What causes a change in the equine balance systems?
The biomechanics of the animal’s upper body are modified when we alter its orthopaedic stance by trimming the individual foot; the CNS and PNS must then be recalibrated and coordinated, allowing the animal to be aware of its relative orientation in space and perceive the locomotion of its body parts.
Our trimming process induces new feedforward nerve signals from the newly trimmed foot through proprioceptors, which are part of the neurological system. However, the newly trimmed foot will relay unconscious information about the changing foot pathology through another neurological component called mechanoreceptors. These mechanoreceptors are essential in providing pathology information to the CNS even before changes are observable in the external hoof capsule.
Mechanoreceptors are sensory receptors that respond to mechanical pressure or distortion found in deep muscle tissue, ligaments, tendons, and joints. There are many diverse types of mechanoreceptors throughout the animal’s body, everything from hair, teeth, and eyelids (not just deep muscle), all relaying about 90% of the information to the CNS concerning the anatomical body structures and their relationship to the balance of the animal’s feet. The signals sent unconsciously to the brain respond to the minor changes in the state they monitor and the animal’s adopted postural stance. Therefore, when we trim the equine foot, neurological activity occurs continuously in the horse’s body at an unconscious level.
Due to the overwhelming awareness of the neurological system and the minor changes throughout the body, we must be mindful of the trimming process each time we trim the feet. It does not matter how little material we remove. The nervous system will recognise the change in size, shape, and pressure and respond by sending new neural signals throughout the animal’s body and triggering changes in the animal’s postural stance.
These different mechanoreceptors directly link to our trimming process by monitoring joint mobility, extreme range of joint movement, speed, pressure, and pain. I have included below a list of the mechanoreceptors, classified according to the information they provide to the CNS.
Type I mechanoreceptors provide information about the joint's mobility and extreme range of motion; they also register the strength and speed at which the joint is moving by pressure inside the joint. This type of mechanoreceptor fires off easily and fatigues slowly, but when fatigue sets in, the number of signals over the same period diminishes.
Type II mechanoreceptors are stimulated at the beginning and end of a joint movement. They are active in non-mobile joints and relay signals to the brain as the joint speeds up. This type of mechanoreceptor fires off easily and fatigues easily.
Type III mechanoreceptors are found in the joint capsule's ligaments, tendons, and dense fibrous connective tissue. They are stimulated at extreme ranges of motion and positions that are harmful to the joint. This type of mechanoreceptor does not fire off easily and fatigues slowly.
Type IV mechanoreceptors are called nociceptors or pain receptors. They are non-fatiguing and are found in the dense capsular tissue and all joint tissue except cartilage. They are not active under normal joint conditions.
For the mechanoreceptors, fatiguing at different rates substantially prevents and protects the animal’s body from harm. Individual animals respond differently to pain when their orthopaedic balance is compromised, or they experience dysfunctional loading issues. Since type IV nociceptors or pain receptors are sensory and non-fatiguing and are not activated under normal joint conditions, they respond to mechanical pressure, such as an orthopaedic imbalance.
Different receptors come with an associated period when the horse responds to pain in response to the loading of different joints throughout the body. Consequently, the new loading forces on the distal interphalangeal joint brought about by the trimming process influencing the external hoof capsule or the direct link of the newly adopted postural stance of the animal dictate the nervous system's response via the mechanoreceptors.
These are all the elements of the nervous system that trimming and shoeing must influence. As farriers, we must consider what we can inflict on animals through our intervention. Appreciating how the animal’s body works neurologically and how this affects its biology will help us decide how to trim or shoe the individual feet of the horse in our care.
Remember, the biomechanics of the upper body will be modified because of this change in the altered orthopaedic stance due to the trimming process. The animal will adopt a long-term postural stance that will directly link to the loading of the distal interphalangeal joint, the subsequence shape of the hoof capsule, and characteristic symptoms that we associate with podiatry issues.
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