LEARNING OBJECTIVES
What you will be able to do
- explain reflexes and synapses
- describe eye responses
- compare nervous and hormonal control
- explain homeostasis and tropisms
AT A GLANCE
INTRODUCTION · THE BIG IDEA
How nervous, hormonal and plant systems detect change and coordinate responses.
Organisms detect change and coordinate responses through electrical impulses, chemical messengers and feedback systems. Different control routes suit different speeds and durations.
Treat each response as a pathway from stimulus to receptor, coordinator and effector, then explain how the response restores or improves conditions.
SECTION 01
Nervous pathways
The CNS is brain and spinal cord; the PNS is all other nerves. Impulses travel along sensory, relay and motor neurones.
DETAILED EXPLANATION
- At a synapse, an impulse releases neurotransmitter; it diffuses, binds receptors and triggers the next impulse.
- Asymmetric release and receptors make transmission one-way.
SECTION 02
Sense organs and the eye
Sense organs contain receptor cells that respond to particular stimuli, including light, sound, touch, temperature and chemicals. In the eye, the cornea refracts; the iris controls the pupil; the lens focuses; the retina detects; and the optic nerve transmits. The blind spot lacks receptors.
DETAILED EXPLANATION
- Near focus: ciliary muscles contract, suspensory ligaments slacken and the lens becomes thicker. Distance focus reverses this.
- Rods support dim vision; three cone types give colour; the fovea has many cones and provides sharp vision.
SECTION 03
Hormones and homeostasis
Hormones travel in blood to target organs; they act more slowly and for longer than nerve impulses. Negative feedback reverses movement from a set point.
DETAILED EXPLANATION
- Adrenaline raises breathing rate, heart rate, pupil diameter and blood glucose.
- Insulin lowers glucose; glucagon raises it through liver responses. Type 1 diabetes is managed with insulin.
- Skin hairs and fatty tissue reduce heat loss. Sweat glands cool by evaporation, and shivering releases heat through muscle contraction.
- Vasodilation sends more blood through surface capillaries to increase heat loss; vasoconstriction reduces this flow. The brain coordinates these responses.
SECTION 04
Tropisms
Phototropism responds to light and gravitropism to gravity. Auxin from the shoot tip becomes unevenly distributed and stimulates cell elongation.
DETAILED EXPLANATION
- Shoots are normally positive to light and negative to gravity; roots show opposite directions.
STEP-BY-STEP EXAM EXAMPLE
Building a reflex-arc explanation
- A receptor detects the stimulus and produces an impulse.
- A sensory neurone carries the impulse to the central nervous system.
- A relay neurone passes it across synapses to a motor neurone.
- The motor neurone activates an effector, producing a rapid response.
Answer: Receptor → sensory neurone → relay neurone → motor neurone → effector.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Reflexes are rapid and automatic.
- Synapses use neurotransmitter.
- The eye controls light and focus.
- Negative feedback maintains conditions.
- Auxin differences cause shoot bending.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- explain reflexes and synapses
- describe eye responses
- compare nervous and hormonal control
- explain homeostasis and tropisms