Neural Control and Coordination: Class 11 Biology NCERT Guide

The Neuron: The Basic Unit of the Nervous System

The entire nervous system is built from specialized cells called neurons. Understanding their structure is the first step to understanding how they function. Each neuron has three main parts:

1. Cell Body (Cyton or Soma): This is the main metabolic center of the neuron. It contains the nucleus and other major organelles like the cytoplasm, Nissl's granules (which are clumps of rough ER and free ribosomes involved in protein synthesis), and mitochondria.

1. Dendrites: These are short, branched fibers that project out from the cell body. Their primary function is to receive signals (nerve impulses) from other neurons or from sensory receptors and transmit them towards the cell body.

1. Axon: This is a single, long fiber that extends from the cell body. Its job is to carry nerve impulses away from the cell body to other neurons, muscles, or glands. The end of the axon branches into terminal buttons (axon terminals) which contain synaptic vesicles filled with chemicals called neurotransmitters. The axon may be covered by a fatty sheath called the myelin sheath, which is formed by Schwann cells in the PNS and oligodendrocytes in the CNS. This sheath is not continuous and has gaps called Nodes of Ranvier. Myelinated axons transmit impulses much faster than unmyelinated ones.

Generation and Conduction of a Nerve Impulse

1. Step 1: Resting Potential (Polarised State) — When a neuron is not conducting any impulse, it is in a 'resting state'. The axonal membrane is more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+). The sodium-potassium pump actively transports 3 Na+ ions outwards for every 2 K+ ions it pumps inwards. This creates a concentration gradient and an electrical potential difference across the membrane, making the inside of the axon negative relative to the outside. This is called the resting potential, typically around -70mV.
2. Step 2: Action Potential (Depolarisation) — When a stimulus is applied, the permeability of the membrane to Na+ ions suddenly increases at that site. Voltage-gated Na+ channels open, and Na+ ions rush into the axon. This rapid influx of positive charge causes the inside of the membrane to become positive relative to the outside. This reversal of polarity is called depolarisation, and the electrical potential generated is known as the action potential.
3. Step 3: Repolarisation — Immediately following depolarisation, the Na+ channels close. The membrane's permeability to K+ ions increases as voltage-gated K+ channels open. K+ ions diffuse out of the axon, down their concentration gradient. This outflow of positive charge restores the negative charge on the inside of the membrane, returning it to its resting potential. This process is called repolarisation.
4. Step 4: Propagation of the Impulse — The action potential at one site acts as a stimulus for the adjacent region of the membrane, causing it to depolarise. This creates a wave of depolarisation that travels down the length of the axon. In myelinated axons, the impulse jumps from one Node of Ranvier to the next in a process called saltatory conduction, which is much faster than conduction in unmyelinated axons.

Worked Example: Transmission Across a Chemical Synapse

• Problem: Explain the sequence of events that occur during the transmission of a nerve impulse across a chemical synapse. Solution: A synapse is the junction between two neurons. Here's the step-by-step process of chemical transmission: Step 1: Arrival of Action Potential: The nerve impulse (action potential) travels along the axon of the pre-synaptic neuron and reaches the axon terminal. Step 2: Influx of Calcium Ions: The arrival of the action potential stimulates voltage-gated calcium ion (Ca²⁺) channels in the pre-synaptic membrane to open. Ca²⁺ ions from the synaptic cleft diffuse into the axon terminal. Step 3: Release of Neurotransmitters: The influx of Ca²⁺ causes the synaptic vesicles (containing neurotransmitters like acetylcholine) to move towards the pre-synaptic membrane, fuse with it, and release their contents into the synaptic cleft by exocytosis. Step 4: Binding to Receptors: The released neurotransmitter molecules diffuse across the narrow synaptic cleft and bind to specific receptor proteins located on the post-synaptic membrane of the next neuron. * Step 5: Generation of New Potential: This binding opens ion channels on the post-synaptic membrane, allowing ions (like Na⁺) to enter the post-synaptic neuron. This generates a new potential, which can be either excitatory (leading to a new action potential) or inhibitory (making it harder to generate an action potential). The neurotransmitter is then either broken down by an enzyme or reabsorbed by the pre-synaptic neuron.

Exam Traps and Key Distinctions

In exams, students often confuse the roles of ions and the direction of signals. Pay close attention to these points:

• Resting vs. Action Potential: Remember, Resting state = K⁺ leaks out, Na⁺/K⁺ pump active, inside is negative. Action potential = Na⁺ rushes in, inside becomes positive.
• Afferent vs. Efferent Neurons: A simple trick is Afferent neurons Arrive at the CNS (sensory), while Efferent neurons Exit the CNS (motor).
• Saltatory Conduction: This term applies only to myelinated axons. The impulse 'jumps' between the Nodes of Ranvier, making it much faster. Don't apply this concept to unmyelinated axons, where the impulse travels continuously along the membrane.
• Synapse vs. Neuromuscular Junction (NMJ): A synapse is a junction between two neurons. An NMJ is a specific type of synapse between a motor neuron and a muscle fiber.

Practice Questions with Solutions

• Q: Why is the inside of a neuron's membrane negative during the resting state? A: Step 1: Identify the key players. The main ions involved are Sodium (Na+) and Potassium (K+), and the sodium-potassium pump is crucial. Step 2: Describe the permeability of the membrane. In the resting state, the axonal membrane is significantly more permeable to K+ ions than to Na+ ions. This allows K+ ions to diffuse out of the cell down their concentration gradient. Step 3: Explain the role of the Na+/K+ pump. This active transport pump continuously ejects 3 Na+ ions for every 2 K+ ions it brings into the cell. This results in a net loss of positive charge from the inside. Step 4: Conclude the effect. The combination of K+ ions leaking out and the unequal pumping of positive ions makes the inside of the membrane negatively charged relative to the outside. Also, negatively charged proteins inside the axon contribute to the negative potential. Final answer: The resting membrane potential is negative primarily due to the outward diffusion of K+ ions through leak channels and the action of the Na+/K+ pump, which pumps more positive ions out than in, combined with the presence of negatively charged proteins inside the axon.
• Q: What is a reflex arc? Describe the pathway of a simple reflex, such as withdrawing your hand from a flame. A: Step 1: Define a reflex arc. A reflex arc is the neural pathway that controls a reflex action. It is an involuntary and rapid response to a stimulus. Step 2: Describe the components of the pathway. The pathway consists of five main components: a receptor, a sensory (afferent) neuron, an interneuron (in the spinal cord), a motor (efferent) neuron, and an effector. Step 3: Trace the signal for the specific example. When you touch a flame (stimulus), thermoreceptors (receptor) in your skin are activated. An impulse is generated and transmitted along a sensory neuron to the spinal cord. Step 4: Explain the processing in the CNS. In the spinal cord, the sensory neuron synapses with an interneuron, which in turn synapses with a motor neuron. This processing happens without involving the brain for speed. Step 5: Describe the response. The motor neuron carries the impulse from the spinal cord to the effector, which is the biceps muscle in your arm. The muscle contracts, causing you to withdraw your hand. Final answer: A reflex arc is the pathway of a reflex action. For hand withdrawal, the pathway is: Receptor (skin) → Sensory Neuron → Spinal Cord (Interneuron) → Motor Neuron → Effector (muscle), causing rapid withdrawal.
• Q: Differentiate between the cerebrum and cerebellum based on their primary functions. A: Step 1: State the primary function of the cerebrum. The cerebrum is the largest part of the brain and is the center for higher-order thinking, intelligence, memory, consciousness, voluntary actions, and processing sensory information like touch, sight, and sound. Step 2: State the primary function of the cerebellum. The cerebellum, located at the back of the brain, is primarily responsible for coordinating voluntary movements, posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity. It does not initiate movement but fine-tunes it. Final answer: The cerebrum is responsible for conscious thought, intelligence, and voluntary action initiation, while the cerebellum is responsible for the coordination, precision, and timing of movement, as well as maintaining posture and balance.
• Q: How does myelination affect the speed of nerve impulse conduction? A: Step 1: Define myelin sheath. The myelin sheath is a fatty insulating layer that surrounds the axons of many neurons. It is not continuous and has gaps called the Nodes of Ranvier. Step 2: Explain how insulation works. The myelin sheath acts as an electrical insulator, preventing the flow of ions across the axonal membrane in the myelinated regions. Step 3: Describe where the action potential is generated. Because of this insulation, the action potentials can only be generated at the unmyelinated Nodes of Ranvier, where the voltage-gated ion channels are concentrated. Step 4: Explain saltatory conduction. As a result, the nerve impulse 'jumps' from one node to the next. This mode of transmission is called saltatory conduction. Final answer: Myelination drastically increases the speed of nerve impulse conduction. The insulating myelin sheath allows the impulse to jump from one Node of Ranvier to the next (saltatory conduction), which is much faster than the continuous propagation along an unmyelinated axon.

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