Two control systems
To respond to its surroundings, the body uses two coordinating systems. The nervous system sends fast, brief electrical impulses along nerve cells to precise targets. The endocrine system sends slower, longer-lasting chemical messages — hormones — through the blood to many cells at once. Together they detect changes (stimuli) and produce responses.
| Nervous | Hormonal | |
|---|---|---|
| Message | electrical impulse | chemical hormone |
| Transmission | along neurones | in the blood |
| Speed | very fast | slower |
| Duration | brief | longer-lasting |
Neurones and the reflex arc
Nerve cells, or neurones, carry impulses: sensory neurones from receptors to the central nervous system (brain and spinal cord), and motor neurones out to effectors (muscles and glands). A reflex is a fast, automatic, protective response — like pulling your hand off a hot object — that follows a fixed pathway, the reflex arc: receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector.
Because a reflex bypasses conscious thought in the brain, it is fast enough to protect the body from harm.
Hormones
Hormones are chemicals made by endocrine glands and carried in the blood to their target organs. Adrenaline from the adrenal glands prepares the body for action (the "fight or flight" response — raising heart rate and releasing glucose). Insulin and glucagon from the pancreas control blood glucose, and the sex hormones testosterone and oestrogen control development and reproduction.
Response in plants
Plants respond too, by growing toward or away from a stimulus — a tropism. Shoots grow toward light (positive phototropism) so leaves catch more sunlight; roots grow toward gravity (positive gravitropism) to anchor and find water. These growth responses are controlled by plant hormones called auxins.
Reflex vs considered response
A reflex takes the short route through the spinal cord — fast and automatic, no thinking. A considered response goes to the brain first — slower, but under conscious control.
A worked example
Describe the pathway of the reflex when you touch a sharp pin and pull your hand away.
- A receptor in the skin detects the sharp pain (the stimulus).Receptors detect the change.
- A sensory neurone carries the impulse to the spinal cord.Sensory neurones run from receptor to the CNS.
- A relay neurone in the spinal cord passes it to a motor neurone.The reflex is processed in the spinal cord, not the brain.
- The motor neurone carries the impulse to the arm muscle (effector), which contracts to pull the hand away.The effector produces the response.
Receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector (muscle) — a fast, automatic reflex arc.
The synapse
Neurones do not quite touch — there is a tiny gap between them called a synapse. When an impulse reaches the end of one neurone, it triggers the release of a chemical neurotransmitter, which diffuses across the gap and sets off a new impulse in the next neurone. Synapses make sure impulses travel in one direction only, and they are where many drugs and poisons act.
Sense organs and the eye
Stimuli are detected by receptors, often grouped into sense organs — the eye (light), ear (sound and balance), skin (touch, temperature, pain), nose and tongue (chemicals). The eye shows the idea well: light enters through the pupil, the lens focuses it onto the retina, and receptor cells there send impulses along the optic nerve to the brain. The pupil reflex protects the eye — in bright light the pupil narrows automatically to let in less light.
One-way traffic
Because only the end of one neurone releases neurotransmitter and only the next neurone has receptors for it, a synapse lets impulses pass in one direction only — keeping signals flowing the right way.