When it comes to diagnosis and treatment of our animal patients, diagnostic techniques in veterinary chiropractic neurology requires an understanding of the neurophysiological mechanisms of function within the central and peripheral nervous systems.
What can diagnostic techniques in veterinary chiropractic neurology contribute to the diagnosis and treatment of many presenting animal complaints? The answer is simple yet complex. It is simple in that a chiropractic neurological evaluation considers functional as well as pathological origins. But it is also complex because an understanding of neurophysiological mechanisms of function within the central and peripheral nervous system is required. This differentiates a disease process (pathology) from aberrancies in the frequency of firing (FOF) of neurons pools pre-synaptic to an end-organ (functional). Functional neurologic evaluation does not replace traditional veterinary neurological exam procedures, but instead expands the clinician’s perspective on causation, and often on treatment.
The neurological exam
Veterinarians and chiropractors study neurology in school and should have a basic understanding of how to perform a neurological exam. For those of us engaged in animal chiropractic, performing a good neurological exam is critical. A well-trained animal chiropractor is a functional neurologist who understands the pertinent neuroanatomy and neurophysiology needed to interpret patient history and objective findings.
Traditional neurological exams
Where we differ from our traditionally trained brethren is in our ability to appreciate the subtleties of a multi-modal integrated nervous system. A traditional approach to the neurological exam provides a simple yes or no answer evaluating for disease. However, the chiropractic neurologist is sensitive to degrees of function. We understand that the nervous system can work less than optimally without being diseased. With this distinction in mind, we can utilize different windows into the nervous system that evaluate the frequencies of firing of pools of neurons. Our goal is to return the nervous system to the highest degree of function allowed by the animal’s genetic potential.
Functional neurological deficiencies
Certainly, the diagnosis of neurological pathology is an important goal of any neurological exam. Yet the presence of functional neurological deficiencies is much more common and thus much more likely to be of benefit to the majority of our patients. A veterinary chiropractic neurological approach includes those aspects of the neurological exam that can be used to evaluate the functional integrity of neuronal pools. Treatment can then be aimed specifically at those areas of the neuro-axis either compromised by pathology or a decreased FOF of a pre-synaptic pool. Since animal chiropractors use receptor-based therapies as their therapeutic tool, we are unique in our abilities to specifically reach pools of neurons by modulating receptor potentials. This of course implies necessary training in the area of functional neurology in order to determine the longitudinal level of the neurologic lesion, so a targeted receptor-based therapy can be employed.
Before we consider the neurological exam from this new perspective, an understanding of certain neurophysiological concepts is necessary.
The central integrative state and animal health
There is probably very little argument that the central nervous system (brain and cord) runs things; at least in the physical body. The nervous system directs healing and maintains health. Nothing is beyond the awareness of the nervous system. Information is received and transmitted throughout the neuro-axis by chemical and electrical means. The synaptic transmission of information is well understood and the sensitivity of neurons can be modulated by chemical and electrical factors.
Define the central integrative state
A total of all excitatory and inhibitory influences from temporal and spatial pre-synaptic effects defines the central integrative state (CIS) of a neuron or pools of neurons. A good chiropractic neurological exam seeks to define the central integrative state at the differing longitudinal levels of the neuro-axis.
What drives the central nervous system?
If the central nervous system (CNS) drives all aspects of biology, what drives the central nervous system? The answer is critical because it explains why our therapies are so effective. The central nervous system is driven by the environment!. The greatest and only constant environmental stressor is gravity. The forces of gravity are transduced into electrical signals by mechanoreceptors. These include muscle spindle cells, golgi tendon organs, and other proprioceptors. Gravity’s influence on the CNS via mechanoreceptors is responsible for the majority of the baseline activity of the neurons of the CNS. In other words, in the absence of normal gravitational forces, all neuronal function would be severely hindered, making neuronal death highly probable over time.
Loss of firing
A very high percentage of mechanoreceptor populations lie in close proximity to the spine in the form of joint mechanoreceptors in the ligaments and joint capsules of the spinal segments. The muscle spindle cells (MSC) and golgi tendon organs (GTO) of the intrinsic muscles of the spine have a significant influence on the CIS of the central nervous system. This is due to their sheer numbers, as well as the fact that as midline structures they were phylogenetically responsible for the evolution of the structures that developed more laterally. Loss of intersegmental motion (fixation) or aberrant motion (subluxation) reduces the FOF (temporal summation) and the numbers of mechanoreceptors firing (spatial summation) into the central nervous system. Since incoming sensory information is divergent in nature, loss of firing of only a few receptors can have tremendous consequences on the central nervous system.
The role of receptors
Receptors are neural structures that transduce an environmental stimulus to an electrical message. Different receptors transduce different types of environmental stimuli. The retinal receptors transduce light, and the auditory receptors transduce sound. Mechanoreceptors transduce joint position sense, and vestibular receptors transduce movement of the head. These are examples of receptors that perceive the external environment. There are also receptors that perceive the internal environment of the animal, such as temperature, pH, hormone levels etc.
As sensory information enters the cord and brainstem via a sensory neuron, it diverges via collateralization. In other words, one sensory neuron will synapse with many post-synaptic neurons that will in turn synapse with many other post-synaptic neurons that will in turn modulate the central integrated state of each neuron in its path.
The central integrated state of a neuron can be defined as the sum total of all the excitatory and inhibitory influences on the neuron. The central integrated state of a neuron will determine its probability of firing.
If any sensory receptor undergoes a decrease in FOF, the post-synaptic pool of neurons will at some point begin to undergo some aspect of trans-neural degeneration.
Trans-neural degeneration
All neural tissue needs oxygen, glucose, and active electrical stimulation to remain healthy. When a neuron is fired, second messengers activate genetic material in the mitochondria and nucleus. These immediate early gene responses direct the production of more mitochondria and new protein within the cytoplasm. This protein is used for structural purposes, as in the production of organelles and microfilaments, as well as enzyme production. The mitochondria are the site of oxidative phosphorylation where glucose is metabolized to produce ATP. This ATP is used in part to run ion pumps that maintain the electrical and chemical gradients of the cell. Protein is also greatly responsible for the negative charge within the cell.
Less protein
With decreased frequencies of firing, secondary to loss of receptor stimulation, protein production is slowed or stopped and the numbers of mitochondria within the cell decrease. Energy for cellular function is more likely derived from the anaerobic pathway of glycolysis. Lactic acid levels rise, creating ferrous iron free radicals, which are poisons to the neuron. With less mitochondria there is less ATP produced to power the ion pumps. Hydrated Na+ ions now accumulate within the cell and the neuron swells. Less protein means less structural integrity and less negativity within the neuron. As the equilibrium potential of the cell is slowly lost, the neuron begins to swell. This process of degeneration is known as trans-neural degeneration.
The mechanism described above may occur to some degree in all neurons in a particular pathway. The probability that a particular neuron will be affected by a pre-synaptic decreased FOF from a receptor field depends on the pre-existing health of the neuron (i.e. central integrated state), and other collateral influences on that neuron from homologous pathways. In other words, maintaining adequate frequencies of firing to neuronal pools via prophylactic receptor stimulation can protect these cells in times of injury or disease when receptor stimulation is reduced.
Exercised animals
Driving frequencies of firing via exercise and specific treatment modalities leads to long-term potentiation, a phenomenon where increased frequencies of firing are maintained in a cell long after the original stimulus has ceased. This increased drive to the genetic machinery of the cell causes positive cell growth. More mitochondria and other organelles increase the stability of the neuron. Budding or dendritic spur formation occurs, increasing the surface area for synaptic communication with other neurons and allowing for more efficient exchange of information (neuroplasticity). This explains the general clinical observation that animals who are regularly exercised and/or receiving prophylactic chiropractic care have an easier time recovering from injury and illness.
Descending suprasegmental and segmental influences affect muscle spindle cell sensitivity
In the absence of trauma, most injuries are a failure of somatic muscle to fire at the right time and at the right amplitude to support a joint against the forces of movement and gravity. Decreased cortico-ponto-medullary influences on alpha and gamma motor neurons cause muscle paresis and a decreased reflexogenic response to stretch. Decreased sensitivity of muscle spindle cells (MSC) means a greater environmental message (stretch) is needed to cause the monosynaptic and polysynaptic firing of homonymous and synergist muscles that support the loaded joint. The reduced sensitivity of the MSC is caused by decreased FOF of the gamma motor neuron that innervates this receptor. Its FOF is determined by its pre-synaptic pool of neurons from local cord reflexes and descending suprasegmental pathways from the brain as stated above. The pre-synaptic pool to both the cord and brain comes ultimately from receptor transduction of environmental forces.
Intermedial lateral cell column (IML) and autonomic function
All incoming sensory input (including mechanoreceptor input) to the dorsal horn sends a collateral to the intermedial lateral cell column (IML), the output nuclei for all autonomic function. In other words, it is impossible to perceive your environment without simultaneously affecting the autonomic system. This input to the IML from all receptors is excitatory. Excitatory influences on the IML also come from descending mesencephalic reticulospinal pathways. In the thoracic and lumbar output of sympathetics, therefore, increased blood pressure, vasoconstriction, sweating, pilo-erection, etc. will occur.
Cerebral cortex
All mechanoreceptor input fires to the cerebral cortex. The cortex then fires back down (via the ponto-medullary reticular formation) to inhibit the ipsilateral IML. This would cause a reduction in blood pressure, vasoconstriction, sweating, and pilo-erection.
Since every environmental potential has a collateral that excites the IML, there needs to be a system that dampens the IML, keeping it in balance. Cortical stimulation of ponto-medullary reticular formation is necessary to inhibit IML output.
Since cortical integrity is determined largely by mechanoreceptor input, its ability to dampen IML output depends on proper joint motion. If there were cortical deficits secondary to the loss of joint motion, incoming sensory input to the IML would not be sufficiently dampened by suprasegmental influences, leading to increased blood pressure, vasoconstriction, hypoxia, etc. Hypoxia causes cell damage and the release of noxious chemicals that irritate nociceptors, causing pain that fires back to the cord and fires the IML even more.
Multiple ways of nervous system expression
At this point in our discussion, we recognize that the nervous system expresses itself in several ways: through the ventral horn cells and muscles and through the IML and the autonomics. This fact provides “windows” for diagnosing functional neurologic deficits.
Manual muscle testing (MMT) is a valuable diagnostic tool utilized in human neurology, chiropractic and physical therapy as a window to CNS integrity. It is extremely important to recognize that muscle strength is a function of fitness and, most critically, the FOF of the integrated presynaptic influences on ventral horn cells. These are the alpha and gamma motor neurons that fire directly to the muscle and muscle spindle cells, respectively. Increased or decreased frequencies of firing of these ventral horn cells can cause muscle hypertonicity or paresis, increasing the probability of joint instability with breakdown and injury. Restoring an appropriate FOF to these ventral horn cells should be the goal of receptor based targeted therapy.
The central integrated state (CIS) of ventral horn cells is influenced by the 10,000 to 12,000 presynaptic neurons that fire to them from many areas of the neuroaxis.
With this understanding, neurological exam procedures can be utilized with the goal of diagnosing the longitudinal lesion of functional as well as pathological lesions.
Muscle testing
In animals, direct muscle testing is not an available tool, yet muscle tone palpation, posture and gait provide an indirect method for estimating the likelihood of aberrant firing frequencies of ventral horn cells from a specific longitudinal level of the cord or brainstem. For example, cranial cruciate ligament sprain is a common canine injury.
A chiropractic neurologic approach would involve evaluating all the muscles that cross the joint. This is based on the knowledge that, to a large degree, the stability of a joint is determined by the FOF of the muscle/tendons that cross the joint. Their FOF is determined by the FOF of the alpha and gamma motor neurons that innervate those individual muscles. It then follows that the FOF of these ventral horn cells depends on the FOF of converging neurons from throughout the nervous system. Where is the problem? This is where a combined cranial nerve exam, postural exam, etc. are viewed in a broader perspective.
Functional lesions
In other words, using our example, we begin to appreciate that functional lesions, almost anywhere in the nervous system, can influence the central integrative state of ventral horn cells and their muscle end-organs, contributing to stifle instability and injury. The veterinary chiropractic neurologic exam aspires to diagnosing the specific longitudinal level of the functional or pathologic lesion. Through our knowledge of neuroanatomy, we can then implement a plan of therapy that specifically addresses the pool or pools of neurons in question with targeted receptor based therapies. In tandem with more traditional veterinary rehabilitation therapies that address end organ injury, this should lead to quicker recovery and less recidivism.
End-organ dysfunction of autonomic nervous system
End-organ dysfunction of the autonomic nervous system is more greatly appreciated by veterinarians as coming from anatomical structures far removed from the end organ. As examples, thyroid disease implies involvement of the pituitary, and Cushing’s disease may involve the pituitary or hypothalamus. In this discussion, we expand our view of autonomic function as a consequence of the central integrative state of the intermediate lateral cell column (IML). As in our discussion of the ventral horn cells, we can acknowledge that organs, glands, blood vessels, and other end-organs of the autonomic system are subject to the central integrative state of the IML.
The CIS of the IML is a consequence of the thousands of presynaptic integers synapsing on the IML from suprasegmental and segmental pools of neurons. As in evaluating muscle dysfunction, the veterinary chiropractic neurological exam endeavors to locate the presynaptic pool of neurons contributing to localized IML aberrancy and resulting end-organ dysfunction.
The veterinary chiropractic neurological exam
It is outside the scope of this article to teach the neurological exam. It is assumed that as a veterinarian or animal chiropractor, you have been trained to utilize the tools of the cranial nerve exam, postural exam, reflex testing, withdrawal testing, and gait analysis, and are skilled in their application. I remind you that the neurological exam begins when first seeing the animal. Everything you observe, hear, and touch is important. The chiropractic neurology practitioner sees everything as a clue to a functional or pathologic lesion!
Windows to central nervous system output
The diagnostic techniques in veterinary chiropractic neurology considers windows to central nervous system output. They also takes into account ventral horn cells and IML, as opposed to positive and negative neurological testing.
Example
For example, in a cranial nerve exam, deficits may imply peripheral or central pathological lesions of the cranial nerve, its nucleus, or surrounding brain tissue. In addition, consider the central integrative state of regional areas of the brain: olfactory nerve (cortex), optic nerve (thalamus), oculomotor and trochlear nerves (mesencephalon), abducens, trigeminal, and facial nerves (pons), vestibulocochlear nerve, (pontomedullary junction), glossopharyngeal, vagus, and hypoglossal nerves (medulla).
Segmental evaluation
For segmental evaluation, in addition to pathological peripheral or segmental, consider the consequences of aberrant CIS of specific levels of spinal segments. These consequences should be studied on muscle tone, joint stability, and autonomic tone.
Multimodal influences on neurological function
This is the veterinary chiropractic neurological exam! It is no different than the classic veterinary neurological exam except that we now appreciate the multimodal influences on neurological function.
As we perform our neurological exam, we will be more sensitive and attentive to the subtleties of the patient’s response. Two important factors that are windows into the central integrative state of a neuron or pool of neurons are:
- Time to summation
- Fatiguability of end organ response.
Neuronal pools that are undergoing trans-neural degeneration will be sitting closer to threshold by the mechanisms described earlier. There’s an increased probability that these neurons will fire with a given stimulation of a pre-synaptic pool so that their time to summation will be less. Due to the inefficiency of cellular energetic processes. there will also be an increased probability that the cells will fatigue at a faster rate with a given stimulation.
Examining an end-organ’s time to summation and fatiguability can give evidence of TND somewhere in the system. By integrating information from a complete history and examination, an accurate assumption on the longitudinal level of pathology or TND can be determined. From this information, you can develop a therapeutic plan that is aimed at increasing the FOF of the involved neurons when appropriate.
Considerations for broadening the usefulness of the neurological exam
Integrate:
An exam procedure performed in isolation tells you nothing. Diagnostic conclusion is reachable only by integrating all aspects of the history and exam.
Compare:
When performing a test procedure, always compare one side to the other when appropriate. Is one side firing too much, or is the other side firing too little?
Functional lesion:
The purpose of each test is to ultimately find the longitudinal level of the pathologic or functional lesion.
Central integration on end-organ response
It is natural for the chiropractor to think in terms of segmental (spinal) affects on end-organ response. Now consider the effects of central integration on end-organ response.
Receptor challenges
Consider using receptor challenges as a diagnostic window. For example: does slow stretching a muscle cause a change in your neurological test? Is the change ipsilateral or contralateral? Is the end-organ response inhibited or facilitated? Not only will asking these questions help you localize the level of the lesion, but they may also aid you in choosing the most effective treatment approach.
Test procedure
Keep in mind that a test procedure may appear normal when first applied, but may quickly turn abnormal with repeated application of the test secondary to TND and fatigue.
Test improvement
Some test results may actually improve with repeated testing secondary to immediate early gene responses, long- term potentiation, and neuroplastic change.
Animal patients difference
Most neurological testing involves a sensory input and a motor output. As far as animal patients are concerned, it is difficult to quantify sensory loss. In the presence of some motoric response to our testing, we can only say that some aspect of the sensory pathway would have to be intact.
If there is an absent or decreased motoric response to our testing, we don’t know the cause so must consider the following:
The receptor potential was not of sufficient amplitude to fire the second order neuron.
Central integration at some level of the neuro-axis caused insufficient facilitation. Otherwise, too much inhibition on the cell bodies of the motor neurons.
We also don’t know the cause with an increased motoric response to testing, so need to consider the following:
The receptor potential was of increased amplitude because of an increased environmental input (from the tester). Otherwise, from an increase in receptor sensitivity (as in nociceptor sensitization and gamma simulation of muscle spindle cells).
The central integrative state of the central neurons are over-exciting or under-inhibiting the cell bodies of the motor neurons.
An understanding and appreciation of the mechanisms described in this paper should be beneficial. It should lead the veterinary chiropractor to a better understanding of the neurologic significance of correcting mechanical faults. Receptor-based therapy directed to specific pools of neurons contributing to joint instability and autonomic dysfunction should be our goal. I hope this article provides some motivation for reviewing neuroanatomy. The more we know, the better questions we can ask.
References
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Binder MD, editor. Progress in Brain Research, Vol.123, Peripheral and Spinal Mechanisms in the Neural Control of Movement. Elselvier, 1999.
Guyton and Hall Textbook of Medical Physiology, 14th Edition.
Haines DE, editor. Fundamental Neuroscience for Basic and Clinical Applications, 3rd Edition Churchill-Livingstone, 2006.
Henneman K. Recognizing Soft Tissue Injuries in the Dog from and Integrative Perspective, Part 1, Innovative Veterinary Care, Vol.8, Issue 4, 2018.
Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ. Principles of Neural Science, 5th edition, McGraw-Hill. 2013.
Latash ML, Zatsiorsky VM. Biomechanics and Motor Control. Elselvier. 2016.
Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia A, McNamara JO, Williams SM. Neuroscience, Sinauer Associates. 2004.
AUTHOR PROFILE
Dr. Carl J. DeStefano graduated Cum Laude from the National College of Chiropractic in 1984. He is certified in Animal Chiropractic from the Options for Animals Veterinary Chiropractic Center and the American Veterinary Chiropractic Association (AVCA). He has completed all the educational requirements for the Diplomat program sponsored by the AVCA. Dr. DeStefano is board certified in Chiropractic Neurology through the American Chiropractic Neurology Board, and is a fellow of the American College of Functional Neurology. He is the Founder of the Health Pioneers Institute of Veterinary Chiropractic, and maintains a busy human and animal practice near Chicago.
