Spinal ostural muscle control and balance comes from a few control centers. One of the major players is from the 8’° cranial nerve, the vestibulocochlear nerve. This nerve gets initiated through the inner ear, which contains semicircular canals, which picks up information by means of flowing endolymph which bend small hair cells whenever the head tilts. The hair cells communicate with nerve fibers that lead to the vestibular nuclei (the spinal, the superior, the medial and the lateral nuclei) or to the cerebellum.
The 4 vestibular nuclei send their axons either to the cerebellum or to one of two tracts: the medial longitudinal tract or the lateral vestibulospinal tract. The medial longitudinal tract receives input from the superior, medial and the spinal nuclei. The lateral vestibulospinal tract receives information from the lateral nucleus.
The medial longitudinal tract: conveys messages to the nuclei of the eye muscles, to the cells innervating the muscles moving the head, the neck, the spinal accessory nuclei (cranial nerve XI), and the muscles of the trunk. The lateral vestibulospinal tract: enters the cord in the anterior fasciculus and ends on the motor neurons of the muscles of the limbs. It is through the vestibule reflexes that act on the neck and on the limbs (vestibulospinal reflex) that are evoked principally by sensory input from the otolith organs.
1. The medial longitudinal tract sends fibers to:
a. The nuclei of the eye muscles-Cranial nerves III (oculomotor), IV (trochlear), and Vi (abducens)
b. Muscles that move the trunk
c. Muscles that move the head and neck-i.e. through the spinal accessory nuclei (cranial nerve XI) The spinal accessory nuclei function: controls the trapezius, sternomastoideus and cleidomastoideus muscles
d. The lateral vestibulospinal tract sends fibers to muscles of the limbs.
The proprioception and touch travel needed for foot placement comes from the dorsal spinocerebellar tract. At the upper spinal levels the dorsal columns can be divided into two bundles (fascicles) of axons: the gracile and the cuneate fascicle. The gracile fascicle ascends medially and contains fibers from the ipsilateral sacral, lumbar and lower thoracic segments. The cuneate fascicle ascends laterally and includes fibers from the upper thoracic and cervical segments. The two bundles terminate in the lower medulla in the nucleus gracile and cuneatus. Together they are called the dorsal column nuclei.
The pathways used to carry information from the upper limbs are from axons in the cuneate fascicle, which then synapse in the cuneate nuclei. The pathways for proprioception of the lower limbs use the gracile fascicle and synapse in the gracile nuclei. Various sources report that the medulla is about I inch in length and is located from the border of the foramen magnum to the inferior border of the first cervical vertebrae (the atlas). These tracts communicate to the cerebellum and to the medulla at a rate of 150 meters per second (1’~ football fields per second.’).
If these tracts have interference, then by doing the righting reflex, by turning over the paw, the result will be a delayed righting reflex (a positive conscious proprioception reflex).
In the reticular formation of the pons and the medulla there were two groups of nuclei identified that were involved in the control of posture. The nuclei in the pons facilitated spinal reflexes. The nuclei in the medulla inhibited spinal reflexes. These nuclei project through the medial and lateral reticulospinal tracts to ALL levels of the spinal cord. These are the tracts that are utilized when delivering an upper cervical adjustment; muscular and reflex changes throughout the body can be seen. The pontine reticular formation projects down the cord through the reticulospinal tract and terminates on and facilitate motor neurons that innervate axial muscles and extensors of the limbs. The medullary reticular formation gives rise to the lateral reticulospinal tract that projects bilaterally down the front of the lateral columns. This tract produces inhibition of neck and back motor neurons, similar to the medial vestibulospinal tract. This tract, importantly, makes polysynaptic inhibitory connections with extensor motor neurons and excitatory connection with flexor motor neurons. This tract can also excite motor neuron innervating extensor muscles and inhibit flexors!
Both medial and lateral reticulospinal fibers also modulate reflex action during ongoing movements and produce different effects, depending on the movement in progress at the time . These fibers coordinate posture and movement by integrating vestibular and other sensory inputs from the cerebral cortex. These centers and tracts are ultimately important to understand because postural adjustments are governed through the corticoreticulospinal system that was explained above.
Reprinted with permission from the Upper Cervical Monograph.
These papers from the Upper Cervical. Monograph discuss mechanisms of the upper cervical subluxation complex and neurological dysfunction. The reviews cover more than 25 years, and it is interesting to see the evolution of the neurological hypotheses from the NUCCA literature. The last paper by Dr. Sherry Dickholtz-Gaber was originally a comparison between observations of horses and observations of humans.