The Guide
Two ways to send a message
A body made of trillions of cells has a coordination problem. The hand has to know what the eye saw. The heart has to speed up when the legs start working. Sugar arriving in the blood after lunch has to be dealt with by organs nowhere near the stomach. Something has to carry word from one part to another.
The body has two systems for it, and they work in completely different ways. The nervous system sends electrical signals along fixed paths, like a wire run to one particular place, and it is fast — a fraction of a second. The endocrine system releases chemicals into the blood, which carries them everywhere, and it is slower but its effects last. The test asks about both, and most of its questions come down to knowing which kind of message a situation calls for.
The neuron, and how a signal moves
A neuron is a nerve cell. It has the usual parts of a cell, plus a shape built for carrying messages a long way: short branches called dendrites that receive signals, a cell body, and one long fiber called an axon that carries the signal away. Some axons run three feet, from the base of the spine to the foot, in a single cell.
Along the axon the signal is electrical. At the end of the axon it stops, because neurons do not touch. There is a narrow gap, and the signal crosses it as a chemical: the first cell releases a substance that drifts across and lands on the next cell, which starts its own electrical signal. That junction is the synapse. It matters more than it sounds, because the gap is where the body — and most medicines that affect mood, pain or alertness — can turn a signal up, damp it down, or block it.
Neurons come in three kinds, named for the direction they work in. Sensory neurons carry signals from the senses inward. Motor neurons carry orders outward to muscles and glands. Interneurons sit in between, inside the brain and spinal cord, connecting the two.
The parts of the system
The brain and the spinal cord together are the central nervous system: the part that receives, decides and orders. Every nerve outside them is the peripheral nervous system: the wiring that reaches the rest of the body. Three parts of the brain come up often enough to be worth knowing by name.
- Cerebrum
- The large wrinkled top. Thinking, language, memory, and the movements you choose to make, along with what arrives from the senses.
- Cerebellum
- The smaller mass tucked underneath at the back. Balance, posture, and the smooth coordination of movement — the difference between reaching for a cup and knocking it over.
- Brain stem
- The stalk joining brain to spinal cord. Breathing, heartbeat, blood pressure, swallowing. Nothing here waits for your permission, which is why an injury to the brain stem is so much more dangerous than an injury of the same size higher up.
That last point is the useful distinction. Some nervous activity is voluntary: you decide, and it happens. Much of it is involuntary: it runs whether you attend to it or not. The involuntary side has two settings, and they oppose each other.
| Ready for trouble | At rest | |
|---|---|---|
| Heart rate | Up | Down |
| Breathing | Faster and deeper | Slow and even |
| Digestion | Put on hold | Running |
| Pupils | Wide | Narrow |
| When | Danger, fright, hard effort | Ordinary hours, after a meal |
Neither setting is the healthy one and the other the unhealthy one. Each is right for its circumstances, and the body shifts between them all day without being asked.
A reflex: when the spinal cord answers without the brain
Put a hand on a hot pan and it comes off before you know what happened. The order to pull back did not come from the brain. It came from the spinal cord, which is nearer, and the brain got the news afterward — which is why the pain arrives a moment after the hand has already moved. That sequence surprises people, and it is a favorite of the test.
Notice what a receptor does at step one. It turns a change in the world — heat, pressure, light, a chemical in the air — into a nerve signal, because a nerve signal is the only language the system reads. Your eye does not send light to the brain. It sends signals that started when light landed on cells at the back of it. Everything you have ever seen, heard or felt reached you in that translated form.
Why the shortcut exists. A signal takes time to travel, and the brain takes time to decide. Routing a burn through the brain would add perhaps half a second, which is long enough to do real damage to a hand. So the body keeps a few decisions close to where the trouble is. Reflexes are not the nervous system failing to think. They are the nervous system declining to, on purpose, where thinking would cost more than it is worth.
Hormones: a message sent to everyone
The endocrine system works the other way around. A gland releases a chemical called a hormone into the blood, and the blood carries it to every part of the body within a minute or two. There is no wire and no address on the message.
So how does it reach the right organ? It reaches all of them, and only some can act on it. A cell responds to a hormone if it carries the matching receptor, and does nothing if it does not. The same hormone passes a bone cell and a liver cell in the same second, and only one of them is equipped to hear it. That is the whole design, and it is worth stating plainly because so much follows from it: one gland can change the behavior of organs scattered all over the body, all at once, without any connection between them.
- Pituitary
- Sits under the brain and directs several other glands, as well as controlling growth. It takes its instructions from the brain just above it, which is one of the places the two systems meet.
- Thyroid
- In the neck. Sets how fast the body uses energy. Too little of its hormone and everything runs slow — tiredness, cold, weight gain; too much and everything runs fast.
- Adrenal glands
- One on top of each kidney. Release adrenaline, which prepares the body for sudden effort, and other hormones that manage salt, water and long-term stress.
- Pancreas
- Behind the stomach. Releases insulin and glucagon, the two hormones that hold blood sugar steady. It also makes digestive enzymes, as Quiz 14 covered — one organ with two unrelated jobs.
- Ovaries and testes
- Release the sex hormones, which direct development and the reproductive cycle.
Holding a level steady: blood sugar
Most of what hormones do is keep some quantity from wandering — the amount of sugar in the blood, the amount of water, the amount of calcium. The method is the same each time, and it is the idea Quiz 19 is built on. When the level goes up, something is released that brings it down. When it goes down, something is released that brings it up. Each correction shuts off the signal that called for it.
The loop also runs the other way. Go a long time without eating and blood sugar falls, so the pancreas releases glucagon instead, and the liver breaks down its stored glycogen and puts sugar back into the blood. Two hormones from one gland, pulling in opposite directions, which is how a level gets held rather than merely lowered.
What diabetes is, in these terms. In one form the pancreas no longer makes insulin, so the signal is never sent. In the other, the pancreas makes it but the body's cells respond to it poorly, so the signal is sent and not much heard. Either way the sugar stays high in the blood instead of being taken into cells — which is also why some of it spills into the urine, the sign Quiz 14 described. The two forms are different problems in the same loop, and that is the level the test asks about.
Where the two systems meet
Something startles you on a stairwell. A nerve signal reaches the adrenal glands in a fraction of a second, and they release adrenaline into the blood. Your heart is pounding before you have identified what it was, and you are still unsteady ten minutes later, long after you know there is nothing there.
Both halves of that are on display. The nerve signal is fast and over quickly. The hormone takes a moment longer to spread and then keeps working until the blood clears it, which is why the shakiness outlasts the scare. The nerve signal starts it quickly, and the hormone keeps it going.
Nerve signals run along fixed paths to one place, arrive in a fraction of a second, and are over quickly. Hormones travel in the blood to the whole body, and only cells with the matching receptor respond; they take longer to arrive and their effects last. A reflex is the spinal cord answering before the brain hears about it, and a hormone loop is a level being pushed back whenever it drifts.
Worked Examples
Two questions, worked through
Trace the path rather than trusting the impression. The receptor in the skin sends a signal along a sensory neuron to the spinal cord. From there, two things happen. One signal goes straight out along a motor neuron to the leg muscle, a short trip. Another goes up the cord to the brain, a longer trip, and only when it arrives and is processed is there any pain.
The short trip finishes first. The feeling of having been hurt is genuinely later than the jerk, though both are quick enough that they feel like one moment.
The movement came first. On any reflex question, ask how far the signal had to travel before something happened. The spinal cord is closer than the brain, so the reflex beats the sensation every time.Start from how a hormone travels. It is not aimed anywhere; the blood takes it past every organ in the body. So the question is not how it found three organs — it found all of them.
What differs is which cells can act on it. Heart cells, airway cells and liver cells each carry a receptor for adrenaline, and each does with the message what its own kind of cell does. The heart beats faster because that is what heart muscle does when urged; the liver releases sugar because releasing sugar is what liver cells have to offer. One message, several kinds of listener, several results.
Because the hormone reaches every cell and only those with the matching receptor respond, each in its own way. When a question asks how a single hormone produces scattered effects, the answer is always about receptors, never about aiming.The Quiz
Ten questions
Answer all ten, then press the button at the bottom. Nothing is timed.
Use the reflex diagram from the Guide above: receptor in the skin, sensory neuron, spinal cord, motor neuron, muscle — with a separate dashed signal running up to the brain.
1.According to the diagram, what carries the signal from the skin to the spinal cord?
2.What is the job of the receptor at step one?
3.The diagram draws the signal to the brain as a dashed line arriving after steps 1 to 5. What does that show?
4.An injury destroys the sensory neuron at step two, leaving everything else intact. What would be expected?
Two people are given the same measured meal at time zero. Their blood sugar is recorded in milligrams per deciliter. One has diabetes that is not being treated.
| Time after the meal | Person A | Person B |
|---|---|---|
| 0 minutes | 90 | 150 |
| 30 minutes | 140 | 240 |
| 60 minutes | 120 | 250 |
| 120 minutes | 95 | 230 |
5.At which time is the gap between the two people largest?
6.Person A's level returns nearly to where it started within two hours. What best explains that return?
7.A student concludes that Person B's numbers prove B ate more sugar than A. What is wrong with that conclusion?
An electrician on a stepladder feels it shift under him. Before he has thought anything at all, his hand has clamped onto the conduit above and his heart is hammering. The ladder settles; he is fine.
Ten minutes later, standing on the floor with the ladder footed properly, his hands are still unsteady and his heart is still faster than usual. He says he does not understand why he is shaking when the danger is over and he knows it.
8.What best accounts for the hand clamping onto the conduit before he had thought anything?
9.Why is he still shaking ten minutes later?
10.Which conclusion does the passage best support?
The Key
Answers, and the trap in each one
Your Score
What the number means
| 8 to 10 | Solid. Check topic 16 on your map, and go on to Quiz 17. |
|---|---|
| 6 or 7 | Close. Read the whole key, then take this again in a few days before you check the box. |
| 5 or fewer | Worth another pass. Draw the reflex path from memory, five steps, then draw the blood sugar loop as a circle of four. Those two drawings carry most of the quiz. |
Misses on 1 to 4 usually come down to two pairs of words — sensory and motor, voluntary and involuntary — and both are about direction and control rather than anything complicated. Misses on 5 to 7 are arithmetic as much as biology: subtract before you answer. Misses on 8 to 10 are the distinction the whole quiz is built on, and the fastest way to hold it is by timing. Instant and brief is a nerve signal. Slower to arrive and slow to fade is a hormone.
The dollar
Before 1922, a diagnosis of the kind of diabetes that strikes children was a death sentence, usually within a year or two. The only treatment was near-starvation, which bought a little time at a terrible cost.
In the summer of 1921, in a borrowed laboratory in Toronto, Frederick Banting and Charles Best worked out how to extract the pancreas's hormone in a form that could be injected, with James Collip later purifying it enough for use in people. In January 1922 they gave it to Leonard Thompson, fourteen years old and close to death in a Toronto hospital ward. His blood sugar fell, and he lived another thirteen years. Children in wards elsewhere, some of them already unconscious, were injected one after another down the row; by the time the doctors reached the end of the ward, the first of them were waking up.
The discoverers sold the patent to the University of Toronto for one dollar. Banting held that a discovery of this kind did not belong to the men who made it, and that no one should own the means of keeping other people alive.