Control and coordination are essential for organisms to respond to environmental stimuli. In animals, this is achieved by the nervous system and endocrine (hormonal) system working together. The nervous system provides fast, electrical responses; the endocrine system provides slow, chemical (hormonal) responses. In plants, coordination occurs through chemical signals (phytohormones/plant hormones) since they lack a nervous system.

The nervous system consists of the brain, spinal cord, and nerves. Neurons (nerve cells) are the structural and functional units — they have a cell body, dendrites (receive signals), axon (transmit signals), and axon terminals. Types of neurons: sensory (receptor to CNS), motor (CNS to effector), relay/association (within CNS). Synapse: the gap between two neurons; neurotransmitters carry signals across it. Central Nervous System (CNS): brain + spinal cord. Peripheral Nervous System (PNS): all nerves outside CNS. Somatic NS controls voluntary actions; Autonomic NS controls involuntary (heart rate, digestion).

The human brain is protected by the skull (cranium) and three membranes called meninges, and is cushioned by cerebrospinal fluid (CSF). Forebrain: Cerebrum — largest part; controls intelligence, memory, voluntary actions, sight, smell, hearing. Midbrain: connects forebrain and hindbrain; controls eye movement and posture. Hindbrain: Cerebellum — balance and coordination of movement. Medulla oblongata — controls involuntary actions (breathing, heartbeat, blood pressure, swallowing, vomiting). Pons — connects cerebellum with rest of brain. Hypothalamus — controls body temperature, hunger, sleep, and links nervous and endocrine systems.

A reflex action is a rapid, automatic, involuntary response to a stimulus that does not involve the brain. Example: withdrawing hand from a hot object. Reflex arc: Stimulus → Sensory neuron → Spinal cord (relay neuron) → Motor neuron → Effector (muscle/gland) → Response. The spinal cord is the coordination centre for reflex actions, making responses faster. The brain becomes aware of the reflex after it has already happened. Reflexes protect the body from harm without wasting time sending signals to the brain.

Endocrine glands are ductless glands that secrete hormones directly into the blood. Hormones are chemical messengers that regulate body functions. Key glands and hormones: Pituitary (master gland) — growth hormone (GH), TSH, ADH; Thyroid — thyroxine (regulates metabolism, needs iodine); Adrenal — adrenaline (fight-or-flight response, increases heart rate); Pancreas — insulin (lowers blood sugar) and glucagon (raises blood sugar); Testes — testosterone (male secondary sexual characters); Ovaries — oestrogen and progesterone (female sexual characters, menstrual cycle). Feedback mechanisms control hormone levels. Diabetes mellitus results from insufficient insulin.

Plants respond to stimuli through movements called tropic movements (directional) and nastic movements (non-directional). Phototropism: shoot grows toward light (positive); root grows away from light (negative) — due to auxin redistribution. Geotropism: roots grow toward gravity (positive); shoot grows away from gravity (negative). Hydrotropism: roots grow toward water. Chemotropism: pollen tube grows toward ovule. Thigmotropism: plants coil around support (tendrils). Plant hormones (phytohormones): Auxins — promote cell elongation, phototropism. Gibberellins — promote stem elongation, seed germination. Cytokinins — promote cell division. Abscisic acid — inhibits growth, promotes dormancy ("stress hormone"). Ethylene — promotes fruit ripening.

The neuron is the structural and functional unit of the nervous system. Parts: dendrites (receive information), cell body (contains the nucleus), axon (long fibre that conducts the impulse away) and nerve endings. Information received at the dendritic tip sets off a chemical reaction that creates an electrical impulse, which travels to the cell body, along the axon and to its end. At the end, the electrical impulse triggers the release of chemicals into the gap called the synapse. These chemicals cross the gap and start a similar electrical impulse in the dendrite of the next neuron.

Receptors are specialised nerve endings or cells in the sense organs that detect changes (stimuli) in the environment. Examples: photoreceptors in the eye (light), phonoreceptors in the ear (sound and balance), olfactory receptors in the nose (smell), gustatory receptors in the tongue (taste) and thermoreceptors in the skin (temperature). Effectors are muscles and glands that respond to the instructions. The pathway: stimulus → receptor → sensory neuron → spinal cord or brain → motor neuron → effector → response.

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of the nerves that connect the CNS to other parts of the body: cranial nerves from the brain and spinal nerves from the spinal cord. The brain is protected by the bony cranium (skull) and enclosed in a fluid-filled balloon called cerebrospinal fluid, which acts as a shock absorber. The spinal cord is protected by the vertebral column (backbone). The spinal cord also controls reflex actions.

Plants respond to stimuli by directional growth movements called tropisms. Phototropism: response to light; shoots bend towards light (positive) and roots bend away. Geotropism: response to gravity; roots grow downward (positive), shoots upward (negative). Hydrotropism: roots grow towards water. Chemotropism: growth in response to chemicals, e.g. a pollen tube growing towards the ovule. Thigmotropism: response to touch, e.g. tendrils of pea plants coil around a support.

Plant hormones are chemical substances that coordinate growth, development and responses. Auxin: made at the shoot tip, promotes cell elongation; causes the shoot to bend towards light as it diffuses to the shaded side. Gibberellins: help in stem growth and seed germination. Cytokinins: promote cell division, present in large amounts in fruits and seeds. Abscisic acid (ABA): inhibits growth and causes wilting of leaves. Ethylene is a gaseous hormone that helps ripen fruits.

The leaves of the sensitive plant (Mimosa pudica or touch-me-not) fold up and droop when touched. This is a nastic movement, not a growth movement. The plant uses electrical-chemical means to transmit information from cell to cell. The movement is caused by a change in the amount of water in specialised cells (changes in turgor): the cells at the base of the leaf lose water, shrink and the leaf droops. Plants have no nervous system, so growth-related responses use hormones while quick movements use such changes.

Pituitary (master gland): growth hormone; regulates growth and development; deficiency causes dwarfism, excess causes gigantism. Thyroid: thyroxine, regulates carbohydrate, protein and fat metabolism; needs iodine, whose deficiency causes goitre. Pancreas: insulin, regulates blood sugar; deficiency causes diabetes. Adrenal: adrenaline, prepares the body for fight or flight (increases heart rate and breathing, diverts blood to muscles). Testes: testosterone, male puberty changes. Ovaries: oestrogen, female puberty changes.

The timing and amount of hormone released are regulated by feedback mechanisms. Example: when blood sugar level rises, the cells of the pancreas detect it and release more insulin, which lowers blood sugar by helping cells and the liver take up and store glucose. When the sugar level falls, insulin secretion decreases. Thus a hormone's own effect controls its secretion. Likewise, thyroxine release is controlled by the pituitary hormone released in response to its levels.

Animals use the nervous system (fast, electrical impulses, short-lasting) and the endocrine system (slower, chemical hormones, long-lasting). Parts of the brain: forebrain (cerebrum: thinking, memory, voluntary action), midbrain, hindbrain (cerebellum: posture and balance; pons; medulla: involuntary actions such as BP, salivation, vomiting). Reflex action is a quick automatic response to a stimulus, via a reflex arc, without the brain's involvement. Plants: tropic movements, nastic movements, and hormones auxin, gibberellin, cytokinin, abscisic acid and ethylene.