Myth Busting - Cranial Nerve 11 and Saddle Fit
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I saw it again this last week. Another website/video talking about Cranial Nerve 11 and how important it is regarding saddle fit. The story is that there is a triangle behind the shoulder blade, in the center of which is Cranial Nerve 11, also called the Accessory Nerve. If you put pressure on that nerve, there is a “reflexive action” which causes the horse to “become agitated… twitch, pin their ears,” and which makes the horse stop and hollow through the back and the croup, preventing them from going forward. So, how much of this is true? Not much, actually. Let’s look at real anatomy and physiology and try to sort out truth from fiction…
The Facts that are right
There is a Cranial Nerve 11 and it is called the Accessory Nerve. And it does, indeed, innervate the trapezius muscle, part of which is in the area behind the shoulder blade. So far so good. So why do I put this into the Myth category? We need some basic neurology to understand that.
The Background to what is wrong
Every nerve cell (neuron) can only do one thing. It can be a sensory cell of various types, collecting information and taking it to the central nervous system (the spine and the brain). Or it can carry signals from the central nervous system to the body to tell the body to do something. In the case of muscles, they are called motor nerves and they cause muscles to contract. Any one cell can only do one or the other, not both.
Nerves combine a lot of nerve cells. Some nerves carry just sensory cells and some just carry motor cells, and some carry both.
The Error in the whole theory
The well attested anatomical fact is that Cranial Nerve 11 only carries motor neurons. (See references 1, 2, 3 below.) It is strictly a motor nerve with no sensory component at all. So you could jump on that nerve or squeeze it with pliers or hammer on it and the horse would not react because that nerve doesn’t feel anything at all!
So the whole “reflexive action" that is supposed to occur when you put pressure on "Cranial Nerve 11" is false. It doesn’t exist. (Nor, by the way, does it fit the true definition of a reflex. What is being described is a response or reaction from the animal, but not a reflex.)
The Cause of the reaction you see
But, you say, watch the horses when they are poked in that area. They do lift their heads and move around and act uncomfortable. Yes, some do. Now go watch videos of horses with sore backs. Guess what they do? The muscles twitch, the horse acts uncomfortable and behaves like something is bothering them. They often drop their backs and lift their heads.
That area is a key point for high pressure with English saddles and with western saddles with too wide an angle. The “reflexive action” is actually a very sore horse that is reacting to pain. If there really was a reflex there, it would occur in all horses. It doesn’t. You can poke and prod all you want on a horse with healthy, non-painful muscles and you get no reaction at all, except maybe a quizzical “What are you doing?” look. On a sore horse, you get a reaction. That reaction, however, has nothing to do with Cranial Nerve 11…
The Real Issue
So, is it OK to put high pressure in that area of the horse then? No, in the same way it is wrong to put high pressure (no Poking) anywhere on the horse’s back. That area is particularly vulnerable because there is so little muscle between the saddle and the vertebral spinous processes, and pressure concentrates on bone. It is very easy for a horse to get sore there, and it is definitely a spot you want to check regularly. But don’t worry about accidentally poking on a nerve which can’t feel anyway and causing a reflex that doesn’t exist. Just watch for heat, swelling or pain - the signs of inflammation - and long term, muscle atrophy (wasting). All those do commonly happen…
The Test of reliability
As a side note, being as skeptical as I am, I always assess whether I can trust any source of information before I will accept it as correct. So my method is to check out anything I am able to in order to see if it is true or not. If what I can check out is confirmed by reliable authorities, then I will be more willing to accept information, ideas or opinions from the same resource even if I can't verify it. However, if what I check out is proved to be incorrect, how can I trust what they say about things I can't confirm, no matter how confident they are in what they say or how many letters (including DVM) that they have after their name? So if the basic anatomy is wrong, how can I accept ideas about saddle fitting that is supposed to be founded on that “anatomy”? I can’t…
(1) Sisson and Grossman's The Anatomy of the Domestic Animals - still the gold standard in veterinary anatomy books - starts off its discussion about Cranial Nerve 11 with this statement: "The accessory nerve is purely motor." (volume one, page 663).
(2) Handbook of Veterinary Neurologic Diagnosis, Oliver and Lorenz, pg. 49 starts their discussion of Cranial Nerve 11 with this statement: "The accessory nerve is the motor pathway to the trapezius muscle and parts of the sternocephalicus and brachiocephalicus muscles."
(3) Veterinary Neuroanatomy and Clinical Neurology, DeLahunta, lists Cranial Nerve 11 with the Special Visceral Efferent nerves (page 100). Efferent nerves take information from the brain to the body as opposed to Afferent nerves which carry information from the body to the brain.
For the Keeners, who want to know more…
There are actually twelve cranial nerves – cranial meaning having to do with the head. Cranial Nerve 11 actually starts in the neck with a number of pieces coming from the first five (of the seven) cervical vertebrae, starting out small and getting larger as it collects more nerve fibers. It goes up toward the head and into the skull where it picks up still more nerve fibers, some of which leave and are active in the head, before coming out of the skull.
Once out of the skull it splits as it travels back down the neck. One branch (the ventral branch) turns down and innervates the sternocephalicus muscle. The other branch (the dorsal branch) goes down the neck and innervates the omotransversarius and the brachiocephalicus muscles and, at its far end, the cervical and thoracic trapezius. As the nerve travels down the neck it gets smaller and smaller as more fibers leave it to innervate different muscles. It lies under the trapezius muscle, and the area of the thoracic trapezius behind the shoulder blade is at the very far end where it separates into its last few fibers.
Please forgive the error in semantics (reflex vs. instinctive reaction vs. physical reaction), but the fact is (and we have DVMs we work closely with, just as you do in the form of your wife) that a poorly fitting saddle that pinches in this area and impacts both the muscles and nerves that constitute the trapezius area will not only hurt the horse but cause the horse to want to stand still - the analogy of the stallion biting the mare was only to illustrate the point that pressure at this area causes behavioral ramifications. The rider often will describe the horse as stubborn and try to urge him forward - without ever realizing that the tree might be pinching and the cause.
Thank you for your comment on our blog post. We completely agree that high pressure over the withers, like high pressure anywhere under the saddle, is detrimental to the horse and can cause biomechanical and behaviour responses at the time and physical damage long term. But the point is that the biomechanical and behavioural responses to high pressure over the wither area are not due to specific pressure on CN 11. That nerve, as stated above, has no sensory component so it doesn’t send information to the Central Nervous System (brain and spinal cord). It only brings information from the CNS back to the muscles – a motor nerve. Damage to the nerve itself can definitely cause atrophy of the muscle it innervates, as neurogenic atrophy (the term for atrophy due to lack of nerve supply) is severe and occurs very quickly after damage to the nerve. Pressure on the nerve itself (if that is possible which, from my background information is quite unlikely but as you point out, isn’t really known and I expect there to be a lot of variation between individuals) may also affect how the trapezius functions. The immediate responses to high pressure are mediated by other nerves which sense the pain and pressure.
All that said, the point is that high pressure is bad and causes problems in movement, behaviour, attitude and long term, physical condition and shape. If your horse is having problems, make sure that you have saddle fit as one of the necessary things to check before you assume it is just a “bad horse”. That is the take home message that riders need to understand.