[excerpted from 2010 AHVMA Proceedings]
The Autonomic Nervous System (ANS) consists of two parts, the Parasympathetic Nervous System (PNS) and the Sympathetic Nervous System (SNS). Spinal manipulation can affect the SNS.
The SNS governs the “flight or fight” response that the body uses to react to sudden changes in the internal or external environment. The SNS increases output to the heart and other organs, the peripheral blood vessels, and ocular muscles. Both the SNS and the PNS work with each other and the somatic motor system to modulate activity. The SNS and the PNS generally exert opposite influences on the target tissue to keep internal homeostasis.
The motor neurons of the SNS lie outside the spinal cord in the sympathetic ganglia. The preganglionic neurons originate in the spinal cord or brainstem. The sensory neurons for the SNS send some fibers into the ganglia as well as the spinal cord, creating a local reflex arc. Unlike the somatic system, postganglionic nerves in the SNS do not have a specialized end plate. They instead have a series of vesicles where neurotransmitters are stored. These transmitters diffuse along the fiber to reach the target tissue. This allows the system great diversity; one neuron can have control of a great amount of smooth muscle or glandular tissue.
The preganglionic neurons originate in the Intermediolateral horn (IML) from the 1st thoracic to approximately the 5th lumbar segment. The axons of the preganglionic nerves leave the spinal cord in the ventral root and run with the spinal nerve, after which they join the sympathetic chain. These preganglionic nerves can join the sympathetic chain at the level at which they exit the spinal cord, or slightly cranial or caudal. These neurons are small diameter myelinated fibers. The ratio of preganglionic to postganglionic fibers is about 1:10; this builds divergence into the system.
The postganglionic neurons are mostly unmyelinated. They travel in spinal nerves to their target tissue. These neurons innervate blood vessels, sweat glands, heart, lungs, digestive organs, and the adrenal medulla.
SPINAL MANIPULATION AND THE SNS
Vertebral subluxation can impact the SNS on two levels: at the level of the IML because of mechanoreceptor input, and directly on preganglionic nerves as they exit the spinal cord in the ventral root. As an example, we can examine the effect of vertebral subluxation and subsequent adjustment on the SNS as it relates to intestinal motility.
The central nervous system (CNS) depends on excitatory input from mechanoreceptors to stimulate the brain and spinal cord. Approximately 80% of baseline activity of the central nervous system is due to mechanoreceptor input. Most of the mechanoreceptors are located close to the spine as joint mechanoreceptors in ligaments and joint capsules. Also playing a very important role are mechanoreceptors in muscle spindle cells and golgi tendon organs in the intrinsic spinal musculature. Loss of mechanoreceptor activity secondary to chiropractic subluxation can have a huge effect on the CNS because of the divergent nature of the CNS.
The input from mechanoreceptors into the dorsal horn of the spinal cord sends collaterals to the Intermedial Cell Column, which is the output for autonomic function. The effect of this stimulation is excitatory. Mechanoreceptor input from joint mechanoreceptors arrives at the contralateral cortex by way of the thalamus. Input from muscle spindle cells and Golgi tendons arrive at the contralateral cortex via the thalamus, cerebellum, and mesencephalon. The cortex responds by firing to inhibit the ipsilateral IML. This keeps the IML in balance. When subluxation is present, there is decreased inhibition of the IML. Spinal manipulation works to restore normal mechanoreceptor activity, which balances the output of the IML.
Increases in autonomic activity secondary to subluxation have specific outcomes in the digestive tract, namely altered motility and secretion. If we look even further, and use Inflammatory Bowel Disease as an example, there are significant effects of decreased mechanoreceptor input on the Hypothalamic-Pituitary-Adrenal axis (HPA). The HPA is an important part of the stress response. The neuroendocrine regulation of immunity by the HPA occurs via circulating glucocorticoids released by HPA stimulation and by effects of neurotransmitters on immune cells. Reduced responsiveness of the HPA to stress secondary to subluxation would certainly predispose the patient to autoimmune inflammatory disease.
The sympathetic portion of the autonomic nervous system is susceptible to effects of vertebral subluxation through effects on the intermediolateral horn of the spinal cord as well as direct effects on the preganglionic nerves of the SNS itself. Correction of vertebral subluxation through spinal manipulation serves to bring homeostasis to the system and return the body to normal function.
Kandel, ER, Schwartz JH, and Jessell TM. Principles of Neural Science. Fourth Edition. McGraw-Hill. 2000.