Authored by subject matter experts. Content strictly validated against latest NCERT rationalized curriculum and official Board Marking Schemes.
📑 In This Chapter Guide (Table of Contents)
1. Nervous System: Structure of Neuron & Nerve Impulse Transmission
The nervous system provides rapid, point-to-point electrical and chemical communication across the human body using specialized cells called Neurons.
Anatomy of a Typical Neuron:
- Dendrites: Fine branched terminal fibers that detect environmental stimuli and initiate electrical impulses.
- Cell Body (Cyton): Contains the nucleus and metabolic cytoplasm; processes signals.
- Axon: Long cylindrical cytoplasmic extension that transmits the electrical impulse away from the cell body over long distances. Covered by insulating myelin sheath.
- Synapse: Microscopic gap between the axon terminal of one neuron and the dendrite tip of the next neuron. When an electrical impulse reaches the axon tip, it triggers the release of chemical neurotransmitters (like acetylcholine) which diffuse across the synapse to stimulate a fresh electrical impulse in the next neuron.
2. The Reflex Arc: Immediate Protective Response
A Reflex Action is an involuntary, instantaneous, and unconscious response to an external stimulus that safeguards the body from injury (e.g., pulling your hand back automatically when touching a blazing hot stove).
Sequence of the Reflex Arc:
Why Spinal Cord Mediates Reflexes: Generating a conscious thought via the brain takes several hundred milliseconds. The spinal cord processes reflex loops locally to execute immediate protective action before pain is consciously registered by the brain cerebrum.
3. Human Brain: Structure and Functions
The human brain is protected inside the bony cranium (skull) and surrounded by three protective meningeal membranes cushioned with shock-absorbing Cerebrospinal Fluid (CSF).
| Brain Region | Anatomical Parts | Primary Control Functions |
|---|---|---|
| Forebrain | Cerebrum (Largest region) | Seat of intelligence, conscious thought, memory, voluntary muscle actions, interpretation of sensory inputs (sight, hearing, smell, taste, touch), and hunger satiety centers. |
| Midbrain | Optic tectum & cerebral peduncles | Controls visual and auditory reflex movements of head, neck, and trunk; pupil dilation reflexes. |
| Hindbrain |
1. Cerebellum 2. Medulla Oblongata 3. Pons |
1. Precision of voluntary movements, maintaining body posture, equilibrium, and balance (walking in a straight line, riding a bicycle). 2. Involuntary life reflexes: blood pressure, peristalsis, salivation, vomiting, coughing, and swallowing. 3. Respiratory rhythm regulation and relaying impulses between cerebrum and cerebellum. |
4. Plant Hormones (Phytohormones) & Tropic Movements
Plants lack a nervous system; coordination is achieved chemically via Phytohormones:
- Auxin: Synthesized at shoot tips. Promotes cell elongation. Responsible for phototropism (accumulates on the shaded side of shoot, causing shaded cells to grow longer so shoot bends towards light).
- Gibberellins: Promotes stem elongation, breaking seed dormancy, and fruit enlargement.
- Cytokinins: Promotes rapid cell division; concentrated in regions of active growth like fruits and seeds. Delays leaf senescence.
- Abscisic Acid (ABA): Stress hormone; inhibits growth, causes wilting of leaves, and induces stomatal closure during water scarcity.
Tropic Movements (Directional Growth): Phototropism (light), Geotropism (gravity - roots positive, shoots negative), Hydrotropism (water), and Chemotropism (growth of pollen tube towards ovule driven by chemical attractants).
💡 Frequently Asked Questions (FAQ)
❓ How do endocrine hormones coordinate actions compared to electrical nerve impulses?
Nerve impulses travel rapidly via axons but reach only specific cells connected by neural synapses, and cannot provide continuous long-term stimulation. Hormones are chemical messengers secreted directly into the bloodstream, reaching all tissues to regulate gradual, long-term metabolic changes such as growth, sugar balance, and sexual maturity.
❓ Why is iodized salt recommended in our daily diet?
Iodine is essential for the thyroid gland to synthesize the hormone Thyroxine. Thyroxine regulates carbohydrate, protein, and fat metabolism for optimal growth and development. Iodine deficiency impairs thyroxine synthesis, causing enlargement of the thyroid gland known as Goitre.
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