An accordion works one particular way. A player pulls and pushes two rigid frames apart, and the folds between them open and close. Air moves because a shape changed.
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| The object every description in the earlier version of this post was borrowing from. Its bellows open and close because a player pulls and pushes. No biological system described here works that way. Johann Pascher, public domain, via Wikimedia Commons |
The previous version of this post opened by calling accordion-like movement "a distinctive type of mechanical motion observed in various biological systems."
It closed by calling that motion "a fundamental biological mechanism."
Neither is true. Searching the phrase returns film criticism, semiotics and psychoanalysis. It returns no zoology and no physiology. No living thing is classified by whether it folds.
What the post did have was four genuinely interesting examples. It simply grouped them under a heading that does not exist, and never noticed that the resemblance it noticed was a resemblance.
The subjects are real. A worm crawls. A lung fills and empties. A gut moves its contents along. A cell crawls across a surface. All four do it with a structure that repeatedly changes length or volume.
That is a genuine shared feature, and it is worth something. But a shared silhouette is not a shared mechanism.
These four systems use different tissues, different scales, different force generators and different control systems. They span roughly four orders of magnitude in size, from a lamellipodium a few microns across to a lung tens of centimeters tall.
When distant things look alike, that is usually a fact about the things we can see. It is rarely a fact about how they work.
| System | What changes shape | What drives it |
| Earthworm | body segments lengthen and shorten | muscles acting against setae |
| Lung | the chest wall changes volume | diaphragm and intercostal muscles |
| Gut | rings of circular muscle narrow the tube | the myenteric plexus |
| Crawling cell | actin network turns over | polymerization at the leading edge |
Four systems that change shape. Note that the third column is different in every row, which is the point of this article.
The earthworm needs a grip as well as a squeeze
An earthworm has no legs, no skeleton and no ability to grip with anything deliberate. It moves by squeezing rings of its own body in sequence, starting at the front.
The body is a hydrostatic skeleton. Fluid fills the cavity between the gut and the body wall and cannot be compressed, so it holds its shape while the muscle layers around it change it.
Circular muscles contracting makes a segment longer and thinner. Longitudinal muscles contracting makes it shorter and fatter. Neither group can work alone.
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| One stride, filmed a moment at a time. Different segments are caught at different phases of the same wave. The pale segments are elongated and the darker ones shortened. Mokele, CC BY 3.0, via Wikimedia Commons |
But here is the step the original version skipped. Alternate muscle action on its own makes an undulating shape that swims in place.
What makes it a walk is the setae. These are stiff bristles set in little sockets in each segment, and they can be pushed out and retracted but not aimed.
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| Setae: the bristles that grip the soil. Without them the worm undulates in place instead of moving. Claire Hildebrand and Audrey Chrisman, CC BY-SA 4.0, via Wikimedia Commons |
During the power stroke the rear setae dig in and hold while the front of the body extends. Then the front setae engage and the rear ones release.
Muscle supplies the squeeze. The bristles supply the traction. Without the second, the first achieves nothing.
The lung: the wrong organ is doing the moving
The original version said that the diaphragm and intercostal muscles "enable the ribcage and lungs to expand and contract."
The lungs do not expand and contract. They are compliant bags that fill and empty. The volume change that does the work happens in the chest wall around them.
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| The ribcage during inhalation. During quiet breathing this chest wall is the structure changing volume, and the lungs fill and empty along with it. Cruithne9, CC BY-SA 4.0, via Wikimedia Commons |
The diaphragm is the main engine. Sitting like a dome above the liver, it contracts and flattens downward, and that pulls the chest wall out from below.
Because a stretched muscle pulls harder than a slack one, the diaphragm also stops the chest from collapsing between breaths. Breathing is active at both ends of the cycle, not a passive afterthought of an inhale.
So the pump is the chest wall, the piston is the diaphragm, and the lungs are the pipework. Calling this accordion motion describes the pipework and misses both moving parts.
The gut: a wave that only travels one way
The original version described peristalsis as "rhythmic compressions and relaxations" creating a motion like an accordion bellows.
Simultaneous contraction along the length of the gut would push nothing anywhere. It would simply clamp the tube shut.
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| The enteric nervous system, including the myenteric plexus that sits between the circular and longitudinal muscle layers. It decides which side of a stretch contracts. Natale, Ryskalin, Morucci, Lazzeri, Frati and Fornai, CC BY 4.0, via Wikimedia Commons |
What actually happens was measured by Costa and Furness in 1976. Distending the gut wall produces a transient contraction of the circular muscle on the oral side and a sustained relaxation on the anal side.
That asymmetry is what moves anything. Contraction behind the contents and relaxation ahead of it means the gut wall squeezes the contents forward, and the wave travels one way along the tube.
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| A peristaltic wave. Circular muscle contracts behind the contents while relaxing ahead of it. If every ring tightened at once, nothing would move at all. Dieter Menne, public domain, via Wikimedia Commons |
The decision-making sits in the myenteric plexus, a network of neurons embedded in the gut wall between the two muscle layers. It does not need to consult the brain first.
Which is the part worth noticing. The gut runs a local control loop that reads its own stretch and responds with a directional pattern. An accordion has no such loop. It is pushed.
The crawling cell runs a treadmill, not a fold
The original version listed lamellipodia as the microscopic example. Migrating cells do push out sheet-like protrusions, and that is correct.
What those protrusions do is not accordion motion. Actin monomers polymerize into filaments at the front of the cell while the network at the back is dismantled.
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| A migrating cell stained for actin. The bright sheet at the leading edge is the lamellipodium, assembled by polymerization and disassembled at the rear. That is a treadmill, not a fold. Yang, Park, Choi, Choi, Ko and Lee, CC BY-SA 3.0, via Wikimedia Commons |
The result is a conveyor rather than a bellows. Leading-edge velocities have been measured in the range of about 1.7 to 5.2 micrometers per minute, with peaks near 10.7.
The structure that builds it is the Arp2/3 complex, which nucleates new branches off existing filaments and holds them at a fixed angle, producing the dense mesh that pushes.
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| The Arp2/3 complex, the engine behind lamellipodial extension. It branches new actin off old filaments at a fixed angle. Thomas Splettstoesser, CC BY-SA 3.0, via Wikimedia Commons |
The cell crawls because this mesh grips the substrate and the rear of the cell is hauled forward by contractile filaments deeper inside.
There is no reciprocal extension and compression of a single structure. Material is added at one end and removed at the other.
What the metaphor costs
An analogy is worth keeping when it compresses something true. This one stops doing that almost immediately, and then gets used to make claims it cannot support.
It explains less than the real mechanisms do
Call it accordion-like and you have said that the shape repeats. You have said nothing about why the shape repeats, what triggers it, or what would happen if it stopped.
Setae, the myenteric plexus and Arp2/3 are the answers to those questions. None of them is about folding.
It groups by what is visible
The original closed by noting the motion occurs "from single cells to complex animals" and reads that breadth as evidence of a single fundamental mechanism.
It is the opposite evidence. When the same shape turns up at wildly different scales, with no shared machinery, it usually means the shape is cheap to produce rather than that the mechanism is universal.
Its engineering claims do not survive contact
The post stated that "expandable stents and prosthetic components use the same principle to achieve controlled expansion and contraction."
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| A coronary stent. Stents are opened by balloon dilation or by shape-memory alloy, and they do not contract back. This is not accordion motion, which is why the claim has been cut rather than hedged. US Food and Drug Administration, public domain, via Wikimedia Commons |
Stents are opened by balloon dilation or by shape-memory alloy. Neither is accordion motion, and neither contracts back. The claim has been cut rather than softened, because softening it would still imply the resemblance exists.
Soft robots borrow from biology, but not from this
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| A soft robot driven by a pneumatic bending actuator. Soft robots do take ideas from biology, but this one works by inflating a folded chamber, not by accordion motion. Sadeghi, Mondini, Del Dottore, Mishra and Mazzolai, CC BY-SA 4.0, via Wikimedia Commons |
Soft robotics is a real field, and it does borrow from biology. But a typical soft actuator inflates a folded chamber. That is a bellows in a literal sense, and it is not the mechanism any organism in this article uses.
A better way to put it
The four subjects are worth a full article each, and they are worth keeping together for one reason: comparing them is more instructive than lumping them.
A worm, a lung, a gut and a migrating cell each solve the same engineering problem, which is how to move something when you cannot push against it directly.
The worm solves it by gripping and pulling. The lung solves it by changing the volume of a sealed space. The gut solves it by squeezing a tube from behind while relaxing it ahead.
The cell solves it by building a mesh and dragging itself along it. Four different answers, and not one of them is a fold.
A useful label for the group is periodic change of shape. That is exactly as broad as the evidence, and it does not accidentally promise a mechanism.
| Figure | What it is |
| about 482 words | the length of the previous version of this post |
| 100 to 150 | approximate segment count of an earthworm |
| about 1.7 to 5.2 | leading-edge velocity in micrometers per minute |
| about 10.7 | peak measured lamellipodial velocity, micrometers per minute |
| 1976 | Costa and Furness measured the peristaltic reflex |
| 4 orders of magnitude | spread between a lamellipodium and a lung |
| 0 | citations in the previous version of this post |
Figures from the sources listed at the foot of this article.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Hydrostatic skeleton | A structure that holds its shape using fluid under pressure rather than bone. Worms and jellyfish both use one. |
| Seta | A stiff bristle in an earthworm segment. Pushes out to grip, cannot be aimed. |
| Peristalsis | A polarized wave of muscle contraction that travels along a tube in one direction only. |
| Myenteric plexus | The nerve network in the gut wall that decides which side of a stretch contracts. |
| Oral | Toward the mouth. The anal end is the other direction. |
| Lamellipodium | A broad, flat sheet of actin at the front of a migrating cell. |
| Actin polymerization | Adding actin monomers to a growing filament. The engine of cell crawling. |
| Arp2/3 complex | A protein that nucleates new actin branches off existing filaments at a fixed angle. |
| Treadmilling | Polymerization at one end and disassembly at the other, so the material moves while the cell advances. |
| Intercostal muscles | The muscles between the ribs. They expand the chest; the diaphragm does most of the work. |
| Shape-memory alloy | A metal that returns to a set shape when warmed. One way stents and some actuators open. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- "Accordion-like movement" is not a recognized biological category. The phrase comes from the arts, and no textbook uses it.
- Four real systems change shape periodically, but they share only a silhouette. Their driving mechanisms differ in every case.
- An earthworm needs setae as well as muscles. Muscle action alone makes it swim in place.
- During quiet breathing it is the chest wall that changes volume. The lungs fill and empty along with it.
- Peristalsis travels one way. Circular muscle contracts behind the contents and relaxes ahead of them.
- A lamellipodium is a conveyor, not a bellows: actin is added at the front and removed at the back.
- The claim about expandable stents was false and has been cut.
Frequently Asked Questions
Is accordion-like movement a real biological term?
No. It does not appear in zoology or physiology as a category of movement. Searching the phrase returns criticism and semiotics rather than biology.
Individual biologists may describe a specific structure as looking like a bellows or a concertina. That is a description, not a classification.
What should the four examples be called instead?
Periodic change of shape. Each of the four repeatedly changes length or volume, and that shared feature is real.
Calling it periodic avoids implying that one mechanism underlies all four, which no source supports.
Do all four systems generate their own rhythm?
No, and this is a real difference between them.
The gut runs a local control loop that detects stretch and responds directionally. Lung rhythm comes from the respiratory centers in the brainstem.
The worm produces its wave as a chain of local reflexes along its own body. The cell has no rhythm at all. It is driven by wherever signals happen to be acting on it.
Is anything in biology actually accordion-like?
Some structures do fold and unfold in ways that earn the comparison.
The concertina is a real instrument that shares its name with this shape. Biological structures that genuinely fold, such as the closed wings of a beetle or folded leaves, are better described by what folds them.
Why did the original version group these four together?
Because they share a visible pattern and nothing else that is easy to state. Grouping by resemblance is a reasonable place to start looking, and a poor place to stop.
The four subjects are all worth studying. Only the invented category holding them together was wrong.
Key references
• Peristalsis and the enteric nervous system
• The peristaltic reflex (Costa and Furness, 1976)
• Gut peristalsis and the myenteric plexus (Huizinga et al., 2009)
• Earthworm locomotion and the hydrostatic skeleton
• Lamellipodia and the Arp2/3 complex
• Actin and cell migration
• Lung structure and ventilation
• Soft robotics
Images: Wikimedia Commons, with the author and license named in each caption. Figures follow the sources above.
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