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Week 12 · Chapter

Chapter 12 — Nervous Tissue, the Neuron & the Action Potential

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~45–55 minutes · Objective 6 (describe the structure of neurons and neuroglia and explain the physiology of the nerve impulse — the resting membrane potential, the ordered phases of the action potential with the ion that drives each, the all-or-none principle, saltatory conduction, and chemical transmission across a synapse), Objective 2 (relate the chemistry of the body to its function — atoms and the three bond types, water's properties, the logarithmic pH scale with the blood's buffered range, and the four classes of organic compounds — and explain the cell's membrane, organelles, and transport (diffusion, osmosis and tonicity, active transport, and bulk transport), the overview of how a cell makes ATP, and how a gene is read into a protein) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Watch a Neuron Fire lab all build on it.

In plain language

Every touch you feel and every move you make rides on a fast electrical signal carried by one kind of cell, the neuron (a nerve cell that receives, combines, and sends signals). This week you take one neuron apart, from the branches that listen to the long fiber that talks, and you meet the 6 kinds of support cells that keep it working. You learn why a resting neuron holds a small electrical charge, and how a pump that trades 3 sodium ions for 2 potassium ions keeps that charge ready. Then you follow the action potential (the brief electrical spike a neuron fires) through its fixed order of steps, and you watch it cross a tiny gap to the next cell as a chemical. You finish with a ruler you catch after it falls about 20 cm, which turns the whole chain into a number you can measure at home.

Why you already care about this

Sit at a kitchen table and have a friend hold a ruler above your open fingers. Wait. When the ruler drops without warning, your fingers snap shut a moment later, and the ruler stops a few centimeters lower than where it started. In that moment your eyes saw the ruler move, your brain decided to grab, and your hand obeyed.

Picture another moment on a night drive. A shape appears in the headlights, and your foot is already on the brake before you finish the thought. Or picture a cat's paw brushing your ankle under the table, and the way you feel it before you see it. Each of those moments is a chain of electrical signals running through cells, handed from one cell to the next across tiny gaps.

This chapter builds that chain one link at a time. You start with the shape of a single nerve cell, then you measure it at rest, then you watch it fire in a fixed order, and finally you time the whole chain with a ruler.

By the end of this chapter you will be able to:

name the parts of a neuron in order and state the job each shape makes possible;
sort the six kinds of neuroglia into the CNS and the PNS, and say which cell makes myelin where;
state the resting membrane potential with its sign and explain what holds it;
put the phases of the action potential in order, with the ion and direction that drive each one, and compute the 15 mV rise and the 100 mV upstroke;
explain the all-or-none principle and tell the work of the pump from the work of the channels;
trace a signal across a synapse in order, and average five ruler-drop catches with every step shown.

Carry this line with you: Na⁺ in to rise; K⁺ out to fall — never the reverse.

Where we are

Already covered: Week 3, The Cell & Membrane Transport, gave you the sodium-potassium pump, concentration gradients, and exocytosis. Week 5, Tissues, named nervous tissue as one of the four tissue types. Week 10, Muscle Tissue & the Physiology of Contraction, showed a motor neuron releasing acetylcholine at the neuromuscular junction.
This week: the neuron's parts, the neuroglia, the resting membrane potential, the ordered phases of the action potential, the all-or-none principle, saltatory conduction, and chemical transmission across the synapse.
How they connect: the pump from Week 3 builds the gradients a neuron spends, and the junction from Week 10 turns out to be one kind of synapse.
Where we're going next: Week 13, The Central Nervous System, opens a new unit on where each job lives in the brain and spinal cord.

In Week 3 you met a pump that spends one ATP to move 3 Na⁺ out of a cell and 2 K⁺ in. That uphill work leaves more sodium outside the cell and more potassium inside it, and you were told the nerve weeks would spend that difference. This is that week. In Week 5 you read nervous tissue on a slide as large neurons among smaller support cells. In Week 10 you saw a motor neuron release acetylcholine, the chemical messenger that starts a muscle contraction, across a narrow gap called the synaptic cleft. Last week you named the muscles that signal moves. This chapter opens the neuron itself and shows you how it makes the signal in the first place.

1. Read the neuron: from dendrites to axon terminals

Think of a tree lying on its side. At one end is a crown of branches, then a thick trunk, then a long root that runs a long way and splits into small tips at the far end. A neuron has much the same shape, and each part's shape fits its job.

A neuron is a nerve cell that receives, integrates, and sends electrical signals; it has dendrites, a cell body, and an axon.

Now picture the labeled figure in words, from left to right. On the left, a cluster of short, branching arms reaches out in every direction. Those arms lead into a rounded central body with a dark nucleus inside. From one side of that body a cone-shaped region tapers into a single long fiber. Along most of the fiber sit sausage-shaped wrappings with small bare gaps between them. At the far right the fiber splits into small branches, each ending in a swollen knob.

Read each part as structure, then function:

A dendrite is a short, branched extension of a neuron that receives signals from other neurons at synapses. Many branches mean many places to receive.
The cell body is the part of a neuron that holds the nucleus and most organelles and integrates the signals arriving from the dendrites; it is also called the soma.
The axon is the single long extension of a neuron that carries the nerve impulse away from the cell body toward the axon terminals.
The axon hillock is the region of the cell body where the axon begins.
An axon terminal is one of the branched endings of an axon, swollen into a synaptic end bulb that releases neurotransmitter.
The myelin sheath is a wrapping made by glial cells around many axons that insulates them and speeds conduction.
The nodes of Ranvier are the gaps in the myelin sheath along an axon, where the action potential jumps from one gap to the next.

Worked example — follow one signal through one neuron. A signal arrives from a neighboring cell. You name each part it passes, in order, and the job of each.

The dendrites receive the incoming signal at their synapses.
The cell body integrates what arrives from all its dendrites.
The signal leaves at the axon hillock and travels down the axon.
On a myelinated axon, the signal jumps from one node of Ranvier to the next.
The signal reaches the axon terminals, which pass it to the next cell.

In words: dendrites in, cell body adds up, axon out, terminals hand off.

Misconception check. Students often say the dendrites carry the signal away from the cell, because dendrites and axon terminals both look like little branches in a drawing. Use direction to tell them apart. Dendrites receive, so the signal flows from them toward the cell body. The axon carries the signal away from the cell body, and its terminals are where it leaves.

Memory hook: dendrites listen, the axon talks.

2. Meet the support crew: the neuroglia

Think of a concert. The musicians make the sound, and a crew around them keeps the power running, tapes down the cables, guards the doors, and sweeps the floor. In your nervous tissue, neurons carry the signal and a crew of support cells makes that possible.

Neuroglia are the support cells of nervous tissue, which make myelin, support and protect the neurons, and act as immune cells alongside the neurons that carry the signal.

The crew differs by location. The central nervous system, or CNS, is the brain and spinal cord. The peripheral nervous system, or PNS, is everything outside the brain and spinal cord. Next week gives both their full tour; here they are two addresses.

The table sorts the six kinds of neuroglia by where they work and what they do.

The six kinds of neuroglia, sorted by nervous-system division and job
Division	Glial cell	Job
CNS	astrocyte	supports neurons and helps form the blood-brain barrier
CNS	oligodendrocyte	makes the myelin sheath around CNS axons
CNS	microglia	the resident immune cells, acting as macrophages
CNS	ependymal cell	lines the brain's fluid-filled spaces and helps produce cerebrospinal fluid
PNS	Schwann cell	makes the myelin sheath around peripheral axons
PNS	satellite cell	supports neuron cell bodies in ganglia

An astrocyte is a star-shaped glial cell of the brain and spinal cord that supports neurons and helps form the blood-brain barrier.
An oligodendrocyte is a glial cell of the brain and spinal cord that makes the myelin sheath around axons there.
Microglia are the resident immune cells of the brain and spinal cord, which act as macrophages.
An ependymal cell is a glial cell that lines the brain's fluid-filled spaces and helps produce cerebrospinal fluid.
A Schwann cell is a glial cell outside the brain and spinal cord that makes the myelin sheath around peripheral axons.
A satellite cell is a glial cell outside the brain and spinal cord that surrounds and supports neuron cell bodies in ganglia.

Worked example — name the myelin maker. For each axon, you first name its division, then name the glial cell that wraps it.

An axon in the spinal cord sits in the CNS, so an oligodendrocyte makes its myelin.
An axon in a nerve running down the leg sits in the PNS, so a Schwann cell makes its myelin.

In words: same job, two different cells, chosen by the address.

Misconception check. Students often say oligodendrocytes myelinate the PNS. It is tempting because both cells make myelin, and the names do not hint at a location. Pin each cell to its address: oligodendrocytes work in the CNS, and Schwann cells work in the PNS.

Memory hook: oligodendrocytes inside, Schwann cells outside.

3. Measure the neuron at rest: a charge with a minus sign

A phone battery sitting in a drawer is not doing anything, yet it holds a charge, ready to be spent the moment you switch the phone on. A neuron at rest is like that battery. It is not firing, but it holds a small electrical charge across its membrane, ready to be spent. The battery is a classroom model, a picture you can hold in one hand, not a full description of a real cell.

A membrane potential is the difference in electrical charge across a cell's plasma membrane, measured in millivolts with the inside compared to the outside.
mV stands for millivolts, the unit of a membrane potential; one thousandth of a volt.
The resting membrane potential is the membrane potential of a neuron that is not firing, about −70 mV with the inside negative.

Circle the minus sign every time you write −70 mV. The minus tells you the inside of the cell is negative compared with the outside.

What holds that charge? Ion gradients. More Na⁺ sits outside the cell, and more K⁺ sits inside. The exact concentrations on each side are figures that vary with the method and the estimate, so this course teaches them in words and never keys them. In the background, the sodium-potassium pump you met in Week 3 keeps the gradients in place, moving 3 Na⁺ out and 2 K⁺ in per ATP.

Worked example — read the resting neuron. You state the resting value, its sign, and what keeps it there.

The resting value is about −70 mV.
The sign is negative, so the inside of the cell is negative compared with the outside.
The gradients hold it: more Na⁺ outside, more K⁺ inside.
The pump keeps the gradients: 3 Na⁺ out and 2 K⁺ in per ATP. That is 3 Na⁺ out versus 2 K⁺ in, a 3:2 ratio for every ATP spent.

In words: a resting neuron sits at about −70 mV, inside negative, because the pump keeps sodium crowded outside and potassium crowded inside.

Misconception check. Students often write the resting potential as a positive number, or drop the sign and write 70 mV. It is tempting because the sign is easy to lose when you copy a number. But the sign is the whole point: the inside of a resting neuron is negative. Write −70 mV every time, and treat a positive resting value as an error to catch.

Memory hook: at rest, the inside is minus.

4. Put the action potential in order

Watch a crowd at a stadium as a wave sweeps around the stands. Each section stands up, then sits down, then the next section takes its turn, always in the same order. An action potential is a wave like that, sweeping along a neuron's membrane, and its steps never change their order.

An action potential is a brief, all-or-none reversal of the membrane potential that travels along an axon; it rises from rest through threshold to a peak of about +30 mV and returns to rest.
Threshold is the membrane potential, about −55 mV, at which voltage-gated Na⁺ channels open and an action potential is triggered.
A voltage-gated Na⁺ channel is a sodium channel in a neuron's membrane that opens when the membrane reaches threshold, letting Na⁺ rush in during depolarization.
Depolarization is the rising phase of the action potential, in which Na⁺ rushes into the neuron and the inside becomes less negative and then positive.
Repolarization is the falling phase of the action potential, in which Na⁺ channels close, K⁺ channels open, and K⁺ flows out so the inside turns negative again.
Hyperpolarization is a brief dip of the membrane potential below the resting value after repolarization, because the K⁺ channels are slow to close.

Here is the order as one chain, the way you will write it all term:

rest (about −70 mV) → depolarization (Na⁺ in, from threshold at −55 mV to the peak at +30 mV) → repolarization (K⁺ out, back toward −70 mV) → hyperpolarization (briefly below −70 mV) → rest.

Worked example — Dataset I, the action-potential values (conventional approximations). These are conventional approximate values. The table lines up each phase with its membrane potential and its ion.

Dataset I: the phases of the action potential with their membrane potentials, in mV, and their ions
Phase	Membrane potential	Ion and direction
Rest	about −70 mV, inside negative	the pump holds the gradients (3 Na⁺ out, 2 K⁺ in)
Threshold	about −55 mV	none; the trigger point
Depolarization	rises to a peak of about +30 mV	Na⁺ in
Repolarization	falls back toward −70 mV	K⁺ out
Hyperpolarization	briefly below −70 mV	K⁺ channels slow to close

Now you work the two signed differences, step by step.

Rise to threshold: from rest at −70 mV to threshold at −55 mV is −55 − (−70) = −55 + 70 = 15 mV.
Upstroke: from rest at −70 mV to the peak at +30 mV is 30 − (−70) = 30 + 70 = 100 mV.

In words: a stimulus has to lift the membrane 15 mV to reach threshold, and once it does, the membrane swings a full 100 mV from rest to peak. Subtracting a negative number is the same as adding its size, which is why both answers come out larger than you might first guess. A simulation's on-screen reading may differ a little from these conventional values; this course keys the sign, the order, the ion, and the two differences, never one screen's exact number.

Misconception check. Students often say depolarization is potassium leaving. It is tempting because both phases involve an ion crossing the membrane, and K⁺ is the ion the pump brings in. Tie each phase to its ion and direction. Depolarization is Na⁺ rushing in, which makes the inside positive. Repolarization is K⁺ flowing out, which makes it negative again. And the phases always run in the same order: rest, depolarization, repolarization, hyperpolarization, rest.

Memory hook: sodium in, the spike goes up; potassium out, it comes back down.

5. Fire all or none: the pump versus the channels

A light switch is either off or on; there is no half-on. Flip it gently or slam it, and the bulb glows the same. A neuron past threshold behaves like that switch, not like a dimmer.

The all-or-none principle is the rule that once threshold is reached a neuron fires a spike of the same full size every time; a stronger stimulus produces more spikes, not bigger ones.

So how does a neuron signal "strong" versus "gentle"? It fires more often. A firm press on your arm sends more spikes per moment than a light brush, but every spike is the same full size.

Two kinds of membrane proteins share the work of a spike, and they do different jobs at different times.

The voltage-gated channels move ions passively and fast during the spike. Na⁺ flows in and K⁺ flows out, each down its own gradient, with no ATP spent.
The sodium-potassium pump restores the gradients afterward, moving 3 Na⁺ out and 2 K⁺ in per ATP. That is uphill work, so it is paid in ATP.

Worked example — sort the job. For each event, you ask whether it happens during the spike or afterward, and whether it costs ATP. Then you name the protein.

Na⁺ rushes in during depolarization. During the spike, downhill, no ATP: a voltage-gated channel.
K⁺ flows out during repolarization. During the spike, downhill, no ATP: a voltage-gated channel.
Three Na⁺ are moved out and two K⁺ in. Afterward, uphill, paid in ATP: the sodium-potassium pump.

In words: the channels let the ions rush during the spike, and the pump pays to put them back.

Misconception check. Two slips travel together here. First, students often think a stronger stimulus makes a bigger spike. It makes more spikes; past threshold every spike is the same size. Second, students often write the pump backward, as 3 Na⁺ in and 2 K⁺ out. Fix the order with the Week 3 sentence: three sodium out, two potassium in.

Caveat: the switch is a classroom picture. Below threshold, small changes in the membrane do happen, and the details of those smaller signals are beyond this course.

Memory hook: channels rush, the pump pays.

6. Speed the signal: myelin and saltatory conduction

Picture a hiker crossing a creek. One way is to wade, step by step, through every bit of water. The faster way is to leap from one stepping stone to the next. An action potential on a myelinated axon takes the stepping-stone route.

Saltatory conduction is the way an action potential travels along a myelinated axon, jumping from one node of Ranvier to the next, which is faster than continuous conduction.
Continuous conduction is the way an action potential travels along an unmyelinated axon, moving along the membrane without jumping, which is slower than saltatory conduction.

Structure determines function here. The myelin sheath insulates the axon between the nodes, and the bare nodes are where the signal reappears. Skipping the insulated stretches is what makes the signal faster. A conduction speed is a figure that varies with the method and the estimate, so this course describes speed in words and never keys a number.

Worked example — predict the speed. You compare two axons and say which carries the signal faster, and why.

Axon A is wrapped in myelin with nodes of Ranvier. The signal jumps node to node: saltatory conduction.
Axon B has no myelin. The signal moves along the whole membrane: continuous conduction.
Saltatory conduction is faster, so axon A carries the signal faster.

In words: the myelinated axon wins because the signal skips the insulated stretches.

The clinical connection, in standard terms: a disease that strips myelin in the CNS slows or blocks signals. That illustrates the structure-to-function pairing exactly: take away the insulation, and the jumps fail. You name the structure and stop.

The discovery has a named history. Working on the squid giant axon, Alan Hodgkin and Andrew Huxley (1952; the 1963 Nobel Prize) showed that ions carry the nerve impulse, with sodium entering as the fiber is excited and potassium leaving as it recovers. They shared the 1963 Nobel Prize in Physiology or Medicine with John Eccles.

Misconception check. Students often say myelin slows the signal, because a wrapping sounds like something that would get in the way. It does the opposite. Myelin insulates the axon so the signal can jump from node to node, and that speeds it up.

Honesty note: this chapter describes speed only in words. Real axons differ in size and wrapping, and this course does not key any conduction speed.

7. Cross the synapse: electrical, then chemical, then electrical

Picture a relay race with a river in the middle. A runner sprints to the riverbank, but cannot run across water, so a small boat carries the baton across, and a second runner takes it on the far side. The signal in your nervous system crosses its gaps the same way: it changes form to cross, then changes back.

A synapse is the junction where a neuron passes its signal to another cell, made of the axon terminal, the synaptic cleft, and the membrane of the next cell.
A synaptic vesicle is a small membrane sac inside an axon terminal that stores neurotransmitter until Ca²⁺ triggers its release.
A neurotransmitter is a chemical messenger released from an axon terminal that crosses the synaptic cleft and binds receptors on the next cell; examples include norepinephrine, dopamine, serotonin, GABA, and glutamate.

Acetylcholine, the messenger you met at the neuromuscular junction in Week 10, is a neurotransmitter too. That junction is one kind of synapse.

Worked example — cross the synapse in order. You name each step and its place.

The action potential reaches the axon terminal.
Voltage-gated Ca²⁺ channels in the terminal open, and Ca²⁺ flows in.
Synaptic vesicles fuse with the membrane and release neurotransmitter into the synaptic cleft by exocytosis, the vesicle-emptying move from Week 3.
The neurotransmitter diffuses across the cleft and binds receptors on the next cell.

In words: the signal is electrical along the axon, chemical across the gap, and electrical again beyond it.

Misconception check. Students often say the signal sparks across the synapse, like electricity jumping a gap. It is tempting because the signal is electrical on both sides. But a chemical crosses the cleft. The electrical signal stops at the terminal, the neurotransmitter carries the message across, and a new electrical signal starts on the far side.

Memory hook: wire, then boat, then wire.

8. Time the whole chain: the ruler drop

Go back to the kitchen table from the start of this chapter. When you catch a falling ruler, a whole chain has to run. Your eyes see the drop, and action potentials carry that news to your brain. Your brain sends a command, and action potentials carry it down to your hand. Synapses pass the signal along at every handoff, and your hand muscles respond. The distance the ruler falls before you catch it measures how long that chain took.

Reaction time is the time between a stimulus and a person's response to it, read in the ruler drop from how far the ruler falls before the catch.

Worked example — Dataset J, the ruler drop (illustrative). These catch distances were engineered by the course's discipline pack to land on clean numbers; they are not measured data. Five catches land at 18, 22, 19, 21, and 20 cm.

Dataset J (illustrative): five ruler-drop catch distances, in cm
Catch	Distance (cm)
1	18
2	22
3	19
4	21
5	20

You add, divide, and read the spread, one move at a time.

Sum the catches: 18 + 22 + 19 + 21 + 20 = 100 cm.
Average the catches: 100 ÷ 5 = 20.0 cm.
Range of the catches: the longest minus the shortest, 22 − 18 = 4 cm.
Read the average: a 20 cm catch corresponds to a reaction time of about 0.20 s, stated in words.

In words: the average catch is 20.0 cm, and the five catches spread across a range of 4 cm. A shorter average distance would mean the chain of action potentials from eye to brain to hand, plus the muscle's response, finished sooner.

In practice (your tools). In the Watch a Neuron Fire lab, the ruler drop is the at-home alternative to the neuron simulation. If you record five catches in a free spreadsheet such as Google Sheets, with the distances in cells B2 to B6, the formula =AVERAGE(B2:B6) returns their average and =MAX(B2:B6)-MIN(B2:B6) returns their range. Your own numbers are your own values, never a norm to meet.

Misconception check. Students often read a longer catch distance as a faster reaction, because a bigger number sounds better. Reverse it. The ruler keeps falling until you catch it, so a longer distance means the chain took longer. A shorter catch is a faster reaction.

Limitation: a reaction time measures the whole chain, not one neuron. It includes the eye, the brain, many synapses, and the hand's muscles, so it cannot tell you how fast any single axon conducts.

Key terms

Neuron — A nerve cell that receives, integrates, and sends electrical signals; it has dendrites, a cell body, and an axon. (first used: 1. Read the neuron: from dendrites to axon terminals)
Dendrite — A short, branched extension of a neuron that receives signals from other neurons at synapses. (first used: 1. Read the neuron: from dendrites to axon terminals)
Cell body — The part of a neuron that holds the nucleus and most organelles and integrates the signals arriving from the dendrites; also called the soma. (first used: 1. Read the neuron: from dendrites to axon terminals)
Axon — The single long extension of a neuron that carries the nerve impulse away from the cell body toward the axon terminals. (first used: 1. Read the neuron: from dendrites to axon terminals)
Axon hillock — The region of the cell body where the axon begins. (first used: 1. Read the neuron: from dendrites to axon terminals)
Axon terminal — One of the branched endings of an axon, swollen into a synaptic end bulb that releases neurotransmitter. (first used: 1. Read the neuron: from dendrites to axon terminals)
Myelin sheath — A wrapping made by glial cells around many axons that insulates them and speeds conduction. (first used: 1. Read the neuron: from dendrites to axon terminals)
Nodes of Ranvier — The gaps in the myelin sheath along an axon, where the action potential jumps from one gap to the next. (first used: 1. Read the neuron: from dendrites to axon terminals)
Neuroglia — The support cells of nervous tissue, which make myelin, support and protect the neurons, and act as immune cells alongside the neurons that carry the signal. (first used: 2. Meet the support crew: the neuroglia)
Astrocyte — A star-shaped glial cell of the brain and spinal cord that supports neurons and helps form the blood-brain barrier. (first used: 2. Meet the support crew: the neuroglia)
Oligodendrocyte — A glial cell of the brain and spinal cord that makes the myelin sheath around axons there. (first used: 2. Meet the support crew: the neuroglia)
Microglia — The resident immune cells of the brain and spinal cord, which act as macrophages. (first used: 2. Meet the support crew: the neuroglia)
Ependymal cell — A glial cell that lines the brain's fluid-filled spaces and helps produce cerebrospinal fluid. (first used: 2. Meet the support crew: the neuroglia)
Schwann cell — A glial cell outside the brain and spinal cord that makes the myelin sheath around peripheral axons. (first used: 2. Meet the support crew: the neuroglia)
Satellite cell — A glial cell outside the brain and spinal cord that surrounds and supports neuron cell bodies in ganglia. (first used: 2. Meet the support crew: the neuroglia)
Membrane potential — The difference in electrical charge across a cell's plasma membrane, measured in millivolts with the inside compared to the outside. (first used: 3. Measure the neuron at rest: a charge with a minus sign)
mV — Millivolts, the unit of a membrane potential; one thousandth of a volt. (first used: 3. Measure the neuron at rest: a charge with a minus sign)
Resting membrane potential — The membrane potential of a neuron that is not firing, about −70 mV with the inside negative. (first used: 3. Measure the neuron at rest: a charge with a minus sign)
Action potential — A brief, all-or-none reversal of the membrane potential that travels along an axon; it rises from rest through threshold to a peak of about +30 mV and returns to rest. (first used: 4. Put the action potential in order)
Threshold — The membrane potential, about −55 mV, at which voltage-gated Na⁺ channels open and an action potential is triggered. (first used: 4. Put the action potential in order)
Voltage-gated Na⁺ channel — A sodium channel in a neuron's membrane that opens when the membrane reaches threshold, letting Na⁺ rush in during depolarization. (first used: 4. Put the action potential in order)
Depolarization — The rising phase of the action potential, in which Na⁺ rushes into the neuron and the inside becomes less negative and then positive. (first used: 4. Put the action potential in order)
Repolarization — The falling phase of the action potential, in which Na⁺ channels close, K⁺ channels open, and K⁺ flows out so the inside turns negative again. (first used: 4. Put the action potential in order)
Hyperpolarization — A brief dip of the membrane potential below the resting value after repolarization, because the K⁺ channels are slow to close. (first used: 4. Put the action potential in order)
All-or-none principle — The rule that once threshold is reached a neuron fires a spike of the same full size every time; a stronger stimulus produces more spikes, not bigger ones. (first used: 5. Fire all or none: the pump versus the channels)
Saltatory conduction — The way an action potential travels along a myelinated axon, jumping from one node of Ranvier to the next, which is faster than continuous conduction. (first used: 6. Speed the signal: myelin and saltatory conduction)
Continuous conduction — The way an action potential travels along an unmyelinated axon, moving along the membrane without jumping, which is slower than saltatory conduction. (first used: 6. Speed the signal: myelin and saltatory conduction)
Synapse — The junction where a neuron passes its signal to another cell, made of the axon terminal, the synaptic cleft, and the membrane of the next cell. (first used: 7. Cross the synapse: electrical, then chemical, then electrical)
Synaptic vesicle — A small membrane sac inside an axon terminal that stores neurotransmitter until Ca²⁺ triggers its release. (first used: 7. Cross the synapse: electrical, then chemical, then electrical)
Neurotransmitter — A chemical messenger released from an axon terminal that crosses the synaptic cleft and binds receptors on the next cell; examples include norepinephrine, dopamine, serotonin, GABA, and glutamate. (first used: 7. Cross the synapse: electrical, then chemical, then electrical)
Reaction time — The time between a stimulus and a person's response to it, read in the ruler drop from how far the ruler falls before the catch. (first used: 8. Time the whole chain: the ruler drop)

Summary

A neuron has dendrites that receive, a cell body that integrates, and an axon that carries the impulse to its terminals.
Myelin insulates the axon; the nodes of Ranvier are the gaps where the signal jumps.
Oligodendrocytes make myelin in the CNS; Schwann cells make it in the PNS.
A resting neuron sits at about minus 70 millivolts, inside negative.
More sodium sits outside the cell and more potassium inside; the pump keeps it that way.
The action potential runs rest, depolarization, repolarization, hyperpolarization, rest.
Depolarization is sodium in; repolarization is potassium out.
The rise to threshold is 15 millivolts, and the upstroke is 100 millivolts.
Past threshold every spike is the same size; a stronger stimulus makes more spikes.
The channels move ions fast and free; the pump restores the gradients with ATP.
Saltatory conduction on a myelinated axon is faster than continuous conduction.
At a synapse the signal is electrical, then chemical across the cleft, then electrical again.
A ruler-drop catch distance measures a whole chain; a shorter catch means a faster reaction.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative set of catches made up for this question, a friend catches a dropped ruler five times at 23, 27, 25, 29, and 26 cm. Find the sum, the average, and the range, and say whether this average points to a faster or a slower chain than a 20 cm average.
In an illustrative recording made up for this question, a stimulus lifts a resting neuron from −70 mV to −62 mV and then stops. Has the neuron reached threshold? How many more millivolts would it need?
A study card made up for this question lists the action potential as "rest, then K⁺ leaves and the inside turns positive, then Na⁺ enters and the inside turns negative, then rest." Rewrite the card in the correct order with the correct ions.
In an illustrative case made up for this question, an axon in a nerve of the forearm has lost its myelin sheath. Name the glial cell that made that myelin, and predict what happens to how the signal travels.

Answers (worked reasoning — read after attempting)

Sum: 23 + 27 + 25 + 29 + 26 = 130 cm. Average: 130 ÷ 5 = 26.0 cm. Range: the longest minus the shortest, 29 − 23 = 6 cm. A 26.0 cm average is longer than a 20 cm average, so the ruler fell farther before the catch, which points to a slower chain. A longer catch is a slower reaction, never a faster one.
Threshold is about −55 mV. The neuron rose from −70 mV to −62 mV, which is −62 − (−70) = −62 + 70 = 8 mV. It has not reached threshold. It would need −55 − (−62) = −55 + 62 = 7 mV more. Check: 8 + 7 = 15 mV, the full rise from rest to threshold. Below threshold, no action potential fires.
The card swapped the two ions. The correct order: rest at about −70 mV → depolarization, Na⁺ in, rising to a peak of about +30 mV → repolarization, K⁺ out, falling back toward −70 mV → hyperpolarization, briefly below rest → rest. Sodium entering makes the inside positive; potassium leaving makes it negative again.
A nerve in the forearm sits in the PNS, so a Schwann cell made that myelin, not an oligodendrocyte. Without myelin the signal can no longer jump from node to node, so it travels more slowly or may be blocked. Myelin speeds the signal; losing it slows the signal.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz; next week the reflex arc, the mechanism that turns these signals into a fast response, leads the tour of the central nervous system.

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