temporal, occipital).
- Cerebellum — behind the cerebrum, controls balance, coordination and posture. Does NOT control thought.
- Brain stem — medulla oblongata, pons and midbrain. Controls involuntary functions (breathing, heart rate, blood pressure, swallowing).
Peripheral Nervous System (PNS) — all nerves outside the CNS:
- Somatic (voluntary) — controls skeletal muscles
- Autonomic (involuntary) — controls internal organs, split into:
- Sympathetic — fight or flight (heart rate up, pupils dilate, digestion stops)
- Parasympathetic — rest and digest (heart rate down, pupils constrict, digestion active)
Neuron structure
The functional unit of the nervous system. Three parts:
Cell body (soma) — contains the nucleus, Nissl granules (rough ER for protein synthesis), and mitochondria. Most cell bodies are in the CNS or in ganglia.
Dendrites — short, branched extensions that receive signals from other neurons or sensory receptors. The more dendrites, the more connections.
Axon — single long extension that transmits signals away from the cell body. Can be up to 1 metre long (sciatic nerve). Ends in synaptic knobs (terminal boutons) that release neurotransmitters.
Myelin sheath — formed by Schwann cells in the PNS and oligodendrocytes in the CNS. A fatty insulating layer that speeds conduction. Gaps between sheath segments are Nodes of Ranvier — these are where the action potential regenerates during saltatory conduction.
Types of neurons:
- Sensory (afferent) — carry signals from receptors to the CNS
- Motor (efferent) — carry signals from the CNS to effectors (muscles/glands)
- Interneurons — connect sensory and motor neurons within the CNS
Resting membrane potential
About -70 mV inside relative to outside. This electrical difference exists because of unequal ion distributions maintained by the Na/K ATPase (3 Na out, 2 K in per ATP) and the selective permeability of the membrane.
| Ion | Inside cell | Outside cell |
|---|---|---|
| K | High (~150 mM) | Low (~5 mM) |
| Na | Low (~15 mM) | High (~150 mM) |
| Cl | Low (~10 mM) | High (~120 mM) |
At rest, the membrane is much more permeable to K than to Na (through K leak channels). K leaks out down its concentration gradient, making the inside negative. The Na/K pump maintains this gradient against the slow inward Na leak.
Maintained by: (1) Na/K ATPase — 3 Na out, 2 K in, (2) K leak channels allowing K to move down its gradient. The Goldman equation gives the precise value based on permeabilities of all ions.
Action potential — the nerve impulse
A rapid, transient reversal of membrane potential. Travels along the axon as a self-propagating wave.
A stimulus opens voltage-gated Na channels. Na rushes in (down its concentration and electrical gradient). The membrane potential rises from mV toward mV. This is the rising phase.
Na channels inactivate (automatically, after ~1 ms). Voltage-gated K channels open. K rushes out, bringing the potential back toward mV. This is the falling phase.
K channels are slow to close, so K keeps leaving briefly, overshooting to about mV (undershoot). Then the Na/K pump and leak channels restore the resting potential. The entire action potential lasts about 1-2 ms.
Key properties:
- All-or-none — once threshold is reached (~ mV), the full action potential fires. Below threshold, nothing happens.
- Refractory period — absolute (no second AP possible, Na channels inactivated) and relative (second AP possible but needs stronger stimulus). This limits firing rate and ensures one-way propagation.
- Propagation — the depolarisation of one region triggers opening of Na channels in the adjacent region, creating a wave.
Saltatory conduction
In myelinated axons, the myelin sheath insulates the membrane, preventing ion flow. Action potentials can only occur at the bare Nodes of Ranvier. The impulse therefore ‘jumps’ from node to node — saltatory conduction.
Speed: up to 100-120 m/s in large myelinated fibres, compared to about 0.5-2 m/s in unmyelinated fibres. This is why myelination evolved — it makes fast reflexes possible.
Multiple sclerosis is a disease where the immune system attacks myelin in the CNS, slowing or blocking nerve conduction. Symptoms include numbness, weakness and vision problems.
Synaptic transmission
At the synapse, the gap between two neurons (about 20-40 nm), chemical signalling replaces electrical signalling:
The action potential reaches the presynaptic terminal. Voltage-gated Ca channels open, and Ca flows in.
Ca triggers fusion of synaptic vesicles with the presynaptic membrane. Neurotransmitter molecules (e.g., acetylcholine, dopamine, serotonin, GABA) are released into the synaptic cleft by exocytosis.
Neurotransmitter binds to specific receptors on the postsynaptic membrane. This opens ion channels (ligand-gated), causing either depolarisation (excitatory — EPSP) or hyperpolarisation (inhibitory — IPSP). If the sum of EPSPs exceeds threshold, a new action potential fires in the postsynaptic neuron.
Neurotransmitter is removed from the cleft by enzymatic degradation (e.g., acetylcholinesterase breaks down ACh), reuptake into the presynaptic neuron, or diffusion away. This prevents continuous stimulation.
Important neurotransmitters
| Neurotransmitter | Location | Effect |
|---|---|---|
| Acetylcholine (ACh) | Neuromuscular junction, brain | Muscle contraction, memory |
| Dopamine | Brain (basal ganglia) | Reward, movement. Deficiency → Parkinson’s |
| Serotonin | Brain, gut | Mood, sleep. Deficiency → depression |
| GABA | Brain | Main inhibitory NT. Deficiency → seizures |
| Glutamate | Brain | Main excitatory NT |
| Norepinephrine | Sympathetic nerves | Fight or flight |
Reflex arc
The simplest neural pathway — from stimulus to response without conscious brain involvement.
Receptor → Sensory neuron → Interneuron (in spinal cord) → Motor neuron → Effector
Examples: knee-jerk reflex (monosynaptic — no interneuron), withdrawal from hot object (polysynaptic — involves interneurons). Reflexes are faster than voluntary responses because the brain is bypassed.
Worked Examples
Neurotransmitter is released only from the presynaptic neuron (which contains the vesicles) and receptors are only on the postsynaptic membrane. There is no mechanism for reverse transmission. This anatomical arrangement enforces unidirectional signal flow at every synapse.
Maintaining the resting potential and firing action potentials requires constant Na/K pumping, which burns ATP. The brain is about 2% of body mass but uses about 20% of basal oxygen consumption. Neurons cannot store glycogen effectively, so they depend on a continuous supply of glucose from the blood. Even a few minutes without blood flow causes irreversible damage.
Drugs like lidocaine block voltage-gated Na channels on sensory neurons. Without Na influx, no depolarisation occurs and no action potential can propagate. Pain signals never reach the brain. The nerve is intact but temporarily silenced. The effect wears off as the drug is metabolised.
Common Mistakes
Saying the resting potential is +70 mV. It is -70 mV inside relative to outside. The positive peak during an action potential is about +30 mV, not +70 mV.
Confusing depolarisation and hyperpolarisation. Depolarisation brings the potential toward zero (and beyond to positive). Hyperpolarisation makes it more negative than resting. During an action potential, depolarisation comes first, then repolarisation, then brief hyperpolarisation.
Thinking myelin insulates the whole axon. Nodes of Ranvier are bare gaps where ion channels are concentrated. The impulse regenerates at these nodes. Without the nodes, no saltatory conduction would be possible.
Saying the action potential travels ‘inside’ the axon like current in a wire. The action potential is a wave of membrane depolarisation — each patch of membrane depolarises its neighbour. It is a regenerative process, not a flow of electrons.
Writing that the cerebellum handles thinking and memory. The cerebellum handles balance, coordination and motor learning. The cerebrum (specifically the cerebral cortex) handles thought, memory, language and voluntary movement.
Exam Weightage and Strategy
Neural Control and Coordination carries 5-7 marks in CBSE Class 11 boards. NEET asks 1-2 questions per year, typically on the action potential mechanism, synaptic transmission, or brain regions. JEE does not test this. The chapter rewards understanding the ion movement sequence — memorising the four phases of the action potential and the role of each ion channel covers most PYQs.
PYQ favourites:
- Describe the mechanism of action potential (Na in → depolarisation → K out → repolarisation)
- What is saltatory conduction? Why is it faster?
- Draw and label a neuron
- Which part of the brain controls balance? (Cerebellum)
- What is the role of Ca at the synapse? (Triggers vesicle fusion and NT release)
Sketch one action potential graph — voltage (y-axis) vs time (x-axis), labelled with: resting (-70 mV), threshold (-55 mV), peak (+30 mV), repolarisation, and undershoot (-80 mV). Mark where Na channels open and where K channels open. That single graph is your whole revision for the action potential.
Practice Questions
Q1. Why is the refractory period important?
The absolute refractory period (when Na channels are inactivated and cannot reopen) ensures that the action potential travels in one direction only — it cannot re-stimulate the region it just passed through. The relative refractory period limits the maximum firing frequency of neurons (typically 500-1000 impulses per second). Without the refractory period, signals would bounce back and forth chaotically.
Q2. Explain why nerve impulses are faster in myelinated fibres.
Myelin insulates the axon membrane between nodes, preventing ion leakage. The action potential can only regenerate at the Nodes of Ranvier, so it effectively ‘jumps’ from node to node (saltatory conduction). This skips the slow continuous depolarisation that occurs in unmyelinated fibres. Speed in myelinated fibres: 70-120 m/s. In unmyelinated: 0.5-2 m/s.
Q3. A person has difficulty with balance and coordination but normal thinking and memory. Which part of the brain is likely affected?
The cerebellum. It controls balance, posture and coordination of voluntary movements. The cerebrum (which handles thinking and memory) is intact. Damage to the cerebellum causes ataxia — uncoordinated, clumsy movements — but does not affect intelligence or consciousness.
Q4. What is the role of acetylcholinesterase at the neuromuscular junction?
Acetylcholinesterase (AChE) is an enzyme in the synaptic cleft that rapidly breaks down acetylcholine (ACh) into choline and acetate. This terminates the signal and allows the muscle to relax. Without AChE, ACh would continuously stimulate the muscle, causing sustained contraction (this is how nerve agents and some pesticides work — they inhibit AChE).
FAQs
What is the difference between a nerve and a neuron?
A neuron is a single nerve cell. A nerve is a bundle of axons (from many neurons) wrapped in connective tissue, like a cable made of many wires. The sciatic nerve, for example, contains hundreds of thousands of individual axon fibres.
Why do neurons not divide?
Most mature neurons are permanently in G0 phase of the cell cycle — they have exited the division cycle. This is why brain and spinal cord injuries are so serious. Some neurogenesis (new neuron formation) occurs in the hippocampus and olfactory bulb, but it is very limited compared to other tissues.
What is a synapse?
A junction between two neurons (or between a neuron and an effector cell). Most synapses are chemical — they use neurotransmitters to cross the gap. A few are electrical — they use gap junctions for direct ionic communication (faster but less flexible). Chemical synapses allow for modulation, integration and one-way transmission.
How do drugs affect synapses?
Many drugs work by altering synaptic transmission. SSRIs (antidepressants) block serotonin reuptake, increasing serotonin in the cleft. Caffeine blocks adenosine receptors (a neuromodulator that promotes drowsiness). Curare blocks ACh receptors at the neuromuscular junction, causing paralysis. Understanding synapse pharmacology is a growing area in NEET questions.
Neural transmission is electricity piggy-backing on ion gradients. Once you see Na and K as the two workers doing opposite jobs, the chapter becomes intuitive.