(i) Dendrite: Receives incoming information from sensory receptors or other neurons. The signal starts here as a chemical-triggered electrical impulse.

(ii) Axon: The long thread-like fibre that transmits the electrical impulse away from the cell body toward the nerve ending. The signal travels along the axon as a wave of electrical potential change.

(iii) Axon terminal (nerve ending): At this point the electrical impulse triggers the release of chemical neurotransmitters into the synapse, converting the electrical signal into a chemical one so it can cross the gap to the next neuron or target cell.

1. Receptor: Heat and pain receptors in the skin of the hand detect the dangerous temperature stimulus.
2. Sensory (Afferent) Neuron: An electrical impulse travels from the receptor up the sensory neuron toward the spinal cord.
3. Relay Neuron in Spinal Cord: The impulse arrives at the spinal cord. A relay neuron connects the sensory neuron to the motor neuron within the spinal cord. At the same time, a signal is also sent upward to the brain (which is why you feel the pain shortly after).
4. Motor (Efferent) Neuron: An impulse travels from the spinal cord down the motor neuron to the arm muscle.
5. Effector (Muscle): The arm muscle receives the impulse and contracts immediately — pulling the hand away from the flame.

Key point: The entire reflex is completed in the spinal cord before the brain processes the pain. This speed is what prevents serious burns.

(a) Riding a bicycle → Cerebellum (Hindbrain): The cerebellum coordinates balance, posture and precision of voluntary muscle movements. Without it, you cannot maintain balance while cycling.

(b) Feeling full → Forebrain (Hypothalamus / specific hunger centre): The forebrain contains a hunger centre that processes satiety signals from the digestive system.

(c) Heart beating → Medulla oblongata (Hindbrain): The medulla controls all involuntary vital functions including heartbeat, breathing rate and blood pressure — these continue even during sleep or unconsciousness.

(d) Solving a math problem → Cerebrum (Forebrain): The cerebrum is the main thinking region responsible for reasoning, mathematics, language and all conscious intellectual activity.

Mechanism of Phototropism:

1. Auxin is produced at the shoot tip (apical meristem).
2. When light comes from one side, auxin moves (diffuses) to the shaded (dark) side of the shoot — away from the light source.
3. The higher concentration of auxin on the dark side causes cells on that side to elongate (grow longer) more rapidly.
4. The light side has less auxin → cells grow less.
5. The result: the shoot bends toward the light source because the shaded side is growing faster.

Adaptive significance: This ensures leaves and shoot tips are always oriented toward sunlight, maximising the surface area available for photosynthesis.

Adrenaline (secreted by adrenal glands, above kidneys) triggers the “fight-or-flight” response:

1. Heart rate increases: More blood pumped per minute → more oxygen and glucose reach muscles for energy.
2. Breathing rate increases: Diaphragm and rib muscles contract faster → more oxygen enters blood, more CO₂ expelled.
3. Blood diverted from digestive system and skin to skeletal muscles: Muscles get more oxygen for the physical response needed.
4. Blood glucose levels rise: Liver releases stored glycogen as glucose → immediate energy source for cells.
5. Pupils dilate: Eyes take in more light → improved alertness and vision of surroundings.

All these changes together prepare the body for rapid, energetic action — whether facing a physical threat, or performing under pressure.

Mimosa (Touch-me-not) leaf movement:
— Not caused by growth; caused by rapid change in cell water content.
— When touched, cells at the leaf base (pulvinus) lose water rapidly → cells shrink → leaf droops.
— Uses electrical-chemical signals between cells but NO specialised nervous tissue.
— The response is reversible and temporary — leaves reopen once stimulus is gone.
— Movement is involuntary and non-directional (nastic, not tropic).

Leg movement during walking:
— Requires specialised muscle tissue that contracts and relaxes.
— Voluntary action: decision made in cerebrum (forebrain) → signal travels via motor neurons → muscles contract.
— Uses electrical nerve impulses along specialised nervous tissue at very high speed.
— The movement is precise, directional and controlled by both the brain (consciously) and cerebellum (balance/coordination).

In Type 1 diabetes, the beta cells of the Islets of Langerhans in the pancreas are destroyed (usually by the immune system) and produce little or no insulin at all. Blood glucose therefore rises to dangerously high levels after every meal.

Why injections, not tablets? Insulin is a protein hormone. If taken by mouth (tablet), the digestive system would break it down with protein-digesting enzymes (like pepsin and trypsin) before it could be absorbed into the bloodstream — rendering it useless. By injecting it directly into the subcutaneous tissue (under the skin), the insulin enters the blood intact and can reach target cells to control blood sugar.

This is a direct example of how feedback regulation (pancreas detecting blood sugar levels) works — and what happens when it fails.

The thyroid gland, located in the neck, produces thyroxin — a hormone that regulates the metabolism of carbohydrates, proteins and fats, and is essential for normal body growth.

Iodine is essential for thyroxin synthesis. Without enough iodine in the diet:
1. The thyroid gland cannot produce adequate thyroxin.
2. The pituitary gland keeps sending stimulating signals to the thyroid to produce more.
3. The thyroid gland keeps enlarging in a futile attempt to produce more hormone → this swelling is visible as a goitre (enlarged neck).
4. Low thyroxin causes slowed metabolism, fatigue, weight gain and in children, stunted growth and developmental delay.

Solution: Adding iodine to common table salt (iodised salt) is a simple and effective public health measure to prevent iodine deficiency.

Feature	Nervous System	Endocrine System
Messenger type	Electrical impulses	Chemical hormones
Speed	Very fast (milliseconds)	Slower (seconds to hours)
Reach	Only cells connected by nerves	All cells of the body via blood
Duration of effect	Short-lived, immediate	Long-lasting, sustained

The spinal cord carries three types of signals that would all be disrupted below the level of injury:

1. Sensory signals (upward to brain): Information from pain, temperature, touch and pressure receptors in the legs, feet and lower body would not reach the brain → the person would feel no sensation (paralysis of sensation/anaesthesia) below the injury.

2. Motor signals (downward from brain): Voluntary commands from the brain to leg and lower body muscles would not pass through → the person would be unable to move their legs (paralysis of movement/paraplegia).

3. Reflex arc pathways: Reflex arcs formed in the spinal cord below the level of injury would also be disrupted → loss of normal protective reflexes in that region.

Key consequence: Both voluntary movement AND sensation below the damage site are lost — this is why spinal injuries can result in permanent paraplegia or quadriplegia.