Spinothalamic Tract

From the receptor, information is transmitted to the afferent fibers of the pseudounipolar spinal ganglion cells, whose efferent fibers reach the spinal cord by way of the dorsal root. A synapse onto a second neuron in the sensory pathway is made either immediately, in the posterior horn of the spinal cord (protopathic system), or more rostrally, in the brain stem (epicritic/lemniscal system). The highest level of the somatosensory pathway is the contralateral primary soma-tosensory cortex. The somatotopic organization of the somatosensory pathway is preserved at all levels.

Posterior column (epicritic/lemniscal system).

Fibers mediating sensation in the legs are in the fasciculus gracilis (medial), while those for the arms are in the fasciculus cuneatus (lateral). These fibers synapse onto the second sensory neuron in the corresponding somatosensory nuclei of the lower medulla (nucleus gracilis, nucleus cuneatus), which emit fibers that decussate and ascend in the contralateral medial lem-niscus to the thalamus (ventral posterolateral nucleus, VPL). VPL projects to the postcentral gyrus by way of the internal capsule. Anterolateral column (protopathic system). Fibers of the protopathic pathway for somatic sensation (strong pressure, coarse touch) enter the spinal cord through the dorsal root and then ascend two or more segments before making a synapse in the ipsilateral posterior horn. Fibers originating in the posterior horn decussate in the anterior commissure of the spinal cord and enter the anterior spinothalamic tract, which is somatotopically arranged: fibers for the legs are anterolateral, fibers for the arms are posterome-dial. The anterior spinothalamic tract traverses the brain stem adjacent to the medial lemniscus and terminates in VPL, which, in turn, projects to the postcentral gyrus. The protopathic pathway for pain (as well as tickle, itch, and temperature sensation) is organized in similar fashion: Central fibers of the first sensory neuron ascend 1 or 2 segments before making a synapse in the substantia gelatinosa of the posterior horn. Fibers from the posterior horn decussate and enter the lateral spinothalamic tract, which, like the anterior spinothalamic tract, projects to VPL; VPL projects in turn to the postcentral gyrus.

Spinocerebellar tracts (spinocerebellar system).

These tracts mediate proprioception. Fibers originating from muscles spindles and tendon organs make synapses onto the neurons of Clarke's column within the posterior horn at levels T1-L2, whose axons form the posterior spinocerebellar tract (ipsilateral) and the anterior spinocerebellar tract (both ipsilateral and contralateral). These tracts terminate in the spinocerebellum (p. 54).

Lateral spinothalamic tract

-Thalamocortical tract

Lateral spinothalamic tract

-Thalamocortical tract

Spinothalamic Pathway And Proprioception

Deep sensation (proprioception)

Vibration

Touch, pressure

Deep sensation (proprioception)

Vibration

Touch, pressure

Examination. Somatic sensation is tested with the patient's eyes closed. The examiner tests each primary modality of superficial sensation (touch, pain, temperature), the patient's ability to distinguish different qualities of each modality (sharp/blunt, hot/cold, different intensities, two-point discrimination), and more complex sensory modalities (stereognosis, graphesthe-sia). Next, sensation to pressure and vibration stimuli are tested, as is acrognosis (posture sense), to evaluate proprioception. Sensory disturbances commonly cause disturbances of posture (tests: Romberg test, standing on one leg) or gait (p. 60).

Interpretation of findings. There is a wide range of normal findings. Apparent abnormalities should be interpreted in conjunction with findings of other types, such as abnormal reflexes or paresis. Sensory dysfunction may involve not only a diminution or absence of sensation (hy-pesthesia, anesthesia), but also sensations of abnormal type (paresthesia, such as prickling or formication) or spontaneous pain (dysesthesia, often of burning type). Patients often use the colloquial term "numbness" to mean hypesthe-sia, anesthesia, or paresthesia; the physician should ask specific questions to determine what is meant.

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