A 54 year old man is evaluated by a neurologist because of a gait disorder. When the physician passively moves the patient's right great toe upward or downward, the patient cannot accurately repo the direction of motion, although his perception of light touch and painful stimuli is unimpaired.This finding can best be explained by a lesion of which of the following structures?
The patient's inability to detect the position of his toe reflects a lack of conscious proprioception for this pa of his body. Conscious proprioception, discriminative touch, and vibration sense are all carried by the dorsal column/medial lemniscus system. The fact that he can still perceive light touch and painful stimuli indicates that his anterolateral system is unimpaired. In the dorsal column/medial lemniscus system, the primary neuron's cell body is located in the dorsal root ganglia and sends its projection to the cord through the dorsal roots. The fibers do not synapse in the cord, but rather ascend the cord in the dorsal columns. Fibers carrying information from the legs ascend in the fasciculus gracilis; those receiving input from the arms project in the fasciculus cuneatus. Both ascend to the caudal medulla, where they terminate in the nucleus gracilis and nucleus cuneatus, respectively. The secondary neurons originating from these nuclei cross as the internal arcuate fibers, ascend as the medial lemniscus, then synapse in the ventroposterolateral (VPL) nucleus of the thalamus. Teiary neurons from the VPL project to the ipsilateral somatosensory coex. Therefore, a lack of conscious proprioception from the right toe could result from lesions to the right fasciculus gracilis, the right nucleus gracilis, the left medial lemniscus, the left VPL, or left somatosensory coex. The right fasciculus cuneatus carries discriminative touch, proprioception, and vibration information from the upper extremities.The right lateral lemniscus , pa of the auditory system, receives input from the contralateral cochlear nuclei and from the superior olivary nuclei, and projects to the inferior colliculus.The right medial lemniscus carries discriminative touch, proprioception, and vibration information from the left side of the body. Ref: Waxman S.G. (2010). Chapter 5. The Spinal Cord. In S.G. Waxman (Ed),Clinical Neuroanatomy, 26e.
Right fasciculus cuneatus
Right fasciculus gracilis
Right lateral lemniscus
Right medial lemniscus