Control and Coordination

The nervous system uses specialised cells called neurons to transmit electrical impulses. A neuron has three parts: dendrite (receives stimulus), cell body (contains nucleus), and axon (transmits impulse away from cell body, covered by myelin sheath). At the synapse (junction between two neurons), the electrical signal triggers release of chemical neurotransmitters that cross the gap and generate a new electrical impulse in the next neuron. A reflex action is an automatic, involuntary, and rapid response to a stimulus (e.g., pulling hand away from a hot object). The reflex arc pathway is: stimulus -> receptor -> sensory neuron -> relay neuron (in spinal cord) -> motor neuron -> effector (muscle/gland). Reflex actions are controlled by the spinal cord, not the brain, which is why they are so fast.

The brain is the main coordinating centre of the body, protected by the skull (cranium) and three membranes called meninges, with cerebrospinal fluid providing cushioning. The brain has three major regions: (1) Forebrain — the cerebrum is the largest part, responsible for thinking, memory, reasoning, and voluntary actions. It has different areas for different functions (sensory areas, motor areas, association areas). (2) Midbrain — controls some visual and auditory reflexes. (3) Hindbrain — the cerebellum controls balance, posture, and coordination of voluntary movements; the pons relays messages between cerebrum and cerebellum; and the medulla oblongata controls involuntary actions like breathing, heart rate, blood pressure, sneezing, and vomiting.

The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. Hormones are chemical messengers that act on specific target organs. Key endocrine glands and hormones: (1) Pituitary gland (master gland): growth hormone. (2) Thyroid: thyroxine (regulates metabolism, needs iodine; deficiency causes goitre). (3) Pancreas: insulin (lowers blood sugar by converting glucose to glycogen; deficiency causes diabetes mellitus). (4) Adrenal glands: adrenaline (fight-or-flight response: increases heart rate, BP, breathing). (5) Testes (male): testosterone (male secondary sexual characters, sperm production). (6) Ovaries (female): oestrogen (female secondary sexual characters, egg maturation). Feedback mechanisms regulate hormone levels — for example, high blood sugar triggers insulin release, which lowers blood sugar, which then reduces insulin secretion.

Plants do not have a nervous system, but they respond to stimuli using plant hormones (phytohormones) and growth movements. Tropisms are directional growth movements: phototropism (towards/away from light), geotropism/gravitropism (towards/away from gravity), hydrotropism (towards water), chemotropism (towards/away from chemicals), and thigmotropism (response to touch). The main plant hormones are: (1) Auxin — promotes cell elongation; concentrated on the shaded side of the stem, causing it to bend towards light (positive phototropism). Roots show negative phototropism. (2) Gibberellin — promotes stem elongation and seed germination. (3) Cytokinin — promotes cell division; present in areas of rapid growth (fruits, seeds). (4) Abscisic acid — inhibits growth; promotes wilting and leaf fall (stress hormone). (5) Ethylene — promotes fruit ripening.