Coordination and Response
What you must understand
Nervous coordination uses receptors, sensory neurones, relay neurones in the spinal cord, motor neurones and effectors. Learn the reflex arc and why reflexes are fast and protective. Understand synapses and how impulses cross them chemically. Know the structure of the eye - cornea, iris, pupil, lens, retina - and how the iris and lens adjust to light and distance.
Hormonal coordination is slower and longer-lasting. Know the roles of insulin, glucagon, adrenaline, ADH and sex hormones. Plant responses include phototropism and gravitropism, controlled by auxins.
Traps examiners set
- Saying reflexes involve the brain - many bypass it via the spinal cord.
- Confusing the iris (muscle controlling pupil size) with the pupil (the gap).
- Mixing nervous (electrical, fast, short) with hormonal (chemical, slow, long) responses.
- Forgetting that auxin accumulates on the shaded side and promotes elongation.
Worked rule of thumb
For any response question, name the receptor, the coordinator, the effector and the response. "Light on retina → brain → iris muscles → pupil constricts" is a complete chain; missing any link loses marks.