CHAPTER 8. THE SOMATOSENSORY SYSTEM

OBJECTIVE:
    BE ABLE TO IDENTIFY IN SPINAL CORD AND BRAINSTEM SECTIONS AND IN GROSS SPECIMENS THE STRUCTURES THAT TRANSMIT SOMATOSENSORY IMPULSES.

Common Peripheral Components

Receptors

Somatosensory nerve endings are found in skin Figure 8-1, connective tissues, skeletal muscles, tongue, cornea, teeth, etc.

Spinal Ganglia and Dorsal Roots

Note the circular bodies of the unipolar ganglion cells in Slide 61.  The axons of these unipolar neurons, upon emerging from the parent cell bodies, divide into two branches.  The peripheral branches enter the spinal nerves and travel via their branches to eventually terminate as somatosensory receptors.  The central branches enter the dorsal roots (Pls. 18, 20) and pass toward the spinal cord.

Each dorsal root divides into about six rootlets which are lined up in a longitudinal fashion so that the lowermost rootlet enters the most caudal part of a specific segment and the uppermost enters the most rostral part of this same segment. Thus we have a means of determining the extent of any spinal cord segment. Immediately before entering the cord, each rootlet separates into lateral and medial divisions. The lateral division contains the axons subserving pain and temperature sensations, whereas the medial division subserves touch, pressure, etc.

Similar unipolar neurons are located in the ganglia of the trigeminal, facial, glossopharyngeal, and vagus nerves that subserve somatosensory sensations in the head.

Spinal Tactile, Vibration and Proprioception Paths

First Order Neurons: Spinal Ganglia

Identify the larger unipolar cells in the spinal ganglia. (Sls. 60, 61).  The central branches of the tactile, vibration, and proprioception axons pass via the medial division of the posterior root and enter the spinal cord at the lateral border of the posterior funiculus as bundles of heavily myelinated fibers (Sls. 1, 2, 3, 4, 5, 6, 7). In slides of sacral, lumbar, thoracic and cervical segments of the spinal cord, identify the dorsal column tracts. Below mid-thoracic levels the dorsal column is comprised of the gracile tract, whereas above mid-thoracic and in cervical segments it consists of the gracile tract medially and the cuneate tract laterally.


FIGURE 8-1.  CUTANEOUS RECEPTORS

Second Order Neurons: Dorsal Column Nuclei

The axons of the gracile and cuneate tracts terminate upon second order neurons in the gracile and cuneate nuclei (Sls. 10, 11).  These are called the dorsal column nuclei. Note that the axons from the dorsal column nuclei form small bundles of myelinated fibers that emerge from the anterior aspects of the nuclei and arch around the central gray as internal arcuate fibers. They then cross the midline as the sensory decussation and begin to ascend in the large bundle located next to the midline, the medial lemniscus. This conspicuous and medically important bundle should be identified as it ascends through the medulla, pons, and midbrain (Sls. 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33).  Note that in the pons the medial lemniscus gradually shifts laterally.

Third Order Neurons: Ventral Posterolateral Nucleus (VPL)

Identify the VPL in the thalamus (Sls. 34, 35, 36).  Axons from the VPL neurons subserving tactile and proprioception sensations, pass laterally as thalamocortical fibers and enter the posterior limb of the internal capsule (Sls. 35, 36) where they are located in its more posterior part (Sls. 47, 48).  They terminate in the primary somatosensory (SI) cortex which is located in the postcentral gyrus (Pls. 1, 4) and the adjacent posterior part of the paracentral lobule (Pl. 5).

The upper limb is represented approximately in the dorsal half of the postcentral gyrus and the lower limb is represented in the posterior part of the paracentral lobule and the area where it joins the postcentral gyrus on the superior margin of the hemisphere. Keep in mind that the somatotopic representation is contralateral.

Spinal Pain and Temperature Paths

First Order Neurons: Spinal Ganglia

Smaller unipolar cells in the spinal ganglia are associated with pain and temperature sensations. The central branches of their axons approach the spinal cord via the lateral divisions of the posterior rootlets. They enter near the posterolateral sulcus and pass into the dorsolateral tract of Lissauer which should be identified as a poorly myelinated, lightly stained bundle of fibers between the dorsal horn and the surface of the spinal cord (Sls. 1, 2, 3, 4, 5, 6, 7).  Upon entering the tract of Lissauer the axons divide into descending branches (for intra- and intersegmental reflexes) and ascending branches which pass rostrally for about one or two segments. Each branch gives off numerous collaterals as it courses in the tract of Lissauer.

Second Order Neurons: Dorsal Horn

The pain and temperature first order axons synapse on neurons located in the dorsal horn, intermediate zone, and ventral horn (Sls. 1, 2, 3, 4, 5, 6, 7).  The second order fast pain and temperature neurons are chiefly in lamina I while the slow pain neurons are chiefly in laminae IV, V, VI but also in the medial part of lamina VII and in lamina VIII (Sls. 4, 7).

Identify lamina II (the substantia gelatinosa) which consists of densely packed small interneurons and is conspicuous because of its relatively light appearance in both myelin-stained (Sls.1, 2, 3, 4, 5, 6, 7) and silver impregnated (Sl. 60) preparations.  Most of its axons terminate in relation to second order slow pain neurons and play a major role in the modulation of slow pain.

The axons from the secondary pain and temperature neurons pass ventromedially and decussate in the ventral white commissure (Sls. 1, 2, 3, 4, 5, 6, 7).

After crossing to the contralateral side the pain and temperature axons pass to the anterior part of the lateral funiculus, the anterolateral quadrant, where they ascend as the spinothalamic tract.

The position of the medically important spinothalamic tract should be identified in the various spinal cord and brainstem levels (Sls. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32).

Damage to the spinothalamic tract will result in decreased pinprick and temperature sensations on the contralateral side in the limbs, trunk, neck, and occiput if the lesion is in the brainstem. If damaged in the spinal cord, virtually all pain and temperature sensations are lost in all contralaterally dermatomes one or two segments below the level of the lesion. Rostral to the spinal cord, the slow pain paths ascend diffusely via the brain stem reticular formation to the more medial parts of the thalamus.

Third Order Neurons: Ventral Posterolateral Nucleus

The fibers transmitting fast pain and temperature intermingle with the medial lemniscus at the level of the pretectal region (Sl. 33) and terminate in the ventral posterolateral nucleus (Sls. 34, 35, 36).  This nucleus is somatotopically organized: lower limb laterally and upper limb medially. The tertiary pain and temperature neurons in the VPL give thalamocortical fibers that pass laterally and enter the posterior limb of the internal capsule intermingling with the tactile and proprioception axons in the more posterior part (Sls. 47, 48).  All these somatosensory fibers terminate in those parts of the postcentral gyrus and paracentral lobule associated with the neck, trunk, and limbs.

Cranial Tactile, Vibration, and Proprioception Paths

Tactile, vibration, and proprioception impulses from the head are conducted centrally chiefly in cranial nerve V. The central connections are made with the trigeminal nuclei and the impulses ascend to higher centers in the trigeminothalamic system.

First Order Neurons: Trigeminal Ganglion

These are the larger unipolar cells in the trigeminal ganglion. They are homologous to those in the spinal ganglia. The central branches of the axons approach the pons in the sensory root of the trigeminal nerve (Pl. 8), enter at the junction between the basilar pons and middle cerebellar peduncle, and pass dorsomedially toward the pontine tegmentum.

Second Order Neurons: Principal Trigeminal Nucleus

The first order axons from the trigeminal ganglion cells subserving touch and proprioception terminate on second order neurons primarily in the principal trigeminal nucleus (Sls. 23, 24). Axons from here ascend mainly as crossed fibers in the trigeminothalamic tract located in the vicinity of the medial lemniscus (Sls. 24, 26, 28, 29, 33, 34).

Third Order Neurons: Ventral Posteromedial Nucleus (VPM)

The secondary trigeminal fibers terminate in the VPM (Sls. 34, 35, 36) which sends thalamocortical axons via the posterior limb of the internal capsule to the ventral part of the postcentral gyrus (Pl. 4), the SI head area.

Cranial Pain and Temperature Paths

Pain and temperature impulses from the head are conducted centrally in cranial nerves V, VII, IX, and X. The central connections and ascending paths in the brainstem occur via the trigeminothalamic system.

First Order Neurons: Trigeminal Ganglion

The smaller unipolar neurons in the trigeminal ganglion (as well as in the geniculate, petrosal, and jugular ganglia of VII, IX, and X respectively) subserve pain and temperature impulses.  The central branches of the axons of the trigeminal ganglion cells enter the pons through the sensory root of the trigeminal nerve (Pl. 8) and course dorsomedially at the junction of the middle cerebellar peduncle and basilar part of the pons.  Upon reaching the pontine tegmentum, they form a conspicuous bundle, the spinal trigeminal tract, which descends through the pons and medulla and intermingles with the dorsolateral tract of Lissauer in the upper cervical segments of the spinal cord.  Identify the spinal trigeminal tract as it proceeds caudally through the pons and medulla (Sls. 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9).

The central branches of the pain and temperature axons from the unipolar neurons in the geniculate, petrosal, and jugular ganglia join the spinal trigeminal tract at their levels of entrance into the brainstem.

Within the spinal trigeminal tract at the obex, fibers from the mandibular division lie dorsomedially, fibers from the ophthalmic division ventrolaterally, and fibers from the maxillary division in between.

Second Order Neurons: Spinal Trigeminal Nucleus  (Sls. 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)

The primary pain and temperature fibers descending in the spinal trigeminal tract terminate in the spinal trigeminal nucleus, chiefly in its caudal part (Sls. 9, 10, 11, 12, 13)which extends from the level of the obex to the spinal cord where it becomes continuous with the dorsal horn (Sl. 8).

Somatotopic localization seems to occur in the caudal part of the spinal trigeminal nucleus since it has been shown that fibers carrying cutaneous impulses from the mandibular division terminate rostrally, from the ophthalmic division caudally, and from the maxillary division in between. It has also been shown that pain and temperature fibers from deeper structures such as the dental pulp terminate in the interpolar part of the spinal trigeminal nucleus.

Second order trigeminothalamic neurons in and adjacent to the spinal trigeminal nucleus, pars caudalis, give rise to axons that cross the midline and ascend in the trigeminothalamic tract, which is thought to be near the dorsal part of the medial lemniscus at medullary levels (Sls. 13, 15, 18, 20) and near its medial part at pontine and midbrain levels (Sls. 22, 24, 26, 28, 29, 33, 34).

Third Order Neurons:  Ventral Posteromedial Nucleus (VPM)

The secondary fast pain and temperature axons in the trigeminothalamic tract terminate in the VPM 34, 35, 36.  The tertiary pain and temperature VPM neurons project thalamocortical axons to the face area of the postcentral gyrus via the posterior limb of the internal capsule.