Control and Coordination

How the nervous system and hormones control the body, and how plants respond with hormones and tropisms.

Key ideas

A message travels along a neuron from the dendrites to the cell body and down the axon, then crosses a synapse chemically.
A reflex is a quick automatic response controlled by the spinal cord: receptor → sensory neuron → spinal cord → motor neuron → muscle.
The forebrain thinks and senses; the cerebellum (hindbrain) keeps balance; the medulla controls involuntary actions such as heartbeat.
Plant hormones: auxin makes shoots bend towards light; gibberellins help stems grow; cytokinins speed up cell division; abscisic acid slows growth and closes stomata.
Tropisms are growth movements towards or away from a stimulus such as light, gravity or water.
Animal hormones come from glands and travel in the blood: insulin controls blood sugar, thyroxine needs iodine, adrenaline prepares for an emergency.

Remember

Reflex arc	receptor → sensory neuron → spinal cord → motor neuron → effector
Hormone → gland	insulin → pancreas; thyroxine → thyroid; adrenaline → adrenal; growth hormone → pituitary
Deficiency	too little iodine → goitre; too little insulin → diabetes

Watch out

Saying the brain controls a reflex (the spinal cord does).
Calling the folding of touch-me-not leaves growth (it is a quick change in water pressure).
Mixing up auxin (towards light) and abscisic acid (slows growth).

Worked example

In a ruler-drop test, a student catches a falling ruler after it has dropped 24.2 cm. Using t = √(2d ÷ g) with g = 10 m/s², find the reaction time in milliseconds.

Working: d = 0.242 m; t = √(2 × 0.242 ÷ 10) = √0.0484 = 0.22 s

Answer: 220 ms