Concepts / Neuron Structure and Function

Neuron Structure and Function

Start with direction: dendrites are associated with receiving information, while the axon carries the neuron's output onward. The cell body is the third main part of the typical three-part structure.

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Reading a Neuron as a Route

A neuron is a cell built to process and transmit information. The most useful way to read its structure is as an information route rather than as a list of labels: signals arrive through dendrites, the cell body is the central part of the typical structure, and output leaves through the axon. That output is carried as electrical pulses called action potentials, or spikes.

Dendritesreceive informationCell bodycentral partAxoncarries outputinput arrives towardoutput leaves through
Where are the dendrites, cell body, and axon located relative to one another, and how are they connected?
Key point

Start with direction: dendrites are associated with receiving information, while the axon carries the neuron's output onward. The cell body is the third main part of the typical three-part structure.

From Incoming Signal to Output

To trace one information event, begin with an incoming signal at the dendrites. The signal arrives through these receiving structures and is associated with the neuron. The neuron's output then leaves through its axon. When the neuron generates an action potential, we say that it fires.

Incoming signalarrivesDendritesreceive informationCell bodycentral partAction potentialelectrical pulseAxoncarries outputTarget sitereceives outputarrives atinformation routeneuron generatestravels alongoutput reaches
How does information move from an incoming signal through the dendrites and cell body, along the axon, to a receiving target?
Worked example

Tracing One Output Event

Trace a single event from an incoming signal to a receiving target site.

  1. 1
    1. Receive

    The incoming information arrives through the neuron's dendrites.

  2. 2
    2. Identify the output event

    The neuron generates an action potential. This is the event described as the neuron firing.

  3. 3
    3. Carry the output

    The action potential, an electrical pulse, travels along the axon.

  4. 4
    4. Reach a target

    The axon carries the neuron's output onward to a target site.

Result

The route is incoming signal, dendrites, neuron firing, action potential along the axon, and output at a target site.

Action Potentials on the Axon

An action potential is an electrical pulse that travels along the axon. It is not a continuous stream carried as an ordinary substance. Instead, the neuron's output consists of electrical pulses, also called spikes. When a neuron generates one of these spikes, the neuron is said to fire.

Action potentialgeneratedAxon startpulse entersAxonpulse travelsAxon outputoutput continuesenterstravels alongreaches
How does an electrical pulse move from one end of the axon to the other?

What do you think happens?

A neuron generates a spike. What should you expect the spike to do next?

  • Travel along the axon as an electrical pulse

  • Become a continuous substance inside the axon

  • Remain only in the dendrites

Reveal answer
Answer:

Travel along the axon as an electrical pulse

The neuron's output consists of action potentials, also called spikes, that travel along the axon. The neuron is said to fire when it generates one.

Branch Points and Axonal Arbors

An axon may divide into branches. When an action potential reaches a branch point, it can produce action potentials on the outgoing branches. The signal therefore does not have to stop at the branch point. Through an axonal arbor, the branching arrangement of the axon, the neuron's activity can reach many target sites.

Action potentialarrivesBranch pointaxon dividesOutgoing branch Aaction potentialOutgoing branch Baction potentialreachescan producecan produce
When an action potential reaches a branch point, does it stop, split, or continue into each branch?
Axoncarries outputBranch oneaction potentialTarget site oneBranch twoaction potentialTarget site twoBranch threeaction potentialTarget site threebranches intobranches intobranches intoreachesreachesreaches
How can one axon distribute a neuron's activity to many different target sites?
Key point

The axonal arbor explains how one neuron's activity can influence many target sites: the axon branches, and an action potential can produce action potentials on the outgoing branches.

Dendrites Versus Axons

FeatureDendritesAxon
Role in information flowReceive informationCarries neuronal output
DirectionInformation arrives toward the neuronOutput leaves the neuron
Signal associated with the structureIncoming informationAction potentials, or spikes
Relationship to target sitesAssociated with receiving inputCan carry output to target sites

The central distinction is receiving input through dendrites and carrying output through the axon.

Do not treat dendrites and axons as interchangeable branches. Their roles point in different directions. Dendrites are the structures through which information arrives, while the axon is the route by which the neuron's output travels away. This distinction remains useful even when a drawing contains many branching structures.

Dendritesreceive informationIncoming signaltoward neuronAxoncarries outputAction potentialaway from neuronarrives throughcarries
What is the difference between the direction and role of information flow in dendrites versus axons?

Common Interpretation Errors

  • Common mistake

    Treating the three parts as interchangeable labels

    What it looks like

    Calling the axon a receiving structure simply because it is a branch in the drawing.

    Why it fails

    The typical neuron description assigns different information-flow roles to the parts: dendrites receive information, while the axon carries output.

    Do this instead

    Identify the part by its role as well as its position: dendrites receive, the cell body is the central named component, and the axon carries output.

  • Common mistake

    Describing an action potential as a continuous substance

    What it looks like

    Saying that output is a stream of material moving through the axon.

    Why it fails

    The output consists of electrical pulses called action potentials or spikes.

    Do this instead

    Describe an action potential as an electrical pulse that travels along the axon.

  • Common mistake

    Assuming an action potential stops at an axonal branch point

    What it looks like

    Tracing the pulse along the main axon and ending the route when the axon divides.

    Why it fails

    At a branch point, the action potential can produce action potentials on the outgoing branches.

    Do this instead

    Continue the trace into the outgoing branches and then toward their target sites.

  • Common mistake

    Using firing to mean continuous output

    What it looks like

    Saying that a neuron fires because it continuously carries output.

    Why it fails

    A neuron is said to fire when it generates an action potential, or spike.

    Do this instead

    Use firing to refer to the generation of an action potential.

Best practice

When labeling a neuron, annotate each part with its information-flow role. Write receive information beside dendrites and carries output beside the axon. Then mark an action potential as the electrical pulse traveling along the axon.

Check the Information Route

MEDIUM

A simplified neuron has dendrites, a cell body, one axon, and two outgoing axonal branches. Describe the route of one incoming signal and explain how one action potential could reach both branches and their target sites.

Hints
  • Begin with the structure that receives information.
  • Name the event that means the neuron fires.
  • At the branch point, explain what can happen on each outgoing branch.
Worked example

A Complete Trace

A signal arrives at a neuron's receiving structures. The axon later divides into two branches. Give a complete information-flow description.

  1. 1
    Input

    The signal arrives through the dendrites, the neuron's receiving structures.

  2. 2
    Firing

    The neuron generates an action potential, so the neuron is said to fire.

  3. 3
    Conduction

    The action potential travels along the axon as an electrical pulse.

  4. 4
    Branching

    When the pulse reaches the axonal branch point, it can produce action potentials on both outgoing branches.

  5. 5
    Distribution

    The axonal arbor allows the activity to reach the target sites served by those branches.

Result

Information arrives through dendrites, the neuron fires, the action potential travels along the axon, and the axonal branches distribute the activity toward multiple target sites.

Essential Takeaways

  1. 1
    A typical neuron has a cell body, dendrites, and one axon.
  2. 2
    Dendrites receive information, while the axon carries the neuron's output.
  3. 3
    An action potential is an electrical pulse, also called a spike, that travels along the axon.
  4. 4
    A neuron fires when it generates an action potential.
  5. 5
    At an axonal branch point, an action potential can produce action potentials on outgoing branches, allowing the axonal arbor to reach many target sites.

At a glance

  • Dendrites receive information, the cell body is the central named part, and the axon carries output.
  • A neuron's output consists of action potentials, or spikes, that travel along the axon.
  • Generating an action potential is described as firing.
  • At an axonal branch point, the activity can continue into outgoing branches.
  • An axonal arbor can distribute one neuron's activity to many target sites.

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