Primary & Middle School Biology · Grades 1–9
60Nervous Communication
Chapter 33 drew the loop — report, decide, order — and measured its fifth of a second. This year we open the cables. What actually travels along a nerve? What are nerves made of? And what happens at the gaps where one wire ends and the next begins — the gaps where, it will turn out, both memory and many poisons do their work?
60.1 The wiring and its cells
Definition 60.1 (Neurons and nerve messages)
The nervous system’s working cells are the neurons (Remark 45.6 listed them): each has a cell body and a long thin fiber — some fibers reach from spine to toe, the body’s longest cells. A nerve is a cable bundling thousands of such fibers. Along each fiber travel nerve messages: brief electrical signals, racing at up to (Definition 33.1’s figure, now with its carrier).
Proposition 60.2 (Coding by frequency)
The signals themselves are all alike — what varies is their rate. A gentle touch sends a slow patter up the sensory fiber; a strong one, a rapid drumroll. The nervous system’s language has one word repeated at different speeds: intensity is coded as frequency, on every line, in and out.
Proof. Admitted at this level. ∎
Example 60.3 (Three lines, one language)
The loop’s cast (Proposition 33.2), rewired: sensory neurons carry the reports — skin to spinal cord, eye to brain; motor neurons carry the orders — cord to muscle, ending where the fiber meets the muscle and its drumroll sets the contraction’s strength (Definition 17.6); and between them, in brain and cord, uncounted connecting neurons do the deciding. Reflex or symphony, the traffic is the same signals at different rates on different lines.
60.2 The gaps: synapses
Definition 60.4 (The synapse)
Neurons do not touch. Where one fiber’s end meets the next cell lies a microscopic gap — the synapse. The electrical signal cannot jump it; instead the arriving fiber releases a chemical messenger into the gap, which crosses, docks on the far side, and — if enough arrives — starts the signal afresh in the next neuron. Electrical along the wire, chemical at every junction: the whole system alternates the two.
Proposition 60.5 (Why gaps, not solder)
The gaps look like a flaw and are the system’s genius:
- they make junctions adjustable — a synapse crossed often grows stronger, one unused fades: learning (Remark 33.8’s “better paths”, found — they are strengthened synapses);
- they make signals one-way — messengers release on one side only, so traffic cannot run backward;
- they let one neuron weigh thousands of inputs — excitatory and damping voices summed at each cell before it fires: the deciding of the loop, performed at junctions.
Proof. Admitted at this level. ∎
Example 60.6 (The juggler’s synapses)
Remark 33.8’s juggler, explained at last: a month of practice conducted the same signals across the same junctions until those synapses strengthened — the routing that was laboriously weighed now runs pre-weighted. Skill is chemistry consolidated at the gaps; “practice makes permanent” is a statement about synapses.
Remark 60.7 (The gaps are the vulnerable points)
Whatever acts on the nervous system from outside acts at the synapses — the one place the signal becomes chemistry, dockable and dupable. Anesthetics quiet them; venoms jam them (some paralyze by blocking the muscle-junction’s messenger, some by flooding it); and Proposition 36.5’s alcohol, and every drug that changes mood or perception, works by imitating, blocking or exhausting the gaps’ messengers. Chapter 62 builds on exactly this point.
60.3 Fast wires, slow mail: two systems compared
Proposition 60.8 (Nerves against hormones)
The body now shows two communication systems, built for opposite jobs:
- nervous: signals in milliseconds, along dedicated lines, to precise addresses — one muscle, one gland — and gone when the signal stops: the system of catches, speech and reflexes;
- hormonal (Definition 56.4): messengers in minutes to months, broadcast by blood to every organ built to hear, ordering slow, body-wide programs — growth, cycles, puberty’s rebuild.
Telegraph and postal service: the body runs both, and Chapter 61 gives the postal service its own chapter.
Proof. Admitted at this level. ∎
Method 60.9 (Diagnosing a communication, system by system)
Given any bodily coordination, assign its system:
- time it: milliseconds — nervous; minutes to years — hormonal (or both, in relay);
- map its reach: one precise address — nervous; body-wide — hormonal;
- find the route: nameable wire path — nervous; bloodstream broadcast — hormonal;
- check persistence: gone with the signal — nervous; running while the messenger circulates — hormonal.
Example 60.10 (The method on three cases)
The blink at a puff of air: milliseconds, one muscle ring, wired path, gone at once — nervous. The growth burst: years, whole-body, bloodstream, running for seasons — hormonal. The racing heart of the sprint (Proposition 49.4): both in relay — nerve signals set the pace up within a beat, and a bloodstream messenger (the famous surge you feel as a fright’s hot wave) holds the whole body’s alert for minutes. Telegraph first, post to follow.
60.4 Exercises
Exercise 60.1 ★
What is a neuron? What is a nerve, in terms of neurons?
Solution
Solution of Exercise 60.1.
A neuron is the nervous system’s working cell: a cell body with a long thin fiber — some spine-to-toe. A nerve is a cable bundling thousands of such fibers.
Exercise 60.2 ★
What travels along a fiber, at what speed — and how is intensity coded?
Solution
Solution of Exercise 60.2.
Brief electrical signals, at up to . The signals are all alike; intensity is coded as their rate — gentle patter to rapid drumroll.
Exercise 60.3 ★
Name the three neuron kinds of the loop and each one’s line.
Exercise 60.4 ★
What is a synapse, and what crosses it?
Exercise 60.5 ★
Give the three services the gaps provide, by Proposition 60.5.
Exercise 60.6 ★
Contrast nerves and hormones on speed, reach, route and persistence.
Exercise 60.7 ★★
Rewrite the ball-catch of Example 33.3 in this chapter’s vocabulary: name the neuron kinds and where synapses do the weighing.
Solution
Solution of Exercise 60.7.
The eyes’ sensory neurons drum the ball’s positions to the brain; connecting neurons weigh the streams at their synapses — the path computed is inputs summed junction by junction; motor neurons drum the orders out, their rates setting each muscle’s strength for step, lean and close of the hand.
Exercise 60.8 ★★
“Practice makes permanent.” Explain the slogan at the gaps, with the juggler.
Solution
Solution of Exercise 60.8.
Each practiced pass conducts signals across the same junctions, and crossed junctions strengthen: after a month the juggle’s routing is pre-weighted — what needed effortful weighing now runs of itself. The permanence is stronger synapses.
Exercise 60.9 ★★
Why does everything that acts on the nervous system from outside act at the synapses? What is special about them?
Solution
Solution of Exercise 60.9.
Because the synapse is where the signal becomes chemistry — a released messenger, docking sites, a gap open to whatever molecules arrive. A wire’s electricity is hard to counterfeit; a chemical courier can be imitated, blocked or exhausted — so that is where anesthetics, venoms and drugs all act.
Exercise 60.10 ★★
Run Method 60.9 on: (a) pulling back from a hot pan; (b) puberty’s voice change; (c) the fright’s hot wave that outlasts the scare.
Solution
Solution of Exercise 60.10.
(a) Milliseconds, one arm’s muscles, wired path, gone at once: nervous. (b) Years, body-wide, bloodstream, running for seasons: hormonal. (c) The relay: a nervous split-second start, then the bloodstream messenger holding the alert for minutes after the scare — both, in order.
Exercise 60.11 ★★
A venom blocks the messenger at the muscle junction. Predict the victim’s state, and name the loop stage cut.
Solution
Solution of Exercise 60.11.
The order arrives at the junction and cannot cross: muscles receive nothing. The victim is progressively paralyzed — conscious, sensing, deciding — with the loop cut at its last link, order-to-muscle.
Exercise 60.12 ★★★
“Electrical along the wire, chemical at the junction — and everything interesting happens at the junctions.” Defend the last clause three ways: learning, deciding, and vulnerability.
Solution
Solution of Exercise 60.12.
Learning: skills are synapses strengthened by use — the juggler’s month lives at junctions. Deciding: each neuron fires or not by summing its junctions’ voices — the loop’s “decide” is junction arithmetic. Vulnerability: anesthetics, venoms and drugs act where signal becomes chemistry — at the gap. Wires only carry; junctions learn, choose and can be fooled.
60.5 Problem: The Reflex Bench
Problem 60.1
Weekend problem — the knee-jerk, instrumented
The classic bench demonstration: the seated volunteer, the little hammer below the kneecap, the foot’s involuntary kick — now instrumented with timers and this chapter.
Part I — The circuit.
- The hammer stretches a tendon; the foot kicks about a fiftieth of a second later. Lay out the circuit: which neuron kinds, through where — and why the brain is not on this loop.
- A fiftieth of a second is faster than the ball-catch’s fifth. Account for the difference with the circuit’s length and junction count.
- The volunteer cannot suppress the kick, though warned. What does this say about where the reflex is decided — and about one-way traffic at the synapses?
- Immediately after the kick, the volunteer feels the tap. Explain the delay’s second, slower journey.
Part II — Bench variations.
- Tap harder: the kick strengthens. Which coding rule of Proposition 60.2 showed itself?
- The class doctor explains that this reflex is tested at every check-up. What does a present, symmetrical knee-jerk certify about cord, nerves and junctions?
- A volunteer clenching their jaw kicks harder — a known amplification. What does this reveal about connecting neurons’ voices reaching even “brainless” loops?
- Local anesthetic in the leg abolishes both kick and feeling. Name the point of attack, by Remark 60.7.
Part III — The write-up.
- Draw the reflex as Proposition 33.2’s three-step loop, marking where this one shortcuts.
- Compare reflex and catch in a two-column table: path, junctions, time, suppressibility, role of practice.
- The write-up’s last section asks: “why do fast protective loops bypass the brain?” Answer with the hot-pan logic (Exercise 33.5).
- One sentence to close: what the bench showed about the nervous system’s language, wiring and junctions at once.
Solution
Solution of Problem 60.1.
1. The tendon stretch fires sensory neurons; they run to the spinal cord, synapse there directly onto motor neurons, and the motor drumroll kicks the thigh muscle. Two neuron kinds, one junction, cord-level — the brain is bypassed because protection cannot wait for deliberation.
2. The catch’s loop runs eye to brain to arm — metres of fiber and many weighing junctions; the knee-jerk runs tendon to cord to muscle — centimetres and essentially one synapse. Shorter wire, fewer junctions, a tenth the time.
3. The decision is made in the cord, before any brain veto can arrive — and the synapses’ one-way traffic means the brain’s later objection cannot run backward down the sensory line. Warning changes nothing: the loop closes below the will.
4. The feeling is a second journey: the same tap’s report also climbs the cord to the brain — a longer path with more junctions — arriving after the kick is already history.
5. Frequency coding: the harder stretch sends a faster drumroll, and the motor line answers in kind — stronger contraction from a higher rate, the one-word language at both ends.
6. That the whole circuit is sound: sensory line, cord junction, motor line and muscle — and symmetry certifies both sides alike. One tap audits four components.
7. That connecting neurons’ voices from elsewhere in the system reach even this loop’s junction, raising its readiness: the “brainless” reflex is still wired into the whole, its synapse hearing more than the tendon.
8. The synapses — and the fibers’ signalling generally: the anesthetic quiets the chemistry at the junctions and the message traffic with it, kick and feeling together.
9. Report (tendon stretch) — decide (one cord junction) — order (motor line): the loop with its middle step reduced to a single synapse, the brain’s weighing bypassed.
10. Reflex: tendon-cord-muscle, one junction, a fiftieth of a second, unsuppressible, practice-free. Catch: eye-brain-arm, uncounted junctions, a fifth of a second, fully voluntary, practice-built.
11. Because the brain’s weighing costs time, and a burn deepens by the millisecond: loops that protect tissue are soldered short, cord-level, and inform the brain afterward — feel it late, saved already.
12. “One hammer tap showed the one-word frequency language, the two-line wiring through the cord, and a junction that decides faster than its owner can.”