<p>See a previous Q&amp;A:</p>
<p><a href="https://psychology.stackexchange.com/questions/23824/how-synapses-are-hold-in-place-if-theyre-not-phisically-attached">How synapses are hold in place if they&#39;re not phisically attached?</a></p>
<p><a href="https://en.wikipedia.org/wiki/Chemical_synapse#Structure" rel="nofollow noreferrer">Synapses</a> are quite structured, and there are physical connections between pre- and post-synapse through adhesion molecules, but the membranes themselves do not quite touch. Cell membranes are a <a href="https://en.wikipedia.org/wiki/Phospholipid" rel="nofollow noreferrer">phospholipid</a> bilayer; the outside is hydrophilic and has a bit of a charge, so typically membranes won't sit right against each other, they'll always have at least a 'shell' of water in between. If they were held more closely, they would fuse, like happens with <a href="https://en.wikipedia.org/wiki/Vesicle_fusion" rel="nofollow noreferrer">vesicles</a>.</p>
<p>The gap is filled with extracellular fluid: mostly water, with dissolved salts and molecules. Released neurotransmitters cross the synaptic cleft by diffusion.</p>
<p>There are some synapses referred to as <a href="https://en.wikipedia.org/wiki/Electrical_synapse" rel="nofollow noreferrer">&quot;electrical&quot; synapses</a> rather than chemical synapses; these are pretty much the same physically, except there are protein pores between the membranes that allow diffusion between their respective intracellular spaces. Everywhere else there'll be a gap. I'm not sure what you were reading about synapses <em>without</em> a gap, but maybe this is what they meant - I wouldn't quite describe it that way, though.</p>

CC BY-SA 4.0
2022-10-20T20:20:13.817Z
Yes, this is just what I needed. And I do think it was a (science writers?) reference to electrical synapses that I was vaguely remembering. I just found https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5705454/ which mentions that many in neuroscience think of electrical synapses in mammals and humans as recently discovered. Thank you, Bryan!
Anasker
CC BY-SA 4.0
2022-10-20T20:26:57.897Z
@Anasker Knowledge of their existence is not particularly new, at least compared overall to the field of neuroscience that with a handful of exceptions is itself quite new (the word 'neuroscience' itself I believe was first used in the 1970s). But yes, I would say their functional roles tend to be underappreciated and often left out of broader discussions.
Bryan Krause
CC BY-SA 4.0
2022-10-20T20:35:17.170Z
Yes, the article says we knew of them mid-century, but: "To many established scientists in the neuroscience community who are uninitiated with respect to the many decades of literature on electrical synaptic transmission, the field of electrical synapses in mammalian systems may appear to have emerged suddenly at the beginning of this century, with the subsequent pace of progress in the area being nothing short of astonishing." Leaves open how many are "uninitiated". Thanks again.
Anasker
CC BY-SA 4.0
2022-10-20T20:08:13.457Z
2022-10-20T20:08:13.457Z
Bryan Krause
<p>First, although I was always taught that the axon and a dendrite were separated by a synaptic gap of about 20 to 40 nm, I recently saw a reference I've since lost that seemed to suggest we've found that not all axon-dendrite pairs have literal gaps between them; some touch. Unfortunately, I didn't have time to read it, I have not recovered the source, and a search hasn't yet helped. Is there research that suggests that some axons and dendrites actually touch?</p>
<p>Second, what exactly is in the gap? Liquid, gas, nothing/vacuum? Of course, neurotransmitters can cross the gap, but I'm asking about whether something's always in the gap and if so what.</p>

<neuroscience><neurophysiology><neuro-transmitters>
Are all synapses "gappy", and what exactly is in the gap?
Anasker
# Are all synapses "gappy", and what exactly is in the gap?

(Asked by: username_0 on 2022-10-20)

First, although I was always taught that the axon and a dendrite were separated by a synaptic gap of about 20 to 40 nm, I recently saw a reference I've since lost that seemed to suggest we've found that not all axon-dendrite pairs have literal gaps between them; some touch. Unfortunately, I didn't have time to read it, I have not recovered the source, and a search hasn't yet helped. Is there research that suggests that some axons and dendrites actually touch?

Second, what exactly is in the gap? Liquid, gas, nothing/vacuum? Of course, neurotransmitters can cross the gap, but I'm asking about whether something's always in the gap and if so what.

**Tags:** neuroscience, neurophysiology, neuro-transmitters


---

## 1 answers

### Answer by username_1 on 2022-10-20. Score: 1

See a previous Q&A:

[How synapses are hold in place if they're not phisically attached?](https://psychology.stackexchange.com/questions/23824/how-synapses-are-hold-in-place-if-theyre-not-phisically-attached)

[Synapses](https://en.wikipedia.org/wiki/Chemical_synapse#Structure) are quite structured, and there are physical connections between pre- and post-synapse through adhesion molecules, but the membranes themselves do not quite touch. Cell membranes are a [phospholipid](https://en.wikipedia.org/wiki/Phospholipid) bilayer; the outside is hydrophilic and has a bit of a charge, so typically membranes won't sit right against each other, they'll always have at least a 'shell' of water in between. If they were held more closely, they would fuse, like happens with [vesicles](https://en.wikipedia.org/wiki/Vesicle_fusion).

The gap is filled with extracellular fluid: mostly water, with dissolved salts and molecules. Released neurotransmitters cross the synaptic cleft by diffusion.

There are some synapses referred to as ["electrical" synapses](https://en.wikipedia.org/wiki/Electrical_synapse) rather than chemical synapses; these are pretty much the same physically, except there are protein pores between the membranes that allow diffusion between their respective intracellular spaces. Everywhere else there'll be a gap. I'm not sure what you were reading about synapses *without* a gap, but maybe this is what they meant - I wouldn't quite describe it that way, though.

> **Comments:**
> **username_0**: Yes, this is just what I needed. And I do think it was a (science writers?) reference to electrical synapses that I was vaguely remembering. I just found https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5705454/ which mentions that many in neuroscience think of electrical synapses in mammals and humans as recently discovered. Thank you, Bryan!  
> **username_1**: @username_0 Knowledge of their existence is not particularly new, at least compared overall to the field of neuroscience that with a handful of exceptions is itself quite new (the word 'neuroscience' itself I believe was first used in the 1970s). But yes, I would say their functional roles tend to be underappreciated and often left out of broader discussions.  
> **username_0**: Yes, the article says we knew of them mid-century, but: "To many established scientists in the neuroscience community who are uninitiated with respect to the many decades of literature on electrical synaptic transmission, the field of electrical synapses in mammalian systems may appear to have emerged suddenly at the beginning of this century, with the subsequent pace of progress in the area being nothing short of astonishing." Leaves open how many are "uninitiated". Thanks again.  

---

