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Q3(b):
Explain the following processes: (b) Depolarisation of the membrane of a nerve fibre
Solution :
Theoretical Foundation: The Resting Membrane Potential
Before the initiation of an action potential, the axonal membrane exists in a state of polarized equilibrium. At rest, the axoplasm contains a high concentration of $K^+$ and negatively charged proteins, while the extracellular fluid contains a high concentration of $Na^+$. Due to the selective permeability of the membrane—maintained largely by the $Na^+/K^+$ ATPase pump—the resting membrane potential is approximately $-70\,mV$ [Per the Goldman-Hodgkin-Katz equation].
Step 1: Application of Stimulus and Permeability Shift
When a stimulus of threshold intensity is applied to the nerve fibre, the voltage-gated $Na^+$ channels undergo a conformational change. This leads to a rapid increase in the permeability of the membrane to sodium ions ($Na^+$). [Justification: Ion channels operate via electrochemical gradients, where $Na^+$ ions flow from a region of high concentration to low concentration].
Step 2: The Influx of Sodium Ions ($Na^+$)
As the $Na^+$ channels open, sodium ions rush into the neuron down their concentration gradient. This influx of positively charged ions neutralizes the negative charge on the inner surface of the membrane. The membrane potential shifts from its resting state ($-70\,mV$) toward zero and eventually becomes positive. This transition to a positive internal charge is defined as Depolarisation.
Step 3: Analytical Data of the Action Potential
| Phase | Membrane Potential ($mV$) | Ion Movement |
|---|---|---|
| Resting Potential | $-70\,mV$ | $Na^+/K^+$ Pump Active |
| Depolarisation | $-70\,mV \rightarrow +30\,mV$ | Rapid $Na^+$ Influx |
| Repolarisation | $+30\,mV \rightarrow -70\,mV$ | $K^+$ Efflux |
Step 4: The All-or-None Principle
Depolarisation only proceeds to completion if the stimulus reaches a critical "threshold" level (typically $-55\,mV$). If the stimulus is sub-threshold, the membrane remains at its resting potential. Once the threshold is breached, the massive influx of $Na^+$ triggers an Action Potential, which propagates along the axon via local current circuits [Per the Hodgkin-Huxley model of membrane excitability].
Final Solution: Depolarisation is the process wherein the sudden increase in the permeability of the axonal membrane to $Na^+$ ions results in a rapid influx of $Na^+$ into the axoplasm, causing the membrane potential to change from negative to positive, effectively triggering an action potential across the nerve fibre.
More Questions from Class 11 Biology Neural Control and Coordination EXERCISES
- Q1: Briefly describe the structure of the Brain
- Q2(a): Compare the following: (a) Central neural system (CNS) and Peripheral neural system (PNS)
- Q2(b): Compare the following: (b) Resting potential and action potential
- Q3(a): Explain the following processes: (a) Polarisation of the membrane of a nerve fibre
- Q3(c): Explain the following processes: (c) Transmission of a nerve impulse across a chemical synapse
- Q4(a): Draw labelled diagrams of the following: (a) Neuron
- Q4(b): Draw labelled diagrams of the following: (b) Brain
- Q5(a): Write short notes on the following: (a) Neural coordination
- Q5(b): Write short notes on the following: (b) Forebrain
- Q5(c): Write short notes on the following: (c) Midbrain
- Q5(d): Write short notes on the following: (d) Hindbrain
- Q5(e): Write short notes on the following: (e) Synapse
- Q6: Give a brief account of Mechanism of synaptic transmission.
- Q7: Explain the role of $Na^+$ in the generation of action potential.
- Q8(a): Differentiate between: (a) Myelinated and non-myelinated axons
- Q8(b): Differentiate between: (b) Dendrites and axons
- Q8(c): Differentiate between: (c) Thalamus and Hypothalamus
- Q8(d): Differentiate between: (d) Cerebrum and Cerebellum
- Q9(a): Answer the following: (a) Which part of the human brain is the most developed?
- Q9(b): Answer the following: (b) Which part of our central neural system acts as a master clock?
- Q10(a): Distinguish between: (a) afferent neurons and efferent neurons
- Q10(b): Distinguish between: (b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre
- Q10(f): Distinguish between: (f) cranial nerves and spinal nerves.
CBSE Solutions for Class 11 Biology Neural Control and Coordination
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Chemical Coordination and Integration
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