Neuron Structure and Function
Dendrites receive information and the axon carries neuronal output.
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.
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.
Tracing One Output Event
Trace a single event from an incoming signal to a receiving target site.
1. Receive: The incoming information arrives through the neuron's dendrites.
2. Identify the output event: The neuron generates an action potential. This is the event described as the neuron firing.
3. Carry the output: The action potential, an electrical pulse, travels along the axon.
4. Reach a target: The axon carries the neuron's output onward to a target site.
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.
What do you think happens?
A neuron generates a spike. What should you expect the spike to do next?
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.
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
| Feature | Dendrites | Axon |
|---|---|---|
| Role in information flow | Receive information | Carries neuronal output |
| Direction | Information arrives toward the neuron | Output leaves the neuron |
| Signal associated with the structure | Incoming information | Action potentials, or spikes |
| Relationship to target sites | Associated with receiving input | Can carry output to target sites |
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.
Common Interpretation Errors
Treating the three parts as interchangeable labels
The typical neuron description assigns different information-flow roles to the parts: dendrites receive information, while the axon carries output.
Fix:
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.Describing an action potential as a continuous substance
The output consists of electrical pulses called action potentials or spikes.
Fix:
Describe an action potential as an electrical pulse that travels along the axon.Assuming an action potential stops at an axonal branch point
At a branch point, the action potential can produce action potentials on the outgoing branches.
Fix:
Continue the trace into the outgoing branches and then toward their target sites.Using firing to mean continuous output
A neuron is said to fire when it generates an action potential, or spike.
Fix:
Use firing to refer to the generation of an action potential.
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
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.
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.
Input: The signal arrives through the dendrites, the neuron's receiving structures.
Firing: The neuron generates an action potential, so the neuron is said to fire.
Conduction: The action potential travels along the axon as an electrical pulse.
Branching: When the pulse reaches the axonal branch point, it can produce action potentials on both outgoing branches.
Distribution: The axonal arbor allows the activity to reach the target sites served by those branches.
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
- A typical neuron has a cell body, dendrites, and one axon.
- Dendrites receive information, while the axon carries the neuron's output.
- An action potential is an electrical pulse, also called a spike, that travels along the axon.
- A neuron fires when it generates an action potential.
- At an axonal branch point, an action potential can produce action potentials on outgoing branches, allowing the axonal arbor to reach many target sites.
Key Takeaways
- 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.