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Neural Control and Coordination - Neuron and nerves

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Neurons are the structural and functional units of the nervous system, composed of three main parts: the cell body (cyton), dendrites, and a long axon.

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The cell body contains cytoplasm, typical cell organelles, and specific granular bodies called Nissl's granules, which are sites of protein synthesis.

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Neurons are classified based on the number of axons and dendrites: Multipolar (one axon, two or more dendrites; found in cerebral cortex), Bipolar (one axon, one dendrite; found in retina), and Unipolar (cell body with one axon only; found in embryonic stage).

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Nerve fibers are of two types: Myelinated (enveloped with Schwann cells forming a myelin sheath around the axon) and Non-myelinated (Schwann cells present but do not form a sheath).

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The gaps between two adjacent myelin sheaths are called Nodes of Ranvier, which facilitate saltatory conduction of nerve impulses.

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Resting Membrane Potential: When a neuron is not conducting an impulse, the axonal membrane is more permeable to K+K^+ ions and nearly impermeable to Na+Na^+ ions. This results in a positive charge outside and a negative charge inside, maintaining a potential of approximately −70-70 mVmV.

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Action Potential (Depolarization): Upon stimulus, the membrane becomes freely permeable to Na+Na^+, leading to a rapid influx of Na+Na^+ and reversal of polarity (inside becomes positive).

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Synaptic Transmission: Impulses are transmitted across a synapse via neurotransmitters (chemical synapse) like Acetylcholine (AChACh) or directly via ion flow (electrical synapse).

📐Formulae

Resting Membrane Potential≈−70 mV\text{Resting Membrane Potential} \approx -70\text{ mV}

Action Potential (Peak)≈+30 to +45 mV\text{Action Potential (Peak)} \approx +30\text{ to } +45\text{ mV}

Sodium-Potassium Pump Ratio=3 Na+ (out):2 K+ (in)\text{Sodium-Potassium Pump Ratio} = 3\text{ Na}^+\text{ (out)} : 2\text{ K}^+\text{ (in)}

Velocity of Nerve Impulse∝Diameter of Axon\text{Velocity of Nerve Impulse} \propto \text{Diameter of Axon}

💡Examples

Problem 1:

Calculate the net movement of positive charges across the axonal membrane for every cycle of the Na+−K+Na^+-K^+ pump.

Solution:

The pump moves 3Na+3 Na^+ ions out and 2K+2 K^+ ions in.

Explanation:

Since 33 positive charges (3Na+3 Na^+) leave the cell and only 22 positive charges (2K+2 K^+) enter, there is a net loss of 11 positive charge from the intracellular fluid per cycle. This contributes to the negativity inside the resting neuron.

Problem 2:

Explain why the conduction of impulses is faster in myelinated neurons compared to non-myelinated neurons.

Solution:

Due to 'Saltatory Conduction'.

Explanation:

In myelinated fibers, the myelin sheath acts as an insulator. The action potential 'jumps' from one Node of Ranvier to the next, rather than traveling the entire length of the membrane. This mechanism significantly increases the speed of impulse transmission compared to the continuous conduction seen in non-myelinated fibers.

Problem 3:

What happens to the membrane potential during the repolarization phase?

Solution:

The membrane potential returns from a positive value (+30+30 mVmV) back toward the resting potential (−70-70 mVmV).

Explanation:

During repolarization, Na+Na^+ channels close and K+K^+ channels open. K+K^+ ions diffuse out of the axon along their concentration gradient, restoring the negative charge inside the membrane.

Neuron and nerves - Revision Notes & Key Diagrams | CBSE Class 11 Biology