Thyroid connections from the sympathetic cervical ganglion — Part 3
In previous columns I’ve been discussing the sympathetic chain or trunk that runs along the anterior surface of the entire length of the spinal column, from the sacral/coccygeal area to the brain. The many ganglion found along the route not only supply impulses from each spinal level, but are also quite capable of communicating up and down the cord. The relevance of this chain and its relationship to chronic recurring spinal subluxations cannot be overemphasized.
My point is, most chronic subluxations without a traumatic origin are perpetuated by muscle contractions originating from visceral organs that are unable to adequately perform their normal functions. The origin of their stress may be dietary or emotional, yet they produce chronic or recurring structural deviations that prevent their correction. I believe this factor of modern chiropractic practice is widely misunderstood.
So with that let’s translate that paradigm to a discussion of the thyroid gland. Most endocrine glands are governed by both endocrine and neurological factors, the exception being the anterior pituitary gland that does not receive autonomic innervation.
As for the thyroid gland, it’s well-established that the trophic hormone TSH, secreted by the anterior pituitary, is the principal endocrine regulator of thyroid function. The release of thyroxine is controlled by the hypothalamus through the anterior pituitary and its release of thyroid-stimulating hormone (TSH). In fact, there is now convincing evidence for the existence of multi-synaptic neuronal pathways between the hypothalamic paraventricular nucleus (PVN) and the thyroid, probably via both branches of the autonomic nervous system.
In addition to this endocrine regulation, it has been known for many years that the thyroid gland is also richly innervated by both sympathetic and parasympathetic nerve fibers. Autonomic nervous regulation of the glandular secretion is not clearly understood, but most of the effect is postulated to be on blood vessels.
Parasympathetic fibers come from the vagus nerves. In that regard, it’s important to keep in mind that besides output to the various organs in the body the vagus nerve conveys sensory information about the state of the body’s organs to the central nervous system. In fact, 80-90 percent of the nerve fibers in the vagus nerve are afferent or sensory nerves communicating the state of the viscera to the brain.
Sympathetic fibers to the thyroid gland were assumed to be distributed from the superior, middle, and inferior ganglia of the sympathetic trunk and these small nerves enter the gland along with the blood vessels. And as early as in the 1930s, nerve-endings surrounding blood vessels and follicles in the thyroid were reported, suggesting a role in thyroid function. More recent studies indicate that the thyroid ganglion and superior cervical ganglion (SCG) contribute most to the nerve supply of the thyroid.
Superior cervical ganglion
Before I list possible functional considerations that may be related to muscle contractions and loss of range of motion in the upper cervical spine, it might be well to review the superior cervical ganglion (SCG) and its connections. It is, after all, the largest of the cervical ganglia and is placed opposite the second and third cervical vertebrae.
The SCG lies posterior to the sheath of the internal carotid sheath and internal jugular vein, and anterior to the Longus capitis muscle. It contains neurons that supply sympathetic innervation to the face and receives input from the ciliospinal center located in the intermediolateral cell columns of the spinal cord between C8 and T2. This plays a role in the control of the iris dilator muscle that temporarily lets more light reach the retina when the sympathetic system responds to stress.
The superior cervical ganglion is believed to be formed by the coalescence of four ganglia, corresponding to the upper four cervical nerves. However, note that these fibers are postganglionic fibers that have already synapsed with preganglionic sympathetic fibers derived from the T1 to T4 levels of the spinal cord. The bodies of these preganglionic sympathetic neurons are specifically located in the lateral horn of the spinal cord.
The potential of sympathetic input to the thyroid in altering thyroid function has been shown in older studies. For example, unilateral electrical stimulation of the postganglionic cervical sympathetic trunk induced a marked increase in plasma radio-labeled iodine in mice pre-treated with thyroxine in order to suppress TSH.
More recently, electrical stimulation of the cervical sympathetic trunk was shown to induce norepinephrine release into the thyroid vein as well as a sharp decrease in thyroid blood flow in rats, suggesting indirect neural control of thyroid function by modulating thyroid blood flow. A functional role for parasympathetic innervation of the thyroid in modulating thyroid blood flow was shown in experimental thyrotoxicosis. In this condition, electrical stimulation of the thyroid nerve resulted in an increase in thyroid blood flow.
Possible visceral dysfunctions from the many organs supplied through the three cervical ganglion, offer a rich and rewarding field for diagnostic endeavor as well as a truly scientific approach for the individual chiropractor.
Next time, I will continue this series with a look at innervation to the heart and lungs.
(Dr. Loomis can be reached by mail at 6421 Enterprise Lane, Madison, WI 53719-1116 or by phone at 800-662-2630. Visit his website at loomisenzymes.com.)
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