Part 2 of 4

Dena Jersild CAC DVM

“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.”


Descending reticulospinal pathways affects the sensitivity of muscle spindle cells, modulates IML activity and inhibits pain.

What determines the frequency of firing of the reticulospinal pathways from the brainstem? As you know, the answer to this question is the same for all neuronal systems. The frequency of firing of a neuron or pool of neurons is the frequency of firing of its pre-synaptic pool. There are many reticulospinal pathways originating in the reticular formation of the brainstem. Pontomedullary RST pathways generally have an excitatory consequence on ventral horn cells (ά and γ motor neurons), and an inhibitory effect on the intermedial lateral cell column. They also inhibit second order pain neurons located in the base of the dorsal horn. The central integrated state of the pontomedullary reticular formation is determined primarily by the frequency of firing of the ipsilateral cortex and cerebellum.

A loss of RST activity on γ motor neurons would cause decreased sensitivity of muscle spindle cells (MSC). This means that a greater environmental message (stretch) is needed to cause the monosynaptic and polysynaptic firing of homonymous and synergist muscles that support the loaded joint. This greatly increases the probability of injury from acute trauma or repetitive long term micro trauma as a consequence of any activity performed against the earth’s gravitational field.  To repeat, the decreased sensitivity of the MSC is due to a decreased frequency of firing of the gamma motor neuron that innervates this receptor. Its frequency of firing is determined by its pre-synaptic pool of neurons from local cord reflexes and descending reticulospinal pathways from brain. As stated earlier, the pre-synaptic pool to both the cord and brain is ultimately from receptor transduction of environmental forces. Dysafferentation from subluxation and other receptor deficits may increase the probability of decreased firing of reticulospinal systems on spinal cord neurons.

Reticulospinal influences on 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), which is the output nuclei for all autonomic function. In other words, it is impossible to perceive your environment without at the same time affecting the autonomic system. This input to the IML from all receptors is excitatory. There are also excitatory influences on the IML from descending mesencephalic reticulospinal pathways. Thus in the thoracic and lumbar output of sympathetics there would be increased blood pressure, vasoconstriction, sweating, pilo-erection, etc.

All mechanoreceptor input fires to ipsilateral cerebellum and the contra-lateral cortex primarily. 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 and cerebellar stimulation of the ponto-medullary reticular formation is necessary to inhibit IML output.  Since cortical and cerebellar integrity is determined largely by mechanoreceptor input, its ability to dampen IML output is dependent on proper joint motion and afferentation. If there were cortical or cerebellar 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 release of noxious chemicals that irritate nociceptors causing pain that fire back to the cord firing the IML even more.

Reticulospinal influences on nociception

There are many ways in which the reticulospinal pathways inhibit pain. Some occur pre-synaptically and some post-synaptically relative to the first and second order neurons in the pain pathway. A decreased frequency of firing from RST pathways can be responsible for increased pain responses and deleterious neuroplastic changes leading to chronic pain.

Chiropractic Therapies and the Central Integrative State of the Nervous System

Decreased frequencies of firing from a loss of mechanoreceptor input on one side of the body can lead to decreased brain function on the contra-lateral side. This loss of mechanoreceptor stimulation might be from joint fixation, subluxation, old injuries or activities that are not balanced from right to left (i.e. racehorses running in only one direction). This loss of cortical activity on one side is known as hemisphericity and can be diagnosed with a good neurological exam.

Correction of aberrant joint motion and other receptor faults with chiropractic therapies restore normal mechanoreceptor firing (primarily through MSC’s), which fires to cortex, which then fires to excite the pontomedullary reticular formation, which fires ipsilaterally to:

  1. Inhibit an inhibitory interneuron to the gamma motor neuron which restores normal MSC sensitivity.
  2. Inhibit the IML restoring normal autonomic output.
  3. Inhibit pain at the dorsal horn.

General rules of brain (cortical) activity

  1. Increased brain activity = increased muscle tone = decreased inhibition of gamma and alpha motor neurons = decreased IML output = decreased pain.
  2. Decreased brain activity = decreased muscle tone = increased inhibition of gamma and alpha motor neurons = increased IML output = increased pain.

An understanding and appreciation of the mechanisms described in this paper should lead the animal chiropractor to a better understanding of the importance of diagnosing and correcting afferentation faults in the system. Our neurological exam must not only qualify pathological disease but also those physiological lesions resulting from dysafferentation syndromes and pre-existing central physiologic deficits.

Patient management that involves promotion of balance from side to side might involve the following concepts:

  • Chiropractic adjustments performed by qualified practitioners who understand the implications of dysafferentation or loss of normal mechanoreceptor stimulation to the neuro-axis.
  • Promoting free exercise and activities that use the body equally on both sides.
  • Balanced dentistry.
  • Balanced podiatry.
  • Diagnosis and correction of old injuries and their consequences.
  • Being aware of the multitude of management issues that promote imbalance of sensory input to our animals.


Part 3 of this 4 part lecture will be posted (on facebook and at healthpioneersinstitute.com) in one week.