In multicellular organisms, control and coordination ensure that different organ systems work together in a well-organised manner. This is achieved through the nervous system and the endocrine (hormonal) system.
Nervous System in Humans
- Neuron (Nerve cell): The basic structural and functional unit of the nervous system.
- Dendrites: Receive signals
- Cell body (Cyton): Contains nucleus
- Axon: Transmits signals away from cell body
- Myelin sheath: Insulation around axon, speeds up transmission
- Synapse: Junction between two neurons; chemical neurotransmitters (e.g., acetylcholine) carry signals across the gap.
Types of Neurons: Sensory (receptor to CNS), Motor (CNS to effector), Interneurons (within CNS)
Central Nervous System (CNS): Brain + Spinal cord
Peripheral Nervous System (PNS): All nerves outside CNS
- Brain Regions:
- Cerebrum: Thinking, memory, voluntary actions, intelligence
- Cerebellum: Balance, posture, coordination of movement
- Medulla oblongata: Involuntary actions (heartbeat, breathing, vomiting)
- Reflex Action: Rapid, automatic, involuntary response to a stimulus. The pathway is called the reflex arc.
- Receptor → Sensory neuron → Spinal cord (relay neuron) → Motor neuron → Effector (muscle/gland)
Endocrine System (Chemical Coordination)
Hormones: Chemical messengers secreted by endocrine glands directly into the bloodstream.
Properties: Produced in small quantities, act on specific target organs, slow but long-lasting effect.
- Key Hormones and Glands:
- Pituitary gland (master gland): GH (growth hormone), TSH, ACTH, FSH, LH
- Thyroid: Thyroxine — regulates metabolism; iodine deficiency → goitre
- Adrenal (suprarenal) gland: Adrenaline — "fight or flight" hormone; increases heart rate, BP, blood glucose
- Pancreas: Insulin (lowers blood glucose) and Glucagon (raises blood glucose)
- Testes: Testosterone — male secondary sexual characteristics
- Ovaries: Oestrogen, Progesterone — female secondary sexual characteristics and menstrual cycle
- Pineal gland: Melatonin — regulates sleep-wake cycle
Feedback Mechanism: Hormonal levels are regulated by negative feedback.
Coordination in Plants
Plants do NOT have a nervous system. They respond to stimuli through plant hormones (phytohormones).
- Types of Plant Movements:
- Tropic movements (tropisms): Growth response towards/away from stimuli; directional and permanent.
- Phototropism (light), Geotropism (gravity), Hydrotropism (water), Thigmotropism (touch)
- Nastic movements: Non-directional, independent of stimulus direction.
- Example: Mimosa pudica (touch-me-not) — leaves fold on touch
- Plant Hormones:
- Auxin (IAA): Promotes cell elongation, phototropism (more auxin on shaded side → bends toward light)
- Gibberellins: Promote stem elongation, fruit development, seed germination
- Cytokinins: Promote cell division, delay ageing
- Abscisic acid (ABA): Inhibits growth, promotes dormancy, closes stomata during drought
- Ethylene: Promotes fruit ripening
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Describe the reflex arc when you accidentally touch a hot object.
- Stimulus (heat) → Pain receptors in skin → Sensory neuron to spinal cord → Relay neuron → Motor neuron → Muscle of arm contracts → Hand withdraws.
- The brain receives information AFTER the reflex — it is not needed for the response.
How does adrenaline prepare the body for emergency?
- Adrenaline (epinephrine) is secreted by adrenal medulla during stress/danger.
- Effects: Heart rate increases, pupils dilate, blood flow to muscles increases, blood glucose rises (glycogen → glucose).
- This is the "fight or flight" response.
What is the role of iodine in thyroid function?
- Thyroxine hormone requires iodine for its synthesis.
- Iodine deficiency → thyroid gland enlarges to absorb more iodine → goitre (swelling in neck).
- Iodised salt prevents iodine deficiency.
Explain how insulin and glucagon work as antagonistic hormones.
- After a meal: blood glucose rises → pancreas secretes insulin → cells take up glucose → blood glucose falls.
- During fasting: blood glucose falls → pancreas secretes glucagon → liver releases glucose from glycogen → blood glucose rises.
- Diabetes mellitus occurs when insulin is insufficient or ineffective.
How does auxin cause phototropism in plants?
- Sunlight falls on one side of a shoot.
- Auxin migrates to the shaded side (away from light).
- Higher auxin concentration on shaded side → cells elongate more on that side.
- Shoot bends towards the light source (positive phototropism).
What is thigmotropism? Give an example.
- Thigmotropism is growth response to touch.
- The tendril of a pea plant wraps around a support because cells on the contact side grow slower than cells on the free side.
- This differential growth causes the tendril to curl around the support.
Why is the pituitary gland called the master gland?
- The pituitary gland secretes trophic hormones (TSH, ACTH, FSH, LH) that regulate other endocrine glands.
- Example: TSH stimulates thyroid to produce thyroxine. If thyroxine levels drop, more TSH is released (negative feedback).
- It also produces GH, ADH, and oxytocin directly.
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Common mistakes
- Nervous vs. hormonal coordination: Nervous = fast, short-lived, electrical; Hormonal = slow, long-lasting, chemical.
- Tropic vs. nastic: Tropic movements are directional (towards/away from stimulus); nastic movements are non-directional.
- Reflex arc: Spinal cord mediates reflex, NOT the brain (though brain is later informed).
Summary
The nervous system provides fast electrical coordination via neurons and synapses. Reflex arcs in the spinal cord enable rapid responses. The brain controls complex behaviours. Endocrine glands release hormones for slow but sustained coordination. Plants use phytohormones (auxin, gibberellin, etc.) for tropisms and growth regulation.