---
title: "The Brain in Command"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 33
exercises: 11
source: https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command
---

# Chapter 33 — The Brain in Command

A ball flies toward you; you catch it. Half a second, no thought at all — yet in that half-second, your [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) reported a flying object, something computed its path, and dozens of [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) were ordered into exactly the right catch. The something is the [brain](#def-g5-brain-in-command-system), and this chapter follows its commands along the body’s wiring.

## 33.1 The command network

**Definition 33.1 (The nervous system).**

The *nervous system* is the body’s command network:

- the *brain* , in the [skull](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) ’s shelter ( [Definition 17.1](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) ), is the center: it receives all reports and issues all orders;
- the *spinal cord* , a thick cable of nerve tissue, runs from the brain down inside the [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) ’s protective column;
- the *nerves* , white threads branching from brain and spinal cord to every organ, carry the messages — at more than $100\,\mathrm{m}/\mathrm{s}$ for the fastest.

**Proposition 33.2 (Report, decide, order).**

Every action runs the same loop ([Proposition 5.4](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#prop-g1-the-five-senses-brain) was its first sketch):

1. the *[sense organs](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)* report: [nerves](#def-g5-brain-in-command-system) carry their messages *to* the [brain](#def-g5-brain-in-command-system) ;
2. the *[brain](#def-g5-brain-in-command-system)* puts reports together, decides, and composes orders;
3. [nerves](#def-g5-brain-in-command-system) carry the orders *from* the [brain](#def-g5-brain-in-command-system) to the *[muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)* ( [Definition 17.6](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ), which contract on command.

[Sense organs](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) report, [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) obey — neither ever decides.

**Proof.** *Admitted at this level.* ∎

![The loop of every action: report to the brain, decision, order to the muscles.](https://one-course.com/images/onecourse/chapters/biology-1/g5-brain-in-command/fig-81f2f3241cd4.svg)

*The loop of every action: report to the [brain](#def-g5-brain-in-command-system), decision, order to the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle).*

**Example 33.3 (The catch, replayed slowly).**

Replay the half-second: [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) track the ball and report, continuously; the [brain](#def-g5-brain-in-command-system), comparing report after report, works out where the ball will be; orders stream out to [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) (a step left), trunk (lean), [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and fingers (open — now close). No single organ “caught the ball”: [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), [brain](#def-g5-brain-in-command-system) and [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) each played their line of [Proposition 33.2](#prop-g5-brain-in-command-loop).

**Remark 33.4 (More than movement).**

The same [brain](#def-g5-brain-in-command-system) that catches balls also stores your memories, holds your attention on this page, dreams at night — and runs services you never think about: it keeps [breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) going day and night ([Definition 26.1](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements)) and adjusts the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s pace to the body’s needs ([Example 27.7](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ex-g4-heart-and-blood-effort)). Some commands are yours to give; others the [brain](#def-g5-brain-in-command-system) gives for you.

## 33.2 Measuring the loop: reaction time

**Definition 33.5 (Reaction time).**

The loop is fast but not instant: report, decision and order each take their share. The delay between a signal and the body’s response is the *reaction time* — for a simple task (catch, press, grab), about a fifth of a second in a practiced person, a little more in children.

**Method 33.6 (The ruler test).**

[Reaction time](#def-g5-brain-in-command-reaction) with no instrument but a ruler:

1. a partner holds a ruler hanging vertically, zero mark down, between your open thumb and finger;
2. without warning, the partner lets go; you catch the ruler as fast as you can;
3. read where you caught it: the fewer centimetres slid past, the faster your loop;
4. repeat several times and keep your typical value — single tries jump around.

The falling ruler turns time into centimetres: about $15\,\mathrm{cm}$ corresponds to a sixth of a second.

![The ruler test, an instant before the drop: when the upper hand lets go, the lower one’s loop — see, decide, close — starts its fifth of a second.](https://one-course.com/images/onecourse/chapters/biology-1/g5-brain-in-command/img-14c1c8fd348b.jpg)

*The ruler test, an instant before the drop: when the upper hand lets go, the lower one’s loop — see, decide, close — starts its fifth of a second.*

**Example 33.7 (Why the passenger seat gasps first).**

A fifth of a second seems nothing — until speed multiplies it. A car at highway speed covers about $7\,\mathrm{m}$ during a fifth-of-a-second reaction: the driver has not yet touched the brake, and the car has crossed a classroom. [Reaction time](#def-g5-brain-in-command-reaction) is why distances between cars exist, and why a driver’s attention is a safety organ.

**Remark 33.8 (Training the loop).**

Practice shortens and sharpens the loop — not the [nerves](#def-g5-brain-in-command-system)’ speed, but the [brain](#def-g5-brain-in-command-system)’s routing. The beginner juggler drops everything; a month later the same [brain](#def-g5-brain-in-command-system) orders the same hands through three balls without apparent effort. Learning a skill is the [brain](#def-g5-brain-in-command-system) laying down better paths — one more thing that grows with exercise, like the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) of [Method 17.9](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#met-g3-bones-and-muscles-care).

## 33.3 Protecting the commander

**Proposition 33.9 (Sleep, food, safety).**

The [brain](#def-g5-brain-in-command-system) is the body’s most protected organ — [skull](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) helmet, [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) column — and the most demanding: though a small part of the body’s weight, it consumes a large share of its [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) and oxygen ([Proposition 27.1](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-carries)), and it does its filing and repairs during *[sleep](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs)* ([Proposition 6.8](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs)). A tired [brain](#def-g5-brain-in-command-system) commands badly: slower reactions, scattered attention, grumpy decisions.

**Proof.** *Admitted at this level.* ∎

**Example 33.10 (Helmets are for the loop).**

[Bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) mend ([Remark 17.3](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#rem-g3-bones-and-muscles-alive)); the [brain](#def-g5-brain-in-command-system)’s tissue, once badly hurt, mostly does not. That is the calculation behind bicycle helmets: [skull](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) plus foam where the body’s own protection might not be enough. Cheap insurance for the organ running everything else.

## 33.4 Exercises

**Exercise 33.1 ★.**

Name the three parts of the [nervous system](#def-g5-brain-in-command-system) and each one’s role.

**Solution of Exercise 33.1.**

The [brain](#def-g5-brain-in-command-system): receives all reports, makes all decisions, issues all orders. The [spinal cord](#def-g5-brain-in-command-system): the thick cable running from the [brain](#def-g5-brain-in-command-system) down inside the [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton). The [nerves](#def-g5-brain-in-command-system): the branching threads carrying messages between [brain](#def-g5-brain-in-command-system) or cord and every organ.

**Exercise 33.2 ★.**

State the loop of every action in three steps. Which parts report, which decides, which obey?

**Solution of Exercise 33.2.**

[Sense organs](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) report to the [brain](#def-g5-brain-in-command-system); the [brain](#def-g5-brain-in-command-system) decides and composes orders; [nerves](#def-g5-brain-in-command-system) carry the orders to the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), which contract. [Sense organs](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) report, the [brain](#def-g5-brain-in-command-system) decides, [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) obey.

**Exercise 33.3 ★.**

Where are the [brain](#def-g5-brain-in-command-system) and the [spinal cord](#def-g5-brain-in-command-system) each protected, and by which [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton)?

**Solution of Exercise 33.3.**

The [brain](#def-g5-brain-in-command-system) in the [skull](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton); the [spinal cord](#def-g5-brain-in-command-system) inside the [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton)’s column of stacked small [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton).

**Exercise 33.4 ★.**

What is [reaction time](#def-g5-brain-in-command-reaction)? Give its rough value for a simple task.

**Solution of Exercise 33.4.**

The delay between a signal and the body’s response — about a fifth of a second for a simple task in a practiced person, a little more in children.

**Exercise 33.5 ★.**

Replay slowly, as in [Example 33.3](#ex-g5-brain-in-command-catch): you step on something sharp and your foot jerks away. Who reported, who decided, who obeyed?

**Solution of Exercise 33.5.**

The [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) of the foot reported (touch and pain), the [nervous system](#def-g5-brain-in-command-system) decided the retreat, and the [leg](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) obeyed by jerking the foot away. (So fast a retreat is handled partly by the [spinal cord](#def-g5-brain-in-command-system) itself — the command center’s emergency desk.)

**Exercise 33.6 ★.**

Give two services the [brain](#def-g5-brain-in-command-system) runs without your thinking about them, from [Remark 33.4](#rem-g5-brain-in-command-more).

**Solution of Exercise 33.6.**

It keeps [breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) going day and night, and adjusts the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s pace to the body’s needs (also: storing memories, dreaming).

**Exercise 33.7 ★.**

Why does a tired [brain](#def-g5-brain-in-command-system) command badly? Name two of its needs from [Proposition 33.9](#prop-g5-brain-in-command-protect).

**Solution of Exercise 33.7.**

[Sleep](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs) — its filing-and-repairs time — and a large steady share of the body’s [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) and oxygen. Deprived of either, it reacts more slowly, scatters its attention and decides badly.

**Exercise 33.8 ★.**

Try it: run the ruler test of [Method 33.6](#met-g5-brain-in-command-ruler), five tries. Report your typical catch distance — and whether practice improved it.

**Solution of Exercise 33.8.**

Open answer. Typical catches run around $10$–$20$ cm; several tries usually improve and steady the value a little.

**Exercise 33.9 ★★.**

Why is there no such thing as an instant catch, however skilled the catcher? Answer with the loop’s three steps.

**Solution of Exercise 33.9.**

Because every catch must run all three steps — the [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’ report must travel, the [brain](#def-g5-brain-in-command-system) must compute and decide, the order must travel back and the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) contract — and each step takes real time. Skill shortens the loop; nothing removes it.

**Exercise 33.10 ★★.**

Using [Example 33.7](#ex-g5-brain-in-command-driver): why do road rules demand *distance* between cars rather than merely good brakes? What does attention change in the loop?

**Solution of Exercise 33.10.**

Because the best brakes act only after the driver’s loop has run — and at highway speed the car covers about $7\,\mathrm{m}$ before the order “brake!” even reaches the foot. The gap between cars is room for the loop. Attention shortens the report-and-decide steps; a distracted [brain](#def-g5-brain-in-command-system) adds metres.

**Exercise 33.11 ★★★.**

The juggler of [Remark 33.8](#rem-g5-brain-in-command-training) improved without her [nerves](#def-g5-brain-in-command-system) conducting any faster. What part of the loop improved instead? Compare with training a [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle): what grows in each case?

**Solution of Exercise 33.11.**

The [brain](#def-g5-brain-in-command-system)’s part — the deciding and ordering: practice laid down better paths, so the same reports now route to the right orders quickly and without effort. [Muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) training grows the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’s strength; skill training grows the [brain](#def-g5-brain-in-command-system)’s routing. One builds the motor, the other the commander.