Anatomy & Physiology · Chapter 12

The Nervous System & Nervous Tissue

Every thought you have ever had, every muscle you have ever moved, and every sensation you have ever felt was carried by a wave of electricity racing along a living wire. This chapter is the story of that wire — the neuron — and the tissue it lives in. We will map how the whole nervous system is organized, meet the loyal support cells that keep neurons alive, and then get to the heart of the matter: how a neuron builds an electrical charge, fires an action potential, sends it flying down a myelinated axon, and hands the message to the next cell across a microscopic gap. Master these ideas and the rest of neuroscience — the brain, the spinal cord, reflexes, the senses — becomes a set of variations on one theme.

By the end of this chapter you can

Divide the nervous system into its central and peripheral parts and into its sensory, motor, somatic, and autonomic pathways.
State the nervous system's three core jobs — sensory input, integration, and motor output — and give an example of each.
Name the parts of a neuron and the six neuroglia, and say what each glial cell does and where it lives.
Explain how the resting membrane potential of about −70 mV is created and maintained.
Walk through the phases of an action potential and explain why it is all-or-nothing and one-way.
Contrast continuous and saltatory conduction and list the two factors that set conduction speed.
Describe the steps of chemical synaptic transmission and tell an excitatory signal from an inhibitory one.
Connect each mechanism to a real clinical picture — multiple sclerosis, local anesthetics, nerve gas, and more.

Section 1How the nervous system is organized

Nervous system organization reference figure — **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 communication network, and anatomists split it two different ways — by where the parts are and by which direction the traffic flows. By location there are just two divisions. The central nervous system (CNS) is the brain and spinal cord — the headquarters where information is processed and decisions are made. The peripheral nervous system (PNS) is everything else: the nerves and ganglia that fan out from the CNS to reach every corner of the body. Think of the CNS as the computer and the PNS as the cables running to every sensor and every motor.

Three jobs: sense, decide, act

No matter how complicated a nervous pathway looks, it is always doing one of three things. First comes sensory input — receptors detect a change, inside the body or out, and send that news toward the CNS. Second is integration — the CNS processes and interprets the incoming signals and decides what, if anything, to do. Third is motor output — a command travels away from the CNS to an effector (a muscle or a gland) that carries out the response. Touch a hot stove and all three fire in a heartbeat: skin receptors sense heat (input), the spinal cord decides "pull back" (integration), and arm muscles yank the hand away (output).

The words for these directions are worth memorizing because they appear on every diagram. Neurons carrying signals toward the CNS are afferent (sensory); neurons carrying commands away from the CNS are efferent (motor). The efferent side splits again by target. The somatic nervous system carries voluntary commands to skeletal muscle — the movements you choose. The autonomic nervous system (ANS) runs the automatic housekeeping, sending signals to cardiac muscle, smooth muscle, and glands without your conscious say-so. And the ANS itself has two arms that usually oppose each other: the sympathetic division (the "fight-or-flight" accelerator) and the parasympathetic division (the "rest-and-digest" brake).

Division	What it includes	Direction / job
Central (CNS)	Brain and spinal cord	Integration — processes input, issues 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	Voluntary movement
Autonomic (ANS)	Motor neurons to cardiac/smooth muscle & glands	Involuntary control
Sympathetic	Thoracolumbar outflow	Fight-or-flight (accelerate)
Parasympathetic	Craniosacral outflow	Rest-and-digest (calm)

Afferent Arrives, Efferent Exits. The two words differ by a single letter and students mix them up for a lifetime. Anchor them with the vowel: Afferent signals Arrive at the CNS; Efferent signals Exit toward muscles and glands.

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

Interactive Split the nervous system by anatomy (central vs peripheral) and by traffic direction (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 function: afferent (sensory) signals arrive at the CNS; efferent (motor) signals exit toward effectors.
The motor side divides into somatic (voluntary, skeletal muscle) and autonomic (involuntary), and the autonomic into sympathetic and parasympathetic.
Every nervous act reduces to three steps: sensory input → integration → motor output.

Section 2Neurons and their support crew

Neuroglia reference figure — **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 support cells, meet the star. A neuron is an excitable cell shaped for one purpose — to receive a signal at one end and pass it along from the other. Three parts do the work. The dendrites are branching antennae that receive incoming signals. The cell body (or soma) holds the nucleus and does the cell's living and housekeeping; it is also where incoming signals are added up. The axon is a single long fiber that carries the outgoing signal away, often wrapped in a fatty myelin sheath and ending in axon terminals that hand the message to the next cell. Neurons are also amitotic — most cannot divide to replace themselves — which 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 roughly as many, or more, neuroglia (or glial cells) — the crew that feeds, insulates, cleans, and protects. Glia do not fire action potentials, but without them neurons would starve, short-circuit, and drown in debris. There are six types: four live in the CNS and two in the PNS.

Glial cell	Where	Job
Astrocyte	CNS	Most abundant; anchors neurons to blood supply, controls the chemical environment, helps form the blood–brain barrier
Oligodendrocyte	CNS	Wraps CNS axons in myelin (one cell myelinates many axons)
Microglia	CNS	The brain's immune cells — engulf pathogens and dead cells
Ependymal cell	CNS	Line the brain's cavities; circulate cerebrospinal fluid
Satellite cell	PNS	Surround and support neuron cell bodies in ganglia
Schwann cell	PNS	Wraps PNS axons in myelin (one cell myelinates one segment)

Two cells make myelin — keep them straight

The two myelinating cells cause endless confusion, so pin them down now. In the CNS, an oligodendrocyte reaches out several arms and myelinates 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 clinically: diseases that attack CNS myelin (multiple sclerosis) and those that attack PNS myelin (Guillain–Barré syndrome) are entirely separate conditions with different courses.

Microglia are the brain's cleanup crew. Because the blood–brain barrier keeps most immune cells out, the CNS grows its own defenders. Microglia patrol constantly, and when they detect injury or infection they swarm, engulf debris, and sound the alarm — which is why they are central to the story of nearly every brain disease.

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

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

Key points

A neuron has three parts: dendrites (receive), cell body/soma (integrate and live), and axon (transmit).
The six glia: CNS — astrocyte, oligodendrocyte, microglia, ependymal; PNS — satellite, Schwann.
Oligodendrocytes myelinate many CNS axons; Schwann cells each myelinate one PNS segment.
Microglia are the CNS's immune cells; astrocytes are the most abundant glia and help form the blood–brain barrier.

Section 3The resting membrane potential

Resting membrane potential reference figure — **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. Across its membrane sits a tiny voltage called the resting membrane potential, about −70 mV, meaning the inside of the cell is 70 millivolts negative compared with the outside. A membrane that holds this separation of charge is said to be polarized, like a charged battery ready to be tapped. Everything a neuron does — every signal — is a brief, controlled discharge of that stored charge. So the first question of all neurophysiology is simply: how does the cell build the charge in the first place?

Why the inside is negative

The answer is unequal ion distribution. The cell carefully keeps different ions at different concentrations inside versus outside. Sodium (Na⁺) is piled up outside the cell; potassium (K⁺) is concentrated inside. In addition, the inside is stuffed with large negatively charged proteins and phosphates that are simply too big to leave. Now add the membrane's channels. At rest the membrane is far leakier to potassium than to sodium — there are many more open leak channels for K⁺. So potassium trickles out down its gradient, carrying positive charge with it and leaving the inside more negative. That potassium leak, working against the trapped internal anions, is the main reason the resting voltage lands near −70 mV.

Ion	Concentrated where	Role at rest
Sodium (Na⁺)	Outside the cell	Poised to rush in; membrane mostly closed to it at rest
Potassium (K⁺)	Inside the cell	Leaks out through many open channels — sets the resting voltage
Chloride (Cl⁻)	Outside the cell	Contributes to the negative interior
Large anions (proteins)	Trapped inside	Cannot cross — hold negative charge in

The pump that never sleeps

Leaks would eventually run the gradients down — sodium would trickle in, potassium would drift out, and the battery would go flat. To prevent that, the membrane runs the sodium–potassium pump (Na⁺/K⁺ ATPase) around the clock. Burning ATP for fuel, it pumps 3 Na⁺ out for every 2 K⁺ in, restoring both gradients continuously. Because it moves three positives out for every two brought in, the pump also removes a little net positive charge, nudging the interior slightly more negative. This pump is so essential that it consumes a large share of the body's resting energy budget — a big reason the brain is such a hungry organ.

Potassium is the voltage-setter. It is tempting to credit the sodium–potassium pump for the resting potential, but the pump mainly maintains the gradients. The −70 mV itself comes chiefly from potassium leaking out through open K⁺ channels. That is why disorders of blood potassium (hyper- and hypokalemia) so quickly disturb the heart and nerves.

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, inside negative — the membrane is polarized.
It arises from unequal ion distribution: Na⁺ high outside, K⁺ high inside, large anions trapped inside.
Potassium leaking out through open channels is the main setter of the resting voltage.
The Na⁺/K⁺ pump (3 out, 2 in, using ATP) maintains the gradients and adds a small negative bias.

Section 4The action potential

Action potential reference figure — **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 — a brief, dramatic reversal of the membrane voltage that shoots down the axon. It does not happen unless the membrane is pushed from its resting −70 mV up to a critical threshold of about −55 mV. Below threshold, nothing fires. Reach threshold and a full-sized spike goes off every time — the same height whether the stimulus was barely enough or enormous. This is the famous all-or-nothing law: a neuron cannot fire a partial action potential any more than a gun can fire a partial bullet. Strength is coded not by bigger spikes but by how often the neuron fires.

The upstroke: sodium floods in

The whole event is run by voltage-gated channels — ion gates that open in response to voltage. When threshold is reached, voltage-gated Na⁺ channels snap open and sodium, which was piled up outside and pulled inward by both its concentration gradient and the negative interior, floods in. Each entering positive charge makes the inside less negative, which opens still more sodium gates — a fast, self-amplifying rush called depolarization that 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 (they auto-close), and now the slower voltage-gated K⁺ channels finish opening. Potassium rushes out, carrying positive charge away and dropping the voltage back down — this is repolarization. The potassium gates are sluggish to close, so a little too much K⁺ leaves and the membrane briefly dips below rest to about −80 mV, a hyperpolarization or undershoot. Finally the leak channels and the Na⁺/K⁺ pump restore the tidy −70 mV. During the recovery there is a refractory period when the axon either cannot fire at all (absolute) or needs an unusually strong stimulus (relative).

Phase	What the channels do	Voltage
Resting	Only leak channels open	−70 mV
Threshold	Enough Na⁺ gates begin to open	−55 mV
Depolarization	Voltage-gated Na⁺ channels open; Na⁺ rushes in	rising to +30 mV
Repolarization	Na⁺ channels inactivate; K⁺ channels open, K⁺ exits	falling back down
Hyperpolarization	K⁺ channels close slowly — slight overshoot	~−80 mV
Return to rest	Leak channels + Na⁺/K⁺ pump restore balance	−70 mV

The refractory period makes signals one-way. Right after a stretch of membrane fires, its sodium channels are inactivated and briefly cannot reopen. That forces the action potential to keep marching forward into fresh membrane rather than backward, and it caps how fast a neuron can fire — a built-in speed limit that protects the code.

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

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

Key points

An action potential fires only if the membrane reaches threshold (~−55 mV), and then it is all-or-nothing.
Depolarization = voltage-gated Na⁺ channels open, Na⁺ floods in, voltage climbs to ~+30 mV.
Repolarization = Na⁺ channels inactivate and K⁺ channels open, K⁺ exits, voltage falls; a brief hyperpolarization follows.
The refractory period keeps the signal moving one way and limits firing rate; stimulus strength is coded by frequency.

Section 5Saltatory conduction & speed

Saltatory conduction reference figure — **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 is born, it has to travel the length of the axon, and there are two ways to do that. On a bare, unmyelinated axon the spike regenerates itself at every adjacent patch of membrane, marching along step by step — continuous conduction. It works, but it is slow. On a myelinated axon the fatty sheath insulates long stretches so the current cannot leak out there; the action potential can only regenerate at the bare gaps between myelin segments. So the signal appears to jump from gap to gap, skipping the insulated lengths in between. This leaping is saltatory conduction, from the Latin saltare, "to leap," and it is dramatically faster.

Nodes of Ranvier

The bare gaps have a name worth knowing: the nodes of Ranvier. These little exposed rings are packed with voltage-gated sodium channels, so the action potential is regenerated fresh and full-strength at each node. Between the nodes, the insulated segment (the internode) carries the current quickly and passively, with no channels to slow it down. The result is that the impulse effectively hops from node to node — regenerate, glide, regenerate, glide — covering far more ground per unit time than a continuous crawl.

What sets conduction speed

Two features determine how fast a fiber conducts. The first is myelination: a myelinated axon massively outpaces an unmyelinated one of the same size. The second is axon diameter: fatter axons conduct faster because they offer less resistance to current, the same way a wide pipe moves water faster than a narrow one. The body uses both dials deliberately — the large, heavily myelinated fibers that carry reflexes and limb-position sense can conduct at speeds over 100 meters per second, while the thin, unmyelinated fibers that carry dull, aching pain crawl along at barely one. (Sharp, localized pain travels on thinner myelinated fibers — faster than the dull, unmyelinated kind, but far short of that top speed.)

Factor	Faster conduction	Slower conduction
Myelin	Myelinated (saltatory)	Unmyelinated (continuous)
Axon diameter	Large diameter	Small diameter
Typical role	Reflexes, sharp/localized pain, position	Dull aching pain, temperature, autonomic

Strip the myelin and the signal fails. In multiple sclerosis, the immune system destroys CNS myelin; without insulation the current leaks away and conduction slows or stops, producing weakness, numbness, and vision loss. In Guillain–Barré syndrome the same thing happens to PNS myelin, causing ascending paralysis. Myelin is not optional — it is the difference between a nerve that works and one that doesn't.

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

Interactive A myelinated signal leaps node to node, skipping the insulated stretches — watch two signals race. Open full ↑

Key points

Continuous conduction regenerates the spike at every patch (slow, unmyelinated); saltatory conduction leaps between gaps (fast, myelinated).
The bare gaps are the nodes of Ranvier, packed with Na⁺ channels; the insulated internodes carry current passively.
Speed rises with myelination and with axon diameter.
Demyelinating disease (MS in the CNS, Guillain–Barré in the PNS) slows or blocks conduction.

Section 6The synapse: passing the message on

The chemical synapse reference figure — **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

When an action potential reaches the end of an axon, it faces a problem: the next cell is not connected to it. There is a microscopic gap in between called the synaptic cleft, and the junction as a whole is the synapse. The neuron sending the message is presynaptic; the cell receiving it is postsynaptic. Because the electrical signal cannot jump the gap on its own, most synapses convert it into a chemical messenger — a neurotransmitter — that drifts across and re-creates the signal on the far side. Electrical, then chemical, then electrical again.

Six steps across the gap

Chemical transmission always follows the same choreography. (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 triggers tiny sacs called synaptic vesicles to fuse with the membrane and dump their contents. (4) Neurotransmitter is released by exocytosis and diffuses across the cleft. (5) It binds receptors on the postsynaptic membrane, opening ion channels that create a small graded potential. (6) The neurotransmitter is quickly cleared — broken down by enzymes, taken back up (reuptake), or diffused away — so the signal is crisp and brief.

Excitatory or inhibitory?

What the postsynaptic cell does next depends entirely on which neurotransmitter and receptor are involved. An excitatory signal depolarizes the postsynaptic membrane, nudging it toward threshold — an excitatory postsynaptic potential (EPSP). An inhibitory signal hyperpolarizes it, pushing it away from threshold — an inhibitory postsynaptic potential (IPSP). A single neuron receives thousands of these at once and sums them; only if the excitation outweighs the inhibition enough to reach threshold does it fire. The synapse, in other words, is where the nervous system actually makes decisions.

Neurotransmitter	Typical effect	Where / note
Acetylcholine (ACh)	Excites skeletal muscle	Neuromuscular junction; broken down by acetylcholinesterase
Glutamate	Excitatory	The main excitatory transmitter of the CNS
GABA	Inhibitory	The main inhibitory transmitter of the CNS
Glycine	Inhibitory	Common in the spinal cord
Norepinephrine	Excites or inhibits	Sympathetic (fight-or-flight), mood, arousal
Dopamine	Modulatory	Reward, movement; disrupted in Parkinson disease

The synapse is where the drugs act. Because transmission has so many steps, it is easy to sabotage — and most medicines and poisons do exactly that. Nerve gases block acetylcholinesterase so ACh never clears; many antidepressants block serotonin reuptake; caffeine, opioids, and anesthetics all work at synapses. Learn this handoff and you have the key to pharmacology.

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

Interactive An electrical signal becomes chemical across the cleft, then 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 postsynaptic.
Transmission: AP arrives → Ca²⁺ enters → vesicles fuse → 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: ACh (muscle), glutamate (main excitatory), 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 PNS (nerves and ganglia), and its work is always sensory input → integration → motor output.
Motor pathways divide into somatic (voluntary) and autonomic (involuntary, split into sympathetic and parasympathetic); afferent arrives, efferent exits.
A neuron (dendrites, soma, axon) is supported by six neuroglia — astrocyte, oligodendrocyte, microglia, ependymal (CNS) and satellite, Schwann (PNS).
At rest the membrane is polarized to ~−70 mV, set by K⁺ leaking out and trapped internal anions, and maintained by the Na⁺/K⁺ pump.
Reaching threshold (~−55 mV) fires an all-or-nothing action potential: Na⁺ in (depolarize) then K⁺ out (repolarize), with a refractory period.
Myelin lets the impulse leap node to node — saltatory conduction — and speed rises 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

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Action potential: A brief, all-or-nothing reversal of membrane voltage that travels down an axon; the neuron's electrical signal.
Afferent (sensory) neuron: A neuron that carries signals toward the central nervous system (afferent arrives).
All-or-nothing: The principle that a neuron either fires a full-sized action potential or none at all; stimulus strength is coded by firing frequency.
Astrocyte: The most abundant CNS glial cell; anchors neurons to blood vessels, regulates the chemical environment, and helps form the blood–brain barrier.
Autonomic nervous system: The involuntary motor division controlling cardiac muscle, smooth muscle, and glands; split into sympathetic and parasympathetic arms.
Axon: The single long fiber of a neuron that carries the action potential away from the cell body.
Depolarization: A shift of the membrane voltage toward positive (less negative), driven mainly by Na⁺ entering 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 (by oligodendrocytes in the CNS, Schwann cells in the PNS) that speeds conduction.
Neuroglia: The support cells of nervous tissue that feed, insulate, clean, and protect neurons; six types across the CNS and PNS.
Neuron: The excitable signaling cell of the nervous system, with dendrites (receive), a cell body (integrate), and an axon (transmit).
Neurotransmitter: A chemical messenger released across a synapse to relay a signal to the next cell.
Node of Ranvier: A bare gap between myelin segments, rich in Na⁺ channels, where an action potential is regenerated during saltatory conduction.
Refractory period: The brief interval after a spike when a region of membrane cannot fire again (absolute) or needs a stronger stimulus (relative), forcing one-way conduction.
Repolarization: The return of membrane voltage toward its negative resting value, driven mainly by K⁺ leaving the cell.
Resting membrane potential: The voltage (~−70 mV, inside negative) across a resting neuron's membrane, set by ion gradients and K⁺ leak.
Saltatory conduction: Fast impulse transmission along a myelinated axon in which the signal leaps from node to node.
Schwann cell: The PNS glial cell that wraps a single axon segment in myelin.
Sodium–potassium pump: An ATP-powered membrane transporter that moves 3 Na⁺ out and 2 K⁺ in, maintaining the ion gradients.
Synapse: The junction where one neuron communicates with another cell across the synaptic cleft.
Threshold: The membrane voltage (~−55 mV) that must be reached to trigger an action potential.

Check yourself

1. Name the two anatomical divisions of the nervous system and state 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 traveling toward the spinal cord from a pain receptor is carried by which kind of neuron, and 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 afferently.

3. Which glial cell myelinates axons in the CNS, and which does the same job in the PNS? How do their coverage patterns differ?

Oligodendrocytes (CNS) and Schwann cells (PNS). One oligodendrocyte myelinates many axons at once; each Schwann cell myelinates only a single segment of one axon.

4. The brain is largely 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 resident immune cells. They patrol the tissue and engulf pathogens and dead cells, taking on the defensive role that circulating immune cells play elsewhere.

5. What is the approximate resting membrane potential, and which single ion's movement is chiefly responsible for setting it?

About −70 mV, and potassium (K⁺) leaking out. The membrane is far more permeable to K⁺ at rest; potassium exits down its gradient, leaving the inside negative. The Na⁺/K⁺ pump maintains the gradients but is not the main voltage-setter.

6. Describe what the sodium–potassium pump moves and why it slightly increases the negativity inside the cell.

It pumps 3 Na⁺ out for every 2 K⁺ in, using ATP. Because it removes three positive charges for every two it brings in, there is a small net loss of positive charge from the interior, nudging it a bit more negative — on top of restoring both gradients.

7. Explain what "all-or-nothing" means for an action potential, and how a neuron therefore signals a stronger stimulus.

If threshold (~−55 mV) is reached, a full-sized spike fires; below threshold, nothing. Spikes are always the same amplitude, so a stronger stimulus is coded by a higher frequency of firing, not by a bigger action potential.

8. Put the action-potential phases in order and name the main ion movement in each: repolarization, depolarization, resting, hyperpolarization.

Resting → depolarization (Na⁺ in) → repolarization (K⁺ out) → hyperpolarization (slight K⁺ overshoot) → back to resting. Voltage-gated Na⁺ channels drive the upstroke; K⁺ channels drive the downstroke and brief undershoot.

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

Because the membrane that just fired has inactivated Na⁺ channels and cannot immediately fire again. The signal can only spread into fresh membrane ahead of it, not back into the region it just left, so it marches forward.

10. Compare continuous and saltatory conduction, and name the bare gaps where a myelinated signal regenerates.

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

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

Without myelin, current leaks out along the axon and saltatory conduction fails. Signals slow down or stop entirely, so motor commands and sensory information cannot get through — producing weakness, numbness, and visual problems.

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

AP arrives → voltage-gated Ca²⁺ channels open and calcium enters → vesicles fuse and release neurotransmitter → it diffuses across the cleft → binds postsynaptic receptors → opens channels, creating an EPSP or IPSP; then it is cleared. Calcium (Ca²⁺) entry is the trigger that makes the vesicles release their neurotransmitter.

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