Primary visceral sensory afferent fibers

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Two types of polymodal visceral sensory receptors have been described in association with some viscera: low-threshold (about 75% to 80% of the afferent fibers) and high-threshold (about 20% to 25%) mechanoreceptors.

The low-threshold mechanoreceptors respond to mechanical stimuli in a physiologic range around, for example, distension of 5 mm Hg, whereas high-threshold receptors respond to a noxious intensity (60 to 80 mm Hg). These two types of receptors also respond to chemical and thermal stimuli; thus they are also chemonociceptive receptors. Accordingly, both low- and high-threshold mechanoreceptors can be activated and respond to chemical stimuli such as bradykinin, prostaglandins, and a group of inflammatory mediators.

CENTRAL TERMINATIONS OF VISCERAL AFFERENT FIBERS

Most visceral sensory input to the CNS is not perceived consciously (Fig. 11-1). For example, much of the gastrointestinal vagal afferent input is associated with mucosal endings that sample the luminal contents and with secretory and motor events that do not reach conscious appreciation.

The cell bodies of vagal afferent fibers are in nodose ganglia, and the central terminals of these fibers are principally in the nucleus of the solitary tract in the dorsal medulla. A small portion of vagal afferent fibers continue either through the medulla or by another nonmedullary route and terminate in the first and second cervical segments of the spinal cord, where they may be involved in modulation of spinal nociceptive processing.7

Vagal afferent input to the CNS is not thought to contribute to visceral pain, but mechanosensi-tive vagal afferent fibers are sensitized when exposed to thermal or chemical stimuli such as hydrochloric acid, bile salts, or nerve growth factors, and these fibers may contribute to chemonociceptive input to the CNS. For example, intragastric instillation of hydrochloric acid leads to the expression of c-fos protein in the brainstem and stimulates visceromotor response to dilute the hydrochloric acid, whereas vag-otomy blocks this visceromotor response.8,9

Retro-olivary groove Jugular foramen Superior ganglion (X) Inferior ganglion (X)

Vagus nerve [X]

Common carotid artery

Subclavian artery Brachiocephalic trunk Aortic arch Cardiac plexus--

Posterior vagal trunk Thoracic aorta

Pulmonary plexus

Oesophageal plexus

Hepatic branches

Coeliac branches

Vagus nerve [X]

Posterior vagal trunk Thoracic aorta

Pons

Oesophageal plexus

Hepatic branches

Coeliac branches

Pulmonary plexus

Figure 11-1 The central termination of the vagal afferent fiber.

Pons

Auricular branch Meningeal branch Communicating branch with glossopharyngeal nerve Pharyngeal branches Superior laryngeal nerve

Superior cervical cardiac branches Recurrent laryngeal nerve Inferior cervical cardiac branches Thoracic cardiac branches Recurrent laryngeal nerve Bronchial branches

Anterior vagal trunk

Oesophageal hiatus Anterior gastric branches (Splenic branches)

Renal branches (Pancreatic branches) (Intestinal branches)

Figure 11-1 The central termination of the vagal afferent fiber.

The cell bodies of spinal visceral afferent fibers are in dorsal root ganglia, and the central terminals of these fibers are in laminae I and II, the superficial dorsal horn of the spinal cord, the intermediolateral cell column and sacral parasympathetic nucleus (pelvic nerve), and the lamina X (dorsal to the central canal) (see Fig. 11-1). Almost all second-order neurons in the spinal cord that receive visceral input also receive convergent somatic input from skin and muscle. This is considered the basis of referral of visceral sensation to somatic sensation. For example, a patient with myocardial ischemia feels pain in the left shoulder and upper arm and occasionally in the jaw, but not in the heart, the source of the pain. In addition to the convergence of somatic and visceral inputs on second-order spinal neurons, this kind of convergence is also common between, for example, bladder and colon and between colon and uterus.

In view of the physiologic activity of visceral afferent fibers, the diffuse character of visceral innervation and sensation, referral to somatic sites, and convergence of inputs from multiple viscera to the same spinal neurons, visceral pain is more challenging, both to patients and to physicians, than somatic pain.

In the past, it was demonstrated that the primary pathway for pain-related information from the dorsal horn of the spinal cord to the brain was via the anterolateral quadrant white matter of the spinal cord, within which are the spinothalamic, spinoreticular, spinomes-encephalic, and spinohypothalamic tracts. More recent research demonstrated that a dorsal column pathway is also involved in visceral afferent inputs.10

Functional imaging during visceral stimulation has revealed consistency in the response from the brain. Rectal distension and urinary bladder distension both produce increased blood flow in select areas of the thalamus, hypothalamus, mesencephalon, pons, and medulla (Fig. 11-2).11 Cortical responses include the anterior and midcingulate cortices, the frontal and parietal cortices, and the cerebellum.12

INTERACTIONS BETWEEN VISCERAL AND SOMATIC REFLEXES

Practitioners' knowledge of how needling acu-reflex points can induce balance in visceral physiology is developing fast, and this improved understanding of

Cingulate gyrus Cingulate sulcus

Superior frontal gyrus

Corpus callosum

(body) Central

Septum sulcus pellucidum

Cingulate sulcus (marginal branch)

Corpus callosum: rostrum genu

Anterior cerebral artery

Hypothalamus

Uncus

Subparietal sulcus

Corpus callosum (splenium)

Parietooccipital sulcus

Calcarine sulcus

Superior frontal gyrus

Corpus callosum

(body) Central

Septum sulcus pellucidum

Cingulate sulcus (marginal branch)

Subparietal sulcus

Corpus callosum (splenium)

Corpus callosum: rostrum genu

Anterior cerebral artery

Calcarine sulcus

Cerebellum: primary fissure vermis hemisphere

Hypothalamus

Uncus

Occipitotemporal gyrus

Inferior temporal gyrus

Figure 11-2 Median section of the brain to show the connection between the brain and some of the viscera (see text).

Occipitotemporal gyrus

Inferior temporal gyrus

Cerebellum: primary fissure vermis hemisphere

Figure 11-2 Median section of the brain to show the connection between the brain and some of the viscera (see text).

the interaction between reflex systems improves clinical practice. During electroacupuncture, low-frequency, low-intensity somatic afferent stimulation produces long-lasting reduction in blood pressure—increased by visceral afferent fibers—by as much as 40%.13

Acupuncture stimulates group III and IV somatic afferent fibers that provide input to both spinal and supraspinal centers in the CNS, as discussed in previous chapters. During acupuncture, polysynaptic input to the ventral hypothalamic arcuate nucleus, midbrain ventrolat-eral periaqueductal gray, and midline medullary (raphe) nuclei causes prolonged inhibition of sympathoexcitatory cardiovascular premotor neuronal activity and sympathetic outflow in the rostral ventrolateral medulla. Visceral afferent fiber-induced activity in bulbospinal glutaminer-gic neurons of the rostral ventrolateral medulla is inhibited by modulatory neuropeptides, including enkephalins and endorphins, nociceptin, and gamma-aminobutyric acid (GABA), released during low-level somatic stimulation.

These somatic visceral interactions have the capability of ultimately lowering elevated blood pressure and reducing demand-inducing myocardial ischemia.

REFERRED OR REFLEX ZONES OF MAJOR VISCERA

Most of the data from Chinese acupuncture literature on reflex zones are empirical, and further research is needed to confirm and refine these zones. In general, for the viscera that are anatomically above the diaphragm, such as the heart and lungs, acu-reflex points are projected to the upper part of the body and upper limbs (Figs. 11-3 and 11-4). For the viscera below the diaphragm, such as intestines or urogenital organs, acu-reflex points are projected to the lower part of body and lower limbs (Figs. 11-5 to 11-8). For an organ situated just level with the diaphragm, such as the stomach,

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Figure 11-3 Heart reflex zones. The heart reflex zone involves the spinal nerves T1 to T7 on the chest and back and the cutaneous branches from the median and ulnar nerves (C5 to T1 and C7 to T1) on the arm and forearm.

Figure 11-3 Heart reflex zones. The heart reflex zone involves the spinal nerves T1 to T7 on the chest and back and the cutaneous branches from the median and ulnar nerves (C5 to T1 and C7 to T1) on the arm and forearm.

Figure 11-6 Intestine reflex zones. The small and large intestine reflex zone involves the spinal nerves T10 to S2 or S3 on the abdominal region and on the lower back, the iliohypogastric nerve (T12 to L1), the saphenous nerve (a branch from the femoral nerve [L1 or L2 to L4]), and the common fibular nerve (L4 to S2).

Figure 11-6 Intestine reflex zones. The small and large intestine reflex zone involves the spinal nerves T10 to S2 or S3 on the abdominal region and on the lower back, the iliohypogastric nerve (T12 to L1), the saphenous nerve (a branch from the femoral nerve [L1 or L2 to L4]), and the common fibular nerve (L4 to S2).

Figure 11-7 Liver, gall bladder, spleen, and pancreas reflex zones. These reflex zones involve the spinal nerves T3 to T12, the suprascapular nerve (C4 to C6), the saphenous nerve (L1 or L2 to L4), and the common fibular nerve (L4 to S2). The organs usually project reflex zones ipsilaterally; for example, the liver projects reflex zones only to the right side of the body.

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tions of the eyes may sensitize the greater occipital nerve tions of the ears may sensitize the lesser occipital nerve (C2). (C2 to C3) and the greater auricular nerve (C2 to C3).

conditions of the nose may sensitize the greater occipital nerve (C2) and spinal nerves C3 to C7.

pathologic conditions of the oral cavity and the throat may sensitize the greater occipital nerve (C2).

acu-reflex points are projected to both the upper and lower body and limbs. Single organs always produce referred hyperalgesia ipsilaterally (Figs. 11-9 to 11-12).

References

1. Cervero F, Laird JMA: Visceral pain, Lancet 353:2145-2148, 1999.

2. Raybould H: Visceral perception: sensory transduction in visceral afferents and nutrients, Gut 51:i11-i14, 2002.

3. Laird JMA, Souslova V, Wood JN, et al: Deficits in visceral pain and referred hyperalgesia in Nav1.8(SNS/PN3) null mice, J Neurosci 22:8352-8356, 2002.

4. Al-Chaer ED, Traub RJ: Biological basis of visceral pain: recent developments, Pain 96:221-225, 2002.

5. Giamberardino MA, Berkley KJ, Iezzi S, et al: Pain threshold variations in somatic wall tissues as a function of menstrual cycle, segmental site and tissue depth in non-dysmenorrheic women, dysmenorrheic women and men, Pain 71:187-197, 1997.

6. Giamberardino MA: Recent and forgotten aspect of visceral pain, Eur J Pain 3:77-92, 1999.

7. Hirshberg RM, Al-Chaer ED, Lawand NB, et al: Is there a pathway in the posterior funiculus that signals visceral pain? Pain 67:291-305, 1996.

8. Lamb K, Kang Y-M, Gebhart GF, et al: Gastric inflammation triggers hypersensitivity to acid in awake rats, Gastroenterology 125:1410-1418, 2003.

9. Randich A, Gebhart GF: Vagal afferent modulation of noci-ception, Brain Res Rev 17:77-99, 1992.

10. Nauta HJW, Soukup VM, Fabian RH, et al: Punctate mid-line myelotomy for the relief of cancer pain, J Neurosurg 92:125-130, 2000.

11. Blok BFM: Central pathways controlling micturition and urinary continence, Urology 59:13-17, 2002.

12. Athwal BS, Berkley KJ, Hussain I, et al: Brain responses to changes in bladder volume and urge to void in healthy men, Brain 124:369-377, 2001.

13. Longhurst J: Acupuncture. In Robertson D, Low P, Burnstock G, et al: Primer on the autonomic nervous system, New York, 2004, Academic Press, pp 246-249.

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  • cindy
    Can dry needling activate visceral hypersensitivity syndrome?
    8 months ago

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