<p><strong>Short answer</strong><br>
Axonal outputs are coupled to dendrites or other effector tissues, such as muscular fibers or glandular tissue. There are no redundant axonal outputs, or redundant dendritic inputs. There is a tight coupling between the two both during developmental formation of synapses and during the maintenance of the two.  </p>

<p><strong>Background</strong><br>
Neurons can have extensive dendritic trees (<em>e.g.</em>, Purkinje cells in the cerebellar cortex, Fig. 1) and also elaborate arborated axon terminals (*e.g., in the neuromuscular junction, Fig. 2).   </p>

<p>In the cases shown below, the functions can be defined as simple logical operators, namely integration (Purkinje cell) and amplification (neuromuscular junction). The Purkinje cells receive and integrate input from the brainstem. The motoneurons innervating muscle use multiple axon terminals to target a larger area of the muscle so that stronger, synchronous muscle contractions can be achieved.</p>

<p>The development and maintenance of dendrite-axon connections are tightly coupled (<em>e.g.</em>, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2845208/" rel="nofollow noreferrer">Shen &amp; Lowan, 2010)</a>. Generally, when either input to the dendrite is diminished (axonal regression), or output of the axon is cancelled (dendritic regression), the plasticity of the neural tissue will lead to degeneration or re-directing of the axon, or regression or re-innervation of the dendrite, respectively (<em>e.g.</em> <a href="https://www.ncbi.nlm.nih.gov/pubmed/12892644" rel="nofollow noreferrer">Marc <em>et al</em>., 2003</a>).  </p>

<p>As regarding to your math; be aware axons <em>can</em> branch, axon arborations can target a single dendritic tree, axons can terminate on other tissues, such as glands and muscles (no dendrites there!). In other words, your mathematical example is an oversimplification. Further; cells with elaborate dendritic trees as shown in Fig. 1 are common in cortical layers, but are not the norm. The ratios of the cell types have to be taken into account as well. </p>

<p>Lastly, there can be the textbook connections between neurons, namely <strong><a href="http://michaeldmann.net/mann13.html" rel="nofollow noreferrer">axo-dendritic</a></strong> connections, but also <strong><a href="http://michaeldmann.net/mann13.html" rel="nofollow noreferrer">axo-axonal</a></strong> <a href="https://www.ncbi.nlm.nih.gov/pubmed/11567620" rel="nofollow noreferrer">(Schmitz <em>et al</em>, 2001)</a>, <strong><a href="http://michaeldmann.net/mann13.html" rel="nofollow noreferrer">dendrodendritic</a></strong> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3819211/pdf/nihms498522.pdf" rel="nofollow noreferrer">(Shepherd, 2009)</a>, and even <strong><a href="http://michaeldmann.net/mann13.html" rel="nofollow noreferrer">axosomatic</a></strong> synapses between neurons (Fig. 3).    </p>

<p><a href="https://i.sstatic.net/tpVtJ.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/tpVtJ.jpg" alt="dendritic tree"></a><br>
<sup>Fig. 1. Dendritic tree <a href="https://psychcentral.com/classics/Wundt/Physio/chap2.htm" rel="nofollow noreferrer">(Wundt, 1904)</a></sup></p>

<p><a href="https://i.sstatic.net/JgeH2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/JgeH2.png" alt="enter image description here"></a><br>
<sup>Fig. 2. Axonal terminals in a neuromuscular junction. source: <a href="https://www.linkedin.com/pulse/brain-muscle-connection-jamie-farrell" rel="nofollow noreferrer">Farrel (2017)</a></sup></p>

<p><a href="https://i.sstatic.net/r0Ios.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/r0Ios.jpg" alt="synapses"></a><br>
<sup>Fig. 3. Different kinds of synapses. source: <a href="http://droualb.faculty.mjc.edu/Course%20Materials/Physiology%20101/Chapter%20Notes/Fall%202011/chapter_8%20Fall%202011.htm" rel="nofollow noreferrer">Modesto Junior College</a></sup></p>

<p><sub><strong>References</strong><br>
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/pubmed/12892644" rel="nofollow noreferrer">Marc <em>et al</em>., <em>Prog Retin Eye Res</em> (2003); <strong>22</strong>(5):607-55</a><br>
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/pubmed/12892644" rel="nofollow noreferrer">Shen &amp; Kowan, <em>Cold Spring Harb Perspect Biol</em> (2010); <strong>2</strong>(4): a001842</a><br>
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3819211/pdf/nihms498522.pdf" rel="nofollow noreferrer">Shepherd, <em>Ann N Y Acad Sci</em> (2009); <strong>1170</strong>; 1-11</a><br>
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/pubmed/11567620" rel="nofollow noreferrer">Schmitz <em>et al</em>., Neuron (2001); <strong>31</strong>(5): 831-40</a><br>
<strong>-</strong> <a href="https://psychcentral.com/classics/Wundt/Physio/chap2.htm" rel="nofollow noreferrer">Wundt, Principles of Physiological Psychology (1904)</a></sub> </p>

CC BY-SA 3.0
2017-06-08T08:54:58.097Z
2017-06-10T21:42:20.360Z
2017-06-10T21:42:20.360Z
AliceD
<p>As far as I understand, one neuron can have up to 200 000 dendrites (e.g. purkinje cells). As for axons, there can be only one per neuron, which can divide into thousands (but not hundreds of thousands) of branches at the end. </p>

<p>I assume that each dendrite has one input terminal and each axon branch has one output terminal. I also assume that only dendrites can have input terminals and that only axons' branches can have output terminals. So we can have up to 200 000 input terminals and up to thousands of output terminals per one neuron. So it seems to me like in the whole brain we have <strong>more input terminals than output terminals</strong> available.</p>

<p>Are my assumptions correct?</p>

<p>If yes, does that mean that there are a lot of <strong>redundant dendritic input terminals</strong> available?</p>

CC BY-SA 3.0
2017-06-07T17:30:03.110Z
Possible duplicate of [How many dendrite connections vs axon terminals does a multipolar cerebral neuron have?](https://cogsci.stackexchange.com/questions/9144/how-many-dendrite-connections-vs-axon-terminals-does-a-multipolar-cerebral-neuro)
Arnon Weinberg
<neuroscience><neurophysiology><neuroanatomy>
Are all dendrites targeted by an axon, or are there a lot of "empty" dendrites?
Danyl Filatov
# Are all dendrites targeted by an axon, or are there a lot of "empty" dendrites?

(Asked by: username_0 on 2017-06-07)

As far as I understand, one neuron can have up to 200 000 dendrites (e.g. purkinje cells). As for axons, there can be only one per neuron, which can divide into thousands (but not hundreds of thousands) of branches at the end.

I assume that each dendrite has one input terminal and each axon branch has one output terminal. I also assume that only dendrites can have input terminals and that only axons' branches can have output terminals. So we can have up to 200 000 input terminals and up to thousands of output terminals per one neuron. So it seems to me like in the whole brain we have **more input terminals than output terminals** available.

Are my assumptions correct?

If yes, does that mean that there are a lot of **redundant dendritic input terminals** available?

**Tags:** neuroscience, neurophysiology, neuroanatomy

> **Comments:**
> **username_1**: Possible duplicate of [How many dendrite connections vs axon terminals does a multipolar cerebral neuron have?](https://cogsci.stackexchange.com/questions/9144/how-many-dendrite-connections-vs-axon-terminals-does-a-multipolar-cerebral-neuro)  

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## 1 answers

### Answer by username_2 on 2017-06-08. Score: 3

**Short answer**  
Axonal outputs are coupled to dendrites or other effector tissues, such as muscular fibers or glandular tissue. There are no redundant axonal outputs, or redundant dendritic inputs. There is a tight coupling between the two both during developmental formation of synapses and during the maintenance of the two.

**Background**  
Neurons can have extensive dendritic trees (*e.g.*, Purkinje cells in the cerebellar cortex, Fig. 1) and also elaborate arborated axon terminals (\*e.g., in the neuromuscular junction, Fig. 2).

In the cases shown below, the functions can be defined as simple logical operators, namely integration (Purkinje cell) and amplification (neuromuscular junction). The Purkinje cells receive and integrate input from the brainstem. The motoneurons innervating muscle use multiple axon terminals to target a larger area of the muscle so that stronger, synchronous muscle contractions can be achieved.

The development and maintenance of dendrite-axon connections are tightly coupled (*e.g.*, [Shen & Lowan, 2010)](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2845208/). Generally, when either input to the dendrite is diminished (axonal regression), or output of the axon is cancelled (dendritic regression), the plasticity of the neural tissue will lead to degeneration or re-directing of the axon, or regression or re-innervation of the dendrite, respectively (*e.g.* [Marc *et al*., 2003](https://www.ncbi.nlm.nih.gov/pubmed/12892644)).

As regarding to your math; be aware axons *can* branch, axon arborations can target a single dendritic tree, axons can terminate on other tissues, such as glands and muscles (no dendrites there!). In other words, your mathematical example is an oversimplification. Further; cells with elaborate dendritic trees as shown in Fig. 1 are common in cortical layers, but are not the norm. The ratios of the cell types have to be taken into account as well.

Lastly, there can be the textbook connections between neurons, namely **[axo-dendritic](http://michaeldmann.net/mann13.html)** connections, but also **[axo-axonal](http://michaeldmann.net/mann13.html)** [(Schmitz *et al*, 2001)](https://www.ncbi.nlm.nih.gov/pubmed/11567620), **[dendrodendritic](http://michaeldmann.net/mann13.html)** [(Shepherd, 2009)](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3819211/pdf/nihms498522.pdf), and even **[axosomatic](http://michaeldmann.net/mann13.html)** synapses between neurons (Fig. 3).

[![dendritic tree](https://i.sstatic.net/tpVtJ.jpg)](https://i.sstatic.net/tpVtJ.jpg)  
Fig. 1. Dendritic tree [(Wundt, 1904)](https://psychcentral.com/classics/Wundt/Physio/chap2.htm)

[![enter image description here](https://i.sstatic.net/JgeH2.png)](https://i.sstatic.net/JgeH2.png)  
Fig. 2. Axonal terminals in a neuromuscular junction. source: [Farrel (2017)](https://www.linkedin.com/pulse/brain-muscle-connection-jamie-farrell)

[![synapses](https://i.sstatic.net/r0Ios.jpg)](https://i.sstatic.net/r0Ios.jpg)  
Fig. 3. Different kinds of synapses. source: [Modesto Junior College](http://droualb.faculty.mjc.edu/Course%20Materials/Physiology%20101/Chapter%20Notes/Fall%202011/chapter_8%20Fall%202011.htm)

**References**  
**-** [Marc *et al*., *Prog Retin Eye Res* (2003); **22**(5):607-55](https://www.ncbi.nlm.nih.gov/pubmed/12892644)  
**-** [Shen & Kowan, *Cold Spring Harb Perspect Biol* (2010); **2**(4): a001842](https://www.ncbi.nlm.nih.gov/pubmed/12892644)  
**-** [Shepherd, *Ann N Y Acad Sci* (2009); **1170**; 1-11](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3819211/pdf/nihms498522.pdf)  
**-** [Schmitz *et al*., Neuron (2001); **31**(5): 831-40](https://www.ncbi.nlm.nih.gov/pubmed/11567620)  
**-** [Wundt, Principles of Physiological Psychology (1904)](https://psychcentral.com/classics/Wundt/Physio/chap2.htm)


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