Neural Control

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The nervous system and endocrine system together coordinate all body functions. The nervous system provides rapid, short-lived responses through electrical impulses, while the endocrine system produces slower, long-lasting effects through chemical messengers (hormones). Understanding neural control is essential for comprehending how the body senses, processes, and responds to stimuli.

Divisions of the Nervous System:

Division	Components	Function
CNS	Brain, Spinal cord	Integration and coordination
PNS	Cranial nerves (12 pairs), Spinal nerves (31 pairs)	Transmission
Somatic NS	Voluntary control of skeletal muscles	External environment response
Autonomic NS	Sympathetic, Parasympathetic, Enteric	Internal homeostasis

Neuron Structure:

• Cell body (soma): Contains nucleus, ribosomes, Nissl bodies (RER) — metabolic centre
• Dendrites: Receive incoming signals (receptive region)
• Axon: Transmits impulse away from cell body (conducting region)
• Axon terminals: Synaptic boutons — release neurotransmitters
• Myelin sheath: Lipid covering on axon (from oligodendrocytes in CNS, Schwann cells in PNS) — speeds conduction
• Nodes of Ranvier: Gaps in myelin (1-2 μm) — saltatory conduction occurs here

Types of Neurons:

• Sensory (afferent): Carry impulse from receptor to CNS
• Motor (efferent): Carry impulse from CNS to effector (muscle/gland)
• Interneurons: Connect sensory and motor neurons within CNS (most numerous — 99% of all neurons)

Resting Membrane Potential: The inside of a neuron is negative (-70 mV) relative to outside due to:

1. Na⁺/K⁺-ATPase pumps: 3 Na⁺ out, 2 K⁺ in (active transport, uses 1 ATP per cycle)
2. K⁺ leak channels: More K⁺ channels than Na⁺ channels → K⁺ diffuses out
3. Anions (proteins, Cl⁻) trapped inside cell

⚡ Exam Tip: The Na⁺/K⁺-ATPase is an electrogenic pump (transports more positive charges out than in), contributing directly to the negative resting potential. It maintains concentration gradients — without it, K⁺ would equilibrate and the membrane would depolarize.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding and problem-solving practice.

Action Potential:

Rapid, all-or-none depolarization that travels along axon:

1. Resting state: Voltage-gated Na⁺ channels closed; K⁺ channels closed; membrane at -70 mV
2. Depolarization: Stimulus reaches threshold (-55 mV) → Na⁺ channels open → Na⁺ rushes in → membrane potential rises to +30 mV
3. Repolarization: Na⁺ channels inactivate; K⁺ channels open → K⁺ exits → membrane potential returns toward resting
4. Hyperpolarization: K⁺ channels remain open briefly → membrane potential dips slightly below -70 mV (overshoot)
5. Refractory period: Na⁺/K⁺-ATPase restores ionic gradients; cannot fire another AP during absolute refractory period

Key Ion Equations:

• Equilibrium potential for Na⁺ (E_Na): +67 mV (inside positive due to high extracellular Na⁺)
• Equilibrium potential for K⁺ (E_K): -98 mV (inside negative due to high intracellular K⁺)
• At threshold (-55 mV), Na⁺ channels open; at +30 mV, they inactivate

Conduction Velocity:

Type	Diameter	Myelin	Speed (m/s)
Type A (α)	12-20 μm	Myelinated	70-120
Type B	3-6 μm	Myelinated	15-40
Type C	0.5-1.5 μm	Unmyelinated	0.5-2
Squid giant axon	500 μm	None	25

Saltatory conduction: Nerve impulse jumps from one Node of Ranvier to the next (nodes have high density of voltage-gated channels). This is 20× faster than continuous conduction in unmyelinated fibres of similar diameter.

Synapse Structure:

Presynaptic terminal contains:

• Synaptic vesicles (filled with neurotransmitter)
• Mitochondria
• Voltage-gated Ca²⁺ channels

Synaptic cleft: 20-30 nm gap between presynaptic and postsynaptic membranes.

Postsynaptic membrane has:

• Neurotransmitter receptors (ligand-gated ion channels or GPCRs)
• Postsynaptic density (protein scaffolding)

Neurotransmitters:

Neurotransmitter	Location	Effect
Acetylcholine (ACh)	NMJ, Parasympathetic synapses	Excitatory (muscle) or inhibitory
Noradrenaline	Sympathetic postganglionic	Excitatory
Dopamine	CNS (substantia nigra, VTA)	Inhibitory or modulatory
Serotonin	CNS (raphe nuclei)	Inhibitory, mood regulation
GABA	CNS	Inhibitory (Cl⁻ influx)
Glutamate	CNS	Excitatory (AMPA, NMDA, kainate receptors)
Endorphins	CNS	Analgesia

⚡ NEET-Specific Tip: Remember the sequence of events at a synapse:

1. Action potential arrives at axon terminal
2. Voltage-gated Ca²⁺ channels open (depolarization opens these channels)
3. Ca²⁺ enters terminal
4. Vesicles fuse with presynaptic membrane (Ca²⁺ triggers exocytosis)
5. Neurotransmitter released into synaptic cleft
6. Binds postsynaptic receptors
7. Postsynaptic potential generated (EPSP or IPSP)
8. Neurotransmitter degraded/reabsorbed/recycled

Common Student Mistakes:

• Confusing EPSP (excitatory postsynaptic potential) with action potential
• Forgetting that a synapse is one-way (information flows from presynaptic to postsynaptic)
• Mixing up saltatory vs continuous conduction

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory with derivations and exam pattern analysis.

Postsynaptic Potentials:

Excitatory Postsynaptic Potential (EPSP): Depolarizing response (Na⁺ or Ca²⁺ influx through ligand-gated channels). Graded, decays with distance, sums temporally (multiple inputs rapidly) or spatially (multiple inputs simultaneously).

Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizing response (Cl⁻ influx or K⁺ efflux through ligand-gated channels). Also graded.

Summation: One EPSP is usually subthreshold — not enough to trigger action potential. Spatial summation (multiple synapses on same neuron) or temporal summation (rapid inputs from one synapse) of EPSPs can reach threshold.

Reflex Arc:

The basic unit of neural function. Components: Receptor → Sensory neuron → CNS (interneuron) → Motor neuron → Effector

Patellar reflex (knee-jerk): Stretch receptor in quadriceps → sensory neuron → spinal cord → motor neuron → quadriceps contraction. This is a monosynaptic reflex (only one synapse between sensory and motor neuron).

Flexor withdrawal reflex: More complex — involves interneurons, antagonistic muscles, crossed extensor reflex.

Central Nervous System:

Brain regions and functions:

• Cerebrum: Higher functions (consciousness, memory, language, reasoning)
• Cerebellum: Motor coordination, balance, posture
• Thalamus: Sensory relay station (except smell)
• Hypothalamus: Homeostatic control (temperature, hunger, thirst, endocrine control via pituitary)
• Brainstem: Vital functions (respiratory, cardiovascular)
• Spinal cord: Reflex arcs, conduction pathways

Meninges: Dura mater (outer), Arachnoid mater (middle), Pia mater (inner). CSF fills subarachnoid space.

Autonomic Nervous System:

Sympathetic (fight-or-flight): Origin: T1-L2 (thoracolumbar). Postganglionic neurotransmitter: Noradrenaline (except sweat glands which use ACh). Effects: ↑ Heart rate, ↑ BP, bronchodilation, ↓ digestion.

Parasympathetic (rest-and-digest): Origin: Cranial nerves III, VII, IX, X and S2-S4 (craniosacral). Postganglionic neurotransmitter: ACh. Effects: ↓ Heart rate, ↓ BP, bronchoconstriction, ↑ digestion.

Acetylcholine Receptors:

• Nicotinic (nAChR): Ligand-gated cation channels; found at NMJ (on muscle end plate), autonomic ganglia. Activated by nicotine, ACh.
• Muscarinic (mAChR): GPCRs; found in parasympathetic target organs. M₂ (heart, decreases HR); M₃ (glands, smooth muscle). Activated by muscarine, ACh.

Long-Term Potentiation (LTP) and Memory:

LTP: Prolonged enhancement of synaptic transmission following high-frequency stimulation. Key mechanism in hippocampal learning and memory.

NMDA receptors are crucial for LTP: At resting potential, Mg²⁺ blocks NMDA channel. When EPSP depolarizes postsynaptic cell AND glutamate binds to NMDA receptor, Mg²⁺ is removed → Ca²⁺ enters → intracellular signalling cascades (CaMKII, CREB) → insertion of more AMPA receptors → enhanced synaptic transmission.

Action Potential Propagation:

Velocity equation (cable theory): $v \propto \sqrt{d/r}$ where d = diameter, r = internal resistance. Larger diameter = faster conduction (less internal resistance). Myelination further increases velocity by allowing saltatory conduction.

Nodes of Ranvier: 1-2 μm gaps. Voltage-gated Na⁺ channels are highly concentrated at nodes. AP regenerated at each node.

Neurological Disorders:

Disorder	Cause	Features
Multiple sclerosis	Autoimmune demyelination of CNS axons	Progressive paralysis, sensory loss
Myasthenia gravis	Autoantibodies against nAChR at NMJ	Fatigable muscle weakness
Parkinson’s disease	Loss of dopaminergic neurons in substantia nigra	Tremor, rigidity, bradykinesia
Alzheimer’s disease	Amyloid-β plaques, tau tangles	Memory loss, cognitive decline

NEET Previous Year Patterns (2019-2024):

• 2019: Action potential phases and ion movements (Na⁺ influx during depolarization, K⁺ efflux during repolarization) (3 marks)
• 2020: Synapse structure and neurotransmitter release sequence (Ca²⁺ entry triggering exocytosis) (3 marks)
• 2021: Sympathetic vs parasympathetic effects on heart rate and blood pressure (2 marks)
• 2022: Saltatory vs continuous conduction comparison (2 marks)
• 2023: Resting membrane potential establishment (Na⁺/K⁺-ATPase role) (2 marks)
• 2024: Synaptic integration and EPSP summation (spatial vs temporal) (3 marks)

⚡ Advanced Tip: For NEET synapse questions, remember that EPSPs and IPSPs are graded potentials — they decay with distance and time, and their amplitudes add. Whether a neuron fires depends on whether the net postsynaptic potential at the axon hillock (trigger zone) reaches threshold. The trigger zone has a lower threshold than the rest of the neuron, which is why summation happens there.

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📊 NEET UG Exam Essentials

Detail	Value
Questions	200 (180 mandatory + 10 optional)
Time	3h 20min
Marks	720
Section	Physics (50), Chemistry (50), Biology (100)
Negative	−1 for wrong answer
Qualifying	50th percentile (general category)

🎯 High-Yield Topics for NEET UG

• Human Physiology — 18 marks
• Genetics & Evolution — 16 marks
• Ecology & Environment — 12 marks
• Organic Chemistry (Reactions) — 15 marks
• Electrodynamics (Physics) — 18 marks
• Chemical Equilibrium — 10 marks

📝 Previous Year Question Patterns

• Q: “A particle moves in a circle…” [2024 Physics — 2 marks]
• Q: “Identify the incorrect statement about DNA…” [2024 Biology — 4 marks]
• Q: “The major product ofFriedel-Crafts acylation is…” [2024 Chemistry — 3 marks]

💡 Pro Tips

• NCERT Biology is the single most important resource — 80%+ questions are from NCERT lines
• Focus on Human Physiology, Genetics, and Ecology — together they make ~40% of Biology
• In Physics, master Electrostatics + Current Electricity + Magnetism (combined ~20%)
• Organic Chemistry: learn named reactions with mechanisms — they repeat across years

🔗 Official Resources

• NTA Official
• NEET Syllabus PDF

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