PATHWAYS OF SOMATOSENSORY SYSTEM

Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations. The sensory information pro­cessed by the somatosensory systems travels along different anatomical pathways depending on the information carried.

11.4.1 Medial Lemniscal Pathway: Body Discriminative Touch and Proprioception

The pathway is important in identifying objects and fea­tures of surfaces by touch even though small shapes can be perceived with information solely from tactile recep­tors, most discriminations involve an active process of tac­tile exploration and multiple contacts on the skin, and an integration of cutaneous and proprioceptive information as well as efferent control. The medial lemniscus is a second- order neuron of the dorsal column-medial lemniscus path­way which commences at the nucleus gracilis and nucleus cuneatus at the caudal medulla; the arcuate fibers decussate at the caudal medulla and ascend via the medial lemnis­cus contralaterally in the brainstem until synapsing at the ventral posterolateral nucleus of the thalamus, the point at which the third-order neuron of the DCML pathway com­mences. The second-order neuron with the somatotopic arrangement, transports the sensory spinothalamic infor­mation of conscious proprioception, vibration, fine touch, and 2-point discrimination of skin and joints of the body and head; from the caudal medulla to the ventral posterolat­eral nucleus of the thalamus, and subsequently the primary somatosensory cortex. The third-order neuron ascends from the ventral posterolateral nucleus until synapsing with the primary somatosensory region of the brain cortex.

The primary function of the medial lemniscus is as a second-order neuron of the dorsal column-medial lemnis­cus pathway is to transport the sensory spinothalamic infor­mation of conscious proprioception, fine touch, 2-point discrimination of skin and joints of the body and head and vibration, from the caudal medulla to the ventral posterolat­eral nucleus of the thalamus, and subsequently the primary somatosensory cortex. Due to the unique somatosensory information and location of this pathway, the dorsal col­umn-medial lemniscus tract may be used as an important clinical clue to pinpoint a possible site of injury and manage the patient accordingly.

The blood supply of the medial lemniscus varies accord­ing to the level of the brainstem. The nuclei gracilis and cuneatus are located in the medial posterior aspect of the medulla and receive vascular supply from the posterior spi­nal artery. The arcuate fibers decussate in the medial-ante­rior aspect of the medulla and are irrigated predominantly by the anterior spinal artery. The medial lemniscus begins ascending contralaterally in the brainstem in the medial aspect of the medulla and is supplied by the anterior spi­nal artery. As the medial lemniscus continues to ascend, it begins shifting posteriorly and laterally; at the level of the pons, the medial lemniscus receives supply from the basilar artery. By the time the medial lemniscus makes it to the midbrain, it is predominantly in the posterior lateral aspect of the midbrain, and the posterior cerebral artery irrigates it.

11.4.2 Main Sensory Trigeminal Pathway: Face Discriminative Touch and Proprioception

The main sensory trigeminal pathway, the cranial homo­logue of the medial lemniscal pathway, mediates discrimina­tive touch and proprioception from the face. Somatosensory information from the face and dura travels via four cranial nerves: the trigeminal (V) nerve, facial (VII) nerve, glos­sopharyngeal (IX) nerve and the vagus (X) nerve. Recall that some afferent fibers in the facial, glossopharyngeal and vagus nerves carry somatosensory information from the ear (pinna, external auditory meatus, tympanic membrane, and middle ear cavity) and posterior dura.

11.4.3 Neospinothalamic Pathway: Body -

Sharp Prickling Pain and Cool/Cold

There are two well-defined spinothalamic pathways, chiefly concerned with pain and temperature sensations and with crude touch. The “fast” conducting neospinothalamic path­way is involved in conveying the “sharp/cutting” pain elic­ited at the time tissue is damaged. The “slower” conducting paleospinothalamic pathway is involved in conveying the “dull/burning” pain that accompanies the later inflamma­tory reaction in the damaged tissue as well as temperature and crude touch information. The archeospinothalamic pathway is a poorly defined pathway involved in a general­ized sense of discomfort and diffuse pain.

The 1° afferents of the spinothalamic pathways send A-delta (neospinothalamic) and C (paleospinothalamic) fibers to the periphery where they form free nerve end­ings in skin, muscle, tendon, joint capsules and viscera. The central processes of these 1° afferent neurons are the pseudounipolar cells of the posterior root ganglia and enter the spinal cord in the lateral division of the posterior root. Recall that fibers entering via the medial division of the posterior root are concerned with tactile, pressure, proprio­ception, and vibration sensations.

a. Neospinothalamic Pathway - The 2° neospinotha- lamic afferents (nucleus posteromarginalis axons) cross in the anterior white commissure to collect in the spinothalamic tract within the contralat­eral anterior and lateral (predominantly) funiculi. These 2° neospinothalamic afferent axons ascend the spinal cord and brain stem in the spinotha­lamic tract to terminate on 3° afferents in the ven­tral posterolateral (VPL) nucleus of the thalamus. Note that these 3° neospinothalamic VPL affer- ents differ from the VPL neurons synapsing with 2° afferent axons of the medial lemniscus. The 3° neospinothalamic VPL neurons send their axons to the primary somatosensory cortex (i.e., the postcentral gyrus of the parietal lobe).

b. Paleospinothalamic Pathway - The 2° paleospino- thalamic afferents (axons of the substantia gelati­nosa) travel a short distance to terminate in or near the ipsilateral nucleus proprius on 3° afferents. The axons of most paleospinothalamic 3° (nucleus proprius) afferents cross in the anterior white com­missure to collect bilaterally in the spinothalamic tracts of the anterior (predominantly) and lateral funiculi. Along their ascending course to the thal­amus, many of the paleospinothalamic 3° (nucleus proprius) afferents leave the spinothalamic tracts to terminate in the brain stem reticular formation or midbrain periaqueductal gray. The remaining paleospinothalamic 3° afferents remain in the spinothalamic tracts and ascend to terminate on 4° paleospinothalamic afferents in the intralami­nar nuclei of the thalamus (IL in the figure). The projections of the intralaminar nuclei axons (4° paleospinothalamic afferents) are to diffuse areas of the cerebral cortex which are believed to play a role in poorly localized sense of pain.

11.4.4 Spinal Trigeminal Pathway: Face Pain, Temperature, and Crude Touch

The spinal trigeminal pathway, the cranial homologue of the spinothalamic pathway, mediates crude touch, pain and temperature from the face. Somatosensory information from the face and dura travels via four cranial nerves: the trigeminal (V) nerve, facial (VII) nerve, glossopharyngeal (IX) nerve and the vagus (X) nerve. Recall that some affer­ent fibers in the facial, glossopharyngeal and vagus nerves carry somatosensory information from the ear (pinna, external auditory meatus, tympanic membrane, and mid­dle ear cavity) and posterior dura. The central processes of the 1° afferent neurons carrying information about simple touch, pain and temperature from the face and dura enter the spinal trigeminal tract and terminate on 2° afferents in the spinal trigeminal nucleus. Most of the 2° afferent axons from the spinal trigeminal nucleus decussate and form the contralateral ventral trigeminothalamic tract (also called the ventral trigeminal lemniscus). The 2° afferent fibers ascend in the ventral trigeminothalamic tract to the thal­amus where they terminate on 3° afferents in the ventral posteromedial nucleus (VPM) or intralaminar nuclei of the thalamus. The axons of these 3° afferents ascend in the posterior limb of the internal capsule and terminate in the postcentral gyrus of the parietal lobe (VPM neurons only) or in more diffuse areas of the cerebral cortex (intralaminar nucleus neurons).

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