Nervous System and Reflex Action
Living organisms respond dynamically to environmental stimuli through electrical impulses (nervous system) and chemical messengers (endocrine hormones).
Anatomy of a Neuron (Nerve Cell):
- Dendrites: Branched protoplasmic projections that acquire environmental chemical signals and convert them into electrical impulses.
- Cyton (Cell Body): Contains nucleus and granular cytoplasm; processes incoming impulses.
- Axon: Long, slender cylindrical projection insulated by a myelin sheath that conducts nerve impulses away from the cell body.
- Synapse: Sub-microscopic gap between the axon terminal of one neuron and the dendrite of the adjacent neuron. When the electrical impulse reaches the terminal, it triggers the release of chemical neurotransmitters (e.g., acetylcholine) that diffuse across the synapse.
Stimulus ---> Receptor ---> Sensory Neuron ---> Relay Neuron (Spinal Cord)
---> Motor Neuron ---> Effector (Muscle/Gland) ---> ResponseAnatomy of the Human Brain
The central nervous system consists of the Brain and Spinal Cord, protected by the skull (cranium), vertebral column, and three layers of protective membranes called meninges cushioned by cerebrospinal fluid (CSF).
| Brain Division | Major Components | Primary Functions |
|---|---|---|
| Forebrain | Cerebrum | Largest part; site of thinking, memory, intelligence, sensory interpretation (vision, hearing, smell), conscious voluntary actions. |
| Midbrain | Tectum & Tegmentum | Relay center for involuntary visual and auditory reflex movements of neck and trunk. |
| Hindbrain | Cerebellum | Coordinates voluntary muscular activities; maintains precision, posture, and body balance. |
| Hindbrain | Pons | Relays signals between cortex and cerebellum; regulates respiration rate. |
| Hindbrain | Medulla Oblongata | Involuntary reflex actions: heartbeat, blood pressure, peristalsis, swallowing, coughing, vomiting. |
Plant Hormones & Directional Tropisms
Plants lack a nervous system; they coordinate growth through chemical plant growth regulators (phytohormones):
- Auxins: Synthesized at shoot tips; promote cell elongation. When unidirectional light strikes a shoot, auxins diffuse to the shaded side, stimulating faster elongation and causing the shoot to bend towards light (Phototropism).
- Gibberellins: Promote stem elongation, seed germination, and break seed dormancy.
- Cytokinins: Promote active cell division; present in high concentrations in fruits and seeds; delay leaf senescence.
- Abscisic Acid (ABA): Stress hormone and growth inhibitor; triggers stomatal closure during drought and causes wilting/falling of leaves.
- Ethylene: Gaseous hormone that accelerates fruit ripening.
Human Endocrine Glands & Hormones
| Gland | Hormone Secreted | Major Biological Function | Clinical Consequence |
|---|---|---|---|
| Pituitary | Growth Hormone (GH) | Regulates body growth and bone development | Dwarfism (Deficiency) / Gigantism (Excess) |
| Thyroid | Thyroxine | Regulates carbohydrate, protein, and fat metabolism | Goitre (Swollen neck due to dietary Iodine deficiency) |
| Pancreas | Insulin | Lowers blood glucose by driving glucose uptake into cells | Diabetes Mellitus (High blood sugar) |
| Adrenal | Adrenaline | Prepares body for "Fight or Flight"; elevates heart rate, breathing, and blood pressure | Stress response regulation |
| Testes | Testosterone | Male secondary sexual characteristics and sperm production | Pubertal development |
| Ovaries | Estrogen & Progesterone | Female secondary sexual characteristics and uterine lining maintenance | Menstrual cycle regulation |
