The Human Body and Its Systems · Grade 8
When does the body send an instruction by nerve and when by hormone?
It depends on how fast and for how long. The nervous system sends an electrical signal along a defined route to one organ; it arrives in a fraction of a second and acts for a moment. The hormonal system releases a chemical into the blood; it arrives slowly, reaches the whole body, and goes on acting. Speed against persistence.
Learning objectives
- Follow a signal around a reflex arc from the receptor to the effector
- Compare the nervous system and the hormonal system by speed, by route and by how long the effect lasts
- Explain how insulin and glucagon hold blood sugar inside a narrow range
- Explain what happens to four different systems during ten minutes of running
The reflex arc
A hand touches something hot. A receptor in the skin detects the stimulus, a sensory neurone carries the signal to the spinal cord, a relay neurone passes it to a motor neurone there, and the motor neurone drives the muscle of the arm.
The hand pulls back before the brain has processed the pain. That is the point: the reflex short-cuts through the spinal cord instead of travelling up to the brain and back, and that difference is the difference between a mild burn and a serious one.
The signal itself travels along a neurone as an electrical change, and between one neurone and the next it crosses a tiny gap as a chemical. That is why a nerve signal is fast but its route is fixed in advance.
A hormone: a message to everyone
A gland releases a hormone into the blood, and the blood distributes it everywhere. Only cells with a matching receptor respond, so the message is sent to all and read only at the right address.
That route is slower — seconds to minutes — but its effect lasts. That is also its advantage: growth, puberty and blood sugar are not things you can manage in millisecond pulses.
Insulin and glucagon
After a meal the blood sugar level rises. The pancreas releases insulin, which tells cells to take up sugar and the liver to store it. The level falls back into the normal range.
A few hours without food and the level drops. The pancreas releases glucagon, which tells the liver to release stored sugar back into the blood. The level rises back into range.
Two opposite hormones from one gland, each correcting the deviation in its own direction. This is homeostasis seen close up, and it is the same idea as in the kidney lesson.
Worked examples
Put the stages of a reflex in order: motor neurone, receptor, muscle, spinal cord, sensory neurone.
- The receptor detects the stimulus
- The sensory neurone carries it to the spinal cord
- The motor neurone leaves the spinal cord
- The muscle carries out the response
Answer: Receptor, sensory neurone, spinal cord, motor neurone, muscle
Why does the effect of adrenaline last longer than the effect of a nerve signal?
- Adrenaline is a hormone released into the blood
- It stays in the blood until the body breaks it down
- A nerve signal lasts milliseconds and is over
Answer: Because the chemical itself keeps circulating
What happens to four body systems during ten minutes of running?
- The muscles increase cellular respiration and use more oxygen
- The respiratory system increases the breathing rate
- The circulatory system increases the heart rate
- The skin sweats to shed heat
Answer: Four systems responding together to one demand
Common mistakes
- Thinking the brain makes the decision in a reflex
- The circuit closes in the spinal cord, which is what makes it fast. The brain receives the information and feels the pain, but only after the hand has already moved.
- Assuming a hormone reaches only its target organ
- It reaches the whole body in the blood. Only cells with a matching receptor respond, and that is what creates the address.
- Mixing up insulin and glucagon
- Insulin lowers blood sugar and glucagon raises it. A simple anchor: insulin is released after a meal, when there is something to put away.
What to remember
- Nerve: fast, targeted, brief. Hormone: slow, broad, lasting.
- A reflex closes in the spinal cord, so it beats the brain to it.
- A hormone reaches everyone and is read only where there is a receptor.
- Insulin down, glucagon up.
More in The Human Body and Its Systems
- Why does a large body need organ systems at all?
- What happens to a sandwich between the mouth and the bloodstream?
- What is the difference between breathing, gas exchange and cellular respiration?
- Why does blood pass through the heart twice on every complete circuit?
- How does a kidney decide what to remove from the blood and what to keep?