Anatomy & Physiology · Chapter 12

The Nervous System & Nervous Tissue

Every thought you have had was carried by a wave of electricity. So was every move you have made and every feeling you have felt. That wave races along a living wire. This chapter tells the story of that wire, the neuron, and the tissue it lives in. First we map how the whole nervous system is set up. Then we meet the loyal helper cells that keep neurons alive. Then we get to the heart of it. You will see how a neuron builds up an electrical charge and fires an action potential. You will see how that signal flies down an axon wrapped in myelin. And you will see how it hands the message to the next cell across a tiny gap. Learn these ideas well. The rest of the nervous system, from the brain and spinal cord to reflexes and the senses, is built on this one theme.

By the end of this chapter you can

Divide the nervous system into its central and peripheral parts. Also divide it into its sensory, motor, somatic, and autonomic pathways.
State the nervous system's three core jobs: sensory input, integration, and motor output. Give an example of each.
Name the parts of a neuron and the six neuroglia. Say what each glial cell does and where it lives.
Explain how the resting membrane potential of about −70 mV is created and kept steady.
Walk through the phases of an action potential. Explain why it is all-or-nothing and only goes one way.
Compare continuous and saltatory conduction. List the two things that set how fast a signal travels.
Describe the steps of chemical synaptic transmission. Tell an excitatory signal from an inhibitory one.
Link each process to a real medical problem, such as multiple sclerosis, local anesthetics, and nerve gas.

Section 1How the nervous system is organized

Diagram of the central and peripheral parts of the nervous system — **Reference.** Central and peripheral divisions of the nervous system — OpenStax, CC BY 4.0 (via Wikimedia Commons)

One control center, wires everywhere

The nervous system is the body's fast message network. Scientists split it up in two ways. One way is by where the parts are. The other is by which way the signals flow.

By location, there are just two parts. The central nervous system (CNS) is the brain and spinal cord. It is the headquarters, where information is handled and choices are made. The peripheral nervous system (PNS) is everything else. It is the nerves and ganglia (clusters of nerve cell bodies) that fan out from the CNS to every corner of the body. Think of the CNS as a computer. The PNS is the set of cables that run to every sensor and every motor.

Three jobs: sense, decide, act

A nerve pathway may look complex. But it is always doing one of three things. First comes sensory input. Receptors sense a change, inside or outside the body, and send that news toward the CNS. Second is integration. The CNS sorts through the incoming signals and decides what to do, if anything. Third is motor output. A command travels away from the CNS to an effector, which is a muscle or a gland. The effector carries out the response.

Touch a hot stove, and all three happen in a heartbeat. Skin receptors sense heat (input). The spinal cord decides "pull back" (integration). Arm muscles yank the hand away (output).

The words for these directions are worth learning. You will see them on every diagram. Neurons that carry signals toward the CNS are afferent, or sensory. Neurons that carry commands away from the CNS are efferent, or motor.

The efferent side splits again by where its signals go. The somatic nervous system carries commands you control to skeletal muscle. These are the moves you choose. The autonomic nervous system (ANS) runs the automatic chores inside you. It sends signals to cardiac muscle, smooth muscle, and glands without you having to think about it. The ANS itself has two arms that usually work against each other. The sympathetic division is the "fight-or-flight" gas pedal. The parasympathetic division is the "rest-and-digest" brake.

Division	What it includes	Direction / job
Central (CNS)	Brain and spinal cord	Integration: sorts input, sends out commands
Peripheral (PNS)	Nerves and ganglia	Carries signals to and from the CNS
Sensory (afferent)	Sensory neurons	Toward the CNS (input)
Motor (efferent)	Motor neurons	Away from the CNS (output)
Somatic	Motor neurons to skeletal muscle	Movement you choose (voluntary)
Autonomic (ANS)	Motor neurons to heart muscle, smooth muscle & glands	Automatic (involuntary) control
Sympathetic	Leaves the spinal cord from the chest and lower back (thoracolumbar)	Fight-or-flight (speed up)
Parasympathetic	Leaves from the brain and the base of the spine (craniosacral)	Rest-and-digest (calm down)

Afferent Arrives, Efferent Exits. The two words differ by just one letter. People mix them up for a lifetime. Tie each one to its first letter. Afferent signals Arrive at the CNS. Efferent signals Exit toward muscles and glands.

Tap through each division in the interactive below. Try central and peripheral, sensory and motor, and somatic and autonomic.

Interactive Split the nervous system by body part (central vs peripheral) and by which way signals flow (in vs out). Open full ↑

Key points

By location, the CNS is the brain and spinal cord. The PNS is the nerves and ganglia.
By job, afferent (sensory) signals arrive at the CNS. Efferent (motor) signals exit toward effectors.
The motor side splits into somatic (voluntary, skeletal muscle) and autonomic (involuntary). The autonomic side splits into sympathetic and parasympathetic.
Every act of the nervous system comes down to three steps: sensory input → integration → motor output.

Section 2Neurons and their support crew

Drawing of the six types of glial cells — **Reference.** The six types of neuroglia in the CNS and PNS — OpenStax, CC BY 4.0 (via Wikimedia Commons)

The neuron: a cell built to signal

Before we meet the helper cells, meet the star. A neuron is a nerve cell. It is an excitable cell, which means it can fire an electrical signal. Its shape suits one purpose. It takes in a signal at one end and passes it on from the other. Three parts do the work.

The dendrites are branches, like antennae, that receive incoming signals. The cell body, or soma, holds the nucleus. It keeps the cell alive and running. It is also where incoming signals are added up. The axon is one long fiber that carries the outgoing signal away. It is often wrapped in a fatty myelin sheath. It ends in axon terminals that hand the message to the next cell.

Neurons are also amitotic. This means most of them cannot divide to replace themselves. That is exactly why damage to the brain and spinal cord is so hard to repair.

The support crew: six kinds of glia

For every neuron, there are about as many neuroglia, or more. These are also called glial cells. They are the crew that feeds, insulates, cleans, and protects neurons. Glia do not fire action potentials. But without them, neurons would starve, short-circuit, and drown in waste. There are six types. Four live in the CNS, and two live in the PNS.

Glial cell	Where	Job
Astrocyte	CNS	The most common type. It ties neurons to their blood supply, controls the chemicals around them, and helps form the blood–brain barrier.
Oligodendrocyte	CNS	Wraps CNS axons in myelin. One cell covers many axons.
Microglia	CNS	The brain's immune cells. They swallow germs and dead cells.
Ependymal cell	CNS	Line the hollow spaces of the brain. They help move cerebrospinal fluid.
Satellite cell	PNS	Surround and support neuron cell bodies in ganglia
Schwann cell	PNS	Wraps PNS axons in myelin. One cell covers one segment.

Two cells make myelin, so keep them straight

People often mix up the two cells that make myelin. So let's pin them down now. In the CNS, an oligodendrocyte reaches out several arms. It wraps many different axons at once. In the PNS, a Schwann cell wraps itself around a single segment of one axon, like a jelly roll.

Both make the same fatty insulation. But the difference matters in medicine. Multiple sclerosis attacks CNS myelin. Guillain–Barré syndrome attacks PNS myelin. They are totally separate diseases, and they play out in different ways.

Microglia are the brain's cleanup crew. The blood–brain barrier keeps most immune cells out. So the CNS makes its own defenders. Microglia are always on patrol. When they sense injury or infection, they swarm. They swallow the debris and sound the alarm. That is why they play a part in nearly every brain disease.

Tap each of the six glial cells. See where it sits and what it does.

Interactive Six neuroglia, four in the CNS and two in the PNS, feed, insulate, clean, and protect neurons. Open full ↑

Key points

A neuron has three parts. The dendrites receive. The cell body/soma adds up signals and keeps the cell alive. The axon sends the signal on.
The six glia in the CNS are the astrocyte, oligodendrocyte, microglia, and ependymal cell. In the PNS, they are the satellite cell and Schwann cell.
Oligodendrocytes each wrap many CNS axons in myelin. Schwann cells each wrap one PNS segment.
Microglia are the immune cells of the CNS. Astrocytes are the most common glia and help form the blood–brain barrier.

Section 3The resting membrane potential

Diagram of the ions and channels that set the resting membrane potential — **Reference.** Ion gradients and channels that set the resting membrane potential — OpenStax, CC BY 4.0 (via Wikimedia Commons)

A charged battery, waiting

A neuron at rest is not doing nothing. It is armed and ready. There is a tiny voltage across its membrane. It is called the resting membrane potential, and it is about −70 mV. This means the inside of the cell is 70 millivolts more negative than the outside.

A membrane that keeps charges apart like this is called polarized. It is like a charged battery, ready to be used. Every signal a neuron sends is a quick, controlled release of that stored charge. So the first question is simple. How does the cell build up the charge in the first place?

Why the inside is negative

The answer is unequal ion distribution. Ions are charged particles. The cell keeps different amounts of each ion inside and outside. Sodium (Na⁺) is piled up outside the cell. Potassium (K⁺) is packed inside. The inside is also stuffed with large proteins and phosphates that carry negative charge. They are too big to leave.

Now add the membrane's channels. At rest, the membrane lets potassium leak through much more easily than sodium. It has many more open leak channels for K⁺. So potassium trickles out, from where there is more of it to where there is less. It carries positive charge with it. This leaves the inside more negative. The potassium leak, along with the trapped negative particles inside, is the main reason the resting voltage lands near −70 mV.

Ion	Concentrated where	Role at rest
Sodium (Na⁺)	Outside the cell	Ready to rush in. The membrane is mostly closed to it at rest.
Potassium (K⁺)	Inside the cell	Leaks out through many open channels. This sets the resting voltage.
Chloride (Cl⁻)	Outside the cell	Adds to the negative inside
Large anions (proteins)	Trapped inside	Cannot cross, so they hold negative charge in

The pump that never sleeps

Over time, leaks would run the gradients down. Sodium would trickle in. Potassium would drift out. The battery would go flat. To stop this, the membrane runs the sodium–potassium pump (Na⁺/K⁺ ATPase) all day and night. It burns ATP for fuel. It pumps 3 Na⁺ out for every 2 K⁺ in. This keeps rebuilding both gradients.

The pump moves three positive charges out for every two it brings in. So it also removes a little positive charge overall. This makes the inside slightly more negative. The pump is so vital that it uses a large share of the body's energy at rest. That is a big reason the brain is such a hungry organ.

Potassium is the voltage-setter. It is tempting to give the sodium–potassium pump credit for the resting potential. But the pump mainly keeps up the gradients. The −70 mV itself comes mostly from potassium leaking out through open K⁺ channels. That is why problems with blood potassium quickly upset the heart and nerves. Too much is hyperkalemia, and too little is hypokalemia.

Explore the ion gradients, the leak channels, and the tireless pump in the interactive below.

Interactive A resting neuron is a charged battery at about −70 mV. See the gradients, channels, and pump that build it. Open full ↑

Key points

The resting membrane potential is about −70 mV, with the inside negative. The membrane is polarized.
It comes from unequal ion distribution. Na⁺ is high outside, K⁺ is high inside, and large negative particles (anions) are trapped inside.
Potassium leaking out through open channels is the main thing that sets the resting voltage.
The Na⁺/K⁺ pump (3 out, 2 in, using ATP) keeps up the gradients. It also makes the inside a little more negative.

Section 4The action potential

Graph of the phases of an action potential — **Reference.** The phases of an action potential plotted against membrane voltage — OpenStax, CC BY 4.0 (via Wikimedia Commons)

All or nothing

An action potential is the neuron's signal. It is a quick, dramatic flip of the voltage across the membrane. This flip shoots down the axon. But it only happens if the membrane is pushed up from its resting −70 mV to a key level. That level is the threshold, at about −55 mV.

Below threshold, nothing fires. Reach threshold, and a full-sized spike goes off every time. The spike is the same height whether the push was barely enough or huge. This is the famous all-or-nothing rule. A neuron cannot fire part of an action potential, just as a gun cannot fire part of a bullet. So how does a neuron signal a strong stimulus? Not with bigger spikes. It fires more often.

The upstroke: sodium floods in

The whole event is run by voltage-gated channels. These are ion gates that open when the voltage changes. At threshold, voltage-gated Na⁺ channels snap open. Sodium was piled up outside. It is pulled in by two forces: its concentration gradient and the negative inside. So it floods in.

Each positive charge that enters makes the inside less negative. That opens even more sodium gates. This fast, self-boosting rush is called depolarization. It drives the voltage from −55 all the way up to about +30 mV. For an instant, the inside of the cell is actually positive.

The downstroke and the reset

The spike ends as fast as it began. At the peak, the Na⁺ channels inactivate, which means they shut on their own. Now the slower voltage-gated K⁺ channels finish opening. Potassium rushes out. It carries positive charge away, and the voltage drops back down. This is repolarization.

The potassium gates are slow to close. So a little too much K⁺ leaves. The membrane briefly dips below rest, to about −80 mV. This dip is called hyperpolarization, or the undershoot. Then the leak channels and the Na⁺/K⁺ pump bring things back to a tidy −70 mV.

During the recovery there is a refractory period. In the first part (absolute), the axon cannot fire at all. In the later part (relative), it needs an extra-strong stimulus to fire.

Phase	What the channels do	Voltage
Resting	Only leak channels are open	−70 mV
Threshold	Enough Na⁺ gates start to open	−55 mV
Depolarization	Voltage-gated Na⁺ channels open. Na⁺ rushes in.	rising to +30 mV
Repolarization	Na⁺ channels inactivate. K⁺ channels open, and K⁺ leaves.	falling back down
Hyperpolarization	K⁺ channels close slowly, so the voltage dips a bit too far	~−80 mV
Return to rest	Leak channels + Na⁺/K⁺ pump restore the balance	−70 mV

The refractory period makes signals one-way. Right after a stretch of membrane fires, its sodium channels are inactivated. For a moment, they cannot reopen. So the action potential has to keep moving forward into fresh membrane. It cannot go backward. The refractory period also caps how fast a neuron can fire. It is a built-in speed limit that protects the code.

Hit "Stimulate" in the interactive below. Follow the voltage through every phase.

Interactive Reach threshold at −55 mV and fire an all-or-nothing spike. Sodium goes in, then potassium goes out. Open full ↑

Key points

An action potential fires only if the membrane reaches threshold (~−55 mV). Then it is all-or-nothing.
Depolarization: voltage-gated Na⁺ channels open, Na⁺ floods in, and the voltage climbs to about +30 mV.
Repolarization: Na⁺ channels inactivate, K⁺ channels open, K⁺ leaves, and the voltage falls. A brief hyperpolarization follows.
The refractory period keeps the signal moving one way and limits how fast a neuron fires. The strength of a stimulus is coded by frequency.

Section 5Saltatory conduction & speed

Diagram of a signal jumping between gaps in the myelin of an axon — **Reference.** An impulse leaping between nodes of Ranvier on a myelinated axon — OpenStax, CC BY 4.0 (via Wikimedia Commons)

Crawling versus leaping

Once an action potential starts, it has to travel the whole length of the axon. There are two ways to do that. Some axons are bare, or unmyelinated. On these, the spike rebuilds itself at every patch of membrane in a row. It marches along step by step. This is continuous conduction. It works, but it is slow.

Other axons are myelinated. The fatty sheath insulates long stretches, so current cannot leak out there. The action potential can only rebuild itself at the bare gaps between the myelin segments. So the signal seems to jump from gap to gap. It skips the insulated parts in between. This leaping is saltatory conduction. The name comes from the Latin word saltare, which means "to leap." It is much faster.

Nodes of Ranvier

The bare gaps have a name worth knowing: the nodes of Ranvier. These small bare rings are packed with voltage-gated sodium channels. So the action potential is rebuilt fresh and at full strength at each node. Between the nodes is a covered stretch called the internode. It carries the current quickly and passively, with no channels to slow it down. So the impulse hops from node to node. It rebuilds, glides, rebuilds, and glides. This covers far more ground in the same time than a steady crawl.

What sets conduction speed

Two features decide how fast a nerve fiber carries a signal. The first is myelination. A myelinated axon is much faster than an unmyelinated one of the same size. The second is axon diameter, or how thick the axon is. Thick axons are faster because current flows through them more easily. In the same way, a wide pipe moves water faster than a narrow one.

The body uses both of these on purpose. Large fibers with thick myelin carry reflexes and the sense of where your limbs are. They can carry signals at over 100 meters per second. Thin fibers with no myelin carry dull, aching pain. They crawl along at barely one meter per second. Sharp pain that you can pinpoint travels on thinner myelinated fibers. These are faster than the dull kind, but far short of that top speed.

Factor	Faster conduction	Slower conduction
Myelin	Myelinated (saltatory)	Unmyelinated (continuous)
Axon diameter	Large diameter (thick)	Small diameter (thin)
Typical role	Reflexes, sharp pain you can pinpoint, limb position	Dull aching pain, temperature, autonomic signals

Strip the myelin and the signal fails. In multiple sclerosis, the immune system destroys CNS myelin. Without insulation, the current leaks away. Signals slow down or stop. This causes weakness, numbness, and loss of vision. In Guillain–Barré syndrome, the same thing happens to PNS myelin. It causes paralysis that climbs up the body. Myelin is not optional. It is the difference between a nerve that works and one that does not.

Race a myelinated signal against an unmyelinated one in the interactive below. Watch it leap the nodes.

Interactive A myelinated signal leaps from node to node and skips the insulated stretches. Watch two signals race. Open full ↑

Key points

Continuous conduction rebuilds the spike at every patch. It is slow and happens on unmyelinated axons. Saltatory conduction leaps between gaps. It is fast and happens on myelinated axons.
The bare gaps are the nodes of Ranvier, packed with Na⁺ channels. The covered internodes carry current passively.
Speed goes up with myelination and with axon diameter.
Diseases that strip myelin slow or block signals. Examples are MS in the CNS and Guillain–Barré in the PNS.

Section 6The synapse: passing the message on

Diagram of a chemical synapse between two neurons — **Reference.** A chemical synapse handing a signal from one neuron to the next — Young, KA., Wise, JA., DeSaix, P., Kruse, DH., Poe, B., Johnson, E., Johnson, JE., Korol, O., Betts, JG., & Womble, M., CC BY 4.0 (via Wikimedia Commons)

Neurons don't quite touch

An action potential reaches the end of an axon. Now it has a problem. The next cell is not connected to it. There is a tiny gap in between, called the synaptic cleft. The whole meeting point is the synapse. The neuron sending the message is presynaptic ("before the synapse"). The cell getting it is postsynaptic ("after the synapse").

The electrical signal cannot jump the gap by itself. So most synapses turn it into a chemical messenger, called a neurotransmitter. The chemical drifts across the gap. On the far side, it creates the signal again. The message goes from electrical, to chemical, to electrical again.

Six steps across the gap

Chemical transmission always follows the same six steps. (1) The action potential arrives at the presynaptic axon terminal. (2) Its voltage opens voltage-gated calcium (Ca²⁺) channels, and calcium flows into the terminal. (3) Calcium makes tiny sacs called synaptic vesicles merge with the membrane and dump what they hold.

(4) The neurotransmitter is released (by exocytosis) and drifts across the cleft. (5) It binds receptors on the postsynaptic membrane. This opens ion channels, which create a small graded potential. (6) The neurotransmitter is quickly cleared away. Enzymes may break it down. It may be taken back into the sending cell (reuptake). Or it may drift away. This keeps the signal crisp and short.

Excitatory or inhibitory?

What the postsynaptic cell does next depends on which neurotransmitter and receptor are involved. An excitatory signal depolarizes the postsynaptic membrane. It nudges the cell toward threshold. This is an excitatory postsynaptic potential (EPSP). An inhibitory signal hyperpolarizes the membrane. It pushes the cell away from threshold. This is an inhibitory postsynaptic potential (IPSP).

One neuron gets thousands of these signals at once. It sums, or adds up, all of them. It fires only if the "go" signals beat the "stop" signals by enough to reach threshold. In other words, the synapse is where the nervous system actually makes decisions.

Neurotransmitter	Typical effect	Where / note
Acetylcholine (ACh)	Excites skeletal muscle	Neuromuscular junction (nerve meets muscle). Broken down by acetylcholinesterase.
Glutamate	Excitatory	The main “go” transmitter of the CNS
GABA	Inhibitory	The main “stop” transmitter of the CNS
Glycine	Inhibitory	Common in the spinal cord
Norepinephrine	Excites or inhibits	Sympathetic (fight-or-flight), mood, alertness
Dopamine	Modulatory (tunes other signals up or down)	Reward, movement. Disrupted in Parkinson disease.

The synapse is where the drugs act. Sending a signal across a synapse takes many steps. So it is easy to disrupt, and most medicines and poisons do just that. Nerve gases block acetylcholinesterase, so ACh never gets cleared. Many antidepressants block serotonin reuptake. Caffeine, opioids, and general anesthetics all work at synapses. (Local anesthetics, such as lidocaine, work a bit earlier. They block voltage-gated Na⁺ channels, so the axon cannot fire action potentials.) Learn this handoff, and you hold the key to how drugs work.

Step through the electrical-to-chemical-to-electrical handoff in the interactive below.

Interactive An electrical signal turns chemical to cross the cleft, then turns electrical again. Step through the handoff. Open full ↑

Key points

Neurons meet at a synapse, across the synaptic cleft. The sender is presynaptic. The receiver is postsynaptic.
Transmission: action potential (AP) arrives → Ca²⁺ enters → vesicles merge → neurotransmitter released → binds receptors → cleared.
An EPSP pushes the postsynaptic cell toward threshold. An IPSP pushes it away. The cell sums them to decide whether to fire.
Key transmitters are ACh (muscle), glutamate (main excitatory), and GABA (main inhibitory). The synapse is where most drugs act.

The chapter in one breath

The nervous system splits into the CNS (brain and spinal cord) and the PNS (nerves and ganglia). Its work is always sensory input → integration → motor output.
Motor pathways split into somatic (voluntary) and autonomic (involuntary). The autonomic side splits into sympathetic and parasympathetic. Afferent arrives, and efferent exits.
A neuron (dendrites, soma, axon) is supported by six neuroglia. The astrocyte, oligodendrocyte, microglia, and ependymal cell are in the CNS. The satellite cell and Schwann cell are in the PNS.
At rest, the membrane is polarized to about −70 mV. This is set by K⁺ leaking out and by trapped negative particles inside. The Na⁺/K⁺ pump keeps it steady.
Reaching threshold (~−55 mV) fires an all-or-nothing action potential. Na⁺ comes in (depolarize), then K⁺ goes out (repolarize). A refractory period follows.
Myelin lets the impulse leap from node to node. This is saltatory conduction. Speed goes up with myelination and diameter.
At the synapse, the signal crosses as a neurotransmitter. The postsynaptic cell sums EPSPs and IPSPs to decide whether to fire.

ReferenceWords worth knowing

Here is a quick guide to the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Action potential: The neuron's electrical signal. It is a quick, all-or-nothing flip of the membrane voltage that travels down an axon.
Afferent (sensory) neuron: A neuron that carries signals toward the central nervous system. Afferent arrives.
All-or-nothing: The rule that a neuron fires either a full-sized action potential or none at all. The strength of a stimulus is coded by how often the neuron fires.
Astrocyte: The most common glial cell in the CNS. It ties neurons to blood vessels, controls the chemicals around them, and helps form the blood–brain barrier.
Autonomic nervous system: The automatic (involuntary) motor division. It controls heart muscle, smooth muscle, and glands. It splits into sympathetic and parasympathetic arms.
Axon: The one long fiber of a neuron. It carries the action potential away from the cell body.
Depolarization: A shift of the membrane voltage toward positive (less negative). It happens mainly when Na⁺ enters the cell.
Efferent (motor) neuron: A neuron that carries commands away from the central nervous system to an effector. Efferent exits.
Myelin sheath: Fatty insulation wrapped around an axon that speeds up signals. Oligodendrocytes make it in the CNS, and Schwann cells make it in the PNS.
Neuroglia: The helper cells of nervous tissue. They feed, insulate, clean, and protect neurons. There are six types across the CNS and PNS.
Neuron: The nerve cell that sends signals. It has dendrites (receive), a cell body (adds up signals), and an axon (sends the signal on).
Neurotransmitter: A chemical messenger released across a synapse. It passes a signal to the next cell.
Node of Ranvier: A bare gap between myelin segments, packed with Na⁺ channels. The action potential is rebuilt here during saltatory conduction.
Refractory period: A short time after a spike when that patch of membrane cannot fire again (absolute) or needs a stronger push (relative). It makes signals travel one way.
Repolarization: The return of the membrane voltage toward its negative resting value. It happens mainly when K⁺ leaves the cell.
Resting membrane potential: The voltage across a resting neuron's membrane, about −70 mV with the inside negative. Ion gradients and the K⁺ leak set it.
Saltatory conduction: Fast signaling along a myelinated axon. The signal leaps from node to node.
Schwann cell: The PNS glial cell that wraps one segment of an axon in myelin.
Sodium–potassium pump: A membrane pump powered by ATP. It moves 3 Na⁺ out and 2 K⁺ in, which keeps up the ion gradients.
Synapse: The meeting point where one neuron passes a message to another cell, across the synaptic cleft.
Threshold: The membrane voltage (about −55 mV) that must be reached to set off an action potential.

Check yourself

1. Name the two main parts of the nervous system by location. Which structures belong to each?

The central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is the brain and spinal cord, where integration happens. The PNS is the nerves and ganglia that carry signals to and from the CNS.

2. A signal travels from a pain receptor toward the spinal cord. Which kind of neuron carries it? What does the memory trick tell you?

An afferent (sensory) neuron. Afferent Arrives at the CNS. Efferent Exits toward muscles and glands. Sensory input always travels on afferent neurons.

3. Which glial cell makes myelin in the CNS? Which does the same job in the PNS? How is the way they cover axons different?

Oligodendrocytes (CNS) and Schwann cells (PNS). One oligodendrocyte wraps many axons at once. Each Schwann cell wraps only one segment of one axon.

4. The brain is mostly walled off from the bloodstream. Yet it still needs immune defense. Which glial cell provides it, and what does it do?

Microglia, the CNS's own immune cells. They patrol the tissue and swallow germs and dead cells. They do the job that immune cells in the blood do in the rest of the body.

5. About what is the resting membrane potential? Which one ion's movement mostly sets it?

About −70 mV, and potassium (K⁺) leaking out. At rest, the membrane lets K⁺ through much more easily. Potassium leaves down its gradient, which leaves the inside negative. The Na⁺/K⁺ pump keeps up the gradients, but it is not the main voltage-setter.

6. What does the sodium–potassium pump move? Why does it make the inside of the cell a little more negative?

It pumps 3 Na⁺ out for every 2 K⁺ in, using ATP. It removes three positive charges for every two it brings in. So the inside loses a little positive charge overall and becomes a bit more negative. It also rebuilds both gradients.

7. What does "all-or-nothing" mean for an action potential? How does a neuron signal a stronger stimulus?

If threshold (~−55 mV) is reached, a full-sized spike fires. Below threshold, nothing happens. Spikes are always the same size. So a stronger stimulus is coded by firing more often (higher frequency), not by a bigger action potential.

8. Put these action potential phases in order: repolarization, depolarization, resting, hyperpolarization. Name the main ion movement in each.

Resting → depolarization (Na⁺ in) → repolarization (K⁺ out) → hyperpolarization (a little too much K⁺ out) → back to resting. Voltage-gated Na⁺ channels drive the upstroke. K⁺ channels drive the downstroke and the brief undershoot.

9. Why does the refractory period make an action potential travel in only one direction?

The membrane that just fired has inactivated Na⁺ channels. It cannot fire again right away. So the signal can only spread into fresh membrane ahead of it. It cannot go back into the part it just left, so it marches forward.

10. Compare continuous and saltatory conduction. What are the bare gaps called where a myelinated signal is rebuilt?

Continuous conduction crawls patch by patch along unmyelinated axons. Saltatory conduction leaps between the nodes of Ranvier on myelinated axons, and it is much faster. The nodes are packed with Na⁺ channels. The covered internodes carry current passively.

11. Multiple sclerosis destroys CNS myelin. Use what you know about conduction to explain why this causes weakness and numbness.

Without myelin, current leaks out along the axon, and saltatory conduction fails. Signals slow down or stop. So motor commands and sensory information cannot get through. This causes weakness, numbness, and vision problems.

12. List the steps of chemical synaptic transmission, from the arriving action potential to the postsynaptic response. Which ion triggers the release of neurotransmitter?

AP arrives → voltage-gated Ca²⁺ channels open and calcium enters → vesicles merge and release neurotransmitter → it drifts across the cleft → it binds postsynaptic receptors → channels open, creating an EPSP or IPSP. Then it is cleared. Calcium (Ca²⁺) coming in is the trigger that makes the vesicles release their neurotransmitter.

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