Lecture: Nervous Tissue, the Neuron & the Action Potential
Lecture script · about 15–20 minutes spoken · Week 12
Opening
Picture a friend holding a ruler above your open fingers at a kitchen table. They let go without warning, and your fingers snap shut a moment later. In that moment your eyes saw the ruler move, your brain decided to grab, and your hand obeyed. Every link in that chain was an electrical signal running through cells. That's what this lecture is about, and it matters whether you're headed for a clinic, a driver's seat, or a lab bench.
Here's where we are. In Week 3, you met a pump in the cell membrane that spends one ATP to move three sodium ions out and two potassium ions in. I told you then that the nerve weeks would spend that difference. In Week 5, you saw nervous tissue on a slide as large neurons among smaller support cells. In Week 10, you watched a motor neuron release acetylcholine across a narrow gap to start a muscle contraction. And in Week 11, you named the muscles that signal moves. In this lecture, we open the neuron itself.
Here's the plan. I'll walk you through the parts of a neuron and the cells that support it. Then we'll measure a neuron at rest, put its electrical spike in order, and see why that spike is all or nothing. We'll speed the signal up with myelin, carry it across a gap to the next cell, and finish by timing the whole chain with a ruler. Next week, in Week 13, you'll see where these signals do their work in the brain and spinal cord.
1. Reading the neuron, from dendrites to axon terminals
Think of a tree lying on its side. At one end is a crown of branches. Then comes a thick trunk, and 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 every 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. Let me describe one in words, from one end to the other. First come the branches. A dendrite is a short, branched extension of a neuron that receives signals from other neurons at synapses. Many branches mean many places to listen.
The branches lead into a rounded center. 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's also called the soma. From one side of the cell body, a tapering region leads into one long fiber. The axon hillock is the region of the cell body where the axon begins, and the axon is the single long extension of a neuron that carries the nerve impulse away from the cell body toward the axon terminals.
Many axons wear a wrapping. The myelin sheath is a wrapping made by glial cells around many axons that insulates them and speeds conduction. It isn't one smooth tube. It comes in segments, with small bare gaps between them, called the nodes of Ranvier. Those gaps are where the signal jumps. At the far end, the axon splits into small branches. An axon terminal is one of the branched endings of an axon, swollen into a synaptic end bulb that releases neurotransmitter.
So a signal flows one way. It comes in at the dendrites, gets added up in the cell body, leaves at the axon hillock, runs down the axon, and is handed off at the terminals.
Pause and try this. A classmate says the dendrites carry the signal away from the cell. What's the fix?
Think about which way the signal flows through the tree.
Here's the answer. The dendrites receive, so the signal flows from them toward the cell body. The axon is the part that carries it away, and the terminals are where it leaves.
2. Meeting the support crew, the neuroglia
Think of a concert. The musicians make the sound, but a crew keeps the power on, tapes down the cables, guards the doors, and sweeps the floor. In 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 depends on the address. The central nervous system, the CNS, is the brain and spinal cord. The peripheral nervous system, the PNS, is everything outside them. In the CNS there are four kinds. An astrocyte supports neurons and helps form the blood-brain barrier. An oligodendrocyte makes the myelin sheath around axons in the brain and spinal cord. Microglia are the resident immune cells, acting as macrophages. And an ependymal cell lines the brain's fluid-filled spaces and helps produce cerebrospinal fluid.
In the PNS there are two kinds. A Schwann cell makes the myelin sheath around peripheral axons. A satellite cell surrounds and supports neuron cell bodies in ganglia. That's 6 kinds in all.
Here's the trap I want you to see coming. Two different cells make myelin, and the names don't hint at where they work. So people say oligodendrocytes myelinate the PNS. They don't. Oligodendrocytes work inside the brain and spinal cord, and Schwann cells work out in the body.
Pause and try this. An axon runs in a nerve down the leg. Which glial cell made its myelin?
Name the address first, then the cell.
Here's the answer. A nerve in the leg is in the PNS, so a Schwann cell made its myelin. An oligodendrocyte would only wrap an axon inside the brain or spinal cord.
3. Measuring the neuron at rest
A phone battery sitting in a drawer isn't doing anything, yet it holds a charge, ready the moment you switch the phone on. A neuron at rest is like that. It isn't firing, but it holds a small electrical charge across its membrane. 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. The unit, the millivolt, is written mV. The resting membrane potential is the membrane potential of a neuron that is not firing, about minus 70 millivolts with the inside negative.
That minus sign carries the meaning. It tells you the inside of the cell is negative compared with the outside. Circle it every time you write the resting value.
What holds the charge? Ion gradients. More sodium sits outside the cell, and more potassium sits inside. The exact concentrations vary with the method and the estimate, so this course teaches them in words. In the background, the sodium-potassium pump keeps those gradients in place, moving 3 sodium ions out and 2 potassium ions in for every ATP it spends.
Pause and try this. A study note reads "resting potential: about plus 70 millivolts." What's wrong with it?
Look at the sign before you look at the number.
Here's the answer. The sign is wrong. A resting neuron sits at about minus 70 millivolts, because the inside is negative compared with the outside.
4. Putting the action potential in order
Think of a wave sweeping around a stadium. Each section stands, then sits, and then the next section takes its turn, always in the same 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 plus 30 millivolts and returns to rest. Its steps never change their order, so the order is something you can be graded on.
Start at rest, about minus 70 millivolts. A stimulus nudges the membrane upward. Threshold is the membrane potential, about minus 55 millivolts, at which voltage-gated sodium channels open and an action potential is triggered. Now comes depolarization, the rising phase, in which sodium ions rush into the neuron and the inside becomes less negative and then positive. It climbs to a peak of about plus 30 millivolts.
Then the sodium channels close and the potassium channels open. That's repolarization, the falling phase, in which potassium ions flow out so the inside turns negative again. The potassium channels are slow to close, so the membrane dips briefly below rest. That's hyperpolarization. Then the neuron settles back to rest.
Let's put numbers on two of those moves, using the conventional values. From rest at minus 70 to threshold at minus 55, you subtract a negative, which is the same as adding: the rise to threshold is 15 millivolts. From rest at minus 70 to the peak at plus 30, the upstroke is 100 millivolts. A simulation's screen may show slightly different readings, but those two differences are the ones this course keys.
Pause and try this. Which ion drives depolarization, and which way does it move?
Say the ion and the direction out loud.
Here's the answer. Sodium drives depolarization, and it moves in. Potassium leaving is repolarization, the falling phase, never the rise.
5. Firing all or none, and the pump versus the channels
A light switch is either off or on. Flip it gently or slam it, and the bulb glows the same. A neuron past threshold works 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 say "strong"? It fires more often. A firm press on your arm sends more spikes than a light brush, but each spike is the same full size.
Two kinds of membrane proteins share the work, at different times. During the spike, the voltage-gated channels let ions rush through, sodium in and then potassium out, each one downhill, with no ATP spent. Afterward, the sodium-potassium pump restores the gradients, moving 3 sodium out and 2 potassium in per ATP. That's uphill work, so it's paid for. I like to put it in four words: channels rush, the pump pays.
Pause and try this. A card says the pump moves 3 sodium in and 2 potassium out. Fix it.
Think back to the pump from Week 3.
Here's the answer. The card has it reversed. The pump moves 3 sodium ions out and 2 potassium ions in for every ATP.
6. Speeding the signal with myelin
Picture a hiker crossing a creek. One way is to wade 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 stones.
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. The myelin insulates the stretches between nodes, and the bare nodes are where the signal reappears. Skipping the insulated stretches is what makes it faster. Conduction speeds vary with the method and the estimate, so we describe them only in words.
The clinical connection, in standard terms, is this: a disease that strips myelin in the brain and spinal cord slows or blocks signals. Take away the insulation, and the jumps fail. We name the structure and stop there.
This story has a named history. Working on the squid giant axon, Alan Hodgkin and Andrew Huxley showed in their 1952 papers 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.
Pause and try this. Does myelin slow a signal down or speed it up?
Think of the stepping stones.
Here's the answer. Myelin speeds the signal up. It insulates the axon so the action potential can jump from node to node.
7. Crossing the synapse
Picture a relay race with a river in the middle. A runner sprints to the bank, but nobody can run on water, so a small boat carries the baton across, and a second runner takes it on the far side. Your signal crosses its gaps the same way.
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. Here's the order, with its places. First, the action potential reaches the axon terminal. Second, voltage-gated calcium channels in the terminal open, and calcium flows in. Third, each synaptic vesicle, a small membrane sac inside the terminal that stores neurotransmitter, fuses with the membrane and releases its contents into the synaptic cleft by exocytosis. Fourth, the neurotransmitter diffuses across the cleft and binds receptors on the next cell.
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, and the acetylcholine you met at the neuromuscular junction is one too. So the signal is electrical along the axon, chemical across the gap, and electrical again beyond it.
Pause and try this. A friend says the signal sparks across the synapse like electricity jumping a gap. What crosses instead?
Think of the boat in the relay.
Here's the answer. A chemical crosses the cleft. The neurotransmitter carries the message across, and a new electrical signal starts on the far side.
8. Timing the whole chain with a ruler
Go back to the kitchen table. Catching a falling ruler takes the whole chain: eyes, action potentials to the brain, a command, action potentials down to the hand, synapses at every handoff, and muscles that respond. How far the ruler falls tells you 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.
Here's the illustrative set the course uses, engineered to land on clean numbers. Five catches land at 18, 22, 19, 21, and 20 centimeters. Add them, and you get 100 centimeters. Divide by the 5 catches, and the average is 20.0 centimeters. The longest minus the shortest, 22 minus 18, gives a range of 4 centimeters. And in words, a 20 centimeter catch corresponds to a reaction time of about 0.20 seconds.
In the Watch a Neuron Fire lab, this ruler drop is the at-home alternative to the neuron simulation. Your own catches are your own values, never a norm to meet.
Pause and try this. If someone's average catch were longer than 20 centimeters, would their chain be faster or slower?
Remember that the ruler keeps falling until the hand closes.
Here's the answer. Slower. A longer catch means the ruler fell farther before the hand closed, so the chain took more time.
Closing
Let me pull it together, in the order we built it. A neuron's dendrites receive, its cell body integrates, and its axon carries the impulse to the terminals, with myelin and the nodes of Ranvier along the way. Oligodendrocytes make myelin in the CNS, and Schwann cells make it in the PNS. A resting neuron sits at about minus 70 millivolts, inside negative, because the pump keeps sodium outside and potassium inside.
The action potential runs rest, depolarization, repolarization, hyperpolarization, rest: sodium in to rise, potassium out to fall, with a 15 millivolt rise to threshold and a 100 millivolt upstroke. Every spike is full size, the channels rush, and the pump pays. Myelin lets the signal jump, a synapse turns it chemical and back, and a ruler drop times the whole chain.
Next in this module come the slides, the Readings and Resources page, the tutorial, the practice set, the quiz, the discussion, the assignment, and the Watch a Neuron Fire lab. Next week, in Week 13, The Central Nervous System, you'll see where each of these signals does its job in the brain and spinal cord.
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