Neural Control and Coordination Class 11 Notes

Overview of the Human Neural System

The neural system of all animals is composed of highly specialized cells called neurons, which can detect, receive, and transmit different kinds of stimuli. In lower invertebrates like Hydra, the neural network is simple, whereas insects show a more organized brain structure.

In humans, the neural system is divided into two major divisions:

1. Central Nervous System (CNS): Comprises the Brain and the Spinal Cord. It acts as the site of information processing and control.
2. Peripheral Nervous System (PNS): Comprises all nerves associated with the CNS. The nerve fibers of PNS are of two types: afferent fibers (transmit impulses from tissues/organs to CNS) and efferent fibers (transmit regulatory impulses from CNS to peripheral tissues).

The PNS is further divided into the Somatic Nervous System (relays impulses to skeletal muscles) and the Autonomic Nervous System (ANS, transmits impulses to involuntary organs and smooth muscles). The ANS is classified into Sympathetic and Parasympathetic nervous systems.

Important Definitions to Memorize

Neuron

The structural and functional unit of the neural system, consisting of three major parts: cell body (soma), dendrites, and an axon.

Synapse

A specialized junction through which a nerve impulse is transmitted from one neuron to another, formed by the membranes of a pre-synaptic and post-synaptic neuron.

Resting Membrane Potential

The electrical potential difference across the axonal membrane when a neuron is not conducting an impulse, typically measuring around -70 mV.

Action Potential

The rapid change in membrane potential (depolarization) that propagates along the surface of an axon when stimulated past its threshold.

Saltatory Conduction

The rapid jumping propagation of an action potential along myelinated axons from one Node of Ranvier to the next, drastically increasing conduction speed.

Reflex Arc

The entire neural pathway involved in a reflex action, consisting of at least one receptor, one afferent (sensory) neuron, an integration center (spinal cord), one efferent (motor) neuron, and an effector organ.

Mechanism of Nerve Impulse Conduction

1. Step 1: Polarized State (Resting State) — At rest, the axonal membrane is more permeable to K+ ions and nearly impermeable to Na+ ions. The sodium-potassium pump actively transports 3 Na+ ions outwards for every 2 K+ ions brought inwards. This creates a positive charge outside and a negative charge inside, establishing the resting potential (~ -70 mV).
2. Step 2: Depolarization (Action Potential Generation) — When a stimulus is applied, the Na+ channels open rapidly at the site of stimulus (Site A). Na+ rushes in down its concentration gradient, reversing the polarity at that localized site (inside becomes positive, outside negative). This transient reversal of potential is called the Action Potential (~ +30 mV).
3. Step 3: Propagation of the Impulse — The current flows on the inner surface from Site A to adjacent Site B, and on the outer surface from Site B to Site A. This completes the circuit and depolarizes Site B, moving the action potential forward down the axon.
4. Step 4: Repolarization — Very quickly, Na+ channels close, and voltage-gated K+ channels open. K+ diffuses outside the membrane to restore the negative resting potential inside the membrane. The original ionic concentration is restored by the Na+/K+ ATPase pump.

Comparison: Chemical vs. Electrical Synapses

Must Remember: Key Conceptual Points

• The human brain is protected by cranial meninges: the outer Dura mater, middle Arachnoid mater, and inner Pia mater.
• The Forebrain consists of the Cerebrum, Thalamus, and Hypothalamus. The cerebrum is divided into left and right hemispheres connected by the Corpus Callosum (a tract of nerve fibers).
• The Hypothalamus acts as the thermostat of the body, regulating body temperature, urge for eating and drinking, and secreting hypothalamic hormones.
• The Limbic System (formed by the inner parts of cerebral hemispheres, amygdala, and hippocampus) regulates sexual behavior, emotions (excitement, fear), and motivation.
• The Midbrain contains four round swellings called Corpora Quadrigemina on its dorsal portion, which are involved in visual and auditory reflexes.
• The Hindbrain comprises the Cerebellum (convoluted surface for extra neuron space, coordinates muscular balance), Pons (fiber tracts coordinating different regions), and Medulla Oblongata (controls respiration, cardiovascular reflexes, and gastric secretions).
• Nissl's granules are granular bodies composed of rough endoplasmic reticulum and ribosomes found in the cyton (cell body) and dendrites of a neuron, but are absent in the axon.
• Myelinated nerve fibers are wrapped with Schwann cells that produce myelin. The unmyelinated gaps between adjacent Schwann cells are called Nodes of Ranvier.

Exam Warning & Diagnostic Traps

1. The Sodium-Potassium Pump Ratio: Students often confuse the stoichiometry. Always remember: 3 Na+ are pumped OUT, and 2 K+ are pumped IN using 1 ATP molecule. If you reverse this in your subjective answer, you will lose full marks.
2. Nissl's Granules Location: Examiners love asking true/false or MCQ questions regarding Nissl's granules. They are present in the Dendrites and Soma (Cell Body), but completely absent in the Axon.
3. Reflex Arc Direction: Ensure your flow diagram is strictly unidirectional: Receptor -> Sensory (Afferent) Neuron -> Spinal Cord (Interneuron) -> Motor (Efferent) Neuron -> Effector Organ. Do not skip labeling the dorsal root ganglion (sensory input) and ventral root (motor output).

Quick Revision Check

• What is the function of the Corpus Callosum? The Corpus Callosum is a thick band of myelinated nerve fibers that connects the left and right cerebral hemispheres, facilitating interhemispheric communication.
• Why is the conduction of nerve impulses faster in myelinated fibers than in non-myelinated ones? In myelinated fibers, the ionic exchange takes place only at the Nodes of Ranvier, allowing the impulse to 'jump' from node to node (saltatory conduction), which is much faster than continuous conduction.
• Which part of the brain controls respiration, cardiovascular reflexes, and gastric secretions? The Medulla Oblongata, located in the hindbrain, contains vital centers that control cardiovascular reflexes, respiration, and gastric secretions.
• What triggers the release of neurotransmitters at a chemical synapse? The arrival of an action potential at the axon terminal depolarizes the membrane, opening voltage-gated Calcium (Ca2+) channels. The influx of Ca2+ triggers the synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft.

Frequently Asked Questions