Michel Coppieters, who obviously reads in rather obscure places, sent me this article from this month’s Current Biology – Stretchy nerves are an essential component of the extreme feeding mechanisms in rorqual whales”, by Vogl et al.
Rorqual whales such as the blue whale, feed by engulfing water laden with krill. They can take in a massive amount of water equal to the volume of the whale itself. This requires the jaw to open very widely, and the tongue and ventral grooved blubber (VGB) to accommodate.
Vogl et al. have recently demonstrated how stretchy the nerves have to be to allow this feeding function – they could be easily stretched to over double their resting length with folded fascicles (bundles of nerve fibres) unfolding.
This physical feature is a necessity to allow the primary role of impulse conduction to be unhindered.
Humans dance the Fontana!
Human nerves, while not quite so stretchy are similarly constructed. Nerves such as the occipital nerves with neck movement, the infrapatellar branches of the saphenous nerves during squatting, and the nerves of the upper arm during shoulder elevation, all go through remarkable physical challenges – they not only stretch, they also slide in relation to surrounding structures. The median nerve can slide about two centimetres in relation to surrounding tissues. Neurones do their own microscopic dance – I have always thought of this dance as the Fontana, where folded neurones – known as the “spiral bands of Fontana”, unfold during loading. And even more than that, myelin sheaths slide on each other, clefts in the myelin widen, and nodes of Ranvier widen. Much of this work was done in the 60s and then forgotten. I reviewed it quite recently in Mobilisation of the Nervous System in 1991! The brain is so trendy these days that peripheral nerves have been forgotten.
But maybe think of the dance of the Fontana and the remarkable mouth of the blue and fin whales and ponder what the loss of this remarkable movement could do to symptoms and function.
Vogl, AW et al (2015) Stretchy nerves are an essential component of the extreme feeding mechanism of rorqual whales. Curr Biol 25: R360-R361
Learn all about stretchy, sliding and gliding nerves at the Mobilisation of the Neuroimmune System course with Prof Michel Coppieters in Sydney, and Michel and David Butler in Adelaide.
David, firstly, really interesting post and thanks for stimulating our thinking with this. My questions goes back to a concept I remember learning about in my physio studies and that is the constant length phenomenon. I remember learning that nerves don’t really stretch but rather tension stresses placed upon them is taken up throughout the system via gliding and neurodynamic movement. I like to think of it in the sense that at rest in the anatomical position our nerves have some slack in them and then once we do a task, say raising the arm, tension is taken up throughout the system to allow for the movement. What would be pathological would be a peripheral entrapment or sensitization that limits the movement due to guarding in which case neurodynamic stretches would be indicated. So my question in an around about what is… cool that the nerves in the whale can stretch, but is it the same in us. I don’t pretend to know the answer, matter of fact I ask you due to the fact that you, Lorimer, Adrian, and others are so much further along in knowing about this stuff than me. Does the constant length phenomenon still hold up in the human model? One more example, I remember a surgeon telling me as he was figuring out his cuts for a Total Knee that if he overcorrected the patient’s genu valgus too much toward neutral (relative varus to that patient), that he could create a common fibular palsy due to traction stress on the peripheral nerve. That example tell me that traction/stretch stress in the human model is not as well suited as that of the whale right?
Hi there Stevo,
I am not sure of the constant length phenomenon as applied to nerves but it is quite logical and apparent that during movement, a nerve will “find” the position which is best for sustained function. This position usually minimises mechanical loading. However there will always be a positive internal pressure inside a nerve – hence even in a resting state, if the perineurium is nicked, nerve fibres will “mushroom” out.
The mechanical response of a nerve to loading is quite complex with multiple tissues all contributing differently. The whole nerve slides in relation to the surrounding tissue, the nerve then slides in its mesoneurium, the epineurium stretches a bit, the lamellae of the perineurium slide on each other and stretches a bit as does the endoneurial tubes. Axons are quite elastic, much more than myelin, thus myelin has to adapt by processes listed in the original post. It really is quite a dance in the nerves – bundle of C fibres unhindered by myelin can “get away” from forces when for example, a nerve turns a corner as it may in the elbow. A little bit of continual “micromotion” probably keeps all these moving structures nicely “oiled”. And lets not forget the connective tissues are innervated.
So in answer to your question – human nerve will stretch but intraneurally it is quite complex. At around a length increase of 8% the nerve will tighten and intraneural blood flow will slow (eg Ogata and Naito 1986) and essentially a “danger response” will be sent off to the CNS for action evaluation.
Thinking clinically – if you elevate your shoulder to 90 degrees and extend your elbow, the “bed” of the median nerve from wrist to shoulder will be around 10 cm longer. (eg. Zoech et al 1991) The nerve adapts to this partly by stretching but also by sliding, ie local adaptations and “calling” upon nerve in the hand or shoulder/ neck to come and contribute. Limitations in these areas may lead to symptoms.
Sorry about the long response. I am very happy to answer more questions here. I also want to acknowledge the many researchers in the 70s, 80s and 90s whose work allowed this knowledge. Chapter 5 in Butler DS The Sensitive Nervous System has a review.
Ogata K, Naito M (1986) J Hand Surg 118:10-14
Zoech G et al (1991) Neuro-Orthopaedics 10: 73-82
Thanks for the long reply, no need to apologize, it’s a good learning opportunity for me to listen to you explain. I will check out those references as I have your text but for sake of transparency have not studies as I want/could/should. I do use the explain pain book quite a lot and I love the simplicity of that text so for what’s it’s worth, thank you for your explanation of my question and also just your contribution to neuroscience in general and I will be checking up on those references.