The pain pathways

All the sensory pathways have their first neuron within the spinal ganglion for spinal nerves and within the cranial ganglion for cranial nerves.

A. Spinal cord level. The epicritic sensibility, concerning precise tactile discrimination and stato-kinesthetic sense given by the skin goniometric information coming from the Ruffini receptors, travels along the medial gracile (lower limb and abdomen) and lateral cuneate (upper limb and thorax) fasciculi in the dorsal column to the corresponding gracile and medial and lateral cuneate nuclei of the medulla. After synapses, the fibers cross the mid-line to form the medial lemniscus reaching the tha-lamic ventral posterolateral nucleus pars oralis (VPLo). They are then projected on to the primary somatosensory cortex located on the parietal postcentral gyrus with area 3 for precise tactile identification, areas 1 and 5 for stereognosia (body scheme) and area 2 for statokinetic perception. Note that the lateral cuneate nucleus is connected with the cerebellum in order to achieve the motor coordination which requires information first on stiffness of the muscles given by the muscular spindles (spinocerebellar tract), identifying the three possible states of the muscle: relaxed, contracted or stretched, and second on position and movement of body segment given by the skin via the lateral cuneate nucleus.

The protopathic sensibility, which concerns pain, temperature and crude touch by AS fibers articulated in the posterior horn with lamina I of Rexed as well as IV and V (nucleus proprius) and C fibers with II and III (substantia gelatinosa) and

VII and VIII, travels in the first conduction system of the dorsal and ventral spinothalamic tract representing the extralemniscal ways placed in the medulla and pons laterally to the medial lemniscus. They will later join to reach the thalamic ventral posterolateral nucleus (VPL) in the pars caudalis for the ventral and in the pars posterior for the dorsal spinothalamic tract (nucleus supragenicula-tus). This protopathic system can be considered as an alarm system. A second conduction system is made by the spinoreticulothalamic tract conveying mainly C fibers more or less mixed with fibers of the ventral spinothalamic tract and going first to medullar and pontine reticular nuclei, second to periaqueductal gray matter and mesencephalic raphe nuclei and third to the reticular nuclei of the medial thalamus (intralaminar and parafascicular). They then reach the anterior cingulate cortex (area 24) and the close prefrontal cortex, adding to the precisely localized pure pain sensation, the emotional component characterizing suffering.

B. Brainstem level. This is directly related to the auricle. As in the spinal cord, the sensory fibers have their first neuron within the cranial ganglions (trigeminal, geniculate, superior and inferior of IX and X) sending fibers to the brainstem nuclei (Plate IC). Three of them are of particular importance:

First, the very long trigeminal nucleus divided into three different parts:

l The caudal part is the spinal trigeminal nucleus with its pars caudalis originating within the cervical spinal cord mainly at C2/C3 level, corresponding to the part of the cervical plexus which supplies a large territory of the auricle. C1 is linked with the supraspinal nucleus corresponding to the motor innervation of the muscles of the craniocervical Cardan joint moving the head along the two visual exploration axes: vertical and horizontal, and placed close to the tectospinal tract ending at the cervical level and connected with the visual superior colliculus for the mandatory visuomotor articulation integrated within the eye-head servo-mechanism. This pars caudalis is mainly devoted to the protopathic sensibility of the face and in fact of the entire auricle. Above, at medullary level, the pars interpolaris receives sensitive facial skin signals and is connected with the cerebellum for the goniometric information related to the deformation of the facial mask by the facial muscles (activated by the CN VII motor nucleus) corresponding to two different activities: first the semi-automatic eyelid movement for corneal protection by lachrymal secretion and second the mouth movements for articulated language. The same problem invests the lateral cuneate nucleus sending goniometric information from trunk and limbs to the cerebellum. At the pontine level the pars oralis receives the epicritic sensations from the face. Some sensory fibers from CN IX and X innervating the middle ear, pharynx mucosa and external auditory meatus are also connected with the trigeminal nucleus, which is of particular interest for the understanding of the innervation of the auricle. After decussation, the fibers then go to the ventral posteromedial thalamic nucleus forming the ventral trigemino-thalamic pathways. To simplify, one can bear in mind that the epicritic pathways join the medial lemniscus and the protopathic pathways, the spinothalamic system (Fig. 2.12A, B).

l The principal sensory trigeminal nucleus is isolated at the pontine level and located on the lateral border of the motor trigeminal nucleus which innervates all the masticatory muscles. Presumably it receives sensory fibers from the teeth and mouth and through sensory feedback allows control and regulation of the forces applied during mastication. Of course the commands for masticating come from the motor keyboard of the precentral gyrus but the force control has to use a closed loop regulation linking the sensory and motor trigeminal nuclei. Uncrossed fibers then go to the ventral postero-medial thalamic nucleus forming the dorsal tri-geminothalamic pathway which is not so well known.

l The mesencephalic trigeminal nucleus is a long thin nuclear column leading to the upper part of the mesencephalon at the superior colliculus devoted to visuomotor articulation. It has a very original feature, collecting the proprioceptive inputs from Ia and Ib fibers (muscular spindles and Golgi organs) directly without synaptic relay in the trigeminal ganglion, not only from the striated masticatory muscles but also from the extraocular muscles, the nerves of which are purely motor and require a sensory anastomosis provided by the ophthalmic branch (CN VI) within the cavernous sinus. That explains the length of this nucleus and it is also conceivable that the 17 muscles of the tongue activated by the pure motor nerve CN XII send their pro-prioceptive information through the sensory anastomosis with branches of the lingual nerve

Epiphysis

Unconscious proprioception IaIb fibers

Epiphysis

Unconscious proprioception IaIb fibers

Vestibular Pathway Diagram

Fig. 2.12 (A) Graphic representation of the nuclei of the brainstem with particular emphasis on the trigeminal nucleus connections.

Fig. 2.12 (A) Graphic representation of the nuclei of the brainstem with particular emphasis on the trigeminal nucleus connections.

(V3) into the mesencephalic nucleus. This particular feature is exactly the same as that of the spinal cord where the Ia and Ib fibers go directly without relay to the alpha motoneurons of the anterior horn with a monosynaptic fast junction. This is not to be considered only in the myotatic reflex matter, but mainly in the fusorial servo-mechanism able to regulate from the periphery the correct level of force to apply for a particular motor task.

Second, the nucleus of the solitary tract. Its upper part (nucleus ovalis) is related to the gustatory sense through fibers from the sensory root of CN VII (anterior two-thirds of the tongue) and IX (posterior third). Its lower part receives sensory fibers of CN IX and X coming from the pharynx and larynx which probably correspond to the superior laryngeal sensory nerve giving to the inferior laryngeal motor nerve a classic proprioceptive anastomosis called the 'anse de Galien' (Galen's anastomosis) along the lateral border of the crico-arytenoid muscle. In addition it is an important vegetative nucleus receiving input signals from vessel receptors and the digestive tract.

Third, the nucleus ambiguus which is a motor nucleus sending motor fibers to the pharynx (CN IX) and to the larynx by the inferior recurrent laryngeal nerve of CN X.

It is important anyway to know that some controversial opinions among neuroanatomists still remain regarding the exact location and functional role of some brainstem structures, but our proposed scheme is the most acceptable at the

30 Auricular acupuncture diagnosis

Larynx Auricular AcupunctureCorticospinal Tract Axial

Fig. 2.12, cont'd (B) Three axial sections: (i) at the mesencephalic are represented with the identification of the different nuclei and

1 corticospinal tract

2 corticonuclear tract

3 frontopontine tract

4 parietotemporopontine tract

5 medial lemniscus

6 lateral lemniscus

7 spinothalamic tract

8 tectospinal tract

9 cuneate nucleus and fasciculus

10 gracile nucleus and fasciculus

11 medial and dorsal longitudinal fasciculus

12 spinal trigeminal nucleus and tract

13 principal sensory trigeminal nucleus

14 mesencephalic trigeminal nucleus and tract

15 motor trigeminal nucleus

16 ventral spinocerebellar tract

17 dorsal spinocerebellar tract

18 substantia nigra, pars reticulata

19 substantia nigra, pars compacta

20 red nucleus, magnocellular part

21 red nucleus, parvocellular part

22 cerebral aqueduct

23 rubrospinal tract

24 dorsal and lateral periaqueductal gray

25 central tegmental tract

26 medial geniculate body

27 lateral geniculate body

28 supraspinal nucleus (c1)

29 lowest part of the nucleus ambiguous (accessory nucleus)

30 nucleus of the solitary tract

31 superior vestibular nucleus

32 middle cerebellar peduncle (brachium pontis)

33 ventral tegmental area (A10)

34 dorsal nucleus of raphe (B7)

35 superior colliculus

36 oculomotor nuclei (Perlia, principal and Edinger-Westphal)

37 optic tract

38 mamillary body

39 nucleus reticularis tegmenti pontis

40 nucleus reticularis tegmenti caudalis

41 nucleus reticularis medullae oblongatae centralis

42 vestibulospinal tract

43 dorsal trigemino thalamic tract

44 ventral trigemino thalamic tract

Fig. 2.12, cont'd (B) Three axial sections: (i) at the mesencephalic are represented with the identification of the different nuclei and level; (ii) at the level of the pons; (iii) at the medullary level, pathways.

Corticonuclear Tract Pathway

1 corticospinal tract

2 corticonuclear tract

3 arcuate nucleus

4 medial lemniscus

5 tectospinal tract

6 medial longitudinal fasciculus

7 nucleus raphe obscurus

8 nucleus raphe pallidus

9 ventral trigeminothalamic tract

10 inferior olivary nucleus

11 central tegmental tract

12 spinothalamic tract

13 ventral spinocerebellar tract

14 rubrospinal tract

15 corpus pontobulbare

16 inferior cerebellar peduncle

17 dorsal cochlear nucleus

18 spinal trigeminal nucleus

19 spinal trigeminal tract

20 nucleus ambiguus

21 fibers of the IX

22 nucleus reticularis gigantocellularis

23 dorsal longitudinal fasciculus

24 nucleus prepositus

25 nucleus of the solitary tract

26 medial vestibular nucleus

27 descending vestibular root

28 inferior vestibular nucleus

Fig. 2.13 An example of the possible identification and localization of nuclei and pathways: vessel microinjection with black ink (A); anatomical interpretation of the localization of nuclei and pathways (B) (from Henri Duvernoy with permission).

moment, based on the data in the literature and the exceptional high-level research work carried out by Henri Duvernoy13,14,15 from Besancon in France (Fig. 2.13).

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