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Q7:
Explain the role of $Na^+$ in the generation of action potential.
Solution :
1. Theoretical Foundation: The Resting Membrane Potential
In the resting state, the neuronal membrane is selectively permeable to $K^+$ ions while being nearly impermeable to $Na^+$ ions. The concentration of $K^+$ is high inside the axon, and the concentration of $Na^+$ is high in the extracellular fluid. This gradient is maintained by the $Na^+/K^+$ pump, which exports $3Na^+$ for every $2K^+$ imported, maintaining a resting potential of approximately $-70\text{ mV}$ [Per the Goldman-Hodgkin-Katz equation].
2. Step-by-Step Mechanism of Action Potential Generation
Step 1: Stimulus and Threshold Reach
When a stimulus is applied to a site on the membrane, the permeability to $Na^+$ increases significantly. If the stimulus strength reaches the threshold level (typically $-55\text{ mV}$), the voltage-gated $Na^+$ channels undergo a conformational change [Per the Hodgkin-Huxley model].
Step 2: Rapid Depolarization
Upon reaching the threshold, the influx of $Na^+$ ions into the intracellular space occurs rapidly due to both the concentration gradient and the electrical gradient (as the inside is negative). As $Na^+$ enters, the membrane potential shifts from negative to positive. This is defined as: $$\Delta V_m = V_{in} - V_{out} \rightarrow \text{towards } +30 \text{ to } +45\text{ mV}$$
Step 3: Positive Feedback Loop
The rapid influx of $Na^+$ further depolarizes the membrane, which triggers even more voltage-gated $Na^+$ channels to open. This self-propagating "Hodgkin Cycle" ensures the rapid rise of the action potential spike.
3. Comparative Ion Flux Dynamics
| Phase | $Na^+$ Channel Status | Membrane Potential Change |
|---|---|---|
| Resting | Closed | $-70\text{ mV}$ |
| Depolarization | Open (Rapid Influx) | Moves to $+30\text{ mV}$ |
| Repolarization | Inactivated | Return to $-70\text{ mV}$ |
4. Justification of the Role
The $Na^+$ ion serves as the primary driver of the upstroke of the action potential. Without the rapid influx of $Na^+$ ions, the membrane cannot achieve the critical positivity required to transmit an electrical impulse (nerve impulse) down the axon [Biological Law of All-or-None]. After reaching peak positivity, the $Na^+$ channels become inactivated, and voltage-gated $K^+$ channels open to facilitate repolarization.
Final Solution: The $Na^+$ ions act as the essential depolarizing agent; their rapid influx through voltage-gated channels, driven by electrochemical gradients, reverses the resting negative membrane potential to a positive state, thereby initiating the action potential necessary for neural signal propagation.
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(b): Explain the following processes: (b) Depolarisation 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.
- 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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