Unit 1 · Topic 1.3

The Neuron and Neural Firing

A neuron collects messages on its dendrites, and if its charge passes threshold it fires an all-or-none action potential down its axon, sped by myelin.

Practice 1: Concept ApplicationPractice 2: Research Methods and Design

Part 1 · Hook

Why this matters

A dentist numbs your jaw, and for an hour you cannot feel your lip. A cup of coffee makes you alert; a sleepless night does the opposite. Every one of these effects comes down to how neurons fire and pass messages to one another.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which part of the nervous system is made of the brain and spinal cord?

  1. The central nervous system
  2. The peripheral nervous system
  3. The somatic nervous system
  4. The autonomic nervous system
Show the answer

The central nervous system is the brain and spinal cord; everything outside them is peripheral.

  • Correct: The central nervous system:
  • The peripheral nervous system:
  • The somatic nervous system:
  • The autonomic nervous system:

2. In an experiment, what does random assignment do?

  1. Gives each participant an equal chance of being in any group
  2. Picks participants so they represent the whole population
  3. Keeps participants from knowing which group they are in
  4. Measures the variable the researcher changes
Show the answer

Random assignment makes the groups alike at the start, so a difference after the treatment can be blamed on the treatment.

  • Correct: Gives each participant an equal chance of being in any group:
  • Picks participants so they represent the whole population:
  • Keeps participants from knowing which group they are in:
  • Measures the variable the researcher changes:

Part 4 · See it

See it first

A neuron, left to right: branching dendrites, the cell body with its nucleus, an axon wrapped in segments of myelin sheath, and branching axon terminals; an arrow shows the action potential traveling from the cell body toward the terminals. An enlarged synapse below shows vesicles of neurotransmitter in the sending terminal, molecules crossing the synaptic gap to receptor sites on the receiving dendrite, and a reuptake arrow back into the sending terminal.
A neuron and, enlarged, a synapse. Tap "Hide labels" to test yourself. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Excitatory messages reach the dendrites and push the charge inside the cell up from about -70 mVthe neuron depolarizes toward threshold
  2. The charge reaches threshold, about -55 mVa full action potential fires and races down the axon (all-or-none); myelin makes it faster
  3. The action potential reaches the axon terminalsvesicles release neurotransmitter into the synapse
  4. Neurotransmitter binds to receptor sites on the next neuronthat neuron is pushed toward firing (excitatory) or away from it (inhibitory)
  5. Reuptake pulls leftover neurotransmitter back into the sending neuronthe message ends; drugs that block reuptake make it last longer

Part 6 · Key ideas

Key ideas

  • A neuron fires an action potential only when its charge passes threshold. Every action potential is the same size (all-or-none); a stronger stimulus means more frequent firing and more neurons firing.
  • After firing, a neuron has a brief refractory period when it cannot fire again.
  • Know the job of each listed neurotransmitter: dopamine (movement, reward), serotonin (mood, sleep, appetite), norepinephrine (alertness), glutamate (main excitatory), GABA (main inhibitory), endorphins (pain relief), substance P (pain signals), acetylcholine (muscles, learning).
  • Hormones travel in the blood and act more slowly and for longer: adrenaline, leptin (full), ghrelin (hungry), melatonin (sleep timing), oxytocin (bonding). The pituitary gland directs other glands.
  • Drugs work at the synapse: an agonist mimics or boosts a neurotransmitter, an antagonist blocks it, a reuptake inhibitor keeps it in the gap longer. Repeated use brings tolerance, and stopping brings withdrawal.

Part 7 · Misconception

A common mistake

The wrong idea: A stronger stimulus makes a neuron fire a bigger action potential.

What actually happens: Action potentials are all-or-none: each one is the same size. A stronger stimulus makes neurons fire more often, and more neurons fire.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Figure

A neuron and a synapse, parts marked A to F

The neuron and synapse with six parts marked by letters: A a dendrite, B the cell body, C a segment of myelin sheath on the axon, D an axon terminal, E the synaptic gap in the enlarged synapse, F a receptor site on the receiving dendrite. The letters are the answers to questions, so the figure gives no names.
LevlPrep original diagram.

1. In the figure, which lettered part receives messages from other neurons and carries them toward the cell body?

  1. The part marked A
  2. The part marked C
  3. The part marked D
  4. The part marked E
Show the answer

A marks a dendrite, the branching fiber that collects incoming signals and passes them to the cell body.

  • Correct: The part marked A: Right: A is a dendrite, which receives messages from other neurons.
  • The part marked C: C is a segment of myelin sheath, which insulates the axon; it does not receive messages.
  • The part marked D: D is an axon terminal, where the neuron sends its message out, not where it receives one.
  • The part marked E: E is the synaptic gap, the space between two neurons, not a part of the receiving cell.

2. In a person with multiple sclerosis, the immune system damages the part marked C. What change in neural firing would you predict?

  1. Signals along the axon slow down or fail
  2. Neurotransmitter stops being taken back up
  3. Dendrites stop receiving incoming signals
  4. The cell body stops making any proteins
Show the answer

C is the myelin sheath. Myelin lets the action potential jump quickly down the axon; when it is damaged, as in multiple sclerosis, signals slow, scramble or stop, which causes weakness, numbness and vision problems.

  • Correct: Signals along the axon slow down or fail: Right: damaged myelin slows or blocks signals traveling down the axon.
  • Neurotransmitter stops being taken back up: Reuptake happens at the axon terminal and synapse, not at the myelin.
  • Dendrites stop receiving incoming signals: Dendrites receive signals; multiple sclerosis attacks the myelin on the axon.
  • The cell body stops making any proteins: The cell body keeps working; the problem is how fast the signal travels.

3. A drug blocks the receptor sites marked F, so the neurotransmitter released into E cannot bind there. How is this drug classified?

  1. An antagonist
  2. An agonist
  3. A reuptake inhibitor
  4. A hormone
Show the answer

A drug that occupies or blocks receptors so the neurotransmitter cannot act is an antagonist: it weakens that neurotransmitter's effect.

  • Correct: An antagonist: Right: blocking the receptor stops the neurotransmitter from working, so the drug is an antagonist.
  • An agonist: An agonist mimics or boosts a neurotransmitter; this drug stops it from acting.
  • A reuptake inhibitor: A reuptake inhibitor keeps neurotransmitter in the gap longer; it does not block receptor sites.
  • A hormone: A hormone is a chemical messenger carried in the blood, not a drug that blocks a receptor.

4. A neuron receives a weak burst of excitatory input that raises its charge from -70 mV to -62 mV, then the input stops. What happens?

  1. It does not fire, because the charge never reached threshold
  2. It fires a small action potential that matches the weak input
  3. It fires a full action potential, but more slowly than usual
  4. It enters its refractory period and is unable to fire for a while
Show the answer

Threshold is about -55 mV. Below it nothing fires; at or above it the neuron fires a full action potential. There is no partial action potential (the all-or-none principle).

  • Correct: It does not fire, because the charge never reached threshold: Right: -62 mV is below a threshold of about -55 mV, so the neuron does not fire.
  • It fires a small action potential that matches the weak input: There are no small action potentials: firing is all-or-none.
  • It fires a full action potential, but more slowly than usual: Action potential speed depends on the axon (such as myelin), not on how strong the input was.
  • It enters its refractory period and is unable to fire for a while: The refractory period follows an action potential; this neuron never fired.

5. Marisol touches a warm mug, then a hot stove. Her sensory neurons send a stronger message about the stove. Which change explains how they signal "stronger"?

  1. They fire more often, and more neurons fire
  2. Each action potential grows taller and larger
  3. Each action potential travels a longer path
  4. The neurons release a different neurotransmitter
Show the answer

Because each action potential is all-or-none, a neuron cannot make a single signal bigger. A stronger stimulus makes neurons fire more often and recruits more neurons.

  • Correct: They fire more often, and more neurons fire: Right: intensity is coded by firing rate and the number of neurons firing.
  • Each action potential grows taller and larger: All-or-none: every action potential is the same size, whatever the stimulus.
  • Each action potential travels a longer path: The path from her hand to the spinal cord is the same length for the mug and the stove.
  • The neurons release a different neurotransmitter: A neuron does not switch neurotransmitters to signal a stronger stimulus.

6. A medication slows the reuptake of serotonin, and over several weeks a patient's low mood improves. What does the medication do at the synapse?

  1. Leaves more serotonin in the synapse for longer
  2. Blocks serotonin receptors on the next neuron
  3. Releases a burst of adrenaline into the blood
  4. Speeds up the firing of motor neurons
Show the answer

Slowing reuptake means serotonin stays in the synaptic gap longer and keeps reaching the receptors. That is what a reuptake inhibitor does.

  • Correct: Leaves more serotonin in the synapse for longer: Right: a reuptake inhibitor keeps serotonin available in the synapse.
  • Blocks serotonin receptors on the next neuron: Blocking receptors would make the drug an antagonist and weaken serotonin's effect.
  • Releases a burst of adrenaline into the blood: Adrenaline is a hormone from the adrenal glands; this drug acts on serotonin at the synapse.
  • Speeds up the firing of motor neurons: Motor neuron firing controls muscles; the drug changes serotonin, which is linked to mood.

7. A signal must travel 0.9 meters along a myelinated axon that conducts at 60 meters per second. How many milliseconds does the trip take? (1 second = 1,000 milliseconds.)

Type a number (ms). No calculator needed: work it out by hand, as on the exam.

Show the answer

Time = distance / speed = 0.9 / 60 = 0.015 seconds, which is 15 milliseconds. Without myelin, at about 1 meter per second, the same trip would take most of a second.

  • Answer: 15 ms

Part 9 · Summary

Summary

A neuron collects messages on its dendrites, and if its charge passes threshold it fires an all-or-none action potential down its axon, sped by myelin. At the terminals it releases neurotransmitters that cross the synapse, bind to receptors and excite or inhibit the next neuron, until reuptake ends the message. Hormones carry slower messages in the blood. Drugs act as agonists, antagonists or reuptake inhibitors, and repeated use brings tolerance and withdrawal.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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