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Spinal Cord and Reflexes
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The sensory fibers that enter the spinal cord through the dorsal roots are part of what are referred to as the first-order neurons in the pathway from the periphery to the cerebral cortex. As mentioned before, the cell bodies of these first-order neurons are in the dorsal root ganglia. The dorsal column–medial lemniscus pathway (DCML), also known as the posterior column-medial lemniscus pathway (PCML), contains first-order afferents that convey sensations of fine touch, vibration, and proprioception. The pathway is divided into two components: (i) the gracile fasciculus that carries information from the lower limb, below T6, and terminates in the gracile nuclei in the medulla (Figure 12.17), and (ii) the cuneate fasciculus that carries information from the upper limb, above T6, and terminates in the cuneate nuclei in the medulla. The neurons of the gracile and cuneate nuclei are second-order neurons in the pathway from the periphery to the cerebral cortex. Their axons decussate at the level of the medulla, travel up the brainstem as the medial lemniscus on the contralateral side, and terminate in the thalamus, which relays the signals to the cerebral cortex.
The Somatosensory System
Published in Golara Honari, Rosa M. Andersen, Howard Maibach, Sensitive Skin Syndrome, 2017
Tactile primary afferents, or first-order neurons, immediately turn up the spinal cord toward the brain, ascending in the dorsal white matter and forming the dorsal columns. In a cross section of the spinal cord at cervical levels, two separate tracts can be seen—the midline tracts comprise the gracile fasciculus conveying information from the lower half of the body (legs and trunk) and the outer tracts comprise the cuneate fasciculus conveying information from the upper half of the body (arms and trunk). At the medulla, situated at the top of the spinal cord, the primary tactile afferents make their first synapse with second-order neurons, where fibers from each tract synapse in a nucleus of the same name—the gracile fasciculus axons synapse in the gracile nucleus and the cuneate axons synapse in the cuneate nucleus. The neurons receiving the synapse provide the secondary afferents and immediately cross to form a new tract on the contralateral side of the brainstem—the medial lemniscus—which ascends through the brainstem to the next relay station in the midbrain, the thalamus.
Cervical myelopathy causing numbness and paresthesias in lower extremities: A case report identifying the cause of a false positive Sharp–Purser test
Published in Physiotherapy Theory and Practice, 2019
The patient in this case report experienced Lhermitte’s sign while at the hair salon, however she did not experience the electrical shock or symptoms into the upper extremities during the initial physical therapy examination with the cervical spine range of motion testing. The author hypothesized that during the physical therapy examination, active cervical flexion decreased the central canal space due to the large central disc herniation to the point where the gracile fasciculus, an ascending sensory tract along the medial aspect of the dorsal column of the spinal cord, was compressed and caused the patient’s concordant symptoms of paresthesia in the lower extremities (Purves et al., 2014). Although cervical flexion typically increases the central canal space, the presence of the large central disc herniation caused a narrowing of the central canal space with active cervical flexion.