Campbell Biology Chapter 48 Review: Neurons, Synapses, and Signaling

48.1 How neurons are built to pass on information

pp. 1045–1047

On the AP exam? Background

You won't be tested on neuron anatomy or how nervous systems are organized. What carries over: a neurotransmitter crossing a tiny gap to a nearby cell is the course's example of local signaling (Topic 4.1), and sensing and responding to the surroundings is Topic 8.1.

In the course: Topic 4.1 Cell Communication, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

Nervous systems handle information in three steps: sensors detect a change (sensory input), the brain or a cluster of neurons weighs it up (integration), and commands go out to muscles or glands that act (motor output).
The brain and main nerve cord make up the central nervous system (CNS). The neurons carrying signals to and from it make up the peripheral nervous system (PNS), and bundles of their axons are called nerves.
Sensory neurons bring information in, interneurons link neurons together inside the CNS (most brain neurons are this kind), and motor neurons carry commands out to muscles.
A neuron's nucleus sits in its cell body. Branching dendrites, along with the cell body, pick up incoming signals, and one long axon carries the neuron's own signal away. That signal usually starts where the axon leaves the cell body, at the axon hillock.
The axon ends in branches with synaptic terminals. At each synapse, the terminal usually releases a chemical neurotransmitter onto the next cell. The sender is the presynaptic cell; the receiver (a neuron, muscle cell or gland cell) is the postsynaptic cell.
Neurons come in many shapes, and the shape tracks the wiring: lots of dendrite branches mean lots of incoming connections, while an axon that branches widely spreads one cell's message to many others.
Glia are the nervous system's support cells. Some wrap axons in insulation, and others supply neurons with nutrients or tidy up the ions and chemicals around them. Older books say glia outnumber neurons by 10 to 1 or more, but careful counts now suggest the human brain has about as many glia as neurons.

Key terms (15)

integration: The step where the brain or a cluster of nerve cells weighs up incoming information and decides on a response.
central nervous system (CNS): The brain plus the main nerve cord (the spinal cord in vertebrates), where most information is processed.
peripheral nervous system (PNS): All the neurons and nerves outside the CNS that carry signals in from the body and out to muscles and glands.
nerve: A cable made of many axons bundled together, running between the CNS and the rest of the body.
sensory neuron: A neuron that carries information from a sensor, such as a touch or light detector, toward the CNS.
interneuron: A neuron that links other neurons together inside the CNS. Most neurons in your brain are this kind.
motor neuron: A neuron that carries commands out of the CNS to a muscle, telling it to contract.
dendrite: One of the short, branching extensions of a neuron that pick up signals from other cells.
axon: The single long extension of a neuron that carries its outgoing signal toward other cells.
axon hillock: The cone-shaped spot where the axon leaves the cell body. Outgoing signals usually start at or just past it.
synapse: The junction where a neuron passes a signal to another cell, such as a neuron, muscle cell or gland cell.
neurotransmitter: A chemical messenger a neuron releases at a synapse to signal the cell on the other side.
presynaptic cell: The neuron on the sending side of a synapse.
postsynaptic cell: The cell on the receiving side of a synapse. It can be a neuron, a muscle cell or a gland cell.
glia: The nervous system's support cells. They insulate axons, supply neurons with nutrients and tidy up the fluid around them.

Check yourself: 48.1 How neurons are built to pass on information

4 questions on 48.1 How neurons are built to pass on information. Pick an answer to see if you got it, and why.

Question 1 of 4

In a stained slice of brain tissue, a researcher sees a neuron with dozens of short, highly branched extensions close to its cell body, plus one long extension that splits into many fine branches far from the cell body. Which statement correctly matches these structures to their usual roles?

Question 2 of 4

When a cat steps on a thorn, it pulls its paw away within a fraction of a second. One neuron in this pathway has its cell body inside the spinal cord, receives signals only from other neurons in the cord, and passes signals only to other neurons in the cord. This neuron is best classified as

Question 3 of 4

Which of the following is part of the peripheral nervous system (PNS) rather than the central nervous system (CNS)?

Question 4 of 4

A neuron signals a cell in a salivary gland, causing it to release saliva. Which statement about this synapse is correct?

0 of 4 answered

48.2 The resting potential: a charged membrane

pp. 1048–1050

On the AP exam? Yes

Topic 2.8 tests how the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in) builds the gradients behind the membrane potential, and Topic 2.6 covers ions moving through channels. The Nernst equation and exact ion concentrations won't be on the exam.

In the course: Topic 2.8 Mechanisms of Transport, Topic 2.6 Facilitated Diffusion, Topic 2.5 Membrane Transport (notes, videos and more questions)

Key points

Every cell has a voltage across its membrane, called the membrane potential, because charges are spread unevenly across it. In a neuron that isn't firing, this resting potential is usually −60 to −80 mV, with the inside negative.
Two gradients matter most. K⁺ is roughly 30 times more concentrated inside a neuron than outside, and Na⁺ is roughly 10 times more concentrated outside than inside.
The sodium-potassium pump spends ATP to keep those gradients in place, moving 3 Na⁺ out and 2 K⁺ in per cycle. Its uneven trade adds only a few millivolts directly; its real job is maintaining the gradients.
The main reason the inside is negative: at rest, many K⁺ channels are open but very few Na⁺ channels are. K⁺ leaks out down its gradient, and the large negative ions inside, like proteins, can't follow, so negative charge builds up inside.
As the inside gets more negative, it tugs positive K⁺ back. When that electrical pull exactly balances the concentration push, K⁺ has no net flow. That voltage is K⁺'s equilibrium potential, about −90 mV in a typical mammalian neuron.
A small trickle of Na⁺ leaking in keeps the resting potential a little less negative than −90 mV. It stays much closer to K⁺'s value than to Na⁺'s (about +62 mV), because the resting membrane lets K⁺ through far more easily.
Setting up this voltage takes very few ions, far too few to change the concentrations noticeably. For a +1 ion at body temperature, the Nernst equation gives the equilibrium potential: E = 62 mV × log₁₀([ion]outside ÷ [ion]inside).

Key terms (11)

membrane potential: The voltage across a cell's membrane, caused by charges being unevenly spread between the inside and the outside.
resting potential: A neuron's membrane potential when it isn't sending a signal, usually about −60 to −80 mV, with the inside negative.
millivolt (mV): One thousandth of a volt. Membrane potentials are measured in millivolts.
sodium-potassium pump: A membrane protein that uses one ATP to push 3 Na⁺ out and bring 2 K⁺ in, keeping both gradients in place.
ion channel: A protein pore through the membrane that lets certain ions diffuse across, down their gradients.
selective permeability: A membrane's habit of letting some substances cross more easily than others. A K⁺ channel lets K⁺ through but not Na⁺.
concentration gradient: A difference in how much of a substance is on each side of a membrane. Ions tend to diffuse from the high side to the low side.
electrical gradient: A difference in charge across a membrane. It pulls positive ions toward the negative side and negative ions toward the positive side.
electrochemical gradient: The combined push of an ion's concentration gradient and the electrical gradient. Together they decide which way the ion actually moves.
equilibrium potential: The membrane voltage at which the electrical pull on an ion exactly balances its concentration gradient, so it has no net flow.
Nernst equation: A formula that gives an ion's equilibrium potential from its concentrations inside and outside the cell.

Check yourself: 48.2 The resting potential: a charged membrane

4 questions on 48.2 The resting potential: a charged membrane. Pick an answer to see if you got it, and why.

Question 1 of 4

Which description of a typical mammalian neuron at rest is correct?

Question 2 of 4

As K⁺ leaks out of a resting neuron, it leaves negative charge behind inside the cell. Which best explains why negative ions inside don't simply follow K⁺ out and cancel that charge?

Question 3 of 4

Neurons in a dish are moved from a saline with 5 mM K⁺ into one with 15 mM K⁺. All other ion concentrations stay the same, and K⁺ inside the cells stays near 140 mM. What will most likely happen to the resting potential?

Question 4 of 4Calculator allowed

A neuron from a marine worm has 100 mM K⁺ inside and 10 mM K⁺ outside. For an ion with a +1 charge, its equilibrium potential is E = 62 mV × log₁₀([ion]outside ÷ [ion]inside). What is the equilibrium potential for K⁺ in this neuron?

0 of 4 answered

48.3 Action potentials: the all-or-none nerve signal

pp. 1050–1055

On the AP exam? Background

The current course doesn't test action potentials, threshold or myelin. They're still good practice with tested ideas: ions crossing through gated channels down their gradients (Topics 2.6 and 2.8) and positive feedback, which drives the rising phase (Topic 4.4).

In the course: Topic 2.6 Facilitated Diffusion, Topic 2.8 Mechanisms of Transport, Topic 4.4 Feedback (notes, videos and more questions)

Key points

Gated ion channels open or close when something triggers them, such as a voltage change, a chemical or a stretch. Opening them changes which ions can cross, so the membrane potential changes.
Making the inside more negative is hyperpolarization (for example, opening extra K⁺ channels). Making it less negative is depolarization (for example, letting Na⁺ in).
A graded potential is a small shift whose size depends on how strong the stimulus is. It shrinks as it spreads, so it only works over short distances.
If a depolarization reaches threshold (around −55 mV in mammals), voltage-gated Na⁺ channels open, Na⁺ floods in, the membrane depolarizes further and even more channels open. This positive feedback makes an action potential: a spike to a positive voltage that's the same size every time (all or none).
The spike ends because the Na⁺ channels quickly inactivate (part of the protein plugs the pore) and slower voltage-gated K⁺ channels open, letting K⁺ out. Those K⁺ channels stay open a moment too long, so the voltage dips below rest (the undershoot) before recovering.
While Na⁺ channels are inactivated, a new spike can't start. This refractory period limits how fast a neuron can fire and keeps the signal moving forward only. A stronger stimulus makes spikes come more often, not bigger.
Each spike depolarizes the next stretch of axon to threshold, so the signal is rebuilt at full strength all along the axon. Wider axons conduct faster. In vertebrates, glia (oligodendrocytes in the CNS, Schwann cells in the PNS) wrap axons in myelin, and spikes jump between bare gaps called nodes of Ranvier. This saltatory conduction is fast without needing huge axons.

Key terms (15)

gated ion channel: An ion channel that opens or closes in response to a trigger, such as a voltage change, a chemical or a stretch.
voltage-gated ion channel: A gated channel that opens or closes when the membrane potential reaches a certain value.
hyperpolarization: A change that makes the cell's interior more negative than it was, so it moves further from threshold.
depolarization: A change that makes the cell's interior less negative than it was, so it moves closer to threshold.
graded potential: A small shift in membrane potential whose size matches the stimulus strength. It fades as it spreads.
action potential: A brief, full-size electrical spike that travels along an axon. It either happens completely or not at all.
threshold: The membrane potential (about −55 mV in mammals) that a depolarization must reach to set off an action potential.
inactivation: A state in which a part of a voltage-gated Na⁺ channel plugs its pore, so the channel can't reopen for a short time.
undershoot: The last part of an action potential, when the voltage dips below the resting potential because extra K⁺ channels are still open.
refractory period: The short time after an action potential when a new one can't start, because Na⁺ channels are still inactivated.
myelin sheath: Many layers of fatty glial membrane wrapped around an axon. It insulates the axon and speeds up signals.
oligodendrocyte: A glial cell that makes myelin around axons in the CNS.
Schwann cell: A glial cell that makes myelin around axons in the PNS.
node of Ranvier: A short bare gap between sections of myelin, where the axon's voltage-gated Na⁺ channels are packed.
saltatory conduction: The way a signal leaps from one node of Ranvier to the next in a myelinated axon, which makes it much faster.

Check yourself: 48.3 Action potentials: the all-or-none nerve signal

4 questions on 48.3 Action potentials: the all-or-none nerve signal. Pick an answer to see if you got it, and why.

Question 1 of 4

A researcher records the membrane potential at one point on an axon (resting potential −70 mV) while applying brief stimuli of increasing strength (invented data). Stimulus strength (arbitrary units) | Peak membrane potential (mV) 1 | −66 2 | −61 3 | +38 4 | +38 5 | +38 Which conclusion is best supported by the data?

Question 2 of 4

During the rising phase of an action potential, the membrane potential shoots up toward a positive value in about a millisecond. Which sequence describes the positive feedback loop that drives this rise?

Question 3 of 4

Tetrodotoxin, a toxin found in pufferfish, blocks voltage-gated Na⁺ channels but leaves other channels and the sodium-potassium pump working. In a neuron treated with tetrodotoxin, which result is most likely?

Question 4 of 4

At one instant, the peak of an action potential is halfway along an axon, traveling toward the synaptic terminals. Which best describes the stretch of membrane just behind it, on the side nearer the cell body?

0 of 4 answered

48.4 Synapses: handing the signal to the next cell

pp. 1055–1060

On the AP exam? Yes

Neurotransmitters are the course's main example of local signaling (Topic 4.1), ligand-gated channels and G protein-coupled receptors are tested receptor types (Topics 4.2 and 4.3), and vesicle release is exocytosis (Topic 2.5). EPSP and IPSP summation and most transmitter names are background.

In the course: Topic 4.1 Cell Communication, Topic 4.2 Introduction to Signal Transduction, Topic 4.3 Signal Transduction Pathways, Topic 2.5 Membrane Transport (notes, videos and more questions)

Key points

At an electrical synapse, gap junctions let current flow straight from one cell to the next. It's almost instant and keeps groups of neurons firing in step, but it's hard to fine-tune.
Most synapses are chemical. When a spike reaches the terminal, voltage-gated Ca²⁺ channels open, Ca²⁺ rushes in, and vesicles full of neurotransmitter fuse with the membrane and empty into the cleft (exocytosis).
The transmitter diffuses across the synaptic cleft and binds receptors on the postsynaptic cell. Many of these receptors are ligand-gated ion channels. The signal ends when the transmitter is cleared: it diffuses away, is pumped back into the terminal or nearby glia, or is broken down by an enzyme.
If the opened channel lets Na⁺ in, the receiving cell depolarizes toward threshold: an excitatory postsynaptic potential (EPSP). If it lets Cl⁻ in or K⁺ out, the cell hyperpolarizes: an inhibitory postsynaptic potential (IPSP).
A single EPSP is usually too small to start a spike. Inputs add up at the axon hillock, either from one synapse firing again quickly (temporal summation) or from several synapses at once (spatial summation), while IPSPs subtract. The neuron fires only if the total reaches threshold.
Metabotropic receptors are G protein-coupled receptors that start second-messenger pathways such as cAMP. Their effects start more slowly but last longer and are amplified. Because the response depends on which receptor a cell makes, one transmitter can excite one cell and inhibit another.
Main transmitter groups: acetylcholine (motor neurons to skeletal muscle), amino acids (glutamate is the brain's main 'go' signal, while GABA and glycine are 'stop' signals), biogenic amines (norepinephrine, dopamine, serotonin), neuropeptides (including endorphins, the body's own painkillers) and gases such as nitric oxide, which is made on demand and diffuses through membranes.

Key terms (15)

electrical synapse: A synapse where gap junctions connect two cells, so current passes straight from one to the other with almost no delay.
chemical synapse: A synapse where the sending cell releases a neurotransmitter that diffuses across a gap to the receiving cell.
synaptic vesicle: A small membrane sac in a synaptic terminal that holds neurotransmitter until it's released.
synaptic cleft: The narrow gap, a few tens of nanometers wide, between the sending terminal and the receiving cell.
ligand-gated ion channel: A receptor that is also a channel: it opens (or closes) when its signal molecule binds. Also called an ionotropic receptor.
excitatory postsynaptic potential (EPSP): A small depolarization in the receiving cell that moves it toward threshold, making it more likely to fire.
inhibitory postsynaptic potential (IPSP): A small hyperpolarization in the receiving cell that moves it away from threshold, making it less likely to fire.
temporal summation: When inputs from the same synapse arrive so close together in time that they add up before the first one fades.
spatial summation: When inputs from different synapses on the same neuron arrive at about the same time and add up.
metabotropic receptor: A neurotransmitter receptor that isn't a channel. It starts a pathway inside the cell, often through a G protein and a second messenger.
neuromuscular junction: The synapse between a motor neuron and a skeletal muscle cell.
acetylcholine: The neurotransmitter motor neurons use to signal skeletal muscle. An enzyme in the cleft, acetylcholinesterase, quickly breaks it down.
glutamate: An amino acid that is the main excitatory neurotransmitter in the vertebrate brain.
GABA: An amino acid neurotransmitter that is the main inhibitory signal in the brain. It opens Cl⁻ channels.
nitric oxide (NO): A gas some neurons make on demand and release as a local signal. It diffuses straight through membranes.

Check yourself: 48.4 Synapses: handing the signal to the next cell

4 questions on 48.4 Synapses: handing the signal to the next cell. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers stimulate a presynaptic neuron and measure how much neurotransmitter is released per action potential while changing the Ca²⁺ concentration in the fluid around the synaptic terminal (invented data). The action potentials reaching the terminal are the same size in every trial. Outside Ca²⁺ (mM) | Neurotransmitter released (% of maximum) 0 | 0 0.5 | 12 1.0 | 41 2.0 | 100 Which conclusion is best supported?

Question 2 of 4

At a synapse in a fish's spinal cord, researchers test how the transmitter is cleared from the cleft. They measure how long each postsynaptic response lasts under three conditions (invented data). Condition | Response duration (ms) No drug | 6 Drug that blocks the transmitter-destroying enzyme | 6 Drug that blocks a transporter in the presynaptic membrane | 24 Which conclusion is best supported?

Question 3 of 4Calculator allowed

A neuron rests at −70 mV and fires when the axon hillock reaches −55 mV. Each EPSP from its excitatory synapses raises the hillock's voltage by 4 mV, and each IPSP from an inhibitory synapse lowers it by 5 mV. Assume inputs arriving together add up simply. What is the smallest number of EPSPs arriving together that will make it fire, and how many are needed if one IPSP arrives at the same time?

Question 4 of 4

0 of 4 answered