Lower figure

This illustration shows the difference at the entry level between the two sensory pathways — the dorsal column tracts and the anterolateral system. The cell bodies for these peripheral nerves are located in the dorsal root ganglion, the DRG (see Figure 1).

On the left side, the afferent fibers carrying discriminative touch, position sense, and vibration enter the dorsal horn and immediately turn upward. The fibers may give off local collaterals (e.g., to the intermediate gray), but the information from these rapidly conducting, heavily myeli-nated fibers is carried upward in the two tracts that lie between the dorsal horns, called collectively the dorsal columns. The first synapse in this pathway occurs at the level of the lower medulla (see Figure 33).

On the right side, the afferents carrying the pathways for pain, temperature, and crude touch enter and synapse in the nuclei of the dorsal horn. The nerves conveying this sensory input into the spinal cord are thinly myelinated or unmyelinated, and conduct slowly. After several synapses, these fibers cross the midline in the white matter in front of the commissural gray matter (the gray matter joining the two sides), called the ventral (anterior) white commissure (see upper illustration). The fibers then ascend as the spino-thalamic tracts, called collectively the anterolateral system (see Figure 34).

Clinical Aspect

The effect of a lesion of one side of the spinal cord will therefore affect the two sensory systems differently because of this arrangement. The sensory modalities of the dorsal column system will be disrupted on the same side. The pain and temperature pathway, having crossed, will lead to a loss of these modalities on the opposite side.

Any lesion that disrupts just the crossing pain and temperature fibers at the segmental level will lead to a loss of pain and temperature of just the levels affected. There is an uncommon disease called syringomyelia that involves a pathological cystic enlargement of the central canal. The cause for this is largely unknown but sometimes can be related to a previous traumatic injury. The enlargement of the central canal interrupts the pain and temperature fibers in their crossing anteriorly in the anterior white commissure. Usually this occurs in the cervical region and the patients complain of the loss of these modalities in the upper limbs and hand, in what is called a cape-like distribution. The enlargement can be visualized with MRI.

Dorsal horn Intermediate gray Ventral horn

Dorsal horn Intermediate gray Ventral horn

Substantia Gelatinosa

Dorsolateral fasciculus (of Lissauer)

Posteromarginal n. Substantia gelatinosa Proper sensory n.

Dorsal n. (of Clarke)

Ventral white commissure

Dorsal column fiber

Discriminative touch/ joint position/ vibration afferent

Spino-cerebellar fiber (proprioception)

Spino-thalamic fiber (anterolateral system)

Spino-cerebellar fiber (proprioception)

Spino-thalamic fiber (anterolateral system)

Pain And Temperature Spinal Cord

Pain/temperature/ crude touch afferent

Interneuron

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