Somatosensory and trigeminal pathways

This diagram presents all the somatosensory pathways, the dorsal column-medial lemniscus, the anterolateral, and the trigeminal pathway as they pass through the midbrain region into the thalamus and onto the cortex. The view is a dorsal perspective (as in Figure 10 and Figure 40).

The pathway that carries discriminative touch sensation and information about joint position (as well as vibration) from the body is the medial lemniscus (see Figure 33). The equivalent pathway for the face comes from the principal nucleus of the trigeminal, which is located at the mid-pontine level (see Figure 8B and Figure 35). The anterolateral pathway conveying pain and temperature from the body has joined up with the medial lemniscus by this level (see Figure 34). The trigeminal pain and temperature fibers have likewise joined up with the other trigeminal fibers (see Figure 35).

The various sensory pathways are all grouped together at the level of the midbrain (see cross-section). At the level of the lower midbrain, these pathways are located near to the surface, dorsal to the substantia nigra; as they ascend they are found deeper within the midbrain, dorsal to the red nucleus (shown in cross-section in Figure 65A and Figure 65B).

The two pathways carrying the modalities of fine touch and position sense (and vibration) terminate in different specific relay nuclei of the thalamus (see Figure 12 and Figure 63):

• The medial lemniscus in the VPL, ventral pos-terolateral nucleus

• The trigeminal pathway in the VPM, ventral posteromedial nucleus

Sensory modality and topographic information is retained in these nuclei. There is physiologic processing of the sensory information, and some type of sensory "perception" likely occurs at the thalamic level.

After the synaptic relay, the pathways continue as the (superior) thalamo-cortical radiation through the posterior limb of the internal capsule, between the thalamus and lenticular nucleus (see Figure 26, Figure 27, Figure 28A, and Figure 28B). The fibers are then found within the white matter of the hemispheres. The somatosensory information is distributed to the cortex along the postcentral gyrus (see the small diagrams of the brain above the main illustration of Figure 36), also called S1. Precise localization and two-point discrimination are cortical functions.

The information from the face and hand is topographically located on the dorsolateral aspect of the hemispheres (see Figure 13 and Figure 14A). The information from the lower limb is localized along the continuation of this gyrus on the medial aspect of the hemispheres (see Figure 17). This cortical representation is called the sensory "homunculus," a distorted representation of the body and face with the trunk and lower limbs having very little area, whereas the face and fingers receive considerable representation.

Further elaboration of the sensory information occurs in the parietal association areas adjacent to the postcentral gyrus (see Figure 14A and Figure 60). This allows us to learn to recognize objects by tactile sensations (e.g., coins in the hand).

The pathways carrying pain and temperature from the body (the anterolateral system) and the face (spinal trigeminal system) terminate in part in the specific relay nuclei, ventral posterolateral and ventral posteromedial (VPL and VPM), respectively, but mainly in the intralaminar nuclei. These latter terminations may be involved with the emotional correlates that accompany many sensory experiences (e.g., pleasant or unpleasant).

The fibers that have relayed pain information project from these nuclei to several cortical areas, including the post-central gyrus, SI, and area SII (a secondary sensory area), which is located in the lower portion of the parietal lobe, as well as other cortical regions. The output from the intralaminar nuclei of the thalamus goes to widespread cortical areas.

Clinical Aspect

Lesions of the thalamus may sometimes give rise to pain syndromes (also discussed with Figure 63).

Postcentral gyrus

Caudate n.

Putamen

Globus pallidus

Substantia nigra

Substantia nigra

Medial lemniscus

Trigeminal pathway

Anterolateral system

^alamo-cortical fibers

Ventral posterolateral n. Ventral posteromedial n.

^alamo-cortical fibers

Ventral posterolateral n. Ventral posteromedial n.

Trigeminal pathway

Medial lemniscus

Anterolateral system

Trigeminal pathway

Medial lemniscus

Anterolateral system

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